ISO 15156-2:2020 addresses the selection and qualification of carbon and low-alloy steels against cracking in H2S-containing oil and gas production environments. It covers sulfide stress cracking (SSC), stress-oriented hydrogen-induced cracking (SOHIC), soft-zone cracking (SZC), hydrogen-induced cracking (HIC) and stepwise cracking (SWC), together with provisions for the use of cast irons.
This guide covers the fourth edition, published in November 2020. It is an original technical overview of the supplied standard. Use the complete document and the assessed service conditions for material acceptance and project decisions.
What Is ISO 15156-2:2020?
Part 2 provides material requirements for carbon and low-alloy steel equipment used in sour production and natural-gas treatment service. It supplements the relevant design codes and product specifications. Selection depends on the material’s manufacturing and metallurgical condition as well as its exposure to the service environment.
The series is associated with NACE MR0175/ISO 15156. A purchase order should state the required publication, part and edition, together with the application conditions. A grade designation or a general claim of “NACE compliance” does not establish suitability for every sour environment.
Standard at a Glance
| Item | ISO 15156-2:2020 |
|---|---|
| Edition | Fourth edition, November 2020 |
| Material families | Carbon steels, low-alloy steels and provisions for cast irons |
| Primary application | H2S-containing oil and gas production and natural-gas treatment equipment |
| Damage addressed | SSC, SOHIC, SZC, HIC and SWC |
| Normative annexes | Annex A: material provisions; Annex B: laboratory qualification |
| Informative annexes | Annex C: H2S exposure parameters; Annex D: pH; Annex E: purchasing information |
| Related parts | Part 1 for general selection principles; Part 3 for corrosion-resistant and other alloys |
ISO 15156 Part 1 vs Part 2 vs Part 3
| Part | Role | Procurement relevance |
|---|---|---|
| ISO 15156-1 | General principles, service assessment and qualification framework | Establishes the common basis for selecting cracking-resistant materials |
| ISO 15156-2 | Carbon and low-alloy steels; use of cast irons | Check the applicable steel, welding, hardness and cracking-resistance provisions |
| ISO 15156-3 | Corrosion-resistant alloys and other alloys | Check stainless steel, duplex, super duplex and nickel alloy requirements |
Part 2’s general carbon-steel hardness provisions should not be applied to duplex stainless steel or nickel alloys. Those products require assessment under the applicable Part 3 provisions and Part 1 framework.
Scope and Cracking Mechanisms
Clause 1 and Table 1 address equipment including wells and wellheads, flowlines, gathering lines, field processing facilities, water injection and disposal equipment, natural-gas treatment plants and certain transportation pipelines. Table 1 also includes exclusions; assess applicability against the full scope.
The document addresses cracking resistance. It does not establish protection against general metal loss or localized corrosion. Refining and downstream applications require a separate assessment of the governing standard, and strain-based designs require consideration of the Part 1 provisions.
| Mechanism | Selection implication |
|---|---|
| SSC | Assess the steel, hardness, total tensile stress and sour environment together |
| SOHIC and SZC | Consider plate products and welded carbon-steel equipment, including heat-affected zones |
| HIC and SWC | Evaluate steel cleanliness, manufacturing route and any required testing separately from SSC acceptance |
Service Conditions and the 0.3 kPa H2S Threshold
Clause 6 identifies interacting factors including H2S exposure, in situ pH, water-phase chlorides, sulfur and other oxidants, temperature, non-production fluids, applied and residual tensile stress, and exposure duration. Material composition, strength, hardness, cold work, heat treatment and microstructure also influence cracking resistance.
Under Option 1, H2S partial pressure of at least 0.3 kPa (0.05 psi) requires selection of SSC-resistant steels using Annex A.2. Below that threshold, special SSC selection precautions are normally unnecessary, but highly susceptible steels can still crack. The threshold does not eliminate the need to assess HIC/SWC or other relevant mechanisms.
For Option 2, Figure 1 relates SSC environmental severity to in situ pH and H2S partial pressure. Include upset conditions, low-pH condensed water during downtime and stimulation-fluid exposure where relevant. Annex C explains partial-pressure determination and alternative parameters, while Annex D provides pH guidance.
Two Routes for SSC Selection and Qualification
Option 1: Apply Annex A.2
Use the stated material and fabrication conditions for SSC-resistant steels. Materials meeting Annex A.2 are generally qualified without additional SSC testing, subject to its exceptions. Any SSC tests required by the manufacturing specification must still be completed successfully and reported. Annex A acceptance does not automatically establish resistance to SOHIC, SZC, HIC or SWC.
Option 2: Qualify for a Specific Application or Range of Service
Clause 7.2 permits selection and qualification for defined sour-service conditions or an SSC severity region. Laboratory qualification follows Annex B and Part 1. Documented field experience may support a specific application under Part 1. Keep the environmental and metallurgical limits of the qualification explicit.
