Heat treatment process and properties of S30432 austenitic stainless steel bend

A new austenite S30432 stainless steel pipe bend was prepared, and the effect of heat treatment technology on properties of S30432 pipe bend was investigated. The results show that the 1150℃ solution strengthening lead to the grain size nouniform and large austenite grain occurring. Solution process at 1110℃ or 1060℃, the uniform and fine austenite grain was obtained, with a austenite grain size 8~9 grade even if at the largest deformation zone, which still keep the same grain size in comparison with the original direct tube. While, the tensile strength at the zone is higher than that of direct raw tube, and the plastic properties are equivalent to the later. At the results, it is the best suitable solution processing temperature for S30432 pipe bend at 1060℃ to 1100℃, and the mechanical properties and grain size all meet with the specifications for super- super boiler pipe application.

Keywords: austenite, stainless pipe, boiler pipe, bend pipe, heat treatment, grain size, mechanical properties

Experimental method
Test results
Analysis and discussion

Super304H stainless steel is a high-temperature material for super-supercritical boilers [1-3] jointly researched and developed by Sumitomo and Mitsubishi Heavy Industries of Japan, and is often used as a superheater and reheater component. Super304H is actually based on ASME SA-213 TP304. By reducing the upper limit of Mn content, adding about 3% Cu, 0.45% Nb and trace amount of N, it can be strengthened by diffusion precipitation of Nb-containing compound strengthening phase in high temperature service environment. And the precipitation of the Cu-rich phase in the austenite, so that it has the best combination of high temperature strength, high temperature plasticity and high temperature oxidation resistance. Super304H was recognized by ASME CODE CASE2328 in 2000 [2][3]. Some domestic units have done some preliminary process evaluation work [2][5], which proves that Super304H pipe has good bending deformation ability. Super304H furnace tubes have been used at home and abroad for decades [4][5]. S30432 Austenitic stainless steel furnace tube is an alternative material for the domestically produced Super304H furnace tube. At present, there is still no research data on the process and performance of S30432 material furnace tube. In this paper, the domestic S30432 furnace tube (solid solution state) was subjected to post-bending heat treatment process and performance study, in order to determine the appropriate heat treatment process after bending, so as to provide testing and analysis basis for practical industrial production applications.

1 Experimental method
1.1 Furnace tube chemical composition and specifications

The chemical composition of the test tube material is shown in Table 1, Specification Ф 51 × 9.5.
Table 1 Chemical composition of pipe bend /%
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1.2 Heat treatment process of pipe bend

The pipe bend is cold-formed in the mold at 180° (see Figure 1) with a bend radius of R = 2.5D (D is the bend diameter). After the bending, the solution heat treatment is carried out at 1150 ° C, 1110 ° C, and 1060 ° C respectively. The specific heat treatment process is shown in Table 2.
Table 2 Bending tube heat treatment process
Tab.2 Heat treatment process of bend

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20190514221435 61319 - Heat treatment process and properties of S30432 austenitic stainless steel bend

Fig.1 the bend pipe photo

1.3 Preparation of samples

Tensile, hardness and grain size test specimens were taken at the 90° maximum deformation position (upper part in Figure 1) and the undeformed straight tube position. Influenced by the thickness and deformation of the pipe wall, the φ 3 small-size specimen is taken by stretching.

1.4 Test equipment

The hardness test was performed on a Rockwell hardness tester. Tensile performance testing was performed on a universal testing machine. Austenitic grain size ratings were performed on metallographic samples on a Leica DIM5000M metallographic microscope according to the GB/T6394/ASTM rating chart.

2 Test results
2.1 Tensile test

The tensile test results are shown in Figures 2 and 3.

2.1 Hardness test

The hardness test results at different positions after bending are shown in Fig. 4.

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Fig.2 tensile strength changes
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Fig.3 plastic properties changes
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Fig.4 hardness curves at 90°site
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Fig.5 original grain shapes

2.2 Grain size

The grain size of the original pipe (straight pipe) was rated as 7 to 8 grades; the grain of the maximum treated deformation (90°) after bending had traces of elongation and deformation, but the grain size remained at 7-8. After solid solution treatment at 1150 °C, the grain size is coarsened at the maximum deformation (90°). The grain size is mostly grade 1 to 5 (about 70%), and the grain size of a few areas is maintained at 7 to 8 grades; after 1110 °C After solution treatment at 1060 ° C, the grain size is finer than that of the original pipe of 8 to 9 grades.

