17-7 PH Condition A - In-service Use 2

[RobSelby]

I have an application that is currently using 17-7PH material in condition A. Referring to the Mil-HDBK (MMPDS-01) it clearly states that this material should not be used in condition A. Does anyone on here know the reason that 17-7PH should not be used in condition A? I cannot find any specifc material that would help me on this matter.

Kind Regards

Rob

 

[MikeMet]

Condition A puts the material in a fully untempered, martensitic microstructure, which is generally not a good condition for engineering applications. It has high strength, but low ductility and untempered martensite can be subject to catastrophic, unpredictable, brittle fracture. It may also be susceptible to stress corrosion cracking from hydrogen exposure, as would any ferritic material that contains untempered martensite. Do a web search and you will find lots of information. Allegheny Ludlum website and AK Steel website would be a good start.

 

[EdStainless]

Can I repeat, don't do it.
The material is unstable and unpredictable.

You must use it in an aged condition.

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Plymouth Tube

 

[metengr]

There is some confusion here. I think you better have a closer look at the technical brochure from AK Steel

http://www.aksteel.com/markets_products/stainless_precipitation.aspx

Condition A is not untempered martensite, it is the lowest strength of the conditions - supersaturated austenite. Condition A has maximum ductility for forming and is supplied for welding and forming in this condition. The reason you don't use Condition A is that it can transform to martensite in service, which is want you do not want as above.

 

[redpicker]

Well, not trying to add to the confusion, but my understanding is that condition A is martensitic, but it is not an iron-carbon martensite, but an iron-chromium-nickel martensite, which is not hard and brittle but rather soft. The hardness and strength is developed on aging by precipitation of Fe-Ni-Cr-Al intermetallics, the size and distribution of which is controlled by the aging temperature. The higher the temperature, the coarser the intermetallics, with the finest distribution obtained at or around 950 F.

I can echo the recommendation to not use it in condition A. Even if the high strength is not needed, corrosion resistance as well as fatigue properties are better in the aged conditions.

rp

 

[EdStainless]

If you don't need much strength look at the highest possible aging temperature. The ductility and toughness are much better there.

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Plymouth Tube

 

[RobSelby]

The particular application is as use as a pushrod in a solenoid. This component undergoes fairly minor compressive stresses due to the small magnetic solenoid forces. I am thinking that whilst under significant loads of 50% and beyond of yield stress there is evidence of stress corrosion cracking in this particular application that may not apply? Your thoughts would be appreciated.

 

[MikeMet]

Metengr;

Condition A and solution annealed condition are the same for 17-4PH sst. In condition a the tensile strength is close to 150 ksi, very strong, but elongation is less than 10% See the following website:

http://www.alleghenyludlum.com/ludlum/pages/products/xq/asp/P.112/qx/product.html#

It also has the warning on martensite and use in the A condition.

 

[MikeMet]

RobShelby;

Do not use the rod in Condition A, to unpredictable with regard to properties. Do an aging treatment to be sure.

 

[metengr]

MikeMet;
I am not going to nit pick with you regarding the UTS, YS properties but a published value of 10% elongation is not correct for Condition A. Think about this for a minute – why would such an alloy after a conditioning treatment with no precipitates from aging have such poor elongation and is provided in this condition for forming and welding? This makes no sense.


The PH hardening stainless steels are very formable in Condition A they work harden and can transform to untempered martensite from cold working, that is how they are sold. Condition A affords welding and forming (cold working) operations before aging or reheat treatment. Condition A is not a condition that is used because it is an intermediate step in the final heat treatment for these alloys.

 

[metengr]

Also, I would check ASM Handbook, Volume 4 on Heat Treatment

17-7 PH (UNS S17700) is normally supplied in the solution-annealed condition (condition A), in which it is soft and formable. Heat treatment is accomplished through the use of the transformation-hardened (TH) or refrigeration-hardened (RH) procedures indicated in Table 15. The choice of method of heat treatment is usually dictated by the ease with which the particular sequence fits into the production techniques of the user. Parts that receive significant deformation should be heat treated to RH 950, or soft spots may result.

This alloy is also supplied in the cold-rolled condition (condition C). Here, transformation has been achieved by cold rolling, and heat treatment is reduced to a single step: 480 °C (900 °F) for 1 h. Although strength and stress-corrosion resistance are greatly increased by this treatment, ductility is reduced and formability is limited.

 

[EdStainless]

The tensile elongation in Condition A is highly variable, even from part to part. The lowest that I have ever seen is about 15%.
When you age these alloys you are altering the microstructure. A lower strength condition does not infer better ductility.

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Plymouth Tube

 

[metengr]

People can confuse Condition A for 17-4 and 17-7. These are different alloys and they respond differently to heat treatment and cold work.

 

[EdStainless]

How true, in 17-4 if you want the minimum strength and best ductility together you must go to the over aged condition.

With 17-7 you do get the lowest strength in the "A" condition, but the results can be highly variable and not suitable for any service.

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Plymouth Tube

 

[1toolsteelguy]

Agree 17-4 ph is meant to be used in the (precipitation hardened =ph) "aged" condition. here is another datasheet for you to refer. HH1150 is most common.

http://www.latrobesteel.com/assets/documents/datasheets/17-4.pdf

It is readily available in the aged condition from distributors such as

http://www.specialtysteelsupply.com

 

[MikeMet]

Metengr - I'm only going by my actual experience using these materials for the past 20 years. You need to check more than the ASM Handbook which picks and chooses data. I have Armco data that shows that 17-4PH in Condition A has elongations in the 10% range, with a minimum allowable of 3%. Also, we use a lot of 17-4 and 15-5 in high pressure applications, been doing so for over 30 years (before I got here), and have done testing on Condition A material and because of the variability alluded to, it can indeed have low elongations. You can do a lot of work on a material that has only 10% elongation, I know we bend, small diameter thick wall tubular components to 90 degrees with not problem.

 

[swall]

MikeMet--the obvious question, then, is why wouldn't you go ahead and age it to H900 or H1075 after forming?

 

[metengr]

MikeMet;
I am not trying to be funny or sarcastic here but, we are not dealing with 17-4 PH, we are dealing with 17-7PH. You keep referring to 17-4PH. The 17-4 PH is truly a martensitic microstructure from the get go and ages with precipitation. 17-7 PH is not the same as 17-4 PH. IN this case, 17-7PH is a semi-austenitic precipitation hardening stainless steel.