NACE mr0175 interpretation

[sta67]

According to mr0175 alloy steel with more than 1% nickel is not allowed even if hardness is less than 22 HRC. This would rule out using 4340 steel in our H2S environment. We were wanting to use quenched and tempered 4340 instead of 4140 for a large compressor forging to get better through hardening. Am I correct that 4340 is not allowed, and if so can you explain why that is? sta67

 

[SJones]

Refer to the second and third paragraphs of ISO 15156-2, 7.2.1.4.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[sta67]

Thanks SJones for your response. I looked up the latest ISO 15156-2 that you referred to but that section you referenced simply tells me that I can either test for acceptance, or petition based on historical use. Further down in the standard it still says the same thing, that alloy steel greater than 1% Ni is not allowed (without the history). So my question still stands, and that is, why is 4340 with Ni content not allowed when 4140 is? What does the nickel do to the microstructure that makes it suspect in H2S? sta67

 

[metengr]

I am not a member of the committee that developed this standard. Nickel does two things to ferrous-based materials - it increases ferrite strength, increases hardenability and toughness. The increase in toughness is good.

Increased hardenability and increased ferrite strength will result in increased hardness of weld heat affected zones in base material containing over 1% nickel. This plays a significant role in trying to keep hardness below a threshold to avoid SCC in service.

 

[SJones]

It's not a question of 'allowed'; it's a question of qualification and whether the material can be considered to be 'automatically qualified.' You are free to qualify the material as directed in the referenced paragraphs.

MR0175 is, essentially, an empirical standard. The restriction on nickel was based on observed failures in materials with nickel content greater than this value. No doubt, a healthy margin was shaved off to arrive at the 1 mass % threshold.

NACE Publication 8X294 has this to say:

"For many years controversy has existed concerning nickel content of the steel to the extent that NACE Standard MR0175 limited nickel content to a maximum of 1%. Several investigations suggested that the effect of nickel is related to the potential resultant microstructure (i.e., retained austenite that transforms to untempered martensite) and its high susceptibility to SSC."

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04