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Effect of Vacuum Annealing on Properties of 304 Stainless Steel Tensile Billets

1.Stainless steel refers to the steel with high chemical stability in air or various environments, or in water, fire or aqueous solutions of various acids, alkalis and salts. Wide and versatile high alloy steel. Stainless steel is not only a corrosion-resistant material, but also a wear-resistant material, a low-temperature material, a non-magnetic material and a heat-resistant material. For a long time, stainless steel has been widely used in various industrial fields due to its extensive and superior properties.

As a general-purpose corrosion-resistant material, 304 stainless steel sheet is the raw material for manufacturing food industry equipment and kitchen utensils. In the manufacturing process, the steel plate is punched, bent, and deep drawn by cold stamping, and then the parts are made by shrinking, bulging and other processes. In the cold working process, if the workpiece appears work hardening and the workability is deteriorated, the heat treatment method of vacuum annealing must be used to eliminate cold work hardening, so that the structure is uniform and softened, the hardness is reduced, and the pressure workability is improved. To this end, the influence of different annealing processes on the mechanical properties of 304 stainless steel sheet stretched blanks was studied, so as to formulate the best shrinkage and bulging processes of stainless steel products, determine a reasonable heat treatment system for the stretched blanks, and select the corresponding heat treatment equipment , improve product quality and productivity, and provide data and suggestions for the practical application of steel.

2 Materials and methods

2.1 Test material

The chemical composition of the AISI standard austenitic and steel grade 304 stainless steel plates used in the test is shown in Table 1.

On a 200t drawing press, a 304 stainless steel plate with a thickness of D=252mm and a thickness of S=0.7mm was drawn into a round piece with a diameter of d=135mm. At this time, the tensile coefficient m=135/252=0.536 (m=0.5~0.6 to meet the test requirements). Along the axis of the stretched circular simple, a 30mm wide sheet is cut from the side wall of the circular simple, which is cut by wire and then polished, and the end face is cut by the side wire to make the required sample, the size of which is shown in Figure 1.

After the material is stretched and deformed, due to the influence of cold work hardening, its hardness and tensile strength are greatly improved, the elongation is significantly reduced, and the workability is deteriorated, so vacuum annealing must be carried out.

2.2 Heat treatment of samples

The heat treatment equipment adopts a power-saving box-type resistance furnace, equipped with a temperature controller and a NiCr-NiSi thermocouple, to realize temperature prompting and automatic control. The 4 heat treatment regimes are:

1( 600℃, 700℃, 800, 900℃, 1000℃)×1min air cooling.

2(600℃, 700℃, 800℃, 900℃, 1000℃) × 1min water quenching.

3( 600℃, 700℃, 800, 900℃, 1000℃) × 3min air cooling.

4(600℃, 700℃, 800℃, 900℃, 1000℃)×3min water quenching.

2.3 Performance test

The mechanical properties of 304 stainless steel sheets under different heat treatment conditions were determined by hardness test and conventional tensile test. The hardness testing equipment is HDI-1875 type cloth, Rock and Wey hardness tester, which measures the Brinell hardness of the samples under various heat treatment conditions HB2.5/62.5/30, which means that the diameter of the indenter is 2.5nm and the pressure is 62.5kg to maintain Brinell hardness value measured for 30s. The tensile test was carried out on a 60t universal material testing machine to determine the elongation and strength limit of the specimen. There are 3 samples for each heat treatment process, and the test results are averaged.

3 Test results and analysis

3.1 Analysis of sample hardness test results

Metal hardness test is a widely used mechanical property test method. It generally only produces small indentations within the local volume of the metal surface, so it can be regarded as non-destructive testing. The hardness test is generally divided into the indentation hardness method and the scratch hardness method. In the machinery manufacturing industry, the indentation method is mainly used to measure the hardness of materials. The hardness of the metal is closely related to the machinability, and its indentation hardness comprehensively reflects the elasticity, micro-plastic deformation resistance, deformation hardening ability and large-scale plastic deformation resistance of the metal. There is no strict physical correspondence between the hardness of metals and other mechanical property indexes, but the values of metal static strength and other properties can be generally estimated according to the hardness test results, which can be used as reference indexes for evaluating material properties. The effect of different heat treatment regimes on the Brinell hardness of the samples is shown in Figure 2.

It can be seen from Figure 2 that the hardness of the material decreases with the increase of the heating temperature. Especially the material that is air-cooled after being heated for 3 minutes has a lower relative hardness value.

3.2 Analysis of sample elongation test results

Property indicates the amount of irreversible deformation of a material before fracture, and is one of the most important mechanical properties of a material. It is because of the plasticity of metal that it can be made into parts of various geometric shapes using different press working methods. In the process of processing, the plasticity of the material should be improved, the plastic deformation resistance should be reduced, and the pressure processing performance of the material should be improved. The plasticity of a material is expressed in terms of elongation. Figure 3 shows the effect of different heat treatment regimes on the elongation of the samples.

It can be seen from Figure 3 that when the elongation of the material is below 700 °C, there is no significant change with the increase of the heating temperature: but when the heating temperature is 700~900 °C, the elongation increases significantly: the heating temperature is greater than 900 °C. At ℃, the elongation of the samples increased significantly with the increase of temperature, except that the increase of the elongation of the samples slowed down in the heat treatment state of heating for 1 min and air cooling.

3.3 Tensile strength of samples, analysis of test results

The maximum load received during the tensile test is divided by the original area A of the test piece. That is the tensile strength. The effect of different heat treatment conditions on the tensile strength of the material is shown in Figure 4.

Strength is the resistance of the material to plastic deformation and fracture. The strength of the material can be determined according to the engineering stress-strain curve, and the tensile strength is the maximum stress that the specimen can withstand before fracture.

4 Conclusion

(1) With the increase of the heating temperature, the tensile strength and hardness of the stretched billet decreased, the elongation increased, and the temperature F changed significantly at 700-900 °C.

(2) In order to avoid high temperature oxidation and reduce energy consumption, on the premise of meeting the process requirements, the corresponding heating temperature range can be selected according to the required deformation amount and referring to the change curve of elongation and heating temperature. If it is necessary to achieve a higher elongation, it is recommended to use a heat treatment process of 900~950 ℃ heating and heat preservation.

(3) Under the same heating conditions, using the cooling method of air cooling, the hardness and tensile strength of the material are the lowest.

Equipment selection of vacuum annealing furnace: RVA vacuum annealing furnace produced by SIMUWU is an excellent choice for completing this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and annealing uniformity. High process repeatability, stable production, quality output can be guaranteed.