Welding Preheating and Post-Weld Heat Treatment of Chromium Martensitic Stainless Steel
Chromium-martensitic stainless steel is a kind of stainless steel with high strength and good oxidation and corrosion resistance, which is widely used in aerospace, chemical and petrochemical, food and paper and other industries. As the microstructure, mechanical properties, corrosion and oxidation properties of this type of stainless steel are mainly determined by their high-temperature phase transformation, so it is necessary to pay attention to the preheating and post-weld heat treatment of this type of stainless steel in welding process.
Preheating is a necessary process for welding of all metals in order to reduce stress and thermal shock, and to improve weld quality. When welding chromium-martensitic stainless steel, preheating can prevent excessive heat input resulting in phase transformation and reduce the possibility of cold cracking. In addition, preheating can reduce the thermal shock on the joint due to rapid cooling, so as to avoid surface cracks, reduce tensile stress on the surface and improve the impact properties of the welds.
The preheating temperature for chromium-martensitic stainless steel should be limited to below 600℃. If the preheating temperature exceeds 600℃, the austenite phase of chromium-martensitic stainless steel will be transformed to martensite or other phases, which leads to the formation of small and dense martensite or other phases in the welding heat affected Zones (HAZ), resulting in brittle and poor toughness of welds.
The welding temperature of chromium-martensitic stainless steel should be controlled to avoid overheating, which will lead to over-austenitization of the weld and the heat-affected zone, and ultimately result in excessive reduction in strength, toughness, ductility and lack of corrosion resistance.
The cooling rate of welding chromium-martensitic stainless steel should also be considered. For thinner materials, water cooling is usually adopted to facilitate rapid cooling and obtain more stable martensite phase. However, for thicker materials, if the cooling rate is too fast, it may cause brittle, so it is better to use insulating materials to slow down the cooling rate and reduce the stress caused by shrinkage during solidification.
In addition to preheating, the post welding heat treatment of chromium-martensitic stainless steel is also very important. Postweld heat treatment can reduce residual stresses in the weldment, eliminate hardening and softening of HAZ, and ensure the mechanical properties and fracture toughness of chromium-martensitic stainless steel weldment.
The post-weld heat treatment of chromium-martensitic stainless steel is suitable to be carried out in an annealing furnace 600~900℃ or using a middle frequency furnace with a holding time of approximately 0.5~1h per 25mm of thickness. The purpose of post-weld heat treatment is to form a fine and uniform microstructure in the heat-affected zone and to produce a balanced composition in the weld.
In areas where the welding process of chromium-martensitic stainless steel is required, it is very important to pay attention to preheating and post-weld heat treatment. Through the reasonable choice of preheating and post-weld heat treatment temperature, time and cooling rate, the welding process of chromium-martensitic stainless steel can be effectively controlled to improve the mechanical properties and corrosion resistance of the weldment, protect the environment and ensure peoples safety.
