Straightening after heat treat

A

Alfazulu

Joined
Mar 24, 2016
Messages
234
Hey guys, I recently made my first knife (stock removal), and just ordered more steel to make more. At this point, I'm not sure if I'll ever make knives to sell, or just make them because I like making things, but I'm considering turning it into a side business, if what I make is good enough to sell, and people like it. I'm in the process of making a proper grinder right now, and have read up on making a heat treating kiln. I'm trying to learn as much as I can about heat treating. One thing that I'm not seeing too much instruction on is post heat treat straightening. I'm afraid this may be something that could trip me up. Basically, all I know about straightening is that Peters Heat Treat (to whom I'm sending my initial knives) says that some straightening is included in the price. Do all knives need to be straightened after heat treat? Most, just some? And what is involved in straightening? Do you have to heat and bend, hammer, or what? Any shared knowledge on heat treating, straightening, etc. would be greatly appreciated.

Another side question is about wrapping stainless steels in foil. I haven't really tried to research this yet, but figured while I'm asking related questions, I might as well ask it, too. Why do you do this? Is it ONLY to prevent oxidation from forming during heating, or is it to prevent decarburization? If it's just to prevent scale formation, then is it optional just to prevent the added work of removing it? What type of foil is used?

Thanks!
 
Alfazulu,

Straightening of warped blades doesn't happen after heat treatment. It happens during heat treatment, specifically during the tempering cycles. If your blade takes a warp in the quench, do the first temper cycle as-is with the warp to stress-relieve the blade. Then clamp your blade to a rigid piece of steel with the blade bent in the opposite direction of the initial warp. Using C clamps and spare change, washers etc dial in a counter bend approximately the same distance as the initial bend. Run through your second temper cycle then check results. Additional bend and temper cycles may be necessary. Some steels may be tougher to remove bends. If a bend and temper cycle doesn't work, increasing the temperature by 15-25˚ may help. Some blades may have a number of wiggles. Clever clamping can remove more than one at a time. I have run as many as 15 or more two hour temper cycles to get particularly stubborn blades straight. The additional temper cycles should not harm your blade.



There are two wraps used for blades, 309 & 321. 309 is preferable. It is good for much higher temps than 321. When looking for the foil search for "stainless tool wrap"


This info is from South Coast Industrial Metals:



309/309S STAINLESS STEEL TOOL WRAP



Our 309 Stainless Steel Tool Wrap is the ideal solution for heat treating tooling. Just wrap the tooling with the edges crimped to form an envelope. This creates an air tight package that will prevent the loss of surface carbon. Don’t let your tooling become scaled or damaged. Protect your investment while eliminating charcoal and sawdust box packaging our 309 Tool Wrap. Contact us today for a quote or visit our pricing page for stock inventory items.



General Properties



Alloy 309 (UNS S30900) is an austenitic stainless steel developed for use in high temperature corrosion resistant applications. The alloy resists oxidation up to 1900°F under non-cyclic conditions. Frequent thermal cycling reduces oxidation resistance to approximately 1850°F.



Because of its high chromium and low nickel content, Alloy 309 can be utilized in sulfur containing atmosphere up to 1832°F. The alloy is not recommended for use in highly carburizing atmospheres since it exhibits only moderate resistance to carbon absorption. Alloy 309 can be utilized in slightly oxidizing, nitriding, cementing and thermal cycling applications although the maximum service temperature must be reduced.



When heated between 1200-1742°F the alloy is subject to sigma phase precipitation. A solution annealing treatment at 2012 – 2102°F will restore a degree of toughness.


Element 309
Chromium 12.0 min-15.0 max
Nickel 12.0 min – 15.0 max
Carbon 0.20
Manganese 2.00
Phosphorous 0.045
Sulfur 0.030
Silicon 0.75
Iron Balance


----------------------------------------



321 Stainless Steel Tool Wrap



Our 321 Stainless Steel Tool Wrap is the ideal solution for heat treating tooling. Just wrap the tooling with the edges crimped to form an envelope. This creates an air-tight package that will prevent the loss of surface carbon. Don’t let your tooling become scaled or damaged. Protect your investment while eliminating charcoal and sawdust box packaging with 321 Tool Wrap. Contact us today for a quote or visit our pricing page for stock inventory items.



