Sulfide Stress Cracking is a form of hydrogen embrittlement cracking. It occurs in low-alloy steel pipelines, high-strength steels, welded joints, and Heat-Affected Zones that are subjected to tensile stress in acidic environments and at temperatures below 82°C , depending on the steel's composition, microstructure, strength, residual stress, and external stress.
Immerse steel plate specimens in an acidic aqueous solution containing H₂S, and obtain Sulfide Stress Cracking Resistance performance data by applying appropriate incremental loads. According to the NACE TM0177-2016 standard, specific requirements are as follows: Take a set of forged steel plate specimens with the highest σb or Hb , conduct the sulfide stress cracking resistance test, and a stress σTh ≥247MPa is considered qualified. Take one set of specimens from each grade A, B, and D welded joint specimens for the sulfide stress cracking test, and a stress σTh ≥247MPa is qualified.
Hydrogen-Induced Cracking is an internal crack with stepwise characteristics formed by the interconnection of parallel hydrogen layer cracks, with no obvious interaction with external stress or residual stress. At the blistering site, the stress generated by hydrogen accumulation inside intensifies hydrogen-induced cracking. HIC is closely related to steel cleanliness, manufacturing methods, the presence of impurities, and their shapes.
HIC occurs in thin, heterogeneous sulfide or oxide inclusions parallel to the steel plate rolling direction. These inclusions form nucleation sites for microhydrogen bubbles, which eventually grow together through stepwise fracture. Since HIC is stress-independent and does not occur in hardened structures, post-weld heat treatment has little significance. Resistance to hydrogen-induced cracking can only be achieved by limiting trace sulfur elements and controlling steel manufacturing variables.
SSC and HIC tests are based on NACE International test standards recommended by the American Society of Corrosion Engineers . The SSC test mainly adopts constant-load stress corrosion test and three-point bending test, primarily in accordance with the NACE TM0177 standard; the HIC test mainly follows the NACE TM0284 standard. For materials used in the design and manufacture of elastic design standards, those complying with ISO 15156-2, ISO 15156-3, or NACE MR0175 standards can be selected. These standards specify environmental conditions to avoid stress corrosion. Only materials meeting these limits should be used.
Conditions for Carbon Steel, Low-Alloy Steel, and Cast Iron to Be Exempt from SSC and HIC Tests
Materials shall be delivered in the following conditions:Hot-rolled /annealed/normalized/normalized + tempered/austenitized, quenched + tempered/austenitized, quenched + tempered
Material hardness ≤22HRC, nickel content <1.0%;S ≤0.003%, P ≤0.010%;The hardness of welds and heat-affected zones shall not exceed 22HRC.
Material yield strength <355MPa, tensile strength <630MPa
Carbon equivalent limits:Low-carbon steel and carbon-manganese steel: Ce ≤0.43 (Ce = C + Mn/6)Low-alloy steel: Ce ≤0.45 (Ce = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15)
Conditions for Stainless Steel to Be Exempt from SSC and HIC Tests
Chemical Composition Limits
321 stainless steel with higher carbon content is allowed to add other elements within the corresponding technical range.
Solution annealing quenching or annealing heating stabilization heat treatment process shall be adopted;
Cold working to improve mechanical properties is not permitted;
The hardness of raw materials, welds, and heat-affected zones shall not exceed 22HRC.
