Why do blades warp when quenched

I remember the times when I forged and quenched my blades in my parents’ backyard. I had little knowledge about metallurgy or the craft of bladesmithing, so I always screwed up something.

The most bitter mistake was when I pulled out a completely warped blade from the quench tank. I had no idea what happened and it seemed to be too late to do anything about it.

Fast forward to today: with a BSc in metallurgy and several self quenched blades behind my back, I could go on for pages about how to correct blade warping. But this has been written about, so I want to write about something else.

How often does a blade warp?

Let’s make this clear: we’re talking about thin, red-hot steel strips that are plunged into some kind of warm (at most) liquid until it cools down to a temperature that you can touch within a minute. And you expect it to not deform? Bad news: it does. Always. But if everything is right, the deformation is not visible.

I’ve always been told that the best way to correct blade warping during quenching is not to allow it to occur in the first place. I keep myself to this guideline, and it seems to work, although my definitions about a “true” blade have changed somewhat.

When does one call a blade warped?

I can imagine that someone calls a blade true only when the edge and the spine are both completely straight and there is no noticeable irregularities in the blade’s shape at any point.

However, I noticed more or less deformation in most of the knives upon close examination, even those made by pro knifemakers. I can notice deformation even in some of my blades that come back from the heat treater (more about this later).

However, if the deformation is arguable, like “it seems like that there is some problem at that point”, I leave it as is and call the blade true enough. I do this for a good reason.

Is warping bad?

You need a straight edge to cut. The only acceptable curve in the edge is in the center plane of the blade, anything else affects the knife’s performance in a negative way. The only question is the extent of this.

When I made myself a kitchen knife a year ago, the finish went quite in a hurry, so when I noticed that the blade had warped during the quench, it was too late. The warping was not extreme, but it was well noticeable it someone looked for it. Keeping in mind that the knife was for myself, I decided that I finish it anyway and see if I can feel anything in use. I have my best kitchen knife since then.

The moral of the story is that a blade must have a botheringly huge bend in it to affect cutting ability, anything else is mainly a visual problem.

Okay, but really… how can someone avoid quench warping?

A few words for fellow knifemakers. I’m all for DIY, but having a BSc in metallurgy, yet sending nearly all of my blades to a professional heat treater tells a lot.

I’ve done it in the past and I may do it in the future, but the workshop that I send my blades to uses such equipment that I don’t think I’ll ever have. Not to mention that they have such experience that I will surely never have. Besides, they work for a reasonable price, so it’s hard not to choose them over myself messing around, screwing up things. The only heat treating I still do is my usual triple normalizing cycle myself (which is a must prior to hardening). This is all I can say in addition that’s already been discussed in the forums. Go and have your research.

Summary

Custom knifemaking is all about keeping everything neat and tidy: perfect lines, perfect curves, perfect joints, immaculate polish, etc. But if one looks beyond that, it’s easily recognisable that even a great knife can have minor flaws. Like a nearly unnoticeable warp. So next time you notice something like that in a finished knife, ask yourself: would you have noticed it if you weren’t looking for it?

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