RU2741048C1 - Manufacturing method of binding rings for rotors of turbine generators - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: invention relates to production of binding rings for rotors of turbine generators from steel 15H15G16ANF. Ingot is made of steel 15H15G16ANF, then forged, ingot is pierced, combined with austenization of its steel, to produce annular billet. Preliminary machining of circular billet is performed, its hardening by means of finishing impact deformation with reduction of its temperature by 200...400 °C below minimum value of set interval of hot deformation of steel 15H15G16ANF. Additional annealing is performed by heating it to 700 ± 20 °C with holding at this temperature for 7 hours and cooling in air. Final machining of circular billet is performed.

EFFECT: simplified process.

1 cl, 2 tbl

Description

The invention relates to methods for the production of workpieces from corrosion-resistant steel of the austenitic class, in particular steel 15Kh15G16ANF (see the chemical composition in patent RU No. 2375492 C1).

The alleged invention can be used in the manufacture of blanks for parts for power engineering, mainly for shroud rings (hereinafter BC) of turbine generator rotors.

A known method of manufacturing shroud rings, including smelting an ingot, forging an annular billet on a press, heat treatment (austenitization), preliminary machining for strengthening by semi-hot rolling on a mandrel, semi-hot rolling on a mandrel, low-temperature heat treatment to relieve stresses and finishing machining in [one].

The disadvantage of this method is that it is used for the manufacture of blanks for shroud rings from medium-carbon non-corrosion-resistant steel grades of the 60Kh3G8N8V type and cannot be used for the manufacture of BK blanks from corrosion-resistant steels.

Also, the disadvantage of this method is the uncontrolled anisotropy of the mechanical properties of the metal around the circumference, height and thickness of the ring during strengthening by semi-hot deformation on the mandrel due to the implementation of fractional partial deformations, unevenly moved around the circumference when turning the ring blank. In this case, semi-hot deformation at low temperatures affects only the surface layers to a shallow depth, and during further operations the workpiece loses its strength characteristics. In addition, the peak loads on equipment and tooling increase significantly, which often leads to their failure.

There is also known a method of manufacturing shroud rings, including smelting an ingot, forging an annular billet on a press, heat treatment (austenitization), mechanical treatment before strengthening the annular billet by internal hydraulic pressure (hereinafter hydrostretching), hydrostretching operation, low-temperature heat treatment to relieve stress and finish machining in delivery dimensions [2] - prototype.

This method is mainly used for the manufacture of BC blanks from corrosion-resistant steels of the 12X18AG18-Sh type.

The disadvantages of this method are the complex multi-operation technology of strengthening by hydrostretching, the manufacture and use of expensive and bulky equipment for hydrostretching (see USSR AS No. 198915, 1967 MPK V30V), as well as equipment that is difficult to manufacture, set up and operate. In addition, when the annular blank is stretched in the cold state, as a result of intense tensile stresses, the strength and plasticity of the outer surface layers of the shroud ring is partially exhausted, which can adversely affect its performance as the most loaded part in turbine generators.

The problem solved by the alleged invention is to improve the mechanical properties of the blanks of the shroud rings by using steel 15Kh15G16ANF, simplify the technological process and reduce the production cost by combining a separate austenitizing operation with the piercing of the ring blank in the process of its forging, strengthening by reducing the temperature of the end of the final forging of the ring, using additional hardening heat treatment and, as a consequence, the elimination of a multistage hydrostretching operation associated with the manufacture and use of expensive, complex equipment and tooling, as well as the operation of low-temperature annealing to relieve stress.

The technology for manufacturing shroud rings blanks according to the prototype method consists of the following operations:

- steel smelting in electric arc furnaces;

- forging an annular billet on a press in the range of forging temperatures specified by the technical process;

- preliminary machining;

- a separate austenitizing operation;

- mechanical treatment for the operation of hydrostretching;

- the operation of hydraulic stretching of the annular workpiece using special, expensive equipment and tooling;

- low temperature annealing for stress relief;

- machining to final dimensions;

The presented technology is multi-operational and difficult to implement, since it requires bulky and expensive equipment to create high pressure when stretching an annular billet with internal hydraulic pressure to cold plastic deformation. It should be noted that the specified hardening operation is performed in several transitions, since requires high energy consumption for cold plastic deformation of an annular blank, i.e. each transition is quite long in time. At the same time, the equipment used is also complex in design, adjustment and operation.

