H11 hot work tool steel, heat treatment

[limey1]

I have compared data sheets from several suppliers and the heat treatment temperatures differ.
I have a 4500lb extrusion shell that was inadvertantly left in a gas oven with a faulty controller. The setpoint was set at 960F but it climbed to 1400f and soaked for @12 hours. Some data sheets say 1400F is the midrange for annealing, others say 1550F+.
So my questions are as follows:
1. Is it annealled
2. If it is annealled, could I try a 2 stage temper without hardening.
3. What is the risk if I do nothing.(Material is subject to axial and radial forces)

Thanks in advance

 

[swall]

The big bugaboo with H-11 at excessive time and/or temp is grain growth. As a first step, I would have a metallographic specimen prepared in order to determine grain size. As you obviously don't want to ruin your workpiece, you will need to obtain a coupon from the same material lot and give it the same thermal exposure.

 

[limey1]

Thanks for the info Swall....I maybe should have mentioned that the shell is over 40 years old!!

 

[Metalhead97]

limey,

I'm assuming the annealing operation took place after the extrusion operation. Are you sure the 1400F soak wasn't intentional? With large parts like yours uneven heating can occur, which can cause warping/cracking.

That being said:

- 1400F, 12 hrs should be enough to recover any cold work. In addition, the temperature is below the austenitizing temperature so you should also get a tempering effect
- Sure you could do a 2-stage temper, but don't go above the austenitizing temperature.
- Have you tried to do any non-destructive testing such as xray diffraction to see what you have?

Metalhead

 

[Maui]

The critical transformation temperature for H11 is 1505F. If you know for a fact that you soaked it at 1400F, then this thermal treatment cycle will act like a subcritical anneal provided you cooled it no quicker than 50F/hour between 1400 and 1200F. If you cooled it faster than this, then it will not be a subcritically annealed microstructure. (It should be noted that during a typical annealing cycle you would air cool to room temperature after you reach 1200F.) I suggest that you take a hardness reading, and if possible cut a sample to examine the microstructure. This should clearly show you what condition the material is actually in. While grain size can be an important consideration, I have never found an etching procedure that allowed me to clearly identify and measure the prior austenitic grain boundaries in hot work tool steels. Swall, do you know of such an etching process? (I would be very intereted in it if you do.)

If you need information regarding tempering characteristics, the tempering curve for modified H11 can be accessed in this link:

http://www.asminternational.org/pdf/HTSRefCharts/OMM052908.pdf

Maui

http://www.EngineeringMetallurgy.com

 

[swall]

Maui--the work I did was in the late '70's. I was studying grain growth with multiple reheat treatments of H-11 aircraft wheel rotor drive keys. The only etchants we employed back then were nital, ferric chloride and Vilella's. Can't remember which one was used on this project.

 

[Maui]

Swall, I can tell you from experience that nitric and Vilella's won't do it. I have not tried ferric chloride on the hot work grades though.

Maui

http://www.EngineeringMetallurgy.com

 

[swall]

Another etchant we had back then was Carapella's. It is possible we might have used that. When I get a chance, I'll check my archives and see if I still have the report.

 

[limey1]

Thankyou all for your input.
As far as how quickly it cooled from 1400F to 1200F I dont know, this happened over a weekend, sods law, and was simply removed from the oven and allowed to cool.
I am waiting to hear from the machine shop where the shell is as far as hardness. Cutting a sample would be difficult as it could potentially compromise the integrity of the shell, and any other testing would be limited (Equipment / resources)
Just for clarification, if the hardness isnt too soft, could I attempt the 2 stage tempering? Reason why I ask this is, our gas oven cannot reach temperature required for hardening (Preheat 1450F, High heat 1825F)

 

[Metalhead97]

limey1,

Can you not send the part out to be properly heat treated to spec? What condition do you need the material in?

From what I gather is that you have a H11 shell which was extruded 40 years ago. Did you personally do the heat treating or was this immediately following extrusion? A hot extruded material should have very little cold work. The hot extruded micros that I've seen are fine equiaxed after several reductions. The fact that you annealed it after at 1400F, 10hr means that no cold work should be present.

Personally I don't think that material has been hardened because the critical temperature was never exceeded. Now if the critical temperature was exceeded during the extrusion process that's a different story, however I've never hot extruded an alloy more than 100F above the annealing temperature, which (in your case) is likely below your critical temperature.

Metalhead

 

[swall]

Maui--I dug out the report. It was from 1978. Unfortunately, the etchant was not recorded. But, given the time frame, we had started using electrolytic etchants by then, which opens up a whole new realm of possibilities.

 

[metengr]

2. If it is annealled, could I try a 2 stage temper without hardening.

Why? You did not exceed the lower critical transformation temperature for H11, thus you have no hardening. If you were below the stated lower critical temperature, all you performed was a subcritical stress relief as mentioned above.

3. What is the risk if I do nothing.(Material is subject to axial and radial forces)

Bulk hardness will determine. If it is within the acceptable range per specification, I don't see an issue.

 

[Maui]

Swall, the standard etchants that I've tried never seemed to do the trick. I did have access to an electrolytic etchant that we used for ditching tests on stainless grades, but I never tried it on hot work tool steels. It wouldn't surprise me if you developed a specialized electrolytic method for bringing out the grain structure. If you ever find it, please let me know. You can contact me directly through my website at the bottom of my post. Thanks for looking.

Maui

http://www.EngineeringMetallurgy.com

 

[mrfailure]

Have you thought of evaluating using replication? Also, you can use a Leeb/Equotip portable hardness tester to get hardness.

Aaron Tanzer

http://www.lehightesting.com