Steel Bar Products

Steel long products, which include bar, rod and wire products, based on information provided in a 2010 Mega Associates study, account for about 20 to 25 percent by weight of the steel used in North American (NA) light and heavy vehicles.  Of this more than 85 percent are parts sourced from hot-rolled bar products. Parts made from steel bars represent 400-500 pounds of an average NA vehicle, and this quantity has remained consistent despite ongoing lightweighting programs. These parts are mainly used in the powertrain, driveline, suspension and steering systems. The steel bars are manufactured into a wide variety of components in a vehicle including, gears, connecting rods, crankshafts, injector systems, camshafts, hubs, bearings, transmission shafting , steering racks and steering linkages, stabilizer bars, constant velocity (CV) joints, drive axles, suspension springs, and more.

North American steel bar producers provide a wide range of steel grades with unique chemistries and properties. In addition, metallurgical processes such as heat treating, thermo-mechanical rolling, cold working and surface hardening enable diverse properties in these products. These processes impart a wide range of new properties ranging from a very ductile, formable structure to a high-strength product with excellent fatigue properties. This wide scope of mechanical properties and the fact steel bar production is a proven high volume manufacturing process bodes well when considering steel as the material of choice for demanding automotive part applications.

The automotive industry is striving to affordably reduce the mass of a vehicle to improve fuel efficiency while maintaining or improving safety and performance. In order to move in this direction there is a need to create more power-dense steel products, that is make components lighter and stronger. North American steel suppliers are actively engaged in optimized design projects with original equipment manufacturers (OEM) and Tier 1 and 2 customers.

As vehicles are downsized automotive companies will look to optimize component weight by looking at cost effective ways to reduce part weight, modify the design of the part or consider other light weight materials. When deciding on the best lightweighting strategy it is important to consider other aspects such as past performance, vehicle safety, life-cycle environmental impacts, and the value of the lightweighting approach.

Steel is a proven product in critical design, is 100 percent recyclable and has the ability to be cold formed, hot forged, machined and extruded into a variety of part configurations. Steel solutions using these manufacturing processes offer high value to the customer as there is an opportunity to optimize manufacturing costs and part performance. 

Part manufacturing processes represent a significant portion of the part cost. For example, machining can be as much as 40 percent of the part cost. To address steel machining performance, Steel Market Development Institute - Bar Application Group (SMDI BAG) has been working with steel bar producers for a number of years in developing a bar steel machinability database. This machinability data can provide design and manufacturing engineers useful information when making decisions on steel grade options and specific bar processing options as a function of machinability. This machinability database, shown in Figure 22, continues to be expanded and has a Machinability Estimator with easy access to machinability data.

 Figure 22 - AISI Machinability Predictor from SMDI Website (

From a part design perspective, the use of Finite Element Analysis (FEA) modeling is proving to be a useful tool to accurately analyze stress levels in redesigned lighter weight parts. In order to take full advantage of FEA as a predictive tool, steel grade characterization is very important as FEA models must be populated with reliable mechanical property data such as true strain data and tensile data. Throughout the past 15 years, SMDI BAG has been developing a fatigue property database in conjunction with a number of NA steel suppliers and automotive OEM producers. This fatigue database development is a continuing project. Fatigue data consists of fatigue testing and characterization of a number of automotive steel grades processed to specific hardness levels or other special requirements and provides the automotive component designer comparative data when making material choices. Figure 23 shows a typical fatigue curve for steel grade 20MoCr4 which was tested as a part of this program.


Figure 23 - Overload and constant fatigue curves for steel grade 20MoCr4

The automotive industry is continuously looking for lighter weight, cost-competitive and environmentally responsible material solutions for automotive components. Although analysts think the internal combustion engine will continue to play a significant role in the future, the challenge will be to make powertrains smaller and lighter with the same performance. Also as electric and hybrid powered vehicles gain more ground, these vehicles will similarly be challenged to be lighter, cost-competitive and provide the same level of safety and performance. There will be a significant role for bar steels in this new material challenge and success will be in engineering parts on a case-by-case basis.

The Automotive Roadmap illustrates some examples of steel bar successes in automotive part manufacturing as well some of the new technical challenges and research opportunities in steel bar product metallurgy, enabling processes and steel characterization and part design.

Bar Steel Successes, Challenges and Opportunities when Competing with Alternative Materials or New Vehicle Design Concepts

Powertrain Systems

Automotive powertrain systems offer the greatest opportunity for improved fuel economy through mass reduction and increased efficiency. As internal combustion engines become smaller and generate more power, this will result in increased loads and stresses and the potential for more steel components. Higher-strength steel components such as crankshafts, connecting rods, camshafts and pistons as shown in Figure 24 will be needed.


Figure 24- Internal combustion engine with key components (highlighted in red) where steel is currently used and likely to remain as engine size decreases and power density increases.

Forged Steel Crankshafts

A crankshaft is a rotating eccentric shaft running the length of the engine supported by bearings. The offset portions of the shaft are called “throws” to which the connecting rods are attached. As the pistons move up and down, the connecting rods rotate the crankshaft.

Crankshafts have been produced from cast iron or forged steel for many years. However, automotive companies have increasingly been reverting to hot-forged steel crankshafts for higher quality, durability and improved performance. Forged steel crankshafts can deliver higher torque at lower rpm. Design optimization has been the primary focus for crankshaft producers in order to manufacture a cost-competitive light-weight component with improved fatigue strength.

Forged steel crankshafts have consistently higher stiffness, higher strength, better fracture toughness and superior fatigue resistance than cast iron. This is a result of the higher modulus of elasticity; about 30 x106 for steel versus 14.5 x106 for cast iron. Elastic modulus is the main driver for selecting steel as it allows weight reduction and improved noise, vibration and harshness (NVH) performance in the crankshaft. Such material advantages allows crankshaft forgings to be downsized to fit lighter and smaller engines resulting in better fuel efficiency and higher power output. A 2007 research study for the Forging Industry Educational and Research Foundation (FIERF) and the Steel Market Development Institute (SMDI) showed forged steel crankshafts were found to have a 36 percent higher fatigue strength, Figure 25, than cast iron crankshafts, resulting in a usage life six times longer for the forged steel crankshaft. The study lead by Dr. Ali Fatemi of the University of Toledo also explored strength, ductility and impact toughness of the two materials and found forged steel to be superior to the ductile cast iron. Further design optimization of the forged crankshaft resulted in an 18 percent weight reduction with no decrease in performance.


Figure 25 - Forged steel crankshaft showing improved fatigue performance over cast iron crankshaft.

Further improvements in fatigue performance of forged steel crankshafts have been recently reported by researchers at the Colorado School of Mines (CSM) by using a cold working  process called “deep rolling” to impart residual compressive stress.   Improvements in crankshaft life and performance have been reported by induction hardening crankshaft journals.

Technological Challenges and Opportunities

The main challenges for continued selection of forged steel crankshafts are:

  • Development of new micro-alloyed steels with improved strength and fatigue properties after induction hardening and fillet rolling
  • Improved machinability to reduce manufacturing costs related to forged components
  • More competitive near-net shape forging operations to reduce the amount of metal removal during machining
  • Fillet strengthening techniques to allow for higher strength levels in this fatigue-sensitive location


Camshafts control the opening and closing of the intake and exhaust valves in an automotive engine. Cast iron camshafts have been used traditionally for smaller vehicles while larger vehicles use forged steel bars. Recent lightweighting activities have resulted in the development of a lightweight assembled camshaft consisting of a tubular steel hollow shaft and forged or sintered steel lobes, Figure 26, with projected weight savings up to 50 percent.