This problem is solved by using corrosion-resistant, chromium-manganese, nitrogen-containing steel of austenitic class 15Kh15G16ANF (see patent RU 2375492 C1, authors: Stepurin A.V., Zibert O.A., Serebryakov D.V., Nureyev R.M.)

The chemical composition of the specified steel contains in%:

C 0.15-0.20; Si 0.2-1.0; Mn 15.0-16.0; Cr 14.0-16.0; Ni 0.8-1.2; N 0.3-0.45; V 0.2-0.4; Zr 0.02-0.04; Fe is the rest. If necessary, the ingot is refilled into electrodes and their subsequent electroslag remelting.

The temperature range of forging of ring billets from ingots of this steel, set by the technical process: Tinit. = 1210 ° C, Tnd of forging = 1000 ° C.

The specified steel is of the austenitic class, therefore, in the technological process of manufacturing ring forgings, there is a heat treatment (austenitization) operation, which is performed to improve the macro- and microstructure of metals of this class and is an effective means of preventing intergranular corrosion and imparting an optimal combination of mechanical and anticorrosive properties to the steel. Usually it is heating in the temperature range 900 ° -1100 ° C and rapid cooling in an appropriate environment. In the prototype method, it is performed as a separate operation, which entails additional costs and an increase in the duration of the technical process.

In the proposed method, austenitization is performed by combining the operations of piercing and quenching in a cooling medium.

In the technological process of forging a ring from an ingot, there is an operation of piercing a disk for further deformation. This operation is the least time-consuming, because consists of heating the disk blank to Weaving = 1210 ° C, moving it to the working area of the equipment and piercing with a tool in one press stroke. During this time, the workpiece is cooled to approximately 1000 ° ± 30 ° C, which is sufficient for the austenitization process. Then the sewn workpiece is sent to the cooling medium.

In addition, when using the proposed method, there is no need to carry out the operation of low-temperature annealing, which is not required due to the use of hardening heat treatment, which also reduces the cost and simplifies the technological process.

Further, a feature of 15Kh15G16ANF steel is that the content of manganese in the specified proportion and in combination with other components of the chemical composition during the final deformation of the ring, mainly by the shock method, with a decrease in the set temperature (1000 ° C) of the end of forging by 200 ° ... 400 ° C can significantly increase the mechanical properties of the forging due to work-hardening, which is confirmed by the results of mechanical tests of specimens cut from a ring sample taken from the body of the ring blank. At the same time, a decrease in the temperature of the end of forging by 200 ° C and less than the set temperature of the end of forging slightly increases the mechanical properties of the workpiece. A decrease in the temperature of the end of forging by more than 400 ° C increases the level of mechanical properties of the ring to a more necessary level, which, as a result, leads to an increase in unacceptable peak loads on the equipment.

After the end of the forging process, preliminary machining is performed before additional hardening heat treatment. The chemical composition of steel 15Kh15G16ANF contains carbide-forming elements chromium, manganese, vanadium, nitrogen, which form multicomponent structures of carbides of the type Me7C3, VC and MeC, including those with chromium carbides and carbonitrides.

These carbides dissolve in the austenite when heated during forging, and at the stage of hardening heat treatment are released from the supersaturated solid solution in the form of finely dispersed phases that increase the strength of the steel. In this case, the presence of nitrogen in this steel accelerates the processes of precipitation of carbide and carbonitride phases.

The process of additional hardening heat treatment is carried out as follows.

After preliminary machining, the annular workpiece is placed in a heating furnace, heated to a temperature of 700 ° ± 20 ° C, kept at this temperature for 7 hours, and then cooled in air. In this case, the structure of the steel changes due to the release of these carbides and carbonitrides into the metal matrix, which leads to its strengthening. The use of this operation makes it possible to exclude the laborious and complex operation of hydrostretching and the final operation of low-temperature annealing, which is present in the prototype method.

As a result of the application of the proposed method, the technological process of obtaining the annular blank of the retaining ring is greatly simplified by combining the austenitization process with the piercing operation and eliminating the low-temperature annealing operation. In addition, an increase in the mechanical properties of the final impact deformation of the forging at a low forging temperature and additional hardening heat treatment makes it possible to exclude the expensive and complex operation of hydrostretching of the annular billet.

