Low Alloy Steel (AISI 4130) and Stainless Steel (AISI 410) in API 6A - Wellhead and Christmas Tree 2

[JHWC]

Hi All,

I have doubts on the mechanical properties for material selection, especially on the low alloy and stainless steel which are often used in wellhead and christmas tree (API 6A) in oil and gas industries.

Imagine on the following scenario:

AISI 410 (martensitic stainless steel) for NACE and API 6A requirements:

Yield strength: 75 KSI
Surface Hardness: 22 HRC (max)

However, vendor replied that they have achieved a hardness of 25 HRC which violates the NACE requirement (Hardness 22 HRC max).

Questions:

1. Can we do 2nd heat treatment in order for AISI 410 to achieve a hardness of 22 HRC (maximum) again? Will there be any changes in the microstructure?

2. Is it still recommended to use the material (AISI 410) if it meets the mechanical properties after 2nd heat treatment? Will there by any changes in the microstructure?

3. In both API 6A and NACE requirement, I do not find any related statements that stated that if the microstructure (martensitic) is not achieved, it cannot be used. Please correct me if I am wrong.

4. Is there any limits to the number of times for the material to be reheated so that the mechanical properties would not be affected? Do stress accumulate from several heat-treatment?

5. Since the vendor has got it wrong, what is the suggestion? Do we ask the vendor to forge using raw material again or reheat again to achieve the desire mechanical properties? Cost is a important factor here.

Your suggestions and helps is very much appreciated.

Thanks,
Jeffrey

 

[redpicker]

What does 4130 have to do with any of your questions?

1) Yes, you can re-heat treat the material. I would recommend a re-temper at 1350F for 2 hrs per inch. Make sure you have a current furnace survey at this temperature (a requirement of 6A, I believe).

2) The only way to tell if the microstructure is affected is to take before and after samples. If the material meets the 6A requirements, you should be able to use it.

3) I believe the assumption is that you are dealing with a tempered martensitic microstructure. If you meet the required mechanical properties, I am not sure you have anything else.

4) This is a philosophical question. Believe me, API has discussed it at length.

5) Ask the vendor to meet the requirements he agreed to when the contract was accepted. This isn't the first time this has happened.

 

[EdStainless]

Re-tmepering to lower strength does not damage material in any way.
You need good uniformity and control, in some alloys the window between max hardness and minimum strength is only 100F in temper.
The risks with re-heat treating largely involve distortion and surface damage from oxidation. The microstructures will go back and forth as many times as you want.

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P.E. Metallurgy, Plymouth Tube

 

[JHWC]

Hi repicker and EdStainless,

Thank you for the great information!

This is indeed very helpful for me, a fresh materials engineer. =)

Have a pleasant and great day ahead.

Cheers,
Jeffrey