RU2009215C1 - Method for production of shells operating under internal pressure - Google Patents

Abstract

FIELD: metal pressure working. SUBSTANCE: billet - hot-deformed pipe - is cut into billet lengths and machined, then it is deformed by rotary drawing, hardened at 850 C with cooling in water, tempered at 430 C for 2 hours with cooling in oil, deformed by rotary drawing at deformation degree of 50% , and squeezed out at 300 C for 2 hours. EFFECT: improved quality of the product.

Description

 The invention relates to the processing of metals by pressure and can be used in the manufacture of shells operating under internal pressure by rotational drawing.

 In modern engineering, traditional methods of increasing the strength properties of a metal are known by a combination of heat treatment methods and cold plastic deformation.

 Known methods are used in the production of pipes and do not allow to obtain the structural strength of the shells operating under internal pressure at a level close to the maximum resource of strength of structural steels, do not provide the ability to control the structural strength of the shells.

A method for manufacturing cylinders of alloyed structural steel burnishing comprising cutting blanks to length, heat treatment (quenching followed by tempering at 570 ^{° C,} machining, cold plastic deformation in several burnishing operations poligonizatsionnym followed by annealing at 570 ° ^C).

The disadvantage of this method is that quenching and tempering at 570 ^{° C,} followed by cold plastic deformation with a deformation degree 20.. . 65% do not provide the ability to obtain maximum structural strength and reduce the metal consumption of shells operating under internal pressure. This is due to the fact that after hardening performing tempering at 570 ^{° C,} tempering sorbitol obtained structure, which of all possible structural components in the steel provides the least during subsequent hardening of cold plastic deformation. Furthermore, poligonizatsionny annealing at a temperature of 570 ^{° C,} performed after cold plastic deformation, also leads to a softening of steel, which is due to the reduction of dislocation density and the stored energy from the formation and growth of small-angle dislocation subgrain.

 The aim of the invention is to increase the structural strength of the shells of structural steel, operating under internal pressure, by 20-35% while reducing metal consumption.

This goal is achieved by the fact that in the method of manufacturing shells operating under internal pressure, which includes operations: cutting pipe billets, hardening followed by tempering, machining and cold plastic deformation in several passes, tempering with tempering is performed before the last forming operation, tempering is carried out in the temperature range 350-500 ^C, and the cold plastic deformation is performed by rotating the hood with the strain of 30-60%, after which perform low-temperature otzhi at 280-450 ^° C

 It is known that hardening of steel occurs as a result of the inhibition of the motion of dislocations through crystals and their boundaries and is expressed in that more and more efforts are required to continue the deformation. From a physical point of view, strain hardening is the result of complex physicochemical processes: the occurrence during the deformation of a number of obstacles to the movement of dislocations, causing accumulation of dislocations inside the grain and the accompanying stresses of the third kind, curvature of slip planes, crushing into blocks of crystal parts enclosed between slip planes and the turns of these blocks. In addition, an interlayer with strongly distorted packing of atoms forms on both sides of the slip plane. There is an increase in the energy level of the system and the accumulation of potential energy.

 The occurrence of plastic deformation in structural steels has some features: the transfer of strain from grain to grain is associated with the formation of stress concentrators, plastic deformation mainly begins near grain boundaries and, at its first stages, is localized in a relatively narrow zone adjacent to the boundary, the density increases when grinding grain of dislocations participating in plastic deformation at its first stages, the formation of segregations of impurity atoms at grain boundaries, significantly affects the grain size ranichnogo hardening. The presence of grain boundaries, which play a barrier role in the path of moving dislocations, increases the yield strength and flow stress at significant degrees of deformation. It was experimentally established that the intensity is higher, the softer the steel, unhardened steels can be hardened as a result of hardening by 100% or more, hardened by 10-15%. The greatest increase in strength is observed in steels with the structure of austenite, ferrite and martensite. No significant effect of grain size on the deformation microstructure was found, however, the rate of change at low and medium degrees of deformation in finely dispersed material is higher than in coarse-grained material, and transverse sliding occurs more intensively in small grains. The decrease in the effect of grain size at large strains can be explained by grain fragmentation, which, together with the flattening of grains, increases the area of high-angle boundaries and therefore reduces the effect of the initial grain size.

