- Sour service — any process environment containing hydrogen sulfide at concentrations sufficient to cause sulfide stress cracking — demands specific material restrictions on every wetted metallic component in the valve, and those restrictions are not optional suggestions: they are the difference between a valve that lasts decades and one that fails catastrophically within weeks of installation.
- NACE MR0175 (now jointly published as ISO 15156) defines the material qualification requirements for equipment in H2S-containing oil and gas environments, and its hardness limits, heat treatment requirements, and alloy restrictions must be verified at the traceability level — for every individual component, from every heat of material, from every production run.
- The most common sour service failures in the field are not from specifying the wrong alloy — they are from using the right alloy specification but with inadequate quality control: wrong heat treatment, hardness values just above the limit, or undetected hard heat-affected zones in welds that bypass the specified base material properties entirely.
What Is Sour Service and Why Does It Matter?
Hydrogen sulfide is produced wherever organic material containing sulfur is decomposed or processed under elevated temperature and pressure — in oil and gas reservoirs, in refinery hydrotreating units, in gas sweetening plants, in sour water systems, and in many other industrial processes. Even at low concentrations — measured in parts per million — H2S creates a specific and dangerous corrosion mechanism called sulfide stress cracking (SSC) that can fracture high-strength steel components suddenly and without significant prior warning.
Sulfide stress cracking occurs because atomic hydrogen, generated at the steel surface by the cathodic reaction in the corrosion process, diffuses into the steel lattice rather than combining to form hydrogen gas. This atomic hydrogen accumulates at microstructural defects, grain boundaries, and regions of high residual stress — particularly in heat-affected zones of welds. The hydrogen embrittles the steel by reducing its fracture toughness, and when this is combined with applied or residual tensile stress, cracks initiate and propagate rapidly. The fracture appearance is typically brittle with little or no plastic deformation — a high-strength steel component that would normally withstand significant overload before yielding simply shatters at a fraction of its nominal design stress.
The consequence for valve engineering is direct and unambiguous. Any steel component in contact with the sour process fluid — valve body, bonnet, stem, trim, bolting, springs — is potentially susceptible to SSC if its material properties are not within the limits defined by NACE MR0175/ISO 15156. The material properties of primary concern are yield strength and hardness, both of which correlate strongly with susceptibility to SSC. High-strength, high-hardness steels are most susceptible; lower-strength, softer steels are more resistant.
NACE MR0175 and ISO 15156: The Governing Standards
NACE MR0175 was originally published by NACE International (now AMPP) in 1975 and has been revised many times since. In 2003, it was jointly adopted with ISO as ISO 15156, and both designations are now in widespread use — they refer to the same technical content. The standard is divided into three parts: Part 1 covers general principles, definitions, and assessment criteria; Part 2 covers cracking-resistant carbon and low-alloy steels; and Part 3 covers cracking-resistant corrosion-resistant alloys (CRAs) and other alloys.
The standard defines the environmental conditions under which it applies — specifically, it applies to systems where the partial pressure of H2S in the gas phase exceeds 0.3 kPa (0.05 psi), and to aqueous liquid phase systems where the total system pressure exceeds 0.45 MPa (65 psi) and H2S is present. Below these thresholds the standard does not apply, though many operators choose to apply its requirements at lower concentrations as a precautionary measure.
For valve engineers, the most immediately relevant sections of NACE MR0175/ISO 15156 Part 2 cover the requirements for carbon and low-alloy steels — the materials from which the vast majority of industrial valves are made. The key requirement is a maximum hardness limit of 22 HRC (equivalent to approximately 237 HV or 250 HB) for carbon and low-alloy steel base material. This limit applies to the bulk material, to heat-affected zones of welds, and to work-hardened surfaces. Any region of the valve that exceeds this hardness in the as-installed condition is non-compliant with the standard and is susceptible to SSC.
Material Requirements for Sour Service Valves
For carbon steel valve bodies in sour service, the typical specification is ASTM A216 Grade WCB (cast) or ASTM A105 (forged), both of which are low-carbon, low-alloy steels that in the normalized and tempered condition meet the NACE hardness limits. The supply condition — normalized and tempered — must be specified explicitly in the purchase order. Carbon steel supplied in the as-cast or as-rolled condition without heat treatment may have localized hard spots that exceed the 22 HRC limit, even though the nominal material specification appears compliant.
