JP2010236006A - Restoration heat-treatment method for metal member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treatment method which can surely and sufficiently restore creep strength at a weld heat-affected zone of a metal member. <P>SOLUTION: This heat treatment method includes: heating the weld heat-affected zone of the metal member to the temperature T1 of an A<SB>3</SB>transformation temperature or higher, with a heating apparatus; holding the metal member at the temperature T1 for a predetermined time; subsequently lowering the temperature of the metal member to a predetermined temperature T3; then re-heating the metal member to the temperature T2 of lower than the A<SB>3</SB>transformation temperature; holding the metal member at the temperature T2 for a predetermined time; and cooling the metal member to room temperature. When heating the metal member to the temperature T1, a heating rate is set at 50°C/h or more but less than 800°C/h. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for recovering the creep strength of a welding heat-affected zone of a metal member used under high temperature and high pressure in a thermal power plant or the like by regenerative heat treatment.

  In equipment (such as a boiler or a turbine) used in a thermal power plant or the like, high-temperature and high-pressure (550 ° C. to 600 ° C., 250 MPa) gas is handled. In equipment used in such an environment, the heat affected zone (HAZ) of a metal member (for example, a steel pipe through which gas flows) generated by prolonged exposure to high-temperature and high-pressure gas is used. Creep damage is a particular problem. This is because, in the vicinity of the welded portion, a weld heat affected zone is generated due to the heat effect during welding, and when used under high temperature and high pressure, it is particularly susceptible to creep damage.

Therefore, various methods have been proposed in the past for the purpose of restoring the strength of the metal member that has been lowered due to creep damage. For example, Patent Document 1, by heating holding the metallic member at A ₃ transformation point or above the temperature, intensity recovery method to recover the strength of the degradation of the metallic member is described. According to this method, in order to recover the creep strength of the deteriorated part, the deterioration portion is heated to a temperature of at least A ₃ transformation point at a heating rate of about 850 ° C. / h.

JP 2008-132511 A

However, the present inventors have found that the various experiments and considerations, the heating rate to a temperature of at least A ₃ transformation point has been found that a significant effect on creep strength recovery deteriorated part.

  An object of the present invention is to provide a heat treatment method capable of reliably and sufficiently recovering the creep strength of a weld heat affected zone of a metal member.

  The gist of the present invention is as follows.

A heat treatment method according to the present invention is a heat treatment method for recovering the creep strength of a welding heat-affected zone of a metal member used under high temperature and high pressure, wherein the welding heat-affected zone of the metal member is 50 ° C./h or more. at a heating rate of less than 800 ° C. / h are those comprising the step of heating to a temperature of at least a ₃ transformation point.

According to this heat treatment method, the weld heat affected zone of the metal member is heated to a temperature of at least A ₃ transformation point, it is possible to recrystallize the grain. Since recrystallization causes nucleation from grain boundaries, particularly where carbides exist, voids that are one of the causes of creep strength degradation exist at the crystal grain boundaries in the weld heat affected zone of metal parts before heat treatment. Even in such a case, the void can be taken into the crystal grains after recrystallization.

Moreover, even when carbide is present at the crystal grain boundary of the weld heat affected zone of the metal member before the heat treatment, the carbide can be dissolved and refined. As a result, it is possible to prevent the occurrence of cracks and crack growth due to the deterioration of the creep strength of the weld heat affected zone of the metal member starting from the void or carbide. Further, according to this heat treatment method, the weld heat affected zone of the metal member is heated to a temperature of at least A ₃ transformation point at a heating rate of less than 50 ° C. / h or higher 800 ° C. / h, about conventional heating rate 850 ° C. / h or more a ₃ transformation point or more the grain size of the crystal grains to be generated as compared to the thermal treatment heating to a temperature can be sufficiently increased. Therefore, the creep strength of the weld heat affected zone of the metal member can be sufficiently recovered.

