Quenching Speed, Carbon Content, and Maximum Hardness

I was looking over some threads and came across a topic that might be good to discuss.

Some posts recently led me to believe that there are some new comers who believe that quenching steel faster will lead to a higher hardness and that even something like 1018 low carbon steel could be reach adequate hardness with a fast enough quench.

While an adequate quenching speed is certainly important, the carbon content of the steel will determine the maximum as-quenched hardness. Hardness increases steadily up to about 0.8% carbon then levels off and actually can decrease due to excessive retained austenite. So, for a refresher to the old members and clarification to new members, quenching speed just needs to be adequate to reach full hardness, and faster quenching won't help if maximum hardness has been reached for the available carbon.

Very true, but there are other things too. Austinizing temperatures also figure into the equation. Some IT diagrams that I have show a great difference between a lower and higher austinization temperature. I looked up S5 steel the other day and the data sheet gave a different maximum HRc for several different austinizing temperatures. Then there is the speed with which the steel will change phase upon cooling which is regulated by the alloy. A simple steel like 1060 changes so quickly that it would hard to avoid some slack quenching even with brine. A little more complex 9260 will give about a second before conversion starts and 4360 has about 30 seconds or better before conversion starts. All of them have approximately the same amount of carbon in them. But you are right that there is a point below which there is not enough carbon to put into solution and trap within the martensite to give adequate hardness and wear resistance to make a good blade regardless of how fast the steel is cooled.

Doug

I was trying to keep things perhaps a little too simple. When I say adequate cooling speed, I mean fast enough to miss the pearlite high temperature area on the CT diagram, or anything else below it except martensite. Naturally, that means "adequate" for 1095 is complete overkill for A2. Different austenizing temperatures have the effect of putting more carbon in solution or ensuring there is no ferrite left to soften things up. For S5 there appear to be some other things going on as well, since Crucible's data sheet gives maximum hardness as 61 to 64 depending on austenizing temperature and quench medium. However, the point is that the maximum potential hardness is set by the carbon content, not by the speed of the quench. Superquenching 1040 can't give the hardness of 1095. On the other hand, motor oil quenching 1095 will give a hardness between 40 and 55 HRc, well short of it's potential of 66-68. However, that is a problem/issue of processing, not maximum steel potential. As another example, the maximum potential hardness of CPM 3V is 65+, but you have to way overheat the steel to get that high.