In this article we will compare two through-carburized 20MoCr4 steel iterations. The composition of the 20MoCr4 steel is shown below in Table 1. Iteration #137 was atmosphere through carburized using the traditional 927 °C, twenty-six hour cycle which provides a surface hardness of 61 to 64 HRC and a core hardness of 55 to 60 HRC. This cycle does not provide a uniform carbon content of 0.80 to 0.90 weight percent through the 0.200 inch (5.08 mm) gage length diameter. Judging by the hardness, the core is approximately 0.42 weight percent carbon. Iteration #141 was vacuum through carburized using a thirty-six hour cycle at 927 °C, which does provide uniform carbon content and a uniform hardness through the entire cross section.
The mechanical properties, fatigue strength, and hardness for iterations #137 and #141 are shown in Table 2. The most significant difference is the strength. The yield and ultimate strengths of iteration #137 (1,265 and 1,394 MPa respectively) are typical of what we have seen in the past few articles with through-carburized samples that have not been ground after heat treatment. However, the yield and ultimate strengths of iteration #141 (965 and 965 MPa respectively) are significantly lower than iteration #137 and are the same indicating there is no plastic deformation prior to fracture. Essentially the sample is weaker and brittle. The fatigue strength (461 MPa for iteration #137, and 465 MPa for iteration #141) was not affected by the yield/ultimate strength differences as both iterations are nearly identical.
The potential causes for the decrease in strength would be; atmosphere versus vacuum carburizing, an increase in grain size as a result of the longer cycle, or the deeper carbon penetration. In the prior article, “Atmosphere versus Vacuum Through-Carburized Boron Steels” we concluded there was no significant difference in performance between atmosphere and vacuum carburizing. The data is shown in Table 3.
The microstructure photos in the AISI database for iterations #137 and #141 were examined. The grain size appears to be the same for both and no abnormal growth was noted. The typical grain diameter is less than 20 microns. From this it does not appear grain size is responsible for the difference in strength. This leaves depth of carburization as the cause.
Iterations #40 and #41 were examined in several prior blog articles. These two iterations were made from vacuum melted 100 pound heats of 8695 steel. The steel was cast and forged into bar, from which test bars were machined. The carbon content was relatively constant through the cross section, much the same as iteration #141 in this article. Iteration #40 had no intergranular oxidation (IGO) at the surface, while iteration #41 had IGO at the surface. Both iterations were carburized using a normal industry cycle, however iteration #40 was copper plated prior to carburizing to prevent IGO. The data is shown in Table 4. Iteration #41 has similar strength as iteration #141 in this article (979 MPa and 965 MPa respectively). From this it appears the depth of carburization is the likely cause of the lower strength of iteration #141 compared to #137. However, there is some room for doubt as the ultimate strength of iteration #40 is higher than #41 (1,496 MPa and 979 MPa respectively) and no cause has been determined. More work would be necessary to confirm this conclusion.
