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Bar Steel Fatigue Blog

Further Comparisons of the Case and Core Properties of Induction Hardened Steels

Posted At : July 31, 2008 1:50 PM | Posted By : Andrea Tomaszewski
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In the last posting, the fatigue properties of surface induction hardened 1050 axle shafts were presented. Strain life data showed that in the short life regime, the fatigue properties of the low hardness core were better than those of the case. In the long life regime the fatigue properties of the high hardness case were superior to those of the core.

Here we would like to look at additional data that has been developed on induction hardened SAE 1070 steel. In this study, the case and core properties were simulated using 50mm diameter steel bars. As-hot rolled bars were used to simulate an induction hardened core, and additional bars were through-induction hardened to simulate the high hardness case.

The mechanical properties and hardness obtained for the simulations of core and case were as follows:

Location Yield Str. Tensile Str. Red. in Area BHN

(MPa) (MPa) %

Core 520.0 659.0 36.2 280

Case 1950.0 2069.0 2.3 613

The core exhibited a ferrite-pearlite microstructure and the high hardness case contained a mixture of martensite, bainite and a small amount of pearlite.

The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.

Figure 1 (click here for larger image).

The strain-life curve for Iteration No. 36 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 37 shows the behavior for the high hardness case. As was observed for the SAE 1050 steel in the previous post, the softer core exhibits better fatigue properties than the case in the short life (high strain amplitude) regime. Conversely, in the long life regime (low strain amplitudes), the properties of the high hardness case are superior to those of the core. A cross-over point occurs between 10⁴ and 10⁵ reversals.

Additional comparisons of the case and core of surface hardened steels will be included in future postings covering the properties of carburized low alloy steels.

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Comparison of the Case and Core Properties of Induction Hardened Steels

Posted At : July 3, 2008 10:45 AM | Posted By : Andrea Tomaszewski
Related Categories: Bar Steel Fatigue

Medium and high carbon bar steels are often used in the induction hardened condition for applications such as axles and shafts. Induction hardening is a surface hardening heat treatment whereby a component is rapidly heated for a short period of time in an induction coil and then quenched. This results in a high-hardness, wear resistant case and a softer core. Of interest are the comparative fatigue properties of the hardened case versus the softer core. AISI obtained axle shafts that had been hot forged, cold extruded and induction hardened. The steel grade was SAE 1050.

The mechanical properties and hardness obtained for the case and core were as follows:

Location Yield Str., MPa Tensile Str., MPa Red. in Area, % BHN

Core 460 828.5 34.1 220

Case 2100 2360 14.7 536

The core exhibited a ferrite-pearlite microstructure, and the high hardness case was 100% martensite. The differences in the mechanical properties and hardness of the case and core are (as might be expected) quite significant.

The strain-controlled fatigue properties determined for both the case and the core are shown in Figure 1.

Figure 1 (click here for larger image).

The strain-life curve for Iteration No. 4 shows the fatigue behavior of the core; the strain-life curve for Iteration No. 5 shows the behavior for the high-hardness case. In the short life regime (at high strain amplitudes), the softer core exhibits better fatigue properties than the case. In the long life regime where strain amplitudes are lower, the case shows superior fatigue properties to the core. A cross-over point can be seen at approximately 10⁴ reversals.

High hardness is generally considered to result in better fatigue properties at long life, and this is confirmed in Figure 1. This data also shows however that at high strain amplitudes a high hardness exhibits a greater tendency toward crack initiation. This suggests that the case of induction hardened shafts may be vulnerable to crack formation under conditions where "spike" loading occurs.

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Looking at a Comparison of As-Rolled and Normalized Properties

Posted At : June 10, 2008 6:58 AM | Posted By : Andrea Tomaszewski
Related Categories: Bar Steel Fatigue

While medium carbon bar steels are often used in the as-hot rolled condition, some applications call for normalizing the hot rolled product. Normalizing consists of re-austenitizing the steel followed by ambient air cooling. This often results in improvements in ductility and notch toughness.

As part of the development of the AISI bar steel fatigue database, the properties of SAE 1541 steel were examined in the hot rolled and normalized conditions. The hot rolled bars were given a slight cold sizing treatment; the normalized bars were subjected to austenitizing at 900°C and air cooled.

The mechanical properties obtained for the two conditions were as follows:

Condition Yield Str., MPa Tensile Str., MPA Red. in Area, % BHN

As-Rolled 461.0 905.5 41.7 195

(Cold Sized)

Normalized 471.2 783.2 55.1 180

Normalizing resulted in a slight increase in yield strength, a reduction in tensile strength and hardness, and improved ductility.

Both the as-rolled and normalized conditions exhibited ferrite-pearlite microstructures.

The strain-controlled fatigue properties determined for both conditions are shown in Figure 1.

Figure 1 (Click here for larger figure)

It can be seen that Iteration No. 1 gives the fatigue results after normalizing, and Iteration No. 2 shows the fatigue properties in the as-rolled condition. The curves drawn through the data points for each iteration were calculated from their respective strain-life equation. As can be seen, the fatigue properties for both conditions are very similar.

In the case of the long life regime, the curves show the as-rolled SAE 1541 as having somewhat better fatigue performance than the normalized SAE 1541. A calculation of the fatigue strengths at one million cycles from the strain life equations results in values of 312 MPa for the as-rolled condition, and 260 MPa for the normalized condition This might be expected, since the as-rolled condition exhibited slightly higher tensile strength and hardness.

However, as can be seen from the actual data points, the difference in the fatigue performance between the two conditions is quite modest. Thus application considerations should focus primarily on mechanical property requirements, with fatigue performance being a secondary consideration.

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