Hello all. As I continue to like my induction forge, I've noticed that while heating pieces, I often see the decalescence/recalescence shadow pass through my piece multiple times as I move it around in the coil to get a bigger heat, and that got me wondering if there are any long term effects this might have on the micro-structure of the steel?
I'm guessing this hasn't been studied? But I thought I'd throw it out there...
Using induction is a completely different beast. It has benefits and a lot of drawbacks. Figuring out solutions for the drawbacks is a proces. Let's dive into your case.
Scanning thru the coil (heating a section of the piece and moving it thru the coil until all is heated) involves careful consideration as the austenitization travels from outside to inside, thus one cannot know if it's heated correctly thru or not, without practice and testing. If even the scan is controlled evenly, there is a danger of not heated equally thru. That makes the grain refinement uneven and the piece will warp like nuts.
If the heating is even (very rarely even in a controlled and automated setup) there is a BIG danger of overheating the core. Ex. heating stainless will make the core 150°C higher than the surface, thus melting and evaporating the core is due when going for higher aust temps (higher carbon stainless). Carbon steels are easier as they require low temps, easy for induction. But if the core temp is not controlled (needs calculation, simulations and practice) we get to why some praised knives break easily when JoeX tests them.
Reason is that people don't understand induction. So called surface heat is related to where there is 60% of heating power. So if the knife is 6mm thick and 50kHz induction is used there is a 3mm surface layer when austenitized. But where does the remaining 40% of heat go? It goes deeper than 3mm, thus they overlap in the middle and heat it way more. It makes the surface desirable but the core has a lot of RA that with hard vibration turns into hard martensite and cracks under abuse.
The coil is more important than the machine. It has to be tailored to the piece. Ex. uniform coil will make the center more heated. So the coil has to be ununiform, tighter gaps between turns at the ends and looser gaps in the center. It has to be drastically changed if one grinds a knife before HT and if there are major design changes from a flat even piece (like a knife
). General coils could be used but with knowledge of the "error". Essentially understanding how to use imperfect coils to one's advantage.
So long term disadvantages are a lot. If it's low hardenability steel, the uneven heating will make hardenability uneven. Not realizing when the thru heat is, will lead to ununiform hardening. It just piles up. One can go nuts with this.
My advice: use it for forging. It helps a lot.
Advantages:
For forging it's heaven. Forge bonds turn into forge welds easily. Microcracks are healed due to high power frequency. Speed is outstanding. One can concentrate on hammering, not on waiting the piece to heat up. I recommend it for damascus.
For HT. Use simple steels and heat until the power maxes on the machine and it gets more silent. That is the point where thru heat is made. Don't soak as it will overheat the core. Quench. So in a matter of 1min, from cold, a knife is quenched.
I'll stop here. Going into how carbides react to induction is a time consuming effort
