In what situations are hardness tests required for pressure vessels?

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Hardness represents a material’s ability to resist local plastic deformation, indentation, or scratching; it is a comprehensive indicator of a material’s properties, reflecting aspects such as its elasticity, plasticity, strain hardening, strength, and toughness. There is a corresponding relationship between hardness and tensile strength, and empirical formulas can be used for conversion. Among the commonly used standards, HG/T 20581 and NB/T 47008 mention hardness testing. According to HG/T 20581 7.8, carbon steel and low-alloy steel that require resistance to stress corrosion shall undergo hardness testing. In NB/T 47008, the inspection parameter for grade I carbon steel forgings is only hardness HBW; however, forgings of grade II and above are not tested for hardness, but rather require tensile and impact tests. Grades III and IV also require ultrasonic testing. In practice, there are several situations in which hardness testing is generally required in design documents. These mainly include: first, carbon steel and low-alloy steel materials, as well as welds, that are subject to stress corrosion resistance requirements must undergo hardness testing. In addition to section 7.8.2 of HG/T 20581 mentioned earlier, there is another technical specification specifically focused on \"controlling the hardness of welds in steel pipelines and equipment to prevent sulfide stress cracking\" – GB/T 27866. Stress corrosion cracking is essentially the initiation, propagation, and failure of a crack. Generally speaking, whether in terms of toughness or microstructure, cracks tend to propagate more easily in materials with high hardness. Therefore, the stress corrosion resistance of the material can be ensured by controlling the hardness of the material and the weld. II. Welding cold crack susceptibility is relatively high in Cr-Mo steels and similar materials, and determining the maximum hardness in the heat-affected zone is a simple method for assessing this cold crack sensitivity. Therefore, for pearlitic heat-resistant steels with high Cr and Mo contents, it is common to include requirements for hardness testing of the heat-affected zone in their welding procedure qualification. III. Post-weld heat treatment procedures: GB/T 30583 does not address the inspection aspects of post-weld heat treatment, but in practice, hardness and spectral testing are usually required. The weld is the focus of heat treatment, and hardness and spectral analysis (to determine its composition and content) are also primarily conducted on the weld. Generally, the hardness of carbon steel welds should be kept below HB 200, while that of 15CrMoR should be below HB 225. The purpose of heat treatment is to eliminate welding residual stresses, soften the heat-affected zone, increase the ductility of the weld metal, and improve fracture toughness and corrosion resistance; therefore, the reason for controlling its hardness is also quite straightforward. IV. The Fixed Capacity Regulations TSG 21-2016 stipulate in its \"Chapter 8: Periodic Inspections\" that for vessels prone to material degradation or those with requirements regarding weld hardness, hardness testing shall be conducted as part of the periodic inspections of pressure vessels. During the use of pressure vessels, decarburization can occur due to factors such as pressure, temperature, and the type of medium involved. Decarburization causes a decrease in hardness. In in-use testing, when decarburization is suspected, the hardness of the suspicious area should be measured. After long-term use at high temperatures, pressure vessels may experience phenomena such as carburization, hydrogen absorption, sulfidation, vanadization, and graphitization, which can alter the hardness of the material. During the inspection of pressure vessels, an appropriate location should be selected for hardness testing. Pressure vessels used in stress-corrosion environments should undergo hardness testing during manufacturing or in-service inspection to determine their susceptibility to stress corrosion.