Discussion on the material and heat treatment of t

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Discussion on stainless steel sensor materials and heat treatment

Abstract: precipitation hardening stainless steel 0Cr17Ni4Cu4Nb, as a common material for elastic components of stainless steel sensors, its material composition and content will affect the comprehensive mechanical properties after heat treatment. It can be reduced by adjusting its material composition and content and strict heat treatment process δ- The content of ferrite can obtain uniform metallographic structure, improve the mechanical properties of the material, and thus improve the performance index of the sensor

key words: stainless steel material heat treatment solid solution metallographic structure δ- Ferritic

I. overview

the performance of the weighing sensor determines the accuracy, stability and reliability of the weighing instrument. The weighing sensor with stainless steel as elastomer material can be welded and sealed with metal diaphragm. It has the characteristics of corrosion prevention, explosion prevention, high reliability and high stability. In corrosive occasions, food, chemical industry and other industries, it will become a substitute for alloy steel sensors, and the market capacity will gradually expand

at present, the elastomer materials of weighing sensors are mainly divided into three categories: aluminum alloy (LY12), alloy steel (40CrNiMoA) and stainless steel (0Cr17Ni4Cu4Nb). The first two materials are most widely used, and the processing technology, heat treatment technology and manufacturing technology are very mature. However, at present, the research and production of domestic stainless steel sensors are in the primary stage, the market demand is small, and the market scale of mass production of stainless steel sensors has not been formed. The accuracy of stainless steel sensors is low, and the rate of reaching grade C3 in the national standard of gb/t "weighing sensors" and the metrological verification regulation of JJG "weighing sensors" is low. Only some forms and specifications of stainless steel sensors can achieve high accuracy levels. The reason is that the manufacturing cost of stainless steel sensors is high, and domestic manufacturers have not fully mastered the manufacturing technology of stainless steel sensors, mainly including: 1 The basic research on the elastomer material of stainless steel sensor is not enough, and its composition, smelting process, rolling requirements and supply status are not well known. 2. The heat treatment process of stainless steel materials in China has not been fully mastered, and the impact of heat treatment on the performance indicators of sensors (mainly lagging indicators) has not been solved. 3. Matching between strain gauge and stainless steel material. For the elastomer material of the load cell, the material composition determines its structure, the structure determines the material performance, and the material performance determines the performance of the sensor. Therefore, the selection of material and the determination of composition are the first step. Secondly, the matching of heat treatment process and strain gauge becomes the key point

II. Selection of elastomer materials for stainless steel sensors

generally speaking, the metal materials used for elastomers should not only have strict requirements on chemical composition and smelting conditions, but also have excellent comprehensive properties. While ensuring elasticity and stress, try to choose materials with high micro plastic deformation resistance, high purity of materials and good uniformity of components. When selecting elastic materials, special attention should be paid to the elastic modulus E of the material, as well as the influence of the elastic aftereffect and thermoelastic effect of the material on the performance of the sensor. The material composition affects the comprehensive mechanical properties of the material, which determines the performance of the sensor. Therefore, the selection of materials and the determination of composition are the key. At present, the selection of stainless steel elastomer materials in the domestic weighing sensor industry includes: 0Cr17Ni4Cu4Nb, 2Cr13, etc; Martensitic precipitation hardening stainless steel is also the main choice of stainless steel elastomer materials in the foreign weighing sensor industry, such as pH in the United States, 630631 in the United Kingdom, and s in Japan. At present, us630 is facing difficulties. Taking pH as an example, its material composition is:

brand csipsmncrnicunb+ta ≤≤≤≤≤%%%% ph0. 53.. 003.. 000.. 45

the brand of Chinese stainless steel material close to pH is 0Cr17Ni4Cu4Nb, which is the first choice of domestic sensor manufacturers

0cr17ni4cu4nb material standard and composition are:

