Straightening Hardened Parts

I have two parts 1/8" x 2" x 15" of O-1 that are hardened to about 58 -59 Rc. They're both bowed, not unexpected, but I'd like to have them a little flatter if possible. It's only the edges that are critical features.

Are there thoughts on getting them straighter with only modest effort? I've tried clamping and over-bending them without much improvement and could give them more but if I break one I'm screwed.

Sometime in the last several years someone involved with grinding flat parts mentioned having a way to do some straightening to minimize grind stock required but I haven't been able to track that comment down. He didn't say how they did that on the thread in any case. Heat straightening sounds improbable in this case since it depends on differential heating and cooling and there isn't much differential in a thin part.

Do you have experience with an effective process for thin, hard parts?

heat shrinking would be nice.

Those aren't massively expensive from a material cost standpoint, and unless there's extremely accurate machining done to them (unlikely for a part that then gets H/T) then maybe it's better to remake them and properly fixture them to a larger bar and get the lot H/T again for a better outcome. Or take the current parts, anneal, fixture, H/T.

Trying to straighten a low alloy steel at high hardness is sort of the opposite of why you harden it - you're trying to pin all the grain boundaries so you don't get slip (ductile) movement. So you press and press until you get two pieces rather than one.

At work they hit the side with a chisel, it leaves a fine line everywhere, not the prettiest thing to see.

if i was doing it i would grind the thing straight, grind the part with the bow up then flip it over so it sits on the magnetic table well and doesn't rock.

Heat shrinking will muck up heat treatment and only induces more stress that may come out with grinding.

OH don't grind it aggressively as any heat build up can really make it bend lots, use a soft wheel.

So common for heat treated items to bend at work they make allowances for it.

Also heat treating depending on the material and heat treater you can end up with cracked items so if its critical cracktest the items as well.

Happy New Year

My savage nibbler takes 3/16" x 7/8" ? x 7"? punches , When hardened they bowed , After putting in arbor pres and no movement after scary amount of bow A light bulb went of in my head peening starting light and cover the area it was amazing how smoothly they straightened up . You peen on inside of bend causing stretching which straightens the part . Try it you like it Ken

A guy building a metal boat was trying to crown flat stock the hard way for the deck. I told him to just peen the lower edge working your way along it. He thought it was crazy and lots of hard work. He reported back getting a 3" crown in 6 feet in about 15 minutes per part. So the peening could work well.

I recently spoke to a knife maker who had a similar problem after heat treating, and he said he straightened the parts by very lightly sandblasting one side.

Sorry, that's all I got from him, but it sounded interesting enough to remember and try it out next time I make some small knife blades. . .

What we did for knives and still do is clamp to flat pices of brass and put in an oven for a couple hours just under tempering heat. Then shut the oven off and let cool slowly while still clamped. There is no real logic why this works but we do it all the time.

Edit we did this most often directly after heat treat while in the temper process but I have done it a couple times after the fact with some success.

"At work they hit the side with a chisel, it leaves a fine line everywhere, not the prettiest thing to see."

can you explain,please?

also, what do you peen 59 hrc with?

I make tools from O1 that is 1/8" X 1/2" X 9" long. I've found that double-tempering takes out any bowing, but in fairness the bowing is extremely small over the short length of these tools, and if I quench properly, only about 10% of them have any bowing at all.

The second temper is done immediately after they cool from the first temper, and at 25 degrees F lower. I don't know if the timing is critical.

One thing that may matter also is that these tools are only ground at the ends, not along their length like a knife blade would be. So the grinding doesn't create heat along their entire length. My experience has been that even the gentle heat of hand filing slightly hardens O1, I've had a couple crack from that. Maybe that's part of the reason for bowing?

You could heat straighten it but it would take quite a while, you would have to keep the max temp around 200-300*F...
Not saying it would be easy, cost effective, pretty, or well just about anything... just that it *Could* be done without ruining the HT.

sorry cannot explain any further with the hitting aspect, other than its akin to peaning it a little just from looking at it. Stretching the metal on that side.

Really stuffs the finish and look too but they still do it.

to me i would make it oversize and grind carefully, any heat will bend it like a banana as the coolant quenches it.

We had a small air hammer device and rounded chisels points that would be held to the part to be straightened. The carbide chisel point would be held with pressure and begin hammering the part as we would move it along to make a line dent into the part. It would induce stresses pushing apart so would cause the concave side to go straight. This was for very hard flat and round broach inserts. ( actually bars and round broaches not as one might think inserts are the small blades for a tool holder) Round spline broaches often were 6 feet long and needed straightening often. We had both a hand held and a full size machine that used this hammer chisel process. Yes it could be used for planner blades but often customers did not like the lines added to their tools.