 Figure 26 - Cast iron camshaft (top) photo versus assembled tubular forged steel camshaft in bottom photo. The weight saving can be as much as 50 percent. 

Technological Challenges and Opportunities

Although the cost of the assembled camshaft is higher than the cast iron shaft, the weight savings are significant and the assembled component has greater fatigue life, stiffness and design flexibility in lobe geometry. There is also real potential for lower cost, higher-strength steel camshafts offering lighter weight and flexibility of individually assembled components. As engines become smaller and components become more highly stressed, there is a need for lobes having excellent rolling fatigue strength as radial stresses will increase in the component. New bearing quality steels may offer improvements either as a completely forged camshaft or as forged, induction hardened lobes used with a tubular steel assembled camshaft.

Connecting Rods

Connecting rods are critical components in defining an engine’s performance and require materials with suitable strength and durability as they are subjected to repetitive high tensile, compressive and bending loads. Connecting rods used in a high performance engine are usually forged steel components; powder metal (PM) forged components are also used. Research by the University of Toledo comparing hot forged versus PM connecting rods showed the forged steel rod exhibited a 37 percent higher fatigue life which is about two orders of magnitude longer life.   Therefore forged steel connecting rods are the material of choice in high performance engines. New micro-alloyed steel grades, near-net shaped forged and “crackable” connecting rods shown in Figure 27 have improved the cost competiveness of forged steel connecting rods.


Figure 27 - Typical forged steel connecting rod. Both cap and rod are forged as one piece and are “crackable,” meaning the cap and the rod can be separated to allow attachment of the crankshaft. New micro-alloyed steel grades can eliminate the need for heat treating and subsequent cold coining.   

Technological Challenges and Opportunities 

As a result of a continued trend for weight reduction there is a continuing requirement for optimized steel connecting rods which can provide lightweighting and higher strength at a competitive cost compared to PM parts. Steel forged connecting rods generally have better fatigue performance than PM parts and have the potential for more lightweighting possibilities. There are further weight saving possibilities in redesigning these components and savings related to near-net shape forming to reduce part manufacturing scrap and machining costs in the future. Continued development of steel composition is needed to attain the best combinations of fatigue properties and machinability.  


In an internal combustion engine, pistons transfer the force from expanding gases in the engine cylinder to the crankshaft via the connecting rods.

Pistons for light- and medium-sized vehicles are generally manufactured from cast aluminum for weight savings. Aluminum part costs are higher than for a steel part. With higher performance engines, piston temperatures are expected to increase which could require higher temperature performing materials such as steel for automotive pistons.

MAHLE Industries recently announced their TopWeld® steel pistons used in diesel passenger car engines can reduce fuel consumption by up to five percent. The steel grade used for the TopWeld® piston is 42CrMo4, which provides significantly greater strength and rigidity than aluminum. The piston height can thus be reduced by 30 percent. As a result, the contact surface between the piston and cylinder wall is smaller, as is the friction loss, which leads to measurable fuel consumption savings.

Technological Challenges and Opportunities

Pistons account for a significant portion of the engine’s friction. Therefore, enhancements in durability, mass and frictional forces within the cylinder will impact fuel efficiency. Automotive engine designers are looking for lighter, lower friction pistons with the stability to endure higher operating temperatures. Steel’s higher density and strength over aluminum may prove to be more resilient to higher pressures and temperatures. However, steels with even greater high-temperature properties may be required for future engine designs where high temperature fatigue properties are needed. A higher-strength steel option has the potential for redesigning the piston so it is closer in weight to an aluminum component and more cost effective. 


The drivetrain generally describes the components delivering power from the engine to the wheels, which include the transmission, driveshaft, differentials and axles. Most of the parts used to produce these components are sourced from steel bars, which are manufactured into shafts, bearings, gears and forgings. Although steel bars are the main material source for these components, they are threatened by other products, for example, PM for forged gears, fiber-reinforced polymers (FRP) for shafting and aluminum tubing for driveshafts. Also, there are some new product development activities involving the use of bi-metal or hollow gears which involve an outer steel shell and aluminum or other lightweight material core. 


There are two main types of transmissions. The first is a manual transmission based on a foot-operated clutch and shifter. These consist basically of sets of steel spur gears on parallel steel shafts. A five-speed transmission will have five sets of gears plus one set for reverse. The second is an automatic transmission which is more complex. There are four main types consisting of the planetary automatic, continuously variable transmission (CVT), automated manual and dual-clutch transmissions (DCT) which all shift gears automatically without a clutch pedal. 

Automatic transmissions consist mainly of gears (helical or spur) or pulleys (CVTs), bearings and shafting components which are sourced as steel bar or tube products. Steel is the primary material for these highly stressed components for its superior fatigue endurance limit compared to other materials. Further downsizing of transmissions will likely push the need for higher strength steels performing at higher operating stress levels and perhaps at slightly elevated temperatures. Although there has been some limited substitution by PM gears, steel gears still have better wear characteristics than PM components.

One significant transmission lightweighting activity is the co-development by Ford and General Motors of the ten-speed transmission first shown in the 2017 F-150 Raptor performance model at the 2015 North American International Auto Show.  It features a wide gear-ratio span, using steel, aluminum and composite alloys and has eliminated any cast iron to save weight.

Driveshafts, Differentials, Transfer Cases and Constant Velocity Joints

Various material choices such as solid steel bars, hollow steel tubing, a hybrid aluminum-steel component, aluminum-only product and carbon fiber reinforced polymers (CFRP) exist for driveshafts. Although some of these materials may offer lighter weight parts, lower part costs as well as proven durability make steel the material of choice.

Differentials in rear-wheel drive (RWD) vehicles allow the outer driven wheel to rotate faster than the inner driven wheel when turning. This is necessary when the vehicle turns to ensure speeds are balanced. This mechanical action is done by an input shaft or prop shaft connected to a gear set. This gear arrangement is contained in a casing. On RWD vehicles the differential may connect to half-shafts inside an axle casing or driveshafts that connect to the rear driving wheels. Front-wheel drive (FWD) vehicles tend to have a pinion on the end of the main shaft of the gearbox with the differential enclosed in the same casing as the gearbox. A transfer case is a part of the drivetrain of four-wheel-drive, all-wheel-drive and other multiple powered axle vehicles. The transfer case moves power from the transmission to the front and rear axles by means of driveshafts. It also synchronizes the difference between the rotation of the front and rear wheels and may contain one or more sets of low range gears for off-road use.

An automotive differential generally consists of one input shaft, the driveshaft, and two output shafts which control the rotation of the drive wheels and are coupled by their connection to the roadway. The gear components of a differential consist mainly of steel pinion and ring gears. The outer casing is usually aluminum. It is projected the gears and pinions will continue to be sourced from steel resulting from steels superior mechanical properties. 

Constant-velocity joints are mainly used in front-wheel drive vehicles and many modern rear wheel drive cars with independent rear suspension. They allow a driveshaft to transmit power through a variable angle at constant rotational speed. With the increased loads placed on this component it would appears CV joints will continue to be sourced from steel.

Transmission Shafts, Half-shafts and Axle Shafts

These shafting components are primarily steel as they are highly stressed components requiring high strength and stiffness. Also transmission shafts generally have integrated gear teeth and splines with one-piece steel shafts to simplify part manufacturing. Solid steel half-shafts remain the most practical material option in terms of part costs and performance.

Forged and extruded axles shafts for trucks will likely be the product of choice as a result of the higher stress levels which these components require.

