Influence of carbon on 17-4 PH 3

[stanislasdz]

The ASME SA 705 code state's that :

C% content in 17-4Ph Stainless Steels is 0.070 %

Why we can not go above 0.070 ?

Is it related to intergranular corrosion ?
Is it related to retained austenite ?
Is it related to hot working ?
Is it related to Impact Charpy Toughness ?

Any Help

Thanks

 

[redpicker]

The carbon would pull chromium out of solution, lowering the effective carbon content. The martensitic reaction in 17-4 is a Fe-Ni martensite, not a Fe-C martensite. With this much chrome, the carbides are FeCr23C6, which can tie up a lot of the chromium, and may not dissolve during solution treatment. This could affect the corrosion resistance as well as the mechanical properties (CVN included).

rp

 

[stanislasdz]

I really appreciate your advice

Thanks redpicker

 

[metengr]

In addition to what rp stated above I would be very concerned the higher carbon content may result in retained austenite during cooling. Carbon is known to have a significant impact on the martensite start transformation temperature.

Ref; "Handbook of Stainless Steels" by Peckner and Bernstein

 

[redpicker]

I meant
"carbon would pull chromium out of solution, lowering the effective chromium content"

 

[Metalhead97]

stanislasdz,

Is it related to intergranular corrosion ?
As redpicker pointed out C will react with Cr, which will result in sensitization at the grain boundaries.

Is it related to retained austenite ?
As metengr said carbon has a strong inverse effect on the Ms temperature; as the carbon content increases the Ms temperature decreases. This will lead to an increase in the amount of retained austenite. Carbon tends to get "squeezed" when the material goes through the Bain Path, and the necessary FCC to BCC driving force is raised.

Is it related to Impact Charpy Toughness ?

In addition, interstitial impurities like carbon cause the martensite to develop a BCT (a domain structure with three variants) structure rather than the Fe-Ni BCC (single variant) structure. Due to the asymmetry of the BCT structure dislocations spanning several domains tend to get locked up, which results in a additional hardening mechanism. The same effect is seen ordered FCT transition metal alloys (CuAu & PtNi). If you added that effect with the solid solution effect of C, and the PH effect from Cu you would get a material with very little ductility, which would have a negative effect on the Charpy impact energy.

MH

 

[stanislasdz]

Many thanks to all people !

I am looking for a figure with can shows effect of the carbon of mechanical Ph properties of 17-4 PH alloy

Thanks in advance

 

[EdStainless]

Typically you will find 0.02-0.04C.
The problem with your question about property variation is that you have to re-balance the chemistry for each C level. so it isn't a simple comparison.
If you are not welding the material you can make it work at higher C levels, but the chemistry looks very odd.
In a PH stainless the first problem that you will run into is the shift in Ms-Mf temps. You may not be able to heat treat the alloy without doing some very strange things to it.


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[stanislasdz]

Thanks Ed Stainless

"....is that you have to re-balance the chemistry for each C level...."

Do you mean balance between Gammagenic and alphagenes elements ?

 

[EdStainless]

You have to pay attention to austenite/ferrite; Ms and Mf; composition of martensite (and the matrix).
This is why commercial alloys fall into a very tight range, and each mill uses its own chemistry.

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