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Help! What caused this?


Paul Carter
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I made this dagger out of a piece of an old coil spring. I'm assuming it's 5160, but that could be an invalid assumption. I think I overheated it, but I'll let you guys tell me what I did wrong. Here is how I heat treated it. I got the heat treat instructions off the net, I believe from Alpha Knife Supply from the 5160 page although it is no longer there. Don't know if it is gone because they sold out, or if I saw these instructions somewhere else. But after reading a couple posts here, I'm pretty sure I over heated it by about 100°F. Here's what I did after sanding it to 600 grit. I forged this blade about a year ago and it's just been sitting around since, if that makes a difference.

 

Normalized by taking it to 1625°F, then let air cool.

Took to 1500°F, then let cool.

Took to 1425°F, then let cool completely to room temp.

Took up to 1625° and let soak for 10 minutes[as per instructions I read], then quenched in canola oil pre-heated to about 140°F.

 

This is how it came out after sanding it some, after I removed it from a jar of vinegar overnight to dissolve the scale, but it didn't even touch this scale. The vinegar was clear as water this morning.

This happened to me one other time when I heat treated a coil spring steel blade I made at school. That blade was heat treated in a forge, by eye. This blade was done in my Evenheat oven.

 

 

dagger001.jpg

dagger002.jpg

Edited by Paul Carter
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Yep, that's overheated, and that's also why you see me telling people not to soak things that don't need it.  Low alloy steels generally do not need it.  What you're seeing there is grain growth and a little alloy segregation along the grain boundaries, I think.  I've done that to 5160 by accident, it happens.  When following industrial HT guides, remember the soak times are per inch of thickness.  If the guide says to soak 5160 for ten minutes, and your blade is 3/16" thick, a soak of 1.85 minutes is how that translates.  If you've done the normalizations, which the HT guides usually don't take into account, the soak time becomes a second or so once the blade reaches the target temperature, especially for an alloy that does not have things like vanadium to prevent grain growth.

 

And now for the obligatory scrap steel statement: Coils like that can be 5160, and usually are, but can also be 9260, 1070, 1060, 1084, or even a precipitation hardening low carbon steel (Ford started using that around 2008 or so, you just hot-wind it and bake, it's springy but won't harden).  Coils from American cars between around 1958 and 2000 are almost always 5160 or 9260.  HT is pretty much the same.  

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These coils were from a 1964 Pontiac GTO I used to have. I believe they were OEM too. I know I shouldn't really be using old springs for knives, and usually I buy steel I know what it is. This knife was for a gift. Well, so much for that. Win some, lose some. All part of the learning curve! The hard part. Thanks, I changed my heat treat notebook for 5160 to 1525° and quench. Does that seem to be the standard for 5160? Thanks again!

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Just want to chime in here to add some food for thought.  

 

  1. Grain boundaries are the devil.  If we didn't have any then our steels would be SO much better.  When you have grain boundaries though, it is best to have a lot of them.  As grains grow, the grain boundaries get thicker, too.  And grain boundaries like to get attacked by things like acid or atmospheric oxidation more than the grains.  
  2. 21 hours ago, Alan Longmire said:

    When following industrial HT guides, remember the soak times are per inch of thickness.  If the guide says to soak 5160 for ten minutes, and your blade is 3/16" thick, a soak of 1.85 minutes is how that translates.  

    This kind of thing gets really tricky.  Industry standard is generally 1 hour per inch, with a 1 hour minimum when you read older things.  More and more you see people getting a bit more realistic and they start to say things like "1 hour for the first inch, and 30 minutes for every inch thereafter".  This is still probably overkill, but it is safe (sort of).  Originally, the soak time allowed for 2 things.  The first was to ensure the entire piece came up to temp.  The second was for things to happen metallurgically (generally speaking: diffusion based things and such).  These things take time at temperature.  So historically there was a soak to ensure everything was up to temp for at least long enough to do what needed doing.  The more modern approach is to give soak times based on "once the part is fully up to temperature".  If you see something called out for a soak time that isn't along the lines of "1 hour per inch", this is what they mean.  So in the example above, when they say soak for 10 minutes, they mean get the whole thing, regardless of size, up to temp then soak for 10 minutes.  But then there is this:

    21 hours ago, Alan Longmire said:

    If you've done the normalizations, which the HT guides usually don't take into account

    Which makes things more complicated, too.  If you have done an especially hot normalizations cycle or two (say, 150F or so above your aim quench temperature), then you have indeed done a lot of the stuff that would be done during the soak.  Remember that things happen faster at higher temps.  So, if your soak is specifically to dissolve carbides because you want to have them evenly distributed, then you can do that really hot for a soak and normalize without soaks to get your grain size back down (and quench without soaking).  But if you need to soak to get carbon into solution to not have carbides in your martensitic matrix, you still need to soak.  

