Heat treatment of stainless steel

Stainless steelAfter forming and welding,The heat treatment is performed not only to obtain the required mechanical properties, but also to obtain its inherent, good corrosion resistance and suitable high temperature strength.soHeat treatmentyesStainless steelWorkpiecesAn extremely important thing in productionofProcess and production links.

Stainless steelMaterialDuring molding and welding, great stress will be generated. Moreover, the structure of the stainless steel weld is also very uneven, and the weld molten metal area has a cast metal structure; the weld heat-affected zone has an overheated metal structure because it is close to the melting zone, and then it gradually transitions to the structure that the base metal itself has. The unevenness of the weld structure will cause great stress, which will reduce the strength of the weld and increase the tendency of intergranular corrosion and stress corrosion; the thick structure of the metal in the molten metal area and the heat-affected zone will also increase the hardness and toughness of the weld. To eliminate stainless steelMaterialStresses caused by molding and welding processes improve the mechanical properties of the welds,To improve the corrosion resistance of welds, heat treatment after welding is essential.

According to the steel type and use of stainless steel, the heat treatment method can be divided into solid solution heat treatment and stabilization.Heat treatmentandAnnealing, etc.Way.

• Solid solution heat treatment

Solid solution heat treatment isWorkpiecesHeat to1050℃~1150℃, after insulationQuick cooling, can make the welded joint in a stable state again, and the intergranular corrosion tends to beGet smaller.Generally, austenitic stainless steel adopts solid solution heat treatment, which canEliminate the adverse effects of the second phase,getMore even ingredients andsingleorganize,Relieve stress or soften the material that has been processed hardened, get betterCorrosion resistanceand plastic toughness.

When formulating specific heat treatment processes,fullConsider the specific grade composition and workpiece type, and adjust the heating temperature appropriately, insulation time. When the heating temperature is low, the carbide cannot be fully dissolved; when the temperature is too high, there will be problems such as grain growth and corrosion resistance.After insulation is overCooling should be done at a faster rate to prevent carbide from being re-precipitated.

• StabilizeHeat treatment

In order to prevent oxidation and deformation caused by solid solution treatment,Stabilize elements Ti,Nb StainlesssteelgenerallyStabilization treatment is used instead of solid solution treatment to eliminate stress, and uniform structure ensures good corrosion resistance. The stabilization heating temperature is generally selected in 850-930℃, this will make Cr_{twenty three} C₆ Dissolve fully to make Ti or Nb With it again CC ombined, and Cr and continue to remain in austenite.Cooling methodGenerally, air cooling is used, water cooling or furnace cooling can be used, which should be determined based on the specific conditions of the parts.,The cooling rate has no significant impact on the stabilization effect.

• annealing

The purpose of heat treatment of ferrite stainless steel is only toEliminate the second phase generated during processing,Make the weld structure evenly refined,Improve its plasticity and corrosion. The heat treatment method of ferrite stainless steel isannealingdeal with. Ferrite stainless steel is mostly used in corrosion-resistant environments, and it also tends to intergranular corrosion, but it is different from the situation of austenitic steel. Ferrite stainless steel from high temperature(Cr17 Approximately 1100℃~1200℃;Cr25 Approximately 1000℃~1200℃)When the temperature is cooled down, the intergranular corrosion tends to occur, but in the 650℃~850℃ After heating and cooling, the tendency of intergranular corrosion is eliminated. This is because the solubility of carbon in ferrite is much smaller than that of austenite and is easy to precipitate,Moreover, the diffusion rate of carbon in ferrite is also relatively large. During the high-temperature quenching process, chromium carbides have been formed, and a chromium-depleted layer on the grain boundary appears. through 650℃~850℃ Heating, andCrThe diffusion is uniform, causing the chromium-depleted layer to disappear. Therefore, the ferrite stainless steel is welded 650℃~850℃ Heat treatment is very necessary. For ultrapure ferrite stainless steel (including C≤0.01%, strictly control Si,Mn,S,P), the annealing heating temperature can be increased a little.

• Quenching   

The purpose of quenching is to transform supercooled austenite into martensite or bainite to obtain martensite or bainite structure, and then temper at different temperatures to greatly improve the strength, hardness, wear resistance, fatigue strength and toughness of the steel, thereby meeting the different usage requirements of various mechanical parts and tools. Generally, the stainless steel is heated above the phase change temperature.1000~1100°C, the insulation time is calculated based on the thickness or diameter of the material.The insulation endsThen it is quickly cooled in the quenching agent.

• Backfire 

 Tempering is usually followed by quenching, with the purpose of eliminating residual stress generated during quenching of the workpiece and preventing deformation and cracking; adjusting the hardness, strength, plasticity and toughness of the workpiece to meet the requirements of performance; stabilizing the structure and size, ensuring accuracy, etc.. generallyHeat the stainless steel below the phase change temperature.according to The thickness or diameter of the material is insulatedmustTime, thenSlow or fast cooling. According to the tempering temperature range, tempering can be divided into low-temperature tempering, medium-temperature tempering and high-temperature tempering. Workpiece quenching+The composite heat treatment process of high-temperature tempering is called tempering. The conditioning is not only done for final heat treatment, but also for pre-heat treatment of some precision parts or induction hardened parts.

                      Various types of stainless steel heat treatment methods