JP5684296B2 - Creep damage evaluation method - Google Patents

Description

  The present invention relates to a creep damage evaluation method for a weld portion of a cast steel material that is subjected to creep damage due to power steam or the like.

  In a welded portion of a rolled pipe used for boiler piping or the like, creep voids are generated from the middle of the life when creep damage occurs. And it exists in the tendency for a crack to generate | occur | produce after the end of life. Therefore, in the remaining life evaluation of a normal rolled pipe welded part, the occurrence of creep voids is used as an index. Patent Document 1 discloses a creep remaining life evaluation method using a void as an index.

International Publication No. WO05 / 124315

  Even in a welded portion between cast steel materials such as a steam turbine casing and a pipe, creep damage occurs when used for a long time under high temperature and high pressure conditions. However, the welded portion of the cast steel material tends to generate a microcrack in the coarse-grained region of the heat-affected zone (HAZ) from the middle of the life, and the damage mechanism is different from that of the rolled material. Therefore, it is not possible to evaluate the damage rate of the welded portion of the cast steel by applying the master curve created using the creep void as an index. Therefore, a technique capable of evaluating the damage rate of a cast steel welded portion that undergoes creep damage is desired.

  This invention is made | formed in view of such a situation, The objective is to enable it to evaluate the damage rate of the cast-steel-material weld part which receives a creep damage.

The present invention for achieving the object of the pre-mentioned is a creep damage evaluation method of cast steel welds subjected to creep damage, over the entire grain boundary length in the unit area of the coarse range of the cast steel weld The number of grain boundaries where grain boundary cracking has occurred is obtained as an index, and the damage rate of the cast steel welded part subjected to creep damage is evaluated using the relationship with the damage rate of the cast steel welded part relative to the index. Also good. In this way, the number of grain boundaries where grain boundary cracking occurred over the entire grain boundary length in the unit area of the coarse grain region was used as an index, and the damage rate was evaluated based on the relationship between this index and the damage rate. can do.

  In the creep damage evaluation method described above, the relationship between the index and the damage rate is determined by performing a creep test on a test piece having the cast steel material welded portion, obtaining a rupture time until the test piece undergoes a creep rupture, An intermediate stop creep test is performed on another test piece having a material weld, and the ratio of the test time in the intermediate stop creep test to the break time is defined as a damage rate, and the index corresponding to the other test piece in the damage rate It is preferable to be obtained by obtaining By doing in this way, the relationship between a damage rate and the parameter | index obtained from the test piece by which the halfway stop creep test was done by the test time corresponding to this damage rate can be calculated | required appropriately.

  In the creep damage evaluation method described above, the cast steel material is preferably chromium molybdenum steel. Thus, even in chromium molybdenum steel having an excellent strength to weight ratio and easy to weld, a microcrack occurs in the coarse-grained region of the heat-affected zone of the cast steel material weld. Therefore, also in this case, it is possible to evaluate the damage rate of a cast steel material welded portion that undergoes creep damage using a microcrack that is a crack at one grain boundary or more as an index.

  In the creep damage evaluation method described above, the cast steel material is preferably chromium molybdenum vanadium steel. In this way, even in chromium molybdenum vanadium steel, which suppresses the growth of crystal grains during heat treatment by adding vanadium and improves strength and toughness, it is microscopic in the heat affected zone coarse grain region of the cast steel material weld zone. Cracks occur. Therefore, also in this case, it is possible to evaluate the damage rate of a cast steel material welded portion that undergoes creep damage using a microcrack that is a crack at one grain boundary or more as an index.

  According to the present invention, since a microcrack is generated in the coarse grain region of the heat-affected zone of the welded part of the cast steel material that undergoes creep damage, the index related to the microcrack that is a crack at one grain boundary or more and Based on the relationship with the damage rate of the cast steel weld zone, the damage rate of the cast steel weld zone subjected to creep damage can be evaluated.

It is explanatory drawing of a cast steel material welding part. It is a flowchart of the cast steel material welding part remaining life evaluation method in 1st Embodiment. 1 is an explanatory diagram of a uniaxial creep test piece 10. FIG. It is explanatory drawing of the board | plate material test body 20 from which the uniaxial creep test piece 10 is cut out. It is a flowchart of master curve creation. It is explanatory drawing of the number of microcracks. This is a master curve with the crack number density as an evaluation index. It is explanatory drawing of the number of crack grain boundaries. It is a master curve with the number density of crack boundaries as an evaluation index. It is explanatory drawing of crack length. It is a master curve with the integrated value of crack length as an evaluation index.

