The NACE/ISO TG 299 committee has approved a new ballot that will drastically limit the use of the precipitation hardenable martensitic stainless steel UNS S17400 (17–4PH). The ballot was prepared by Weatherford International in response to environmentally assisted cracking (EAC) concerns in hydrogen sulfide (H2S) containing environments.
The proposed change adds a note to ISO 15156–3 Table A.27 that states:
The use of UNS S17400 is restricted to those applications where the sustained stress intensity is no more than 50% of the specified minimum yield strength (SMYS) or 380 MPa (55 ksi), whichever is less. The use of UNS S17400 is acceptable for wellhead components and valve components, such as stems, which are only subjected to higher stress intensity levels for very short periods of time during actuation or other field operations. The use of UNS S17400 is prohibited for components subjected to other or longer duration in-service stress intensities above 50% of the SMYS or 380 MPa (55 ksi), whichever is less.
What constitutes a “short term” exposure is not defined at this time. The wording might change before the final release of the standard.
The consequences of the updated restrictions can be far reaching. Original equipment manufacturers, suppliers, and oil and gas companies should update internal specifications and gauge the consequences of the maximum allowable stress in their designs. Alternative material selection may include UNS S45000, a molybdenum-containing age hardenable martensitic grade that can reach high strength levels by a double step heat treatment. Although not specified in ISO 15156–3 Table A.27, UNS S13800 (13–8PH), a molybdenum-containing precipitation hardenable martensitic stainless steel, could also be considered owing to successful qualification testing. Table 1 compares composition and mechanical properties of the three alloys.
Table 1. Composition and mechanical properties of various age hardenable martensitic stainless steels
| UNS | Condition | SMYS (min) | Hardness | Cr (wt%) | Ni (wt%) | Mo (wt%) | Cu (wt%) | Al (wt%) | PREN |
|---|---|---|---|---|---|---|---|---|---|
| S17400 | H1150 | 868 MPa | 33 HRC | 15.0–17.5 | 3.00–5.00 | – | 3.00–5.00 | – | 15.0–17.5 |
| S45000 | H1150 | 634 MPa | 33 HRC | 14.0–16.0 | 5.00–7.00 | 0.50–1.00 | 1.25–1.75 | – | 15.5–19.3 |
| S13800 | H1100 | 930 MPa | 34 HRC | 12.25–13.25 | 7.50–8.50 | 2.00–3.00 | – | 0.90–1.35 | 18.85–23.15 |
| S13800 | H1150 | 620 MPa | 30 HRC | 12.25–13.25 | 7.50–8.50 | 2.00–3.00 | – | 0.90–1.35 | 18.85–23.15 |
Where:
$$ {\text{PRE}_{\text{N, W}}} = {{\text{wt%Cr}}} + 3.3 \cdot \left( {{{\text{wt%Mo}}} + 0.5 {\text{wt%W}}} \right) + 16 {\text{wt%N}} $$
More information can be found in the NACE MR0175/ ISO 15156 Proposal for Change Ballot submitted by Robert Badrak.
Acknowledgements
Many thanks to Tim Haeberle for the update.
