Demystifying Knifemaking: The Truth About Heat Treating and Tempering Blades

[Woody101]

My hobby is Knifemaking. In the knifemaking world there is so much mis-information and out right alchemy regarding heat treating and tempering blades that actual fact often becomes lost.

Many bladesmiths subscribe to the 3 theory. That is anneal, normalize, quench and temper 3times each. I know that triple quenching can shrink the grain size in the steel, but others believe that annealing and normalizing can also get the same results.

My understanding is that annealing softens the metal and makes it workable and normalizing relaxes the stress from forging. All the information I can find on normalizing indicate a soak time at temperature and slow cooling in air but nothing I can find indicate there is any benefit to repeating the process. The same goes with tempering. My understanding is that it is a function of time at the tempering temperature not the cycling of the metal to tempering heat repeatedly that allows the transformation from Martensite to Austenite.

 

[FredGarvin]

I can sympathize with the aspect of some "knowledge" being close to alchemy (I like that description). That happens in a lot of places/areas.

I have heard of forging processes using double tempering, but not doubling up on any of the other processes. I can't imagine what doing the others 3 times over gets you. If you heat the part up to the propertemperature and hold it long enough, you are doing everything you need to do. Unless there is some form of work being done on the part in between. What alloys do you guys usually deal with? Are they typically tool steels?

Annealing = a "resetting" of the internal structure and relieving internal stresses due to working. Normalizing = a way to refineme and make the grain structure uniform. Dmittedly, our metallurgists have some very thick books that cover every type of heat treatment for steels. IMO, there are few fast and set rules because they are highly dependent on alloy used. The only rule of thumb that I have found safe to follow is that if you heat and then quench in water you will get more distortion/change than if you were to quench in air.

Astronuc is the real materials guru here.

 

[Woody101]

Thanks Fred. Everything I have been able to find indicates the same as what you said. The steels used are some of the more common tool steels, some of the high carbon steels such as 1084, 1095, and even ball bearing steel like 52100, spring steel 5160.

Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel
John D. Verhoeven, Emeritus Professor, Iowa State University shows that significant grain size reduction can be obtained by triple quenching. However, the information that I have been able to find indicated that the other processes, normalizing, annealing and tempering are a function of time at temperature rather than the number of times the metal is heated to temperature.

I won't even go into the myth that you need to be facing Magnetic North when quenching the blade to avoid having the Earth's magnetic field put an uneven pull on the blade and cause it to warp. :)

Most of the blades are oil quenches as a water quench is more severe and tends to crack some of the higher carbon steels.