General Properties



Alloy321 (UNS S32100) is a stabilized stainless steel which offers as its main advantage an excellent resistance to intergranular corrosion following exposure to temperatures in the chromium carbide precipitation range from 800 to 1500°F . Type 321 is stabilized against chromium carbide formation by the addition of titanium.



While 321 continues to be employed for prolonged service in the 800 to 1500°F temperature range, 304L has supplanted this stabilized grade for application involving only welding or short time heating.



321stainless steel is also advantageous for high temperature service because of its good mechanical properties. 321 stainless steel offers high creep and stress rupture properties than 304 and, particularly 304L which might also be considered for exposures where sensitization and intergranular are concerns. This results in higher elevated temperature allowable stresses for this stabilized alloy for ASME Boiler and pressure vessel code applications. 321 alloy has a maximum use temperature of 1500°F for code applications like 304, where 304L is limited to 800°F


Element 321
Chromium 17.00 – 19.00
Nickel 9.00 – 12.00
Manganese 2.00 2.00
Phosphorous 0.45
Silicon .75
Sulfur .030
Carbon .08


Molybdenum .75
Copper .75
Nitrogen .10
Other Ti=5x(C+N) min to .70 max
 
ATP-641 is a decarb coating for blades. Flour paste, soot, soap can all offer lower level protection.
 
i4Marc said:
Alfazulu,

Straightening of warped blades doesn't happen after heat treatment. It happens during heat treatment, specifically during the tempering cycles. If your blade takes a warp in the quench, do the first temper cycle as-is with the warp to stress-relieve the blade. Then clamp your blade to a rigid piece of steel with the blade bent in the opposite direction of the initial warp. Using C clamps and spare change, washers etc dial in a counter bend approximately the same distance as the initial bend. Run through your second temper cycle then check results. Additional bend and temper cycles may be necessary. Some steels may be tougher to remove bends. If a bend and temper cycle doesn't work, increasing the temperature by 15-25˚ may help. Some blades may have a number of wiggles. Clever clamping can remove more than one at a time. I have run as many as 15 or more two hour temper cycles to get particularly stubborn blades straight. The additional temper cycles should not harm your blade.



There are two wraps used for blades, 309 & 321. 309 is preferable. It is good for much higher temps than 321. When looking for the foil search for "stainless tool wrap"
Click to expand...

Quote trimmed to save space.

Man, that was an awesome response! Thanks so much for the info. That was exactly what I wanted to hear. That sounds like something I can handle.
 
If you get oxidation you will most likely get decarb !
Deal with most of the warping pre- HT ! Do an stress relief treatment at 1200 F for 2 hours after all grinding etc. Then straighten easily by hand.

Post HT ?Never try to straighten at under 400F on a hardened blade. You can also straighten as the blade is cooling from austenitizing temps but that's more difficult.
 
There is a very short window, (at least with certain steels) right after quench during which you can straighten warps twists and bends in the blade. This doesn't last long at all, maybe a few minutes if you're lucky. During this "soft" phase the steel is very easy to manipulate and is not brittle. After this phase it hardens up and becomes very brittle. On our thinnest blades we usually have some straightening to do and we try to get it done in that window. After that we put the blades into the sub-zero quench.
If there is any more straightening to do after that, we do it during the first temper by clamping the blade flat against a straight edge using 1" C clamps for the duration of the temper.

The procedures I've outlined may not work on all steels, we use CPM S35VN exclusively at this point and find these methods work well for it.

We use 309 HT foil and have found the best price by far is at enco

hope that helps!
-mike
 
You must log in or register to reply here.
Back
Top