---
title: "NACE MR0175 / ISO 15156: Stainless Bar for Sour Service"
source: "https://www.laxconsteels.com/nace-mr0175-iso-15156-norsok-m-650/"
description: "NACE MR0175 / ISO 15156 and NORSOK M-650 explained: sulphide stress cracking, the three parts, stainless and duplex limits, hardness, MR0103."
---
# NACE MR0175 / ISO 15156 and NORSOK M-650: Stainless Bar for Sour and Offshore Service

Send an enquiry

10 min read

NACE MR0175 / ISO 15156 is the international standard for metallic materials in oil and gas production equipment that is exposed to hydrogen sulphide (H2S). It sets the conditions under which carbon steels, low-alloy steels and corrosion-resistant alloys resist the cracking that H2S causes, and it does so through limits on composition, heat treatment, hardness and the service environment. NORSOK M-650 is a standard of the Norwegian petroleum industry with a different purpose: it qualifies the manufacturer, not the material, and confirms that a producer of duplex and high-alloy austenitic stainless steels has the experience, equipment and procedures to make them with acceptable properties. A stainless bar for sour or offshore service is therefore described twice: its grade, condition and hardness fall within the limits of ISO 15156 for the stated environment, and, where a NORSOK order applies, its manufacturer is qualified to NORSOK M-650 for that material and product form.

## Sour service and sulphide stress cracking

Sour service is service in a fluid that contains H2S together with water. In the presence of water, H2S corrodes the steel surface and releases atomic hydrogen. Sulphide slows the recombination of these atoms into hydrogen gas, so more of them diffuse into the metal. Inside the steel the hydrogen collects at points of high stress and at hard microstructures, and it lowers the stress at which a crack can grow.

Sulphide stress cracking (SSC) is the resulting failure: a brittle crack in a part under tensile stress, which can be a stress from service or a residual stress from welding or cold work. It occurs at stresses well below the yield strength. Hard steels are the most susceptible, which is why hardness is the central control in the standard. Corrosion-resistant alloys can also fail by stress corrosion cracking in the same environment, where chloride, temperature and acidity act together with H2S. A grade that resists one mechanism is not thereby shown to resist the other. The general forms of stress corrosion cracking in stainless steel are described in the [corrosion of stainless steel](https://www.laxconsteels.com/corrosion-of-stainless-steel/) article.

## The three parts of NACE MR0175 / ISO 15156

The standard began as NACE MR0175 and has been published jointly with ISO as a three-part document since 2003. The current text is the fourth edition, ISO 15156-1:2020, ISO 15156-2:2020 and ISO 15156-3:2020, published by AMPP as ANSI/NACE MR0175-2021 / ISO 15156 together with later technical circulars. The parts are:

- **ISO 15156-1**, general principles for the selection of cracking-resistant materials: scope, definitions, and the responsibilities of the equipment user and the material supplier.
- **ISO 15156-2**, cracking-resistant carbon and low-alloy steels and the use of cast irons. It defines the H2S partial pressure of 0.3 kPa (0.05 psi) below which special precautions against SSC are normally not required for these steels, and it sets their general hardness limit of 22 HRC.
- **ISO 15156-3**, cracking-resistant corrosion-resistant alloys (CRAs) and other alloys. It covers stainless steels, nickel alloys, cobalt alloys and titanium, each in a material group with its own table of acceptable conditions and environmental limits.

The equipment user defines the service environment: the H2S partial pressure, temperature, chloride concentration, in situ pH and the presence of elemental sulphur. The manufacturer of the material is responsible for meeting the metallurgical requirements that apply to that environment. The standard contains no certification scheme for a material. A bar therefore meets ISO 15156, or lies within the limits of a stated table for a stated environment; it is not "NACE certified".

## Stainless steels in ISO 15156-3

ISO 15156-3 divides the stainless steels into material groups by microstructure. Each group is accepted in a defined metallurgical condition, and its use is limited by an environmental envelope that becomes narrower as the alloy becomes less resistant. The groups that apply to stainless bar are the austenitic, duplex, martensitic and precipitation-hardening steels.

