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According to NACE MR0175, What are the hardness requirements and what hardness method should be used?

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What are the hardness requirements according to NACE MR0175? Is 22HRC enough to meet NACE MR0175? What method should be used to measure hardness?

 

⊗ 22HRC is not the only NACE MR0175 requirement (see this link).

⊗ 22HRC applies for some carbon and low alloy steels, but 22HRC is not the hardness limit for all materials.

⊗ The conversion of hardness readings to and from other scales is material dependent; there are hardness conversion tables such as ASTM E140, ISO 18265 . As per NACE MR0175, the user may establish the required conversion tables.

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Specifically for CA6NM and F6NM, the hardness requirements are listed in NACE MR0175 part 3, clause A.6. The hardness limits are listed in Hardness Rockwell scale C (HRC). However, for welding purposes HV10 or HV5 is required and currently there is no conversion from HRC to HV10 in the published references. The conversion must be agreed between manufacturer and equipment user.

Question:

Per A.6.3 “The hardness of the HAZ after welding shall not exceed the maximum hardness allowed for the base metal, and the hardness of the weld metal shall not exceed the maximum hardness limit of the respective alloy used for the welding consumable.” Per Table A.23 note (b) “Low-carbon, Martensitic stainless steels either cast J91540 (CA6NM) or wrought S42400 or S41500 (F6NM) shall have 23 HRC maximum hardness…” Per 6.2.2.2.2 “Hardness testing for welding procedure qualification shall be carried out using Vickers HV 10 or HV 5 methods in accordance with ISO 6507-1 or the Rockwell 15N method in accordance with ISO 6508-1. The use of other methods shall require explicit user approval.” However, neither a Vickers nor Rockwell 15N acceptance criteria is specified for Martensitic Stainless Steels. Furthermore, ASTM E140 does not provide a hardness conversion for Martensitic Stainless Steels. Thus, there is neither a Vickers nor Rockwell 15N acceptance criteria. Is a correct interpretation that the acceptable hardness test method for qualification of Martensitic Stainless Steels is the Rockwell C Method, regardless of the applied stress, and without the need for explicit user approval?

Answer:

No, ISO 15156-3, 6.2.1, Para. 2 states “The conversion of hardness readings to and from other scales is material dependent; the user may establish the required conversion tables”.

This question is in relation to NACE MR0175/ISO 15156-3 Clause 6.2.2.2.2

Reference: ISO 15156 Maintenance Panel Inquiry #2006-01Q3

There is an API committee currently working in preparing a conversion table for martensitic stainless steel, duplex stainless steels and nickel alloys.

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