Temper to color: yes or no?

[Alarion]

Ok, I've read a couple of posts that have left me thinking and perplexed.

Basically, they say "temper to color is a big no-no".
Why?
I mean: ok, temper in the oven for some hours is Ok, but using a coal forge people has always tempered to color and I have various manuals which advocate this type of temper.
Obviously it has to be donbe properly, with adequate soak times (that is, it's not enough to reach proper color, you have to get there slowly and remain there some time.
I have tempered a couple of knives this way, and they seem ok, even if I didn't try a destructive test.
So: should I temper in the oven only or is temper to color a valid method if done properly?

 

[mete]

A proper temper is at least one hour at temperature.This is necessary to produce a very stable martensite. Tempering by color is imprecise in both time and temperature. The color is only a surface oxide and depends on both condition of the surface and alloy. Most don't have the patience or the control to maintain accurate temperature for that time .A typical tempering temperature of 400F is easy to get using your kitchen stove (if your wife permits) or other low cost options. Remember to temper immediately after quench. Even If I were to temper back the spine of a knife I would first temper the whole knife at 400F then use a torch for the spine.

 

[peter nap]

I agree with Mete with some exceptions. What you are talking about is draw tempering and it is fine for some applications. It can be used for knives but IMHO with poor results. To start off the colors involved in blade hardness are hard to judge. They are as a rule light colored and it is very easy to go past them. Second, to do it correctly, it is a little more involved than just bringing it up to color and quenching. To properly draw temper it it needs to be hardened.You will need to polish it and continue to polish it during the drawing process.
Heat one side of the work that will be softer, to critical, and do a mini quench, while polishing the part to be tempered (you polish so you can see the progress of the colors) wait until it gets to the proper temperature and then quench. Make sure the part that you are polishing is higher than the part you heated. This should give you a progressively harder piece of steel. It also happens quickly.

This process works much better on pieces to be tempered as springy than on cutting edges.

 

[Alarion]

I do my HT this way:
after normalizing and ensuring that no residue tension remains, I quench and clean off the scale from BOTH sides of the blade.
I then put it in the feather of flame and hot gases that comes out of the forge mouth, quiet HIGH on that, so thet it gets heated SLOWLY and in a homogeneous way. When a color is reached I keep the blade there to see if it goes further. If it doesn't it means I have to lower it, I lower it slightly and SLOWLY until it gets to the color I want and keep it there some time. I expose the blade one side and the other, so as not to overshoot the temper on the underside while the one above reaches temp. Doing this both sides are exposed uniformly and not only the exterior but the WHOLE blade gets tempered. Obviously the coloring is only a skin outside, being it an oxidation, but that doesn't mean the whole blade got tempered. I don't quench it afterwards but leave it cooling slowly on a soft refractory brick, so as not to induce termal stresses.
I do this twice (after cleaning again the balde, obviously).
Then i draw the spine with the torch.
I do this way instead than with the oven because I can actually see how much the blade got tempered instead of having to go with trial and error with the oven.