Case hardening by carburizing presents an opportunity for a similar comparison. This process results in a component with superior wear resistance along with excellent mechanical properties and toughness.  During carburizing, a component is heat treated in a carbon-bearing atmosphere to permit diffusion of carbon into the surface of the part. A high-carbon case is developed, the depth of which is a function of time and temperature during heat treatment. At the end of the process, the component is quenched and tempered resulting in a very high hardness case and a lower hardness core. 

 

AISI has examined the fatigue properties of both the case and core for a number of steel grades used in carburizing applications. In this posting, the properties of SAE 4620 are presented. The properties of the case were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.

 

The following chart shows the mechanical properties and hardness obtained for the case and core:

 

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

                      MPa          MPa                  %_________

 

Core         891.5         963.7               61.9             29.6

 

Case       1169.1       1775.6                 1.4              54

 

The case microstructure was 100% martensite, and a mixed bainite-martensite microstructure was developed in the core. The differences in the mechanical properties and hardness between case and core are quite evident.

 

Figure 1 (below) shows the strain controlled fatigue properties for both the case and the core.

 

 

  

Figure 1 (click here for larger image).

 

The strain-life curve for Iteration No. 53 shows the fatigue behavior of the core, and the strain-life curve for Iteration No. 54 shows the behavior for the high-hardness case. As was shown for induction hardened steels, 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 between 10³ and 10⁴ reversals. 

 

The data show that as expected, high hardness such as that shown in the case, results in better long life fatigue properties. At short life however, the data suggests that the high hardness case may be more prone to crack initiation where occasional overloads are encountered.