KR100251266B1 - Fatigue strength treatment method for britterness material - Google Patents

Abstract

PURPOSE: A method is provided to improve the fatigue strength by manufacturing a plurality of samples for the fatigue testing of a Cr-Mo steel degraded material which is used as a steel for a high temperature and high pressure container for a long time as changing an irradiation density with parameters such as laser output, irradiation speed of a laser beam irradiator and diameter of an irradiation beam. CONSTITUTION: The method for increasing the fatigue strength of a steel for a pressure container comprises the processes of irradiating laser on a matrix steel for a pressure container which is easily subject to degradation, thereby inducing the phase transformation of a surface hardening layer of martensite with a heat source of the irradiated laser; and generating residual compression stress inside the steel with the phase transformation so that impurities segregated into the grain boundary of the steel are diffused by a high temperature heat source of the laser irradiation resulting in an increase of the fatigue strength of the steel.

Description

Fatigue Strength Treatment Method

The present invention is a method of treating fatigue strength of a degraded material by using it for a long time, and more specifically, by increasing the fatigue strength of the machine and structure exhibited deterioration as the existing service life is prolonged, the life extension effect of the mechanical structure and economic It is for reducing the loss of phosphorus.

As the service life of machinery and structure such as nuclear power generation facilities, high temperature, high pressure, etc. is prolonged, life problems are being raised. Accordingly, inspection, repair, replacement, suspension of use, limitation of operating conditions, review of reconstruction safety and reliability Increasingly, there is a growing demand for more consistent, safer and more efficient use of existing machinery and structures.

In this regard, the evaluation of the life and soundness of machinery and structures has emerged as an important issue in various industries. However, there are many factors that govern the lifespan of various machines and structures, but the influence of degradation, that is, a phenomenon in which the strength characteristics of the material change with time, such as the conditions of use of the material and the history of use, has a large range.

Deterioration proceeds by a combination of a long time of use, high temperature environment, load conditions, corrosive environment, etc. Material degradation (Material degradation) of the kind of degradation has the greatest impact on the life of the machine and structure.

Phenomenological characteristics of material deterioration have been achieved to some extent by many scholars, but in general, deterioration occurs locally rather than throughout the structure. Therefore, if the target material is small, it can be recovered by the existing heat treatment method, but in actual operation, that is, if the target structure is large, it is very difficult to apply the existing heat treatment method.

Therefore, in an effort to extend the lifespan of machinery and structures in which such deterioration occurs, each sector of the industry pays great attention to the development of new materials and technologies.

In the present invention, the heat source of the laser beam was used to increase the fatigue strength of the deterioration material. In particular, the industrial laser, which is a new technology, uses high density energy and is currently mainly used for materials processing fields such as welding, cutting, and ultra-precision processing.

In addition, if the laser beam heat source is used as the fatigue strength treatment method of the deterioration material, it will be possible to treat the deterioration of the local processability, controllability of the treatment area, complexity, and inaccessible parts. As this is possible, the economic benefits will be followed.

In view of the above problems, in the present invention, the chromium-molybdenum (Cr-Mo) steel deterioration material, which has been used for a long time as a high temperature and high pressure steel for pressure vessels, has the laser output, irradiation speed and diameter of the irradiation beam as parameters. The purpose of this study is to provide a method of improving fatigue strength (treatment of deterioration) by laser beam irradiation and the effect of laser beam irradiation on fatigue crack generation and propagation characteristics by making a large number of fatigue test specimens with varying irradiation density.

1 is a plan view showing an example of a test piece used in one embodiment of the present invention.

2 and 3 are graphs showing the degree of curing according to the laser beam irradiation.

4 is a graph showing the residual stress distribution of a test piece irradiated with a laser beam.

5 is a graph showing the relationship between energy density and curing depth during laser beam irradiation.

6 is a graph showing the relationship between the energy density and the hardness value during laser beam irradiation.

7 is a graph showing the relationship between the fatigue crack growth length and the load repetition frequency of the laser beam irradiation material.

8 is a graph showing the relationship between the crack growth length (Δa) and the fatigue life ratio of various laser beam irradiation materials.

FIG. 9 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 1.67-1.70 (W · sec / cc) during laser beam irradiation.

10 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 5.22 (W · sec / cc) upon laser beam irradiation.

FIG. 11 is a graph showing the fatigue crack propagation velocity of a test piece having an energy density of 5.30 (W · sec / cc) upon laser beam irradiation.

FIG. 12 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 3.29 (Wsec / cc) during laser beam irradiation.

FIG. 13 is a graph showing the fatigue crack propagation velocity of a test piece having an energy density of 2.62 (Wsec / cc) during laser beam irradiation.