*Stainless steel groups in ISO 15156-3 with the condition, hardness requirement and environmental limit for each*

| Group | Typical bar grades | Required condition | Hardness | Environmental limit (general case) |
| --- | --- | --- | --- | --- |
| Austenitic | [304](https://www.laxconsteels.com/grades/aisi-304/), [316](https://www.laxconsteels.com/grades/aisi-316/), [321](https://www.laxconsteels.com/grades/aisi-321/), [347](https://www.laxconsteels.com/grades/aisi-347/) and their L grades | Solution annealed and quenched, or annealed and stabilised; free of cold work used to raise strength | 22 HRC maximum | Up to 60 °C at an H2S partial pressure up to 100 kPa (15 psi), any chloride and pH |
| Duplex, PREN 30 to 40 and molybdenum 1.5 percent or more | [2205](https://www.laxconsteels.com/grades/duplex-2205/) (UNS S32205, S31803) | Solution annealed and quenched; ferrite 35 to 65 percent | No limit set for solution annealed wrought material | A maximum temperature and a maximum H2S partial pressure set by the duplex table |
| Duplex, PREN over 40 | [2507](https://www.laxconsteels.com/grades/super-duplex-2507/) (UNS S32750) | Solution annealed and quenched; ferrite 35 to 65 percent | No limit set for solution annealed wrought material | The same temperature limit with a higher permitted H2S partial pressure |
| Martensitic | [410](https://www.laxconsteels.com/grades/aisi-410/), [420](https://www.laxconsteels.com/grades/aisi-420/) | Tempered to the condition the table sets | Maximum hardness set by the table | A low H2S partial pressure with a minimum pH |
| Precipitation hardening | [17-4 PH](https://www.laxconsteels.com/grades/17-4-ph/) | Double aged at 620 °C (H1150D) or H1150M | 33 HRC maximum | Restricted to listed components, with environmental limits |

The austenitic group is defined by composition rather than by grade number: carbon 0.08 percent maximum, chromium 16 percent minimum and nickel 8 percent minimum, with limits on phosphorus, sulphur, manganese and silicon. The common 300 series bar grades fall inside it. Above 60 °C an austenitic grade is accepted only under narrower conditions, such as a low chloride content. Duplex grades are classed by the pitting resistance equivalent number, which ISO 15156-3 calculates as PREN = Cr + 3.3 (Mo + 0.5 W) + 16 N in weight percent, and they are accepted at temperatures far above the austenitic limit. What a PREN figure does and does not promise is set out in the [PREN reference](https://www.laxconsteels.com/pren-stainless-steel-what-your-grade-guarantees/).

The numerical limits for each group, and the alternative envelopes that trade temperature against chloride or H2S, are tabulated in Annex A of ISO 15156-3 and have been changed by technical circulars since 2020. A design is checked against the edition and circulars named in the purchase specification, not against a summary of this kind.

## Hardness limits and material condition

Hardness is the control the standard uses most because it is a measurable proxy for the strength and microstructure that govern hydrogen cracking. A hard steel has a high yield strength, high local stresses and microstructures that trap hydrogen, and each of these raises the risk of SSC. The same 22 HRC limit applies to carbon and low-alloy steels in Part 2 and to austenitic stainless steels in Part 3. For carbon and low-alloy steels it applies to the parent metal, to weld metal and to the heat-affected zone, where welding can produce local hard spots in a part that is otherwise soft.

The condition requirement matters as much as the number. Austenitic stainless steel cannot be hardened by heat treatment, but it hardens strongly when cold worked, and a cold-drawn or strain-hardened bar can exceed 22 HRC. The standard therefore excludes cold work intended to raise strength, and a solution-annealed austenitic bar meets the condition. Duplex steel takes its sour resistance from the balance of ferrite and austenite set by solution annealing and quenching, so the standard sets a ferrite range for it rather than a hardness value. Martensitic and precipitation-hardening steels are accepted only in heavily tempered or over-aged conditions, which trade strength for resistance to cracking. The heat treatments behind these conditions are described in the [heat treatment of steel](https://www.laxconsteels.com/heat-treatment-of-steel/) article.

## NACE MR0175 and NACE MR0103

NACE MR0103 is the companion standard for sour environments in petroleum refining and related processing, rather than in oil and gas production. Its current form is ANSI/NACE MR0103 / ISO 17945:2015, titled metallic materials resistant to sulphide stress cracking in corrosive petroleum refining environments, reaffirmed in 2023. It applies to refinery equipment exposed to H2S as a gas or dissolved in water.

