CN113186377B

CN113186377B - Heat treatment method for reducing residual stress of forging

Info

Publication number: CN113186377B
Application number: CN202110453632.3A
Authority: CN
Inventors: 李其; 徐超; 房鑫; 刘伟; 蒋新亮; 沈国劬; 余志勇
Original assignee: China Erzhong Group Deyang Heavy Industries Co Ltd; China United Heavy Gas Turbine Technology Co Ltd
Current assignee: China Erzhong Group Deyang Heavy Industries Co Ltd; China United Heavy Gas Turbine Technology Co Ltd
Priority date: 2021-04-26
Filing date: 2021-04-26
Publication date: 2022-02-01
Anticipated expiration: 2041-04-26
Also published as: CN113186377A

Abstract

The invention provides a heat treatment method for reducing the residual stress of a forging, which comprises the steps of heating the forging to a heat preservation temperature, and preserving the heat for a period of time; then, continuously heating the forging to AC3+ (40-100) DEG C, and preserving heat for a period of time; then, cooling the forge piece furnace to AC3+ (10-60) DEG C, and preserving the temperature for a period of time; and finally quenching and cooling. The method increases the steps of cooling the forging to AC3+ (10-60) DEG C and preserving heat for a period of time, has fewer improved parts, and ensures the universality of heat treatment equipment. When the temperature is reduced to AC3+ (10-60 ℃), the alloy elements are still in the redissolution and homogenization stages, the homogenization of the alloy components in the later period is not influenced, and the temperature difference between the center and the edge of the workpiece can be reduced on the premise that the cooling mode is not changed, so that the influence of thermal stress on the workpiece is reduced. The reduction of the quenching temperature reduces the temperature difference of the cross section of the workpiece in the quenching process and reduces the thermal stress, thereby reducing the integral residual stress of the forged piece after quenching.

Description

Heat treatment method for reducing residual stress of forging

Technical Field

The invention relates to the technical field of forging, in particular to a heat treatment method for reducing residual stress of a forging.

Background

The large forging has larger thickness, and the temperature drop rate of the surface and the core is inconsistent and time difference exists during cooling and cooling in the performance heat treatment, namely the quenching process, so that an uneven temperature field is formed; in the process of structure transformation, because the cooling speed of the surface and the cooling speed of the core are different, the structure transformation is asynchronous, an uneven temperature field and asynchronous structure transformation are caused, and residual stress in the forge piece is formed.

The forging pieces of the gas turbine, the steam turbine or the motor have higher requirements on the residual stress level of the workpiece due to the working condition requirements. The workpiece is heavy in weight, large in size and bad in operation condition, and if the residual stress of the workpiece body is too large, potential hazards are formed on the operation safety of equipment. Therefore, when the technical conditions of the forgings are formulated, the residual stress level of the workpiece body is also an important standard for the qualification of the workpiece. The residual stress testing occasion is generally after the performance heat treatment and before the performance test, if the residual stress is too high, the stress relief treatment is needed. At present, the conventional method is to add one-time stress relief treatment after performance heat treatment, and the mode has long period, high energy consumption and poor environmental protection.

The invention application with the application number of 201310562501.4 discloses a heat treatment method of a thick forging for nuclear power, which is to heat the forging of SA-508Gr.3 material to 880-920 ℃, then cool the forging to 788-820 ℃, and quench after heat preservation. The quenching temperature can be reduced, so that the temperature difference between the inside and the outside of the forging is reduced. It is known that, during heat treatment, the forging needs to be heated to a temperature above the temperature of AC3, so that the forging completes austenite transformation, and the temperature is kept at a temperature higher than that of AC3, so that the alloy elements can be maintained to be redissolved and homogenized, and the forging has uniform components. However, the temperature of the AC3 of the SA-508Gr.3 material is 820 ℃, when the temperature of the forged piece is reduced to 788-820 ℃, the temperature is lower than the temperature of AC3 (820 ℃), so that the alloy elements are difficult to maintain redissolution and homogenization, and the forged piece may have insufficient components after quenching, thereby affecting the performance. In addition, the temperature of the forge piece is reduced from 880-920 ℃ to 788-820 ℃ by adopting an air cooling mode, firstly, the operation is troublesome, the forge piece needs to be taken out of the furnace for cooling, and then the forge piece needs to be put into the furnace again during heat preservation, so that the times of taking and placing the forge piece are increased, the weight of the thick and large forge piece is heavy, the taking and placing consumes longer time, the efficiency is influenced, and the temperature is difficult to control accurately; and secondly, the cooling speed of air cooling is high, the surface temperature of the thick and large forgings is reduced quickly, the central temperature is reduced slowly, and thermal stress is easy to generate.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a heat treatment method for reducing the residual stress of a forging, the residual stress is reduced in the performance heat treatment process of the forging, the stress removing treatment is not required after the performance heat treatment, the working procedures can be reduced, the production efficiency is improved, and the uniformity of the components of the forging is ensured.

