Ferrite content and heat treatment in AISI 630 / 17-4PH material (Tech Center of Shanghai HY Industry Co., Ltd)
AISI 630 precipitation hardening stainless steel is a commonly used material for sensors. The amount of ferrite in the material has a great influence on product quality.
Performance of the load cell determines the accuracy, stability and reliability of the weighing instrument. The load cell using stainless steel AISI 630 as an elastomer material can be welded and sealed with a metal diaphragm. It has the characteristics of anti-corrosion, explosion-proof, high reliability and high stability. It will become an alloy in corrosive occasions, food, chemical and other industries. The market capacity of alternatives to steel sensors is also increasing.
Generally speaking, in addition to strict requirements on chemical composition and smelting conditions, the metal materials used in elastomers must have excellent comprehensive performance. While ensuring elasticity and stress, try to choose materials with high resistance to microplastic deformation. The purity of the product should be high, and the uniformity of the composition should be good. When selecting elastic materials, special attention should be paid to the elastic modulus E of the material, and the influence of the material’s elastic after-effect and thermoelastic effect on the sensor performance. Material composition affects the comprehensive mechanical properties of the material, thereby determining the performance of the sensor, so the selection of materials and the determination of the composition are the key. At present, the selection of stainless steel elastomer materials in China’s load cell industry are: 0Cr17Ni4Cu4Nb, 2Cr13, etc.; internationally, it is also preferred to choose such as AISI 630, 631, etc.
The main factors affecting the mechanical properties of AISI 630 stainless steel heat treatment are the level of Ni and δ-ferrite. The ferrite content in G8732-88 “Steel for Steam Turbine Blades” requires an average content of not more than 5%. Many of the ferrite requirements are 3%, or even less than 1%. If AISI 630 is used as an elastomer material, the Cr content is up to the standard (15 to 16%), and the Ni content is appropriately increased to 4.0 to 4.6%, which will help reduce the formation of δ-ferrite after heat treatment, and can improve the Fine grain size and better mechanical properties. This kind of stainless steel has excellent mechanical properties after heat treatment. The hardness can reach HRC42～47, the strength limit can reach 135kg/mm2, and the linear expansion coefficient is 11.1×10-6/℃. The smelting and rolling process should be: electric arc furnace + AOD+ LF / Electric arc furnace + VOD vacuum refining + electroslag smelting + forging billet + rolling process is better.
AISI 630 precipitation hardening stainless steel is a martensitic stainless steel. When cooled from the solution temperature to room temperature, the structure transforms into low-carbon martensite. After aging treatment, the copper-rich phase is precipitated from the martensite matrix to increase the strength. improve. The solution temperature should not be too high or too low. Too high will cause the increase of δ-ferrite, and the MS point will be low, which will increase the retained austenite after solid solution and cooling to room temperature, thereby reducing the strength; The organization is difficult to homogenize. Usually the solid solution temperature is 1020～1060℃, the phase transition temperature is: Ms=150℃, Mf=30℃, AC1=670℃, Ac3=740℃. The carbon compounds and alloying elements are dissolved into the austenite through solid solution treatment, and martensite is obtained after rapid cooling. After cryogenic treatment, the retained austenite is transformed into martensite as much as possible, and then the aging treatment is carried out to precipitate copper-rich The equal strengthening phase produces dispersion strengthening, which improves the strength and hardness. Compared with alloy steel, the heat treatment process of stainless steel requires stricter temperature control and holding time. At the same time, surface cleanliness during the heat treatment of stainless steel materials is very important. Surface oil, nitrogen ions, impurities, etc., can cause surface nitriding and infiltration Carbon causes defects such as dirt, rust, and grain boundary corrosion on the surface.
Sensors are technology- and process-intensive products, and the consistency of performance requires a reasonable and strict process as a guarantee. The heat treatment process is very important in the production of sensors. The heat treatment process of the stainless steel load cell is: cleaning → solution treatment → cryogenic treatment → aging treatment. When the heat treatment process is solid solution, the cooling medium is water cooling, oil cooling or forced inert gas cooling. The cooling rate is very different. At the same time, the size of the elastomer should be considered. The cooling rate should be different to make the solid solution cooling rate meet the corresponding requirements. , The cooling medium and cooling speed during solid solution have a great influence on the sensor index, and the unreasonable process parameters are harmful to the sensor index.
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