JP2012117838A - Method for evaluating life of welded part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating life of a welded part that can highly accurately estimate the remaining life of welded part by taking a metal material in use into consideration.SOLUTION: A method for evaluating life of a welded part estimates the remaining life of welded part by giving stress to the welded part that has been welded to a metal material. As microflaws are formed in grain boundaries of the metal material at the welded part, an area setting step sets an effective area where stress to the grain boundaries can be supported and an ineffective area where the stress cannot be supported. An effective cross section area derivation step derives an effective cross section area of the welded part that can support the stress, based on the effective area and the ineffective area. An effective stress derivation step derives an effective stress by performing correction on the stress applied on the welded part, according to the derived effective cross section area. A remaining life estimation step estimates the remaining life of the welded part based on the effective stress. In the area setting step, after flaw correction processing is performed in which adjacent microflaws are merged to be regarded as a merged flow, the microflaws and the merged flow are set as an ineffective area.

Description

  The present invention relates to a life evaluation method for a welded portion in which a stress is applied to a welded portion welded to a metal material to estimate the remaining life of the welded portion.

  Conventionally, as a method for evaluating the life of welds, a metal that models the grain boundaries of the metal material to be evaluated, divides the modeled grain boundaries into multiple cells, and calculates the progress of damage for each cell. A damage evaluation method for a material is known (for example, see Patent Document 1). In this damage evaluation method for a metal material, a cell in which a micro defect has occurred is handled as a cell that cannot support stress, while a cell that can support stress is handled as an effective cell. And the effective stress which correct | amended the stress given to a metal material is set according to the ratio (effective cross-sectional area) of the effective cell per unit area. Thereby, since the effective stress which considered the effective cross-sectional area can be set with respect to the modeled grain boundary, the damage of a metal material can be evaluated suitably.

Japanese Patent Laid-Open No. 2005-3421

  By the way, as a metal material used as evaluation object, there are low chromium steel (low alloy steel) such as 2 chrome steel and high chrome steel such as 9 chrome steel. At this time, in the conventional damage evaluation method for a metal material, when low alloy steel is applied as the metal material, the damage to the metal material can be suitably evaluated. On the other hand, in the conventional damage evaluation method for metal materials, it has been found that when high chromium steel is applied as the metal material, the accuracy of the damage evaluation of the metal material is lowered. This is presumably because the crystal grain size formed by welding varies depending on the metal material used. That is, in the case of 2 chrome steel, the formed crystal grain size is large, and in the case of 9 chrome steel, the formed crystal grain size is small. At this time, when the crystal grain size is large, the micro defects generated at the crystal grain boundary across the crystal grains are far away from each other, so that the stress can be supported between the micro defects. It is assumed that there is. On the other hand, when the crystal grains have a small crystal grain size, the distance between the micro defects becomes close, so that it is assumed that the stress cannot be supported between the micro defects. As described above, the conventional metal material damage evaluation method cannot evaluate the damage of the metal material in consideration of the metal material to be used, and there is a problem that the accuracy of the damage evaluation is lowered depending on the metal material to be used. there were.

  Then, this invention makes it a subject to provide the lifetime evaluation method of the welding part which can estimate the remaining life of a welding part accurately considering the metal material to be used.

  The life evaluation method for a welded portion of the present invention is a method for evaluating the life of a welded portion in which a welded portion welded to a metal material is stressed to estimate the remaining life of the welded portion. In accordance with the effective region and the ineffective region, a region setting step for setting an effective region capable of supporting stress and an ineffective region incapable of supporting stress with respect to the crystal grain boundary, in which micro defects are formed, Effective sectional area derivation process for deriving the effective sectional area of the weld that can support stress, and effective stress for deriving the effective stress by correcting the stress applied to the weld according to the derived effective sectional area A derivation step, and a remaining life estimation step for estimating the remaining life of the weld from the effective stress. Micro and coalescence defects , And sets as an invalid region.

