17-7 PH Stainless Steel, Condition TH 1050 not responding to Heat Treatment 14

[moskalik]

I have a question about 17-7 PH, condition TH 1050 that has been troubling me for some time.

We manufacture wire products, so any 17-7 we use is in wire form that we coil into shape. When using 17-7 PH, we usually (99.9% of the time) use Condition CH900 which works well for us, but every now and again we get a request for Condition TH 1050 that we just cannot seem to get to work. The problem is that the AMS 5528 specification states the tensile strength should be 180 - 210 ksi after heat treatment and we can barely get over 150 ksi.

I know we are meeting the specification exactly in regards to the heat treat process and I even have the furnace charts that show our vendor is cooling to the 60°F within one hour to ensure proper formation of Martensite (they go down to -150°F for get the proper cooling rate). I would think that if we meet the heat treat process exactly, we should be able to duplicate the physical properties, but that just isn't happening.

I have heard from several sources that chemistry can play a role in how the material responds. Does anyone have insight on what could be happening to make the TH 1050 come out with a lower than desired tensile strength? Also, does anyone have any insight on the chemical elements that could affect the response to heat treatment and what are the optimal ranges for those elements?

Thanks,
-Ben Moskalik

 

[moskalik]

Terry,

Thank you for the positive feedback on Smalley and sharing the good experience you had with us. We have many engineers (about 20) that work on the sales side of things and they do a good jobs giving the technical assistance that our customers need. I am glad to hear that you have a good impression of Smalley. What company do you work for? It is always nice to pass along positive feedback.

To get back to your question about why I am asking these questions about 17-7 PH, Condition TH1050, I want to direct your attention back to my very first post on this thread. We currently manufacture many 17-7 products that are made out of 17-7 PH Stainless Steel, Condition CH900 flat wire and we do not have a problem with the condition CH900 of material. We have our own rolling mills in-house and we can tightly control the geometric dimensions and physical properties (amount of cold work) of the wire. Again, we have no problem with Conditions CH900 similar to the parts we supplied to your company in the past. We pride ourselves on being able to roll our own raw material (flat wire) and being about to tightly control the parameters that are important to make quality product.

However, we have never been able to successfully make 17-7 PH, Condition TH1050 material that meets industry specification for physical properties, and it frustrates us that we cannot doso. This means we cannot bid on these requests. TH1050 is different than CH900 because it relies on thermal heat treatment to achieve final physical properties, and that requires a heat treat vendor to perform the heat treatment correctly and the original material to have correct chemistry to respond to that heat treatment. Both of these parameters are out of Smalley's control unlike CH900 which mostly relies on the cold work which Smalley does control.

I have approaching wire suppliers, rod suppliers, heat treaters ... anybody who may know something about why the 17-7 wire doesn't respond to the heat treatment of TH1050 and I have received a wide range of answers from "I don't know" to "the chemistry of the rod changed 30 years ago and TH1050 can no longer be achieved". Since I am an R&D Engineer, you can imagine those are not answers that are acceptable and I want to know exactly why we cannot do something, so this is why I am here

So I appeal that you still consider Smalley as a first-class operation. Nothing has changed at our organization or the knowledge base we possess and we still pride ourselves on quality product and the technical assistance we supply. Smalley is just trying pursue something that we have not been able to do previously so we can continue to be world leaders in coiled flat wire products.

Did that somewhat answer your question or at least ease your mind?


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Ben Moskalik
Smalley Steel Ring Company

 

[moskalik]

Maui,

That link was very useful. I had seen this NASA document before but needlessly blew it off. Since you focused my attention to it, I sat down and read it yesterday and it is very good compliment to the 17-7 handbook that Ed posted a few days ago. Thanks again!

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Ben Moskalik
Smalley Steel Ring Company

 

[mcguire]

We used to make 17-7 PH at J&L. It was a very early PH grade and extremely sensitive to minor changes in chemistry within the specification. Aluminum is the crucial precipitate former. To the extent it has formed oxides or nitrides, it will show up in the analysis, yet not be useful. Too little carbon will cause a too high MS temperature, the same as an incomplete austenitizing. The as-received condition may be difficult to re-homogenize from prior heat treat making large carbides. And, if the ferrite balance is slightly off, but within spec, you get delta ferrite that you can't eliminate.