The technical scheme adopted by the invention for solving the technical problems is as follows: the heat treatment method for reducing the residual stress of the forged piece comprises

Heating the forging to a heat preservation temperature, and preserving heat for a period of time; then, continuously heating the forging to AC3+ (40-100) DEG C, and preserving heat for a period of time; then, cooling the forge piece furnace to AC3+ (10-60) DEG C, and preserving the temperature for a period of time; and then quenching and cooling by using a quenching medium.

Further, heating the forging piece to a heat preservation temperature at a heating rate of 40-80 ℃/h, and preserving heat for a period of time; then heating the forge piece to AC3+ (40-100) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for a period of time; cooling the forging to AC3+ (10-60) DEG C at a cooling speed of less than or equal to 60 ℃/h, and preserving heat for a period of time; and then quenching and cooling by using a quenching medium.

Further, the ratio of the nominal thickness of the forging to the holding time of the forging at the holding temperature is less than or equal to 50 mm/min.

Further, the ratio of the nominal thickness of the forging to the heat preservation time of the forging at the temperature of AC3+ (40-100) DEG C is less than or equal to 10 mm/min.

Further, the ratio of the nominal thickness of the forging to the heat preservation time of the forging at the temperature of AC3+ (10-60) DEG C is less than or equal to 30 mm/min.

Further, the quenching cooling is water cooling or oil cooling.

Further, the forging is made of medium alloy steel.

The invention has the beneficial effects that: the method increases the process of cooling the forging to AC3+ (10-60) DEG C and preserving heat for a period of time, and the rest processes are conventional heat treatment processes, so that the improved parts are fewer, the universality of heat treatment equipment is ensured, the sufficient dissolution and uniformity of alloy elements in the material are also ensured, and a foundation is laid for the structure transformation in the later quenching and cooling process. When the temperature of the forge piece is reduced to AC3+ (10-60) DEG C, the alloy elements are still in the redissolution and homogenization stages, the homogenization of the alloy components in the later period is not influenced, and the temperature difference between the center and the edge of the workpiece can be reduced on the premise that the quenching mode is not changed, so that the influence of thermal stress on the workpiece is reduced. The reduction of the quenching temperature reduces the temperature difference of the cross section of the workpiece in the quenching process and reduces the thermal stress, thereby reducing the integral residual stress of the forged piece after quenching. In addition, the furnace cooling is adopted, the cooling speed is low, the synchronous cooling of the surface and the center of the forge piece can be ensured, the temperature difference is reduced, and the residual stress is reduced. In addition, the forge piece does not need to be taken out during furnace cooling, the times of taking and placing the forge piece are reduced, and the heat treatment process is simplified.

Drawings

FIG. 1 is a schematic view of a prior art thermal process;

FIG. 2 is a schematic view of the heat treatment of the present invention;

FIG. 3 is a grain size metallographic photograph of a prior art heat treated forging;

FIG. 4 is a grain size metallographic photograph of a forging after treatment according to example one;

FIG. 5 is a metallographic photograph of the structure of a prior art heat treated forging;

FIG. 6 is a metallographic structure photograph of a forging after treatment according to one embodiment.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

The conventional forging performance heat treatment is shown in figure 1, the forging is heated to a heat preservation temperature at a constant speed, the temperature is preserved for a period of time, then the forging is heated to a quenching temperature, the quenching temperature is AC3+ (40-100) DEG C, the temperature is preserved for a period of time, and then quenching and cooling are carried out by a specific quenching medium. The quenching temperature is selected to be AC3+ (40-100) DEG C, so that the alloy elements in the material are fully dissolved and uniform, and the structure of the forging is uniform.

The heat treatment method for reducing the residual stress of the forged piece, shown in figure 2, comprises the following steps

Compared with the existing heat treatment process, the method has the improvement points that the process of reducing the temperature of the forge piece from AC3+ (40-100) DEG C to AC3+ (10-60) DEG C is added, the forge piece is kept warm for a period of time, and then quenching is carried out, so that the quenching temperature is reduced, the temperature difference of each part in the quenching process is reduced after the quenching temperature of the forge piece is reduced, and the integral residual stress of the forge piece after quenching is reduced.

