Guide to Anti-Sulfur Valve Material Selection
5 min read
Overview
Oil production environments are acidic, requiring valves to resist hydrogen sulfide (H₂S) cracking. This article explores the standards set by the National Association of Corrosion Engineers (NACE) for preventing sulfide stress cracking (SSC).
Common Standards
SSC happens when water and H₂S cause metal to crack due to stress and corrosion. High-hardness materials are more prone to SSC. The common NACE standards used include:
- NACE MR0175
- NACE MR0103
- NACE MR0177
- NACE MR0175/ISO 15156.1
- NACE MR0175/ISO 15156.2
- NACE MR0175/ISO 15156.3
- NACE TM0284
Standards Analysis
NACE MR0175 is usually specified for sulfur-resistant valves in oil and gas extraction. NACE MR0103 is used for petroleum refining. Both standards are similar, but NACE MR0103 is more detailed. For instance, it specifies chemical composition requirements for austenitic stainless steel and nickel alloys, which are not detailed in NACE MR0175. ISO has integrated NACE MR0175 into its standards, creating NACE MR0175/ISO15156.1 to .3.
Scope of NACE MR0175
NACE standards apply to environments where H₂S partial pressure is ≥ 0.0003 MPa (0.05 psi). This includes drilling equipment in oil and gas fields. Prolonged exposure to acidic conditions can cause metal cracking if not properly treated. NACE MR0175 sets requirements for metal grades, heat treatment methods, and mechanical properties to prevent SSC. Factors affecting SSC include:
- Material Factors: Composition, strength, hardness, heat treatment, microstructure, local differences, cold work extent, grain size, cleanliness.
- Environmental Factors: pH, H₂S concentration, and pressure.
- Other Factors: Chloride ion concentration, tensile stress, temperature, and duration.
Anti-Sulfur Materials
- Material Selection and Hardness Control:
- Carbon and Alloy Steel:
- For castings, forgings, and cold-worked pipe fittings.
- Hardness ≤ 22 HRC.
- Heat treatment methods include hot rolling (for carbon steel), annealing, normalizing, normalizing + tempering, austenitizing + quenching + tempering.
- Grades include ASTM A105, ASTM A53 B, ASTM A106 B, and API 5L X-42.
- Austenitic Stainless Steel:
- For castings and forgings.
- Hardness 22–38 HRC.
- Heat treatment: solution annealing and cold work annealing.
- Grades: UNS S20910, UNS S8020, ASTM A351 CN7M, UNS S31254, UNS N08367, UNS S32200, UNS N08926, UNS J93254 (CK3MCUN), UNS S32654, UNS S31266, UNS S34565.
- Ferritic Stainless Steel:
- For castings and forgings.
- Hardness ≤ 22 HRC.
- Heat treatment: annealing.
- Grades: AISI 405, AISI 430, ASTM A268, TP268, TP405, TP430, TP XM27, TP XM33.
- Martensitic Stainless Steel:
- Hardness ≤ 22 HRC.
- Heat treatment: normalizing or quenching, tempering twice.
- Grades: AISI 410, 501, ASTM A216 CA15, A268 TP410, A743 CA15M, A487 CA15M, A487 CA6NM, UNS S42400.
- Precipitation Hardening Stainless Steel:
- For forgings.
- Hardness 31–35 HRC.
- Heat treatment: solution annealing, aging.
- Grades: UNS S17400, UNS S66286, UNS S45000.
- Duplex Stainless Steel:
- For castings or forgings.
- Hardness 17–36 HRC.
- Heat treatment: solution annealing, cold work.
- Grades: UNS S32404, UNS S31803, UNS S32750, UNS S32760, UNS J93380, UNS S31260, UNS S39274, UNS J93404, UNS S39277.
- Non-Ferrous Alloys:
- Nickel-copper, nickel-chromium-iron, nickel-chromium-iron-molybdenum, nickel-chromium, nickel-chromium-molybdenum alloys.
- Hardness 32–45 HRC.
- Heat treatment: solution annealing, aging.
