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Finishing before hardening?


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I’m just wondering what grit you all finish your blades to before you harden them. It seems to be one of those things that everyone does different. I finish my blades by hand so I like to get as much work done before hardening as I can.

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Anywhere between 220 and 400. It depends on the type of steel and what finish I'm going to give the blade also. If it's going to be a bead blasted tactical theres no point in going beyond 220. For a collectors piece or something with vanadium I'll lean more toward 400. I've seen some odd surface anomalies going beyond 400 prior to heat treat.

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Remember that it depends on what type steel and how you harden .For multiple hardening steps scale and decarb can be significant. For stainless such as S30V you need protection like foil packets and with the high alloy content ,especially vanadium you won't really want to do any polishing after HT !

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Most of my stuff is done at anywhere from 120 to 220 grit before hardening.... Sometimes I wish I had done more before the hardening, though, since I go to a high polish.... lol

 

Since I do a lot of reading around a lot of boards, I've once or twice seen the theory put forward that some grits may be better in the quench than others, affecting quench speed and vapor jacket, etc. I don't know how much that might be true but is worthy of some consideration in the big picture, I suppose... Anybody have any thoughts on this?...

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The surface roughness does impact the quench speed - as a general rule the rougher the surface, the faster the quench. HOWEVER, if you have too rough of a surface, the valleys actually hold on to the bubbles and the quench gets slower. THe direction of the grit lines also have an impact on holding on to the bubbles - vertical lines are much better. Now the question is - how much better? Standard metallurgist answer - it depends. It changes the shape of the cooling curve and helps defeat the vapor phase, resulting in a faster quench. THe size of this vapor phase depends on the size of the part. For water quenchants that have a very stable vapor phase, the effect is quite remarkable. For oils, the quench rates go up, but not as much. For both quenchants, nucleate boiling occurs at a much higher temperature, and the temperature of maximum cooling is also much higher.

 

Hope that helps.

 

Scott

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