Temperature threshold for LME of 300-series stainless 3

[kevlar49]

Does anyone know the temperature threshold for LME (Liquid Metal Embrittlement) of 300-series stainless steel by copper? They are wanting to use a copper packing on a 304 stainless gate valve stem.

 

[metengr]

It would be the melting temperature of copper, and this is well above the service temperature.

 

[metengr]

Actually, its not copper that is a concern its copper-zinc or zinc liquid that results in LME of 3XX austenitic stainless steel. Below is an interesting article from TWI on LME.

If this is a copper-zinc alloy, you would need to know the alloy composition and melting temperature. If the application is copper metal only, you should have no concerns with LME.

http://www.twi.co.uk/toolkits/Damage/H001Liquid_metal_embrittlement.htm

 

[EdStainless]

With pure copper you have to get close to the annealing temp for LME. (it isn't the melting point of Cu but the melting point of the lowest melting eutectic that can form)

Adding Zn to that mixture lowers the temp a lot.
I have never seen LME damage from Cu alloys (with Zn, Sn, Pb) on austenitic stainless below 500C.

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Plymouth Tube

 

[kevlar49]

Confirmed from thermography that valve operates at 1100 F, so 500 C (932 F) will be a concern. I am checking on alloy of packing. EdStainless, when you are referring to Sn, Sb alloying elements are you referring to the eutectic of those alloys as a threshold? or is there any temperature lower than that. So, Zn is not the only alloying element of concern? If I hit the eutectic with Sn or Sb alloying, I can also be at risk of LME?

 

[metengr]

Maybe I need to restate what I mentioned above. For LME to occur one or more of the metals in direct contact with 3xx stainless has to be a liquid. I am not referring to grain boundary liquation from excessive annealing temperatures. For 3XX series stainless to suffer from LME, zinc or aluminum are the only culprits, not copper. So, as I stated before, the copper-zinc alloy, zinc or aluminum must be liquid and in contact with 3xx, to have LME.

 

[kevlar49]

ASM online handbook seems to suggest copper as an SOLID Metal Embrittler for 300-series SS. Any idea what threshold I should use there?

 

[unclesyd]

How are you going to manage the oxidation of the copper at the stated temperature?

We ran OFHC ring and flat gaskets mainly against 316 SS at your temperature extended periods of time and the only problem was that the copper oxidized badly. We had all our gaskets Nickel plated to prevent the oxidation.

 

[metengr]

kevlar49;
Please read carefully the excerpt below from ASM Handbook, Volume 11 Failure Analysis and Prevention



Solid-metal-induced embrittlement (SMIE) occurs in steels as well as many nonferrous materials. This phenomenon appears to be related to LMIE as the materials and couples susceptible to SMIE are those for which LMIE is known to occur. In some instances, contact of a metal with a low-melting-point alloy at a temperature below the melting point of the low-melting-point material has been shown to cause delayed brittle fracture and cracking in addition to decreasing strength and ductility. The relative level of damage increases with temperature up to the melting point of the low-temperature species.
Research suggests SMIE is probably caused by a number of complicated interrelated processes. By whatever mechanisms, empirical evidence shows that the embrittling species can diffuse into the grain boundaries of the base metal, causing embrittlement and cracking. Applied and/or residual stresses above a threshold level are necessary for cracking to occur. In some cases the embrittling species can be an immiscible, discrete solid phase, such as lead in leaded steel grades. Intimate contact between the two materials is necessary, and other factors such as microstructural discontinuities affect SMIE behavior. Solid-metal-induced embrittlement is substantially slower than LMIE, but multiple cracks occur in SMIE.

 

[EdStainless]

I am with Syd on this one, oxidation of the Cu would look to be a bigger issue.
I no longer have access to the data (previous employer) and I used 500C based on memory of it being 100% safe. 1100F doesn't raise major concerns for me.

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Plymouth Tube