Rapid hardness drop when tempering martensitic SS

[defogman]

I find that Rc hardness drops rapidly when tempering 400 series SS between 950-1000 deg f.Why does this happen and how can it be avoided?A typical example is 416ss oil hardened from 1750f .15%C atmosphere requiring 37-41 Rc.1st temper @ 600f results in high hardness.2nd temper 2 hr @ 960f high results again(42-45Rc).Then a 3rd temper @970 2hr ,same furnace,results are 34-40Rc.Similar results happen with 410,420,431,440,and some tool steels when tempered about 975f.Preferably ,I'd like to get the required hardness by tempering lower or higher where it does'nt drop so fast.

 

[metengr]

defogman;
Your post is confusing because you have packed a lot of information into it.

What size is the 4xx series stainless steel piece?
Are you sequentially tempering the same piece or are the tempering times and temperatures you referenced above based on separate test pieces? It was not clear to me.

 

[defogman]

The example is about 200 lb batch of 1" dia pins processed in an IQ furnace.Tempers described are sequencial attempts to hit Rc 37-41.(customer spec.)

 

[metengr]

defogman;
This really sounds to me like you have a serious process problem. Either you are not achieving an adequate quench after austenitizing, or your hardness measuring technique is incorrect, and last, you should not be performing sequential tempering on the same piece to optimize hardness. You only need to perform the following for 416 ss;

1. Austenitize at 1700-1850 deg F
2. Quench in oil (process check for as-quenched hardness)
3. Temper at 550-600 deg F for 1 hour per inch of thickness at temperature to achieve a hardness HRc of ~ 40.

 

[defogman]

TY Metenqr.Problem is I get Rc 44 after a 600 deg. temper.I've been reading about sensitization of 300 series SS.Chrome-carbides forming at grain boundries.Does sensitization happen with martensitic SS as well?Would this be causing a rapid hardness drop as well as reduced corrosion resistance?I get similar problems trying to heat treat 420ss to about 47Rc.

 

[swall]

Following up on what meteng said, when I was involved with 410 and 416 components, we always tempered at 600F. Period. The typical hardness range was Rc 35-45. Carpenter warns against tempering between 750-1050F, as there is a significant loss of impact resistance and stress corrosion cracking resistance. In examining the tempering temperature vs hardness curves shown in Heat Treater's Guide, one will find an interesting hump in the curve between about 750F and 900F where hardness increases before falling off again at around 950F. Long story short--another case where customer education is required. Customer needs to understand that this alloy can not have the hardness tweaked to a narrow range like a low alloy steel. I have also seen cases where certain lots of 416 did not respond predictably to heat treatment. Sometimes you would get low hardness (i.e. below RC 35) for no discernible reason.