FIG. 14 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 3.66 (Wsec / cc) during laser beam irradiation.

FIG. 15 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 3.03 (W · sec / cc) upon laser beam irradiation.

FIG. 16 is a graph showing the fatigue crack propagation velocity of a test piece having an energy density of 2.81 (W · sec / cc) during laser beam irradiation.

FIG. 17 is a graph showing the fatigue crack propagation speed of a test piece having an energy density of 2.52 (W · sec / cc) upon laser beam irradiation.

18 and 19 are graphs comparing fatigue crack propagation speeds of deterioration material, recovery material and various laser beam irradiation materials on the same graph.

20 is a graph showing the fatigue crack propagation speed of a test piece irradiated with a laser beam after the fatigue crack length?

FIG. 21 is a graph comparing fatigue crack propagation speed of a specimen that is irradiated with a laser beam with a crack, and a degradation material and a recovery material.

Hereinafter, the fatigue strength increase method of the steel for pressure vessel of the present invention together with the accompanying drawings will be described in more detail.

An embodiment for illustratively explaining a method for increasing fatigue strength of steel for pressure vessels of the present invention will be described below with reference to the accompanying drawings.

EXAMPLE

As a research material for developing fatigue strength improvement method of pressure vessel steel by laser beam irradiation, a temper embrittlement material was selected, which was used for about 60,000 hours at about 430 ° C. as 2¼Cr-1Mo steel. The material is steel for petroleum refining pressure vessel, its chemical composition is shown in Table 1, and its mechanical properties are shown in Table 2.

TABLE 1

TABLE 2

In order to compare the strength of the deteriorated material, a healthy material is required, but since a new material that is the same as the material used for a long time cannot be obtained, the deteriorated material was heated at 650 ° C for 1 hour and then cooled as a recovery material.

However, it is technically very difficult to recover a large, pressure vessel by heating the whole by this method. Therefore, the same material was used for the purpose of treating the deterioration of the necessary part or the weak strength part by a local heat source using a laser beam. .

After processing the deterioration material of Cr-Mo steel in the same shape as the specimen in Fig. 1, cut the notch in the center of the specimen by using a drill to drill 0.2 mm hole, and then use the electric discharge notch to lengthen the artificial notch 2a = 6.6 mm and the curvature. Machining was carried out with a radius of? = 0.2 mm.

As described above, in order to accurately measure the crack propagation behavior of the test piece irradiated with the laser beam, which is a method of increasing the fatigue strength of the steel for pressure vessel of the present invention, the surface of the test piece was polished using emery paper (# 200 to # 1500). After the fatigue crack propagation experiment was performed.

In general, in the case of metal during laser beam irradiation, the absorption rate tends to be lowered due to the reflection of light from the surface. In order to prevent this, the absorption rate was increased by coating an absorber absorbing a laser on the specimen surface.

Graphite powder, black paint, phosphate, etc. are widely used as an absorber. In this example, black paint was used as the absorbent material for the laser beam irradiation for test pieces T-1, 2, 3, and 8, 9, 10, 11, 12, 13, and 15. In addition, five test specimens T-4, 5, 6, 7 and 14 were coated with phosphate as an absorbent during laser beam irradiation, and then irradiated with a laser beam. The laser used in the present invention used a CO 2 laser (Model No HGL 8950) with an output of 5 kW.

In addition, the energy absorbed by the specimen is changed according to the irradiation speed (Velocity) of the laser beam, the laser beam irradiation power (Power), the diameter of the beam (beam), and the laser beam type (Type), so that the parameters of accurate fatigue life extension (Parameter) In order to find), laser beam irradiation was performed while changing various parameters.

Table 3 shows the conditions under which the laser beam was irradiated to the specimen. As shown in the table, specimens T-1, 2, and 3 were irradiated at the front and rear surfaces of the specimen with similar irradiation speed, laser beam diameter, and shape. It was.

The diameter, shape, and irradiation speed of the laser beam are the same from specimens T-4 to No. 7, and the output of the laser beam is kept the same in specimens T-4, 5, T-6, 7 and T-8, 9, respectively. Investigation was conducted. T-14 examined only the entire surface of the specimen.

In the case of specimen T-15, in order to observe the effect of the laser beam irradiation on the cracked part, the fatigue material was first tested to generate a crack length (Δa) of about 3 mm, and then the laser beam irradiation was performed. It was. Through the fatigue test, the crack propagation rate was compared with that of the fire.

TABLE 3

As described above, the structure, hardness and residual stress of the irradiated portion by irradiation of the laser beam are measured as follows.

In order to observe the change in the metal structure of the base material, the deterioration material, the recovery material, and the laser beam irradiated, three kinds of materials were sampled and polished after mounting. Nital) solution was etched for 20 seconds.

