Warped Blade After Finishing

R

Royal North

Joined
Sep 30, 2014
Messages
5
Hey All,

I have made many chef knives that are around 9 inches long and made from 01 and many have ended up great but some of them have slightly bent to one side towards the tip of the blade after sitting on a shelf in the garage for a couple weeks.

Anyone have any idea as to how this could be happening and maybe how to fix it afterwards if it happens again?

Thanks!
 
Let's get this to the correct forum...moving to Shop Talk.
 
Welcome to Bladeforums and to Shop talk.
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They were almost surely warped before, you just didn't notice it.

This is the BF search engine. If you search "straighten a warped blade" or similar searches, you will find a lot of solutions.
https://www.google.com/cse/home?cx=011197018607028182644:qfobr3dlcra

In posting a question like this, give us the details like blade thickness and width as well as how it was made ( forged or ground) and how the HT was done.
 
The blade is flat ground from 1/8 thickness and then hardened with a forge to cherry red and oil quenched and then evenly soaked at 400f for one hour for tempering afterwards. They came out straight and sharp but after 2 weeks or so of just sitting in the garage they have started to bend. I've read of carbon steel "relaxing" after time, like if there is a slight stress to one side the steel will gradually very slowly bend itself to that side over time so maybe it's that? Just seeing if anyone has experienced this before. They were definitely totally straight for at least a week after they were completely finished.
 
Like Stacy said, it is unusual for them to move after they were finished.

The only thing I can think of would be over heating the tip during HT setting it up for retained austenite which the relatively low temper did not decompose that later on decomposed spontaneously into untempered martensite causing as dimensional change. RA will decompose spontaneously in relatively simple steels like this.

Two temper cycles helps.

Briefly heating the tip on the convex side with a torch to ~400 might straighten it.
 
For some reason this didn't post earlier this morning????


New to me about slowly bending if properly heat treated and fully tempered. ........Read on for what can happen with improper HT.

O-1 needs a 5-10 minute soak and fairly close temperature control. A forge and cherry red aren't going to do it well. Also, I noticed you only did one temper....There should be two.



If the blade was insufficiently hardened and incompletely tempered, over a few weeks the RA could stabilize and some "aging" of the martensite could happen to increase the hardness. This could possibly cause a warp if the structures were not evenly distributed ( as would be the case in the too short and uncontrolled HT).

It should be no real difficulty to straighten the blades if it is a bit of tip lift. I usually stick the end in a slotted 2X4 and give it a gentle bend. Check and repeat as needed until it is straight.
 
Thanks guys, that seems to make sense, I will temper twice and see if it happens again. Also, I am looking into getting a more solid option for hardening, the forge was just a cheaper option as I'm just getting started.
 
If the blade is still warm when you lay it on its side, then both sides won't cool uniformly and this will cause warp.
 
He is claiming it was dead straight two weeks ago after finishing, and warped while sitting on a shelf.
 
To elaborate on Rick's comment:

The steel cools rapidly from a state of austenite at 1400-1500F. It pass the pearlite nose at 1000F and stays austenite. As it reaches 400F ( called the Ms), it starts to convert to martensite. This conversion isn't all at once, and continues until it reaches the finish point when all the martensite that is going to convert is converted ( this is called the Mf). For simpler steel, this is usually between room temp and 200F. When alloys are involved, the temp can drop until it may need to be taken down to -100F before the transition is finished. There will normally still be some austenite remaining in the mix( called RA = retained austenite) . The first temper tempers the new and brittle martensite. It also converts most of the RA into new martensite. This new martensite need to be cooled down to room temperature to "finish", and then be tempered on the second temper cycle. Without the second cycle, it can cause the edge to be chippy. On very complex steels prone to RA, it may be advantageous to do three tempers.
 
Thanks, Stacy... Great explanation. It seems you've really honed that passage over the years.
 
Stacy, your summary of the process seems to imply that a high alloy steel would need to go down to Mf between tempers- another dunk into dry ice.
Am I reading something in that isn't there? If the Mf for that steel is -90, the job wouldn't get done at room temp, would it?
I've never done subzero between temper cycles, but maybe we should?
Thanks for any thoughts on the subject.
 
I just realized that the slow warping of the blade over time could also be due to the fact that during the hardening process the blade is laying on it's side in the forge (although I do flip it a couple times) and then when I take it out of the oil quench it cools on it's side. I should be keeping it perfectly vertical, no? Also, any good suggestions for a high temp oven that doesn't cost thousands of dollars? Thanks again!
 
As long as the blade is evenly heated and at the proper temperature, warp during quench should be fairly little. It will all happen rightb then, too. Any warping after HT would be unusual except as discussed earlier.

I would not just lay a blade on the forge floor and leave it there. It should be moved and turned constantly.

When I use a forge for HT, I hold the blade in the tongs and pretty much constantly push/pull it through the center ( often called "pumping the blade"), as well as constantly rolling it over from spine to edge. This allows the entire blade to evenly come up to temp without overheating the edge or the tip. I watch the color and start checking by pulling it out and touching the welding magnet stuck to the forge near the port. As it becomes non-magnetic, I pay close attention to the edge and tip. I often angle the tip over to the side where it is cooler as it gets close to ready. This prevents the tip from greatly overheating, as it quickly can do. The goal is to get the entire blade the same shade of red ( just a full shade redder than it was when the magnet stopped sticking).
 
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