Grain Size for 1075

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Vincent Price

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Feb 28, 2020
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The picture I'm referencing is 1075, brought to just above non magnetic and back to black three times in a propane forge, then back to just above non magnetic and quenched in canola oil. Tempered twice in an electric range oven at 400 for 1 hour each. I let it rest for a week or two then broke in my vice. Would someone in the know take a look at the grain structure and give me their opinion?

https://imgur.com/gallery/9CaGeoz
 
If you can see any crystal structure in a break like that, it's too big.
 
Vincent Price said:
Solution?
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When you do your thermal cycles you want them to be in descending heats, for example the first cycle around 1600, second 1500, third 1400. Heat treating in a forge you’ll have to learn how color looks to your eye and in your shop, turn the lights off, eliminate most or all of the natural light then make a few test coupons about 2-3” long. Heat the first coupon up and pay attention to the color when the magnet stops sticking completely, that is your baseline color from there for your first cycle go up about 2 shades brighter, second cycle one shade brighter and pay close attention to that color as that’s about where you will want to bring it back up to later to quench, on the third cycle bring it up a shade or two below your non magnetic color. The key to forge heat treating in my opinion is consistency in your lighting and process. Get used to those colors once you know they work. Make 3 coupons and on each one try to be consistent on the first two cycles but on the last cycle try a different temp for each coupon, try one at just barely magnetic, one a shade lower and the last one two shades lower. Once you break all three see how the grain looks on each one and then you know what process to use to get a good grain with that steel.
 
Also I should mention when using visual color to help with temperature avoid staring into the forge as the color from the forge will trick your eye, I try to not directly look at the flame only glancing at the work piece and check the color in the shadows outside the forge. Forge heat treating is trial and error until you get some practice and become consistent at it.
 
About the best way I've seen to compare grain size is to break an old file or drill bit. Look at the grain structure and you don't see any grain at all, mostly a grayish color. That's what a small grain should look like. As the other folks have said, go thru the 3 heats, let cool, then again heat and quench. You can break as soon as the metal gets back to room temperature and it should break like the file breaks, just snap with no bend at all.
 
Joshua Fisher said:
Also I should mention when using visual color to help with temperature avoid staring into the forge as the color from the forge will trick your eye, I try to not directly look at the flame only glancing at the work piece and check the color in the shadows outside the forge. Forge heat treating is trial and error until you get some practice and become consistent at it.
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Thanks!!
 
Ken H> said:
About the best way I've seen to compare grain size is to break an old file or drill bit. Look at the grain structure and you don't see any grain at all, mostly a grayish color. That's what a small grain should look like. As the other folks have said, go thru the 3 heats, let cool, then again heat and quench. You can break as soon as the metal gets back to room temperature and it should break like the file breaks, just snap with no bend at all.
Click to expand...
Thanks!!
 
As with Larrins post, there appears to be some stretching and ductility in the fracture surface, meaning what you see in that picture isn't only the fracture grain size. You're seeing some deformation that leads up to the fracture.

Ken covered the best and most readily available examples of proper fracture appearance.
 
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