ISO 15156-2:2020 Standard

Where Part 2 Sits in the ISO 15156 Series

Three parts divide the work between method and material family. Reading Part 2 without Part 1 leaves you with limits and no way to establish the service conditions they apply to.

The three parts share two test standards. NACE TM0177 covers laboratory SSC and SCC testing; NACE TM0284 covers HIC evaluation of pipeline and pressure vessel steels. NACE TM0316 is referenced for four-point bend assessment, a reference change introduced in the 2020 edition. The ISO 15156-1 standard page sets out the selection method, and the ISO 15156-3 standard page carries the CRA envelopes.

9 Factors Affecting Carbon Steel Behaviour in H₂S Environments

Clause 6 lists nine parameters whose complex interactions govern how carbon and low-alloy steels behave in sour environments. All nine must be considered when selecting material under this document; the standard does not treat any as optional.

• Material condition — chemical composition, manufacturing method, product form, strength, hardness and its local variations, cold work, heat-treatment condition, microstructure, microstructural uniformity, grain size and cleanliness
• H₂S partial pressure — or the equivalent concentration in the water phase
• Chloride ion concentration — in the water phase
• Acidity (pH) — of the water phase
• Presence of sulphur or other oxidants
• Exposure to non-production fluids — stimulation acids and their backflow sit here
• Exposure temperature
• Total tensile stress — applied plus residual
• Exposure time

Note what sits at the top of that list. The standard states that hardness control can be an acceptable means of obtaining SSC resistance (7.3.1).

2 Options for Selecting SSC-Resistant Steels

Part 2 gives two selection routes. Option 1 is a lookup against Annex A and needs no testing. Option 2 qualifies a steel for a specific application or a defined range of sour service, and it can override the Annex A limits for that material.

Option 1: Selection Using Annex A.2

If the H₂S partial pressure in the gas is 0.3 kPa (0.05 psi) or greater, SSC-resistant steels shall be selected using Annex A.2. Steels conforming with A.2 are qualified without further SSC testing, with stated exceptions. The majority of conforming steels are not individually listed; Tables A.2 to A.4 give examples rather than an exhaustive set.

Below 0.3 kPa (0.05 psi) the standard does not address selection in detail and normally no special precautions are required. Highly susceptible steels can still crack at that level, so the absence of a requirement is not a guarantee of immunity.

Option 2: Qualification for Specific Applications or Service Ranges

Option 2 lets the user qualify steels for specific sour-service applications or for ranges of sour service. The limits established by Option 2 qualification may replace any limits listed for that material in Annex A.2. Using it requires knowledge of both in situ pH and H₂S partial pressure, plus how each varies with time.

The commercial case is specific. Option 2 suits bulk purchases of OCTG and line pipe, where the saving from using a steel outside Annex A outweighs the qualification cost. It also supports fitness-for-purpose evaluation of existing equipment now facing conditions more severe than the original design assumed.

4 SSC Regions of Environmental Severity: Region 0, 1, 2 and 3

Figure 1 of the standard maps H₂S partial pressure against in situ pH and divides the result into four regions. The region you land in decides which part of Annex A you may select from, and severity rises as H₂S climbs and pH falls.

Two discontinuities appear in Figure 1, below 0.3 kPa (0.05 psi) and above 1 MPa (150 psi) H₂S partial pressure. They reflect measurement uncertainty at low partial pressure and uncertainty about steel performance outside those limits at both ends of the range.

When defining severity, the standard asks users to account for exposure to unbuffered, condensed aqueous phases of low pH during upset conditions or downtime. Acids used for well stimulation and the backflow of spent stimulation acid belong in the same assessment.

Region 0 Still Carries Risk

Four conditions deserve attention even in Region 0. Highly susceptible steels can crack. Physical and metallurgical properties still govern inherent resistance. Stress concentrations raise the risk. And above roughly 965 MPa (140 ksi) yield strength, very high-strength steels can suffer HSC in aqueous environments with no H₂S present at all.

What Regions 2 and 3 Mean for Steel Choice

At publication there were no listings of steels approved specifically for Region 2 (A.3) or Region 1 (A.4). Both annexes describe typical properties rather than naming grades. For Region 2, Cr-Mo low-alloy tubulars have proven acceptable at actual yield up to 760 MPa (110 ksi) and hardness no more than 27 HRC, with pipeline steels up to 450 MPa (65 ksi) SMYS and weld hardness not exceeding 280 HV.

Region 1 allows more. Cr-Mo tubulars have proven acceptable at actual yield up to 896 MPa (130 ksi) and hardness no more than 30 HRC, with pipeline steels up to 550 MPa (80 ksi) SMYS and weld hardness not exceeding 300 HV. Region 3 permits neither relaxation — A.2 is the only source.

