Strange mechanical properties of 17-4 PH

[waynebgordon]

I have a failed shaft constructed from 17-4 PH. The fracture surface is brittle or mixed-mode. I did Charpy impact tests at room temperature, which gives a ductile fracture appearance. When tested at 0 deg C, it has a brittle fracture appearance. The tensile properties are strange, with a high UTS (>1200 MPa), a low yield strength (<750 MPa) and a low ductility of 8%. HRC = 40. These mechanical properties are outside the range of any of the usual aged conditions. Large ferrite grains are present in the martensitic microstructure. I am seeing a lot of this lately with 17-4 PH bars coming from India and China. What are they doing to the material to give such properties? Does anyone know what is going wrong here?

 

[Maui]

Can you post some images of the microstructures that you observed in these materials? What were the material specifications that were referenced (if any) on the purchase orders that your company sent to these suppliers? What do those specifications call out with respect to material properties?

Maui

 

It would certainly appear that your shaft was not heat treated correctly. Ferrite in martensitic steels is a fracture toughness killer.
Not surprised at all about the country of origin. Making any alloy is just like making soup: the list of ingredients is insufficient, you also need to cook it correctly.
The moral of the story: you get what you pay for (or put another way, you get what your company's perversely-incentivized buyers source for you).


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

My first hunch is that they mis-annealed it before the aging.
Just because 17-4 is hard to mess up doesn't mean that you can't get it wrong.
Your spec is very important.
I know, no one wants to do source inspections. They would all just rather have field failures.


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P.E. Metallurgy, Plymouth Tube

 

[waynebgordon]

Unfortunately, I cannot post the microstructures. The material was not ordered to any specification (such as ASTM A 564), but rather just as 17-4 PH. Very bad, but customers just won't listen.

 

[EdStainless]

Take a piece and try to re-heat treat it.
Reanneal, cool, and then age.
At least it will help you narrow down the issue.

I presume that you are buying this fully aged?
How are you specing the properties if you don't use a spec?
At least start using the ASTM, and require them to report both annealed and aged properties.

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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear waynebgordon,

Carry out a chemical analysis by sending the sample in a lab. The carbon content could be the thing to watch out for.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[MagBen]

@Dhurjati Sen: interesting thought. Do you think carbon too high or too low? carbon is austenite former, so i donot think high carbon is possible cause. low C is less possible from a practical standpoint.
@OP: since this seems to be bar stock, do you see ferrite stringer? bcc delta ferrite will decrease ductility and toughness. ferrite was possibly formed at solidification, cool at slow speed could form as high as 15-20% ferrite. Homogenization, or solution for a long time can eliminate this phase, and then re-age to gain desirable properties.

 

[Dhurjati Sen]

Dear MagBen,

I think the carbon is much higher than the specified 0.07% max.

While making the SS, there is always a dilemma of having less Cr or more C as the more you oxidise C in order to remove it, Cr-oxide forms resulting in loss of Cr (Cr goes to the slag).

Some(unscrupulous)manufacturers might just keep the Cr content right while not reducing the C.

Too much of C would make the steel have strange mechanical properties as stated in the original post.

I just wanted waynebgordon to be sure that the above is not the case.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[MagBen]

Dhurjati Sen: so how do you explain why high carbon led to ferrite?

Formation of Cr23C6 does raise an issue for this alloy, and this why a min of Nb (5 x carbon) is added to tie with carbon.

 

We've veered off-road into wild speculation with this.
(1) Get a proper chemical analysis done.
(2) If that is good, re-heat treat the material from the beginning and see what happens.
If all properties do not all fall within spec you have an actual problem.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

I have seen 17-4 with too little C, even just near the bottom of the range can cause issues.
So can lack of homogenization. Multiple anneal+age cycles improves homogenization and will drive grain refinement.
As can not cooling below the Mf between anneal and age.
There are a bunch of ways to mess this alloy up.

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P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

WnB...

The metallurgists working this question with you will not steer You wrong.

Note.
Quote "...but rather just as 17-4 PH. Very bad, but customers just won't listen."

In the business of materials, clarity of specifications, processes and laboratory certifications ['certs'] = no-nonsense traceability.

A few concepts [quotes/cartoons] You need to be [painfully] aware of...

"You deserve to get what you ask for: but if You don’t ask for very much, don’t-expect to get very much.” --'a senior engineer at a major aerospace company'... summarizing his experiences with critical aircraft fastener manufacturing.


"You don't get what you want... You get what you ask for. Ask carefully!” –TheTick, Eng-Tips

"You may not get what you asked for... but [for sure!] you never get more than you ask for!” –EdStainless, Eng-Tips

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I have discovered that when a critical element is undefined by the 'customer', then it is MY NECESSARY DUTY to provide clarity and direction... otherwise non-airworthy junk is made... and we have to finally got-back-to square-1 and 'do it right' under a 'time-crunch'.

Customers who 'just don't get it' will never be pleased... ANY outcome other than what they 'imagined' is sure to disappoint them.



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Benakano]

Micros and chemistry are a good place to start not sure how large the shaft is but you might check for center line micro-structural issues, niobium carbides seem to mess with ROA and impacts especially in large cross sections.

I second the comments about sourcing you cant live with them you cant live without them tighten up your specs...