overheating 440C

[SVanderkolff]

I am in the process of making a big fighter/bowie blade length 9 1/2" out of 440C and am wondering if I might have overheated it during heat treat. After tempering it rockwells at 58 so that does not seem to be a problem but as I polish it I see what looks like wood grain in the blade. I currently have it up to 800 grit and it is very distict under the right light. I can't get a picture of it but there certainly appears to be a grain to the metal. Given that this is 440C from Admiral I had not expected that. Any ideas what could be causing this.
Thanks
Steve

[Larry Peterson]

Stephen,
When I was in machinest school 46 years ago, we were shown a training film from the Grey Hound Bus folks. They showed a chrome moly' axel that had broken and it had large grain structure. They determined that the problem was heating to a too high temprature during the hardening operation. The trmporing went well and they had obtained the correct hardness but the steel still retained the large grain structure. When the steel is hardened at too low a temp you also get large grain structure. The teacher demonstrated this principle making and heat treating cold chisels. The fine grain structure is much stronger, at least for axels.

I use 440-C in about 90% of my projects. I am very fussy about the soaking heat so as to avoid large grain structure.

I hope this helps. Best wishes, Larry Peterson

[mknife]

Stephen,
this orange peel effect I believe that depends on the heat treatments that undergoes the steel in origin, before it is purchased by knifemaker.

[WBE]

Large grain size is the product of too much heat for too long for a given steel. Large grain is not caused by too low of heat at quench, and only grows from heat. Not lack of. However, what you are describing sounds more like alloy banding, which as far as I know has more to do with the original mix, pour, and rolling process at the mill. Over heating may cause the banding to be more visible, or may be a contributing factor, but I don't really know. You might want to experiment with some left over pieces of scrap from that blade, and or, discuss your problem with the supplier. I would suggest repeating your HT process on a piece of that scrap by hardening it, no temper, then break a piece off and check for grain size. If the grain size is good, then there may be little or nothing you can do to correct your problem. You might try a second piece but use a shorter soak time and see what you get.