  According to this configuration, by performing the defect correction process, it can be regarded as a combined defect in which adjacent micro defects are combined. For this reason, when the distance between the minute defects is reduced, the ineffective region between the minute defects cannot be supported with respect to the stress. As a result, in the case of a metal material with small crystal grains formed by welding, the effective cross-sectional area can be suitably derived based on the effective area and the ineffective area after the defect correction processing, so that the remaining life of the welded portion can be accurately It can be estimated well.

  In this case, the method further includes a grain boundary model generation step of modeling a crystal grain boundary of the metal material and dividing the modeled crystal grain boundary into a plurality of regions, and the crystal grain boundary extends in the grain boundary model generation step. The direction is a line direction, the stress direction to which stress is applied is orthogonal to the line direction, and a plurality of regions are arranged in a lattice shape with the stress direction as one side and the line direction as one side. In the defect correction process, the presence / absence of an invalid area in the stress direction is extracted with respect to one side of the line direction. In the effective area deriving step, the extracted line direction is set in the effective area and invalid area. It is preferable to derive the effective cross-sectional area based on the effective area and the ineffective area on one side.

  According to this configuration, the defect correction process is a simple process of extracting the presence / absence of an invalid area in the stress direction with respect to one side in the line direction, and can be regarded as a combined defect in which adjacent micro defects are combined. it can.

  In this case, the method further includes a grain boundary model generation step of modeling a crystal grain boundary of the metal material and dividing the modeled crystal grain boundary into a plurality of regions, and the crystal grain boundary extends in the grain boundary model generation step. The direction is a line direction, the stress direction to which stress is applied is orthogonal to the line direction, and a plurality of regions are arranged in a lattice shape with the stress direction as one side and the line direction as one side. In the defect correction process, the effective area sandwiched between the invalid area and the invalid area is set as the invalid area. In the effective area deriving step, the defect area after the defect correction process is set. It is preferable to derive the effective cross-sectional area based on the effective area and the ineffective area.

  According to this configuration, since the effective area sandwiched between the ineffective areas can be used as the ineffective area, the defect correction processing can be processed as a combined defect in which adjacent micro defects are combined.

  In this case, adjacent micro defects are a micro defect in one crystal grain boundary with a crystal grain having an average crystal grain size of less than 10 μm, and a micro defect in the other crystal grain boundary with a crystal grain in between. Yes, in the region setting step, when the average crystal grain size of the metal material in the weld is less than 10 μm, the defect correction process is performed, whereas when the average crystal grain size is 10 μm or more, the defect correction process is not performed. Is preferred.

  According to this configuration, it is possible to select whether or not to perform the defect correction process according to the average crystal grain size. For this reason, even when the crystal grain size is large or the crystal grain size is small, the effective cross-sectional area can be suitably derived.

  According to the welded part life evaluation method of the present invention, the remaining life of the welded part can be accurately estimated in consideration of the metal material to be used.

FIG. 1 is a cross-sectional view of a welded portion that is an evaluation target of the life evaluation method for a welded portion according to the first embodiment. FIG. 2 is a graph of the effective cross-sectional area that changes with time. FIG. 3 is a graph of effective stress that varies with time. FIG. 4 is an explanatory diagram modeling a grain boundary of a metal material (high chromium steel) in an element. FIG. 5 is a creep rupture time that changes over time. FIG. 6 is an explanatory diagram modeling a grain boundary of a metal material (low chromium steel) in an element. FIG. 7 is an explanatory diagram for explaining the defect correction processing of the welded part life evaluation method according to the first embodiment. FIG. 8 is an explanatory diagram for explaining the defect correction processing of the welded part life evaluation method according to the second embodiment.

  Hereinafter, a method for evaluating the life of a welded portion according to the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following examples. In addition, constituent elements in the following embodiments include those that can be easily replaced by those skilled in the art or those that are substantially the same.

  The life evaluation method of the welded part according to Example 1 is a method for estimating the remaining life of the welded part welded to the metal material. In this welded part life evaluation method, finite element analysis (FEM analysis) is used, and a life due to a creep phenomenon is estimated by continuously applying a load due to a creep phenomenon to the welded part. Here, the creep phenomenon is a phenomenon in which the deformation of the welded portion increases with the passage of time when a stress is continuously applied to the welded portion. First, with reference to FIG. 1, a welded part that is a life evaluation target will be described.

