So i kind of been into trying to make tomahawks here as of late. My first, well go to the "what did you do in your shop today" and you can see it. Just finished today. It turned out purty good. My second however miserable failure, my 3rd welds were bad. But my question is about this 4th one here. Wednesday i made it. Thursday did some rough grinding and hardened it. Friday put it in for temper. 4 hours at 400. A-36 with a 1085 insert. Sunday i was cooking out with the grandkids and showing it off to a friend. I laid it on my out side table. That evening i went out to pick everything up and found it like this. Sat for almost 48 hours with no complaints. 

Not bad, until:

Why did it take so long to just suddenly separate? Really, it sat for almost 2 days then all of a sudden it just came apart. 

 

 

I generally try to temper within minutes of hardening.  Did you really wait to the next day?  

Could be that it just took longer for the crack to propagate till you could see it.

I am no bladesmith, but i have made a couple knives and i usually harden one day then temper the next and have had good results. 

When i hardened i did an edge harden, only the first 1 1/2" or so heated. The high carbon portion. Then let it rest till the next day and tossed in the oven.

Should i have tempered like i would have a chisel and  let the color run? 

The first one i made turned out nice and i used the same process. Just seems odd that it took that long for the weld to fail and it looks like it sprung apart. 

The general wisdom of the high end bladesmiths is to temper as soon as it's been hardened. 

What caused the failure of yours isn't something I can attribute to heat treatment. Looks like a bad weld to me.

Frosty The Lucky.

One of my students once hardened a blade late at knife and laid it on a rag on the workbench to temper the next day.  When he went to temper it after breakfast it was lying in 3 pieces on the workbench on the rag.  Hardening puts a lot of stress on a blade and doing at least a snap temper on it can help if it will be a while before completing the heat treat.

You  should read the pinned heat treat posts.  They were pinned to help people notice them and avoid making mistakes like this.

Could easily be a quench crack that developed. Also, a hardened piece of medium carbon could be susceptible to hydrogen embrittlement, which will manifest itself within typically 24-48 hours of stress being applied. Most common in fasteners that are torqued.

I'm no bladesmith either, but it looks to me like the hardened steel cracked before the temper, and that crack propagated into a bad weld and split it apart.  Good thing it came apart on the table and not in use!

Sounds to me as though you've been lucky hitherto, but I would second the recommendations above that you temper right after hardening. A lot of people will preheat the oven even before they start heating their workpiece to harden it, so that there's no delay at all.

 

10 minutes ago, Chuck_Steak said:

a hardened piece of medium carbon could be susceptible to hydrogen embrittlement,

Is hydrogen embrittlement an issue in forge welding?

I guess I don't see why it might not be. If you have a high hardness steel, have tension, and have dissolved hydrogen, I'd think it could be an issue. It must not inherently be an issue, or they'd be popping left and right. But maybe with the initiation of a small quench crack there was enough stress concentration to put it over the edge? Without having access to a scanning electron microscope, it's hard to say.

51 minutes ago, Chuck_Steak said:

If you have [...] dissolved hydrogen

That's my question: where would the dissolved hydrogen come from? My understanding (which I fully admit could be faulty or merely partial) is that the majority of dissolved hydrogen in a weld comes from either the cellulosic flux coating of welding electrodes or from environmental moisture, neither of which -- I assume -- is likely to be an issue with a forge weld. 

I'm not saying you're wrong, I hasten to make clear; I just don't see how it might be an issue in this particular case.

Agreed with your statement. Without knowing the entire history of the part there is a bit of conjecture, but moisture could play a part. Was there any acid cleaning or descaling? Those could factor in also.

I think the delay in temper may have resulted in a crack and then residual stress did it in after the temper.

Very interesting thoughts. With a propane forge the by products are carbon dioxide and water. That could be a reasonable source of hydrogen... 

I have hard several discussions on hydrogen embrittlement at work, but not realty studied it. Very interesting indeed, but I doubt that’s the case here.

Looks like a overstressed weld to me also.  May not have completely taken, but also could be the result of differential thermal expansion during the quench.  I have seen several SanMai type blades (of which adding a HC bit to a folded hawk blank is a subset) appear to self destruct during the quench.  These can put tremendous tension on the weld joints and even the HC bit itself.  I've seen the latter crack right in half along a thickness plane.  Reportedly a fix for this is to be sure to grind the billet to the extent that you expose the HC bit on the sides of the billet before you go to quench (and don't wait to temper).  I did this with the last batch of (5) wrap and weld hawks I made with HC bits from W1, 5160 or 1084 and didn't have any trouble at all.  These were quenched in warmed canola and tempered immediately at 400 deg for (2) 1 hour cycles. 

Using a mystery metal such as A36 didnt help

3 minutes ago, Latticino said:

These were quenched in warmed canola and tempered immediately at 400 deg for (2) 1 hour cycles.

How long between tempering cycles?

Thanks for all the input yall. 

I should have also mentioned i did this in a coal forge and not gas. 

Steve, i do not understand what you mean by mystery metal. I can look up the chemical composition and mechanical properties. From what i gather it is one of if not the most common mild steels there is. Also with most bridges being built from it, i would have to guess they know exactly what they are using. 

I think i will have to say i agree with JHCC, i have just been lucky so far. 

 

1 minute ago, BillyBones said:

Steve, i do not understand what you mean by mystery metal.

A36 is a structural standard, not an alloy standard. It has a range of acceptable levels of alloying elements, to be sure, but that's not as precise as, say, 1018 mild steel.

A36 is a mechanical standard, it is not an alloy is what I meant.  Its like saying bread,  it may be rye or wheat even raisin.  its all still bread. Heat treating is all based on alloy content not how far it stretches before breaking, Same with expansion and contraction at temperatures. can not predict that based on a Charpy test

2 hours ago, JHCC said:

How long between tempering cycles?

That time only around 15 min, as the oven was still hot.  As far as I know, once you do your first tempering it doesn't matter that much if you delay the second. Actually for hawks sometimes I only temper once as I don't believe that optimal heat treat is essential for a chopping or throwing hawk with a relatively small HC bit.

Thanks guys that makes sense now.