Treatment of common problems in heat treatment production of 15Cr1Mo1V forged sleeve

This article mainly analyzes the various mechanical properties of the 15Cr1Mo1V forged sleeve in the steam turbine during heat treatment production, proposes specific measures to solve these problems, and applies them to production. The practice has proven that these measures are correct and effective.

1. Introduction

The inner and outer sleeve of 15Cr1Mo1V forging is a part of the high and medium pressure steam connection pipe of the steam turbine. The commonly used working temperature is 535 ℃, and the working pressure ranges from tens to over 170 atmospheres (see Figure.1 and Figure.2 for the shape of the forging sleeve). Initially, our company used steel castings, but due to the unstable quality of the castings, they are prone to defects and cannot guarantee normal production cycles. Therefore, now all forgings are used. Although the situation has greatly improved, there are still some instances of unsatisfactory performance during the heat treatment production process. Therefore, we have made a series of attempts in long-term production practice and taken corresponding measures to address various unsatisfactory performance phenomena, achieving satisfactory results.

Figure.1 Rough machining diagram of inner sleeve forgings

Figure.2 Rough machining diagram of outer sleeve forgings

2. Material Introduction

15Cr1Mo1V belongs to low alloy heat strength steel medium chromium molybdenum vanadium steel, which is bainitic steel. The chemical composition of forgings is shown in Table.1, and the mechanical properties at room temperature are shown in Table.2. For 15Cr1Mo1V steel, the alloying elements Cr, Mo, and V are all reduced γ Phase zone γ Phase circle; Cr, V in α Infinite solid solution in iron; Mo and V are strong carbide forming elements; And these three alloying elements can increase the hardenability of the steel. The high content of Mo in steel can suppress tempering brittleness, and at higher tempering temperatures, it forms a dispersed distribution of special carbides with a secondary hardening effect. V can improve the creep and rupture strength of steel through the dispersion distribution of small carbide particles. Based on the above situation, various types of oil fired heating furnaces or resistance heating furnaces can be used in production, and the commonly used heat treatment process is quenching and tempering.
Table.1 Chemical composition (%)

Table.2 Mechanical Property Requirements

3. Analysis of common problems and measures taken

3.1 A total of 8 high-pressure outer sleeve pipes of a 125MW steam turbine need to undergo heat treatment

Each weighing 240kg. After heat treatment, the main body is cut and tested, and the performance is shown in Table.3.

Table.3

Note: * refers to performance nonconformities (the same as in the table below)
Table.4

From the table, it can be seen that the impact values of the five samples 512 #, 515 #, 517 #, 518 #, and 519 # are low, which is significantly different from the normal values. Near the fracture α 513 # with qualified KU value and α The 518 # sample with a low KU value underwent metallographic examination and the results were the same. The metallographic structure is ferrite and about 30% upper Bainite with a grain size of 6-7. The microstructure is so similar, but the impact toughness varies greatly, which cannot be explained theoretically. But we can find ways to improve the impact value from the metallographic structure. Carbides in upper Bainite are mainly distributed between ferrite Flat noodles, so the impact toughness of this structure is poor. Changing the morphology and distribution of carbides can improve the impact toughness. Based on the original tempering temperature, appropriately increasing the tempering temperature can cause some carbides to aggregate, grow, and spheroidize α Phase recovery and recrystallization. The actual situation is indeed the same. After using this process, the impact value increases and all performance meets the requirements. See Table.4.

3.2 Conduct heat treatment on the inner sleeve of a domestic 300MW steam turbine

One of the mechanical properties is shown in Table.5.
Table.5

Among them, the yield strength and impact toughness are lower than the required values. Metallographic analysis shows that there is a large amount of widmannstatten structure in the sample, accompanied by coarse ferrite grain size and directionality. The ferrite in the qualified samples is equiaxed, with a uniform structure and relatively fine grains. We all know that under the same heat treatment conditions, it is impossible to produce completely different structures, and the occurrence of this phenomenon indicates that the original organizational state is different. From the formation conditions of the widmannstatten structure, it can be inferred that the forging itself formed the widmannstatten structure due to improper cooling after forging, which is reflected in the low yield strength and impact toughness of the material after quenching and tempering. To improve both yield strength and impact toughness, the only way is to eliminate the widmannstatten structure. The commonly used methods are normalizing, annealing, and forging to refine the grains. We adopted the method of adding a high-temperature normalizing to eliminate the widmannstatten structure and then refining the grains through re-quenching and tempering, with satisfactory results. The performance is shown in Table.6.
Table.6