Hardness Requirements for Parent Metal

Carbon and low-alloy steels are acceptable at 22 HRC maximum hardness provided they contain less than 1 % mass fraction nickel, are not free-machining steels, and are supplied in one of six permitted heat-treatment conditions. The 22 HRC limit comes from correlations of heat treatment, chemical composition, hardness and field failure experience built up across every revision of NACE MR0175.

The standard allows a small tolerance on individual readings. An HRC reading above the permitted value may be accepted if the average of several readings taken in close proximity stays within the limit and no single reading exceeds it by more than 2 HRC. Equivalent rules apply to other hardness methods when specified.

Weld Hardness Acceptance Criteria

Welding changes the metallurgy, and the changed metallurgy drives susceptibility to SSC, SOHIC and SZC. Qualification of welding procedures for sour service must include hardness testing, and Table A.1 sets the ceilings.

The 275 HV weld cap allowance is conditional, not automatic. All three conditions must be met: the equipment user agrees the alternative limit, parent material is over 9 mm thick, and the weld cap is not exposed directly to the sour environment. Otherwise the limit reverts to 250 HV or 70.6 HR 15N.

As-welded carbon, carbon-manganese and low-alloy steels meeting Table A.1 do not require post-weld heat treatment. Weldments failing those requirements must be PWHT after welding, with a minimum PWHT temperature of 621 °C (1,150 °F) for low-alloy steels.

HV 10 or HV 5 per ISO 6507-1, or Rockwell 15N per ISO 6508-1, are the normal methods. The HRC method is permitted for welding procedure qualification only where design stress does not exceed two-thirds of SMYS and the procedure includes PWHT; any other use needs the equipment user’s agreement. Vickers and 15N surveys give a more detailed picture and may detect small hard zones that an HRC survey misses.

Qualified Carbon Steel Grades Under Annex A

Annex A gives example lists rather than a closed set. Table A.2 covers tubular products, Table A.3 covers casing and tubing by temperature, and Table A.4 covers bolting. Pressure vessel steels classified P-No 1 Group 1 or 2 in ASME Section IX are acceptable.

Tubular Products (Table A.2)

Examples of tubular products that can conform with the general requirements of A.2.1.

Pipe seam welds shall conform with A.2.1. A qualified parent metal with an unqualified seam weld does not satisfy the standard, and the seam is where hardness excursions most often appear in welded line pipe.

Casing and Tubing by Minimum Service Temperature (Table A.3)

Higher-strength casing and tubing grades become acceptable only above a minimum service temperature. The temperatures are minimum allowable service temperatures with respect to SSC; low-temperature toughness is a separate assessment the equipment user must make.

Q125 carries an extra restriction. Types 1 and 2 are based on Q&T Cr-Mo chemistry to 1,036 MPa (150 ksi) maximum yield, and C-Mn steels are not acceptable in that grade. R95 and the former C95 designation are deemed equivalent.

Bolting and Fasteners (Table A.4)

Bolting exposed directly to a sour environment, buried, insulated, fitted with flange protectors or otherwise denied atmospheric or open seawater exposure must conform to A.2.1. Acceptable materials are ASTM A193 grade B7M and ASTM A320 grade L7M for bolts, with ASTM A194 grades 2HM and 7M for nuts. Using SSC-resistant bolting can require lowering equipment pressure ratings.

Cold Deformation and Stress Relief Rules

Carbon and low-alloy steels shall be thermally stress-relieved following any cold deforming that produces permanent outer fibre deformation greater than 5 %. The minimum stress-relief temperature is 593 °C (1,100 °F) and final maximum hardness is 22 HRC, except for A234 WPB and WPC fittings at 197 HBW.

Three carve-outs apply. Cold work from pressure testing to an applicable code does not trigger the rule. Cold-rotary straightened pipe is acceptable only where the applicable ISO or API product standard permits it. Cold-worked line pipe fittings in A53 Gr B, A106 Gr B, API 5L X-42, ISO 3183 L290 or lower grades are acceptable at 15 % strain or less provided strained-area hardness stays at or below 190 HBW.

For tubulars, cold-straightening at or below 510 °C (950 °F) requires stress relief at a minimum of 482 °C (900 °F). Cold-formed regions exceeding 5 % outer fibre deformation need 593 °C (1,100 °F). Cold-formed connections on tubulars above 22 HRC need the same 593 °C treatment.

Surface treatments follow separate rules. Metallic coatings, conversion coatings, plastic coatings and linings are not acceptable for preventing SSC. Nitriding to a maximum case depth of 0.15 mm (0.006 in) is acceptable below the lower critical temperature. Controlled shot-peening is acceptable at maximum shot size 2.0 mm (0.080 in) and Almen intensity no greater than 10C.