  FIG. 1 is a cross-sectional view of a welded portion that is an evaluation target of the life evaluation method for a welded portion according to the first embodiment. The welded portion 10 is formed by overlay welding using a filler metal 16 to a groove portion formed in the base material 15. At this time, a welding heat affected zone (HAZ portion) 17 is formed by welding heat between the base material 15 and the molten filler metal (molten metal) 16. In addition, as a metal material which comprises the base material 15 and the filler material 16, high chromium steel, such as 9 chrome steel, is used.

  In the welded portion 10, the HAZ portion 17 is most susceptible to the creep phenomenon. Here, the stress applied to the HAZ portion 17 is distributed in the thickness direction (vertical direction in FIG. 1). At this time, the stress applied to the HAZ portion 17 is a parameter such as stress due to internal pressure, restraint due to discontinuity of the material of the molten metal 16, the HAZ portion 17 and the base material 15, or local stress due to unevenness of the beads. Determined by.

  In the HAZ portion 17, minute defects called creep voids are formed with time due to the creep phenomenon. Here, FIG. 2 is a graph of effective cross-sectional area that changes with time, and FIG. 3 is a graph of effective stress that changes with time. In the graph shown in FIG. 2, the vertical axis represents the effective cross-sectional area, and the horizontal axis represents time. In addition, the line representing the change in the effective area in the prior art is L1, and the line representing the change in the effective area in the present invention is L2. In the graph shown in FIG. 3, the vertical axis represents effective stress, and the horizontal axis represents time. In addition, a conventional line representing a change in effective stress is T1, and a line representing a change in effective stress in the present invention is T2. The comparison between the conventional lines L1 and T1 and the lines L2 and T2 of the present invention will be described later.

  As shown in FIG. 2, when a minute defect occurs with the passage of time, in the HAZ portion 17, the effective cross-sectional area of the welded portion 10 capable of supporting stress is reduced by the amount of occurrence of the minute defect. For this reason, as shown in FIG. 3, the stress applied to the HAZ portion 17 is corrected so as to increase as the effective cross-sectional area decreases, and the corrected stress becomes the effective stress. Furthermore, with the passage of time, the minute defect is combined with other minute defects or grows to form a crack. Note that a process model for forming minute defects over time is determined in advance based on experiments and the like.

  Then, the lifetime evaluation method of the welding part 10 is demonstrated. In the life evaluation method of the welded part 10, the remaining life with respect to the HAZ part 17 is particularly estimated. In the life evaluation method of the welded part 10, a welded part model in which the welded part 10 is divided into a finite number of elements 20 is generated, and the stress applied to the welded part 10 is analyzed using the generated welded part model. The process is performed.

  In the creep stress analysis step, the stress distribution in the plate thickness direction of the HAZ portion 17 is obtained using a welded part model and considering each parameter relating to the stress. Moreover, the load per unit plate | board thickness is calculated | required by integrating stress over a plate | board thickness direction.

  As shown in FIG. 4, in the method for evaluating the life of the welded portion 10, the crystal grains of the metal material in each element 20 are modeled in a rectangular shape, and the crystal grain boundaries between the crystal grains are determined. A grain boundary model generation process for modeling in a linear shape is performed. FIG. 4 is an explanatory diagram modeling a grain boundary of a metal material (high chromium steel) in an element.

  In the grain boundary model generation step, a plurality of grain boundary lines 21 are generated so that the crystal grain boundaries extend in a straight line in a direction orthogonal to the stress direction, and the generated plurality of grain boundary lines 21 extend across the stress direction. Several are arranged. In addition, let the extending direction of the grain boundary line 21 used as a straight line be a line direction. Further, the crystal grain boundary in the stress direction is also modeled so as to connect between the plurality of grain boundary lines 21 in the line direction. Therefore, the part divided by the crystal grain boundary is the modeled crystal grain 22. The grain boundary line 21 in the line direction is divided into a plurality of cells (regions) C.