Based on the foregoing, it can be concluded that the steel structure to obtain maximum strain hardening should be as dispersed as possible and contain incoherent particles that provide effective braking of dislocations; for structural steels, these may be particles of granite and ferrite carbides. The structure of tempering troostite - a highly dispersed ferrito - cementite mixture - satisfies these requirements. This structure is formed by quenching and tempering workpieces membranes, structural steels and hardening is performed from the known temperatures, and accommodation in the temperature range 350-500 ^C, when the carbon selection ends of martensite, cementite particles are completely incoherent and decrease microstresses 1 and 2 genus. The tempering temperature range is selected taking into account the degree of alloying of structural steel and the associated process of formation of a troostite structure with maximum dispersion. All the described transformations occur inside martensitic plates; therefore, ferrite and cementite particles have maximum dispersion. The proposed structural state of the steel is optimal from the point of view of manufacturability: it allows the use of high degrees of deformation - up to 60% in one operation and provides an increase in strength by 20-35%. An important property of strain hardening is that after the end of deformation and removal of the load, distortions of the crystal lattice are not removed by backward slip. They are retained due to the formation of a special kind of blockage, which persists until the temperature is raised to a value at which diffusion processes are sufficiently activated. It was established experimentally that the temperature range is within 280-450 ^{° C,} m. F. Above strain aging temperature range and below the temperature range of polygonization. In this temperature range, steel approaches the equilibrium state by the reaction between point defects and their annihilation, annihilation of dislocations of different signs and the disappearance of dislocation loops. From the metallophysical point of view, this is expressed in a decrease in the level of internal stresses without noticeable structural changes and a decrease in strength characteristics.

 Thus, after processing by the proposed method, structural steel will have a stable energy and structural state, with a strength level close to the ultimate. This ensures high technical characteristics of the shells during long-term operation.

 Based on the foregoing, we can conclude that the proposed method is new and has significant differences from previously known methods.

PRI me R. Production of a shell working under internal pressure from steel 20KHN4FA with an inner diameter of 199 mm. Hot-deformed pipes with a size of 219x12 mm, which is cut into measured billets, are taken as the initial billet. Then the workpieces are machined to a diameter of 217-0.3 mm and bored along the inner diameter to 199 + 0.3 mm, after which they are subjected to cold plastic deformation by rotational drawing to a wall thickness of 5 mm. Further quenching is performed from a temperature of 850 ^{° C} with cooling in water and tempering at 430 ^{° C} for 2 hours with cooling in oil. Thereafter, the last rotary extractor at a wall thickness of 2.5 mm at a deformation of 50% and a finish processing carrying out low-temperature annealing at 300 ^C for 2 hours.

As a result of such processing workpieces strength increases with σ _in = 1157 MPa σ _t = 1098 MPa and σ _in = 1540 MPa σ _t = 1481 MPa, while maintaining high ductility level δ 5> 7% KSU> 35 J / ^cm2. The increase in strength as a result of processing is 33%.

 Separate research results on steel 20XH4FA are presented in the table.

 An increase in the strength of the workpiece material made it possible to reduce the wall thickness of the shell from 3.1 mm to 2.5 without reducing the structural strength, as a result of which the mass of the shell decreased by 23% (from 34.9 kg to 26.8 kg). At the same time, geometric accuracy increased - the runout of the workpiece was reduced from 3.5 to 1.5 mm, the ovality from 5.0 mm to 2.0 mm.

 Based on the proposed processing method, a technological process has been developed for the manufacture of shells from structural steels. According to the developed technological process, an experimental batch of shells was made of steel 20XH4FA, 30XMA, 40X. In the process of testing a new technological process, it was found that the structural strength of the shells increases on average by 20-35%, with a good level of ductility (elongation of at least 7%). The level of residual internal stresses does not exceed 10-20% of the temporary resistance of the shell material, and the anisotropy of the mechanical properties along and across the axis of the shell does not exceed 3-5%. In the process of manufacturing the shells of the experimental batch, the possibility of replacing high-strength steel 28Kh3SNMVFA with cheaper 20KHN4FA, 30KhMA, which provide the same strength characteristics after processing in a new way, was revealed. This saves up to 800 rubles per 1 ton of pipes. Or, making shells from the same steel grades, but with a smaller wall thickness due to increased strength, reduce the weight of the shells by 10-20%. In this case, the savings is 200-300 rubles per 1 ton of pipes.

Using the proposed method for the manufacture of shells of structural steels operating under internal pressure, in comparison with known methods provides the following advantages:
the possibility of maximum use of the resource of strength of structural steels without reducing the operational characteristics of the shells;
the ability to reduce the wall thickness of the shells and reduce the metal consumption of the shells without loss of structural strength;
the possibility of using cheaper grades of structural steels instead of high alloy;
the ability to increase the geometric accuracy of the manufacture of shells;
the ability to control structural strength. (56) Forging and stamping, N 12, 1990.

 Copyright certificate N 1573037/02, cl. C 21 D 9/08, 1988.

Claims (1)

METHOD FOR MANUFACTURING SHELLS FROM STRUCTURAL STEELS WORKING UNDER INTERNAL PRESSURE, including cutting pipe blanks to measured lengths, hardening, tempering, machining, cold plastic deformation in several passes and annealing, characterized in that the strength is lowered with the aim of increasing the construction to increase metal consumption, quenching with tempering is carried out before the last pass of cold plastic deformation, the deformation is carried out by the method of rotational drawing with a degree of 30 - 60%, tempering at 350 - 500 ^o C, and annealing at 280 - 450 ^o C.

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