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And this is why it's so nice to have an actual metallurgist here.  I tend to oversimplify OR be talking about one specific alloy, and/or forget to mention some things because I assume everyone can hear the voices in my head...:wacko:

 

So for the record, my advice on this most-likely 5160 blade and my comments on not soaking simple low-alloy steels are based on the assumption that the blade has been forged to shape at a fairly high temperature of 1800-2000 degrees F, then given a triple-normalizing in descending steps from well over critical for the alloy at hand (as determined by decalescence) to as close to critical as can be achieved.  I use these assumptions as the starting point for anything else I say about heat-treating carbon steels (see what I did there, I don't know enough about stainless to talk about it like this!) 

 

If all the assumptions above are true, and the steel in question is 5160, 1095, W1/W2, 1084, 1075, or similar, I stand by the not needing a soak once you hit the target temperature.  If you're using O1, L6, D2, Cru-Forge V, or other high-alloy steel, that changes what I would say about it.  

 

Finally, when in doubt, ask Jerrod. B)

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There is a common saying of "Ask 10 metallurgists a question and you'll get at least 15 answers".  That is absolutely wrong.  Every metallurgist agrees, the one answer is always "It depends".  There are so many things to take into consideration.  With most people on this forum it is pretty safe to make some assumptions.  I like to throw out the underlying principles to help get people thinking about whether or not the assumptions are valid.  

 

11 minutes ago, Alan Longmire said:

Finally, when in doubt, ask Jerrod. 

I try to read every thread on the forum (except the Show and Tell, I just can't keep up there), but don't be afraid to tag me or PM me if there are any questions.  All I ask is that you give a bit of effort into searching for the answer first, as there is a ton of good info out there.  If there is something technical that I haven't responded to, that is generally because someone else already covered it well (often better than I would have).  

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Thank you guys! That helps a lot. As far as this blade goes, is there any hope for it? Or is it junk?. Can it be re-normalized and heat treat to get grain size back down? Or since it has been quenched, is the grain size locked in and nothing can be done about it?

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As long as the grain boundaries themselves have not gotten too large then you can always resize them with thermal cycles.  And it takes a LOT to get the grain boundaries too large.  I don't think your grain boundaries are likely to be too large, they were just aggressively attacked and made to look larger.  I would say this is well worth normalizing a couple times, re-sanding, then re-quench and temper.  

 

Option 2 is to break the blade as is to see what the grain structure looks like now.  Then do it to one you have heat treated properly later.  Very educational.  

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Thank you Jerrod! I will try re-normalizing and heat treating it. I think 1630°f was the highest I saw the oven get and that was only for a few seconds as it cools off fast at those temps when the coils turn off. I've had the most trouble heat treating 5160. Not so much with getting hard, they all got hard, but more with blisters like this one has and warpage. The first two were heat treated in a forge, this is the third, and it was done in the oven. So I plan on hammering out a few knife sized pieces of it and do what you suggested for option 2. I think, as you say, that will be very educational.

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Another question. When I heat treat blades I have been sticking them in the oven when it is cold, then letting the blade heat up with the oven. Is this proper? Or should I be pre-heating the oven then just stick the blade in for a couple minutes until it gets up to temp? Talking plain steels here, not high alloys.

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Blisters on 5160 mean overheating.

 

As for the oven, most people say it doesn't matter. It’s certainly easier to do it cold, and grain growth is negligible until you get hotter than critical. 

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I'd say it is going to depend on how quick your oven heats up.  If you put it in cold and it takes an hour to get up to temp, with the last 30 minutes just being the last 50 degrees, then that is bad.  If you can go from room temp to 1600 in 15 minutes, you don't have a problem.  Just think about what you want to happen to your steel (in terms of times and temperatures) and use your equipment as needed.  

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2 minutes ago, Jerrod Miller said:

I'd say it is going to depend on how quick your oven heats up.  If you put it in cold and it takes an hour to get up to temp, with the last 30 minutes just being the last 50 degrees, then that is bad.  If you can go from room temp to 1600 in 15 minutes, you don't have a problem.  Just think about what you want to happen to your steel (in terms of times and temperatures) and use your equipment as needed.  

Yeah, it takes about 40 minutes to get up to 1600°. So maybe I wait until it's up to temp, then stick the blade in. Thanks Alan and Jerrod for your help.

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You can also over shoot the temp you want by a bit, so when you open the oven and add you blade it drops down to the right temp.  So if you wanted to heat your part to 1550, get the oven to 1600, change the hold temp to 1550, add you part, and you're good to go.  It may take a little trial and error to see how much heat you lose opening the oven and adding a part.  Maybe you only need to overshoot by 25 degrees, maybe more, maybe less.  

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