In the process of cooling the forging from AC3+ (40-100) DEG C to AC3+ (10-60) DEG C and the heat preservation process of the forging at the temperature of AC3+ (10-60) DEG C, the alloy elements are still in the stages of re-dissolution and homogenization, and the homogenization of the later alloy components is not influenced, so that the obvious influence on other performance indexes of the forging is avoided, and the target required by performance heat treatment is achieved. The temperature of the forge piece is reduced from AC3+ (40-100) DEG C to AC3+ (10-60) DEG C in a furnace cooling mode, the temperature of the forge piece is not lower than AC1+10 ℃ in the furnace cooling process, and otherwise the performance of the forge piece is affected. The cooling speed of furnace cooling is relatively low, so that the synchronous cooling of the surface and the center of the forge piece can be ensured, the temperature difference is reduced, and the residual stress is reduced. In addition, the forge piece does not need to be taken out during furnace cooling, the times of taking and placing the forge piece are reduced, and the heat treatment process is simplified.

Compared with the existing performance heat treatment process, the invention has simpler improvement point, can completely adopt the existing performance heat treatment equipment to finish the improved heat treatment process, does not need to adopt new equipment, and has low improvement cost. The heat treatment of the invention can realize the improvement of the performance and the reduction of the stress of the forging at the same time, and the stress removing treatment of the forging is not needed after the heat treatment, thereby simplifying the whole manufacturing process of the forging, shortening the manufacturing period and improving the manufacturing efficiency.

In order to ensure that the whole forge piece is stably heated and cooled and avoid the phenomenon that the temperature difference between the inside and the outside is too large, the heating and cooling rate of the forge piece needs to be controlled, specifically, in the invention, the forge piece is heated to a medium temperature at the heating rate of 40-80 ℃/h and is kept for a period of time; then heating the forge piece to AC3+ (40-100) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for a period of time; cooling the forging to AC3+ (10-60) DEG C at a cooling speed of less than or equal to 60 ℃/h, and preserving heat for a period of time; and then quenching and cooling by using a quenching medium.

The effect of preserving heat of the forging at a plurality of temperatures is to ensure that the temperature inside and outside the forging is uniform, reduce the temperature difference inside and outside the forging, the size of the forging is one of the main factors influencing the temperature difference inside and outside the forging, the larger the nominal thickness of the forging is, the longer the heat preservation time is, therefore, in the invention:

the ratio of the nominal thickness of the forging to the heat preservation time of the forging at the heat preservation temperature is less than or equal to 50 mm/min.

The ratio of the nominal thickness of the forge piece to the heat preservation time of the forge piece at the temperature of AC3+ (40-100) DEG C is less than or equal to 10 mm/min.

The ratio of the nominal thickness of the forge piece to the heat preservation time of the forge piece at the temperature of AC3+ (10-60) DEG C is less than or equal to 30 mm/min.

Therefore, the shortest heat preservation time can be determined according to the nominal thickness of the forging, and the heat preservation time is increased on the basis of the shortest heat preservation time for ensuring the heat penetration of the large forging.

Quenching cooling is generally water cooling, oil cooling or the two are used alternately.

The invention is preferentially suitable for forgings made of medium alloy steel, the medium alloy steel has high austenite stability and strong hardenability, and the supercooling degree is only reduced by a small margin after the quenching temperature is reduced, so that the structure transformation type is not obviously influenced. Of course, the method can also be used for forgings made of other steel types.

Example one

Taking a forging with the diameter of phi 2200mm multiplied by 400mm as an example, heating the forging to the intermediate temperature at the heating rate of 40-80 ℃/h, and keeping the temperature for 200 min; then heating the forge piece to AC3+ (50-70) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for 300 min; cooling the forging to AC3+ (10-30) DEG C at a cooling speed of 30-50 ℃/h, and keeping the temperature for 120 min; and then quenching and cooling by using water as a quenching medium. And (5) tempering treatment after cooling.

Example two

Taking a forging with the diameter of phi 2200mm multiplied by 400mm as an example, heating the forging to the intermediate temperature at the heating rate of 40-80 ℃/h, and keeping the temperature for 200 min; then heating the forge piece to AC3+ (50-70) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for 300 min; cooling the forging to AC3+ (10-30) DEG C at a cooling speed of 30-50 ℃/h, and keeping the temperature for 120 min; and then oil is used as a quenching medium for quenching and cooling. And (5) tempering treatment after cooling.

