heat treating and edge thickness

Shane Wink

Well-Known Member
Jason Wilder and I were talking about heat treating some 01 blades and taking the edge to .020 before heat treat. I had been taking 3v to .015 before ht in tool wrap but never a hypereurutic steel with and edge below .050.

I told Jason I would make two identical stock removal blades from some of aldos 1084. They were both ground to .020 from tip to ricasso and one was coated with pbc high temp and the other left uncoated.

The blades were placed in the oven @ 1250* and ramped up to 1500* over the next hour. Both were quenched in 100* p50 with the uncoated blade being first. Both blades were cooled in the oil using slicing motions till they were oil temp then placed in the oven @ 400* for two 2hours cycles.

After all this the blades were sanded to .015 and the edge convexed with a full bur running along its entire length then strophed it off.

The question was whether or not it mattered if the blade was coated or not with the edge at .020 and after cutting boxes and rope I noticed the uncoated blade began to drag quicker than the coated blade. I strophed the two blades back to what I believed to be equal sharpness as best I could and cut more boxes and rope and again same results.

What do y'all think? Has anyone else ever done a test like this? What were the results? Do y'all have any suggestions as to what may improve the blades and the test?
 
Shane, you mention the heating for hardening to be in an hour time frame, that is quite a bit of time in an electric oven atmosphere and the decarb could be significant. Decarb is very “3 dimensional” in that corners or interfaces that allow more than one surface to come into play will pull it much deeper than a single flat surface alone. Such is the case with an edge. I take most of my fine cutting blades to a .020”edge before heat treatment, and use a lot of O-1. But I also do not have to contend with atmospheric issues which you are on the opposite end of the spectrum with in a static air oven. I would like to give you some measurements of decarb for given soak times on samples that I have but decarb is not that easy to pin down. Each atmosphere is different and one of the most critical factors, even more so than oxygen levels, is moisture content. These things can be different in any given oven so the best we can do is very general ideas of predicting decarb.
 
I kinda figured a lot had to do with the hour ramping. I programed the pid after some test I had done with cramping and over shooting temp by cramping too fast and also comparing a little of what I had done with a lot that another maker had done with the same oven as mine.

Using the PBC I never have any scale and the blade comes out of the quench as clean and shiny as it was before heat treating. If I were to place the blades in the oven at 1475 would that not possibly be too great of a thermal shock to he blades?

When using the PBC to exclude the o2 from the blade and placing them into the oven at 1250 and leaving them to 1475 would there be any issues?
 
I, of all people, cannot fault for following standard recommended industrial procedures, but most of the issues that call for pre-soaks in simpler steels are due to expansion/contraction rates in parts of a much more complex shape that a knife. Steel is allotropic in nature which means that at room temperature the iron’s atomic stacking is body centered cubic (bcc) in nature, but at temperatures above 1335F the stacking is face centered cubic (fcc). O.K. you are probably asking what all this technobabble has to do with anything… stick with me. The fcc arrangement is a much more efficient way for atoms to get together, so you can actually fit more atoms in a given space with it than you can at room temp with bcc.

So when you are heating steel to harden you will be continually expanding as the bcc absorbs the thermal energy, until you reach the critical temperature of the steel when the atoms will shift to the new stacking and the steel will actually contract rather dramatically. Now imagine a heat treater dealing with parts that have many small sections connecting larger sections, with necking and corners and kinks, oh my! (forgive that last bit, my kids just watched the Wizard of Oz)

With complex parts you are going to have large sections expanding while thin sections attached to them are contracting, and all in a state that is very ductile, the chances of you part coming out of this the shame shape as it went in are pretty slim. A preheat stops the heating at a temperature just below the transition to allow all the sections of such a part to equalize before taking the plunge, it is the exact same reason marquenching was developed on the way down.

But knife blades are as simple in shape as you get without just heating a flat bar, so the preheat is not as critical a part of the procedure as it often is with other things. Heck, I have used salt baths for heating O-1 since the early 1990’s. And you don’t get a preheat with salts, all you get is an express ride to the target temp faster than just about anything this side of induction heating. I have dealt with some sword sections that may have been borderline, but the simple shape of most knives has never really been an issue.
 
