Stress Fractures

[cedarghost]

Forged from a piece of 5160. Heat treat went south. I heated to non magnetic then an extra minute and quenched in oil. Didn't harden, Repeated. Didn't harden. Repeated and quenched in water, hardened but warped. Went to straighten warp and she broke. I tried to salvage half by making the small blade below and it came out of the first temper looking like the picture below. 

Now...

I looked up recommendations for heat treating 5160 on the forums here. Next time I work with 5160 I will normalize 3 times after forging, then will try to oil quench again then run 2 or 3 temper cycles on 400 for an hour or so. If that doesn't harden the steel without warping, I will try an edge quench. 

As far as the small blade below, what do you think all the cracks are? You couldn't see any in the metal before I tempered. I read that they might be stress fractures from forging or not. I am not sure whether to go ahead and temper a couple more times then test the blade or if I should normalize a few times and reheat treat then temper. 

Any thoughts/knowlege is appreciated. I know some of you guys have seen this before.

 

Thanks!

[-Quint-]

I mostly use 5160 and I always normalize 3 times, harden 3 times and temper 3 times, moving onto each new process while the steel is still warm from the previous process. Hope that makes sense. 

Sorry, forgot to actually answer your question lol. I would say put an edge on that knife and use it in your shop til it fails. That's what I do with my cracked blades that have gotten this far. 

I don't think any amount of normalizing or tempering will get you back to a good piece of steel.

As far as why it happened, my guess would be that it has nothing to do with your heat treat but that you worked it too cold. And I say that because I've been guilty of that and got the same results. And I also DIDNT learn the first time, so I've seen it more than once 

[cedarghost]

Thanks for the reply. I know I did let it get dull red a few times while I was working it. Lesson learned. Now if I can just keep from doing it again. haha

Good to know that I am on the right track with heat-treat next time. Do you quench in warm oil? I was thinking about trying an edge quech as well. 

[-Quint-]

Yes, I preheat my oil just a bit prior to the first quench. If the steel mass to oil volume ratio is too close, your oil will be pretty hot by the second quench and practically boiling by the third (the downside of multiple quenches) so you may want to have a second quench tank for the final quench. 

As far as edge quenching, I don't do it but I'm not opposed to it. I prefer to harden the entire blade and then temper all or part of it how I want it, instead of basically having a normalized, potentially nearly annealed spine. What I prefer is (when the blade shape allows) is to go through the process in my above post but only oven temper once. Then I have a large shallow tank that I have rigged up to hold a blade edge down in water. Then I torch temper the spine to exactly how tough I want it to be, without affecting my edge.

With a large double edged fullered blade I once heat treated as above, tempered once and then clamped 2 red hot rods into the length of the fullers until the colors I wanted started creeping toward the edges.

Just my preferences, they usually work for what I set out to accomplish.

 

Edited May 8, 2017 by -Quint-
Forgot to mention the large blade was double edged.

[Latticino]

I'm not completely sure of the logic behind multiple quenches, but preheating of your quench oil is important to reduce it's viscosity and get a better quench (unless you are using commercial quenching oil like Parks which does not need preheating).  One of the best sources for knifemaking metallurgy is Kevin Cashen.  He has been kind enough to post some great information.  I strongly recommend review of this info (and you will note that his heat treatment procedure for 5160 is a single quench in oil, not water).  This process has worked well for me, though I have also done a 5160 blade or two with an edge quench using an oil/paraffin mix and a torch to both heat the edge for quenching and selectively temper afterwards. 

I suspect that your blade surface cracking was from the overly aggressive water quench.  The original break was most likely from trying to bend a fully hardened blade.  Even after tempering that is a somewhat questionable practice (which is why I have so many blades with slight warps in them I guess, but I'm still learning).  You can do a interrupted quench and attempt to straighten during that time, but that is a bit of an art, or jig your blade with clamps and spare stock to attempt to reduce the bend during the second tempering cycle.

Torch tempering the spine, as indicated above, is a great technique, but does not allow for easy warp reduction.  I suspect that using the clamped heated stock for tempering may work better.

[cedarghost]

Thanks guys. Great information.

