ISO 15156-3:2020 Standard

Where Part 3 Sits in the ISO 15156 Series

The series splits method from material family. Part 3 holds the data, but establishing the service conditions those limits apply to is Part 1’s job, so the two are read together.

Part 3 addresses three cracking mechanisms for selection: SSC, SCC and GHSC. Hardness survey methods are borrowed directly: weld hardness surveys shall be carried out as described in ISO 15156-2:2020, 7.3.3.3, so the butt weld, fillet weld and overlay survey figures are shared between the two parts.

10 Factors Affecting CRA Cracking Resistance

Clause 5 lists ten parameters whose interactions govern CRA behaviour in sour environments. Two of them do not appear in the Part 2 carbon steel list, and both explain why CRA selection works differently.

• Material condition — chemical composition, strength, heat treatment, microstructure, manufacturing method and finished condition
• H₂S partial pressure — or equivalent dissolved concentration in the water phase
• Acidity (in situ pH) — of the water phase
• Chloride or other halide ion concentration
• Presence of oxygen, sulphur or other oxidants
• Exposure temperature
• Pitting resistance of the material in the service environment — unique to Part 3
• Galvanic effects — the GHSC driver
• Total tensile stress — applied plus residual
• Exposure time

Pitting resistance and oxygen are the two additions. Carbon steel in sour service fails by cracking. The standard notes PREN is not directly indicative of corrosion resistance in H₂S environments; it is used as a classification parameter.

PREN: The Formula That Decides Duplex Qualification

Part 3 classifies type 3b and duplex by PREN, and type 3a by (Ni + 2Mo) > 30, not by trade name. The formula is given in Clause 6.3 as Formula (1):

Two rules change how you use that number in practice. The calculation is based on actual composition, not nominal composition; nominal values are for general classification only, so the mill certificate decides, not the datasheet.

The second rule is stricter. All PREN limits in the standard are absolute limits as defined in ASTM E29. An observed or calculated value is not rounded. It is compared directly against the limit. A heat calculating at 39.97 does not round up into the 40.0 band.

Higher PREN is not free. The standard notes that higher values give better corrosion resistance but increase the risk of sigma- and alpha-prime phase formation in the ferrite during manufacture, depending on product thickness and achievable quench rate. The quoted PREN bands are chosen to minimise that problem.

Materials Groups Covered by Annex A (A.2 to A.12)

Annex A divides CRAs into thirteen groups. Within each group, alloys are identified either by materials type, meaning a compositional window, or as individual named alloys. The table maps each group to its Annex A clause and the XTD product families that sit inside it.

Table A.1 of the standard guides you to the right table by equipment type. Separate tables exist for general equipment, for downhole tubular components and packers, and for valve stems, pins and shafts, and the component-specific tables are often less restrictive than the general ones. Reading only the general table can cost you an alloy that would have qualified.

Austenitic Stainless Steels: Annex A.2 Limits

Austenitic stainless of this materials type must contain, as mass fractions, C 0.08 % max, Cr 16 % min, Ni 8 % min, P 0.045 % max, S 0.04 % max, Mn 2.0 % max and Si 2.0 % max. Other alloying elements are permitted. Free-machining austenitic products are prohibited outright.

Three metallurgical conditions apply to the whole materials type. The steel must be solution-annealed and quenched, or annealed and thermally stabilised; must be free of cold work intended to enhance mechanical properties; and must have a maximum hardness of 22 HRC.

Materials Type Envelope and the 316L Extension

The generic austenitic envelope is narrow but simple. The dedicated S31603 rows go much further, and they are the reason 316L appears in sour specifications far more often than its reputation suggests.

Note the trade-off running down the 316L rows. At 60 °C you get 50,000 mg/l chloride at 1,000 kPa H₂S. At 120 °C the S31603 row allows 100 kPa H₂S and 1,000 mg/l chloride, pH ≥ 3.5. Temperature and chloride are against each other, and the table is read as whole rows, never as individual columns.

316L seamless pipe used outside the materials type limits but inside the specific S31603 rows must be solution-annealed and quenched, free from cold work caused by shaping, forming, cold reducing, tension or expansion after final annealing. Hardness and incidental cold work from machining or straightening must stay inside the product specification limits.

Dual certification helps. The standard accepts that it is common practice to dual certify 300 series grades as standard and low carbon, and states that the environmental limits given for low carbon grades are acceptable for the dual certified grades. A 316/316L dual cert carries the S31603 envelope.

