differential hardening

[elasmonut]

I have been making knives as a hobby for a while now and in the past, when I had a forge, had some success with using clay to get a differentially hardened blade on simple carbon and low alloy tool steels.
I have no access to a forge now as I live in suburbia, but was contemplating differential hardening the other day. I'm no expert on this stuff by a long shot and break into a sweat trying to understand some of more complicated discussions I read here on metallurgy and heat treating.
But the way I understand the process is that the clay is being used to slow the rate at which the covered portion cools during the quench, resulting in a lower hardness. Assuming this is correct, and I know I might be wrong, I thought that instead of clay why not clamp the blade between two plates of steel, covering the section where the clay would normally be and because of the increased thickness this should result in slower cooling and lower hardness.
The reason I considered this idea is using a commercial heat treating service as I do now I can see why they wouldn't want to mess around with fragile and messy clay coated blades but two plates would be clean and relatively simple. So now I've explained my bright idea I would appreciate it if some of the wiser heads here could shoot my plan full holes and tell me why it wouldn't work.

 

[A C Richards]

There are a few ways to accomplish a differential HT. The clay method you described is but one of them. You were correct in the clay slows the cooling and thus prevents the transformation of the austenite to martensite. What you get instead is a mix of pearlite and martensite. There is a lot of controversy as the the benefits of a differential HT. The only reason I do it is to obtain a Hamon which is in itself an art. The method you described would be difficult to obtain much activity in the HT line and could be accomplished by en edge quench rather than clamping 2 sheets of steel to your blade. I am not saying it will not work, rather why are you looking for in the differential HT? If it is for flexibility in the spine then just do a blue back draw with the edge in a bucket of water. If you are looking for the distinct line in the transformation area (hamon) then stick with the clay.

Have Fun

 

[richard j]

i heat treat the knives i make with an oxygen-ascetylene torch and i get good results. you can invest in a small set of torches and with practice get good results also. you could still use clay which i'm going to try sometime just to see how it works with a torch. you could also invest in a small electric ht oven.

 

[elasmonut]

Thanks guys. Am aware of the torch method, but don't have one yet and Apart from simple carbon steels I think I would need a lot of practice to get it right. Chuck I'm not looking to produce a hamon, as you said art in itself, I was looking for more spine flexibility. I think I understand what you saying cus you kept it simple, if I want flexibility and toughness of the spine then the mix of pearlite and martenstite formed in a clay quench is not ideal, I would assume then that even the edge quench would produce similar results. I think with the tools and know how I have I will stick with the soft back draw as I feel this gives optimum edge performance toughens the spine and more importantly its less likely that I'll screw it up. Thanks for your advice,the role and interaction of pearlite, martenstite, bainite and all those various steel states does my head in a bit think I'll go and drag my feeble brain through some more heat treat stickys,

 

[Ed Fowler]

I am hoping some who use the clay hardening methods will chime in and show some blades tested to destruction that manifest the differences in hardening zones.

 

[Rick Marchand]

Chuck is spot on.

I think most makers would agree that clay tempering is manly used for aesthetics these days. The beautiful hamon produced by various methods of coating and HT'g, clearly shows the maker's understanding of the process. There are much better ways to improve performance in modern steel. The ancient swordsmiths did the best they could with the knowledge they had at the time.

I won't get into which method is best.... that's a thread killer and has been done several times over. Know your metallurgy, test your blades, understand your customer (use) and stay honest to yourself and your craft.



Rick