Questions about heat treating and tempering... just curious

[Skeaterbait]

Reading through as many posts as I could find on heat treating and tempering, it's clear that you first heat treat beyond your target hardness then bring it back down to where you want it to be. Without getting too much in to the geek speak of metallurgy (which sadly would be lost on me), what is the reason you don't just shoot for the target hardness right off the bat?

Second question is specifically about 1095, how many chances do you get at heat treating? Presuming I don't get too far and burn it up completely.

 

[samuraistuart]

What we are trying to do is convert our pearlite into austenite and then quench it to form untempered martensite. Then we temper the martensite. If you don't reach full max hardness, you'll have a mixture of martensite and pearlite. Not good.


From Kevin Cashen....." I have likened the concept of shooting for a lower as-quenched hardness because, after all, you are going to temper it back to that or softer anyhow, to a farmer deciding that since his crop will only be six inches tall after he harvests it why not only wait until it is only six inches tall to begin with and harvest it then, heck he may be able to plant two crops a season then.

Two things result in less than maximum hardness, pearlite and retained austenite (I am going to just ignore the possibility of upper bainite), but for most shallow hardening steels the main culprit is pearlite. If you quench 1084 or 1095 to a 63HRC you have pearlite in the mix. No cryo, no temper, nothing you can do short of re-heat treating will convert that pearlite to anything else- you are stuck with it, and that is that much less martensite you can have. Also bear in mind that one of the keys to maximizing both strength and toughness is homogeneity, a blade with a mixed microstructure with pearlite will not match a fully martensitic one in stability.

The heat treater is a farmer, his crop is martensite, he wants as much as he can get before the tempering harvest that will signal the end of his work."

 

[Skeaterbait]

Gotcha. I didn't realize how much change takes place in the heating of the steel. Perhaps someone could make a Schoolhouse Rock video

Thank you.

 

[samuraistuart]

You are most welcome. And more on the topic from Mr Cashen.....

"Gary your question is a good one and it seems quite logical it tempering is viewed in a vacuum. However tempering is actually a VERY complex process, perhaps one of the most complex when you look at the steel on the inside. If all you wish to do it relax the stress of hardening and impart some toughness, which is the most common and obvious view of tempering, then all you really need is 1 cycle for 2 hours and you would be done.

But we do not live in a perfect world and many, many things happen in the process if heating hardened steel to 350F or above.

First, in an oven it takes a very long time to heat the steel to around 400F, much longer than it would take to heat it to 1400F. So realize that most of time of the first hour is just to get the entire blade to temp. and you need a lot of time for the changes in the steel to occur at this temperature, while they happen in seconds at 1500F they take hours to happen at 400F. So this is the reason for the standard recommendation of 2 hour tempers.

But to the heart of your question- in a single temper cycle changes occur in the steel both from the heat and from the cooling. No quenching operation is perfect in getting 100% transformation to the hard stuff you want in your blade, and when things make it to room temperature without transferring to the hard stuff it tends to be trapped in limbo. When the steel “relaxes” a bit in tempering further transformations can occur when the blade cools again, leaving microscopic zones inside that are again fresh and untempered. So the second temper just sort of finishes the process.

I typically do at least three tempering cycles on my blades because I like to walk in the exact Rockwell hardness I want, so I start low and bump up the temp until I nail it. I have a log book next to my Rockwell tester where I record the information on every test that is ever done in my shop and I have noticed an undeniable trend in the numbers with just about any tempered steel I have tested. The first series of readings right after hardening will be in range that can deviate as much as 1.5 points HRC, but after the first temper it drops to a deviation of 1 HRC, after the second tempering the deviation is smaller. By a third tempering cycle (there is no significance the number 3 BTW) the deviation is normally less than ½ point Rockwell. This all illustrates a definite evening out and normalizing of the overall strength of the blade by the retempering process.

There is some debate as to whether one needs to increase the temperature to get these gains or if the same temp will work. My observations show that even the same temperature will help in evening things out as it zaps anything converted by the previous cycle."