0cr17ni4cu4nbcsipsmncrnicunb+ta product standard ≤≤≤≤≤≤%%%%gb 071.000.0350.0301.0015.. 53.. 003.. 00(Nb)0.. 45GB. 0551.000.0350.0300.0515.. 003.. 503.. 700.. 35

according to the material composition checked from the standard, there is no big difference between foreign requirements and domestic materials. The difference is that pH requires nb+ta:0.15 ~ 0.45%, while domestic 0Cr17Ni4Cu4Nb is nb:0.15 ~ 0.45%. The author believes that Nb and Ta form metal strengthening items in the process of heat treatment, and the degree of strengthening is basically the same. The following is the comparison of the measured composition of 0Cr17Ni4Cu4Nb material between pH (USA) and a domestic factory:

material c%si%p%s%mn%cr%ni%cu%nb+ta%ph0.0350.240.0190.0110.6415.364.083.20/0cr17ni4cu4nb 0.0530.0270.0110.0040.5117.253.983.770.28 (NB)

the measured composition of 0Cr17Ni4Cu4Nb and foreign pH (detectable) in terms of composition, the difference between foreign and domestic stainless steel materials is that the content of C and Cr of domestic materials is within the upper limit of GB standard, The content of Ni is slightly lower than that of foreign materials. The main factors affecting the mechanical properties of 0Cr17Ni4Cu4Nb stainless steel material by heat treatment are δ- The average content of ferrite in G "steel for steam turbine blades" shall not exceed 5%. If domestic 0Cr17Ni4Cu4Nb is used as elastomer material, the Cr content should be reduced to the lower limit of the standard (15 ~ 16%), and the Ni content should be appropriately increased to 4.0 ~ 4.5%, reaching the content of foreign materials, which will help to reduce after heat treatment δ- Formation of ferrite. This kind of stainless steel has excellent mechanical properties after heat treatment, with hardness of hrc42 ~ 47, strength limit of 135kg/mm2 and linear expansion coefficient of 11.1 ×/℃, which is equivalent to the commonly used alloy steel 40CrNiMoA

it must be pointed out that the fluctuation of chemical composition of stainless steel has a certain impact on the structure and properties, and the smelting process should be strictly controlled. At the same time, as the sensitive element of the load-bearing component, the elastomer has high requirements for mechanical properties. The smelting and casting process is better to adopt the process of electric arc furnace +vod vacuum refining + electroslag smelting + forging bloom + rolling. 0Cr17Ni4Cu4Nb precipitation hardening stainless steel is used. According to the requirements of the sensor for elastomer, the smelting, rolling process, composition, content and supply state of elastomer materials should be put forward from a technical point of view, δ- The ferrite content should be controlled within 5%

III. heat treatment of stainless steel elastomer

precipitation hardening stainless steel belongs to martensitic stainless steel. When it is cooled from solid solution temperature to room temperature, the structure changes into low-carbon martensite, and then after aging treatment, copper rich phase is precipitated from martensitic matrix, which further improves the strength. The solid solution temperature should neither be too high nor too low, and too high will cause δ- With the increase of ferrite, the MS point will be low, which will increase the residual austenite after solid solution and cooling to room temperature, thus reducing the strength; If it is too low, the organization is difficult to homogenize. Generally, the solid solution temperature should be 1020 ~ 1060 ℃, and its phase transition temperature is: ms=150 ℃, mf=30 ℃, ac1=670 ℃, ac3=740 ℃. First, carbon compounds and alloy elements are dissolved into austenite through solid solution treatment, and martensite is obtained after rapid cooling. After cryogenic treatment, residual austenite is transformed into martensite as much as possible, and then copper rich equivalent strengthening phase is precipitated after aging treatment, resulting in dispersion strengthening, which improves the strength and hardness. Compared with alloy steel, the heat treatment process of stainless steel has strict requirements on temperature control and holding time. At the same time, the surface cleaning of stainless steel materials in the heat treatment process is very important. Oil stains, nitrogen ions, impurities, etc. on the surface can lead to surface nitridation and carburization, resulting in surface pollution, rust, grain boundary corrosion and other defects. It not only affects the appearance of the material, but also affects the performance of the material. Therefore, it is best to use vacuum heat treatment for stainless steel elastomer

