Hardenability of 17-7 RH 950

[mighoser]

I know the specs, AMS 2759/3 say 42-49 HRC but I've got some parts which are reported to be as high as HRC 57. I'm redoing the hardness testing on in stock inventory (I question the results given the shape of the part and wall thickness. These are tubular shaped parts which are made from bar stock. Anyone know if faulty heat treat could produce values this high. My text seems to indicate that maximum hardness would be achieved by cooling at -75 instead of -100 for as long as possible (my ref. material only shows curves a little over 8 hrs.)

Any additional info would be much appreciated

 

[TVP]

17-7 PH can only achieve that type of hardness/strength when processed into spring wire (CH900 condition), and even then only in small diameter wire that has been heavily strained.

 

[mighoser]

TVP,

What would you think a max hardness would be for the RH950 condition, 1.5" bar stock? Keep in mind, the product is hogged out to a .030" wall thickness prior to heat treatment. Yes, I'm uphappy with the current machining process. I'm going to get into why we don't use sheet or tube stock. Perhaps machining is a factor (condition A is a bit gummy).

 

[EdStainless]

The best compromise for machining is to heat treat the material to the high age temp (lower strength) condition. Machine it then and re-heat treat to your final condition.
Yes, it is a lot of heat treating, but you will get better reuslts.
I have only done a couple of RH950 jobs. The highest hardness that I ever say was RC50. This was an actual Rockwell C reading.
I can see a couple of possible issues. Is your reading converted from another scale? Is it superficial or microharness? How much cold work was put into hte surface in machining?
I think that you need to sacrifice a couple of parts and do some hardness surveys.



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[mighoser]

Ed,

I think you may be thinking of 17-4 which is most easily machined in the overaged H1150 condition (higher elongation values compaired with condition A). 17-7 in condition A has low hardness,low yield and high elongation (35%). Perhaps we are transforming to Condition C (partially) while machining and resulting in CH900. This might explain high hardness. We're going start today by trying duplicate reported hardness readings and understand the sensitivity of the setup (with this part there is only one spot available to check). We'll go from there.

 

[EdStainless]

Well, while not the same animal 17-7 does machine better aged than in the A condition. The A condition is rather variable and smearing can be a real issue. The higher hardness aged improves cutting and surface condition.

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[mighoser]

Yes, I've heard condition A mechanicals vary quite a bit. What condtion would be best for machinability? TH1150?

 

[EdStainless]

If I was drilling or turning, yes, anthing over TH1050 should be fine.
If I was grinding I might even what a higher strength condition. Heck I might even wait until the part was finish aged at RH950 to do any grinding.

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[NickE]

Just an aside:

Even good 17-7 is hard to get to hit the RH950 Rockwells reliably.

we tried for a while and varied the heat treat a bit in different ways, still couldnt get full transformation... Turns out that the chemistry basically prevented that.

I also know it takes lots of cold work to get 17-7 to condition C. I dont think any decent machining process would do that.

 

[mighoser]

Nicke,

"Even good 17-7 is hard to get to hit the RH950 Rockwells reliably" Are you implying that it is hard to get to HRC 42-49? If so, I should doubt the reported hardness values even more.

 

[EdStainless]

I would expect to see 42-49, but over 50. Let's say that I am skeptical. We make low to mid 40's every time.

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[NickE]

Ours was some material that was rolled to Cond. C. We couldnt get annealed at the time, I solution annealed it, then formed the part and attempted to get RH950. It was worthless.

Ed knows more about why. And I couldnt find any of the threads about the story.

 

[mighoser]

Edstainless,

Aero Metals Handbook states that machining should be completed in Condition T or R 100. I'm assuming this is to minimize distortion from transformation. Any idea on how much cold work machining could produce. The parts has a fairly rough finish. I've got a chart which shows the effect of cold work (%elongation) on final RH950 mechanicals. Though, it is for sheet material not bar.

 

[EdStainless]

I like the idea of machining in the T or R100 condition.
Yes, it should minimize distortion.
I am not sure what the properties are in those conditions.

Bad machining, work harden the surface, you should end up with the equivalent of C950 if you do that. But I still find 50 hard to believe.

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