The influence of heat treatment process on the microstructure and mechanical properties of 2Cr12NiMoWV steel was studied. The results show that the steel can achieve good strengthening and toughening effects when quenched at 1055-1070 ℃ and tempered at 700-710 ℃. And based on this, the heat treatment process specifications for the steel used to manufacture high-temperature fastening bolts for steam turbines were determined.
Keywords 2Cr12NiMoWV steel quenching temperature impact toughness high temperature fastening bolt
2Cr12NiMoWV steel is a 12% Cr martensitic stainless steel, mainly used for manufacturing high-temperature fasteners for large steam turbines. With the development of large steam turbines and their manufacturing technology, the usage of this steel in China is increasing. However, so far there have been few research reports on the influence of heat treatment system on the microstructure and properties of this steel, which has brought many inconveniences to its practical application. This article investigates the influence of different heat treatment process parameters on the microstructure and room temperature mechanical properties (mainly hardness and impact toughness) of 2Cr12NiMoWV steel. On this basis, the heat treatment process for high-temperature fastening bolts of steam turbines made of this steel is optimized.
1. Experimental Materials and Methods
1.1 Test Materials
The experimental material was taken from a retired steam turbine bolt from a power plant, with a chemical composition (%) of 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 is annealed using a process of heating at 870 ℃ for 3 hours, cooling at 20 ℃/h to 700 ℃ for 3 hours, and then furnace cooling to 500 ℃ before being air cooled out of the furnace. The annealed structure (Figure 1) consists of granular carbides and a certain amount of grain boundary carbides distributed on the ferrite matrix. After annealing, the material is processed into an impact specimen blank, which is then subjected to different temperature quenching and tempering treatments. Finally, it is ground and wire cut into a 10 mm × 10 mm × 55 mm standard Charpy impact specimen.
1. Annealing organization
A 4 kW high-temperature box type electric furnace is used for quenching heating, and different quenching heating temperatures such as 950, 980, 1010, 1040, 1055, 1070, 1100, 1130, and 1160 ℃ are selected for testing. The quenching heating time for each sample is 4 minutes/mm, and it is cooled with oil after being taken out of the furnace. Tempering is carried out in a 5 kW multi-purpose furnace, with three tempering temperatures of 650, 680, and 710 ℃ selected, and a tempering time of 6 hours. After being discharged from the furnace, it is air-cooled.
Conduct room temperature impact tests on samples that have undergone different heat treatments to determine their impact toughness values. Perform fracture analysis, metallographic analysis, and hardness measurement on the fractured sample. The etching agent used for metallographic analysis is ferric chloride nitric acid aqueous solution. The determination of austenite grain size is carried out according to the Hilliard method [1], and the corrosive agent used is a saturated aqueous solution of picric acid with a small amount of sodium alkylbenzenesulfonate.
2 Experimental results and analysis
2.1 Effect of Quenching Temperature on Austenite Grain Size
The effect of quenching temperature on austenite grain size is shown in Table 1. According to Table 1, as the quenching temperature increases, the austenite grains grow. When the temperature is below 1070 ℃, the tendency for grain growth is small, and when the temperature is above 1070 ℃, the grain growth rate accelerates. Due to the direct influence of the amount of undissolved carbides on the austenite grain size during quenching and heating, the more undissolved carbides there are, the smaller the austenite grain size [2]. Therefore, the data in Table 1 indicate that when the quenching temperature reaches 1070 ℃, most of the carbides in the original structure have dissolved into austenite, greatly reducing the obstacles that hinder grain boundary migration and accelerating the growth rate of austenite.
Table 1 Effect of Quenching Temperature on Austenite Grain Size
Quenching temperature/℃ 980 1040 1070 1100 1130 1160
Austenitic grain size grade/grade 6.7 6.4 6.2-5.2-

2.2 The influence of quenching temperature on hardness and impact toughness
The effect of quenching temperature on impact toughness and hardness during tempering at 680 ℃ and 710 ℃ is shown in Figure 2. At the same tempering temperature, as the quenching heating temperature increases, the hardness continuously increases. This is because as the heating temperature increases, the amount of alloy carbides dissolved into austenite increases, which increases the degree of alloying of austenite and subsequently obtained martensite, thereby improving the tempering stability of martensite. The effect of quenching temperature on impact toughness is complex. As the quenching temperature increases, the impact toughness increases. When the quenching temperature exceeds 1040 ℃, the impact toughness increases significantly; Under the tempering condition of 710 ℃, the impact toughness reaches when quenched at 1055-1070 ℃; When quenched above 1100 ℃, the impact toughness decreases as the quenching temperature increases.
Figure 2: The Effect of Quenching Temperature on Hardness and Impact Toughness
(a)680 ℃； (b)710 ℃
Figure 3 Quenching metallographic structure
(a) 980 ℃； (b) 070 ℃
The metallographic structure of the samples at different quenching temperatures is shown in Figure 3. The quenching structure is Flat noodles martensite, and the orientation of Flat noodles beam conforms to a certain crystal orientation. With the increase of quenching temperature, the size of martensitic Flat noodles increases. When quenched at 980 ℃, due to the low quenching temperature, grain boundary carbides are still retained in the quenched structure. When the quenching temperature is increased to 1055-1070 ℃, almost all grain boundary carbides dissolve in austenite.
