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I've got a problem with hardening 4140


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I am relatively new to blacksmithing but have gotten far enough to know when something is not getting hard. I use Canola oil as my quenching medium & have succesfully hardend two chisles, & two punches from old coilspring. But I have recently purchased some 4140 & have made some more tools out of it but it seems unable to harden. If anyone has any suggestions I will accept them gratefully.

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I'm guessing A.) your canola oil is too slow of a quench as it stands and/or B.) you aren't bringing it up to austenitic temperature.. Did it harden at all?

Are you preheating the oil prior to quenching or is it starting at room temperature?

Also, what did you make that you are you trying to harden?

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I was going to recommend that, but if it's a relatively thin cross section it can crack when quenched in water. If it's something larger (like a hammer) you might need the faster quench medium in order to get the desired hardness.

 

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Welcome to IFI, Cattonkj! If you add your location to your profile settings, you might be able to connect with a local member who can give you some in-person help.

According to the Heat Treater's Guide*, 4140 should be austenitized to 855°C/1570°F and quenched in oil.

*available as a free app from the Heat Treating Society of ASM International

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1 hour ago, Frazer said:

I'm guessing A.) your canola oil is too slow of a quench as it stands and/or B.) you aren't bringing it up to austenitic temperature.. Did it harden at all?

Are you preheating the oil prior to quenching or is it starting at room temperature?

Also, what did you make that you are you trying to harden?

 

1 hour ago, BIGGUNDOCTOR said:

Try water, or brine  instead.  4140 is a tough alloy, not one that gets super hard.

Frazer, to answer your question, no the peice did not harden at all & the oil is starting at room temperature (about 67* F), & I am making a hot cut hardie.

 

& BIGGUNDOCTOR, I did try water, again at room temperature. same results: not hard.

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It's good practice to preheat your oil prior to quenching since warm oil cools the steel faster than cool/room temperature oil.  I won't rehash info that has been described already in other threads, but suffice it to say it has to do with viscosity and the vapor jacket. I usually just stir a hot piece scrap of stock in the oil and wait until the sides of the container are warm to the touch. 

Regardless, if you tried water and saw no results then preheating your oil isn't gong help.

Are you sure you are bringing it up to austenitic prior to quenching? If so, it's possible the surface layer has experienced some decarb during forging and there is a thin softer outer layer with hardened steel underneath. 

BIGGUN (MD) also brings up a good point that 4140 wont get as hard as your average coil spring after hardening; it's intended to be tough. 

While I understand that you may want to figure out what the cause is, I don't think your hardy needs to be hardened/tempered to do it's job. I don't think I bothered with any of mine and they were something akin to 1050.

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With a water quench on 4140 it should get hard and has a risk of cracking in the quench. Before you go any farther, try filing down farther to see if you got a layer of decarb. It’s not uncommon to have problems with carbon loss when you are new at blacksmithing. Basically, the more heats you take to finish something the more changes to loose carbon…

Of course, there have been cases of steel being mislabeled and shipped wrong. That is unlikely, but you could take a small piece for the same material and test that to be sure.

David

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3 minutes ago, Frazer said:

It's good practice to preheat your oil prior to quenching since warm oil quenches the steel faster than cool oil/room temperature oil.

Frazer What temperature would you sugest heating the oil to?

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I don't use a thermometer and I'm quite sure I'm not preheating it to the same temperature every time. The recommended range is indeed covered elsewhere, but IMHO for most of my blacksmithing tools it doesn't have to be exact. I just touch the sides of the container.

Oh, by the way, welcome aboard! Glad to have you. Here's a useful thread that will help you get the most out of the site.

 

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I use 4140 for my power hammer dies. It works great in that application, but it doesn't develop much of a surface hardness. Like others have said, it's a tough steel but I wouldn't personally use it for any edged tools.

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4140 is used to make gun breaches because it doesn't get hard nor brittle. It makes good dies because it doesn't work harden. I "hardened" the 4140 dies I made for my power hammer but the limit isn't what a person would think of as very hard. They're impact resistant tough but a file cuts just fine.

40pts of carbon is medium carbon steel. 4140 is a medium carbon chrome molybdnum alloy, commonly called chrom moly. Light plane airframes are typically 4130 for it's resistance to work hardening.

Frosty The Lucky.

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I don't have a power hammer, but I use 4140 for guillotine tool dies, bottom swages, and recently for a small anvil block. I heat treat the dies for style points, but the other two are just normalized.

IMHO, 4140 is perfectly fine for a hardy. For the oft mentioned toughness as well it's lower hardness makes it less likely to ding your hammer.... not that I have ever done something so silly as actually hitting the hardy with my hammer before :ph34r:.  Heck, you could make one out of mild if you want to,  but you will have to dress it up more often. 

I agree you wouldn't use 4140 for a knife, but I don't grind a hardy to have a sharp edge. To me it's more of a wedge or a butcher where the apex is flat/crowned. 

Different strokes for different folks. 

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I have a hardy made from a jackhammer bit, 1050 steel.   Softer than my hammer faces---which is a GOOD  thing!  Easy to zip the hardy on the grinder as needed to resharpen than to reface a hammer!

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