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Heat treatment of high temperature fasteners

Abstract
 The effect of heat treatment on Microstructure and mechanical properties of 2Cr12NiMoWV steel was studied. The results show that the steel can obtain good toughening effect when quenched at 1 055 ~ 1 070 C and tempered at 700 ~ 710 C. On this basis, the heat treatment process specifications of the steel for manufacturing high temperature fastening bolts for steam turbines are determined.
 2Cr12NiMoWV steel is a 12% Cr martensitic stainless steel. It is mainly used to manufacture high temperature fasteners for large steam turbines. With the development of large steam turbines and their manufacturing technology, the amount of steel used in China is increasing. However, up to now, there are few reports about the effect of heat treatment on the microstructure and properties of the steel, which brings a lot of inconvenience to the practical application of the steel. In this paper, the effects of different heat treatment parameters on Microstructure and mechanical properties (mainly hardness and impact toughness) of 2Cr12NiMoWV steel were studied. On this basis, the heat treatment process for high temperature fastening bolts of steam turbine made of this steel is optimized.


Heat treatment of high temperature fasteners

1 test materials and test methods

1.1 test materials

The test material was taken from the steam turbine bolts of a power plant. The chemical composition (%) of the bolts was 0.24 C, 12.22 Cr, 0.62 Ni, 0.93 Mo, 1.25 W, 0.30 V, 0.20 Si, 0.64 Mn, 0.016 P, 0.010 S.
 1.2 test method
 Firstly, the experimental steel was annealed. The annealing process was heated at 870 C for 3 hours, cooled at 20 C/h to 700 C for 3 hours, and then cooled to 500 C for air cooling. The annealed microstructure (Fig. 1) is a ferrite matrix with granular carbides and a certain number of grain boundary carbides. After annealing, the material is processed into the blank of impact sample, then quenched and tempered at different temperatures. Finally, the standard Charpy impact sample of 10 mm
 1 annealed structure
 Four kW high temperature box furnace is used for quenching heating. Different quenching heating temperatures, such as 950, 980, 1010, 1040, 1055, 1070, 1100, 130 and 160 C, are selected for testing. The quenching time of each sample is 4 min/mm, and the oil is cold after the furnace. Tempering was carried out in a 5 kW multi-purpose furnace. Three tempering temperatures, 650, 680 and 710 C, were selected. Tempering time was 6 h and air cooling was carried out after discharge.
The impact toughness of the samples with different heat treatments was measured at room temperature. Fractographic analysis, metallographic analysis and hardness measurement were carried out by using the specimens. The etching agent used for metallographic analysis is ferric nitrate nitric acid solution. The austenite grain size was determined by Hilliard method. The corrosion agent was saturated picric acid solution with a small amount of Sodium Alkylbenzene sulfonate.

2 test results and analysis

2.1 effect of quenching temperature on austenite grain size
 Effect of quenching temperature on austenite grain size. With the increase of quenching temperature, the austenite grain grows up. When the quenching temperature is lower than 1070, the tendency of grain growth is smaller. When the quenching temperature is higher than 1070, the grain growth rate increases. Because the amount of undissolved carbide directly affects the austenite grain size during quenching and heating, the more the amount of undissolved carbide, the smaller the austenite grain size. Therefore, when the quenching temperature reaches 1070 C, most of the carbides in the original microstructure have been dissolved into austenite, which greatly reduces the barriers hindering grain boundary migration and accelerates the growth rate of austenite.
 2.2 effect of quenching temperature on hardness and impact toughness
 The effect of quenching temperature on impact toughness and hardness is tempered at 680 and 710 degrees. At the same tempering temperature, the hardness increases continuously with the increase of quenching temperature. The reason is that the amount of carbide dissolved in austenite increases with the increase of heating temperature, which improves the alloying degree of austenite and subsequent martensite, thus improving the tempering stability of martensite. The effect of quenching temperature on impact toughness is more complex. The impact toughness increases with the increase of quenching temperature. The impact toughness increases obviously when the quenching temperature exceeds 1 040 C. The impact toughness reaches the maximum when quenched at 1 055 1 070 C at 710 C tempering condition. Toughness decreases.
 The metallographic structure of the samples at different quenching temperatures is shown in Fig. 3. The quenched structure is lath martensite, and the orientation of the slab is consistent with a certain crystal orientation. With the increase of quenching temperature, the size of martensite lath increases. When quenched at 980 C, grain boundary carbides are retained in the quenched structure because of the lower quenching temperature. When the quenching temperature is raised to 1 055 1 070 C, almost all the grain boundary carbides are dissolved in austenite.
 2.3 effect of tempering temperature on hardness and impact toughness
 Effect of tempering temperature on impact toughness and hardness. The impact toughness increases and the hardness decreases with the increase of tempering temperature at the same quenching temperature in the range of 650 ~710 C. The increase of impact toughness and the decrease of hardness increase obviously when tempering temperature is higher than 680 C, indicating that the tempering transformation rate increases obviously when tempering temperature is higher than 680 C. Therefore, for 2Cr12NiMoWV steel, the tempering transformation can be fully carried out only when the tempering temperature is higher than 680 C, and the microstructure after tempering can be guaranteed to have sufficient stability and high toughness.
 2.4 fracture analysis
 Under the experimental conditions, the fracture morphologies of impact specimens after various heat treatments are similar, which are composed of dimple zone and quasi-cleavage fracture zone. Fracture analysis shows that there is a good correspondence between the width of dimple zone and impact toughness of specimens. The specimens with larger dimple zone have higher impact toughness, while the specimens with higher toughness have finer tearing edges in quasi-cleavage zone and smaller tearing element.
 2.5 heat treatment of high temperature fastening bolts
 High temperature fastening bolts are the key parts to ensure the safety of steam turbines. According to the service conditions of bolts, it is required to be tempered and tempered. In order to avoid brittle fracture of bolts during assembly and disassembly, the bolts must have high impact toughness at room temperature. The specific technical requirements are HB 277~331, aK > 35 J/cm2. Based on the test results, the heat treatment process of bolt was selected as quenching at 1 055 ~ 1 070 C and tempering at 700 ~ 710 C. After this process, not only can the bolts have high strength and toughness, but also can ensure that they have a stable structure in long-term service. The hardness of the bolts after treatment is HB 287~299, aK = 40~62 J/cm2, all meet the technical requirements.

