Brief Introduction of Solution Treatment Heat Treatment Process for 304 Stainless Steel

Austenitic stainless steel is a kind of stainless steel with austenitic structure at room temperature. Steady austenite structure occurs in steels containing about 18% Cr, 8%-10% Ni and 0.1% C. Austenitic stainless steel is nonmagnetic and has high toughness and plasticity, but its strength is low. It is impossible to strengthen austenitic stainless steel by phase transformation. It can only be strengthened by cold working. If S, Ca, Se and other elements are added, it has good machinability. In addition to oxidation resistance and acid medium corrosion, such steels can also resist sulfuric acid, phosphoric acid, formic acid and acetic acid corrosion if they contain Mo, Cu and other elements. The intergranular corrosion resistance of such steels can be significantly improved if the carbon content is less than 0.03% or Ti and N are contained. Austenitic stainless steel has been widely used in all walks of life because of its comprehensive and good comprehensive properties.

Experiments and discussions
Conclusion

20190515235524 26595 - Brief Introduction of Solution Treatment Heat Treatment Process for 304 Stainless Steel

304 austenitic stainless steel has good corrosion resistance, heat resistance, low temperature strength and mechanical properties as a kind of widely used steel; stamping, bending and other hot working properties are good, no heat treatment hardening phenomenon, no magnetism. For household goods (tableware, cabinets, boilers, water heaters), automotive accessories, medical appliances, building materials, chemistry, food industry, ship parts. According to different requirements, the commonly used heat treatment processes are: solid solution treatment, stabilization treatment and stress relief treatment, etc. Because of its wide application, the research of heat treatment process has a good guiding significance for production. 1. The experimental raw materials are 304 austenitic stainless steel (domestic grade is 0Cr18Ni9) whose chemical composition is carbon < 0.08%, silicon < 1.00%, manganese < 2.00%, phosphorus < 0.045%, sulphur < 0.03%, nickel 8.0% – 10.5%, chromium 18% – 20%. Raw materials are hot rolled and cut into cylindrical specimens with diameter of 20 mm and height of 20 mm. The samples were treated by solid solution at 1050 C, holding for 30 min, air-cooling and water-cooling respectively, and sensitized at 650 C and holding for 1 h, then air-cooling and 800 C, and holding for 1 h, air-cooling to room temperature. Rockwell hardness tester and metallographic microscope were used to analyze the hardness and metallographic structure of raw materials and heat treated samples.

1. Experiments and discussions

1.1 Determination of Inclusions in Raw Materials
According to the chinese national standard “GB/T10561-2005 steel non-metallic inclusions content determination” experimental method, the raw material non-metallic inclusions as shown in Figure 1, under 100 times compared with the standard map, it can be concluded that the raw material contains two types of inclusions. Alumina is classified as Alumina (Category B) along the rolling direction. It can be judged from the grain size and length as fine series, grade 1.5. The shape ratio is small, and the irregular black particles are spherical oxides (class D). From the size and quantity, it can be judged that they are fine series, grade 1.5. So the results were fine line B1.5 and fine line D1.5. Therefore, the inclusion grade conforms to the national standard.
1.2 Analysis of Metallographic Structure and Mechanical Properties of Raw Materials
The metallographic structure of the raw material is shown in Fig. 2. The etching method is a mixture of 5 g High Ferric chloride, 10 mL hydrochloric acid and 500 mL alcohol for 10 minutes. Austenite grains are uniform and fine. According to GB/T6394-2002 Metal Average Grain Size Measurement Method, the grain size can be assessed as 5.5 grade. In addition, there are twins in the grains, and the black spots are non-metallic inclusions. The metallographic picture shows that the raw material has been treated by solid solution. Various hardness measurements of raw materials as shown in Table 1 show that the hardness distribution is relatively uniform, with an average of about HB187.

Table 1 Various hardness measurements of raw materials

Measurement times

1

2

3

4

5

Average value

Hardness value/HBW

187

185

189

190

186

187.4

1.3 Effect of Heat Treatment Process on Microstructure and Properties
1.3.1 Effect of Solution Treatment on Microstructure
The raw material of 304 austenitic stainless steel was heated to 1050 C and kept for 30 minutes, then re-solid solution treatment was carried out by fast cooling to room temperature. The structure after solution treatment is shown in Fig. 3 and Fig. 4. Fig. 3 shows the metallographic structure of air-cooled specimens and Fig. 4 shows the metallographic structure of water-cooled specimens. The etching method is a mixture of 5 g High Ferric chloride, 10 mL hydrochloric acid and 500 mL alcohol for 10 minutes.
From the metallographic structure photos, it can be seen that the metallographic samples after solution treatment are relatively difficult to corrode, and the grain boundary is not obvious. The metallographic structure is austenite grains, the grains are relatively uniform and fine, accompanied by twins, and the black spots are carbides. According to GB/T6394-2002 Metal Average Grain Size Measurement Method, the grain size after air cooling is about 5.5 grade, which has little change compared with the original material grain size. Therefore, it can be inferred that the solid solution treatment of raw materials is also air cooling. After water cooling, the grain size increased to about 6.5 grade.
1.3.2 Effect of Solution Treatment on Mechanical Properties
The hardness values obtained by space-time cooling and water cooling in solution treatment are shown in Table 2. As can be seen from Table 2, the hardness of austenitic stainless steel increases with the increase of cooling rate. Austenitic stainless steel has no change in structure when it is cooled, but its hardness increases. This is due to the hardening of the outer layer of austenitic stainless steel during rapid cooling, the high and soft internal temperature, and the plastic compression deformation due to the shrinkage of the outer layer. Like being processed by a punch, it shrinks up and down and expands laterally. Because of external cooling and internal heat, the shrinkage of the inner layer is more than that of the outer layer when it continues to cool to room temperature. Because the internal shrinkage produces compressive stress in the outer layer, this thermal stress makes the surface of austenitic stainless steel have extremely compressive stress, which promotes the surface fatigue strength and hardness of austenitic stainless steel to increase. Because this residual compressive stress has a good effect on the mechanical properties of materials. Therefore, water cooling is better than air cooling in solution treatment of austenitic stainless steel.