The other method taught to me by a journeyman blacksmith was to set a blade on two stack build ups one at each end and push with an arbor press the center all the way down to the base table. Then remove .015 from each end and press again. He said the stress point would be just met and the blades would straighten with not breaking the blade. But not using the definite stop one would go just past the straighten point and break the blade. Yes this was for planner blades that the re-sharp grinding would often cause a bend in them. Grinding even with coolant or not, and no burn can put in stresses.

This process worked, I put a 1” round stock under the ram and pushed to the fat of the table, I would push down twice and even held the push for a three second count. It worked but I don’t take responsibility for others using this method.

My friend's uncle would board a ship that had suffered a fire and so would have bulkheads(walls and floors)all bent out of smooth and flat..He would torch and water cool places and after about a week the floor would start to bang and buckle back to flat..Very high pay job. He might go off to Japan on one ship and then come back from another country on another ship.

We had a heat treat department in our plant that did most of the heat treating for us but when we had things that we could see would warp we sent them out to a heat treat company in our town and they would heat treat them and straighten them with heat.

Shafts would come back with one or two little spots on them from a torch, kind of a blue spot. I have no idea if they heated them and then cooled them with water but they came back pretty straight. We also never had a problem with losing enough hardness to be a problem.

500-550°F temper makes for 58-59 Rockwell with O1, it’s pretty safe to push it around at that temp after hardening. With larger sections heat treaters will take the temp over the original tempering# for short periods (time temp thing for hardness drawback).

Never tried peening on tool steel so I dunno, it works by upsetting the short side to make that side grow (may make for stress risers or “notches” in the world of impact toughness).

I wouldn’t heat shrink direct hardening steel of any kind, it makes for retained austenite at the surface and often develops boundary cracks (it’s like grinder burn).

Good luck,
Matt

TGTool said:

Sometime in the last several years someone involved with grinding flat parts mentioned having a way to do some straightening to minimize grind stock required but I haven't been able to track that comment down. He didn't say how they did that on the thread in any case. Heat straightening sounds improbable in this case since it depends on differential heating and cooling and there isn't much differential in a thin part.

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That may have been me... I had a mold build a couple years ago that included some .100" thick S-7 blades about 1" x 3.5". I had left .015" grind stock, but the parts bowed more than that. I really wanted to save them, because they already had some work invested in the profile, and since they were useless as is, I decided to try to straighten them.

I used my Kurt vise, setting a dowel pin at each end of the blade, and a third dowel pin in the center of the convex side, then closing the vise to bend the blade. The first one took forever, sneaking up on the point where the part would start to deform, without overdoing it. I'd close the vise, note the position of the handle, remove the blade and set it on the surface plate to see if the amount of bow had changed. It took a surprising amount of deflection to reach the elastic limit of the steel, but once there the amount was consistent for all four blades, and I was able to bring them to within .004" of straight without breaking them, even though they had holes and details on the profile. And yes, they stayed straight after grinding, which is something I wonder about with the peening method, which relies on the stresses induced in the skin, which of course will change as the surface is ground off.

Buck's method using an arbor press and stacks of shims to limit the amount of defection sounds like a good way to handle larger pieces.

Dennis

Modelman said:

<Snip>
I wonder about with the peening method, which relies on the stresses induced in the skin, which of course will change as the surface is ground off.
<Snip>
Dennis

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If the preening of a part induced changes only in the skin (say top .005") or so of metal, indeed grinding would remove these changes and the benefit of preening would be canceled. But that is not the case ( so to speak). Peening causes changes much deeper in the metal. If you think about peening a piece, say, .060" thick or even .125" thick, you would expect to see peens transfer as bumps to the back of the piece. So, the changes are not superficial. I have had several occasions to peen gibbs and other parts either to straighten them (gibbs) or to intentionally induce a curve. Subsequent grinding or scraping or grinding might slightly reduce the effect of peening, but by no means elliminates it. Peening is a useful and very controllable straightening or curving method.

Denis

Thanks for comments, guys. I'll try the heat and pressure since it's the least risk option to start with. It reminded me that I dealt with a cylinder head repair specialist several years ago who said he straightened heads by clamping to steel plates and parking in an oven for hours. If that doesn't do it, I'll look at the next prospect.

I have straightened quite a few different things involving many different materials, and using a variety of methods. If something is curved, one side is longer than the other. It has always been easier for me to lengthen the short side. In this case, I would use glass bead blasting the concave side. The size of the beads and the pressure used will determine the outcome. If you go too far, blasting the other side a little will correct. This method will give excellent control with little risk.

Also, the part should not look like crap when you're finished.

Good luck,

Roger

Lost Again said:

I have straightened quite a few different things involving many different materials, and using a variety of methods. If something is curved, one side is longer than the other. It has always been easier for me to lengthen the short side. In this case, I would use glass bead blasting the concave side. The size of the beads and the pressure used will determine the outcome. If you go too far, blasting the other side a little will correct. This method will give excellent control with little risk.

Also, the part should not look like crap when you're finished.

Good luck,

Roger

Click to expand...

What size beads would you suggest, and at what pressure?