  FIG. 1 is an explanatory diagram of a welded portion of cast steel. FIG. 1 shows a cast steel material welded portion 2 generated by welding the cast steel material pipes 1 to each other. Welding between the cast steel pipes 1 is performed over the circumferential direction of the pipes. When pressure steam passes through such a cast steel pipe 1, heat from the pressure steam and stress in the circumferential direction are also applied to the cast steel weld 2. As a result, creep damage occurs in the cast steel material welded portion 2. Such creep damage of the welded portion 2 of the cast steel material occurs, for example, at a welded portion between the steam turbine casing made of cast steel material and the piping.

  When the cast steel pipes 1 (hereinafter sometimes referred to as “base metal 1”) are welded together, a weld metal portion 4 is formed at the center thereof. Between the base material 1 and the weld metal part 4, there is a heat-affected part 3 by welding. The heat-affected zone 3 is a portion of the base material that is not melted and whose structure, metallurgical properties, mechanical properties, and the like have changed due to the heat of welding. The heat-affected zone 3 has different properties in hardness and toughness from the weld metal portion 2 and the base material 1 that is not affected by heat.

  The heat affected zone 3 includes a coarse grain region 3a and a fine grain region 3b. When the heat and the circumferential stress as described above are applied, a microcrack is generated in the coarse grain region 3a. A microcrack is a grain boundary crack that occurs when a plurality of voids are generated at the grain boundaries and these voids are connected.

  Grain boundary cracks generated in the coarse grain region 3a grow and expand the range. When the microscopic crack grows up to the fine grain region 3b or the weld metal part 2, the crack also becomes deep and may penetrate the base material 1.

  By the way, in the welded portion of the rolled pipe, when creep damage progresses, creep voids are generated from the middle of the life. For this reason, in the rolled pipe welded part, the remaining life evaluation is conventionally performed using the occurrence of creep voids as an index. However, since the damage mechanism is different from that of the rolled material in the cast steel material welded portion 2, the remaining life evaluation method using the creep void as an index cannot be applied.

  Therefore, in the embodiment described below, a new index is introduced to evaluate the remaining life in a cast steel welded portion that is subject to creep damage.

  FIG. 2 is a flowchart of the cast steel material weld life remaining life evaluation method in the first embodiment. As shown in FIG. 2, in the cast steel welded part remaining life evaluation method, production of a uniaxial creep test piece (S10), creation of a master curve (S12), and actual machine evaluation (S14) are performed. Hereinafter, these methods will be described.

Table 1 shows chemical components of the base material portion in the actual machine and the uniaxial creep test piece. Table 1 shows the components of each steel type in the actual machine and the components of each steel type in the uniaxial creep test piece. In the production of the uniaxial creep test piece (S10), the chemical components 1Cr1Mo (chromium molybdenum steel) and 1Cr1Mo0.1V (chromium molybdenum vanadium steel) shown in Table 1 are used in accordance with the steam turbine material as an example of an actual machine. Manufactured.

Further, in the production of the uniaxial creep test piece, the following processing is performed in order to produce a cast material that is close to the actual equipment.
(A) Melting and casting
(B) Normalizing
(C) tempering
(D) Plate material processing
(E) Welding between plate materials
(F) Annealing

  In melting and casting (A), a vacuum melting furnace is used to cast a steel ingot having the above components. In the normalization of (B), these steel ingots are heated from 150 ° C. or lower to 970 ° C. ± 20 ° C. and then air-cooled to 300 ° C. This normalization adjusts the organization to the prescribed organization. Here, the prescribed structure is ferrite + bainite. In the tempering of (C), heating is performed from 150 ° C. or lower to an optimum temperature, and after furnace cooling, air cooling is performed to 300 ° C. or lower. This tempering adjusts the material to a specified hardness and strength.

  In the plate material processing of (D), cutting is performed from a steel ingot shape by casting into a plate material shape that can be welded. In the welding of (E), the end faces of these plate materials (20a and 20b in FIG. 4 described later) are brought into contact with each other and welded. As the welding material, a 1.25Cr-0.5Mo-based welding material is used when the base material (plate materials 20a and 20b in FIG. 4) is 1Cr1Mo. For the base material of 1CrMo-0.1V, a 2.25Cr-1Mo-based welding material is used. In the annealing of (F), heating is performed from 150 ° C. or lower to the optimum temperature, and then air cooling is performed to 300 ° C. or lower. By this annealing, the welding stress due to the welding is removed.

  Thus, when a plate-shaped test body is manufactured, a uniaxial creep test piece is cut out from the plate-shaped test body.