The two standards share their metallurgy but differ in method. MR0175 / ISO 15156 pairs each material with an environmental envelope defined by temperature, H2S partial pressure, chloride and pH. MR0103 relies mainly on condition and hardness limits for the refinery environment. The requirements for austenitic stainless steels, solution annealed, free of strengthening cold work and at 22 HRC maximum, are the same in both. For duplex stainless steel MR0103 sets a hardness limit of 28 HRC, while MR0175 sets none for solution-annealed wrought duplex. Production and wellhead equipment falls under MR0175, refinery equipment under MR0103.

## NORSOK M-650: qualification of manufacturers of special materials

NORSOK M-650, Qualification of manufacturers of special materials, is published by Standards Norway. Edition 4, of September 2011, is the current edition. It establishes a set of requirements to verify that a manufacturer has the competence and experience for the relevant material grades, and the facilities and equipment to make them in the required shapes and sizes with acceptable properties. It applies to duplex stainless steel and high-alloy austenitic stainless steel in all grades, product forms and dimensions, and to castings in nickel-base alloys and titanium. The materials are called special because their properties depend heavily on how they are melted, hot worked and heat treated, and a correct chemical analysis does not prove a correct structure.

Qualification rests on six elements: knowledge and manufacturing experience with the material type; acceptable manufacturing facilities and equipment; a manufacturing summary that covers every step from melting or the semi-finished product to the finished product; an established heat treatment procedure, verified by a temperature uniformity test of the furnace; a quality management system that meets ISO 9001; and the results of qualification testing on production material. A qualifying company reviews the facilities, equipment and procedures at the manufacturer's premises, and the evidence is collected in a qualification test record. The qualification holds only within the parameters the standard sets, such as the material grade, product form and dimensions qualified, and it is valid for five years from acceptance.

NORSOK M-650 is used together with NORSOK M-630, the collection of material data sheets that sets the material requirements for each grade and product form in piping and other pressure equipment. ISO 17782:2018, Scheme for conformity assessment of manufacturers of special materials, is based on NORSOK M-650 Edition 4 and carries the same approach into an international standard.

## Duplex bar for offshore service

Duplex stainless steel combines a yield strength about twice that of 316 with resistance to chloride stress corrosion cracking. Its properties depend on three features of the microstructure that a chemical analysis does not reveal, and the testing of offshore duplex bar is directed at them.

The first is the phase balance. Solution annealing and rapid quenching set the proportions of ferrite and austenite, and ISO 15156-3 accepts duplex steel only with ferrite between 35 and 65 percent. Too much ferrite lowers toughness; too little lowers strength and resistance to stress corrosion cracking. Ferrite content is measured by point counting on a polished section, the method of ASTM E562. The second is intermetallic phase. Sigma and chi phases form in duplex steel held in the upper part of its unsafe temperature range, during slow cooling after annealing or in a heavy section that cools slowly at its centre. Small amounts of these phases reduce impact toughness sharply and take chromium and molybdenum from the surrounding metal. ASTM A923 sets methods for detecting them in duplex steel. The third is toughness at low temperature. The ferrite in duplex steel undergoes a ductile-to-brittle transition, so offshore material data sheets call for Charpy impact testing at low temperature, together with a pitting corrosion test to ASTM G48 at a stated temperature.

A bar that meets the chemistry of UNS S32205 but was cooled too slowly can fail all three checks. The grades, the lean, standard and super duplex classes and the fabrication limits of the family are set out in the [duplex stainless steel guide](https://www.laxconsteels.com/what-are-duplex-stainless-steels-a-simplified-guide/).

## Sour and offshore bar from Laxcon Steels

Laxcon Steels is qualified as a manufacturer of special materials to NORSOK M-650 Edition 4 by TUV India for hot rolled bars in ASTM A479 grade 316 and in UNS S32205 and S31803. The qualification is listed on the [quality page](https://www.laxconsteels.com/quality/). Engineers India Limited has enlisted the company as a supplier of billets and blooms in duplex stainless steel. Laxcon Steels melts and casts its own billets and rolls [hot rolled round bar](https://www.laxconsteels.com/products/hot-rolled-round-bars/) from 16 to 125 mm, and supplies [duplex steel bar](https://www.laxconsteels.com/products/duplex-steel/) in grades including 1.4462, UNS S31803, UNS S32205 and super duplex 1.4410. Bar is supplied to specifications including NACE MR0175 and MR0103; a sour-service order to NACE MR0175 / ISO 15156 adds a hardness limit and a condition requirement. Its NABL-accredited laboratory carries out mechanical testing, automatic impact testing and metallography. The grade pages for [duplex 2205](https://www.laxconsteels.com/grades/duplex-2205/) and [2205 hot rolled round bar](https://www.laxconsteels.com/products/hot-rolled-round-bars/duplex-2205/) list the composition and mechanical properties.