Nah Kevin, I like your style and corny humor so keep it coming. I have been reading your post for several years and understand what you are saying. Since I dont think in those terms I do forget at times what is going on in the steel. I understand there is a time for saying "do A and you get B" but a good refresher like the one posted above is always welcomed with me and helps me to start making it applicable and able to reason it out.

Thanks for the explanation and I do mean that.
 
Electric Kilns and Ramping

Kevin has addressed your concern with edge thickness better than most ever could.

All I can add to this thread is a comment on electric kilns. Even a slow ramp speed has the potential to overheat your steel. I spent a week with my Sugar Creek Kiln testing programed cycles and temperature variance. In the end, I found that the best ramping speed with a blade in the kiln is ZERO. Unless you shield your blade from the radiant heat of the elements(which is hard to do at HT temperatures) you will get unwanted overheating. Even if only for a few seconds at a time, you can overshoot by a couple hundred degrees in thin sections. What worked for my kiln, was adding thermal mass to help recover from opening the door to add my blade. I line the floor of my kiln with 1/4" mild steel plates. When the kiln has equalized at the desired temperature, I place my blades inside. When you open the door, the heat rushes out and the thermocouple cools, at this point, the elements kick in to compensate. The walls and floor of the kiln are already at temperature and the elements are simply trying to heat the air and TC. By adding thermal mass you can essentially "bank" some of that heat and use IT to begin the equalization process. When I shut the door I "pause" the program and watch the readout climb back up slowly. When it levels out, I engage the program and the thermocouple is now giving an accurate reading of the cavity and is less likely to overshoot the mark. That's why they don't put residential heating thermostats next to the front door.
 
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You and I have the same oven Rick and I patterned my PID program after the one you posted on BF. All this last week i have been making little changes and hope to get some real time in next week as I have a dozen blades to ht. I will do the same and line the oven. I have a good bit of 1/4 -3/8 flat plate that I can cut with the torch. FYI I ave given a lot of thought to the matter and the OCD in me just wont let me not build a low temp salt pot so hopefully I will have it online soon and be able to do some more testing.

Thanks for the responses.
 
Salt pot!... You've been following that Kevin guy around too much... Ha! I'm actually quite jealous. Once I find a permanent shop location I would like to make a pot. That program in my kiln thread was the best I could do to avoid overheating the steel. But realize that I did not intend for that to be a heat treat regime but rather, a test of the kiln. I was simply trying to find a ramp speed that wouldn't jeopardize a blade in the kiln. IMO, it didn't make sense to run that long of a program when you could just equalize the kiln and then slip a blade in. I suppose there are some alloyed steels that need preheat regimes and accurate ramps but I don't work with those. An accurate temperature with a bit of a soak is all the majority of knifemakers will ever need.
 
I have been reading on them and going back and forth on them for 2 years now but now have made the decision to build a high and low temp set over the next year. Time to move the coal forge out of the outdoor shop!

I have only had the oven for 6 months and after adding the Auber PID w/ramp its taken my crafting to the next level for me. I am very color blind and having to use a magnet and start counting off seconds after it goes non mag sucks but I did it for 2 years with some success. Knowing that I can get consistently repeatable results on every blade if I do my part right, take the blade to an almost finished state, less warpage, really get the most from the 52100 I use and love and the ability to work with lower bainte has made my OCD drive me mentally insane. I want to make some O1 banite blades badly. I was given one that Al pendry and charlie yotes made. That was one of the toughest blades I have ever had the pleasure of trying to break. Being a perfectionist does not mean you are perfect but it does mean its the details that nag the hell out of ya!

I am thinking of a Don Fogg style with two t-rex burners, gas solenoid and using the potable controller I have now. We have a pipe mill in town and I think I can get the 316ss tube from them. And Rick I hope you are jealous because now you know how I feel everytime I see something you have made! :)
 
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