  FIG. 3 is an explanatory diagram of the uniaxial creep test piece 10. FIG. 4 is an explanatory diagram of the plate-like test body 20 from which the uniaxial creep test piece 10 is cut out. The plate-like test body 20 is a test body in which the end surfaces of the plate materials 20a and 20b are welded as described above. A plurality of (here, five) uniaxial creep test pieces 10 are cut out from the plate-like test body 20. At this time, it cuts out so that the welding line 24 may be located in the creep test target part of the uniaxial creep test piece 10.

  In the plate-like test body 20, the welding start end region 22 and the welding end end region 23 are not used because the weld line 24 is not stable. The uniaxial creep test piece 10 is cut out by turning the plate-like test body 20 into a predetermined size with a milling machine (at this time, the unnecessary portion 21 is also cut), and then lathing each member. Can do. In this way, a plurality of uniaxial creep test pieces 10 having the same properties as the actual machine are manufactured.

  Next, a uniaxial creep test is performed on the manufactured uniaxial creep test piece 10 to create a master curve (S12).

FIG. 5 is a flowchart for creating a master curve. Table 2 shows the test conditions for the uniaxial creep test.

  In creating the master curve, first, a uniaxial creep test is performed on the uniaxial creep test piece 10 to determine the total life time until rupture (S120). In the uniaxial creep test, a stress of 95 (MPa) was applied to the 1Cr1Mo steel at a temperature of 575 (° C.). Moreover, about 1Cr1Mo0.1V steel, the stress of 125 (MPa) was given in the temperature of 575 (degreeC).

  As a result, the 1Cr1Mo steel reached fracture at 4,840.2 (hours) and 4,676.6 (hours). In Table 2, an item of “total life (break time)” is provided as an average of the time to break, and 4,758, 4 (hours) is shown. In the case of 1Cr1Mo0.1V steel, 3,842.9 (hours) is shown as “total life (breaking time)”.

  Next, a uniaxial creep test is performed for a test time obtained by multiplying the total life time by the damage rate (S122). That is, an interrupted uniaxial creep test is performed in which the test is interrupted before breaking. Here, the “damage rate” is a value obtained by dividing the test time of the interrupted uniaxial creep test by the total lifetime (damage rate = interruption test time / total lifetime). By doing so, it is possible to manufacture an interrupted uniaxial creep test piece corresponding to a predetermined damage rate. Then, it becomes possible to observe a microcrack serving as an evaluation index for the interrupted uniaxial creep test piece corresponding to a predetermined damage rate.

  The 1Cr1Mo steel was subjected to an interrupted uniaxial creep test at a damage rate of 83% and a damage rate of 50%. That is, half-finished uniaxial creep test pieces were produced in which the uniaxial creep test was interrupted at 4,282.6 (hours) and 2,379.5 (hours). In addition, the 1Cr1Mo0.1V steel was subjected to a uniaxial creep test with a breakage of 90%, 70% and 50%. That is, half-finished uniaxial creep test pieces were produced in which the uniaxial creep test was interrupted at 3,458.6 (hours), 2,690.0 (hours), and 1,921.4 (hours).

Next, the coarse grain region 3a of the half-finished uniaxial creep test piece manufactured in this way is observed to obtain an evaluation index (S124). Observation can be performed using an optical microscope. In the first embodiment, the evaluation index is “crack number density”. The crack number density is represented by the following formula (1).

Here, the “microscopic crack” is a crack having a length of one grain boundary or more. The “grain boundary” is a crystal grain boundary, and is an interface existing between two or more small crystals in a polycrystal. In the above equation, “the area of the evaluation target range” is 1 (mm ² ).

  FIG. 6 is an explanatory diagram of the number of microcracks. In FIG. 6, a plurality of observed crystals 30 and grain boundaries 31 between them are shown by solid lines. In addition, cracks occurring at the grain boundaries 31 to form cracks 32 are indicated by solid lines that are thicker than the grain boundaries 31.

  As described above, the microcrack is a crack having a length of one grain boundary or more. This means continuous cracks at one grain boundary or more. Also, cracks that do not reach more than one grain boundary are not counted as microcracks. In FIG. 6, cracks having a length of one grain boundary or more occur at two locations. Therefore, micro cracks are counted as two.

  In this way, each half-finished uniaxial creep test piece is observed using an optical microscope or the like, and the number of microcracks is counted. And crack number density is calculated | required by Formula (1). In determining the crack number density, an average value of the crack number density obtained from a plurality of half-finished uniaxial creep test pieces may be employed.