Low-alloy steel valve bodies in Cr-Mo grades — such as ASTM A217 Grade WC6 (1.25Cr-0.5Mo) or WC9 (2.25Cr-1Mo), commonly used for higher-temperature service — require careful heat treatment control to achieve NACE-compliant hardness. The addition of chromium and molybdenum increases the hardenability of the steel, meaning it is more susceptible to forming hard microstructures during welding or heat treatment if cooling rates are not carefully controlled. Post-weld heat treatment (PWHT) is mandatory for all welds on sour service Cr-Mo valves and must be performed at the correct temperature and duration to temper any martensite formed during welding.
The valve stem is a particular concern in sour service. Stems must transmit torque or thrust, which demands reasonable mechanical strength — but high strength means high hardness, which conflicts with NACE requirements. The resolution is to use austenitic stainless steel stems (such as AISI 316 or 17-4PH in the H1150 or H1150M condition, which comply with Part 3 of the standard), or to use cold-worked austenitic stainless with controlled yield strength within the limits of Part 3. Some specifications allow carbon steel stems with controlled hardness well below 22 HRC, accepting a strength reduction as the price of sour service compliance.
Valve trim components — seats, discs, balls, plugs, cages, and guides — must also comply with NACE MR0175/ISO 15156. Hard materials commonly used for erosion resistance in trim applications — such as hardfacing alloys, cobalt-chromium alloys, and some carbide coatings — need to be specifically qualified under the standard. Stellite (cobalt-chromium) alloys are listed in Part 3 as acceptable in specific conditions. Tungsten carbide coatings are not universally covered and their compliance must be assessed against Part 3 criteria for the specific alloy composition and application.
Bolting Requirements
Valve bolting — studs, nuts, and bolts used for flange connections and bonnet attachment — is a critical sour service concern that receives less attention than valve body materials but causes significant in-service problems. High-strength bolting materials like ASTM A193 Grade B7 studs, which are the standard choice in non-sour service, have yield strengths of 725 MPa and hardness values well above 22 HRC — they are explicitly prohibited in sour service by NACE MR0175/ISO 15156.
NACE-compliant bolting options include ASTM A193 Grade B7M (a modified version of B7 with controlled hardness not exceeding 22 HRC, achieved by tempering at a higher temperature), ASTM A193 Grade L7M for lower-temperature applications, and ASTM A307 for lower-strength applications. The corresponding nuts must be ASTM A194 Grade 2HM or 7M, which are also hardness-controlled. Using standard B7/2H bolting in a sour service valve — even with a correctly specified NACE-compliant valve body — means the bolting itself is non-compliant and potentially susceptible to SSC at the flange connections.
This is an easy mistake to make during procurement: the valve manufacturer correctly supplies the valve body and trim in NACE-compliant materials, but the bolting is supplied from a different source using standard non-compliant grades because the purchase order for bolting did not specify the sour service requirement explicitly. The complete valve bill of materials, including bolting, must specify NACE MR0175/ISO 15156 compliance for every component.
Weld Procedure Qualification for Sour Service
Welds in sour service valves — body casting repairs, bonnet-to-body welds, seat ring welds, and any other pressure-containing welds — require qualification procedures that specifically address SSC resistance. The weld procedure specification (WPS) must be qualified with actual hardness testing of the weld metal and heat-affected zone (HAZ) to demonstrate that no region exceeds 22 HRC. This requires hardness traverses across the weld cross-section, taken at defined locations covering the weld metal, fusion line, HAZ, and base metal adjacent to the weld.
The HAZ is typically the highest-risk region. The thermal cycle of welding creates a narrow band of base metal adjacent to the fusion line that is subjected to very high peak temperatures — close to the melting point — and then cooled at a rate determined by the surrounding metal mass and any preheat applied. Depending on the carbon equivalent of the base material, this rapid heating and cooling cycle can produce martensite or bainite in the HAZ with hardness significantly above the NACE limit. Preheat during welding slows the cooling rate and reduces HAZ hardness. PWHT tempers any hard phases that did form. Both must be specified and implemented correctly.
Casting repairs to valve bodies are a particular area of risk. Cast steel bodies often contain porosity, shrinkage cavities, or inclusions that must be excavated and weld-repaired before the valve is accepted. In non-sour service, these repairs are routine. In sour service, every weld repair must be performed to a qualified sour service weld procedure, with appropriate preheat, controlled interpass temperature, and mandatory PWHT. The weld repair area must then be hardness-tested to confirm compliance. Weld repairs performed before the sour service classification was established — on older valves being redeployed — are a particular risk and should be evaluated carefully.