It is a nasty, ill-designed grade of stainless. I would ask ATI or Carpenter for their recommendation on a substitute. 17-7PH really is trouble.



Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

Yes Mike, it has issues, but it is fully austenitic when annealed which means that those of us that cold draw tube and wire need it.
In order to do heavy cold work with 17-4 it would have be in the overaged condition (to get enough ductility) and the be re-annealed and re-overaged between each draw cycle.
I do like the 17-7 alloy, but that said I would never try to go over about 0.125" thick with it. And we never run strip without first heat treating a sample to verify properties.

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Plymouth Tube

 

[moskalik]

Dr. McGuire,

Thank you for the feedback and the insight on which chemical elements may be affecting the material. Like Ed said, 17-7 may have its issues, but for us making edge coiled wave springs out of flat wire, it works really well in the Condition CH900 and is one of the most stable and predictable materials we use.
I looked at the chemical content in regards to the Aluminum and Carbon content for the 17-7 PH material I used to test the TH1050 process, and they both appear to be OK. AMS 5528 lists the Aluminum content to be 0.75 - 1.50 and the material was actually 0.97 . The Carbon is to be 0.09 maximum and the material was actually 0.080 . I think you stated that a low carbon content would be bad, but this seems fairly high on the allowable range. The Aluminum content is right in the middle of the range, so that appears OK too.

I wonder if the chemistry has been OK all along and it is something to do with improper solution that is not erasing the cold working we are doing. I really hope it doesn;t have anything to do with the chemistry since that is not something we can easily control. Definitely some testing is in order.

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Ben Moskalik
Smalley Steel Ring Company

 

[mcguire]

At the risk of being criticized for guessing, I have to follow the reasoning that the age hardening response is inadequate. This relies on Al and Ni combining. Since there is plenty of Ni, I have to guess that the AL may not be available. It could be in the form of aluminum nitride and unavailable for precipitation as Ni3Al. Nitrogen is more reactive with Al and you may have some pick-up in it via heat treat atmosphere. Just a suggestion. This wouldn't show up on mill certs and it is more possible in your light gauges.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[tbuelna]

moskalik-

Thanks for the response. And don't worry, I still have a high regard for the Smalley Company and its excellent products.

This thread is very interesting to me, and the responses were quite informative. I did some reading on the subject and it seems that what btrueblood noted above about carbon pickup from surface contamination hindering the age hardening process may be one potential source of your problem. I've attached the relevant pages from a technical bulletin from AK Steel discussing issues like surface contamination and furnace atmospheres on heat treatment of 17-7.

Hope it helps.
Terry

 

[moskalik]

Terry - For my testing, I will make sure I vapor degrease the samples before I start. That way I can eliminate the carbon pickup as a variable. Definitely a good suggestion.

Dr. McGuire - You will never be accused of guessing by me We are all guessing at this point what is the issue. I appreciate all the input and insight.

I just got all my test strips placed on my desk a few minutes ago. I have my work cut out for me next week heating these and testing the response.



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Ben Moskalik
Smalley Steel Ring Company

 

[mcguire]

OK Guys

I'm retired now and my help is always free to real people with real questions.

Mike McGuire

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[btrueblood]

Ed,

Can you expound on why you wouldn't use 17-7 in thicknesses over 1/8"? Is it due to the difficulty in getting the right degree of cold work into the material? I ask out of curiousity, and because our spring winding vendor has typically steered us away from 17-7 for our heavier springs.

Mike, is there a way to deduce what degree of oxidation/nitriding of the Al has occurred?

 

[EdStainless]

I know that they say that you can go a lot thicker, but given how sensitive the material is to the annealing and austenite conditioning treatments I would be cautious. Especially if the parts have varying thickness, getting uniform properties could be very difficult. The thicker areas may not get strong enough, and they may also have poor toughness.

If you look at 17-4 you will see that as sections get heavier people move to 15-5.
These alloys as a class are sensitive to heat treatment and can be tricky.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[btrueblood]

Thanks Ed.