The heat treatment method may further include a step of cooling to 300 ° C. or lower after the step of heating to a temperature equal to or higher than the A ₃ transformation point and reheating to a temperature lower than the A ₃ transformation point. In this case, the mechanical properties of the creep strength of the weld heat affected zone of the metal member can be further improved by homogenizing the structure by tempering treatment.

  According to the present invention, the creep strength of the weld heat affected zone of the metal member can be reliably and sufficiently recovered.

It is a figure which shows an example of the regeneration heat processing method which concerns on this Embodiment. It is sectional drawing of a metal member. It is a photograph which shows the crystal grain of HAZ of the metal member used for a long time in the environment of high temperature and high pressure. It is a figure which shows the heat history in the regenerative heat processing. It is a photograph which shows the crystal grain of HAZ after regenerative heat processing. It is a figure which shows the heat history of the regeneration heat processing in an Example. It is a figure which shows an Example and a comparative example creep rupture characteristic.

  Hereinafter, a heat treatment method according to an embodiment of the present invention will be described. In the following, the heat treatment method according to the present invention will be described by taking the method for recovering the creep strength of the weld heat affected zone of the steel pipe as an example.

  FIG. 1 is a diagram illustrating an example of a heat treatment method according to the present embodiment. FIG. 1 shows a metal member 10 composed of a steel pipe 11 and a steel pipe 12 used under high temperature and high pressure. The steel pipe 11 and the steel pipe 12 are connected by a weld metal 13. As shown in FIG. 1, in the heat treatment method according to the present embodiment, a cloth-like electric resistance heater 14 (hereinafter referred to as a heater) is formed so as to cover the outer peripheral surface of the metal member 10 (welding heat affected zone including the weld metal 13). 14 is abbreviated as 14). In this state, the metal member 10 is heated by the heater 14. Thereby, the creep strength recovery of the weld heat affected zone of the metal member 10 is performed. In addition, the heater 14 is comprised by the nichrome wire, for example. Hereinafter, the heat treatment method according to the present invention will be described in detail.

  FIG. 2 is a cross-sectional view showing the weld metal 13 of the metal member 10 and its peripheral portion. As shown in FIG. 2, weld heat affected zones 15 and 16 (hereinafter abbreviated as HAZ 15 and 16) are formed in the vicinity of both ends of the weld metal 13 of the steel pipes 11 and 12. The inventors conducted various experiments paying attention to the HAZ 15 and 16 and considered a method for recovering the creep strength of the weld heat affected zone of the metal member 10.

  FIG. 3 is a photograph showing crystal grains of the HAZ 15 and 16 of the metal member 10 used for a long time in a high temperature and high pressure environment.

  As shown in FIG. 3, in the HAZ (reference numerals 15 and 16 in FIG. 2) of a metal member used for a long time in a high temperature and high pressure environment, a crystal grain 21 (in FIG. 3, a part is shown by a solid line). .)), A plurality of voids 22 are generated and a plurality of carbides 23 are precipitated. The present inventors considered that these voids 22 and carbides 23 cause crack initiation and crack growth at the grain boundaries of the crystal grains 21. And the present inventors thought that if the void 22 and the carbide 23 can be confined in the crystal grains, the creep damage of the metal member can be prevented and the creep strength can be recovered.

Therefore, the present inventors tried recrystallization of the crystal grains by performing regenerative heat treatment of the metal member by the method described below in order to confine the voids 22 and carbides 23 in the crystal grains. The material is 2.25 (wt%) Cr-1 (wt%) Mo steel welded part of high temperature piping for boilers with outer diameter of 720mmφ and wall thickness of 60mm, operated for about 300,000 hours at 570 ° C and 210MPa high temperature and high pressure Various test pieces cut out from the vicinity in parallel to the length direction of the tube were used. Note _{A 3} transformation point of the member is about 880 ° C..

