I've been reading John D Verhoeven's 2005 treatise on metallurgy for bladesmiths (I'm not planning to make blades but I'm fascinated by the process). It's a fascinating read that's fairly easy to understand given the complexity of the subject matter.
There is one thing that I can't quite get my head around:
Please tell me if & where I've got it wrong:
My understanding is that when austenitic steel is quenched rapidly to room temp (I assume that traditionally this was done with water?), it will rapidly reach the Ms (martensite start) point (temp), and will begin to transform into Martensite. Because the transformation to martensite is so rapid (in comparison to pearlite and bainite), once Ms is reached, the formation of pearlite and bainite are circumvented.
The Mf point (temp at which austenite is fully converted to martensite) is lower the higher the C content. If the carbon content is under 0.3-0.4%, it will reach Mf (will be fully converted to martensite) at room temp but if the carbon content is more, then the Mf point will be sub-zero and a cryo quench is needed to fully martensise (if that's even a word?) the steel.
So after a room temp quench , steels over 0.4% C or so will have retained austenite (which will soften the steel compared to a fully martensitic steel).
The hardness of the martensite is dependant on the amount of carbon in it (more carbon 'stretches' the Fe lattice more which puts the bonds in the lattice under more stress, giving them less leeway to move- I kind of think of it like being a taught rope)
In steels up to 0.8% C, the harder martensite (created by the increased carbon content of the martensite) is more important than the retained austenite and the (as quenched) steel will harder than a 0.4% C steel. However, beyond 0.8% C, the retained austenite becomes more important and the overall hardness drops.
This can be improved with a cryo quench (bringing the higher %C steels closer to their (sub-zero) Mf temp, giving less retained austenite and more hardened martensite). So a higher C steel would be harder than a 0.8% steel with a cryo quench.
I'm assuming that honyakis have been around long before we had cryo quenching?
A simple 0.8% carbon steel like 1080 should be able to get to about HRC 65 before tempering with a room temp quench. This would be reduced after tempering. Because of retained austenite, higher carbon steels should have a lower HRC with a room temp quench.
So how do honyakis made from a steel like shirogami 1 (~1.3% C) and quenched at room temp get to HRC 64-65 after tempering?
Is there any benefit of a 1.3% steel over a 0.8% steel with a room temp quench? If not, why do shirogami and similar knife steels have so much carbon?
Thanks for explaining.