The 2020 edition states that Annex A.3 and A.4 contain no approved steel listings for regions 2 and 1 at publication; they describe typical properties. Do not treat these sections as unconditional approval of any steel with similar properties.
Hardness and Welding Requirements
Annex A.2.1.2 accepts carbon and low-alloy parent steels at a maximum of 22 HRC when the specified composition and heat-treatment conditions are met, including nickel below 1% by mass and exclusion of free-machining steels. This value is a conditional general provision; specific products and materials may have different restrictions.
| Example provision | Requirement to check |
|---|---|
| ASTM A105 forgings | Maximum 187 HBW under A.2.1.3, together with the applicable general restrictions |
| ASTM A234 WPB and WPC wrought fittings | Maximum 197 HBW under A.2.1.3, together with the applicable general restrictions |
| Weld procedure qualification | Normally Vickers HV10/HV5 or Rockwell 15N; HRC use is conditional under 7.3.3.2 |
| Weld root, base metal and HAZ | Table A.1 gives 250 HV or 70.6 HR15N for the relevant survey locations |
| Unexposed weld cap | The 275 HV / 73.0 HR15N alternative requires equipment-user agreement, parent thickness above 9 mm and no direct sour exposure; otherwise the lower limit applies |
Welding procedure specifications and qualification records must be available for equipment-user inspection. Hardness surveys must cover the required locations in weld metal, parent material and heat-affected zones. A single bulk hardness reading cannot replace the prescribed weld survey.
As-welded material meeting the applicable provisions may be acceptable without post-weld heat treatment. Where treatment is required, verify both hardness and the resulting mechanical properties. Alternative weld-hardness acceptance criteria require successful SSC qualification under Annex B.
HIC, SOHIC and Fabrication Considerations
Clause 8 requires consideration of HIC/SWC for flat-rolled carbon-steel products even with trace H2S, together with consideration of testing. Seamless pipe is generally less susceptible, especially when produced by modern methods, but assessment or testing may be justified by the consequences of failure. SSC acceptance alone is insufficient evidence of HIC resistance.
Consider SOHIC and SZC for plate and welded products under Clause 7.2.2. Specify the relevant test method and acceptance criteria before manufacture. Annex B provides qualification guidance, including methods associated with NACE TM0177, TM0284 and TM0316 as applicable.
Cold deformation and thermal cutting can alter the final material condition. Annex A.2.1.6 requires thermal stress relief for manufacturing deformation exceeding 5% permanent outer-fibre strain, subject to its stated exception. Clause 7.4 requires hardness testing during qualification where fabrication changes hardness or leaves a heat-affected zone in the final product. Verify the full conditions for the actual manufacturing route.
Purchasing and Documentation Checklist
Clauses 5 and 9 and Annex E provide the basis for an agreed material specification and traceability. A practical purchasing checklist is:
- Identify the grade, product form, equipment application and applicable product specification.
- State ISO 15156-1 and ISO 15156-2 with the required editions.
- Document sour-service conditions, including H2S exposure, in situ pH, temperature and relevant water chemistry.
- State the accepted SSC selection route and any application-specific qualification limits.
- Define requirements for HIC/SWC and any SOHIC or SZC assessment and testing.
- Specify delivery condition, heat treatment, fabrication and applicable hardness requirements.
- Agree welding qualification, hardness survey methods and any additional acceptance criteria.
- Require material traceability and the supporting test and qualification records.
For special applications qualified under Annex B, traceability must include the environmental conditions of that application. Record the service limits with the material evidence so future operating changes can be assessed against the original qualification basis.
Frequently Asked Questions
Does ISO 15156-2 cover duplex stainless steel?
Duplex and super duplex stainless steels fall under the applicable Part 3 material provisions. Part 2 concerns carbon and low-alloy steels and cast irons.
Is 22 HRC the only acceptance requirement?
No. Composition, heat treatment, product-specific restrictions, welding, service conditions and the relevant cracking mechanisms also matter. Particular products may use different hardness requirements.
Does H2S partial pressure below 0.3 kPa guarantee safety?
No. Highly susceptible steels can crack below that SSC selection threshold. Other mechanisms, including HIC/SWC in flat-rolled products, require separate consideration.
Is HIC testing mandatory for every seamless pipe order?
The document does not impose a universal HIC test for every seamless pipe order. Evaluate the product, service conditions, project requirements and consequences of failure, and agree any required testing.
Does SSC resistance establish HIC resistance?
No. Annex A specifically notes that SSC-resistant steels may require additional provisions for HIC, SWC, SOHIC or SZC.
Does Part 2 replace the pipe manufacturing standard?
No. Product dimensions, manufacture, mechanical properties and acceptance still require the applicable product specification and contract.