FIG. 4 is a diagram showing a thermal history in the regeneration heat treatment. In this restoration thermal treatment, first, a test piece including HAZ (code 15, 16 in FIG. 2) of the metal member _{A 3} transformation point or more temperature T1 (for example, 920 ° C.) was heated at a heating rate of 300 ° C. / h until . Thereby, while being able to recrystallize a crystal grain, the precipitated carbide | carbonized_material can be made into solid solution. Next, after holding the test piece at a temperature T1 for about 20 minutes, it was air-cooled to a temperature T3 (for example, 300 ° C.). Next, a test piece temperature below A ₃ transformation point T2 (e.g., 740 ° C.) was heated again until, and held in that state for 60 minutes. Thereafter, the test piece was air-cooled to room temperature. Thereby, internal stress can be relieved and the structure can be made uniform.

  FIG. 5 is a photograph showing crystal grains of the HAZ (reference numerals 15 and 16 in FIG. 2) after the heat treatment. In FIG. 5, crystal grains before heat treatment (reference numeral 21 in FIG. 3) are partially shown by dotted lines, and new crystal grains 31 after heat treatment are partially shown by solid lines.

  As shown in FIG. 5, the crystal grains are recrystallized by the above regenerative heat treatment, and new crystal grains 31 (a part of which is shown by a solid line in FIG. 5) are generated. Here, since carbon (C), which is an element that generates recrystallized grain nuclei, is present in the vicinity of the carbide 23, new crystal grains 31 are formed as crystal grains 21 before heat treatment (in FIG. 5). , A part of which is indicated by a broken line.) Grows from the vicinity of the carbide 23 existing at the grain boundary. Thereby, as shown in FIG. 5, the void 22 can be taken into the new crystal grain 31. Further, the carbide (reference numeral 23 in FIG. 3) is once dissolved by the above heat treatment, refined, and re-precipitated in the new crystal grains 31.

  Thus, in the new crystal grain 31 after recrystallization, the frequency with which the void 22 and the carbide | carbonized_material 23 exist on a grain boundary becomes low. Moreover, the carbide | carbonized_material 23 is refined | miniaturized. Thereby, it is possible to prevent cracks from starting from the voids 22 or the carbides 23 at the grain boundaries of the new crystal grains 31. As a result, the creep strength of the weld heat affected zone of the metal member 10 is recovered.

However, a further investigation by the present inventors, the heating rate to a temperature of at least A ₃ transformation point has been found that a significant effect on creep strength recovery deteriorated part. Specifically, the weld heat affected zone of the metal member 10 when the heating rate when heating to a temperature of at least A ₃ transformation point is high, the new grain size of the crystal grains 31 after recrystallization is sufficiently large On the contrary, when the heating rate is slow, the creep strength of the weld heat affected zone of the metal member 10 is sufficiently recovered even though the grain size of the new crystal grain 31 after recrystallization is sufficiently large. Turned out not to be. Therefore the present inventors have found that by heating the weld heat affected zone of the metal member 10 to the A ₃ transformation point lower than a heating rate of 50 ° C. / h or higher 800 ° C. / h, can be sufficiently recovered creep strength degradation portion Propose that.

Therefore, in this embodiment, the heating rate when heating the weld heat affected zone of the metal member 10 to the A ₃ transformation point set to less than 50 ° C. / h or higher 800 ° C. / h, as described in FIG. 4 heat treatment I do. Thereby, the creep strength of the welding heat affected zone of the metal member 10 can be sufficiently recovered. In order to recover the creep strength, rapid heating (850 ° C./h or more) or void pressing by restraint is not necessary. Since rapid heating is not required, the risk of tensile thermal stress can be reduced, and the creep strength can be easily recovered not only to the extreme surface layer portion of the metal member 10 but also to the depth of the meat.