## Frequently asked questions

### Is there a NACE certificate for stainless steel?

No. NACE MR0175 / ISO 15156 contains no certification scheme for a material. A bar meets the standard when its grade, heat treatment condition and hardness satisfy the relevant table for the service environment, which the equipment user defines.

### Does 316 stainless steel meet NACE MR0175?

Solution-annealed 316 bar free of strengthening cold work and at 22 HRC or below falls within the austenitic group of ISO 15156-3. Its general limit is 60 °C at an H2S partial pressure of up to 100 kPa, with any chloride content.

### Is duplex 2205 acceptable for sour service?

Duplex 2205 is accepted under ISO 15156-3 in the solution-annealed and quenched condition with a ferrite content of 35 to 65 percent. Its PREN places it in the 30 to 40 class, whose use is limited by a maximum temperature and a maximum H2S partial pressure. NACE MR0103 adds a 28 HRC hardness limit for refinery service.

### What is the difference between NACE MR0175 and NACE MR0103?

NACE MR0175 / ISO 15156 covers oil and gas production equipment and pairs each material with environmental limits. NACE MR0103 / ISO 17945 covers petroleum refining and relies mainly on condition and hardness limits. Their requirements for austenitic stainless steel are the same; for duplex steel MR0103 adds a 28 HRC limit.

### What is NORSOK M-650?

NORSOK M-650 is a Norwegian petroleum industry standard that qualifies manufacturers of special materials, chiefly duplex and high-alloy austenitic stainless steels. A qualification covers the tested grades, product forms and dimensions and is valid for five years.

## Sources

- ISO 15156-1:2020, Petroleum and natural gas industries, materials for use in H2S-containing environments in oil and gas production, Part 1: general principles for selection of cracking-resistant materials.
- ISO 15156-2:2020, Part 2: cracking-resistant carbon and low-alloy steels, and the use of cast irons.
- ISO 15156-3:2020, Part 3: cracking-resistant CRAs (corrosion-resistant alloys) and other alloys: Annex A, the austenitic and duplex material tables.
- ANSI/NACE MR0175-2021 / ISO 15156, with technical circulars, AMPP.
- ANSI/NACE MR0103 / ISO 17945:2015, Metallic materials resistant to sulphide stress cracking in corrosive petroleum refining environments.
- NORSOK M-650, Qualification of manufacturers of special materials, Edition 4, September 2011, Standards Norway.
- NORSOK M-630, Material data sheets and element data sheets for piping, Standards Norway.
- ISO 17782:2018, Petroleum, petrochemical and natural gas industries, scheme for conformity assessment of manufacturers of special materials.
- ASTM E562, Standard Test Method for Determining Volume Fraction by Systematic Manual Point Count.
- ASTM A923, Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels.
- ASTM G48, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution.

![Duplex stainless steel bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-duplex-bars.webp)

Enquire about **Duplex Steel**

First name Last name Email Phone Company Location

Product

- Bright Bars
- Precision Shaft Quality (PSQ) Bars
- Pipes and Tubes
- Hexagonal Bright Bars
- Square Bright Bars
- Cold-Drawn Flat Bars
- Forged & Proof Machined Bars
- HRAP Flat Bars
- HRAP Equal Angles
- HRAP Unequal Angles
- Threaded Bars
- Hot Rolled Round Cornered Squares (RCS) Bars
- Hot Rolled Round Bars
- Continuous Cast Billets / Blooms
- Channels
- Forging Quality Ingots
- Precipitation Hardening Steels
- Duplex Steel
- T-Profile
- Others

Message

\* Required

## From the Mill

- [![Duplex stainless steel bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-duplex-bars.webp) Duplex Steel](https://www.laxconsteels.com/products/duplex-steel/)
- [![Stainless steel hot rolled round bars produced by Laxcon Steels](https://www.laxconsteels.com/images/products/stainless-steel-hot-rolled-round-bars.webp) Hot Rolled Round Bars 16 mm to 125 mm](https://www.laxconsteels.com/products/hot-rolled-round-bars/)
- [![Optical emission spectrometer used to certify grade composition](https://www.laxconsteels.com/images/quality/laxcon-steels-optical-emission-spectrometer.webp) Grade reference](https://www.laxconsteels.com/grades/)