- Grades: UNS N04400, ASTM A494 M35-1, M35-2, UNS N04405, UNS N05500, UNS N08800, UNS N08825, UNS N06007, UNS N06250, UNS N06255, UNS N06975, UNS N06525, UNS N0718.
- Titanium Alloys:
- Hardness 32–45 HRC, 92–100 HRB.
- Heat treatment: solution annealing, annealing.
- Grades: UNS R53400, UNS R58640, UNS R50400, UNS R56260, UNS R56403, UNS R56404, UNS R56323.
- Carbon and Alloy Steel:
- Material Chemical Composition:
- ASTM A494 CW2M: sulfur ≤ 0.015%, aluminum ≤ 0.05%.
- ASTM A351, A734, A744 CN7M: carbon ≤ 0.035%, silicon ≤ 1.00%, copper 3–5%, sulfur ≤ 0.015%, phosphorus ≤ 0.030%, aluminum ≤ 0.05%.
- Filler metals with nickel content > 1% should not be used for carbon steel and low-alloy steel.
- Surface Treatment of Anti-Sulfur Materials:
- Shot peening size ≤ 2.0 mm (0.08 in.).
- Avoid electroplating, chemical plating, plastic coatings, and linings.
- Nitriding allowed if temperature is below the material's lower critical temperature.
- Nitriding layer depth ≤ 0.15 mm (0.006 in.).
- Do not use nitriding as a means to prevent sulfide cracking.
- Hardness is generally referenced by Rockwell hardness (HRC).
- Average hardness test value should not exceed the material standard value by more than 2 HRC.
- Use ASTM A193 B7M studs (yield strength ≥ 550 MPa, hardness ≤ 22 HRC) and ASTM A194 2HM nuts (hardness ≤ 22 HRC).
Welding of Materials
Base materials, weld seams, and heat-affected zones should have hardness ≤ 22 HRC after stress relief heat treatment. Hard-surface alloys can be applied using spraying tungsten carbide, ceramics, or overlay welding with cobalt and nickel-based alloys. Welding procedures and materials must be qualified. Defects can be repaired by welding if the process is reliable.
Material Quality
- Chemical composition, mechanical properties, and hardness must meet standards.
- Proper heat treatment based on furnace number, steel type, and mechanical property requirements, with records.
- Metals with >5% deformation must undergo high-temperature stress relief.
- Avoid free-cutting steels in SSC environments.
- Avoid gray, austenitic, and white cast iron for pressure-bearing materials; use ASTM A395 ferritic ductile cast iron.
- Do not use raw materials not refined using AOD or vacuum deoxidation.
- Sulfur-resistant materials are for acidic, not saline or saltwater, environments.
- If using materials not listed in NACE MR0175, conduct sulfur failure tests.
- Austenitic steel, duplex stainless steel, nickel-based alloys, and titanium alloys are sensitive to high temperatures; provide qualified reports.
- Ferrite volume in duplex stainless steel should be 35–65%.
- No welding or repair welding of gray or ductile iron.
Valve Quality
Valve threads can be machined or cold-pressed. General processes include turning, boring, rolling, drilling, and polishing.
- Use NACE MR0175-specified materials and heat treatments.
- Strengthen heat treatment quality control, especially for welded parts.
- Control the environment to reduce H₂S factors.
- Isolate valves from acidic environments when possible.
- Establish regular inspection systems.
- Conduct failure analysis on SSC-affected valves.
Conclusion
Anti-sulfur valves are increasingly used in oil and gas extraction, pipelines, and refining. Understanding NACE standards is crucial for extending the service life of these valves and ensuring safety.
FAQs
- What causes sulfur stress cracking (SSC) in valves?
- SSC is caused by water and H₂S, leading to metal cracking due to stress and corrosion.
- Which standard is used for sulfur-resistant valves in oil and gas extraction?
- NACE MR0175.
- What are the heat treatment methods for austenitic stainless steel?
- Solution annealing and cold work annealing.
- Why control the hardness of materials in sulfur-resistant valves?
- High hardness increases SSC susceptibility; standards specify limits to prevent cracking.
- Can cold-worked materials be used directly in sulfur-resistant environments?
- No, they must undergo high-temperature stress relief first.
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