440C warpage help

[pwheeler]

Hi bladeforums,

This is my first post here, been lurking for a while. I have a heat treat question. I am working on a pair of very large 18" chef's knives in 440C. Unfortunately both came back from the heat treater warped. I have had blades warp before and was successful in flattening them during temper by clamping them a little past flat in the other direction. Unfortunately since these were heat treated by TKS, tempering and cryo treating had already been done. I poked around for other solutions and found none, so I went ahead and rigged up the jig and threw it in the oven. Several attempts later on both blades at increasing temp and time, I still have warped blades. One is close enough I'm about ready to call it good, the question at this point being, is the heat treat totally ruined? I don't have a hardness tester, I ran a file over the edge and it didn't bite hard, but I was able to remove some material with a few passes. Effectively these have been tempered roughly 5 times now including the original at TKS, and by me, at temps up to about 480, which is as hot as my oven will get. About 2 hours of soak time per pass. Should I forget about using them without heat treating again? If so, how can I get these all the way un-warped? I have a blower I use for heat treating O1 knives with charcoal and a regular kitchen oven, but nothing other than that. Suggestions, comments, etc all appreciated. I have a lot of hours in the grind on these knives, really hoping they're not garbage.

Since I'm new here, here's a little of my work, starting with my first knife years ago

Somewhere along the way

First big kitchen knife

And one from the current batch, my new knife to hunt with.

 

[elementfe]

I hope someone more knowledgeable than me chimes in.

For me, if it's gone up to 480, 440c is softer than I want, but I don't know of any reason they couldn't be re-hardened. I wonder if they would benefit from thermal cycling of some sort?

But seriously....I've always done my own heat treat, though in many cases it would have made sense to send a batch of blades out, and I don't understand this. Do heat treaters actually NOT check for warp? What would be the point of having it "professionally" heat treated by someone who doesn't send it back straight?
Warpage is the biggest reason that I've been frustrated with 440c in the past, the thought of PAYING someone to produce the same screwups I can produce in my shop makes my head hurt.

That said, it's probably like the butchers that process deer for hunters...it is what it is, and maybe they see so much sketchy stuff that they just shrug and send it on. I'd hate to have a business like that. I get frustrated enough with my own wrong turns, good idea grenades, and other screwups.

 

[pwheeler]

Warpage is the biggest reason that I've been frustrated with 440c in the past, the thought of PAYING someone to produce the same screwups I can produce in my shop makes my head hurt.

Can you elaborate on this a bit? What stainless have you had better luck with? I've given up on using the knives with this round of heat treatment, but I'm still faced with the dilemma of getting them straight before trying again. Currently the plan is to fire up my charcoal forge when it's not 5 degrees outside, and anneal them and bend them straight. Any better ideas? The charcoal forge creates a lot of work cleaning blades back up, they're currently 5 minutes with the scotch brite belt away from done, finish-wise. Would doing some normalizing before sending them off avoid warping again? That seemed to solve my warping problems with my O1 knives when I was doing back-yard quenches.

 

[Patrickknives]

I have worked with TKS on heat treating and also had some warpage, They actually called me prior to shipping back and got the problem solved, I would suggest giving them a call and discussing, they are very good at what they do, this may have just slipped by.

 

[elementfe]

Sorry, I kind of fired that off without thinking very hard.
I had a couple runs of CPM154 folding knife blades that warped, but I think I've gotten to the bottom of it- 440c been very good to me.
Making sure it was standing straight in the furnace, not laying it on the anvil, even cooling during subzero seems to about cover it.
Stacy recently said in another thread that normalizing would be a subcritical anneal, 1250 with a slow cooling. I think that's a really good idea whenever you do a bunch of grinding on a bar of steel before you stress it by hardening.
The only other thing that occurs to me is making sure all the grinds are centered, but your photos show some very clean work, so I'd assume you have that well in hand.

 

[Ken H>]

You asked what other stainless that might be better than 440C? How about Sandvik's 12C27, or 14C28? I've used both of those with no warping issues at all. I just HT'd a 12" fillet blade of 12C27 yesterday. It was about .090" with no warping. I do plate quench and that seems to keep a blade straight.

After HT - yep, warps are hard to remove.

Ken H>

 

[pwheeler]

Thanks for the help. When I was doing O1 knives I had a few in a row warp on me and was getting really frustrated, luckily I was able to fix them all in tempering. When I started normalizing, just by bringing them up to a dull cherry red and then air cooling back to black a few times before taking them up to critical, I never had another one warp. Getting my own furnace for stainless is not in the cards financially, it's frustrating having part of the process out of my hands and not knowing how to remedy the problem moving forward.

 

[mete]

WARPAGE !! Maybe this response will be a sticky !
When you get the steel do a subcritical anneal ,1200 F for two hours. Straighten if necessary.
Grind etc then another subcritical anneal for 2 hours
Quench , if in oil agitate spine to edge. Air hardening steel can be quenched in plates using 3/4 to 1" aluminum.

If you attempt to straighten a hardened knife heat to at least 400 F !

 

[Joe Watson]

What's your finish looking like before heat treat? I wouldn't grind a knife this thin before HT whatsoever, and any profiling or scale removal would have lines cleaned up to a minimum of 120 grit.

FWIW, I've never had a blade come back completely flat post HT. Enough to pass an eyeball or granite plate test, sure, but they're never completely flat.

I'd have that re heat treated (if it was me I'd scrap it or use for practice). I do not ever suggest guessing where you're at with temper & hardness. Many knifemakers do and get by with it, but I don't see any good reason to take the risk.

 

[pwheeler]

This is one of the knives, cleaned up a bit. Can't seem to take a decent picture with my iphone... Anyway, they were finished all the way to 400 grit before HT, tooling lines and symmetry shouldn't be an issue. I can buy that I took it too thin on the edge, what's strange (maybe) is that the apex of the warp is right at the thickest part of the blades.

 

[Big Chris]

I would recommend you send them to Peters. At $25 a blade I and probably every other maker here that uses them would guarantee that your blades will come back straight. Tell them they have already been heat treated so they can do a full anneal first.

For 440C I plate quench and temper at 300 degrees for a 60-61Rc. My opinion is though that if you went over 400 degrees they would be softer than I would want for a chef knife.

 

[pwheeler]

Thanks Chris, I sent an email to Brad at Peters, I'm kind of hoping I can just send them there as is and get straight knives back.

 

[Stacy E. Apelt - Bladesmith]

I have had excellent results with Peter's. They all come back hard and dead straight.

 

[Stacy E. Apelt - Bladesmith]

I added a WARPAGE page to the Good Info stickies. Anyone with a favorite thread or method, PM me with the info or link and I'll add it, too.

 

[mete]

Thanks Stacy !