  When a grain boundary model is generated by the grain boundary model generation step, a cell setting step (region setting step) for setting a valid cell (effective region) C1 and a missing cell (invalid region) C2 for a plurality of cells C, respectively. ) Is performed. The effective cell C1 is a cell that works effectively against stress, and is a cell that can support stress. The deficient cell C2 is a cell that is ineffective with respect to the stress and cannot support the stress. That is, the minute defect formed at the crystal grain boundary is set as the defective cell C2, and other than the minute defect is set as the effective cell C1. In addition, although mentioned later for details, in the cell setting process, when the metal material to be used is 9 chrome steel, the defect correction process is performed.

  When the effective cell C1 and the missing cell C2 are set for the plurality of cells C by the cell setting process, the effective sectional area of the welded portion 10 that is effective against the stress is calculated based on the effective cell C1 and the missing cell C2. An effective sectional area deriving step is performed. In the effective cross-sectional area derivation step, the effective cross-sectional area in the entire length of the welded portion 10 in the line direction is derived by obtaining the ratio of the effective cell C1 to the total sum of the effective cell C1 and the defective cell C2.

  When the effective cross-sectional area is derived by the effective cross-sectional area derivation process, based on the derived effective cross-sectional area and the load in the unit plate thickness derived in the creep stress analysis process, “effective stress = load / effective cross-sectional area”. The effective stress deriving step for deriving the effective stress is performed from the equation "."

  When the effective stress is derived by the effective stress deriving step, the remaining life estimation step of deriving the remaining life of the welded portion from the derived effective stress is performed based on a graph (not shown) that associates the effective stress and the remaining life. Is called. Note that the graph associating the effective stress with the remaining life is determined in advance based on experiments or the like. And the graph shown in FIG. 5 can be obtained by performing the remaining life (time until a fracture | rupture: creep rupture time) in the welding part 10 for every time progress of the formation process model of a micro defect. FIG. 5 shows a creep rupture time that changes with the passage of time, and a graph of the remaining life lines J1 and J2 can be obtained. The remaining life line J1 is conventional, and the remaining life line J2 is that of the first embodiment. A comparison between the conventional life line J1 and the life line J2 of the first embodiment will be described later.

  Next, referring to FIG. 4 and FIG. 6, the case where the metal material is a low chromium steel (low alloy steel) such as 2 chrome steel and the case of a high chromium steel such as 9 chrome steel are respectively compared. FIG. 6 is an explanatory diagram modeling a grain boundary of a metal material (low chromium steel) in an element. For simplicity of explanation, the size of the weld element 40 modeled in the low chromium steel and the size of the weld element 10 modeled in the high chromium steel are the same size. It is assumed that

  As shown in FIGS. 4 and 6, the average crystal grain size of the metal material in the welded portion 10 is 10 μm or more in the case of low chromium steel, while it is less than 10 μm in the case of high chromium steel. At this time, in the case of the low chromium steel and the high chromium steel, if the generated micro defects are the same size, the micro defects at one crystal grain boundary with the crystal grains 22 sandwiched, and the other crystal grains The distance between the minute defects in the field is different. That is, in the case of low chromium steel, the crystal grain size is large, so the distance between the micro defects is long. On the other hand, in the case of high chromium steel, the crystal grain size is small, so the distance between the micro defects is short. .

  As shown in FIG. 6, in the case of low-chromium steel, the distance between minute defects (deficient cells C2) is long, so that the structure can support stress between the minute defects. In the case of chrome steel, since the distance between minute defects (defect cell C2) is short, it becomes a structure which cannot support stress between minute defects. For this reason, in Example 1, in the cell setting step, a defect correction process is performed according to the size of the crystal grain size, in which adjacent micro defects are merged and regarded as a merged defect. That is, the adjacent micro defects are one micro defect in one crystal grain boundary with the crystal grain 22 of less than 10 μm in between, and the other micro defect in the other crystal grain boundary with the crystal grain 22 in between. It is.

  This defect correction processing is not executed when the crystal grain size is 10 μm or more, while it is executed when the crystal grain size is less than 10 μm. Next, the defect correction process will be described with reference to FIG.

