Designing equipment for sour service environments requires a steel grade that delivers long-term performance and safety. In oil, gas, and petrochemical facilities, hydrogen sulfide (H₂S) often occurs alongside carbon dioxide (CO₂), chlorides, and water. As a result, these elements create a corrosive and embrittling environment.
Pressure equipment such as vessels, piping, and exchangers can suffer from sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), and general corrosion under these conditions. Therefore, these environments require materials with strong resistance to hydrogen embrittlement, thermal stress, and corrosion fatigue.
Chrome molybdenum (Cr-Mo) steels contain chromium and molybdenum, which improve strength and stability. In addition, these steels offer a balance of mechanical strength, creep resistance, and chemical durability. As a result, they perform reliably under prolonged exposure to sour service conditions while maintaining structural integrity. Ultimately, selecting the right Cr-Mo grade supports safe and consistent operation.
Step 1: Understand Service Conditions
Material selection starts with a clear understanding of the operating environment. Sour service refers to any condition that contains H₂S. However, several key factors influence severity:
- H₂S partial pressure or concentration, which increases the risk of hydrogen embrittlement.
- Temperature and pressure, which affect corrosion rates and cracking risk.
- Chlorides, pH, CO₂ levels, and water presence, which drive localised corrosion.
Mechanical factors also influence performance. For example, residual stress, weld geometry, and cyclic loading can accelerate cracking. In addition, equipment life and inspection intervals affect safety margins.
To ensure consistency, industry standards such as NACE MR0175 and ISO 15156 define hardness limits, acceptable microstructures, and testing requirements for H₂S environments.
Step 2: Evaluate Cr-Mo Steel Properties
After defining the service environment, assess whether Cr-Mo steel suits the application. Chromium improves oxidation and corrosion resistance, while molybdenum increases strength, creep resistance, and resistance to hydrogen attack.
Together, these elements allow Cr-Mo steels to maintain toughness and stability under high temperature and pressure. In particular, when manufacturers control sulphur content and inclusions, these steels show strong resistance to hydrogen-induced cracking compared to carbon steel. Therefore, they remain a reliable option for sour service.
Step 3: Select a Cr-Mo Steel Grade
Each Cr-Mo grade suits specific operating conditions. Therefore, selecting the correct grade ensures performance meets process and environmental demands.
For example:
This grade provides higher strength and good weldability. In addition, it contains about 1.25% chromium and 0.5% molybdenum. Engineers use it in pressure vessels and piping for sour gas and hydrocarbons at elevated temperatures. Tensile strength ranges from 75 to 100 ksi, with a minimum yield strength of 43 ksi.
This grade contains around 1.0% chromium and 0.5% molybdenum. As a result, it offers solid oxidation resistance and moderate corrosion protection. It performs well in low to medium temperature sour service. Typical tensile strength ranges from 65 to 85 ksi, with a minimum yield strength of 40 ksi.
This grade includes 2.25% chromium and 1.0% molybdenum. As a result, it offers improved hydrogen resistance and stability under stress. It is widely used in refineries, reactors, and gas separators. Tensile strength ranges from 75 to 100 ksi, with a minimum yield strength of 45 ksi.
This grade delivers excellent creep strength and oxidation resistance at high temperatures. In particular, it contains 9.0% chromium and 1.0% molybdenum. It suits critical equipment such as reactors, heat recovery systems, and high-pressure steam lines. Tensile strength ranges from 85 to 110 ksi, with a minimum yield strength of 60 ksi.
Overall, selecting the right grade requires evaluating temperature, pressure, and environmental exposure over the equipment’s design life. By doing so, engineers can maintain structural integrity and reduce failure risk.
Step 4: Verify Standards, Testing, and Documentation
All Cr-Mo steels for sour service must meet NACE MR0175 or ISO 15156 standards. Specifically, these standards define hardness limits and testing requirements for SSC and HIC.
Verification should include:
- HIC and SSC testing in line with NACE TM0284 and TM0177.
- Chemical control, including sulphur below 0.01% and inclusion control through calcium treatment.
- Post-weld heat treatment (PWHT) to reduce residual stress and refine microstructure.
- Full traceability from mill to finished product to confirm compliance.
Reliable Cr-Mo Steels for Sour Service
Cr-Mo steels combine strength, toughness, and corrosion resistance. Therefore, they remain a dependable choice for sour service environments. When you select and verify the correct grade, it ensures safe and consistent performance.
Masteel supplies ASME SA387 Grades 11, 12, 22, and 91 Class 2. In addition, each plate is fully certified and traceable. As a result, these materials support reliable operation in refineries, petrochemical plants, and pressure vessel applications worldwide. Speak with our team to discuss the right Cr-Mo grade for your requirements.