S20910 is treated differently again. It is acceptable inside both the materials type limits and its own, in the annealed or hot-rolled condition at a maximum hardness of 35 HRC, far above the 22 HRC that governs the rest of the group.

Two cautions close the table. A limit on martensite content should be considered, and SCC resistance of every austenitic steel in this group can be adversely affected by cold working. The S31603 rows carry an NDS marking for elemental sulphur, meaning no data was submitted on sulphur service.

Highly Alloyed Austenitic Stainless Steels: Annex A.3 Limits

Group A.3 splits into two compositional types. Type 3a requires (%Ni + 2 × %Mo) greater than 30 with Mo at 2 % minimum. Type 3b requires PREN greater than 40.0. Both must be in the solution-annealed condition, and free-machining grades are prohibited.

The 3b envelope is where 6Mo super-austenitics earn their price. 171 °C (340 °F) at 100 kPa H₂S with 5,000 mg/l chloride is well beyond anything 316L reaches. 254 SMO seamless pipe sits in this group with its PREN above 42.

904L seamless pipe qualifies through the type 3a route rather than 3b, because its 4–5 % molybdenum and 23–28 % nickel clear the (%Ni + 2 × %Mo) > 30 test while its typical PREN sits below 40. Classification follows the heat’s actual composition. The route you qualify by changes which envelope rows you may use.

N08926 is called out individually because it was tested to higher chloride limits than the type as a whole: 65,000 mg/l at 121 °C and 700 kPa H₂S, with pH estimated from laboratory test conditions. Table A.8 rows are marked No for elemental sulphur; Tables A.10 and A.11 are NDS.

Solid-Solution Nickel Alloys: Annex A.4 Limits

Group A.4 divides into six types by composition. The annealed types 4a and 4b carry the most useful entry in the entire standard, and the reason is in the Remarks column rather than the numbers.

Annealed alloys of types 4a and 4b carry no limit on temperature, H₂S partial pressure, chloride concentration or in situ pH, and they are marked ‘sulphur-resistant’: Yes. The standard notes only that some combinations of values might not be acceptable even though no individual limits are set.

Incoloy 825 seamless pipe falls into type 4a on its 19.5–23.5 % chromium, 38–46 % nickel and 2.5–3.5 % molybdenum. The annealed condition is mandatory, and that is the full metallurgical requirement for the type.

Inconel 625 seamless pipe also qualifies as type 4a, clearing the 19 % chromium and 2.5 % molybdenum floors comfortably with 20–23 % and 8–10 % respectively. Its 58 % minimum nickel is far above the 29.5 % the type requires.

Hastelloy C-276 seamless pipe reaches type 4b, the higher bracket, through 15–17 % molybdenum against the 12 % floor and a nickel base above the 52 % requirement. Type 4b exists precisely for the Ni-Cr-Mo alloys built for reducing acid service.

Hastelloy C-22 seamless pipe sits in the same type 4b bracket with 12.5–14.5 % molybdenum and 20–22.5 % chromium, trading some molybdenum for chromium to handle oxidising conditions that C-276 finds harder.

Monel 400 seamless pipe is listed individually as N04400 rather than by type, with the same unrestricted environmental envelope but a maximum hardness of 35 HRC and an NDS marking for elemental sulphur. N04405 carries identical treatment.

Cold-Worked Nickel Alloys Lose the Open Envelope

Cold working changes everything. Types 4c, 4d and 4e trade the unrestricted envelope for a graded one where temperature and H₂S buy against each other, and most rows lose elemental sulphur resistance.

Metallurgical limits for Table A.14 cold-worked nickel alloys:

• Hardness ceiling: maximum hardness is capped at 40 HRC for types 4c, 4d and 4e (or 45 HRC for UNS N10276 when used at 121 °C [250 °F] or above)
• Yield strength limits: maximum yield strength derived from cold work is capped at 1,034 MPa (150 ksi) for types 4c and 4d, and 1,240 MPa (180 ksi) for type 4e
• Scope application: the first four environmental rows apply collectively to types 4c, 4d and 4e, while higher-severity/higher-pressure rows in Table A.14 apply specifically to types 4d/4e or type 4e only

Read the last row against the first four. Drop to 132 °C and the H₂S limit disappears while sulphur resistance returns. Cold-worked nickel alloy tubulars are temperature-limited, not H₂S-limited, once operating conditions stay below 132 °C.