  FIG. 7 is an explanatory diagram for explaining the defect correction processing of the welded part life evaluation method according to the first embodiment. In the defect correction processing, a plurality of cells C in the element 20 in FIG. 4 are displayed in a grid pattern across the stress direction and the line direction. In the defect correction process, the presence / absence of the missing cell C2 in the stress direction is extracted (projected) with respect to the side D in the line direction. That is, with respect to one side D in the line direction, if there is no missing cell C2 in the stress direction, the effective cell C1 is assumed, and if there is a missing cell C2 in the stress direction, the missing cell C2 is assumed.

  In the effective area deriving step, the ratio of the effective cell C1 to the sum of the effective cells C1 and the missing cells C2 on the one side D in the line direction (the total length of the one side D in the line direction) is derived as the effective area.

  Here, with reference to FIG. 2, FIG. 3, and FIG. 5, the life evaluation method of the conventional welding part which does not perform a defect correction process, and the life evaluation method of the weld part 10 of Example 1 which performs a defect correction process are demonstrated. Compare. When the defect correction process is performed, the effective cross-sectional area is likely to decrease. Therefore, the time change amount of the effective cross-sectional area decrease is larger in L2 of Example 1 in FIG. 2 than in the conventional L1. In addition, when the defect correction process is performed, the amount of time change of decrease in effective cross-sectional area becomes large. Therefore, T2 of Example 1 in FIG. 3 has a larger amount of time change of increase in effective stress than conventional T1. Become. For this reason, as shown in FIG. 5, the remaining life lines J1 and J2 of the welded portion that change with the lapse of time are compared with the time variation of the conventional remaining life line J1. The time change of is faster. In addition, it was confirmed that the remaining life line J2 of Example 1 was accurate with respect to the remaining life results obtained in advance through experiments or the like.

  According to the above configuration, when a high chromium steel having a crystal grain size of less than 10 μm is used as the metal material, by performing defect correction processing, adjacent micro defects can be regarded as coalescence defects, Small defects and coalescence defects can be used as the defective cell C2. For this reason, when the distance between micro defects becomes short, it can be considered that the micro defects are ineffective with respect to stress. Thereby, since the effective cross-sectional area which can support stress can be derived | led-out suitably and effective stress can be derived | led-out appropriately, the remaining life of the welding part 10 can be estimated with a sufficient precision.

  Further, in the defect correction process, it is possible to regard adjacent micro defects as coalesced defects only by extracting the presence or absence of the defective cell C2 in the stress direction on one side D in the line direction. For this reason, the precision of the remaining life evaluation of the welding part 10 using the metal material whose crystal grain diameter is less than 10 micrometers can be improved only by adding a simple process.

  Further, whether or not to perform the defect correction process can be selected according to the average crystal grain size. For this reason, it is possible to properly use the defect correction processing depending on the type of the metal material to be used, and it is possible to evaluate the remaining life of the welded portion 10 according to the metal material.

  Next, with reference to FIG. 8, a method for evaluating the life of a weld according to the second embodiment will be described. FIG. 8 is an explanatory diagram for explaining the defect correction processing of the welded part life evaluation method according to the second embodiment. Different parts will be described in order to avoid duplicate descriptions. In the defect correction process of the first embodiment, the presence or absence of the missing cell C2 in the stress direction is extracted for the side D in the line direction, and the effective cross-sectional area is derived. In the defect correction processing according to the second embodiment, the effective cross-sectional area is derived by setting the effective cell C1 sandwiched between the defective cell C2 and the defective cell C2 as the defective cell C2.

  As shown in FIG. 8, in the defect correction process, as in the first embodiment, the plurality of cells C in FIG. 8 are displayed in a grid pattern across the stress direction and the line direction. In the defect correction processing, the effective cell C1 sandwiched between the defective cell C2 and the defective cell C2 in the line direction is defined as the defective cell C2, and between the defective cell C2 and the defective cell C2 in the stress direction. The effective cell C1 sandwiched between the cells is defined as a defective cell C2. Thereby, the micro defect in one crystal grain boundary across the crystal grain 22 and the micro defect in the other crystal grain boundary across the crystal grain 22 can be combined to form a coalescence defect. In the effective area deriving step, the ratio of the effective cell C1 to the total sum (all cells) of the effective cell C1 and the defective cell C2 is derived as the effective area.