The temperature T1 in FIG. 4 may be a temperature of more than A ₃ transformation point. Preferably _{A 3} transformation point + 30 ° C. to A ₃ transformation point + 100 ° C., is 1200 ° C. about the upper limit. Moreover, in the example of FIG. 4, after heating the welding heat affected zone of the metal member 10 to the temperature T1, the weld heat affected zone of the metal member 10 is reheated to the temperature T2, but the metal is not reheated. The weld heat affected zone of the member 10 may be cooled to room temperature.

The temperature T2 may be a temperature lower than A ₃ transformation point. Further, the cooling rate of the weld heat affected zone of the metal member 10 is not particularly limited, and the metal member 10 may be air-cooled (for example, cooling rate 1000 ° C./h) or gradually cooled (for example, cooling rate 150 ° C./h). ). Also, A ₃ holding time at lower than the transformation point of the temperature is not limited to the above example, it may be longer than 20 minutes may be shorter. The heating may be repeated a plurality of times for the metal member 10 to a temperature higher than A ₃ transformation point. Further, for the heat treatment, a resistance-type heating device such as the cloth-like electric resistance heater 14 (see FIG. 1) may be used, or a high-frequency induction heating device may be used.

  The effects of the present invention will be described below based on examples. In this example, a test piece (material) made of 2.25 (wt%) Cr-1 (wt%) Mo steel was used. In addition, the test piece is welded in the center part, and the welding heat affected zone is formed.

  First, three test pieces (new materials) were prepared, and a uniaxial creep test of 39.2 MPa was performed in an environment of 650 ° C., 675 ° C., and 700 ° C., and the rupture time was measured. As a result, in the test at 650 ° C., the test piece broke at 2812.4 h, in the test at 675 ° C., it broke at 726.8 h, and in the test at 700 ° C., it broke at 163.4 h.

  Next, three test pieces (new materials) having the same configuration were prepared, and a uniaxial creep test of 39.2 MPa was performed in an environment of 650 ° C. for 2161 h, and the test was stopped. In this example, the three test pieces are used as the test pieces of Example 1, Example 2, and Comparative Example 1. In addition, the creep test execution time (2161h) of the test pieces of Examples 1 and 2 and Comparative Example 1 corresponds to 77% of the breaking time (2812.4h) of the test piece in the 650 ° C. creep test described above.

  The test pieces of Examples 1 and 2 and Comparative Example 1 produced as described above were subjected to regenerative heat treatment using a high frequency induction heating apparatus. In the regenerative heat treatment, as shown in FIG. 6, the heating temperature of the heating device is increased from room temperature to 920 ° C. between time points t0 and t1, and is maintained at the heating temperature of 920 ° C. for 20 minutes between time points t1 and t2. During −t3, the test piece was cooled to room temperature. Specifically, the test pieces of Examples 1 and 2 and Comparative Example 1 were heat-treated under the conditions shown in Table 1 below.

  After the heat treatment, the rupture time was evaluated by a uniaxial creep test at 680 ° C. and 39.2 MPa. The result is shown in FIG. In FIG. 7, the vertical axis indicates the test temperature, and the horizontal axis indicates the breaking time. The horizontal axis is shown on a logarithmic scale. FIG. 7 also plots the results of the uniaxial creep test of the test piece (0% damaged material: new material) under the above-described environments of 650 ° C., 675 ° C., and 700 ° C. An approximate straight line (two-dot chain line) is drawn. This approximate straight line indicates the expected fracture time of the test piece (new material) at an arbitrary test temperature.

