Best ways - if any - to get rid of unusual warps?

[Ninja Blacksmith]

I don't do a lot of forging, so I'm not sure if there's maybe something big I missed...I have a 19" W1 (forged) steel blade that I just put through normalizing cycles, and during the first two cycles there were some little kinks that were expected and were fixed promptly with a rubber mallet. The third time, the areas corrected did not re-warp, however, when I set it down to let it cool, the axis along the edge had recurve in two places, the blade shape (side view) changing from "straight" to "enlongated m". I really have no idea if these are just oddball releases of forging stress that don't mean much of anything, or if it indicates the project is already doomed to fail. Before I do correct these warps, I would like to know if it's still a blade worth finishing. Granted I've only normalized/stress relieved a handful of times, but I have never seen a warp occur like these before.

 

[Fred.Rowe]

When you get to forging blades of this length expect the unexpected. This could be stress or if the steel is something re-cycled it could be "steel Memory" The blade below was made from a seat bracket of ancient origin and I fought the warps from start to finish. In the end I dealt with a long curve in the blade that I had "fixed" three times during the building process. I over straightened the blade during the temper cycles, letting it cool hanging straight down without touching any thing that could draw off the heat. It worked. Big blades are a challenge.
If you can get it straight and can grind the bevels and other work you should be able to straighten at 450fh in the temper oven.

Good luck and post us a pic if you get it finished, Fred

 

[RSKT]

This is a challenge I've confronted as well. I'd like the same answers you seek, and perhaps evaluation of the techniques I currently employ.

After forging, I normalize twice (using the forge), after the second normalization I cool to ~400 and straighten again. Then cool to room temp.

At room temp I'll check straightness and correct (when necessary) with a wooden mallet.

I will normalize again, this time using my kiln where I have more control over temps, and as follows, 1600 x 1, 1500 x1, 1400 x 3. Then cool to room temp.

At this point, I check straightness again and correct with a wooden mallet if necessary.

I heat treat, and only after the second tempering cycle do I concern myself with straightness again. If warp is still encountered, I correct them on the third tempering cycle (flat bar/washers/small c-clamps).

Does this sound like a reasonable approach?

 

[elementfe]

A couple days ago I got frustrated with a fairly thin, long blade that was just determined to take a slight dogleg.
After trying to straighten it in the temper oven with a backbone and clamps, I pulled it out of the oven between temper cycles 1 and 2 and just leaned on it a bit while it was hot, flexing it against a board, and it went straight...and stayed that way.
Wish I really understood enough metallurgy to know whether that was a terrible idea, haven't done any serious testing on the blade yet, but I sure was surprised that I was able to take the bend out of 60ish 52100 at 475f and have it apparently stay right. Seems stable, and not inclined to take a set.

 

[BloodrootBlades]

Straightening methods depend substantially on the thickness of the spine of the knife/blade cross section and HRC and steel of the blade and there is no substitute for practice and trial and error. The flex during temper method is ideal, but that's only one tool in your bag. If you differentially temper the spine of a knife with a torch (say with the edge half of the blade held in a heat sink like the jaws of a smooth vice) the heating of the spine often relieves the stress that caused the warp simply because the stress was contained in that part of the steel. Of course you only want to do this if the knife benefits or is not hurt by differential tempering, which causes some loss of blade strength (rigidity). Warp relief by spine heating is often the case with thicker spines and shallow hardening steels as thick cross sections are sources of stress due to uneven cooling and if thick enough, hardening. If the blade cross section is extremely thin like fillet and some chef's knives it will bend with a wooden mallet on a stump fairly easily even in the mid 60's HRC. Some steels that are particularly brittle you shouldn't do this with and there is of course some risk, but in my experience it is low. Just check for cracking and flex it strongly after to make sure it is structurally sound. Thicker cross sections handle the stress of bending or impact less well and should not be expected to move without failure in a martensitic condition unless they are differentially tempered and the spine is in the low 50's or lower. If differentially tempered as long as you only hit the soft spine you can even straighten using hammer and anvil. Failure typically happens not where you hit, but the tip of the knife, where vibration can be exaggerated. Hold the knife firmly in gloved hand so that it acts as a vibration dampener, not in tongs. It also helps if the blade is at tempering heat or at least a couple hundred degrees.

 

[Ninja Blacksmith]

I've managed to get all but one tiny area near the tang resraightened so far, which I'll be fixing sometime later today probably. I glad that the warps aren't a big concern - I've just never ever seen them occur on that axis before, which really threw me for a loop.

Fred; This is the first time I've forged a blade longer than 12-13 inches, so I guess I truly didn't know what to expect. Thinking back to the way the blank looked between forging and general straightening, I'd say it was most likely steel memory, and the epicenter of both warps was where I had briefly forged at the lower extremity (less than 1600 F). I like that knife, by the way. What sort of antler/bone is the handle made of?

RSKT; To me it seems reasonable, since the process you described above is just about the same as what I do. I think the only difference is the type of mallets we use. That, and I don't have a kiln, just a cylinder propane forge.

elementfe; I haven't worked with 52100, so I wouldn't be able to tell you if restraightening the way you did compromised it or not - hopefully not, though.

Bloodroot; The spine thickness at the tang is about .20" and tapers to about .187" at the tip. During my second attempt forging about 2 years before, I learned the hard way what happens when vibrations reach the tip. I'd managed to fix the warps I was concerned about starting at roughly 500 F and air cooling from there - which I did with a rubber mallet on the blade spine. It made the garage smell like burnt rubber for the next 8 hours, but it worked.

 

[RSKT]

My biggest concern is that I'm correcting warp prior to heat treat while the steel is cold. Now, any gross warp is corrected while it's hot, but I'm still tweaking it when the steel is room temp. I haven't noticed anything worrisome on destruction tests, but I'm also not tracking warp correction to know if it's a factor or not. I do know that I feel like a chase warp more NOW, as opposed to when I first started making knives. Perhaps I'm just more attentive now, and actually know what to look for.

How do you check straightness? I use a long ceramic platen that I ordered as a spare, but never needed.

 

[BloodrootBlades]

My biggest concern is that I'm correcting warp prior to heat treat while the steel is cold. Now, any gross warp is corrected while it's hot, but I'm still tweaking it when the steel is room temp. I haven't noticed anything worrisome on destruction tests, but I'm also not tracking warp correction to know if it's a factor or not. I do know that I feel like a chase warp more NOW, as opposed to when I first started making knives. Perhaps I'm just more attentive now, and actually know what to look for.

How do you check straightness? I use a long ceramic platen that I ordered as a spare, but never needed.

If the steel is annealed I don't think you are going to do damage to it in terms of cracking or anything, but I imagine a normalization would remove stress. I have done as you said with fillet knives and not had a problem with the knife even in severe bends. I just do a 4-way line of sight straightening, but I'm also doing a decent amount of stock removal after the blade is hardened (kitchen knives mostly, super thin), so very minor irregularities are taken out and I am only getting it most of the way there, it doesn't have to be perfect at that stage.