In this article we will examine the mechanical properties and fatigue strength of the early through-carburized iterations in the SMDI fatigue program. During this time period the test bars were typically ground and polished after carburizing in order to eliminate any distortion as a result of the heat treatment. The through-carburizing heat treatment was a 24-26 hour cycle at 1700 oF (927 oC). This cycle did not completely through carburize the 0.200 inch (5.08 mm) gage diameter to a consistent high carbon level but typically left the core hardness at or above 55 HRC.
The mechanical properties and the fatigue strength, defined as the stress at one million cycles, are shown in Table 1. Additional information in the table includes: the ratio of the fatigue strength to the ultimate tensile strength; surface and core hardness; and the laboratory where samples were machined and tested. Most of the elongation values are less than 2% which is reasonable for these high hardness, high carbon iterations. However, the elongation of iterations 48 and 54 was significantly higher at 12.5% and 7.7% respectively. In addition, the ultimate tensile strength of iteration 48 was considerably greater than any of the others at 2,227 MPa. Iterations 48 and 54 were both tested at the University of Toledo while most of the others were tested at the University of Waterloo. Both of these iterations were made from 4620 steel, which as a medium nickel steel (1.8% nickel) is expected to have increased ductility. However, the 4320 steel is also a medium nickel steel and 9310 is a high nickel steel (3.3% nickel) and neither of these had ductility comparable to the 4620.
Metallurgically speaking the surface hardness of iteration 48 is lower than most other samples except for iteration 50 which is the 4320 sample. This could indicate an elevated tempering temperature was used on iteration 48 after carburization which would tend to increase the ductility. However, this would not explain the high elongation on iteration 54 and the low elongation on iteration 50. The cause of the high strength and high ductility of the two 4620 samples could be related to the amount of stock that was removed from the surface during the grinding process. If more stock was removed from the surface of these two samples the carbon content at the final test specimen surface would be reduced which would increase the strength and ductility. It is also possible these two iterations were simply carburized with a lower carbon potential.
Figure 1 provides a graphical representation of the ultimate tensile strength data. It is obvious the first 4620 iteration (48) has an ultimate tensile strength which is greater than any of the other samples. The first three iterations on the left of the chart were not ground, but only polished after carburizing. These iterations averaged about 1200 MPa in ultimate tensile strength. The tensile strength of the remaining iterations, without the 1117 steel, averaged about 1700 MPa. If we ignore the two 4620 iterations with high elongation the average ultimate strength is still above 1600 MPa. This indicates grinding after carburizing has a beneficial effect on the test specimen strength. The 1117 iteration (52) had a lower strength level which is likely a result of the high level of manganese sulfide inclusions as well as the fact this was a plain carbon steel with no alloy content.
The two 8695 samples are the best representation of a through-carburized material since they were produced with 0.95% carbon through the cross section. They were produced as 100 pound vacuum melted heats which were forged into test samples then machined and carburized. The carburization was not used to introduce any more carbon but rather to introduce intergranular oxidation (IGO) at the surface. The first 8695 iteration (40) was copper plated prior to carburizing to prevent IGO at the surface. Without IGO the ultimate tensile strength is around 1500 MPa, and with IGO it is around 1000 MPa. This difference may be related to the IGO or possibly the carburization heat treatment. The alloy content of the different steels does not appear to make much difference in strength, except for the 1117 steel. The 9310 steel has the highest alloy content followed by the 4320 and then the 4620. These grades do not have a higher ultimate tensile strength than the more common grades such as 8620, 5120 and 8822.
Figure 2 is a graphical representation of the fatigue strength data. The 4320 iteration (50) had the highest fatigue strength, while the lowest was the 1117 iteration (52). The first 8695 iteration (40), without the IGO, had a lower fatigue strength than the second one (41), with the IGO. This indicates fatigue strength does not necessarily correlate with the ultimate strength and in this case the IGO does not appear to be detrimental toward fatigue life. However, the first three iterations, along with the 1117 iteration, have lower fatigue strengths than the other iterations and do correlate to the ultimate strength.