Corrosion-Resistant Alloys in Sour Service
Where the combination of H2S and other corrosive constituents — chlorides, carbon dioxide, high temperature — would cause excessive corrosion of carbon or low-alloy steel, corrosion-resistant alloys (CRAs) are specified. CRAs covered by NACE MR0175/ISO 15156 Part 3 include austenitic stainless steels, duplex stainless steels, precipitation-hardened stainless steels, nickel alloys, and cobalt alloys. The compliance of each alloy depends on its specific composition and condition (annealed, cold-worked, aged) and on the H2S partial pressure, chloride content, temperature, and pH of the process environment.
Duplex stainless steels (25Cr grades such as SAF 2507 and Zeron 100, and standard 22Cr grades such as 2205) are widely used in offshore sour service applications where their combination of strength, corrosion resistance, and NACE compliance makes them an effective choice. However, duplex stainless is susceptible to sigma phase embrittlement if exposed to temperatures between 300 and 950 degrees C during fabrication or service — a risk that must be addressed through careful control of heat treatment parameters and service temperature limits.
Austenitic stainless steels (316L, 317L, 6Mo) are generally resistant to SSC in the annealed condition, but cold-worked austenitic stainless — the condition commonly encountered in valve stems and trim that have been machined to tight tolerances — can exceed the hardness or yield strength limits of Part 3 in some environments. The specific limits depend on the H2S partial pressure and temperature. For stem applications in severe sour environments, Inconel 718 or similar nickel alloys in the precipitation-hardened condition with controlled hardness are often specified, combining excellent mechanical properties with robust SSC resistance.
Inspection and Verification Requirements
Sour service valve inspection must verify NACE compliance at the component level, not just at the material specification level. The following inspections are required for every sour service valve: material test reports (MTRs) for every pressure-retaining component, traceable to the specific heat of material and confirming the required material specification and heat treatment condition; positive material identification (PMI) using X-ray fluorescence (XRF) or optical emission spectroscopy (OES) on alloy components to verify chemical composition; hardness testing on each component using a calibrated portable hardness tester, with results documented and confirmed to be within NACE limits; and review of weld procedure qualification records and welder qualification records for all production welds.
Hardness testing of valve components requires particular attention to the test method and the hardness scale conversion. NACE MR0175/ISO 15156 limits are expressed in HRC (Rockwell C scale), but the convenient portable testing method for irregular valve shapes is Vickers (HV) or Leeb (rebound) testing. Conversions between scales must use the appropriate conversion table — ASTM E140 provides standardised conversion values. Not all conversion charts are equivalent, and using an inappropriate conversion can cause a non-compliant hardness reading to appear compliant after conversion.
H2S Severity Assessment
Not all sour service conditions are equally severe, and NACE MR0175/ISO 15156 recognises this through the concept of SSC severity regions defined in Part 2. The severity is determined by the pH of the aqueous phase and the partial pressure of H2S. At low H2S partial pressures combined with neutral or high pH, conditions are in Region 0 (no SSC risk) or Region 1 (low risk, standard carbon steel acceptable with moderate hardness control). At higher H2S partial pressures or lower pH, conditions progress to Region 2 and Region 3, where stricter hardness limits and potentially different material choices are required.
The pH in the aqueous phase is not the pH of an external water sample — it is the in-situ pH at process conditions, which depends on CO2 partial pressure, H2S partial pressure, temperature, and the presence of dissolved salts. Calculating the in-situ pH correctly requires a thermodynamic model or specialist software. Getting the pH wrong by even one unit can shift the assessment from Region 1 to Region 3, dramatically changing the material requirements and the cost of compliance. For any sour service system design, a proper SSC severity assessment using qualified process fluid data and recognised software should be performed by a qualified corrosion engineer before valves are specified.
Documentation and Traceability Requirements
Sour service valves require a level of documentation and traceability that exceeds normal industrial valve procurement. The full documentation package for a sour service valve should include a material test report for every pressure-retaining component with heat number, chemical composition, mechanical properties, and heat treatment record; hardness test records for each component; PMI test records for alloy components; weld procedure qualification records for all production weld types; welder qualification records; heat treatment records including time-temperature charts for PWHT; non-destructive examination reports; hydrostatic test records; dimensional inspection records; and a certificate of conformity from the manufacturer confirming NACE MR0175/ISO 15156 compliance with specific reference to the applicable parts and clauses.
This documentation must be archived for the life of the plant. In the event of a sour service valve failure, the documentation trail will be essential to determining whether the failure was caused by a material non-conformance (in which case there may be liability implications for the manufacturer) or by a service condition outside the design envelope (which may indicate a process upset or incorrect severity assessment). Maintaining complete documentation is not bureaucratic excess — it is essential due diligence for operating equipment in a potentially lethal service environment.