As a result of observing the structure photographs of the degradation material, the recovery material, and the laser beam irradiation material of the Cr-Mo steel used in the present invention, the degradation material and the recovery material showed the shape of bainite and ferrite, and the laser beam was irradiated. In the hardened layer, martensite tissues were predominant, but it was found that the parts of the base material were maintained in the parts not subjected to heat affect.

The hardness of the laser beam irradiator is measured as follows.

Vickers hardness test was performed to compare the curing depth and hardness value of the irradiated test pieces by varying the output, irradiation speed and diameter of the laser beam. Experimental conditions were carried out on each test piece at a constant load load of 500g, load time 15sec, load speed 50㎛ / sec.

FIG. 2 is a plot of the results obtained by averaging the values at each position by performing a hardness test along the Y direction at 0.5 mm intervals at the X = 10 mm point of the surface to which the laser beam is irradiated. FIG. The average hardness distribution measured at intervals of 0.2 mm in the thickness direction (Z direction) on the surface is shown. The hardness value of the base material is about 220 Hv, and the hardness value is increased by about 2.5 times to 450-500 Hv, depending on the laser beam irradiation conditions.

The hardening width and the hardening depth are the largest in the laser beam irradiation material T-8 from FIGS. 2 and 3, and the curing depth is about 0.7 mm and the curing width is about 4.5 mm. In the beam irradiation materials T-1 and T-15, the curing width was about 0.4 mm and 2 mm, and the curing depth was about 0.2 mm and 0.4 mm. As a result, the measured hardness distribution is formed differently according to the output, speed and diameter of the laser beam.

The residual stress of the laser beam irradiation site is measured as follows.

When a high-temperature laser beam is irradiated onto the specimen, an X-ray diffractometer (Model No MSF-2M, capacity 0.3KW) is used to observe the residual stress behavior according to the change of metal structure. The residual stress was measured. Residual stress measurements were made at three points of 2, 5 and 9.5 mm in the X direction at the notched tip using specimens of thermal material, T-8, T-10, T-11, T-12 and T-13. saw. The measurement method was measured while changing the diffraction angle (theta) to 0, 15, 30, and 45 degrees by the Glocker method.

Although the half width midpoint method and the parabolic approximation method are used to determine the maximum peak of the measured diffraction line, in the present invention, the line is parallel to the background at half the height of the diffraction line intensity curve. The half-weighted weight point method was used to determine the center of the line (that is, 1/2 of the diffraction angle 2θ) as the maximum peak position.

The measured results were plotted in FIG. 4 to observe changes in residual stress due to laser beam irradiation.

The higher the hardness value, the more compressive residual stress was generated, which has the effect of delaying the propagation speed when the crack progresses in the fatigue test, which is expected to prolong the fatigue life. . Tensile residual stress was found to exist in the case of the base material, which is presumed to be the stress generated when the specimen was processed.

In order to propose the maximum hardness distribution curve during laser beam irradiation, the energy density of the laser beam will be described as follows.

Generally, there are various types of parameters during laser beam irradiation. However, in the present invention, the test pieces during the laser beam irradiation are selected based on the velocity, power, and diameter of the laser beam. The absorbed energy density (Q) and the maximum hardness distribution were estimated.

Therefore, in the present invention, the energy density Q at the time of laser beam irradiation is defined as in Equation (1).

Where P is the output power of the laser (W), V is the irradiation speed (mm / sec) and A is the cross-sectional area of the beam, respectively, and the energy density Q at the time of laser beam irradiation of each test piece is shown in Table 3.

The relationship between energy density and hardness value is as follows.

In order to estimate the optimum laser beam irradiation conditions of the 2¼Cr-1Mo steel deterioration material used in the present invention, these relationships are plotted in FIG. 5 to investigate the relationship between the energy density Q of the laser beam and the hardening depth. . As the energy density increased, the depth of hardening was found to deepen.

However, as shown in FIG. 6, when examining the relationship between the energy density and the maximum hardness value, the maximum hardness in the region where the energy density of the laser beam irradiation in the 2¼Cr-1Mo steel is about 3 to 5 (Wsec / ㎣) Appeared to be nausea. However, at higher energy densities than the maximum hardness zone, the hardness value is softened by softening the tempered tissue before conducting the accumulated heat.

The a-N diagram results in the fatigue test are as follows.

From the data of crack length (a) and load repetition number (N) obtained from the fatigue test results for each specimen, aN and ΔK-da / dN diagrams were prepared to investigate the effect of laser beam irradiation on fatigue crack growth characteristics. Considered.

