Refinery equipment, such as hydrogenation reactors, catalytic cracking units, pressure pipelines, and storage tanks, are exposed to high temperature, high pressure, and corrosive media rich in hydrogen sulfide, cyanide, and other substances for a long time. The primary task of welding heat treatment here is to prevent stress corrosion cracking.
Industry characteristics
The biggest feature of heat treatment in refineries is its strong targeting - to combat SCC. Residual tensile stress is one of the three necessary conditions for SCC, therefore, eliminating welding residual stress below the critical value through post weld heat treatment is the most effective way to cut off the path of SCC occurrence. This is fundamentally different from power plants that focus on high-temperature performance and mechanical plants that focus on controlling deformation.
Key points to note
Preheating before welding: For Cr Mo steel equipment (such as 2.25Cr-1Mo), preheating can not only prevent cracking, but also reduce the tendency for quenching during the welding process. The preheating temperature should be strictly determined based on the carbon equivalent of the steel.
• Hydrogen removal treatment: When welding high-strength steel hydrogenation reactors and other equipment, intermediate hydrogen removal and immediate hydrogen removal after welding are very important. Especially in multi pass welding, when the weld thickness reaches a certain value, post heating should be carried out immediately to prevent hydrogen accumulation between layers.
Post weld heat treatment:
1. Thoroughly relieve stress: The insulation temperature and time of PWHT are set to ensure maximum relaxation of stress. It is necessary to follow relevant standards such as ASME BPVC or API.
2. Preventing temper brittleness: For Cr Mo steel, attention should be paid to its tendency towards temper brittleness. The cooling rate of PWHT should be fast enough to pass through the embrittlement temperature range (e.g. 375-575 ° C), but not too fast to cause new stresses. This requires precise balance.
3. Uniform heating: For large towers and reactors, it is necessary to ensure uniform heating of the entire circumferential weld area, and the temperature difference must be controlled within the standard allowable range, otherwise harmful temperature gradient stress will be generated.
Product Application
To prevent SCC in hydrogen sulfide environment, it is necessary to thoroughly perform post weld stress relief treatment on hydrogenation reactors, Cr Mo steel pipelines, etc. Our company's large-area ceramic heating pad is very suitable for the overall surrounding heating of the reactor, and its uniform thermal field can ensure effective stress relaxation in the entire weld area. For large equipment, our intelligent temperature control machine can achieve synchronous and precise temperature control in dozens of heating zones, ensuring that the temperature difference of the large tower or reactor during the heat treatment process is far below the standard limit, thereby avoiding the generation of new harmful stresses due to uneven temperature. In addition, the multi-stage programming function of the temperature control box can accurately execute complex heat treatment curves for Cr Mo steel, including special control in sensitive temperature ranges, effectively suppressing tempering brittleness.