3 discussion

(1) according to the tempering condition at 2710 C, the impact toughness increases first and then decreases with the increase of quenching temperature. The results show that the grain size of austenite and the solubility of carbide at grain boundary are the main factors affecting the impact toughness of quenched and tempered 2Cr12NiMoWV steel. With the increase of quenching temperature, the amount of carbide dissolved increases, especially in the annealed microstructure, the bonding force between grains increases, and the impact toughness increases. On the other hand, with the increase of quenching temperature, the austenite grains grow up and the impact toughness decreases. When the quenching temperature is below 1055 C, the former one
 The impact toughness increases with the increase of quenching temperature; when the quenching temperature reaches 1 055-1070 ~C, almost all carbides at grain boundary dissolve into austenite, and the impact toughness reaches the maximum; when the quenching temperature continues to increase, the austenite grains coarsen, and the latter factor turns to play a leading role, making the impact toughness obvious. reduce
 (2) the quenching temperature range of 2Cr12NiMoWV steel recommended in the literature is more than 980~1040 degrees. According to the test results, a considerable amount of grain boundary carbide is unavoidable in the annealed microstructure of the steel, and it is difficult to dissolve a large amount of grain boundary carbide during quenching in the above-mentioned interval, resulting in low impact toughness at room temperature after quenching and tempering. If the quenching temperature is properly increased to 1 055 ~ 1 070 C, most of the carbides at grain boundaries have been dissolved and the austenite grains have not been coarsened obviously, and then tempered at appropriate temperature, a good combination of room temperature strength and toughness can be obtained.
 The effect of quenching temperature on the high temperature properties of 2Cr12NiMoWV steel was studied. The results show that with the increase of quenching temperature, the rupture strength (_570105) increases, and the tempering temperature at 980, 1 040 and 1 100 ~C is 680 ~C, _570105 is 85.3, 114.7 and 122.6 MPa, respectively. There is no significant difference in the plasticity between the three kinds of temperature quenching, and the elongation (delta 10) is greater than 10%. Therefore, increasing the quenching temperature properly can not only improve the impact toughness at room temperature, but also improve the endurance strength without reducing the endurance plasticity.
 If the quenching temperature is raised to 1100 C, the impact toughness at room temperature is greatly reduced, and the endurance strength is also increased very little, and the notch sensitivity of steel is increased because of austenite grain coarsening, which is not desirable for the notched parts of high temperature fastening bolts. To sum up, it is recommended that 2Cr12NiMoWV steel be heated and quenched at 1055~1070 C temperature range.

4 Conclusion

(1) At the same tempering temperature, the hardness of 2Cr12NiMoWV steel increases with the increase of quenching temperature, while the impact toughness at room temperature increases first and then decreases, reaching the maximum in the temperature range of 1 055-1 070%. It is suggested that the quenching heating temperature range of the steel is 1055~1070 C.
 (2) The quenching and tempering heat treatment process for the high temperature fastening bolts of steam turbine made of 2Cr12NiMoWV steel is 1 055-1 070 C quenching and 700-710 C tempering. The hardness and impact toughness of the samples treated by this process are HB 287-299 and 40-62 J/cm 2, which can meet the technical requirements of bolts.

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