JP4786479B2 - Fracture mechanics test method and specimen - Google Patents

Description

  The present invention relates to a method for performing a fracture mechanics test at a high temperature on a metal test piece and a test body thereof.

  Among the material strength tests, tests for evaluating the remaining life of facilities that are subjected to heat load from a fracture mechanics viewpoint include, for example, a fracture toughness test and a crack growth test. These tests must be performed under conditions in a small-scale yield or plane strain state, and there is a standard for the size that can be tested on the specimen being tested.

Fracture toughness tests include plane strain fracture toughness (K _IC ) test (ASTM standard E399-90) based on linear elastic fracture mechanics and the strength of singular stress and strain field near the crack tip regardless of elasticity or elasto-plasticity. There are elastoplastic fracture toughness (J _IC ) tests (ASTM standard E1737-96) to be measured, and these tests include, for example, CT (Compact Tension; ASTM standard E399-90) test specimens and three-point bending tests. A piece (Japan Society of Mechanical Engineers standard JSME S 001-1992) is used.

  FIG. 8 is a perspective view of (a) a CT specimen and (b) a three-point bending specimen used for a fracture toughness test.

In the test piece shown in FIG. 8, the minimum dimensions necessary for carrying out the test are the plate thickness B, crack length a and ligament width b. In the plane fracture toughness (K _IC ) test, In the plastic fracture toughness (J _IC ) test, it is defined as shown in equation (2).
B, b, a ≧ 2.5 (K _Q / σ _ys ) ² (1)
B, b ≧ 25 (J _Q / σ _fs ) (2)
Where K _Q is fracture toughness, σ _ys is the yield stress (proof strength) of the material, J _Q is the J integral value, and σ _fs is the effective yield strength.

  According to formula (1) or formula (2), it is ensured that the condition of the plate thickness B is a plane strain state, and the condition of the crack length a or the ligament width b is a small-scale yielding state. It is.

According to these tests, the values of K _Q and J _Q obtained during the test are the values when the conditions of the formula (1) or (2) are satisfied, and the values at that time are calculated as the plane strain fracture toughness K _IQ and the elastic modulus. can be obtained the result that the plastic fracture toughness J _IC, advance, and whether it was valid specimen dimensions, not known only after the test, the results obtained are all reasonable values Not exclusively. Therefore, the larger the dimensions of the test piece, the easier it is to determine the values of _KIQ and J _IC. Therefore, it is preferable to collect as large a test piece as possible from the equipment, etc. Especially, the thickness B is changed after collection. Since it is not possible, it is better to be as thick as possible.

  Moreover, since it is a condition that the crack growth test is also in a plane strain state, it is better that the plate thickness B is thick as in the above test.

  By the way, in the electric power companies, as the aging of high-temperature equipment progresses year by year, extension of the periodic inspection interval and extension of the service life of equipment are regarded as important issues in order to reduce their repair costs. The application of these fracture mechanics tests to the estimation of the longevity of high temperature equipment is being studied.

  However, collecting a sample having a thickness necessary for the above-described test from these high-temperature devices is difficult to apply because it damages the devices themselves. Therefore, the target specimens for the destructive mechanics test are limited to samples taken from used and discarded equipment, and this result is regarded as the remaining life of an actual machine operating for the same time as the used equipment. Since it can be evaluated only by estimation methods, it has not been possible to accurately evaluate the remaining life of an actual machine in operation.

  On the other hand, conventionally, a test method has been proposed in which a small sample that does not damage an actual machine is collected and the above-described test can be realized by effectively using the sample.