Technological Challenges and Opportunities

Most driveline components are steel, with bar steels being the main product of choice. The following product development activities are important to ensure steel continues to plays a key role in driveline parts:

  • Use of high strength or better performing steels
  • Use of enabling processes such as surface hardening or coatings to improve fatigue strength and resistance to surface wear on moving components
  • Development of cleaner steels to allow higher fatigue performance at higher operating stresses
  • Optimization of part component manufacturing to reduce costs compared to other competitive materials

In 2015, North Carolina State University authored a report called “Survey of Forgings Used in Heavy Duty Vehicles and Potential Methods for Weight Reduction.”  One of the main areas of focus was the development of high-strength steel thick-walled hollow shafting suitable for axle shafts or other transmission shafting. They proposed “the hollow axle shaft can be produced by forging, extrusion and machining processes or through a combination of processes.” Concept sketches of some of these hollow components are illustrated in Figure 28. The potential is there to develop cost effective manufacturing processes to produce these hollow components, likely from steel bar forgings.

Figure 728- Concept sketches shows: a) standard machined transmission input shaft, b) hollow lightweight input shaft, c) hot-forged axle shaft and d) lightweight hollow axle shaft.12

Steering Systems

The steering system is a critical part of a vehicle and consists of several key linkages and components which connect the movements of the steering wheel to the front wheels of the vehicle. These components consist mainly of the rack and pinion steering, the steering knuckle, control arms and tie rods. In the past few years, power assistance to the steering rack has changed from hydraulic to electric power involving a change from a piston attached to a hollow bar rack to an induction hardened ball screw on the outer surface of the rack.

Rack and Pinion

In most cases the steering system for passenger vehicles (cars or trucks) is a rack and pinion system. The rack, ball joint, steering arms and linkages will likely remain bar steel as a result of the performance requirements.

Steering Knuckles

Steering knuckles are used to attach the wheels to the vehicle. The competing manufacturing processes for forged steel steering knuckles are cast aluminum and cast iron. Component testing conducted by the University of Toledo indicates forged steel steering knuckles provide superior fatigue life compared to both cast aluminum and cast iron components at the same stress amplitude.

Also a recent professor-student team at the Missouri University of Science & Technology performed a Life Cycle Assessment (LCA) study on the greenhouse gas emissions associated with the manufacturing of steering knuckles produced from forged steel, cast iron and aluminum.   The team found the forged steel steering knuckle had significantly lower greenhouse gas emissions than either the cast aluminum or cast iron steering knuckle over their respective life cycles.

Control Arms and Tie Rods

Control arms (upper and lower) attach the frame to the automobile and to the knuckle/spindle assembly that connects to the wheels. Their function is to permit the vertical travel of the wheel and suspension while allowing uniform horizontal wheel movement on the road. These parts have traditionally been produced as steel forgings or lower carbon steel stampings. Lightweighting by using stamped, hydro-formed steel tubing or aluminum parts is becoming more popular. In the future, these parts may trend back to steel depending on performance requirements from indirect design modifications of the front suspension.

The tie rod is a part of the steering mechanism consisting of an inner and outer end. It transmits force from the steering center link or rack to the steering knuckle causing the wheel to turn. Tie rods are generally produced from steel bars. New lightweighting activities are assessing certain aluminum alloys as a substitute. These components require stiffness and strength, therefore, steel remains the material of choice.

Technological Challenges and Opportunities

New steering systems designs will require lightweight, strong and rigid forged components. There is a requirement for high strength micro-alloyed steel forgings with optimized designs with good fatigue properties and enabling lean part manufacturing processes to enhance the cost competitiveness of steel. For example, by improving machinability, the part cost using steel could decrease significantly. Also promotion of the LCA benefits of using steel versus other materials could provide a strong driving force to use steel components.

Suspension System

The system consists of the structural supporting components for the vehicle which include the suspension springs, strut rods and stabilizer bars. Steel bar products are the source material for these components.

The automobile chassis and suspension system’s main function is to isolate passengers and cargo from dynamic reactions of the vehicle as it travels the road surface. Above all else, safety and durability control suspension design and material choices.

Suspension Springs

There are several different types of suspension springs, namely coil springs, leaf springs and torsion bars. In general, these components are manufactured from steel bar products. They generally perform at high stress levels. Their location in vehicles, close to the road, expose these components to very adverse conditions from corrosive environments, to mechanical and abrasion damage, sudden impact and a wide range of ambient temperatures. In addition, the combination of higher stress levels and corrosion can increase their susceptibility to stress corrosion cracking (SCC). Steel coil springs are produced from a number of heat treated alloy steels and are coated with hard protective coatings to prevent surface corrosion and surface damage from road debris such as gravel.

Technological Challenges and Opportunities

New aerodynamically designed automotive bodies have reduced the space available for packaging. Coil springs are required to be smaller and with more complex designs to fit into the smaller coil spring packaging envelopes. This trend has resulted in increasing operating stresses (see Figure 29), therefore, there are continuing requirements for high-strength micro-alloyed steel spring grades with excellent fatigue properties.


Figure 29 - Graph showing the trend in increased lightweighting of coils springs and associated increasing operating stresses. 

Tensile and yield strengths correlate to working stresses which in turn dictate spring weight. In the past fifteen years, micro-alloying of traditional spring steels (mainly higher carbon-silicon grades) have pushed these steels to higher hardness (HRC > 50) and increased strength. However, hardness above HRC 50 has increased risk of premature fatigue failure and increased propensity for SCC. Therefore, the technological challenge has been to develop micro-alloyed steels and use other strength increasing mechanisms such as shot peening, intensive quenching, etc. to maximize higher strength and improve fatigue life. Another challenge is any new technology must also fit within the processing constraints of existing coil spring manufacturing infrastructure to be feasible. Figure 30 shows the improved fracture toughness of a newly designed micro-alloyed high strength grade compared to two standard spring steel grades. This new micro-alloyed steel grade, which has a combination of high tensile strength and improved fracture toughness, was used for high stress springs in an SUV and resulted in greater than 20 percent weight savings per spring.


Figure 30 - New micro-alloyed steel coil spring steel with improved strength and fracture toughness compared with conventional grades. Both were heat treated to the same hardness HRC 54. This steel grade development resulted in weight savings of in excess of 20 percent on the coil springs used in a high volume SUV.15

Significant mass reduction opportunity exists in the rear leaf spring suspension assembly. Steel leaf springs are rectangular shaped and are usually multi-stacked in order to obtain the desired spring load. One of the most significant advances in steel leaf spring design reducing mass is the parabolic leaf spring which is essentially a mono-spring design. Parabolic springs provide ride comfort and good vehicle dynamics with 15 to 20 percent weight savings over a multi-leaf steel assembly.

Springs are furnished with expensive barrier-type coating corrosion protection. A less expensive barrier system, a sacrificial coating or a grade inherently resistant to corrosion would be an important development in the case for lighter weight steel suspension springs.

Cold wound spring manufacturing allow intricate spring geometry for better package and ride. Cold winding generally requires pre-tempered (oil-tempered) material with a high level of internal and surface quality. Conventional oil tempering is a costly post wiredrawing process. A hard drawn product (for size control) with a cost-effective in-process heat treatment providing equivalent through-thickness properties may be a better option. Induction heating may offer some benefits in alloying costs and improve toughness as a result of short heating times.