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Fig.6 grain shapes (1150℃ 100×)
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Fig.7 grain shapes (1110℃ 500×)4 / 5
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Fig.8 grain shapes (1060℃100×) Fig.9 relation sketch between grain size and crucial deformation

3 Analysis and discussion
3.1 Mechanical properties

It can be seen from the curves of Fig. 2 to Fig. 3 that the strength of the straight pipe increases greatly after the deformation of the pipe, especially the yield strength increases, the elongation of the plasticity index decreases more, and the reduction of the section shrinkage does not change. Big. Correspondingly, the hardness also increases considerably. Obviously, these changes are the result of deformation enhancement.
After solution treatment, the mechanical properties of the largest part of the deformation and the straight pipe tend to be consistent. It is obvious that the deformation strengthening effect of the pipe bend is largely eliminated. As the solution treatment temperature increases, the strength decreases and the plasticity rises. The solid solution tubes at 1060 ° C and 1100 ° C have higher strength and relatively better plasticity. The strength is greatly reduced after solid solution at 1150 ° C, and the reason can be explained by the following grain size change.

3.2 Grain size

The grain size before bending is 7-8 grades (Fig. 5). After the solution is solid solution treated at 1150 °C, the grain size is 5-6, which has a certain degree of growth. Although there is no deformation effect in the straight pipe section, and there is no recrystallization phase change during the solution heat treatment after bending, the experiment shows that the austenite grains with too high solid solution temperature will gradually grow. After the bending, the maximum deformation part is re-dissolved at 1150°, the grain size is mostly in the range of 1~5, and the local retention is 7~8, and the uniformity is deteriorated (Fig. 6). Note 1150 ° solution treatment temperature is high, grain growth tends to be serious. The growth effect greatly reduces the mechanical properties of the tube.
After solution treatment at 1100 ° C and 1060 ° C after bending, the grain size of the deformed part is remarkably refined (Fig. 7 and Fig. 8). It is indicated that low temperature solid solution can suppress grain growth at the deformation site. From the change of grain size, the recrystallization temperature is lower than 1060 °C under the actual deformation degree. According to the introduction of the literature [2] [5] [6], the solution treatment temperature of the general SUPER304 steel pipe is ≥1100 °C, and the optimum heat treatment solution temperature is 1150 °C. The above analysis shows that the 1150 ° C solution temperature may be only suitable for the final heat treatment of the tube (straight tube), which can control the finer grain size. However, for solid-state straight pipe products, the grain size is seriously increased after being bent at a high temperature after 1150 ° C high temperature solution treatment, which does not meet the requirements of finer level 7 for thermal power generation [5] ]. Obviously, the solid solution temperature is higher at 1150 ° C after bending. The solution treatment at 1110 ° C and 1060 ° C can maintain the grain size after recrystallization, which is finer than 8 grades, and the solution treatment temperature is suitable. The properties of metal materials are closely related to the grain size. Controlling the grain size after recrystallization is an important issue in the production of all metal materials. A large number of tests have shown [7] that there is a relationship between the recrystallized grain size and the degree of pre-cold deformation. At the critical deformation degree, the grain size of the metal rapidly grows to form coarse grains. For general metals, the critical deformation is usually in the range of 2 to 10%. When the critical deformation degree is exceeded, the crystal grains become fine as the deformation increases. The test bend has a test deformation of 7 to 9%. Therefore, at a higher solid solution temperature after deformation, the grain size will grow rapidly. The high temperature, large degree of deformation, and increased energy storage promote the nucleation rate and growth rate of the recrystallization process, which shortens the recrystallization period and the entire recrystallization process. Strict control of the solution heating temperature and holding time as well as the heating rate prevents the formation of coarse grains. The above experiments prove that the grain size of solution treatment at 1110 ° C and 1060 ° C meets the technical requirements of ultra-supercritical boilers. Theoretically, the solution temperature selection between 1110 ° C and 1060 ° C is also completely feasible.

4 Conclusion

The suitable solution treatment temperature for S30432 after bending is 1060~1110 °C. It can ensure that the mechanical properties of the pipe are higher than the original pipe, and the grain size of the largest deformation part can be controlled to meet the requirements of the thermal power technology.

Source: China Pipe Bend 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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