Table 3 is a table of the crack number density obtained from the uniaxial creep test piece. Thus, the crack number density corresponding to each damage rate can be obtained. The 1Cr1Mo steel of Table 3 with a damage rate of 100 (%) is also shown, but this is the crack number density obtained from a uniaxial creep test piece in which fracture occurred. As described above, the coarse grain region 3a can be observed for the fractured uniaxial creep test piece having a damage rate of 100 (%) and included in the evaluation index.

  A master curve is created based on the evaluation index (crack number density) thus obtained (S126). The master curve shows the relationship between the damage rate and the evaluation index.

  FIG. 7 is a master curve using the crack number density as an evaluation index. FIG. 7 shows a master curve in which the horizontal axis is the damage rate and the vertical axis is the crack number density. Here, master curves are obtained for the aforementioned 1Cr1Mo steel and 1Cr1Mo0.1V steel.

  Thus, by using the crack density as an evaluation index, a master curve in which the evaluation index simply increases with respect to the damage rate can be obtained.

  Next, actual machine evaluation is performed based on the obtained master curve (S14). In the actual machine evaluation, the coarse grain region of the welded portion of the cast steel material in the actual machine is observed to obtain an evaluation index (“crack number density” in the first embodiment). Then, by applying the obtained evaluation index to the master curve, the corresponding damage rate can be obtained. That is, the remaining life can be evaluated.

  As described above, the damage rate is a value obtained by dividing the interrupted uniaxial creep test time by the rupture time. The interrupted single-axis creep test time corresponds to the elapsed time when the actual machine was damaged by creep. Therefore, by multiplying the damage rate by the rupture time (Table 2), it is possible to estimate the elapsed time of the creep damage. Also, the approximate remaining life can be estimated by subtracting this estimated elapsed time from the fracture time.

  In this way, it is possible to evaluate the remaining life of a welded portion of a cast steel material that is subject to creep damage based on the master curve using “crack number density” as an index related to the microcrack.

  Next, a second embodiment will be described. In the second embodiment, “crack boundary number density” is employed as an evaluation index in the above-described evaluation index calculation (S124). In addition, since it is the same as that of 1st Embodiment about parts other than an evaluation index, description is abbreviate | omitted and the evaluation index which is a different part from 1st Embodiment is demonstrated.

ここで、「割れ粒界」とは、１粒界長さ全体にわたって粒界割れが発生した粒界である。 Also in the second embodiment, the coarse grain region 3a of the interrupted uniaxial creep test piece created in step S122 is observed to obtain an evaluation index. However, in the second embodiment, the evaluation index is “number density of crack boundaries”. The crack boundary number density is represented by the following formula (2).

Here, the “crack grain boundary” is a grain boundary where grain boundary cracking has occurred over the entire length of one grain boundary.

  FIG. 8 is an explanatory diagram of the number of crack boundaries. In FIG. 8, grain boundary cracks occur at two locations over the entire grain boundary length, but there are a total of five grain boundaries where grain boundary cracks occur at these two locations. Therefore, the number of crack boundaries is counted as 5. Here, the difference from the first embodiment is that in the first embodiment, when the crack grain boundaries are continuously connected, even if it is due to a plurality of grain boundaries, it is counted as one. Is a point. However, in the second embodiment, the number of grain boundaries is counted as a unit even when a plurality of crack grain boundaries are continuously connected.

  In this way, each half-finished uniaxial creep test piece is observed using an optical microscope or the like, and the number of crack boundaries is counted. And a crack grain boundary number density is calculated | required by Formula (2). In obtaining the crack grain boundary number density, an average value of the crack grain boundary number density obtained from a plurality of interrupted uniaxial creep test pieces may be employed.

Table 4 is a table of crack boundary number density obtained from the half-finished uniaxial creep test piece. In this way, the crack boundary number density corresponding to each damage rate can be obtained. The 1Cr1Mo steel of Table 4 whose damage rate is 100 (%) is also shown, but this is the number density of crack grain boundaries obtained from a uniaxial creep test piece in which fracture occurred. As described above, the coarse grain region 3a can be observed for the fractured uniaxial creep test piece having a damage rate of 100 (%) and included in the evaluation index.

  A master curve is created based on the evaluation index (number number of crack grain boundaries) thus obtained (S126). The master curve shows the relationship between the damage rate and the evaluation index.

  FIG. 9 is a master curve using the number density of crack boundaries as an evaluation index. FIG. 9 shows a master curve in which the horizontal axis is the damage rate and the vertical axis is the number density of crack boundaries. Thus, by using the number density of crack boundaries as an evaluation index, a master curve in which the evaluation index simply increases with respect to the damage rate can be obtained. And based on this master curve, the remaining life of the cast steel material welded part which receives a creep damage can be evaluated.