1. 0Cr17Ni4Cu4Nb material sensor elastomer heat treatment process flow

sensor is a technology and process intensive product, and the consistency of performance needs to be guaranteed by a reasonable and strict process. Heat treatment process is very important in the production of sensor. The heat treatment process of stainless steel weighing sensor is: cleaning → solid solution treatment → cryogenic treatment → aging treatment. The cooling medium during the solid solution of the heat treatment process is water cooling, oil cooling or forced inert gas cooling, and the cooling rate is very different. At the same time, the size of the elastomer should be considered, and the cooling rate should be different, so that the cooling rate of the solid solution can meet the corresponding requirements. The cooling medium and cooling rate during the solid solution have a great impact on the indicators of the sensor, and the unreasonable process parameters are harmful to the indicators of the sensor

2. The following tests were carried out on the heat treatment system of 0Cr17Ni4Cu4Nb stainless steel materials from two domestic manufacturers. The test data are as follows:

polyurea technology was born in the United States in 1986. 2.1 test sensor elastomer structure form: l-bs-2t-st cantilever beam type (see Figure 1), t-bxb-100kg-st (see Figure 2)

Figure 1 Figure 2

2.2 test elastomer material 0Cr17Ni4Cu4Nb, See the following table for the tested components:

material number c%si%p%s%mn%cr%ni%cu%nb+ta%10.0530.0270.0110.0040.5117.254.083.770.28 (NB) 20.030.410.0100.0030.3715.964.303.200.35

2.3 heat treatment system: heat it to 1040 ℃ with salt bath furnace, and the holding time depends on the size of elastomer; Cool with ice water (temperature 3.5 ℃); Cryogenic treatment (dry ice -70 ℃) for 8 hours; Precipitation hardening (heating in resistance furnace), holding at 480 ℃ for 4 hours, and air cooling to room temperature

2.4 test sensor indicators are:

sensor model elastomer material sensitivity comprehensive error nonlinear hysteresis creep/30 points l-bs-2t-st12.12mv/v0.035% f.s.-0.019% f.s.0.035% f.s.-0.024% f.s.l-bs-2t-st22.17mv/v0.019% f.s.-0.015% f.s.0.019% f.s.-0.011% f.s.t-bxb-100kg-st12.332mv/v0.022% f.s.0.003% f.s.0.022% f.s.0.013% f.s.t-bxb-100kg-st22.406mv/v0.019% f.s.0.004% f.s.0.015% f.s.0.014% f.s.

2.5 metallographic structure

L-bs-2t-st sensor: after heat treatment, the hardness of No. 1 material is HRC 44, and the microstructure is martensitic aging structure and strengthening phase with uniform distribution, and some are chain distributed δ- The average content of ferrite is about 8% - 10%; After heat treatment, the hardness of No. 2 material is HRC 43, and the microstructure is uniformly distributed martensitic aging structure and strengthening phase, which is not found δ- Ferrite

t-bxb-100kg-st type sensor: after heat treatment, the hardness of No. 1 material is HRC 43, and the microstructure is martensitic aging structure and strengthening phase with uniform distribution, with chain distribution δ- Ferrite, with an average content of 5% - 8%, the sample shall not have wrinkles or excessive tension for 4 weeks; After heat treatment, the hardness of No. 2 material is HRC 43, and the microstructure is uniformly distributed martensitic aging structure and strengthening phase, which is not found δ- Ferrite

3. Data analysis

comprehensive test data, there are more than 5% after heat treatment δ- Ferrite will affect the hysteresis index of the sensor. For the shear beam sensor, the less the ferrite content after heat treatment, the better, while for the double hole parallel beam sensor, the impact on the hysteresis index is small. The big difference between the two materials is that in the metallographic test, No. 1 material appears δ- Ferrite, and the content is greater than 5%, while material 2 δ- The ferrite content is small, the shape is small, and it is not easy to observe. This is because 0Cr17Ni4Cu4Nb material will appear in steel after 1050 ℃ solid solution treatment δ- Ferrite, because it does not participate in the transformation of martensite and its morphology is distributed along the grain boundary strip, mainly reduces the thermoplastic and room temperature hardness of steel, thus reducing the strength of materials, and the hysteresis index has the greatest impact

δ- The main reasons for the formation of ferrite are material composition and heat treatment temperature. Cr is the main element, and a sufficient amount of Cr can make the steel form a single δ Among other metal elements, Mo is also a ferrite forming element to the same extent

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