For example, in Patent Document 1, an auxiliary member made of the same material as the sample or a different material is used on both surfaces of a sample collected from an actual machine, and a sample and an auxiliary member using a CO ₂ laser or an electron beam. The weld bead part of this sample was quenched and welded so as to have a hardness more than twice that of the sample, and a test piece with an auxiliary member was produced. A method for calculating the toughness value K _IC is disclosed. This is because the thermal effect on the sample is reduced on both sides of the sample, and the weld beat part is hardened to form a hardened part, thereby increasing the degree of restraint against deformation near the sample and maintaining the plane strain state. Toughness K To achieve the _IC test.
JP-A-10-132718

  However, in the test method disclosed in Patent Document 1, for example, a test piece with an auxiliary member is produced by welding a sample of the same material as the sample collected from the facility to both surfaces of the sample as an auxiliary member. When performing a fracture toughness test at a high temperature where a sample taken from the facility causes a large amount of creep deformation using a piece, creep deformation will occur in the auxiliary member as well, so that sufficient stress can be transmitted to the sample. There is a risk that the fracture toughness test cannot be performed.

  In addition, even if the welding beat portion between the sample and the auxiliary member is welded to form a hardened portion, at a high temperature as described above, the hardness of the hardened portion is reduced, and the degree of restraint on the sample is reduced. There is also a possibility that the fracture toughness test cannot be performed because the strain state cannot be maintained.

  Further, no specific measures are taken for the creep deformation that occurs in the auxiliary member when a different material from the sample is used as the auxiliary member.

In order to reduce the thermal effect on the sample, welding is performed using a CO ₂ laser or an electron beam. Even if this thermal effect is small, the sample is small. The ratio of the heat-affected area with respect to is increased, which may affect the test results. For this reason, it is necessary to minimize the thermal influence on the sample so that an accurate test result can be obtained.

  The present invention has been made in view of the above points, and an object of the present invention is to accurately perform a fracture mechanics test at a high temperature using a small test piece collected from an actual machine.

In order to achieve the above object, the present invention is a method for performing a fracture mechanics test at a high temperature on a metal specimen,
On both surfaces of the test piece, a restraining member having a creep strength at a predetermined temperature higher than that of the test piece and having a hardness at the predetermined temperature higher than that of the test piece is composed of boron and at least one of the test piece. A test specimen is produced by welding with a metal foil containing two metal elements and at least one metal element constituting the restraining member ,
Introducing a precrack into the specimen in the specimen,
A load is applied to the test piece through the restraint member in a state where the test body is heated to a predetermined high temperature (first invention).

  According to the fracture mechanics test method of the present invention, a restraint member having a higher creep strength and higher hardness than a test piece taken from an actual machine is welded to both surfaces of the test piece under high temperature conditions for performing a material strength test. By using the test specimen as a test body, for example, even when the test is performed at a high temperature at which a creep phenomenon occurs in the collected specimen, sufficient stress is transmitted to the specimen through the restraint member. And can be firmly constrained, so a fracture mechanics test can be performed subject to plane strain conditions or small scale yield conditions.

  Also, by welding with a metal foil containing boron between the test piece and the restraining member, welding can be performed with a lower melting point than when not using the metal foil, so the heat effect on the test piece is more And the fracture mechanics test can be performed accurately.

Furthermore, since the metal foil includes at least one metal element constituting the test piece and at least one metal element constituting the restraint member, the melted metal foil, the test piece and the restraint member have an affinity. The weldability between the weld of the test piece and the restraining member can be improved, and the test piece can be restrained more firmly. Thereby, a plane strain state or a small scale yield state with higher accuracy can be formed, and a fracture mechanics test can be performed accurately.

In a second aspect based on the first aspect , the predetermined temperature is 600 ° C. or higher.

According to a third invention, in the first or second invention, the restraining member is made of any one of a zirconia alloy, SUS, an alloy having a chromium content of 9% or more, a nickel-base alloy, or a cobalt-base alloy. .

According to a fourth invention, in any one of the first to third inventions, a laser or an electron beam is used for the welding.

A fifth invention is a specimen for a fracture mechanics test at a high temperature performed on a metal test piece, and the creep strength at a predetermined temperature is higher than that of the test piece on both sides of the test piece. And a metal foil comprising a restraining member having a hardness at the temperature higher than that of the test piece, boron , at least one metal element constituting the test piece, and at least one metal element constituting the restraint member. It is characterized by being interposed and welded.

