What are the sour service limits for UNS N08029?


UNS N08029 seamless tube is specified in ASTM B668-14. According to ISO 15156-3 Table A.12, UNS N08029 belongs to materials type 4c. However the material is not included in ISO 15156-3 2015 Annex D, Table D.3 – Chemical compositions of some solid-solution nickel-based alloys. Based on our testing using constant load and SSR (at least duplicate specimens), this material passed the tests in the sour environments outside of the limit for material type 4c.

Question 1: What would be done in order to include UNS N08029 in standard ISO 15156-3, Annex D, Table D.3?

Question 2: We are thinking of additional testing, to propose new limit through NACE Ballot. Are the data points “A” and “B” enough for extending the acceptable limit to the yellow area in the attached figure?

Question 3: Is one of the testing methods enough to propose the new limit, using constant load or SSR per NACE TM0177 or MR0198?

Question 4: Is 1 g/l sulfur ok to represent the environment with elemental sulfur?

Answer 1: Table D.3 is an informative table and adding a material that complies with type 4c is not a technical change. This can be added to the document at the next opportunity or Technical Circular at your request once a formal request for the editorial change is submitted.

Answer 2: Please refer to 15156-3 Annex B especially B.2.4. To extend the limits, you will need test data (in accordance to B.3 and for applicable cracking mechanisms in Table B.1) from a minimum of three separately processed heats. Since the alloy is cold worked to achieve mechanical properties note that B.3.2.c states that the following shall be considered “the directional properties of alloys because cold-worked alloys may be anisotropic with respect to yield strength and for some alloys and products, the susceptibility to cracking varies with the direction of the applied tensile stress and consequent orientation of the crack plane”.

Answer 3: Please refer to B.3.3. Generally, constant load tests are preferred for homogeneous materials. For constant load and constant displacement (constant deformation) tests, a test duration between 90 and 180 days should be considered. You can augment the data with SSRT test results.

Answer 4: This has been one of subjects recently discussed in the Maintenance Panel. The requirements for testing with elemental sulphur have not been fully defined but use of 1 g/L S0 is severely limiting. Please see NACE Corrosion 1995, Paper 47 for more details and guidance as to the appropriate methodology which will depend on the expected physical state of the elemental sulfur for the application conditions.

Reference: Maintenance Panel 2016-14 and is in relation to NACE MR0175 part 3, table D.3

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