HIC and SWC Evaluation of Flat-Rolled Products

The equipment user shall consider HIC and SWC when evaluating flat-rolled carbon steel products for sour environments containing even trace amounts of H₂S, and shall consider HIC/SWC testing of those products. This duty applies below the 0.3 kPa threshold that governs SSC selection.

Sulphur level is the dominant variable. Typical maximum acceptable levels are 0.003 % mass fraction for flat-rolled products and 0.01 % mass fraction for seamless products. Conventional forgings below 0.025 % sulphur, and castings, are not normally considered sensitive to HIC or SOHIC.

Seamless pipe sits in a different risk class. HIC and SWC leading to loss of containment has occurred only rarely in seamless pipe and other non-flat-rolled products, and modern seamless pipe is much less sensitive than older product. The standard still sees benefit in evaluating seamless pipe where the consequences of failure justify it.

Rust, sulphur or oxygen in the service environment, particularly alongside chloride, is thought to raise the probability of damage. SOHIC and SZC occurrence is described as rare and not well understood, restricted in all likelihood to carbon steels, with sudden failures recorded in parent steels and in weld HAZs.

Laboratory Qualification Under Annex B

Annex B also covers SOHIC and SZC evaluation (Clause B.4): SSC qualification must precede SOHIC testing, and acceptance requires no ladder-like cracking or cracks exceeding 0.5 mm in the through-thickness direction. For HIC/SWC testing of flat-rolled product under Clause B.5 and Table B.3, evaluation per NACE TM0284 requires Crack Length Ratio CLR ≤ 15 %, Crack Thickness Ratio CTR ≤ 5 %, and Crack Surface Ratio CSR ≤ 2 %.

Acceptance is binary: no SSC cracks, assessed per NACE TM0316 for four-point bend or NACE TM0177 for the other methods. A minimum of three specimens is tested per test batch. If one specimen fails, the cause shall be investigated. If the source material conforms to the manufacturing specification, two further specimens from the same source may be tested, and the batch is qualified if both pass. Further retests require the purchaser’s agreement.

Cr-Mo tubulars at the 690–760 MPa (100–110 ksi) SMYS grades carry a mandatory testing requirement. SSC resistance must be demonstrated by testing each test batch using the UT test, and the maximum yield for each grade shall be no more than 103 MPa (15 ksi) above the SMYS.

When Carbon Steel Runs Out of Road

Part 2 has an edge. Part 2 addresses cracking only. Steels conforming with A.2 are accepted for SSC in all regions of Figure 1, and Part 2 sets no chloride or upper-temperature limit for them. The move to a CRA is usually driven by general or localized corrosion, which this standard does not address, or by service outside Part 2’s scope. Where a CRA is chosen, Part 3 governs.

The first step is usually austenitic stainless. 316L seamless pipe qualifies under ISO 15156-3 Table A.2 at 22 HRC maximum in the solution-annealed and quenched condition — the same numerical hardness ceiling as carbon steel, but reached through an entirely different metallurgy that also resists general corrosion.

Where chloride and strength requirements both rise, duplex takes over. Duplex 2205 seamless pipe must be solution-annealed and liquid-quenched with 35 % to 65 % ferrite and no ageing heat treatments (ISO 15156-3, Table A.24), and its 450 MPa (65 ksi) minimum yield exceeds most of the API 5L grades Part 2 lists for sour line pipe.

For moderate H₂S with high chloride, super duplex (PREN > 40.0 to 45) allows up to 20 kPa H₂S (ISO 15156-3, Table A.24). Above that, nickel alloys apply.

Super duplex 2507 seamless pipe has a 550 MPa (80 ksi) minimum yield. It qualifies for the 20 kPa band only if the heat’s actual PREN is above 40.0 and not above 45.

For the most aggressive combinations, nickel alloys replace stainless steel entirely. Incoloy 825 seamless pipe handles reducing acid chemistry through its 1.5–3.0 % copper addition in the annealed condition (Type 4a).

Inconel 625 seamless pipe sits above that with 8–10 % molybdenum and 3.15–4.15 % niobium. Where sour service combines high H₂S, high chloride and elevated temperature in one stream, 625 is the common answer — often as a clad or overlay layer on a carbon steel substrate rather than solid wall.

That overlay route connects the two parts directly. Part 2 Clause A.2.1.5 accepts thermally applied overlays provided the substrate heat-treated condition is unchanged or the base metal hardness conforms with A.2.1.2 and A.2.1.4. The composition and cracking resistance of the overlay itself are governed by ISO 15156-3, not Part 2.

At the top of the hierarchy, Hastelloy C-276 seamless pipe runs 15–17 % molybdenum with 3–4.5 % tungsten. Few production environments justify this much alloy, and the ones that do usually involve free sulphur at temperature.