  According to the present invention, a fracture mechanics test at a high temperature can be accurately performed using a small test piece collected from an actual machine.

  Hereinafter, a preferred embodiment of a crack growth test method according to the present invention will be described in detail with reference to the drawings.

  The crack growth test method according to the present embodiment is used, for example, when performing a remaining life evaluation of a turbine casing of a power plant facility that is subjected to a thermal load.

FIG. 1 is a flowchart showing a procedure of a crack growth test method according to the present embodiment.
As shown in FIG. 1, the crack growth test method includes a sampling step S1 for collecting a metal piece from a passenger compartment, a test piece preparation step S2 for producing a test piece from the collected metal piece, and a produced test piece. It is comprised from the crack growth test process S3 implemented using.

  In sampling process S1, a metal piece is extract | collected from the predetermined position in the surface of the compartment used as the object of remaining life evaluation. At that time, it is preferable that damage to the actual machine is as small as possible and that the metal piece to be collected is not deformed or thermally deformed. For collecting such metal pieces, for example, a discharge sampling device disclosed in Japanese Patent Laid-Open No. 2006-102900 is used.

  FIG. 2 is a plan view and an elevation view showing an example of a metal piece collected using the discharge sampling apparatus. As shown in FIG. 2, the metal piece 12 that can be sampled by the discharge sampling device is sampled in a size of approximately 2.5 mm × 20 mm × 40 mm.

  In the specimen preparation step S2, a specimen is prepared by welding a restraining member to the metal piece 12 collected from the actual machine in the sampling step S1. Since the metal piece 12 is collected so as to peel off the surface of the actual machine by the discharge sampling device as described above, the thickness of the metal piece 12 is gradually reduced at the end portion where the peeling of the metal piece 12 starts or ends. (Hereinafter referred to as the thin-walled portion 12a). On the other hand, since a specimen having a constant thickness is required for the specimen for material testing, a process for removing the thin portion 12a from the metal piece 12 is performed, and the metal piece 12 after the process is used as a test piece 12b. . For the process of removing the thin portion 12a, for example, it is preferable to use an electric discharge machining apparatus or a cutting apparatus that does not cause machining deformation or thermal deformation. As shown in the figure, the size of the test piece 12b is, for example, about 2.5 mm × 20 mm × 20 mm.

  Next, the process which welds the restraint member which restrains so that the test piece 12b may be in a plane strain state is performed on both surfaces of the processed test piece 12b. FIG. 3 is an exploded perspective view of the test body according to the present embodiment, and FIG. 4 is a perspective view of the created test body.

  As shown in FIG. 3, the restraining member 14 hardly causes creep deformation even in a temperature range of 600 to 700 ° C., which is the same temperature as the steam introduced into the turbine casing, and also has high hardness at this temperature. (For example, a zirconia alloy, SUS, an alloy having a chromium content of 9% or more, a nickel-base alloy, or a cobalt-base alloy). Two restraint members 14 having a cross-sectional size similar to that of the test piece 12b (for example, 20 mm × 20 mm × 20 mm) are prepared for each test piece in order to weld the both sides of the test piece 12b. .

  Moreover, when welding the test piece 12b and the restraint member 14, it welds by interposing the filler 16 which is metal foil between them. The filler 16 includes boron, at least one metal element that constitutes the metal piece 12 (for example, chromium, molybdenum, vanadium, or the like if collected from the passenger compartment), and at least one that constitutes the restraining member 14. A material containing a metal element (such as chromium or cobalt) is used. Further, an electron beam (not shown) and a laser (not shown) are used for welding. Below, the joining process of the test piece 12b and the restraint member 14 at the time of welding using this filler 16 is demonstrated.

