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For CRAs in NACE MR0175, how does the 22HRC value convert to other hardness scales?

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Question:In Section of Part 3: Table A.2 (austenitic stainless steel) states: “These materials shall also -be in the solution-annealed and quenched, or annealed and thermally stabilized heat-treatment condition, -be free of cold work intended to enhance their mechanical properties, and -have a maximum hardness of 22 HRC.” Whereas for welding in Section A.2.3 it is stated that: “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.” I addition Section 6.2.2.2.2 states that “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.”

Q1. Please clarify how the requirement for 22 HRC is interpreted in light of this, i.e., what Vickers (HV 10 or HV 5) or Rockwell (15N) value should be used as a maximum for weld HAZ and weld metal? On an associated point, for solid-solution nickel-based alloys (Section A.4) and duplex stainless steels (Section A.7) there are no hardness requirements for materials in the solution-annealed condition (with the exception of one HIP duplex stainless steel alloy). The relevant sections (A.4.3 and A.7.3) on welding state: “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”.

Q2. Please confirm that the interpretation that NACE MR0175/ISO 15156 therefore places no hardness restrictions for welds in these materials is correct.

Answer

(1) NACE MR0175/ISO 15156 provides no guidance for hardness conversion from the Vickers to the Rockwell scales for the austenitic stainless steels, which is then left to an agreement between the manufacturer and the equipment user possibly based on conversion tables made using empirical data; see ISO 15156-3, 6.2.1, Paragraph 2.

(2) There are no hardness limits for the HAZ of welds of corrosion-resistant alloys when there are no hardness limits in the tables or the text of the document for the base materials.
For the weld metal, any hardness limit depends on any hardness limit set for the alloy used as consumable. For matching consumables for solid-solution nickel-based alloys (Section A.4) and duplex stainless steels (Section A.7) there are no hardness limits for weld metal.

This question is in relation to NACE MR0175/ISO 15156-3 Annex A.2.3

Reference: ISO 15156 Maintenance Panel Inquiry #2005-13

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