3.3 Conduct heat treatment on four high-pressure inner sleeves of a 125MW steam turbine

Its Chemical composition is shown in Table.7, and its mechanical properties are shown in Table.8.
Table.7 Chemical composition

Table.8 Mechanical Properties

Table.9

From Table.8, it can be seen that except for specimens 3087 # and 3088 # with lower impact toughness, all other properties are qualified. Metallographic analysis shows that the fracture surface of the impact sample is crystalline (partially), and the ferrite is distributed in a network. Based on this phenomenon, we believe that the reason for the network distribution of ferrite may be due to abnormal furnace temperature or component segregation of the forging. The equipment, instruments, and records used for inspection are all normal. From the impact values in Table.8, it can be seen that one data is normal and the other data is low, indicating that the material itself has uneven composition. So eliminating the ferrite network distribution is the key to improving the impact toughness of high-pressure inner sleeve. We adopt high-temperature normalizing to fully dissolve ferrite, carbides, and various alloy elements in austenite, with uniform composition. However, it will cause coarse grains, which can be refined by re-quenching and tempering. The phenomenon of ferrite network distribution can also be eliminated, achieving the expected goal. The mechanical properties of specimens 3088 # and 3087 # after reheat treatment are shown in Table.9.
When similar phenomena occur again in the production process in the future, the same heat treatment repair process is adopted, the effect is obvious, and the situation of low impact value is improved.
In the production of the 125MW steam turbine outer sleeve, there was a phenomenon of low elongation and impact toughness at the same time, and the impact value was about 10-15J/cm2 lower than the requirement. The actual performance values are shown in Table.10.
Table.10

We can see from the data in Table.10 that the strength indicators of samples 26 # and 27 # both meet the requirements, but are on the high side. The ductility index is lower than the required value. Based on this phenomenon, after analysis, we have decided to give up a certain amount of strength to obtain a certain degree of plasticity. After re-increasing the tempering temperature, the mechanical properties of samples 26 # and 27 # both meet the requirements. The specific data is shown in Table.11.
Table.11

Table.12

Table.13

During the heat treatment process of the inner sleeve, it is often encountered that the yield strength is low and other properties are qualified. This is illustrated by taking a certain production as an example.
At that time, 9 125MW steam turbines were subjected to heat treatment on their inner sleeves, and their mechanical properties were tested after cutting. The mechanical properties of samples 1 # and 3 # are shown in Table.14.
Table.14

From the data in Table.14, it can be seen that although the tensile strength barely exceeds, the strength indicators are all low. Therefore, we believe that it may be caused by the following two conditions: the first may be that more upper Bainite structure is produced due to insufficient cooling rate during quenching, which deteriorates the performance; The second possibility is due to improper temperature selection during the tempering process. Since these inner sleeves are operated according to conventional heat treatment specifications, the second case can be ruled out, which is mainly caused by more upper Bainite due to improper quenching operation in actual production. We re-quenched these two inner sleeves and lowered the tempering temperature appropriately, which proved our inference to be correct. Its performance is shown in Table.15.
Table.15

4. Conclusion

Through the analysis of the above typical examples, the following conclusions can be drawn:

• 1. After normal heat treatment of the sleeve, except for Charpy α In addition to the particularly low impact value of KU type, other mechanical properties are qualified, and an appropriate increase in tempering temperature can be used to improve the impact value and meet performance requirements.
• 2. When the yield strength and impact value are both lower than the required value, and other properties are qualified, it is mainly due to improper cooling after forging, resulting in a large amount of widmannstatten structure and coarse ferrite grains. Adding a normalizing process to eliminate the widmannstatten structure can improve the performance and meet the requirements.
• 3. The strength and hardness values are high, while the impact value is particularly low, mainly due to the presence of network ferrite. The impact toughness of the sleeve can be improved by adding a normalizing layer.
• 4. The strength and hardness are very high, but the elongation and impact values are low. The plasticity and toughness can be improved by giving up the strength appropriately.
• When the plasticity index is low and the strength and toughness are good, the tempering temperature can be appropriately increased to improve plasticity.
• When the yield strength of the sleeve is low due to insufficient quenching cooling rate, only reheat treatment is required to meet the performance requirements.

Author: Gong Li, Xia Zhilong