There are threats for steel in automotive suspensions from competing materials and vehicle design changes, for example, air springs. As the initiatives for vehicle lightweighting continue, there are other material options such as titanium and carbon fiber-reinforced polymers (CFRP) for coil springs and glass fiber-reinforced polymers (GFRP) for leaf springs. New lightweight plastic composites such as continuous thermoplastic fiber composite (CTTC) are being considered for leaf springs and have started to appear on some medium-sized trucks and limited production sport cars. The main concern with these new products is recyclability and the availability of mass production manufacturing facilities.

Front and Rear Stabilizers and Struts

The use of both front and/or rear stabilizer components has become commonplace in many automotive and truck suspension systems. Because of weight and cost considerations, this component application has been revisited to determine the advantages in the use of steel bar over tubular steel product. Several factors such as mechanical properties, fatigue resistance and corrosion protection must be considered when assessing these advantages.

The use of a steel bar in the manufacture of stabilizer components has several advantages over a tubular product in addition to cost. The component is readily manufactured and steel grades are readily available to meet all design criteria. Also, an advantage of a solid steel bar is the “eyes” are hot forged into the ends for a connection with the chassis. While most tubular components have additional forged eye components attached to the tube ends. Therefore, forged steel bars deliver a stronger chassis connection.

Struts are steel structural parts of the suspension system mounted to the top of the chassis and provide a surface for the coil spring to be mounted and held in position.

Technological Challenges and Opportunities

There is potential to use tubular steel products or high strength and reduced section steel bar products. These lighter weight components will encounter higher stresses as well as an aggressive corrosive environment due to where the struts are located on the vehicle. Therefore, there is increased possibility for SCC to occur. Also for new lighter weight components used in suspension systems, there is the potential for sudden impact loading because of rough road conditions. This is an important design consideration which needs to be considered when downsizing these components.

For strut applications, standard steel grades are being replaced by high-strength steel and add-ons such as plastic spring seats, hollow steel rods and aluminum tubes with variable wall thickness and composite materials. With these changes, weight savings of up to 30 percent have been realized without sacrificing strength and functionality. The use of micro-alloyed steel is a consideration but development of an optimized steel grade needs to address both the mechanical properties and fatigue life of the part.

Steel Material Issues

Gear Steels with Bending and Pitting Fatigue Resistance

Technical Situation

Gear manufacturers and users are continuously seeking improvements in performance in order to increase the power density of the individual components and the associated systems. Power density is defined by the ability to carry more load with the same size gear or the same load using a smaller-sized gear. Manufacturers of automotive transmissions often wish to double the torque capability over the ten-year life of a given transmission. Improvements to the resistance to both bending and pitting fatigue are needed to meet the goals of the various market/application sectors.

Technical Challenges

Gears must endure repetitive bending, sliding and rolling contact loading. The performance of a gear is affected by many factors, namely:

  • Variation in tooth geometry, surface finish, and mechanical and metallurgical properties
  • Steel grade selection
  • Through-hardening process
  • Surface hardening process (gas carburized, vacuum carburized, induction hardened or nitriding)
  • Effects of peening, grinding, finishing, coating, and surface hardness

In addition, application conditions such as the type of loading, temperature, corrosion potential, base lubricant and additives, and any mechanical and/or electrical contamination must be considered.

Research and Development (R&D) Focus

Gear solutions are best achieved on an application-by-application basis and must include considerations for material, mechanical and tribological conditions. Improvements in steel selection, heat treat processes, surface peening and finishing have greatly improved bending fatigue. Also, significant improvements in internal steel cleanliness have provided similar benefits to contact fatigue resistance. The performance of gear steels in new lightweight vehicles will depend on continued improvements in steel quality and property enhancing heat treatments and post treatments to attain reliability in the following areas:

  • Steel cleanliness
  • Inclusion engineering
  • Toughness
  • Surface quality
  • Hardenability control
  • Distortion control
  • Control of austenite grain size during final heat treatment
  • Optimum microstructural to maximize life
  • Alloy design for processing and cost structure

Austenite grain size has been shown to be a major contributor to fatigue strength in carburized and higher carbon induction hardened steels. Typical grain size of ASTM 5-8 combined with hardness above HRC 52 results in decreased fatigue strength from intergranular fracture. Austenite grain size refinement to ASTM 10 or finer has been shown to prevent the intergranular fracture at higher hardness and fatigue levels as demonstrated by the Colorado School of Mines.  Attaining these improved properties will require the development of chemical compositions and heat treatments to achieve the smaller austenite grain sizes.

As-Forged Higher Strength Steels with High Toughness

Technical Situation

Hot-forged, air-cooled micro-alloy steel forgings meet the strength and fatigue properties equivalent to heat-treated parts. Micro-alloying precipitation strengthening technology has the potential of providing forged components with increased mechanical properties to allow part lightweighting without the need to heat treat the forging. A major benefit in this technology is a lower part manufacturing cost resulting from eliminating heat treating. However, these micro-alloyed steels are not used to their full strength potential as their toughness tends to be lower than acceptable for a similar chemistry, heat-treated steel part. Therefore, the tendency is to apply these steels to applications with lower toughness requirements. The development of as-forged steels with a higher level or improved balance of toughness and strength would expand the application of as-forged steels.

Technological Challenges

Many of the metallurgical strengthening mechanisms used for steels affect the toughness level of the material relative to the degree to which hardness/strength is enhanced. This is true for precipitation strengthened micro-alloyed steels where their use can be limited in higher toughness applications. Flat rolled steel producers have worked for decades to overcome this problem for lower carbon steels and developed a range of compositional variations and on-line processing schemes to refine the microstructure of the steel, allowing for an improved balance of strength and toughness. This is a much more challenging endeavor for long product producers and part manufacturers for a variety of reasons such as non-uniform cross-sectional issues, non-controlled forge processing, and limited ability to control cooling and managing batch transformation from a forging.

Research and Development Focus

Further R&D focus for improving the toughness of as-forged steels will include the following activities:

  • Studies to determine the optimal metallurgical structure for an enhanced combination of strength and toughness that can be achieved directly from forging, at varying levels of strength or toughness
  • Compositional studies to find the lowest cost solution to obtaining the optimal microstructure from the forging process
  • Forging studies to determine how processing can be controlled to aid the uniform development of the optimal microstructure
  • Post-forging cooling studies to determine how temperature control can be used to provide enhancements to microstructure and properties
  • Assess other measures of toughness, such as K1C by drop load tests on full-size components, more realistic than the standard Charpy Impact test
  • Further research on the evolution of precipitate formation during steel casting and hot rolling, and during thermo-mechanical processing during heating and forging
  • Optimization of strength and toughness through optimization of: 1) the type and quantity of micro-alloying additions, 2) precipitate size, 3) control of austenitic grain size and 4) precipitate strengthening during part forging and cooling
  • Development of lower carbon and higher manganese micro-alloyed steels with improved toughness and yield strength in excess of 700MPa

Higher Machinability Steels

Technological Situation

Machining response can be a primary cost driver when manufacturing components from hot-rolled bar steels, and consequently can affect the competitiveness for steel-based part manufacturing. The costs of machining many ferrous-based components can nearly equal or exceed the cost of the material itself. The relative machinability of steel versus some alternative materials is not typically to the advantage of steel.

New machine tool and insert development has reduced the effect of steel chemistry and microstructure on machinability performance for several applications. With the associated material improvements (chemistry, oxide inclusion control) and machining tools and inserts; the historical cost structure will need to be re-evaluated to judge competitiveness of steel.