Example.

A blank of a retaining ring was made with the following dimensions:

Ф500 X Ф404 X 125 mm made of steel 15Х15Г15АНФ with the chemical composition stated above.

For this, an ingot weighing 1.15 tons was smelted from pure charge materials in an induction furnace with a main lining. The ingot was reforged into electrodes and electroslag remelted. The result was an ingot with dimensions of 350 × 350 × 1000 mm, weighing 950 kg. Then, this ingot was forged on a press with a force of 500 tons. Before each forging operation, the billet was heated to a temperature of the onset of deformation of 1210 ° ± 20 ° C, and forging was finished at a temperature of 1000 ° ± 20 ° C. First, the ingot was crimped to an intermediate billet with a diameter of 270 mm, then the billets were divided into separate billets and upsetting discs with dimensions of Φ 370 × 160 mm were obtained. After that, the disks were heated to 1210 ° C, brought from the furnace to the working zone of the press, a 150 mm diameter piercing was placed on the disk, a hole was pierced, and the workpiece was immediately cooled in water. Measurement of the billet temperature before cooling showed that it dropped to 1000 ° -1020 ° C, which is sufficient for austenitization. The study of the ground templates showed the presence of a dense fine-grained structure, which also contributes to an increase in the strength characteristics of the metal.

Then the workpiece was again heated to the forging temperature and deformed by the impact method on a hammer with a MUF of 3 tons to obtain an annular workpiece of final dimensions Ф 525 × Ф 380 × 145 mm. In this case, the last, finishing passes were carried out with a decrease in temperature during deformation to 700 ° C ... 650 ° C. As a result, due to the tendency of chromium-manganese steel to hardening due to work-hardening, which is achieved, among other things, by impact deformation, the mechanical properties of products from this steel are significantly increased. The results of testing the mechanical properties of samples taken from a ring sample are shown in Table 1.

At the same time, in the last passes, the deformation forces were somewhat reduced, since the strength of the product increased and the load on the tooling and equipment increased.

Comparison of the mechanical properties of circular blanks made by the prototype method, which were made by forging with heating in the range of set forging temperatures and by the proposed method, i.e. with a decrease in temperature in the finishing passes are given in table. one.

Next, a preliminary turning of the workpiece was carried out in the dimensions of Ф505 × Ф400 × 130 mm.

The turned workpiece was placed in an SDO heating furnace and heated to a temperature of 700 ° ± 20 ° C and held for 7 hours, after which it was cooled in air. After cooling, a ring sample was taken, samples were cut in accordance with GOST 1497-84, and the mechanical properties were tested. The test results in comparison with the prototype method are shown in table 2

As a result of the application of the proposed method, the technological process is simplified and the cost of manufacturing shroud rings is reduced by 50 ... 70% by combining the piercing process with the austenitizing operation, strengthening by impact deformation with a decrease in the temperature of forging heating in the finishing passes, strengthening heat treatment of the annular billet and excluding a separate austenitizing operation, excluding expensive and complex, multi-stage hydrostatic expansion operation on special equipment and tooling, as well as the elimination of low-temperature annealing operations.

References:

1. E.P. Silina, Turbine generator rotor shroud rings, St. Petersburg, 2003, UDC 621.3.046: 669.14.018.584, ISBN 5-901300-03-4, p. 11-13.

2. E.P. Silina, Turbine generator rotor shroud rings, St. Petersburg, 2003, UDC 621.3.046: 669.14.018.584, ISBN 5-901300-03-4, p. 14-20.

Claims (1)

A method of manufacturing shroud rings for turbine generator rotors from steel 15X15G16ANF, characterized in that the ingot is smelted from steel 15X15G16ANF, its forging, piercing of the ingot, combined with the austenitization of its steel, to obtain an annular billet, its preliminary machining, strengthening of the annular billet by means of finishing impact deformation to ensure a decrease in its temperature by 200 ... 400 ° C below the lowest value of the established range of hot deformation of steel 15Kh15G16ANF, additional strengthening of the annular workpiece by heating it to a temperature of 700 ± 20 ° C with holding at this temperature for 7 hours and cooling in air and finishing machining of an annular blank.