o-1 warping

[Mitchell H.]

I have tried three times to make a sntoku kitchen knife and all three times it warps during quench.....using motor oil, but never had this problem before, thought it was too thin so used 3/16" instead of 1/8" and left edge thicker, but still warped horribly what to do???? Also I heated the oil to see if that helped......nope.... btw knife blade is about 7" long and 2.5" tall

[Kevin R. Cashen]

Mitchel:

Distortion is perhaps the greatest curse and largest mystery that bladesmiths face. Don't let it get you down, it still happens to all of us. The factors causing or effecting it are so countless that I will no even try to list even a portion but will instead work off form some clues in your post. Are these different blades, or you re-heat treating the same blade? If so are you doning some sort of preparatory heat treatment between? If not, we have found a major contributor to your problem.

Next would be your heat source. One of the major contributors to distortion in blades is the heat source. Even austenitization can be very important to avoid distortion, so many forges and especially something like a torch would be be problematic for this issue.

The third thing I see that may be an issue is the quench. This is one of those reasons why a good quench medium can be rather important. It doesn't sound like you are planning any large scale operations in the near future so I will stay off from my quench oil soap box for this discussion. Instead may I suggest automatic transmission fluid instead of motor oil. It has different viscosities and vapor characteristics and a greater potential for thermal conduction.

Now for the fix, even if you do get warping. Heat the blade as evenly as possible to the quenching temperature. Before quenching, make sure you have good insulated leather gloves on. Quench into 150F. ATF for around a 4 or 5 count and then pull the blade out of the oil while it is still above 400F. Eyeball it and see if it is warping at all. If it is take it in your gloved hands and guide it back straight. O1 is great for this operation so you lucked out there. The blade will only actually begin to harden after it drops below the 400F point and then it will be degress of increasing hardness down to room temperature. All this time you can very easily move it with finger preasure alone. Eventually it will reach a point where it will be too stiff to work and you should leave it alone, most of the heavy distortion will be over with by then as well.

Good luck

[Mitchell H.]

Thanks for the information....I was referring to three different attempts at the3 same blade shape/type, but different blades. Never had this problem with narrower(top to bottom) blades.....

[back pocket]

I also had 2 of 4 blades, all the same size and design of o-1, curve ever so slightly. But I tried to straighten the blade in a vise jig, after tempering 2X, and snapped it. BAD feeling, but it did give me a chance to examine the inner grain structure...very tight , fine grain. I had never broke a blade for testing purposes. And I guess what I saw under 17X magnification was good. Why 2 blades warped??? Why 2 blades did not??? Thanks for the tip on the timing of getting the blade bent back. I will look more closley next time, but I was in a hurry to get into oven for temper. Tony!

[twistedneck]

O1 is a great steel to austemper (Bainite) to a good 57-58 Rc. Or, martemper it. Both ways will reduce warpage. Do you have access to a salt pot Mitchell?

Here is the TTT - you'd need to soak it for six hours.



Its similar to 52160 TTT curve, but 52160 is more difficult to austemper you only have 2.5 seconds to miss the pearlite nose, compared to 9 seconds for O1. Although for 52100, you only hold for one hour vs. six hour hold for O1.. that's the only thing.

[Kevin R. Cashen]

Even if you miss your opportunity to straighten in the interrupted quench, there is still hope. Put the tip of the blade in a padded vise and heat the spine until it start to turn purple ( only the spine and the body of the blade, not the edge) while protecting the edge with a wet cloth. While at temperature give the blade a push to straighten it. At this heat the blade will move with much less effort back to straight.

[back pocket]

Thanks, Kevin...I will try it next time. Tony!

[Fox Creek]

Mitchell, From my own experience, I feel like when blades start warping there are a couple of things to look at, assuming your quench Medium is OK. 1) Make sure the blades are normalized thoroughly to relieve internal stresses. Many folks normalize from too high a temperature. Normalize from a little below a critical - hardening temperature. More than once. 2. harden from a slightly lower temperature than you have been doing. There are so many variables in our perception of heat, you may well be hardeing from a higher temp. than you realize. Usually these two areas will solve the warping thing...Unless there is something really radical going on somewhere else, but it doesn't sound like it. With a particularly difficult blade shape, where you still are getting warping; try a slower quench medium. Use the same media cold instead of pre-heated, then a thicker oil heated; then that oil cold, etc. Of course, test for hardness too. I have gone through this exercise with difficult blade forms, and was pleasantly surprised to find that things finally reached an equiblibrium of sorts, and hardened sucessfully with full hardening, no warping.

[twistedneck]

Originally Posted by Fox Creek

Mitchell, From my own experience, I feel like when blades start warping there are a couple of things to look at, assuming your quench Medium is OK. 1) Make sure the blades are normalized thoroughly to relieve internal stresses. Many folks normalize from too high a temperature. Normalize from a little below a critical - hardening temperature. More than once. 2. harden from a slightly lower temperature than you have been doing. There are so many variables in our perception of heat, you may well be hardeing from a higher temp. than you realize.

Awesome points! I will file this information for sure.

Now about increaseing the oil temp, that actually speeds up the oil due to lower viscocity, but you right it does increase the dt / dT part of the heat extraction equation..

[sdcb27]

Uneven grinding will cause a blade to warp like mad as well

[Mitchell H.]

Originally Posted by sdcb27

Uneven grinding will cause a blade to warp like mad as well

only ever happens on tall thin blades or I had a long thin fillet blade warp as well

[sdcb27]

Originally Posted by Mitchell H.

only ever happens on tall thin blades or I had a long thin fillet blade warp as well

Makes the chances of warping even more so. if your grind is off no amount of normalizing or messing with quenchant temp will make a differance. I had a bunch warp and or twist on me so i decided to exsperiment and what i found was that the cause was uneven grinds. Generally it was exactly where a uneven grind started on the edge is where it warped and if the grind was not even on the flats it would bend.
Conclusion was that the thicker areas cool slower and the thin area went fast making the thin area bend around the thicker areas. Mind you we are talking nanoseconds , I tested in various quenchants various degree water included. The water cracked them but again in the same area that the oil was bending or warping,right at the spots the uneven grind started.
As i really sat down concentrating on my grinding, really focusing on grinding true, all warping and bending went away totally and percentage of simple steels breaking that I quenched in water was cut in half. I had a sweet straightening jig made to fix these problems, its collecting dust..ofcourse i still thoughly normalize but for grain control then a worry of warping now