  Also in the above configuration, when a high chromium steel having a crystal grain size of less than 10 μm is used as the metal material, by performing defect correction processing, adjacent micro defects can be regarded as coalescence defects. Defects and coalescence defects can be defined as a defective cell C2. Thereby, since the effective cross-sectional area which can support stress can be derived | led-out suitably and effective stress can be derived | led-out appropriately, the remaining life of the welding part 10 can be estimated with a sufficient precision.

  In the second embodiment, the case where the defect correction process is performed once has been described. However, the defect correction process may be executed a plurality of times. Further, although the defect correction process is executed in the line direction and the stress direction, the defect correction process may be executed in an oblique direction.

  As described above, the method for evaluating the life of a weld according to the present invention is useful for estimating the remaining life of a metal material having a crystal grain size of less than 10 μm, and is particularly useful for high chromium steel such as 9 chromium steel. Suitable for estimating life.

DESCRIPTION OF SYMBOLS 10 Welding part 15 Base material 16 Filler material 17 Welding heat affected zone (HAZ part)
20 element 21 grain boundary line 22 crystal grain C cell C1 effective cell C2 deficient cell

Claims (4)

In the life evaluation method of the welded portion that gives stress to the welded portion welded to the metal material and estimates the remaining life of the welded portion,
In the crystal grain boundary of the metal material in the weld, a micro defect is formed,
An area setting step for setting an effective area that can support the stress and an ineffective area that cannot support the stress with respect to the crystal grain boundary,
An effective area deriving step of deriving an effective area of the weld that can support the stress based on the effective area and the ineffective area; and
An effective stress deriving step of deriving an effective stress by performing a correction according to the derived effective cross-sectional area with respect to the stress applied to the weld,
A remaining life estimation step of estimating the remaining life of the weld from the effective stress,
In the region setting step, after performing a defect correction process in which the adjacent micro defects are combined and regarded as a combined defect, the micro defect and the combined defect are set as the invalid region. Life evaluation method. Modeling the crystal grain boundary of the metal material, further comprising a grain boundary model generation step of dividing the modeled crystal grain boundary into a plurality of regions,
In the grain boundary model generation step, a direction in which the crystal grain boundary extends is a line direction, a stress direction to which the stress is applied is orthogonal to the line direction, the plurality of regions have a stress direction as one side, and a line It is arranged in a grid with the direction as one side,
In the region setting step, the effective region and the invalid region are respectively set in the plurality of regions,
In the defect correction process, for one side of the line direction, extract the presence or absence of the invalid region in the stress direction,
The lifetime of the welded portion according to claim 1, wherein, in the effective area deriving step, the effective area is derived based on the effective area and the invalid area on one side in the line direction after extraction. Evaluation methods. Modeling the crystal grain boundary of the metal material, further comprising a grain boundary model generation step of dividing the modeled crystal grain boundary into a plurality of regions,
In the grain boundary model generation step, a direction in which the crystal grain boundary extends is a line direction, a stress direction to which the stress is applied is orthogonal to the line direction, the plurality of regions have a stress direction as one side, and a line It is arranged in a grid with the direction as one side,
In the region setting step, the effective region and the invalid region are respectively set in the plurality of regions,
In the defect correction process, the effective area sandwiched between the invalid area and the invalid area is the invalid area,
The weld area life evaluation method according to claim 1, wherein, in the effective area deriving step, the effective area is derived based on the effective area and the invalid area after the defect correction processing. The adjacent micro defects are the micro defects in one crystal grain boundary with a crystal grain having a grain size of less than 10 μm, and the micro defects in the other crystal grain boundary with the crystal grain in between. And
In the region setting step, when the average crystal grain size of the metal material in the welded portion is less than 10 μm, the defect correction process is performed. On the other hand, when the average crystal grain size is 10 μm or more, the defect correction process is performed. 4. The method for evaluating the life of a welded portion according to any one of claims 1 to 3, wherein:

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