Tempering an RE Davis 'hawk head in 6150

[Elnathan]

I hardened up one of these this afternoon, and am trying to figure out what temperature to temper at, shooting for 55Rc or a little lower. There doesn't seem to be much info out there on 6150, and what there is mixed - the AISI specifications seem to indicate that somewhere around 300 deg. F would be proper, though it is hard to tell from the graph I found, while the discussion online suggests that 375 to 450 is likely to give the desired results in practice (one guy says 375 gave him 58 Rc, for example). I am thinking of going with around 400 F.

Anyone done this before and can give some advise? This vanadium stuff is tricky for home tempering, as it turns out....

[LRB]

  You could start from 375°, then check for brittle in the edge. Press the sharpened edge hard on a brass rod and see if it flexes, or chips. If it chips, temper again at a higher heat. You can walk the temper higher, but you can't walk it back. If you hardened the entire head, I would recommend you protect about 3/4" to 1" of the edge in water, then draw the rest to a neutral gray or even beyond.

[cunninmp]

It appears that 6150 will reach 59 Rc max at hardening.
Here is a reference to tempering of 6150.
It appears about 300F would put you in the 55 Rc range.
The chart is in Celsius, so don't forget to convert to F.

http://www.upsteel.com/index.php?c=index&a=show&catid=20&id=9323

Mike C.
 

[Elnathan]

Mike,
 That was the AISI specification page I was looking at. The problem is that people who have actually tried it seem to get different results. That may be the difference between industrial and small shop practices.

Wick,
I may try your suggestion of running it at 375 and then sharpening it and testing the edge. It is only hardened about an inch of the way up the blade.

[cunninmp]

Looks like 375f will put you around 53 Rc.
From the chart, even if you went to 425f, won't change much.
Seems to be pretty forgiving.
I also noticed some people had different results.
Most sources recommend tempering immediately
after heat treat.
Mike C.
 

[Elnathan]

Most sources recommend tempering immediately
after heat treat.
 

Can't do that, unfortunately.

Maybe I'll try it at 350 first.

[LRB]

  Yep, can't hurt starting at even 300° and working up if needed, but you do need to get a temper done soon. Fully hardened steel is in a condition of high stress and can crack just laying around.

[Elnathan]

OK, I tempered it at 350 Monday, and got the chance to sharpen it and test it today. The edge is too thick (~30 degree angle) to see any flex with the brass rod, but when I press hard enough it dents the edge very slightly - I thought it was chipping at first, but running a fingernail along the edge reveals that it is deforming instead of chipping cleanly. It will cut into a nail with very little damage to the edge (almost impossible to see anything without a bright light and knowing exactly where to look, and the damage isn't bigger than the burr from sharpening), and tapping it on concrete causes the edge to deform, not chip.

Overall I am fairly pleased with the result - It might have been better to have run the oven at 300 which theoretically would have resulted in a hardness of about 56 Rc instead of the 54 Rc that 350 should have given me, but domestic politics dictated that I could only run the oven for two hours straight once, so I opted to err on the side of slightly soft rather than risk it being too hard.

Upon reflection, I notice that those folks online who were getting harder results from higher temperatures with 6150 were tempering in one hour cycles, whereas I went with the recommended two hour tempering cycle - the vanadium in 6150 apparently slows down the changes in the steel, and I suspect that with one hour cycles they simply weren't fully tempering the steel.

[LRB]

  There are many factors in play that charts don't mention, such as accuracy in temps, accuracy in hardness testing, actual make up of a given run of steel from the mill, the type of oil, or quench used, temperature of the quench, ect. With many carbon steels, two one hour tempers with a water quench after each cycle will trump one two hour temper by at least some degree. The higher alloys seem to benefit more from this than the simple steels, but in general two one hour tempers is slightly better, especially in steels prone to having retained austenite. The second temper gives any RA a chance to convert a little more to martensite, and the water quench locks it in. However, I know little about 6150, but the low temps involved in the temper heats may not allow further conversion to any advantage.