Technological Challenges

Enhancing the machining characteristics of steel typically requires the introduction or creation of non-metallic particles dispersed within the steel to help prevent tool wear by aiding chip formation and/or improving surface finish. Another approach to enhancing machining characteristics is through determination of the optimal steel composition, processing and resultant microstructure to optimize tool wear and aid chip formation. These modifications to composition, microstructure and/or inclusion introduction all must be balanced with steel processing and performance requirements for the application. The challenge then involves finding technologies or solutions that enable improved machining response while not negatively impacting the part processing or performance to provide an overall cost savings.

Research and Development Focus

The R&D focus for enhanced machining steels must involve continued development of base material characteristics and inclusion technology to further optimize the machining characteristics of steel, while maintaining the critical processing and performance characteristics of the applications. Some areas of focus include:

  • Understand the compositional, processing and resultant microstructure on critical machining response variables, allowing for optimization of those variables
  • Develop and test various inclusion technologies to enhance machining characteristics while maintaining the steel processing and performance
  • Explore the potential of inclusion engineering and the formation of self-lubricating soft inclusions which could benefit hard machining (high-speed finish machining of hardened steel)

Shafting Steels with High Torsional Fatigue Resistance

Technical Situation

Automotive shafting is similar to high performance gearing, in that there is a primary need for increased power density. Requirements for greater torque capacity as a result of lightweighting of individual components will most likely make steel bars the product of choice. It is also likely that shafts, bearings and gears will become much more highly integrated such that the shaft surfaces are more widely used as bearing races and gears made integral with various shafts.

Technical Challenges

The load, torque and fatigue capacities of shafts and any integral components will need to be optimized. These must be simultaneously balanced with other processing elements, i.e., formability, machinability and heat treat response, as they can detrimentally affect the resulting performance, for example, austenite grain growth in cold formed, carburized shafts. An example of new development to address grain growth is new research on ultra-fine induction-hardened shafting steel chemistries have resulted in improved contact fatigue life, torsional fatigue and bending fatigue.

Research and Development Focus

Team approaches like the SMDI Bar Fatigue program have provided an excellent foundation on which to build. Significant baseline data continues to be generated. Additional advances in bar steel metallurgy can result from further improvements in:

  • Steel cleanliness
  • Toughness
  • Surface quality
  • Hardenability control
  • Alloy design optimized for the specific part processing and application
  • Optimal microstructural features to maximize life
  • Enabling technologies to reliably attain ASTM austenite grain sizes finer than 10

Fatigue Resistant Fuel Injection Steel

Technical Situation

Cleaner burning low emission, diesel engines require increasing fuel system pressures to comply with newer government fuel efficiency requirements. Heavy-duty diesel engine injection pressures have doubled for both unit injector systems and the newer common rail systems. It is anticipated future pressure requirements will continue to increase as a result of increasing emission requirements.

Technical Challenges

The steels for fuel systems componentry must meet several demanding application requirements. Strength, toughness, fatigue resistance, corrosion resistance and oxidation resistance will all continue to increase. The ability to design smaller and stronger parts for power density considerations will be of significant importance. Manufacturing processes such as formability, machinability and heat-treat response will remain high on the list of improved deliverables.

Research and Development Focus

The design requirements for future fuel systems steels research activities may include the following areas:

  • Steel cleanliness
  • Toughness
  • Fatigue resistance
  • Corrosion resistance
  • Oxidation resistance
  • Heat-treat response to a variety of processes
  • Distortion control
  • Alloy design for processing and cost structure
  • Coatings
  • Optimized microstructural property and performance characteristics

Springs Steels for Lightweighting /High Stress Suspensions

Technical Situation

There are continuing requirements for lighter weight suspension systems both for coil springs and leaf springs. In the case for smaller coil springs this is a result of the smaller packaging envelope assigned for the spring on modern aerodynamically designed cars. This has pushed operating stress levels for these springs as high as 1300 MPa. From a materials standpoint, steel coil springs will likely continue to be the main cost effective consideration for these springs.

In the case of leaf springs for trucks, the main lightweighting strategy has been the use of mono-leaf springs, therefore, eliminating the weight of the traditional laminated auxiliary spring pack on certain trucks. Since leaf springs behave as a supported beam in a vehicle, their main function is to support the load. However, they must also isolate road-induced vibrations and are subjected to sudden impact loading depending on road conditions. Therefore as leaf spring assemblies are reduced in weight, they will need to have higher load carrying capacity which will require material with higher tensile strength and improved fatigue performance.

Technical Challenges

For spring steels elevated operating stresses require heat-treated steel with tensile strength in excess of 2000 MPa which equates to steel hardness levels in excess of HRC 52. Steel at this hardness level are more prone to premature fatigue cracking as a result of non-metallic inclusions and small surface imperfections. Also at this higher hardness in a corrosive environment such as on the underside of a vehicle, SCC is more prevalent. Therefore, steel springs need to be coated with a durable anticorrosive coating to protect the spring against corrosion and gravel damage which can increase the part cost.

Research and Development Focus

The main areas of research and development include steel grade optimization, and process and post-part manufacturing as listed below:


  • Steel chemistries to provide material with high strength and fracture toughness
  • Improved steel cleanliness with very low levels of oxide inclusions
  • Micro-alloyed steel with ultrafine austenite grain size
  • Improved corrosion behavior through alloy design


  • Explore induction hardening and tempering to improve fracture toughness and improve grain size
  • Optimize surface treatment processes such as stress peening, cold drawing etc. to improve fatigue performance
  • Investigate strategies to control SCC , for example improved enabling surface enhancement technologies such as shot-peening
  • Develop new corrosion protection coatings or surface treatments

Steel Enabling Technology (Improve or Develop)
A) Metal Forming and Machining

Cold Forging

From a steel maker’s perspective, the largest barriers to supplying the cold forging market are the tight restrictions on as-rolled dimensional, surface quality and microstructure and hardness requirements to make cold forming feasible for many applications. These can be improved by turning the bar (dimensional and surface) and heat treating (structure and hardness), but that can make the process too costly to compete with warm or hot forging, where those requirements are more relaxed.


All cold, warm and hot forming operations have the advantage over castings and machined bar stock in controlling the deformation and metal flow to increase metallurgical soundness and improve mechanical properties. This can improve directional grain flow (anisotropy) where needed for maximum part strength and provide better fatigue resistance and impact toughness on part sections such as gear teeth.

Cold forging is done at ambient temperature but may require a prior annealing treatment. Cold forging processes consist of upsetting, confined upsetting, and forward, backward and lateral extrusion which are performed separately or in any combination during manufacturing.

The steel properties affecting cold extrude-ability or formability are the steel composition, microstructure, internal quality such as segregation and the resulting mechanical properties. Ductility can also be improved by reducing hardness and the work hardening factor by annealing the steel prior to or between the multiple forging steps. Cold forging can also be improved by reducing the frictional coefficient between the dies and the work piece through improved die finish and lubrication.

An interesting process that can apply directly to as-forged parts to improve fatigue resistance is a cold working process called deep rolling. It involves a radially symmetric cold deformation process used primarily for surface finish, hardness and residual stress control. Recent studies have shown when applied to sharp radii found on forged crankshafts it can improve fatigue performance.  (Note: Deep rolling is applied as a strengthening method rather than a forming method, although the dimensions and surface finish of the rolled area are also improved.)

