Designing equipment for sour service environments entails the use of a steel grade that ensures long-term performance and safety. In oil, gas, and petrochemical facilities, hydrogen sulfide (H₂S) often occurs alongside carbon dioxide (CO₂), chlorides, and water, forming a corrosive and embrittling atmosphere. Pressure-containing equipment like vessels, piping, and exchangers exposed to these conditions can suffer from sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and general corrosion. Such environments demand materials with exceptional resistance to hydrogen embrittlement, thermal stress, and corrosion fatigue. Chrome molybdenum (Cr-Mo) steels, strengthened by the addition of chromium and molybdenum, provide an effective balance of mechanical strength, creep resistance, and chemical stability that can withstand prolonged exposure to sour process conditions while maintaining structural integrity and reliability. Selecting the right Cr-Mo steel grade is fundamental to achieving safe, consistent operation in sour service applications.
Step 1: Comprehend Service Conditions
Material selection begins with a clear understanding of the environment in which equipment like vessels, piping systems, and exchangers will operate. Sour service refers to any condition containing H₂S, and the severity of exposure is influenced by several interacting factors, including:
- H₂S partial pressure or concentration- which increases hydrogen embrittlement risk as H₂S levels rise.
- Temperature and pressure- which affect both corrosion rate and cracking susceptibility.
- Chloride content, pH, CO₂, concentration, and water presence- which contribute to localised corrosion.
Mechanical factors also play a role. Residual stress, weld geometry, and cyclic loading can accelerate cracking, and equipment life and inspection intervals can influence the design safety margins applied to stress and corrosion allowances. These variables are assessed within established industry standards like NACE MR0175 and ISO 15156, which define hardness limits, acceptable microstructures, and testing protocols for materials used in H₂S environments.
Step 2: Evaluate the Capabilities of Cr-Mo Steel
Once a service environment is defined, the next step is to evaluate whether the properties of Cr-Mo steel make it a good choice for sour service. Chromium improves oxidation and corrosion resistance and molybdenum enhances strength, creep resistance, and tolerance to hydrogen attack. Such a combination of characteristics allows Cr-Mo steels to maintain toughness and structural stability under demanding temperature and pressure conditions. When produced with low sulphur content and controlled inclusions, Cr-Mo steels demonstrate a robust tolerance to hydrogen-induced cracking compared to conventional carbon steels, making them a practical and reliable option for equipment operating in sour service.
Step 3: Select a Cr-Mo Steel Grade
Each grade of Cr-Mo steel is designed for specific operating conditions. Choosing the best Cr-Mo steel ensures its performance meets the temperature and pressure requirements of the process equipment and service environment.
SA387 Grade 12 Class 2
Containing around 1.0% chromium and 0.5% molybdenum, Grade 12 provides dependable oxidation resistance with moderate protection against corrosion. It performs effectively in low to medium temperature sour service, offering a good balance of strength and toughness, with a typical tensile strength of 65-85 ksi and a minimum yield strength of 40 ksi.
SA387 Grade 11 Class 2
Grade 11 offers higher strength together with good weldability, ascertained through the addition of around 1.25% chromium and 0.5% molybdenum. It is commonly used in pressure vessels and process piping designed to handle sour gas and hydrocarbon fluids at elevated temperatures. The typical tensile strength of Grade 11 ranges from 75 to 100 ksi, and it has a minimum yield strength of 43 ksi, ensuring reliable operation under demanding service conditions.
SA387 Grade 22 Class 2
Grade 22, with 2.25% chromium and 1.0% molybdenum, provides increased hydrogen resistance and excellent stability under stress. It is utilised in refineries, reactors, and gas separators where sour gas exposure and elevated temperatures coincide. Additionally, Grade 22 has a typical tensile strength of 75 to 100 ksi, with a minimum yield strength of 45 ksi, providing the resilience required for continuous operation.
SA387 Grade 91 Class 2
Offering exceptional creep strength and oxidation resistance for high-temperature service, Grade 91 is imbued with 9.0% chromium and 1.0% molybdenum. It is applied to critical equipment including reactors, heat-recovery systems, and high-pressure steam lines that demand durability and resistance to hydrogen damage. This grade has a tensile strength of 85 to 110 ksi, and a minimum yield strength of 60 ksi, ensuring reliable performance under prolonged elevated temperatures.
Determining the most suitable grade of Cr-Mo steel involves evaluating how temperature, pressure, and environmental exposure interact over the equipment’s expected design life. By aligning material properties with the demands of sour service environments, engineers can preserve structural integrity and minimise the risk of cracking or degradation over time.
Step 4: Verify Standards, Testing, and Documentation
All steels for sour service, including Cr-Mo steels, must meet NAC MR0175 or ISO 15156 standards, which set hardness limits and define testing for hydrogen-induced and sulfide stress cracking.
Verification should include:
- HIC and SSC testing – in accordance with NACE TM0284 and TM0177 procedures.
- Chemical control – sulphur content below 0.01% and inclusion shape control using calcium treatment.
- Post-weld heat treatment (PWHT) – to relieve residual stress and refine the microstructure.
- Traceability – full documentation from mill to finished component to confirm compliance.
Reliable Cr-Mo Steels for Demanding Sour Service Environments
Cr-Mo steels combine strength, toughness and corrosion resistance and are ultimately a reliable choice for sour service environments. If the appropriate grade of Cr-Mo steel is selected and verified, it delivers safe, sustained performance. Masteel supplies ASME SA387 Grades 11, 12, 22, and 91 Class 2, each fully certified and traceable, to support dependable operation in refineries, petrochemical plants, and pressure vessel applications worldwide. Speak with our team to discuss the most appropriate Cr-Mo steel grades for your sour service requirements.