  Next, a third embodiment will be described. In the third embodiment, “crack length” is employed as an evaluation index in the above-described evaluation index calculation (S124). In addition, since it is the same as that of 1st Embodiment about parts other than an evaluation index, description is abbreviate | omitted and the evaluation index which is a different part from 1st Embodiment is demonstrated.

Also in the third embodiment, the coarse grain region 3a of the interrupted uniaxial creep test piece created in step S12 is observed to obtain an evaluation index. However, in the third embodiment, the evaluation index is “crack length”. The “crack length” is represented by the following formula (3).

  FIG. 10 is an explanatory diagram of the crack length. In FIG. 10, grain boundary cracks occur at two locations over the entire length of one grain boundary. That is, microcracks have occurred at two locations. Specifically, a microcrack has occurred in the range indicated by L1 to L5. Therefore, the sum (integrated value) of the lengths of these two microcracks is obtained. In addition, although the grain boundary crack shown by L2 of FIG. 10 does not have the length beyond one grain boundary, it is continuous with the grain boundary crack shown by L1, and is a crack more than one grain boundary. It becomes the object of accumulation.

  In this way, each half-finished uniaxial creep test piece is observed using an optical microscope, and an integrated value of the microcrack length is obtained. And crack length is calculated | required by Formula (3). In determining the crack length, an average value of crack lengths obtained from a plurality of half-stopped uniaxial creep test pieces may be employed.

Table 5 is a table of crack lengths obtained from the halfway uniaxial creep test piece. In this way, the crack length corresponding to each damage rate can be obtained. The 1Cr1Mo steel of Table 5 with a damage rate of 100 (%) is also shown, but this is the crack length obtained from the uniaxial creep test piece in which fracture occurred. As described above, the coarse grain region 3a can be observed for the fractured uniaxial creep test piece having a damage rate of 100 (%) and included in the evaluation index.

  A master curve is created based on the evaluation index (crack length) thus obtained (S126). The master curve shows the relationship between the damage rate and the evaluation index.

  FIG. 11 is a master curve using an integrated value of crack length as an evaluation index. FIG. 11 shows a master curve in which the horizontal axis is the damage rate and the vertical axis is the crack length. Thus, by using the crack length as an evaluation index, a master curve in which the evaluation index simply increases with respect to the damage rate can be obtained. And based on this master curve, the remaining life of the cast steel material welded part which receives a creep damage can be evaluated.

  The above description of the embodiment is for facilitating the understanding of the present invention, and does not limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and the present invention includes equivalents thereof.

  For example, although 1Cr1Mo steel and 1Cr1Mo0.1V steel have been described as examples in the above embodiment, the present invention can be similarly applied to other cast steel welds. In addition, as for the damage rate, 50%, 83% and 100% were adopted for 1Cr1Mo steel, and 50%, 70% and 90% were adopted for 1Cr1Mo0.1V steel. It is good also as employ | adopting and it is good also as obtaining the evaluation parameter | index with respect to more damage rate samples.

DESCRIPTION OF SYMBOLS 1 ... Cast steel piping, 2 ... Welded part, 3 ... Heat-affected zone, 3a ... Coarse grain area, 3b ... Fine grain area, 10 ... Uniaxial creep test piece, 20 ... Plate-shaped test body, 21 ... Non-use area | region, 22 ... Weld start end region, 23 ... Weld end region, 24 ... Weld line, 30 ... Crystal, 31 ... Grain boundary, 32 ... Microcrack

Claims (4)

A creep damage evaluation method of cast steel welds subjected to creep damage,
The number of grain boundaries where intergranular cracking has occurred over the entire grain boundary length in the unit area of the coarse grain region of the cast steel material welded part is obtained as an index, and the damage rate of the cast steel material welded part relative to the index A creep damage evaluation method characterized by evaluating a damage rate of a welded portion of a cast steel material that receives the creep damage using a relationship. The creep damage evaluation method according to claim 1 ,
The relationship between the index and the damage rate is
A creep test is performed on the test piece having the cast steel material weld, and the rupture time until the test piece creep ruptures is obtained.
Performing a halfway stop creep test on the other test piece having the cast steel material weld, the ratio of the test time in the halfway stop creep test with respect to the rupture time as a damage rate,
A creep damage evaluation method, which is obtained by obtaining the corresponding index from the other test piece at the damage rate. The creep damage evaluation method according to claim 1 or 2 ,
The creep damage evaluation method, wherein the cast steel material is chromium molybdenum steel. The creep damage evaluation method according to claim 1 or 2 ,
The creep damage evaluation method, wherein the cast steel material is chromium molybdenum vanadium steel.

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