Advantages of Cold Forming

  • Provides net-shape parts with close size tolerances
  • Reduces machining costs
  • Saves material costs from reduced alloying requirements
  • Eliminates reheating costs
  • Eliminates some finish heat-treating operations because of increased mechanical properties through cold working
  • Provides economical method for most fasteners
  • Improves tool life

Technical Challenges

  • Requires higher tonnage presses compared to hot or warm forging processes
  • Often requires steel to be spheroidize-annealed prior to cold forming
  • May require a clean steelmaking practice to reduce nonmetallic inclusion content for better formability (Primarily a lower sulfur level and nominal clean steel technology is required.)
  • May require lower residual elements (Ni, Cr, Mo, and Cu and Sn) and a low nitrogen practice for electric furnace melt sources for some severe cold-formed parts to meet formability requirements

Research and Development Focus

The following areas of R&D would contribute to cold forming applications:

  • Improvements in simulation software to predict breakage or lubrication failure
  • Test method to evaluate effectiveness of lubrication prior to part production
  • Additional test method to evaluate formability prior to running production parts (cold upset test is satisfactory for certain applications)
  • Further developments work on deep rolling to understand what other components can benefit from this technology particularly its strengthening benefits

Hot Forging


Hot forging is the process of reforming the shape and properties of steel by first reheating the steel billets to 982 °C (1800 °F) or higher followed by the application of a force or load. The force is usually applied mechanically or hydraulically. Parts can be formed in open or closed dies depending on the shape and size tolerance needed.

Reheating prior to forging decreases the tensile and yield strength of the steel and allows for recrystallization during multiple forging steps which greatly improves formability and allows for very large deformations. The reheating may be applied with a gas or electric furnace, direct electrical resistance, rapid-fire gas or by induction. The entire part or only that portion which is to be formed, for example, the flange end of a wheel axle, is heated. The type of alloy addition, including micro-alloys has a major influence on the selection of reheating temperature for best as-forged properties.


  • Heating the part greatly reduces the forging press loads
  • Best way to form certain high strength or rapid work hardening steel grades
  • Large amount of reduction is capable in a minimum number of steps
  • Able to produce non-axial symmetric parts
  • Common production method for micro-alloyed steels
  • High production rates (280 parts per minute)

Technical Challenges

Developing technologies to enable net shape or near net shape, elimination of unnecessary area/volume on the forgings, and optimizing forging shapes are important to making this manufacturing method viable into the future. Some key challenges include:

  • Reducing energy costs and consumption during reheating
  • Reducing high die wear
  • Meeting new OSHA and EPA requirements
  • Improving size tolerances to get closer to that of warm or cold forged parts
  • Decreasing high scale loss during reheating
  • Improving yield by decreasing forging flash or trim losses

Research and Development Focus

  • Review present reheating methods and projected costs for the optimum future reheating processes to allow better temperature control and avoid grain coarsening and heavy scale formation
  • Develop scale-free reheating
  • Improve die coatings for high forging temperatures, extended times at temperature and erosion from metal / scale contact against the die
  • Improve modeling of non-uniform cross sectional heating on metal flow, especially in automated multi-sequenced forging
  • Create a vision system to monitor part-size tolerances and surface defects at high forging temperatures and high production rates

Warm Forging


The use of warm forging of steel is growing. It is divided into two temperature ranges. The lower temperature range is 204 – 732 °C (400 to 1350 °F) and used for net-shape part production, to slightly increase the formability of the steel compared to cold forming and to reduce the work hardening effect of the steel during multiple hits. The higher range is 843 – 982 °C (1550 to 1800 °F) and used for improved metalworking or formability for near-net shape through greater reduction in steel strength. Warm forging, in combination with in-line heating, leads to continuous forging operation rates as high as 1,200 pieces per hour.


  • Allows forward extrusion rates similar to cold forging and a large increase in the cross section of the part similar to hot forging
  • Improved die life compared to hot forging
  • Improved size tolerance compared to hot forging
  • Reduced forging loads compared to cold forming

Technical Challenges

  • Limitation of extrusion amount from lubrication deficiencies
  • Cannot reduce or expand part size as much as hot forging (as a result of metal flow capabilities)
  • Absolute control of temperature and temperature variations are critical in controlling process variation and part properties

Research and Development Needs

  • Improving die coatings and/or lubricants to better facilitate extrusion at higher temperatures
  • Obtaining steel properties in the warm temperature range for better development of computer simulation packages   



Machining processes are an extension of forming and are typically employed in the latter stages to obtain final part geometry. There is a distinction between rough machining and finish machining in which the amount of material being removed and the constraints on surface finish may vary greatly. Although machining processes are not intended to alter the mechanical or metallurgical properties of the work piece, improper practices can lead to excessive localized cold work or the generation of heat sufficient to cause localized alteration of the microstructure. This can substantially alter the ability of the component to perform as designed.

Machining processes are utilized to impart characteristics required by the function or application of the component, such as dimensional and geometrical precision, surface finish requirements, location and centering features, that cannot be adequately achieved via other bulk forming process such as forging or cold extrusion. Machining processes include: drilling, boring, honing, threading, broaching, milling, turning, grinding, and polishing. Most machining processes employ coated or uncoated cutting tools made of high-speed steel, tungsten carbide, ceramic, or cubic boron nitride to mechanically remove varying amounts of metal from the work piece. Other, less widely-used processes utilize electrical energy (electrical discharge machining or EDM) to selectively remove metal from the work piece.

The objective of any machining process is to economically produce machined components with desired physical characteristics and minimal part-to-part variation. Machinability has been described as a systems property that is a function of many factors including:

Mechanical/metallurgical properties and consistency of the work piece material

  • Cutting tool material, geometry, mechanical, thermal and wear properties
  • Machine operating parameters – cutting speed, feed rate, and depth/width of cut
  • Lubrication/coolant – type and properties, and application to cutting surface
  • Chip morphology and removal
  • Rigidity of tool holding fixture, spindle, gearing and mechanical frame
  • Machine operator skill level

Technical Challenges

In some cases, compromises are made with respect to mechanical/metallurgical properties in order to reduce the costs associated with machining. This compromise can result in over designing a component to account for degradation of desirable mechanical properties in order to use steel with enhanced machinability. Depending on the machining operation micro-alloyed steels can sometimes cause machinability issues. This is best assessed on a part-by-part basis.

Research and Development Focus

There is an opportunity for development of equipment, tools, process parameters, etc. to allow for the economic machining of steels with moderate to high strength/hardness. This machining process, called hard machining, is getting increased attention as new high-strength steel parts must be machined to the final dimensions after forming as well as the higher turning speeds which are necessary for ensuring economical machining costs. Areas of research include development of:

  • Inclusion engineered steels with self-lubricating features resulting in improved machining and tool life (This is less important for certain steel grade/part applications when using clean (low oxygen) steels and coated tooling)
  • Improved machining tooling using a more holistic process approach to allow end users cost-effective alternate manufacturing processing routes.
  • New steel chemistries and associated steel processing such as thermal treatment and cold drawing to improve machinability (lower carbon steel grades)

B) Thermal Treatment Processes for Steel

Various thermal treatments of steel bars or part blanks are used in the production of steel parts and other heat treatments are applied to impart superior performance in forged and machined finished parts. Some of the main thermal treatments which enable steel performance are presented in the following section.

Heat Treating / Carburizing – Gas with Oil Quenching


Gas carburizing of low carbon steel parts is the most common surface hardening process which results in hard wear surfaces while leaving the core structure soft and tough. It is one of many case hardening processes used in the industry. By heating the steel above its upper transformation temperature in a carbon rich atmosphere, the atmospheric carbon is diffused into the steel microstructure essentially forming a high carbon steel layer. Subsequent quenching results in forming a martensitic microstructure in the high carbon surface layer, whereas the low carbon core structure remains strong and tough. Gas carburizing is still the incumbent carburizing process, has lower initial equipment costs for high volume operations, and isn’t overly expensive to run in a lower energy cost environment. Sources of carbon for the furnace atmosphere include natural gas, propane mixes, liquid hydrocarbon and carrier gases.