  FIG. 7 plots the expected rupture times of 27% damaged material, 56% damaged material, and 77% damaged material in a uniaxial creep test at 650 ° C. and 39.2 MPa. The 27% damaged material is a test piece obtained by conducting a uniaxial creep test at 39.2 MPa in an environment of 650 ° C. for 756 hours (a time corresponding to 27% of the breaking time (2812.4 h) of the new material). . Similarly, a 56% damaged material and a 77% damaged material are each subjected to a uniaxial creep test of 39.2 MPa in an environment of 650 ° C. for 1585 h (time corresponding to 56% of the breaking time (2812.4 h) of the new material). ) And 2161h (a time corresponding to 77% of the fracture time of the new material). The expected rupture time of 27% damaged material, 56% damaged material and 77% damaged material is calculated by subtracting the creep test time (756h, 1585h, 2161h) of each damaged material from the rupture time of new material (2812.4h). It is the time calculated by doing. In FIG. 7, a straight line indicating an expected fracture time of each damaged material at an arbitrary test temperature is drawn with a solid line (27% damaged material), a broken line (56% damaged material), and a one-dot chain line (77% damaged material). ing. These straight lines are parallel to an approximate straight line (two-dot chain line) showing an expected fracture time at an arbitrary test temperature of a 0% damaged material (new material) in FIG. Note that the test pieces of Examples 1 and 2 and Comparative Example 1 are 77% damaged material.

  As shown in FIG. 7, the specimen of Comparative Example 1 subjected to regenerative heat treatment at a heating rate of 800 ° C./h was subjected to regenerative heat treatment at a heating rate of 150 ° C./h, whereas the creep rupture time was 178 h. In the test piece of Example 1, the creep rupture time is 348 h, and the rupture time is shifted to the longer life side, and it can be seen that the damage of the creep void is recovered. Further, the test piece of Example 2 subjected to regenerative heat treatment at a heating rate of 60 ° C./h with a lower heating rate was broken than the test piece of Comparative Example 1 subjected to regenerative heat treatment at a heating rate of 259 h and 800 ° C./h. Although the time is long, the rupture time is shorter than that of the test piece of Example 1 subjected to regenerative heat treatment at a heating rate of 150 ° C./h.

  Further, when the influence of the heating rate of the regenerative heat treatment was investigated, when the heating rate was less than 800 ° C./h, the effect of recovering creep void damage was greater than when the heating rate was 800 ° C./h or more. found. Regarding the lower limit of the heating rate, the rate of decrease in the degree of recovery of creep void damage at the heating rate of 60 ° C./h relative to the degree of recovery of creep void damage at the heating rate of 150 ° C./h, and In consideration of work efficiency, 50 ° C./h was set as the lower limit. In the regenerative heat treatment, the effect of the holding time of 920 ° C. at a heating rate of 150 ° C./h was examined from the lower limit of 2 minutes to the upper limit of 120 minutes, but no clear difference was observed in the creep rupture time. Further, the effect of the cooling rate in the regenerative heat treatment was also examined from 50 ° C./h to 300 ° C./h, but no clear difference was observed in the creep rupture time.

  In the above embodiment, the regenerative heat treatment was performed using the high frequency induction heating apparatus. However, in the regenerative heat treatment of the actual machine, for example, when the heating rate is 300 ° C./h or less, a cloth-like electric resistance heater is used. It is preferable to perform regenerative heat treatment. In this case, compared with the case where regenerative heat treatment is performed using a high-frequency induction heating apparatus, a wide range including HAZ can be heated (controlled) in a uniform manner.

  The present invention can recover the creep strength of the welding heat-affected zone of various metal members, and is particularly effective in recovering the strength of a pipe damaged by creep in a thermal power plant or the like.

DESCRIPTION OF SYMBOLS 10 Metal member 11,12 Steel pipe 13 Welded part 14 Electric resistance heater 15,16 HAZ
21,31 Crystal grain 22 Void 23 Carbide

Claims (2)

A heat treatment method for recovering the creep strength of a weld heat affected zone of a metal member used under high temperature and high pressure, wherein the heating rate of the weld heat affected zone of the metal member is 50 ° C / h or more and less than 800 ° C / h. heat treatment method characterized in that it comprises the step of heating to a temperature of at least a ₃ transformation point in. The heat treatment method according to claim 1, wherein the cooling to 300 ° C. or less after heating to process, and further comprising the step of reheating to a temperature below A ₃ transformation point.

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