NACE MR0175 / ISO 15156 Sour Service Flanges
Complete Material Selection Guide

1. Understanding Sour Service

Sour service refers to oil and gas environments containing hydrogen sulfide (H₂S) — a toxic, flammable gas that poses severe risks of sulfide stress cracking (SSC) in metallic materials. When H₂S dissolves in water, it forms a weak acid that attacks steel surfaces, generating atomic hydrogen. This hydrogen diffuses into the metal lattice, embrittling the microstructure and causing catastrophic brittle fracture under tensile stress — often with no visible warning.

The definition of sour service is quantified in NACE MR0175 / ISO 15156:

Key factors affecting SSC susceptibility include:

• Material hardness — higher hardness dramatically increases crack sensitivity
• Applied and residual tensile stress — including from welding, cold forming, and fit-up
• pH and temperature — lower pH and ambient temperatures are most aggressive
• H₂S concentration and total pressure — governs hydrogen charging rate
• Alloy composition and microstructure — some microstructures are inherently resistant

2. NACE MR0175 / ISO 15156 Overview

NACE MR0175 / ISO 15156 is the global benchmark standard for selecting materials in H₂S-containing oil and gas production environments. Originally published by NACE International (now AMPP) and later adopted as a joint ISO standard, it provides detailed requirements for carbon steels, low-alloy steels, stainless steels, nickel alloys, and other materials exposed to sour environments.

The standard is organized into three parts:

3. Material & Hardness Requirements

Hardness control is the single most critical requirement for sour service materials. The relationship is well established: as hardness increases, SSC resistance plummets. NACE MR0175 imposes strict caps:

These limits apply to all locations on the flange — the bore, raised face, bolt holes, weld bevel, and hub. A single hard spot from improper machining or cold forming can create a SSC initiation point. At JIAJI FORGING, we verify hardness with dense grid testing on every sour service flange.

4. Heat Treatment Protocol

Proper heat treatment is non-negotiable for sour service materials. NACE MR0175 is explicit: the final microstructure must be achieved through thermal treatment at temperatures above the lower critical temperature (Ac₁). This effectively rules out cold-worked or strain-hardened conditions.

Acceptable heat treatment routes for sour service flanges:

• Normalizing + Tempering (N+T) — produces fine-grained, uniform ferrite-pearlite structure
• Quenching + Tempering (Q+T) — produces tempered martensite or bainite with superior toughness
• Solution Annealing — required for austenitic and duplex stainless steels
• Stress Relieving (PWHT) — mandatory after all welding on sour service components

Tempering temperature is equally important. For Q+T carbon steels, the minimum tempering temperature is typically 595°C (1100°F) — lower tempering temperatures produce higher strength but unacceptable SSC susceptibility. The heat treatment furnace must be calibrated and the time-at-temperature recorded as part of the MTC documentation package.

5. Alloy Steel in Sour Service (F5, F9, F11, F22)

Chromium-molybdenum (Cr-Mo) alloy steels such as ASTM A182 F5, F9, F11, and F22 are widely used for elevated-temperature refinery and power plant service. Their application in sour environments requires careful evaluation:

Cr-Mo alloys offer a significant advantage: their tempered martensitic/bainitic microstructures, when properly processed, provide excellent SSC resistance at hardness levels approaching the 22 HRC limit that would be dangerous in plain carbon steels. The chromium and molybdenum additions also provide general corrosion resistance in produced fluids.

6. Stainless & Duplex Stainless in Sour Service

Corrosion-resistant alloys (CRAs) are governed by ISO 15156-3, which defines environmental limits based on H₂S partial pressure, chloride concentration, temperature, and pH — the so-called "four-variable envelope" for each alloy.

Austenitic Stainless Steels (316/316L)

316/316L is the most common stainless flange material, but its sour service envelope is surprisingly narrow:

• Max hardness: 28 HRC (solution annealed)
• Chloride limit: Typically ≤ 50 mg/L at elevated H₂S; higher chlorides risk SCC
• Temperature: Best performance above 60°C — cold shut-ins are problematic
• Free machining grades (e.g., 303): NOT permitted for sour service due to sulfide stringers

Duplex Stainless (2205 / UNS S32205)

Duplex 2205 offers a significantly expanded sour service window compared to 316L, with approximately double the yield strength:

• Max hardness: 28–32 HRC (per ISO 15156-3 Table A.25)
• Ferrite content: 35–55% (excessive ferrite reduces SSC resistance)
• Intermetallic phases (sigma, chi): Must be absent — requires controlled solution annealing and rapid quenching
• Cold working: Strictly limited; maximum 5% cold strain without re-solution annealing

Super Duplex (2507 / UNS S32750)

For the most aggressive sour service with combined H₂S and high chloride, 2507 super duplex is the go-to material among stainless alloys:

• Max hardness: 32 HRC
• PREN ≥ 40 (Pitting Resistance Equivalent Number — superior chloride pitting resistance)
• H₂S envelope: Significantly wider than 2205 — consult ISO 15156-3 Table A.29
• Cost trade-off: Approximately 1.5–2× more expensive than 2205 in flange form

7. Welding Considerations for Sour Service Flanges

Welding on sour service flanges — whether during manufacture (weld overlay, weld-neck attachment) or field installation (butt-welding to pipe) — introduces unique risks that directly affect SSC resistance:

Post-Weld Heat Treatment (PWHT)

All welds on sour service carbon and low-alloy steel flanges must receive PWHT (stress relief) unless specifically exempted by the engineering design. Welding creates residual tensile stresses approaching yield strength — exactly the condition that promotes SSC. PWHT at 595–675°C (1100–1250°F) for one hour per inch of thickness reduces residual stress by 80–90%.

Welding Consumable Matching

Weld metal must match or exceed the base metal's SSC resistance. Key requirements:

• Weld deposit hardness ≤ 22 HRC (carbon/alloy steel) or per CRA limits
• Hydrogen-controlled electrodes (low-hydrogen, baked per manufacturer's instructions)
• Matching or slightly over-alloyed filler metal chemistry
• Procedure qualification with SSC testing per NACE TM0177 when required by specification

Hardness Traverse Testing

Production weld procedure qualifications for sour service must include a Vickers microhardness traverse (HV10 or HV5) across the weld, HAZ, and base metal — at both the ID and OD surfaces. No reading may exceed the applicable hardness threshold. The HAZ, particularly the coarse-grained zone, is the most common failure location.

8. Certification & Documentation

Sour service flange procurement demands rigorous documentation. Standard requirements include:

• EN 10204 Type 3.1 MTC — chemical analysis, mechanical properties, heat treatment records
• Hardness test report — Brinell or Rockwell hardness per heat, with individual readings
• NACE MR0175 compliance statement — explicit certification that each flange meets the standard
• Heat treatment chart — time-temperature record from the furnace
• PMI (Positive Material Identification) — optional but recommended for critical service
• NDT reports — ultrasonic testing for internal defects, liquid penetrant for surface
• Weld map — if any welding was performed, showing weld locations and PWHT records