Table 2 Various hardness values of 304 stainless steel after solution treatment

Measurement times 1 2 3 4 5 Average value
Hardness after Water Cooling/HB 205 204 200 207 210 205
Hardness after Air Cooling/HB 181 190 183 188 185 185

1.3.3 Sensitization treatment
Sensitization treatment refers to the austenitic stainless steel which has been treated by solid solution and heated at 500-850 C. Chromium atoms are precipitated from austenite in the form of Cr23C6 carbide along the grain boundary, resulting in increased sensitivity to grain boundary corrosion of austenitic stainless steel, which is sensitization treatment. Process 1: 304 austenitic stainless steel is heated to 650 C and kept for 60 minutes, then air-cooled to room temperature. The metallographic structure at different sensitization rates is shown in Fig. 6 A and B. Process 2: 304 austenitic stainless steel is heated to 800 C and held for 1 h, then air-cooled to room temperature. The metallographic structure at different sensitization rates is shown in Fig. 6 C and D. The etching method is a mixture of high ferric chloride 5g, hydrochloric acid 10mL and alcohol 500mL. The etching time is 10min. It can be seen from the metallographic structure that under the same etching conditions, the grain boundary corrosion of the structure is not obvious when it is sensitized at 650 (?) C for 60 minutes. However, the grain boundary corrosion of the structure is more obvious when the structure is sensitized at 800 C for 60 minutes. The main reason is that the chromium element in austenite near the grain boundary is easier to precipitate along the grain boundary in the form of Cr23C6 when the sensitization temperature range (generally 500-900 C) is higher, resulting in the reduction of chromium element in austenite near the grain boundary, making the potential here lower and making it easier to be corroded. When the sensitization temperature is not very high and the holding time is not long enough, the precipitation of Cr23C6 does not gather on the grain boundary, but disperses in the grain in the form of pitting corrosion, so there are dispersed Cr23C6 precipitates on the grain in the metallographic photograph. The boundary corrosion is obvious. The main reason is that the chromium element in austenite near the grain boundary is easier to precipitate along the grain boundary in the form of Cr23C6 when the sensitization temperature range (generally 500-900 C) is higher, resulting in the reduction of chromium element in austenite near the grain boundary, making the potential here lower and making it easier to be corroded. When the sensitization temperature is not very high and the holding time is not long enough, the precipitation of Cr23C6 does not gather on the grain boundary, but disperses in the grain in the form of pitting corrosion, so there are dispersed Cr23C6 precipitates on the grain in the metallographic photograph.

2 Conclusion

20190515235731 49003 - Brief Introduction of Solution Treatment Heat Treatment Process for 304 Stainless Steel

304 austenitic stainless steel specimens were treated by solid solution at 1050 C for 30 minutes, and cooled in water and air respectively. The results show that the structure of the stainless steel is single phase austenite, and the hardness of the stainless steel cooled in water is higher, which indicates that the internal stress of the stainless steel cooled in water is larger. The raw materials were sensitized at 650 for 60 minutes and at 800 for 60 minutes. It was found that intergranular corrosion was more likely to occur at 800 for 60 minutes. Therefore, in heat treatment of 304 stainless steel, it is necessary to avoid staying at a higher temperature for a longer time in the sensitization temperature range.

Through the study of heat treatment process of 304 austenitic stainless steel, the conclusions are as follows:

  • (1) The austenitic stainless steel after solution treatment has better corrosion resistance. The surface hardness obtained by water cooling after solution treatment is higher than that obtained by air cooling, and the surface is residual compressive stress, which is also beneficial to other mechanical properties.
  • (2) It can be concluded that sensitized austenitic stainless steel is very easy to be corroded. Moreover, the higher the sensitization temperature and the longer the sensitization time, the greater the tendency of intergranular corrosion after sensitization. Therefore, austenitic stainless steel heat treatment must avoid in the sensitization zone.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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brief introduction of solution treatment heat treatment process for 304 stainless steel - Brief Introduction of Solution Treatment Heat Treatment Process for 304 Stainless Steel
Article Name
Brief Introduction of Solution Treatment Heat Treatment Process for 304 Stainless Steel
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304 austenitic stainless steel specimens were treated by solid solution at 1050 C for 30 minutes, and cooled in water and air respectively. The results show that the structure of the stainless steel is single phase austenite, and the hardness of the stainless steel cooled in water is higher, which indicates that the internal stress of the stainless steel cooled in water is larger. The raw materials were sensitized at 650 for 60 minutes and at 800 for 60 minutes. It was found that intergranular corrosion was more likely to occur at 800 for 60 minutes. Therefore, in heat treatment of 304 stainless steel, it is necessary to avoid staying at a higher temperature for a longer time in the sensitization temperature range.​
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