[ThomasPowers]

Also, how do you know it was 5160?  I once ran across a leaf spring that was strain hardened low alloy steel and couldn't be quench hardened; now only the one time in 36 years of smithing; but sure taught me the lesson about scrap steel rules...

[Michael Cochran]

5 hours ago, -Quint- said:

normalize 3 times, harden 3 times and temper 3 times

I understand normalizing and tempering multiple times but would you mind explaining the multiple quenches? As limited as metallurgical knowledge is I am under the impression that reheating to critical temps completely undoes any hardening done by a previous quench. It's also my understanding that the multiple quenches could cause unnecessary stresses on the steel. 

[Latticino]

7 minutes ago, Michael Cochran said:

 It's also my understanding that the multiple quenches could cause unnecessary stresses on the steel. 

Not sure about additional stress in the steel, but depending on how quickly you get back up to critical temperature and how long it is held there you will certainly get more decarb...

[Frosty]

I'd love it if JPH,  Steve or a professional commercial heat treater jumped in and commented on the multiple heat treatment craze. Multiple quenching sounds more like the "If a little is good more is better" bleachers. Multiple anneal or tempering sounds like someone who's not sure of what they're doing.

I hear the double triple heat treat thing from the same guys who believe a forged blade is somehow better than a stock removal blade made from new stock. I believe that myth was covered nicely in a video that's a little too adult to post here and got removed. 

Of course that's just my opinion I could be wrong. I'm flexible though just present me with empirical evidence not hearsay.

Frosty The Lucky.

[Steve Sells]

I would comment if I understood why people multi quench myself.  It just appears to me they are thermal cycling.  I still want to understand why people insist on interrupting the quenches...

[JHCC]

6 minutes ago, Steve Sells said:

I still want to understand why people insist on interrupting the quenches...

Because interrupting an oil quench when the steel is still hot increases the chances of a really cool flame burst?

[-Quint-]

Hello Michael Cochran, Latticino, Frosty and others! I don't have any empirical or metallurgical evidence to offer and I'm also not saying "you MUST quench three times"... If I quench a 5160 blade and it's HARD after the first quench, I stop there. My recommendation was based on my personal practical experience and yes, hearsay. I've had a few blades not harden and when I investigated and did some research I found several seemingly knowledgeable people/sites suggesting hardening three times and sometimes more. When I experimented with that, it did in fact work for me, so that's about as much stock is I put in it. I wrote it in my Forge diary, I'll write it here as a suggestion, but I'm not writing it down and asking anybody to swear on it, that's for sure. But I also don't point my blade North when I quench it  

Just for the record I am using known 5160 (I buy 21 feet at a clip from Admiral Steel), not leaf springs or something else that's believed to be 5160. 

[ThomasPowers]

Thanks for the alloy check!  Now for multiple quenches, IIRC it increases the shallow hardeningness of the steel; not particularly a wanted thing for many alloys.  Patrick N you lurking?

[cedarghost]

5 minutes ago, -Quint- said:

Hello Michael Cochran, Latticino, Frosty and others! I don't have any empirical or metallurgical evidence to offer and I'm also not saying "you MUST quench three times"... If I quench a 5160 blade and it's HARD after the first quench, I stop there. My recommendation was based on my personal practical experience and yes, hearsay. I've had a few blades not harden and when I investigated and did some research I found several seemingly knowledgeable people/sites suggesting hardening three times and sometimes more. When I experimented with that, it did in fact work for me, so that's about as much stock is I put in it. I wrote it in my Forge diary, I'll write it here as a suggestion, but I'm not writing it down and asking anybody to swear on it, that's for sure. But I also don't point my blade North when I quench it  

Just for the record I am using known 5160 (I buy 21 feet at a clip from Admiral Steel), not leaf springs or something else that's believed to be 5160. 

I found the same resources AFTER I messed this piece up. To answer Thomas' question of how I know it was 5160 (good point here, by the way), I do not know for sure. I posted pictures of it with spark tests in another post a while back and the general consensus was that it is probably 5160. It is thought to be coil spring of an old railroad car. 

I appreciate you guys with experience jumping in. 

18 minutes ago, JHCC said:

Because interrupting an oil quench when the steel is still hot increases the chances of a really cool flame burst?

This made me chuckle.