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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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Old 12-31-2014, 12:53 AM
kenshin305 kenshin305 is offline
Join Date: Dec 2014
Posts: 1
Heat Treated Stainless 440C Hole Saw Shattering. HELP!

Hello, I know this isn?t exactly knife related, but it is related to this forum category. I?m an industrial engineering undergrad who is making my own stainless 440C hole saws from 1 inch OD round bar. Using a lathe, I drilled, reamed, and finally used a boring bar to get the wall thickness to 0.021? (I know, it?s very thin). I then used a mill and a v-cutter to machine the teeth. Next, I heat treatment the hole saw to get a martensitic structure. I heat treated at 1038 degrees Celsius for 30 minutes, removed and air quenched, and finally temper for 1 hour at 230 degrees Celsius.

This hole saw was designed to be attached to a T-handle, to manually cut hardwood up to a ? inch thick. It cannot rust, and must have high hardness and toughness, which is why I chose 440C. This hole saw worked well for a year. I recently let a friend try it, who oddly enough is a girl (who isn?t fat), and it broke (see attached pictures). My friend used way more downward force than me when it broke. I feel that my hole saw has way too much hardness (no wear on teeth) but not enough toughness, almost like glass.

Can someone please advise on how to preventing my hole saws from shattering? Many of my engineering professors are baffled.

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Old 01-01-2015, 12:09 PM
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TexasJack TexasJack is offline
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Join Date: Apr 2004
Location: Southeast Texas
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I will reply so this stays on the "new" list and maybe one of our resident metallurgists will get a chance to reply. I have to say this is the first time I've seen such a tool posted on here, so it's an interesting thing to see.

Look at where it cracked. (Too bad you don't have a better close-up of that area.) I would suspect that your heat treating is the source of the problem, but I wouldn't dare to claim to be an expert on the subject. I copied the following from a quick google search:

Hardening: Heat 440c stainless steel to 1850/1950f (1010/1066c) soak, quench in warm oil or air cool. Do not over heat, when overheated maximum hardness cannot be obtained. Take care to minimize temperature time at 2000f (1099c) to avoid excessive grain growth. For best corrosion resistance, temper below or around 800f (427c).

Tempering: Hardened 1900 f (1038c) oil quench and tempered one hour (1? rd)

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Old 01-12-2015, 09:24 AM
Sergio Segre Sergio Segre is offline
Join Date: Jun 2004
Posts: 73
It could be grain growth; you let the drill at 1038?C for 30 min. The drill is thin and 30 seconds should be enough
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Old 01-12-2015, 11:21 AM
Doug Lester Doug Lester is offline
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I would agree with Sergio. In the pictures where the edge of the break are more in focus the grain does seem to be a bit large. You might try tempering your next one at 260? C to reduce the hardness a bit more. Also try two one hour cycles to see if problem is retained austinite converting to untempered martensite with just the one cycle.


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Old 01-12-2015, 05:07 PM
jmccustomknives jmccustomknives is offline
Join Date: Mar 2012
Location: Alabama
Posts: 554
The first problem is you are using the wrong steel for the job. 440C (or any of the 400 series for that matter) are not designed for the stresses that come with that kind of tooling. That is why there is grain growth. While any "stainless" material will be inferior to HSS you might try D-2. With a little research you can find a "stainless" that is high temp but working it will be an issue.
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