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What does NACE MR0175 require from dissimilar welds without filler?

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Question:

My employer manufactures pressure sensing devices that have a very small sensing diaphragm welded to a threaded port. I am hoping you can provide some clarification regarding welding of dissimilar metals. I am employing the material hardness limits for specific materials from Annex A of ISO 15156-3 in conjunction with the weld survey locations specified in 7.3.3.3 of ISO 15156-2. I believe the hardness requirements are clearly interpreted if welding dissimilar metals by use of a third weld-filler metal. Hardness requirements can be clearly applied to each of the weld survey locations for all three involved parent materials. However, I do not know how to apply the hardness requirements when two materials are welded directly together without the use of a third weld-filler metal. 1. If the two materials have different hardness limits, what is the hardness requirement for the weld survey locations within the weld metal region (non parent material nor HAZ regions)? 2. Is the hardness limit governed by the higher of the two material limits or the lower of the two? 3. Can you tell me where I can find clarification in these standards or where I can direct my question? 4. On an unrelated note, I would like to suggest a minor improvement to table A.1 of ISO 15156-3 Annex A. In Table A.6, Note-B defines the table applicable to “…diaphragms, pressure measuring devices and pressure seals.” Table A.1 would be improved if “A.6” was specified in the first column (Austenitic stainless steel) for the row designated for “Diaphragms, pressure measuring devices and pressure seals”.

Answer:

1. The hardness requirements for each material shall be met for both the base metal and HAZ. The standard does not specify requirements for the fusion line hardness between two dissimilar metals with different maximum hardness requirements. Qualification by successful laboratory testing in accordance with Annex B of ISO 15156-3 is required. Qualification based on satisfactory field experience is also acceptable. Such qualification shall comply with ISO 15156-1.
2. See answer to point 1
3. See answer to point 1.
4. This is an editorial improvement and this will be changed.

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

Reference: ISO 15156 Maintenance Panel Inquiry #2014-04

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