The advantages of carburizing, which is the most common case hardening process, is it enables the use of low-carbon, low-alloy steel suitable for high wear and stress applications such as gears and power transmission shafts.

Technological Challenges

Although there have been a number of process improvements to improve the reliability of gas carburizing, such as higher temperature to reduce cycle times and more temperature and gas control equipment, there are still some challenges. Grain coarsening for certain chemistries at specific temperatures, variation in the depth of the carburized case and surface oxidation are some of the metallurgical quality concerns.

Research and Development Focus

Areas of research and development to further improve gas carburizing include:

  • Better control and predictability of carbon potential
  • Elimination of surface oxides in the carburized case (alloy design)
  • Development of processing of low-alloy steel grades
  • Understanding the relationship of grain coarsening as a function of temperature and steel chemistry (Note: Gas carburizing equipment has an upper temperature restriction that limits the ability to carburize at elevated temperatures. Instead, vacuum carburizing is well suited for elevated temperature carburizing.)
  • Management of surface carbon profile on complex part shapes like gears
  • Control of part distortion during quenching (marquenching is an option)

Heat Treating / Carburizing - Vacuum with Gas Quenching


Low pressure carburizing (LPC), also termed vacuum carburizing, with gas quenching, is rapidly gaining popularity with OEMs for cost, efficiency and environmental impact.

The carburizing process consists of loading parts into the furnace, applying a vacuum, heating and carburizing followed by gas quenching and unloading. Total throughput for LPC compares well to traditional carburizing where LPC utilizes higher carburizing temperatures to offset the smaller load sizes required for gas quenching. Once parts reach carburizing temperature, the carburizing gas is pulsed into the chamber in a sufficient amount to saturate the surface with carbon and with sufficient time between pulses to allow carbon to diffuse into the parts until the desired profile is obtained. Gas quenching uses either nitrogen or helium gas where helium is a more efficient quenchant but nitrogen is more recyclable. Cold wall design and low output gas emission allow LPC furnaces to be integrated into the manufacturing line instead of requiring a separate heat-treat facility.


OEMs have reported LPC furnace efficiencies between 85 to 95 percent and a reduction in operational costs of up to 25 percent over traditional gas carburizing with oil or salt quenches.

From a product standpoint, LPC eliminates grain-boundary oxidation, decarburization and heat-treat scale producing higher quality parts in terms of cleanliness and fatigue performance. LPC case depth is more uniform and shows less variation within and between heat-treat loads than case depth produced from traditional gas carburizing. Gas quenching produces less distortion than oil and salt bath quenching as a result of more uniform heat extraction. Some LPC designs provide increased heat treat flexibility where several different parts with different surface areas and weight can be simultaneously carburized at different carburizing temperatures and to different case depths.

Researchers have found higher carburizing temperatures as high as 1100°C (2000°F) have noticeable advantages over lower temperature carburizing with regards to potential productivity increases. However, higher temperatures can also lead to undesirable mechanical properties, control of surface carbon diffusion and case depth, and austenite grain growth control is more challenging. These concerns can be managed by precise control of the carburizing parameters, improved metallurgical control of manganese – sulfide (MnS) inclusions, suppression of intergranular cementite and intergranular oxides as well as more effective micro-alloying to control austenitic grain size. These steel solutions have offered improved fatigue properties in part carburized at elevated temperatures. , 

Technological Challenges

Control of boost/diffuse cycles is necessary to control the carbon profile in complex parts prior to quenching. Also, reduced quench rates with gas quenching require steels with higher hardenability. Carburizing at higher temperatures is desirable for productivity, but creates the challenge of maintaining the fine grain size needed for best part performance.

Research and Development Focus

There are several areas in steel grade and LPC equipment development help improve parts produced by this process including:


  • Adequate hardenability for the less severe gas quenching process
  • Retention of grain size at elevated carburizing, 1100°C (2000°F)
  • Improved internal cleanliness
  • Optimized microstructures to control the formation of intergranular cementite including near surface residual stress profiles


  • Modeling of part performance characteristics versus carbon profile and quench rate
  • Continuous vacuum carburizing
  • Design of racks for gas quenching for optimal gas flow and recirculation
  • Single layer heat treat processes to further improve quench uniformity, distortion, minimize time and allow for cellular flow operations

Heat Treating - Induction Hardening


Induction hardening (IH) is generally applied to medium and high carbon steel parts to enhance part performance. Induction hardening meets the objectives of one-piece advanced, flexible flow manufacturing systems driven by minimized process inventory. Induction hardening uses electrically generated thermal energy within the part to provide process control through the use of compatible electronic smart sensor technology. This capability combination can produce unique high-speed thermal profiles (if needed) along with consistent optimum accelerated quenching rates. State-of-the-art induction heat-treating systems can generate fine-grained microstructures along with beneficial residual compressive stress (selectively in areas of high applied tensile stresses) as well as improved mechanical properties to improve part performance.

Technological Challenges

Work needs to be done to provide compatible material chemistries and prior processing techniques to produce microstructures that will respond to induction heat treatment heating rates and still provide acceptable properties. Product design engineers need an improved understanding of the role of residual compressive stress contribution to part durability performance. This includes the role of induction low temperature temper (LTT-stress relief) normally done after selective hardening to accommodate in-line manufacturing systems.

The advent of induction hardened gears is of major interest to address process inventory problems with conventional batch-type heat treatment operations. Some unique capabilities have been demonstrated by IH, such as profile or contour hardening with extra depth at the pitch line for handling subsurface spalling as well as surface pitting failures. The process must produce full austenitizing of the gear root area for full martensitic transformation and high residual compressive stresses for good tooth bending fatigue. This issue for IH is complicated by modified involute profiles to increase contact ratio to handle higher loads and reduced noise.

Research and Development Focus

Further developments in modeling IH along with improvements in steel grades and the IH process will provide improvements including:


  • Better understanding and modeling of the effects of induction hardening time-temperature cycles on austenite grain size, residual stress and resulting fatigue properties
  • Modeling of the induction hardening process and steel chemistries to achieve specific part performance objectives like fatigue, wear resistance, and pitting resistance
  • IH process modeling to optimize/accelerate response of sluggish prior structures (simultaneous multi-frequency IH for gears)
  • Modeling of tooling and process to handle multi-frequency heating techniques and optimize performance of gears

Steel and Process

  • Improved understanding of how microstructure and prior processing affect machinability and how these factors can be altered to support IH material response
  • Develop an understanding and improve the quench embrittlement limiting factors for IH steels used in gears and other bending fatigue applications

Heat Treating - Through Hardening of Steel


Through hardening is a process in which bars or parts are uniformly heated above the transformation temperature and quenched followed by a subsequent heating at a lower temperature to provide various combinations of hardness and ductility. The initial heating can be in batch or continuous gas-fired furnaces or by suitably designed induction heating. Quenching is in water with additives, oil or salt baths. Subsequent tempering is also by batch, continuous or induction heating. Hardening of finished parts provides high strength, improved toughness and enhanced fatigue properties. The process also results in a product generally free of residual stresses that can affect dimensional stability during machining.

Technological Challenges

Stresses introduced during quenching can cause distortion of a part. These stresses are subsequently relieved during tempering, but the quenching process results in uneven distortion in all non-symmetrical parts. In addition, the quenching stresses, if sufficiently high, can result in cracking.

