Overheating Annealed Steel?

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FLAK111999

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Sep 27, 2010
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I was wondering whether annealed steel could overheat enough during grinding to ruin the structure of the steel. Will hardening and tempering remove any problems from prior damage (if any at all)?
 
When it comes to 52100 and 5160 it is impossible to damage the annealed steel due to the heat generated from grinding, you can turn it back or even to a mild red color and not damage it on a grinder, we tried to and were unable to find any faults in the steel.
 
It's possible. High temperature nickel alloys are tested for grinding damage during regular destructive testing of 1 piece per run for power generating turbine blades. Maybe 2 pieces, the first and last of the batch. For steel that you're holding, it would be uncomfortable at best.

Ed, how did you check for damage?
 
Well, both answers are true. Ed is right that getting annealed knife blades hot when hand grinding...even red hot....won't mess up the blade. An me2 is right that high alloy complex steels can have things caused in grinding on giant milling machines ( with tons of force) that can affect turbine blades ( in ways that knives never deal with).
Luckily, more of us make knives than turbines.

The steel we use for knife blades will anneal when heated to above 1350F and cooled the right way. Any damage to this structure would require long soaks at elevated temperatures or severe stress. The stress and heat of hand grinding won't make much of any change that matters. It may minutely harden the surface, and can change the color, but the steel is still going to be fine when HT is done.

Short answer - Don't worry about it. It will do more damage to your fingers than the blade.
 
Yes: Red ran an experiment in his lab and they tried to damage the steel on purpose hand grinding as hot as they could get from the heat of the grinder, no significant difference. They shot chemistries and did photomicrographs.
 
And they actually got it to glow? Glad I wasn't the one to grind it. I got some pin stock well beyond a dull glow once, by accident, or rather ineptitude, since I had no idea what I was doing. Did they expect a change in chemistry?
 
me2: We did not expect a change in chemistry, but it was easy to check so why not? Making assumptions is always risky, now we have another answer. Rex, not Red did the tests. The heat source was a worn out 220 grit belt.
 
Temperature was by eye - the steel turned black and had been as hot as they could get from the belt. Photomicrographs taken from area nearest the belt which would have been the hottest, they had seen a mild red glow in this area.
 
me2: the questions you asked were good ones, thank you!

The variable we wanted to explore was the one that started this thread, can you damage annealed or highly tempered steel by creating too much heat while grinding?

We answered that question to our satisfaction, but could have measured how hot the steel got during the experiment. If we had measured this variable we would have had more data, would it have been significant to the experiment? I don't think so, but this is an assumption and assumptions can lead to errors later down the road.

I have tried it on experimental blades and found no difference in performance when testing the hardened blades through edge flex, cut and tested to destruction.

I simply over heated one part of the edge and not the entire edge, hardened and tempered then tested. There was no significant difference.
 
Were the edges overheated to dull red? Have you seen the micrographs of the dull red section from grinding? And finally, which method did they use to determine alloy content?
 
Rex shot the chemistry, determined the alloy. It was a lab sample they were testing.. I have not seen the photomicrographs and would not know what they could tell me. That is Rex's job, he has never mislead me.

I have a microscope and asked Rex what I had to do to learn read what I was seeing, he suggested a couple of years in college, then come work with him in the lab for a few years and I could be mildly competent. I decided to let him take care of the laboratory stuff, that has bee his venue for over 20 years.

Rex sent me 10 photomicrographs from 10 samples of 52100 and asked me which was the best. I did not do well on the test.

They just heated one end of a bar, I do not know the size of the bar.
 
Sorry, I wasn't very clear. I meant did the blade edges you got hot on the grinder turn a dull red? What do you mean by shot the chemistry? Did he use a pmi (positive material identification) "gun"?
 
It's possible. High temperature nickel alloys are tested for grinding damage during regular destructive testing of 1 piece per run for power generating turbine blades.
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The blade that I tested turned black, with a slight red glow next to the belt under heavy contact, the faint red glow was maybe 1/64 or less inches deep, the edge turned bright blue as it began to cool, led up to a black color trailing on up to a straw color into the blade another 1/16 "or so on the blade coloring was was about 1/4 of an inch above the future edge. After it cooled to room temp the discoloration was blue - gold- - dark blue to black, then faded to dark straw to light straw color, extending a distance of about an inch above the location of contact with the dull 220 grit belt. I just repeated the experiment since it has been years since I did it the first time. It will be at least a month before I get to test this blades performance qualities.

edit:: Back to my first test blade in this venue made and tested about 12 years ago -There was no significant difference in edge flex or cut in the heated area of the blade when compared to the entire blade after the blade was hardened, tempered and sharpened in my part of the original experiment.

I do not know how the lab evaluates the chemistry of samples, all I know is that Rex likes a piece about the size of a quarter that fits inside of the instrument when I send him samples for chemical tests.

Years ago this same discussion took place in Blade or Ken Warner's Knives Annual, the folks interviewed at the time did not have an answer. This inspired Rex and I to experiment.
 
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I've had one large, long ( 1.5" wide, 3/16" thick, 9" long blade) bowie-type O1 blade that I ground from barstock (no forging on my part) that exhibited some minor warping along the tip during HT. It was quite thin at the tip - nearly-full distal taper, full-flat grind, "swedge" in the neighborhood of 40 degrees, maybe less. As you can imagine, that sort of design involves a lot of very aggressive stock-removal.

My HT guy had to bring it back up to tempering temp a third time, to straighten the tip. He told me, "That just happens sometimes. You stock-removal guys can obviously only grind one side at a time, and that puts uneven stresses on the steel." (I'll presume that my hammering brethren can only pound on one side at at a time) His explanation was that the extra tempering step helped remove any residual stress, and he was confident that the blade was good-to-go. This is a guy with 25 years' experience, who has HT'ed more blades than most of us will ever see.

I still have that blade. While I trust my HT guy very much, I'm still leery of it.

Again, this is an extreme example,,, and I may be just over-cautious. But it seems to me that more awareness of heat during forging/grinding, and an extra step of normalizing before HT couldn't hurt.
 
I feel that whether ground or forged, the steel needs to be prepared for its final hardening.
Ed does this with his triple quench and the steps before and after that. Others do it by "cycling" the steel ( similar to Ed's procedure.) Both groups are doing the steel and their knives a favor.
In either case, the steels structure ,grain, and condition are "normalized" to become as stress free as possible by the time the final hardening and tempering are done.

On the simple steel types which we make blades from, any heating done in grinding, even heat that gets the blade red, will make no damaging change to the structure. The "cycling" steps taken in preparation of the final quench will reverse all changes made up to that point, anyway.

James comments bring up another point:
One thing that gets missed by some folks is the importance of the second temper cycle. In the first temper, the brittle martensite is converted to tempered martensite. Brittle martensite is under great stress ,caused by the quench and conversion of austenite into martensite. This stress is relieved and the brittle structure is changed into a tougher and more usable one. During this temper, any retained austenite is converted to martensite. This new martensite forms as the blade cools down from the temper point to room temp. The newly formed martensite is now brittle martensite.....and causes new stress in the blade. This stress is part of why a blade can come out of the first temper with a warp that it didn't have going in. The second temper converts this to tempered martensite, and releases the stress. On a higher alloy steel, in some cases, a third temper is required. This is usually reserved for complex stainless steels with alloy content in the 20% area. The higher the alloy content, the higher the amount of retained austenite .
In contrast, simpler steels have an alloy content around 2% - including the carbon. These steels need a second temper, but a third would only be done as a precaution after straightening a unresolved warp..
 
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