  First, the filler 16 exceeding the melting point is melted by heating with an electron beam and a laser. At this time, boron in the filler 16 diffuses and penetrates from the end face of the test piece 12b or the restraining member 14 in contact with the melted portion. The test piece 12b in contact with the melted portion of the filler 16 or the end face of the restraining member 14 has a high boron concentration, a lower melting point, and a partial melting. At this time, since the filler 16 contains a metal element constituting the test piece 12b and the restraining member 14, the molten filler 16 has an affinity with the partially melted test piece 12b and the restraining member 14, The filler 16, the test piece 12b, and the restraining member 14 are firmly bonded. Thereafter, as the diffusion progresses and the boron concentration decreases, the melting point increases, and the melted portion solidifies to be joined. In addition, as shown in FIG. 4, it is preferable that the thickness of the welding part 18 is less than 1 mm.

  After welding the restraining member 14 to both surfaces of the test piece 12b in this way, the test piece 10 is produced by introducing the pre-crack 20 into the central portion of the test piece 12b in the thickness direction of the test piece 12b. The pre-crack 20 is processed using, for example, a general electric discharge machine.

  In the crack growth test step S3, the test object 10 produced in the specimen preparation step S2 is subjected to the same high temperature conditions as the steam to be introduced into the turbine casing based on ASTM E647-93. Then perform a crack growth test. FIG. 5 is a schematic elevation view showing the configuration around the specimen 10 in the crack growth test.

ただし、Ｆは補正係数、Δσは最大応力σ_ｍａｘと最小応力σ_ｍｉｎとの差である。 As shown in FIG. 5, the test body 10 is fixed to a chuck 24 provided in the testing machine 22, and the test body 10 is heated to a temperature of 600 to 700 ° C. using a high-frequency heating coil 25 or the like. In this state, a load is repeatedly applied in the longitudinal direction to the test body 10 from the testing machine 22 through the chuck 24 at a predetermined stress σ and a stress ratio (ratio between the maximum stress σ _max and the minimum stress σ _min ). The crack length a (see FIG. 4) is measured. The crack confirmation method is performed with a scanning microscope. The crack growth rate (da / aN) is obtained from the read crack length a and the number of repetitions N, and the stress intensity factor width ΔK is calculated using the following equation (3).

However, F is a correction coefficient, and Δσ is a difference between the maximum stress σ _max and the minimum stress σ _min .

  The relationship between the crack growth rate (da / aN) and the stress intensity factor width ΔK is plotted on a log-log graph. FIG. 6 is a graph showing an example of the relationship between the crack growth rate (da / aN) and the stress intensity factor width ΔK.

As shown in FIG. 6, the distribution of the plot points of the crack growth rate (da / aN) and the stress intensity factor width ΔK is generally in the region A related to the initiation of crack growth and the stable growth stage of the crack. It is classified into a region B and a region C of unstable fracture where a crack progresses rapidly at the end. For the region A, the stress intensity factor width ΔK _{th at the} start of cracking is used as an evaluation value, and for the region C, the maximum stress σ _{max at the} stress intensity factor width ΔK _{max at which} unstable fracture starts. using the stress intensity factor _{K max} corresponding to the evaluation value as a fatigue fracture toughness _{K fc} to. For region B, the relationship of the crack propagation law called the Paris law as shown in the following equation (4) is established, and the inspection interval of the equipment machine and the evaluation of the remaining life are made based on this relationship. The
da / dN = C (ΔK) ^m (4)
Where C is a constant, and m is the slope of ΔK and da / aN plotted on a log-log graph.

  As described above, according to the crack growth test method according to the present embodiment, in the temperature range of 600 to 700 ° C. to be tested, the creep strength is higher than the metal piece 12 collected from the actual machine, and the hardness is high. By using the restraining member 14 having the test piece 12b on both sides of the test piece 12b and using it as the test body 10 for the test, the stress can be sufficiently transmitted to the test piece 12b through the restraining member 14 and firmly restrained. Therefore, it is possible to carry out a crack growth test under the condition of a plane strain state or a small scale yield state.

