Warping during thermal cycling?

[weo]

Hello all. I apologize if I'm re-asking a question that was already answered. This thread on another forum was as close as I could find, but I either couldn't find or didn't understand the answers.
https://www.bladesmithsforum.com/index.php?/topic/33471-warping-damascus/

On my last 2 blades, after my rough grinding and during my normalizing/thermal cycling, they developed a warp during the process, which is as followed:

After forging and grinding, I thermal cycled in the Evenheat at 1650F, 1600F, 1550F, then quenched at 1495F. One of the blades developed a warp after the 1600F, cycle and the other after the 1550F. I was able to straighten them in a vice with a bending fork before quenching. Both of the blades were relatively low layer (15-30) 1080/15N20. On one of them, I tried thermal cycling in my PID controlled forge after forging to shape as well.

Is this normal?

Do I need to do something different?

Thanks and stay safe.

 

[DevinT]

Pattern welded steel actually has more tension than mono steel because different alloys grow and shrink at different rates. There is a good possibility of a welding flaw making it pull to one side. Clean pattern welded material is more likely to stay straight than mono steel. In knife making we are always inducing or reducing stress in steel. If you make enough knives, you will spend your life waiting for things to heat up and straightening things during the process.

Hoss

 

[weo]

Thanks for the reply, Hoss.
From the searching I've done on this and other forums, I'm getting the feeling that this is just the nature of the beast with pattern welded steel. I've only started noticing this the past couple blades I've made and I have been trying to forge as close as I can to final shape/size to minimize grinding, so perhaps I'm starting to push the envelope a bit.

 

[Willie71]

I’m no expert on this, but the worst examples I have seen is San Mai that isn’t centered. I think D DevinT nailed it, the stresses we add, and different rates of change with different alloys is the issue.

I wonder if a full anneal would be better than a normalize?

 

[Stacy E. Apelt - Bladesmith]

I agree with Devin ... warp happens. You learn to quickly straighten it while hot and move on. I straighten 95% of all warping right out of the quench. Ypu have a pretty good period of time where the blade is super-cooled austenite and can be bent by gloved hands easily.

TIP:
A block of wood with a half inch slot in it clamped in the vise or screwed to the bench near the quench tank will straighten any warp or twist in seconds.

 

[samuraistuart]

Side note.....most of your "thermal cycles" are really at temperatures used for normalizing. 1650F, 1600F, that's two normalizing heats. 1550F is almost at a normalizing heat. I would suggest you lower the temperatures after the first normalizing heat of either 1650f or 1600f. Do 1500F 3x or even 1500F, 1475F, 1450f.....something along those lines. You'll have a finer grain structure.

 

[weo]

You'll have a finer grain structure.

I'll try that progression next time. Here's a picture of the grain at the tang (taken with my phone, sorry). I'd like to hear opinions.
I made the tang about 1 1/2" longer than necessary just so I could break it off in the vice to see the grain. I'm not sure if this is truly representative of what the blade will be because this is also where I was grabbing the blade with flat tongs to take it in and out of the forge, so it might not have really air cooled. That might not make a difference, though.
For reference, the tang is 0.060" thick here.

 

[NickBoyle]

Looks like a big grain to me. If you have an oven to do your refinement, don’t step down temps. I’ve been told by a few MS and proven with my own testing, three cycles at say, 1475 and cooling to ambient will refine the grain better. The step down comes from using a forge and looking for steel color. YMMV.

 

[samuraistuart]

Agreed...it looks like large grain to me, but it's really hard to tell with photos, especially if it isn't close up and also if the fracture isn't clean. As long as the phase change occurs, grain refinement will result. If the temp is lower than the where the phase change occurs, then nothing will happen (well, depending on how long you soak then you'll start to spheroidize). Go too high above and grain enlarges. Best to stick in that 1425-1500 range. 1350 is roughly the temp where we start to get the change to occur, and on simple steels the change is complete around 1425f. Those are rough numbers, but close.

 

[weo]

Agreed...it looks like large grain to me, but it's really hard to tell with photos,

Thanks guys. It is hard to tell from the photos, and you might be right, but part of what I see is that in the photos some of those larger bright dots are reflections from the flash that I don't see in real life.