Jominy Quench test of different steel alloy samples

[lou_skywalker]

Hi,
So the other day we performed the Jominy quench test on 4 different steel samples. We followed the standard procedure (heating to austenite, then quenching it at one end, then polishing and etch it for micrograph examination) Now I need to write about the expected phases in each steel sample as we move from the quenched end to the other end. Below is what I currently know about each steel alloy. I am having a hard time explaining why all this happens in terms of carbon content and distance from quenched end.

1095 steel (not heat treated):
We observe martensite at quenched end and coarse pearlite at the other end. Hardness drops significantly as we move across the bar.

1095 Steel (tempered)
Martensites are different (look more like Bainite) and pearlite should be expected throughout the bar.

1018 Steel (not tempered)
similar to 1095 steel, but transition from martensite to pearlite are more significant?

4140 Steel (not tempered)
Bainite observed across the bar. Martensite at quenched look very similar to bainite (no idea why it happens)

8620 Steel (not tempered)
very few martensite seen, mostly a combination of pearlite and ferrite. Hardness numbers are also very low compared to the other 3 samples.

 

[Navin A S]

In general, the transition from martensite to pearlite (or bainite) as we move away from the quenched end is dependent on the composition of the steel. The higher the carbon content, the more martensite will be observed at the quenched end and the more quickly the transition to pearlite (or bainite) will occur. The distance from the quenched end also plays a role, since the further away from the quench the lower the temperature and the less likely it is for austenite to transform into martensite. In the case of 1095 steel, the low carbon content results in only a small amount of martensite at the quenched end and a quick transition to pearlite as we move across the bar. In the case of 4140 steel, the higher carbon content allows for more austenite to transform into bainite, creating a homogenous structure with bainite throughout the bar. Finally, in the case of 8620 steel, the low carbon content prevents significant transformation of austenite into martensite and results in mostly ferrite and pearlite throughout the bar.