  Moreover, according to the crack growth test method of the present invention, the melting point is lower than that in the case where no filler 16 is interposed by welding the filler 16 containing boron between the test piece 12 b and the restraining member 14. Since the welding can be carried out at a lower temperature, the thermal effect on the test piece 12b can be reduced.

  Further, the filler 16 contains at least one metal element constituting the metal piece 12 and at least one metal element constituting the restraining member 14, so that the molten filler 16, the test piece 12 b and the restraining member are contained. 14 can improve the weldability between welds, and the test piece 12b can be more firmly restrained, so that a highly accurate plane strain state can be formed. As a result, the crack growth test can be more accurately performed on the test piece 12b.

  In addition, although the test body 10 by this embodiment was used for the crack growth test, you may use it for not only this but a fracture toughness test.

  Moreover, the test body according to the present embodiment is not limited to the shape described above, and may be applied to a CT test piece or a three-point bending test piece as shown in FIG. FIG. 7 is an exploded perspective view showing an example in which the test body according to the present invention is applied to a CT test piece and a three-point bending test piece.

  As shown in FIG. 7, the metal piece 12 collected from the facility is processed into a predetermined size, and the metal has high creep strength and can maintain high hardness even under high temperature conditions (for example, 600 to 700 ° C.). A CT test that can be used for a fracture mechanics test at a high temperature by welding a constraining member 14 made of a metal to both sides of a metal piece 12 via a filler 16 and introducing a pre-crack 20 into the metal piece 12 A piece 26 and a three-point bending test piece 28 can be produced.

It is a flowchart which shows the procedure of the high temperature crack growth test method which concerns on this embodiment. It is the top view and elevation which show an example of the metal piece extract | collected using the discharge sampling apparatus which concerns on this embodiment. It is a disassembled perspective view of the test body which concerns on this embodiment. It is a perspective view of the completed test body concerning this embodiment. It is an elevation surface schematic diagram showing the configuration around the specimen 10 in the crack growth test. It is a graph which shows an example of the relationship between a crack growth rate (da / aN) and stress intensity factor width | variety (DELTA) K. It is a disassembled perspective view which shows the example which applied the test body which concerns on this invention to a CT test piece and a 3-point bending test piece. It is a perspective view of (a) CT specimen used for a fracture toughness test, and (b) 3 point bending specimen.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Test body 12 Metal piece 12a Thin part 12b Test piece 14 Restraint member 16 Filler 18 Welded part 20 Pre-crack

Claims (5)

A method for performing a fracture mechanics test at a high temperature on a metal specimen,
On both surfaces of the test piece, a restraining member having a creep strength at a predetermined temperature higher than that of the test piece and having a hardness at the predetermined temperature higher than that of the test piece is composed of boron and at least one of the test piece. A test specimen is produced by welding with a metal foil containing two metal elements and at least one metal element constituting the restraining member ,
Introducing a precrack into the specimen in the specimen,
A fracture mechanics test method, wherein a load is applied to the test piece through the restraint member in a state where the test body is heated to a predetermined high temperature. The fracture mechanics test method according to claim 1, wherein the predetermined temperature is 600 ° C. or more. The fracture mechanics test method according to claim 1 or 2 , wherein any one of a zirconia alloy, SUS, an alloy having a chromium content of 9% or more, a nickel base alloy, or a cobalt base alloy is used as the restraining member. Destructive mechanical testing method according to any one to claims 1 to 3 for the welding, characterized by using a laser or electron beam. A specimen for a fracture mechanics test at a high temperature performed on a metal specimen,
On both surfaces of the test piece, a restraining member having a creep strength at a predetermined temperature higher than that of the test piece and having a hardness at the temperature higher than that of the test piece, boron, and at least one metal constituting the test piece A test body characterized by being welded with a metal foil containing an element and at least one metal element constituting the restraining member .

Images