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In NACE MR0175, what does “as close as possible to, but no more than 1mm from, the fusion boundary” mean?

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Question:

Per 7.3.3.3 Using the Vickers or Rockwell 15N measurement methods, hardness impressions 2, 6, and 10 should be entirely within the heat-affected zone and located as close as possible to, but no more than 1mm from, the fusion boundary between the weld overlay and HAZ.” Is a correct interpretation that when welding dissimilar metals such as corrosion resistant overlays on low alloy steels, the phrase, “as close as possible to, but no more than 1mm from, the fusion boundary” means that the indentation should be no less than 3x the mean diagonal length of the indentation from the fusion boundary as is required for adjacent indentations in ISO 6507-1:1998? Note: ISO 6507-1:1998 is referenced by NACE/ISO 15156-2 in the first paragraph of Section 7.3.3.2 (Hardness testing methods for welding procedure qualification).

Answer:

The ISO 15156 Maintenance Panel cannot provide an interpretation of the ISO 6507-1:1998 in relation to the minimum distance of hardness indentations from the boundary between the base metal and the overlay weld.
As stated in ISO 15156-2, 7.3.3.2 and ISO 15156-3, 6.2.2.2.2 hardness measurements can also be carried out using a smaller indentation load, for example HV5 rather than HV10, and in many cases this will allow compliance with the requirements of ISO 15156-2, Fig. 6.

It is important to recognize that there will be a gradient in HAZ hardness in any case, and thus measurements too far from the fusion boundary could be un-conservative.

In all cases it is the task of the equipment user (and hence the supplier) to ensure that the hardness values measured are the most representative possible of the cracking resistance of the welded material in any sour service it is expected to experience.

This question is in relation to NACE MR0175/ISO 15156-2 Clause 7.3.3.3

Reference: ISO 15156 Maintenance Panel Inquiry #2006-01Q2

 

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