The equipment in chemical plants, such as reaction vessels, synthesis towers, urea equipment, and various process pipelines, often operate under high temperature, high pressure, and extremely harsh corrosive media (such as chloride ions, wet hydrogen sulfide, liquid ammonia). The core goal of its welding heat treatment is to ensure the sealing integrity and corrosion resistance of the equipment.
Industry characteristics
The most unique focus of heat treatment in chemical plants is to prevent failure modes in specific environments, especially stress corrosion cracking caused by chloride ions and intergranular corrosion of stainless steel. The formulation of heat treatment processes is directly targeted at the medium that the equipment comes into contact with, and has strong "customization".
Key points to note
Preheating before welding: For austenitic stainless steel, preheating is generally not required, and the interlayer temperature should be controlled not to be too high (usually<150 ° C) to prevent grain coarsening and carbide precipitation in the heat affected zone. But for duplex stainless steel, the interlayer temperature needs to be controlled within a narrow range (such as 100-150 ° C) to maintain the ideal phase ratio.
• Hydrogen removal treatment: In chemical equipment, for carbon steel or low-alloy steel equipment (such as hydrogenation reactors) under hydrogen operation, hydrogen removal treatment is the lifeline to prevent hydrogen embrittlement and hydrogen induced cracking, and the requirements are extremely strict.
Post weld heat treatment:
1. The particularity of austenitic stainless steel: Standard austenitic stainless steel (such as 304316) usually does not undergo traditional PWHT because prolonged residence in this temperature range (450-850 ° C) can cause sensitization, leading to the precipitation of chromium carbide along grain boundaries and causing intergranular corrosion. If it is necessary to carry out (such as eliminating welding stress of dissimilar steels), ultra-low carbon or stabilized stainless steel should be used.
2. Balance between stress relief and corrosion prevention: For carbon steel equipment with SCC risk, PWHT is mandatory. But for stainless steel, the solution may be to use solution treatment (rapid heating to 1050-1150 ° C and rapid cooling), but this is extremely difficult to implement for large equipment. Therefore, more reliance is placed on rational structural design and welding processes to reduce stress concentration.
3. Cleaning and Pollution: When heat treating stainless steel equipment, the heating environment must be clean, and it is strictly prohibited for pollutants containing chlorine and sulfur to come into contact with the surface of the workpiece, otherwise it will cause serious pitting corrosion or cracking at high temperatures.
Product Application
For carbon steel equipment, our company's standard heat treatment equipment can effectively eliminate stress and prevent SCC. For stainless steel equipment, the challenge of heat treatment is to avoid staying in the sensitization zone. Our company's intelligent temperature control machine can achieve rapid heating and minimize the risk of chromium carbide precipitation through sensitive temperature ranges through precise program control. When conducting stress relief treatment, our heater can provide a clean heat source (free of chlorine and sulfur pollutants), and the multi-point temperature control capability of the temperature control box ensures the uniformity of heat treatment, preventing new problems caused by temperature difference stress. For special materials such as duplex steel, our company's equipment can accurately control the narrow process window temperature to ensure phase equilibrium.