Figure 4: The Effect of Tempering Temperature on Impact Toughness
2.3 The effect of tempering temperature on hardness and impact toughness
The effect of tempering temperature on impact toughness and hardness is shown in Figures 4 and 5. Within the temperature range of 650-710 ℃, at the same quenching temperature, as the tempering temperature increases, the impact toughness increases and the hardness decreases. When the tempering temperature is higher than 680 ℃, the increase in impact toughness and the decrease in hardness significantly increase, indicating that the rate of tempering transformation is significantly accelerated above 680 ℃. Therefore, for 2Cr12NiMoWV steel, tempering at temperatures above 680 ℃ is necessary to achieve sufficient tempering transformation and ensure that the tempered structure has sufficient stability and high toughness.
Figure 5: The Effect of Tempering Temperature on Hardness
2.4 Fracture analysis
Under the experimental conditions, the micro fracture morphology of the impact specimens subjected to various heat treatments is roughly similar, consisting of ductile dimples and quasi cleavage fracture zones (Figure 6). Fracture analysis shows that there is a good correlation between the width of the tough zone and the impact toughness of the sample. Samples with larger widths of the tough zone have higher impact toughness values; Samples with higher toughness have finer quasi cleavage zone tearing edges and smaller tearing units.
2.5 Heat treatment of high-temperature fastening bolts
High temperature fastening bolts are key components for ensuring the safe operation of steam turbines. According to the service conditions of the bolts, it is required to undergo quenching and tempering treatment. In order to avoid brittle fracture of the bolts during assembly and disassembly, they must have high room temperature impact toughness. The specific technical requirements are HB 277-331, aK＞35 J/cm2 ［3］。 Based on the results of this experiment, quenching at 1055-1070 ℃ and tempering at 700-710 ℃ were selected as the heat treatment processes for the bolts. Through this process, not only can the bolt be ensured to have high strength and toughness, but it can also have a relatively stable structure during long-term service. The hardness of the processed bolt is HB 287-299, aK＝40～62 J/cm2， All meet the technical requirements.
3 Discussions
(1) According to Figure 2710, under tempering conditions at 10 ℃, as the quenching temperature increases, the impact toughness first increases and then decreases. Analyzing the causes of this phenomenon indicates that the grain size of austenite and the degree of dissolution of grain boundary carbides are the main factors affecting the impact toughness of 2Cr12NiMoWV steel quenched and tempered at high temperatures. As the quenching temperature increases, the amount of carbide dissolution increases, especially in the annealed structure where grain boundary carbides dissolve into austenite, increasing the bonding force between grains and thus improving impact toughness; On the other hand, as the quenching temperature increases, the austenite grains grow, reducing the impact toughness. When the quenching temperature is below 1055 ℃, the former
Figure 6 Fracture morphology of impact specimen
(a) Tough pit area; (b) Quasi cleavage zone
Factors dominate, manifested as an increase in impact toughness with rising temperature; When the quenching temperature reaches 1055-1070 ℃, almost all grain boundary carbides dissolve into austenite, and the impact toughness reaches; Continuing to increase the quenching temperature leads to coarsening of austenite grains, and the latter factor becomes dominant, resulting in a significant decrease in impact toughness
(2) The recommended quenching temperature range for 2Cr12NiMoWV steel in the literature is mostly between 980-1040 ℃ [4.5]. According to the results of this experiment, due to the inevitable presence of a considerable amount of grain boundary carbides in the annealed structure of the steel, it is difficult to dissolve a large amount of grain boundary carbides during quenching in the above range, resulting in lower room temperature impact toughness after quenching and tempering. If the quenching temperature is appropriately increased to 1055-1070 ℃, most of the grain boundary carbides have dissolved and the austenite grains have not yet significantly coarsened. Then, tempering at an appropriate temperature can achieve a good combination of room temperature strength and toughness.
Reference [6] studied the effect of quenching temperature on the high-temperature properties of 2Cr12NiMoWV steel. The results showed that with the increase of quenching temperature, the endurance strength (σ 570105) increased. After quenching at 980, 1040, and 1100 ℃ and tempering at 680 ℃, the σ 570105 values were 85.3, 114.7, and 122.6 MPa, respectively. There is no significant difference in the long-term plasticity of the three temperature quenching methods, and the elongation (δ 10) is all greater than 10%. So increasing the quenching temperature appropriately can not only significantly improve the room temperature impact toughness, but also increase the durability strength without reducing the durability plasticity.
If the quenching temperature is increased to 1100 ℃, the impact toughness at room temperature is greatly reduced, while the endurance strength value increases very little. Moreover, due to the coarsening of austenite grains, the sensitivity of the steel to notches increases, which is not desirable for high-temperature fastening bolts with notches. In summary, it is recommended to heat quench 2Cr12NiMoWV steel in the temperature range of 1055-1070 ℃.
4 Conclusion

(1) At the same tempering temperature, the hardness of 2Cr12NiMoWV steel increases with the increase of quenching temperature; The room temperature impact toughness first increases and then decreases, reaching a value in the temperature range of 1055-1070 ℃. It is recommended that the quenching heating temperature range for this steel be between 1055 and 1070 ℃.
(2) The quenching and tempering heat treatment process for high-temperature fastening bolts of steam turbines made of 2Cr12NiMoWV steel is quenching at 1055-1070 ℃ and tempering at 700-710 ℃. The hardness of the sample treated by this process is HB 287-299, and the impact toughness is 40-62 J/cm2, which can meet the technical requirements of the bolt.