drmini in aust wrote:
If you are going to nitride an EN40B crank, it has to be hardened first. Otherwise, the core strength is too weak.
They also grow in size which causes production problems, one reason BMC did away with it in the late 60s.
From wiki-
"Materials for nitriding
Examples of easily nitridable steels include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best. Minimal amounts of material should be removed post nitriding to preserve the surface hardness.
Nitriding alloys are alloy steels with nitride forming elements such as aluminum, chromium, molybdenum and titanium."
Oh ok... I didn't know that you had to harden EN40B first (I assume through harden if we're worried about core strength).... if it was going to grow at any stage of production that would be it, not as a result of the nitriding process.... are you sure they were through hardened? I thought it a tough core would have been desireable to a hard one..... given the application.... (would explain the way they crack though!
)
I remember learning about Nitrocarburizing (the broader banner that covers nitriding and tufftriding), and one of the big pluses with it was that it doesn't impact size or finish.... but obviously (as you've said) thats subject to any processes that the material used needs to undergo prior to treatment.