Research and Development Focus

Modeling of quenching distortion and stresses
Definition of the minimum heating times necessary for austenitizing and tempering
Design of induction heating controls to counteract distortion during heat treatment

Heat Treating - Annealing 

Annealing is an overall heat-treating category that includes normalizing, lamellar pearlite annealing, spherodize annealing, stress relieving and cold-shear annealing. These processes are utilized to enhance machinability, shearability or forgeability and are often utilized to enhance the response to induction hardening of the finished parts.

Annealing is performed on steel bar lengths in continuous roller hearth, car bottom or roller-hearth batch furnaces. Cut part billets are heat treated in continuous belt type, pusher or bell type furnaces. These heat treatments are generally performed in air prior to machining or in controlled atmospheres to avoid oxidation and decarburization at the surface of near net-shape parts.

Heat treatments are costly because of the utilization of energy for heating and the cost of the equipment.

Technological Challenges
Understanding heat transfer and the kinetics of microstructural reactions during heating, at temperature and during cooling, are keys to the efficient operation of heat treating facilities. Various alloys respond at different rates to these annealing processes and the resulting properties are affected. In addition, in-process annealed microstructures have an impact on the final properties of the finished part.

Research and Development Focus

  • Model the rates of carbide dissolution in various steel grades during heating
  • Define the effects of cooling rates on the microstructure of various alloys (CCT diagrams)
  • Characterize the influence of steel grade and prior microstructure on kinetics and carbide morphology during spheroidize annealing
  • Define the effect of prior microstructure on the fatigue properties of induction hardened shafts

Need for Improved Computer Modeling Capabilities

Development of a Standard Methodology for the Quantitative Measurement of Steel Phase Transformation Kinetics and Dilation Strains Using Dilatometric Methods


The steel producing, forging, heat treating and component user industries are under increasing demands for higher quality, better performing materials and components at lower costs. This is being addressed with improved steel compositions and processes, and by computational tools to study and optimize processes and designs. The development of predictive tools for the steel manufacturing, forging, heat-treating and the steel component user industries offers great opportunity for further steel/process optimization. Critical to the success are the development of metallurgical models with the ability to predict microstructure evolution during material processing. To be effective these predictive tools require accurate and consistent steel transformation kinetic and thermal strain data to populate computer models to allow the prediction of steel microstructure and properties after processing.

Technological Challenges

Transformation kinetic data have been published in several forms over many years, these data have not been widely incorporated into predictive models, partially a result of the lack of standard procedures for the collection, analysis and reporting of transformation data. Currently, there are no reliable public-domain reference resources to provide both the kinetic and thermal strain components associated with steel phase transformations in an electronic format. Such data is considered crucial for process simulation models to accurately predict residual stress, distortion and microstructure for various steel product forms used in manufacturing.

Data on steel transformations presently available in the public domain have been collected using a variety of techniques such as metallography, dilatometry, magnetic permeability and differential thermal analysis. 

Research and Development Focus

There is a need to provide the steel phase transformation data in an appropriate format to maximize and accelerate their utility in state-of-the-art material and process modeling computer simulation software. Standard measurement, data interpretation and data reporting methods need to be developed and defined, upon which a quantitative database for process modeling can be developed and electronically archived. 

An initial area of focus encompassing three distinct austenite transformation scenarios would be beneficial:

  • Transformation of the austenite under no applied elastic stress or plastic deformation
  • Transformation while a static elastic stress is applied to the austenite
  • Transformation of the austenite while it is undergoing plastic deformation

Need for Improved Predictors for Cold, Warm and Hot Forging


Metal forming is influenced by many different factors which include metallurgical structure, material chemistry, tooling surface finish, lubrication effectiveness, die design and forge press parameters. Therefore, it is difficult to create accurate relationships to describe the forging process and predict results.

Research and Development Focus

Many CAD-based programs have been developed to predict the final forge shape. Some can even work backwards from the final part to develop the preforms needed to arrive at the final design. Additional work needed in this area includes:

  • Forging shape optimization model to allow for downsizing of parts with high strength steels and shape optimization to reduce weight
  • Better estimation of the process effects on the finished cold-forged part (For example, what will be the hardness, tensile and yield strengths after processing. Improved integration of chemistry, microstructure, slug hardness, strain rate and press conditions will more accurately predict the properties of the finished product.)

Generalized data has been developed for some steel grades (strain hardening rate, strain rate sensitivity, density, tensile strength, yield strength, elongation and reduction of area), lubrication (friction coefficient), and the forging process (force, press speed and press stroke). Additional work in this area includes:

  • Accurate stress/strain data on more steel grades
  • Better integration of the actual chemistry and prior microstructure into the calculation for production process control and computer simulation to provide a better estimate of the metallurgical characteristics needed for the process
  • Validated friction factor, currently a single number, to vary with part configuration, lubricant type and application effectiveness, actual lubricant characteristics (weight, percent reactive and fat content), and die surface (There is no production test to measure all these factors which change with every part run.)

Induction Hardening - Improved Computer Modeling


Induction hardening uses dynamic multi-variant processing parameters including power density, heating rate (even variable heating rates), selective thermal profile (inductor configuration), and with the new power supply designs, variable–applied frequencies to produce unique, thermal profile depths. 

The two important areas for IH modeling are: 1) tooling (inductor coil) design optimization where most of the work has been done to date and 2) heating kinetics along with on cooling/quenching kinetics required to optimize the total results.

Technological Challenges

  • Full coupling between thermal kinetics and electro-magnetic characteristics
  • Ability to handle phase transformation (curie) kinetics
  • Ability to handle complex part geometry, asymmetric and current in plane, (practical 3D capability) with improved innovative mesh design expertise for reasonable computational requirements
  • Ability to handle high flux densities beyond saturation
  • Ability to handle dual phase (magnetic & non-magnetic) dynamics
  • Suitable material characterization (temperature dependent) data to drive math models (e.g. resistively, specific heat, thermal conductivity and permeability)

Research and Development Focus

There is a need for a joint effort bringing together a multi-discipline group to address the challenges listed above. 

An available industrial testing and validation site with current product knowledge would be beneficial.

Intelligent Induction Hardening Process Controller 


An induction hardening (IH) process is usually integrated directly into the manufacturing process. This inclusion in a processing line supports a higher level of in-process quality assurance. In addition the concentration of flexible manufacturing systems with smaller lot size requires more frequent setup changes. Therefore, quality control costs are increasing as well as the cost of any downtime which can make this technology more costly. Also, IH is installed later in the manufacturing cycle, many times as final operation to reduce cost.

This issue has driven the industry to develop interactive process control using smart sensor technology such as IH signature analysis. It has demonstrated the basic concept is workable in that it essentially interrogates the component during processing (in real time batches of one) to ascertain the proper processing results.

Technological Challenges

There is need to pool knowledge and technology to develop generic interactive process controllers to evaluate incoming variables and can adjust component processing parameters in real time. 

Research and Development Focus

Work done to date has been limited as a result of available technical/specialized expert resources in the industry. The following activities would provide more knowledge and process related solutions:

  • Develop an induction industry partnership
  • Provide on-going interactive engineering review with an experienced industrial supplier to ensure meeting industry needs
  • Provide extensive opportunities on a range of applications for on-going test iterations to ensure concept robustness
  • Provide selected industry sites to assist in development of industrial hardened hardware and user-friendly software

Other Potential Modeling Subjects

  • Predictive tools for compositional, manufacturing processing and heat treating on component distortion control
  • Manufacturing cost modeling for various components
  • Input steel info, forming methods, machining methods, heat treating and finishing to predict/compare component costs by varying inputs


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