Duplex Stainless Steels: Annex A.7 Limits

Duplex is identified by PREN band, not by grade name. Three metallurgical conditions apply to wrought, cast and hot isostatically pressed duplex alike: solution-annealed and liquid-quenched or rapidly cooled by other methods, a ferrite content between 35 % and 65 % by volume, and no ageing heat treatments.

The practical reading is blunt. Standard duplex gets 10 kPa (1.5 psi) H₂S; super duplex doubles that to 20 kPa (3 psi). Both hold 232 °C (450 °F) and 180,000 mg/l chloride. Crossing PREN 40.0 buys you H₂S headroom, nothing else in this table.

Duplex 2205 seamless pipe lands in the 30 to 40.0 band on typical composition, which is why its sour envelope caps at 10 kPa H₂S. The 35 % to 65 % ferrite window is a hard gate alongside it; correct chemistry with ferrite outside that range does not qualify.

Duplex S31803 seamless pipe carries the wider original chemistry window, so individual heats can calculate either side of a PREN boundary. Since PREN limits are absolute and based on actual composition, S32205 with its tightened nitrogen and chromium floors is the safer sour-service buy.

Super duplex 2507 seamless pipe reaches the 20 kPa band only if the heat’s actual PREN is above 40.0 and not above 45. Specify this range on the purchase order. That doubled H₂S allowance is the single clearest commercial reason to specify super duplex over standard duplex in sour duty.

Super duplex S32760 seamless pipe reaches the same band with tungsten in the mix, and the PREN formula counts tungsten at half weight through the 0.5 × %W term. S32750 and S32760 can fall below 40.0 or above 45 depending on the heat. Band assignment follows actual composition.

Downhole Duplex Gets Different Numbers

Table A.25 covers duplex used as downhole tubular components, packers and subsurface equipment. The conditions change: solution-annealed, liquid-quenched and cold-worked, ferrite 35 % to 65 %, and a maximum hardness of 36 HRC, well above the general-equipment expectation.

The envelopes change too. The 30 to 40.0 band drops to 2 kPa (0.3 psi) H₂S but accepts any combination of temperature, chloride and in situ pH occurring in production. The 40.0 to 45 band holds 20 kPa (3 psi) with 120,000 mg/l chloride, and the standard warns that chloride limits here depend strongly on yield strength and level of cold work.

Duplex Welding Carries Microstructural Acceptance Criteria

Duplex welds face checks no other group gets. HAZ hardness shall not exceed the base metal maximum, and weld metal hardness shall not exceed the limit for the consumable alloy. Beyond hardness, a cross-section must be etched and examined at ×400 magnification as part of procedure qualification.

Four acceptance criteria apply to that examination. Grain boundaries shall have no continuous precipitates. Intermetallic phases, nitrides and carbides shall not exceed 1.0 % in total, and sigma phase shall not exceed 0.5 %. Ferrite in the weld root and unreheated cap shall be 30 % to 70 % by volume per ASTM E562, a wider window than the 35 % to 65 % required of base metal.

Hardness Limits Across the CRA Groups

Hardness ceilings vary by group and sometimes by component type within a group. The table collects the limits for the families XTD supplies.

The same averaging tolerance that applies in Part 2 applies here. An individual HRC reading above the permitted value may be accepted if the average of several readings in close proximity stays within the limit and no single reading exceeds it by more than 2 HRC.

Cladding, Overlays and Surface Treatments

Metallic coatings, conversion coatings, plastic coatings and linings may be used but are not acceptable for preventing cracking. The effect of their application on the cracking resistance of the substrate must still be considered.

Nitriding to a maximum case depth of 0.15 mm (0.006 in) is acceptable as a surface treatment if conducted below the lower critical temperature of the alloy. The standard adds a direct prohibition: the use of nitriding as a means of preventing cracking in sour service is not acceptable.

Clause A.13 governs the composition and cracking resistance of overlays. This is where Part 2 and Part 3 meet: a carbon steel substrate follows Part 2 hardness and heat-treatment rules while A.2–A.11 materials may be used as cladding or overlay. The base material must still conform with Part 2 or Part 3 unless long-term integrity of the layer is documented. N06625 overlays are limited to 5 % or 10 % Fe. Wear-resistant alloys have no ISO 15156 limits.

Machine-cut threads are acceptable. Cold-formed rolled threads are acceptable on CRAs provided the material and its application limits otherwise conform. Controlled shot-peening is permitted at maximum shot size 2.0 mm (0.080 in) and Almen intensity no greater than 10C, controlled per SAE AMS-2430.