EXAMPLE III

Taking a forging with the diameter of phi 2200mm multiplied by 400mm as an example, heating the forging to the intermediate temperature at the heating rate of 40-80 ℃/h, and keeping the temperature for 250 min; then heating the forge piece to AC3+ (40-50) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for 350 min; cooling the forging to AC3+ (10-30) DEG C at a cooling speed of 30-50 ℃/h, and keeping the temperature for 150 min; and then quenching and cooling by using water as a quenching medium. And (5) tempering treatment after cooling.

Example four

Taking a forging with the diameter of phi 2200mm multiplied by 400mm as an example, heating the forging to the intermediate temperature at the heating rate of 40-80 ℃/h, and keeping the temperature for 300 min; then heating the forge piece to AC3+ (70-100) DEG C at a heating rate of 40-80 ℃/h, and preserving heat for 350 min; cooling the forging to AC3+ (40-60) DEG C at a cooling speed of 30-50 ℃/h, and preserving heat for 180 min; and then quenching and cooling by using water as a quenching medium. And (5) tempering treatment after cooling.

Comparative example: the prior art shown in figure 1 is adopted to carry out heat treatment on the forging with the diameter of 2200mm multiplied by 400mm, and water, oil and air are respectively adopted as quenching media.

The center, radius 1/2 and edge of the forging subjected to water cooling treatment of the first embodiment and the comparative example are respectively the same, and the water cooling stress of the forging obtained by the first embodiment and the original performance heat treatment is compared by using Deform simulation software, and the results are as follows:

table 1 comparison table of equivalent stress at different positions of the example and the original heat treatment mode

The grain size metallographic photograph of the forging obtained by water cooling of the comparative example is shown in fig. 3, the structure metallographic photograph is shown in fig. 5, the grain size metallographic photograph of the forging obtained in the first example is shown in fig. 4, and the structure metallographic photograph is shown in fig. 6.

The properties of the forgings obtained in examples one to two and comparative example are compared as shown in the following table:

TABLE 2 comparison of the Heat treatment mode of the present invention and the original energy Heat treatment mode

And (4) conclusion:

1. in the aspects of tissue and grain size, the improved heat treatment mode of the invention has no difference from the original mode;

2. in the aspect of impact performance, the heat treatment mode after the improvement is not greatly different from the original mode.

3. In the aspect of strength performance, the improved heat treatment mode of the invention has slightly lower performance than the original mode.

4. In terms of residual stress, the invention is improved and obviously reduced compared with the original mode.

The forged piece obtained in the third embodiment and the forged piece obtained in the fourth embodiment are detected, the performance indexes of the forged piece are basically the same as those of the forged piece obtained in the first embodiment and the forged piece obtained in the second embodiment, and both the performance indexes can meet the design requirements.

Therefore, under the process mode of the patent, the residual stress of the workpiece can be obviously reduced on the premise of not obviously reducing the material performance.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. The heat treatment method for reducing the residual stress of the forged piece is characterized by comprising

Heating the forging piece to a heat preservation temperature at a heating rate of 40-80 ℃/h, and preserving heat for a period of time; then, continuously heating the forge piece to AC3+ (40-100) DEG C at the heating rate of 40-80 ℃/h, and preserving the heat for a period of time; then, cooling the forge piece furnace to AC3+ (10-60) DEG C, and preserving the temperature for a period of time; and then quenching and cooling by using a quenching medium.

2. The heat treatment method for reducing the residual stress of the forging as claimed in claim 1, wherein the forging is cooled to AC3+ (10-60) DEG C at a cooling rate of less than or equal to 60 ℃/h.

3. The heat treatment method for reducing the residual stress of the forging of claim 1, wherein the ratio of the nominal thickness of the forging to the holding time of the forging at the holding temperature is less than or equal to 50 mm/min.

4. The heat treatment method for reducing the residual stress of the forging as claimed in claim 1, wherein the ratio of the nominal thickness of the forging to the holding time of the forging at the temperature of AC3+ (40-100) DEG C is less than or equal to 10 mm/min.

5. The heat treatment method for reducing the residual stress of the forging as claimed in claim 1, wherein the ratio of the nominal thickness of the forging to the holding time of the forging at the temperature of AC3+ (10-60) DEG C is less than or equal to 30 mm/min.

6. The heat treatment method for reducing the residual stress of the forging according to claim 1, wherein the quenching cooling is water cooling or oil cooling.

7. The heat treatment method for reducing the residual stress of a forging according to claim 1, wherein the forging is made of medium alloy steel.