O-1 Tempering Question

[Davy Davis]

I heat-treated an O-1 blade until it was non-magnetic, held it at that stage for about 2 minutes, quenched in oil, then tempered with a torch until the blade was a straw color and the spine was bright blue for most of it's length.
So, my concern is, I'm not positive I got the entire length of the edge to the right temperature. Can I now but it in the oven for say 2 hours at 350?
And, most important, would it be a good idea to make sure all of the knife reached at least that temperature?
Thanks,
Davy

[This message has been edited by Davy Davis (edited 09-11-2000).]

 

[tejas7]

350 degrees is about right. You may hear some other opinions. I am not sure what the "ideal" tempering temp is, depends on the use of the blade.

I would like to recommend a couple of things that might make the whole process a little easier. First, I would not hold the blade at temp for two minutes once you reach non-magnetic. I would say 30 seconds tops... (comments from others would be appreciated here...)

Second, instead of using the torch to try to temper the entire blade, put the edge in water and just use the torch to draw back the spine to the blue color you mentioned... then temper the entire blade in the oven at around 350 degrees....you will have more consistant results that way.

 

[Howard Clark]

there have been several references to O-1 recently, and I think it should be pointed out that it is not the same as simple 1095 in it's heat treating characteristics.

I would assume (perhaps incorrrectly, but I doubt it) that most of you that are using O-1 are buying precision ground annealed flat bar and grinding blades via stock removal, and not forging. If this is so, then the steel really should be soaked at temp for a few minutes before quenching, or it will not achieve full as quenched hardness. The structure that you recieve from the mill in the steel is a full spherodize annealed structure. That means spherical carbides of rather large size (reltively speaking) in a nearly pure ferrite matrix. These large spheroidal carbides take a while to dissolve (longer than the lammelar pearlite of annealed 1084/1095) and they are also complex carbides due to the tungsten and chromium of O-1, whereas the simple 10xx steels do not have either, and everything goes into solution much quicker.

If the O-1 is heated to 1525-1550F (generally accepted practice) it takes about three to four minutes to get enough carbon into solution to get HRC 62 for as quenched hardness. Lower the temp, or shorten the time, and you don't get full hard upon quenching. That can also be ok, but it affects the tempering temperature a great deal. If you hit full hard (HRC 62-64) then the tempering temperature to drop the hardness to HRC 59-60 needs to be up around 475 to 500F.

If the austenitizing temperature was low (less than 1500F) or the soak time was very short, the as quenched hardness will be lower, and the tempering temperature will also need to be lower to preserve the hardness you have.

A very decent job of heat treating can be done on O-1 with simple stuff, but it is not neccesarily a "one size fits all" kind of thing.

I encourage experimentation with careful observation and destructive testing of your work. It doesn't hurt to also keep notes.

 

[tejas7]

Excellent information Howard. That is some important info that I did not know about. Am I substantially increasing grain growth by holding at non-magnetic temp for that amount of time? Or will grain growth occur only when I go over temperature?

 

[Howard Clark]

in steels with strong carbide formers like the tungsten in O-1, you are in no danger of growing the grain much unless you grossly overheat the steel.

One experiment I did with JD Verhoeven at ISU involved holding a piece of 52100 at 1550F for 8 hours, then comparing it in grain size to a sample that was held at 1550F fopr four minutes. There was no difference. O-1 has chromium and tungsten, both form persistent carbides that will not dissolve entirely unless heated very high. These small carbides that don't go away prevent the grain from growing. In simple steels (10xx), that is not the case, and a long soak time or grossly overheating can cause grain growth. In any event, it is all time and temperature, both are important. I regularly hold austenitizing temperatures for four to five minutes on most everything, except O-1 blades that were sent me by stock removal guys, then five is minumum, and if it is a large blade, or thick stock, they will get a longer soak, maybe as much as ten minutes. I generally mar-quench it, then temper at 475F then it ends up at about HRC 58, which in my opinion is plenty hard for a knife. You do sacrifice a bit of edge holding at that copmpared to 60 or higher, but the blades can be resharpened by a guy without power tools or diamond hones, and they hold a decent edge for quite a while and will not be "chippy".