440C debackle

[dbalfa]

This is a new one on me- I HT three 440C blades this weekend with the following procedure: HT temp at 1870 for 30 mins in foil (no paper), plate quenched, tempered at 200 for 1 hour, let them cool in the open toaster oven.
I had three different lengths, all 1/8" thick.

I checked all after plating, they all looked good, I tempered and they all looked good. Then when I started my finish grinding, I noticed they all had developed some blade turn. The small one can be ground out albeit to a shorter blade now. The medium was salvagable as is with finish grinding but the larger one has a crooked tip where the end 3/4" is curled 3/16" off center-line.

ANY idea what happened? It seems obvious there was some stress there that resulted in the warpages. None of them are cracked as far as I can tell as they all ring like a bell. It may very well have happened in the HT / temper oven and I just missed it earlier.

Would you recommend re-HT the larger blade to straighten it? Perhaps my HT recipe is lacking?

[AcridSaint]

200 seems very low, was that correctly typed? If the blades are already ground, I tend to air cool them so that I can adjust them if they warp. I plate quench unground blades or blades that have been left thick. Also, I generally go to 1925 on 440C, but I don't see that 1870 is beneath the austentitizing point.

[SBuzek]

IMHO they were not tempered enough.I go to 1890 then plate quench then temper 370 deg. for 2- 2hour cycles.I always use at least 2 temper cycles as the second one tempers the converted marstenite. On the blades try heaing them to at least 300 deg and straighten by hand or in a vice,be gentle and they should staighten fine.
Stan

[Russ Andrews]

Even if blades come out of the quench straight. they can
still warp badly if not heated evenly for the tempering
cycles.

[Ed Caffrey]

Not sure if it makes any difference for this situation, but a toaster oven is a very fickle thing to temper in. I've seen toaster ovens that vary as much as 150F from the setpoint. Generally with knife type steels your going to want to stay within 25F of the setpoint. My suspicion is that the toaster oven your using is having huge drops and rises from the setpoint, which can cause issues, just as Russ stated.

[mete]

The 200 F is way too low .You should be at closer to 400 F. Two hours would be better and maybe 2+2 better yet.
It may have been the improper tempering that cuased the problem .I wonder if the heat from the grinding tempered unevenly an caused the warp.
Toaster ovens can be inaccurate as stated . One improvement is to put something like a brick in it to minimize fluctuations.

[Don Robinson]

If you plate quenched all 3 at the same time with all 3 still in the package, the weight of the top plate may have bent one or more of the blades.

The blades should be separated and spread out evenly on the quench plate before adding the top plate.

Ordinarily, steel can be easily straightened until it drops below 400f. That means the steel hot from the furnace is easy to bend when removing from foil or careless plate quenching.

I never HT more than one blade at a time and remove the foil before the plate quench to assure the plates are in even contact.

Different strokes for different folks.

[Don Robinson]

Your blade should be annealed, straightened, then re-heat treated.

[Don Robinson]

One factor that makes a blade warp during heat treat is uneven metal removal while grinding.

Removing metal causes stresses at that point. Always grind a little on one side, then grind the same amount on the other side of the blade. Try to keep the metal removal balanced from one side to the other.

Imagine the blade moving (warping) away from the heat source, the grinding belt. It wants to move away, but actually can't. So then when the other side is ground, it relieves that stress. Balance your grinding and balance the stress buildup.

Don't finish grind one side before grinding the opposite side.

[dbalfa]

....the learning just keeps on and on and on....Thanks gentlemen!

The toaster oven brings up another old issue - it takes my kiln hours to cool back down to tempering temps. So I would typically use a snap temper, but if I plan to final temper at say 350, then I'm back at 200-250, where I got the warp. -any suggestions?

I do have some heat-sinks in my toaster oven, steel and aluminum blocks, and a thermometer. Perhaps a better toaster oven would help

The grinding advice Mr. Robinson gave is novel to me but makes worlds of sense, I have never done it that way but will try that from now on. I do only quench one blade at a time, that's all the room I have on my plates as they are 4" wide x 3" thick x 16" long steel. I do it in the foil thinking it would take too long to get them out and would loose some of the benefits of a quicker quench- However, if that is not such a concern, I might try taking the next one out first.

Sounds like some un-even grinding stresses combined with a too low temper caused my demise on these. Thanks again fellas, your all gentlemen and scholars!