7 is aN diagram showing the relationship between crack length (a) and load repetition frequency (N) in the fatigue test data for the laser beam irradiator. The values of the constants C and m in the fatigue crack propagation law obtained from the initiation life (N _i ), the fracture life (N _f ) and the ΔK-da / dN diagram are shown.

The 0.1 mm occurrence life of the fatigue crack was faster in the laser beam irradiation materials T-1, 2 and 3 than in the recovery material and the deterioration material. However, in the case of T-6, 7 and T-8 and 9, the laser beam irradiation material The fatigue crack generation cycle (Cycle) was significantly delayed and the break cycle was extended to the level of recovery material.

FIG. 8 shows the comparison of the crack length Δa advancing per fatigue life cycle of the degradation material, the recovery material and the laser beam irradiation material. The crack propagation ratios of the deterioration and recovery materials are similar, whereas in the case of the laser beam irradiator, the initial cracking occurs due to a significant delay due to the compressive residual stress. Therefore, when proper laser beam irradiation is made, it is suggested that tempering embrittlement deterioration material can be prolonged life of crack generation by laser beam irradiation method.

ΔK-da / dN diagram results in the fatigue test is as follows.

Paris's study of fatigue crack propagation shows that the fatigue crack propagation rate law as a function of the stress intensity factor range (ΔK) as a function of the stress cracking propagation rate (da / dN) according to the test specimen and the load conditions. Can be represented as:

Where m and C are exponents and constants in the law of crack propagation.

In the present invention, the fatigue crack growth rate (da / dN) was calculated according to the secant method from the data on the crack length (a) and the number of load repetitions (N), and the stress intensity factor range ΔK is Obtained using the H.Tada formula

In other words,

Where F _I (α) ^* = (1-0.025α ² + 0.06α ⁴ ) F _I (α),

In addition, the fatigue test was performed under the same conditions in the stress amplitude range Δσ = 115.8MPa and the initial stress intensity factor range ΔK = 11.8Mpa√m. From the aN relationship for each test specimen, the crack propagation rate (da / dN) and stress intensity factor range (△ K) were calculated and the relationship between ΔK and da / dN was plotted on a log-log graph. Plotting results are shown in FIGS. 9 to 17, respectively.

9 shows the relationship between the fatigue crack propagation rate and the stress intensity factor for T-1, 2, and 3 specimens with an energy density, Q of 1.67 to 1.70 (W · sec / ㎣) during laser beam irradiation. FIG. 10 shows energy density, T-4, 5 with 5.22 (Wsec / m), and FIG. 11 shows energy at T-6, 7 and 12 with energy density Q of 5.30 (Wsec / m). The crack propagation rates of T-8 and 9 whose density Q is 3.29 (Wsec / cc) are shown, respectively.

In addition, in Fig. 13, T-10 having an energy density Q of 2.62 (Wsec / cc), in Fig. 14, T-11 having an energy density Q of 3.66 (Wsec / cc), and in Fig. 15, an energy density. T-12 with Q of 3.03 (Wsec / cc) and in Figure 16 crack propagation rate of T-13 specimen with energy density Q of 2.81 (Wsec / cc), and only one side of the specimen The relationship between fatigue crack propagation rate and stress intensity factor of T-14 specimens with energy density Q of 2.52 (W · sec / ㎣) was plotted in FIG. 17.

In FIG. 18, in order to compare fatigue crack propagation speeds of various laser beam irradiating materials, deteriorating materials, and restoring materials, they were plotted and compared on the same ΔK-da / dN diagram. The laser beam irradiators T-1, 2, 3 and T-10 show increased fatigue crack propagation rates in the entire area of ΔK than the deteriorated material, while the energy density Q is 3.29, 5.30 (Wsec / ㎣). The fatigue crack propagation rate of laser beam irradiator T-8, 9 and T-6, 7 was found to be the most delayed. This may be due to the effect of high temperature during laser beam irradiation, resulting in delayed fatigue crack propagation rate due to the improvement of tissue change and hardness and the development of compressive residual stress.

In addition, tempering embrittlement has been reported that impurities such as P, Sn, Sb, and As are segregated at grain boundaries to reduce cohesion of grains, of which P increases embrittlement susceptibility. However, Erhart reports that the average concentration of P in grain boundaries decreases with increasing temperature and the average concentration of P in crystals increases. Therefore, in the present invention, it is assumed that the cohesion force of the crystal grains is increased while decreasing the concentration of the impurity P segregated at the grain boundary by using a high temperature heat source of the laser beam.

In FIG. 19, the constant m and C values of the fatigue crack propagation law by the Paris equation are calculated and calculated according to the least square method from the relationship of ΔK-da / dN for the test pieces, respectively, and the results are shown in Table 4. .

In order to observe the effect of the laser beam when the crack occurred in the actual structure, the degradation material was first fatigue-tested, and the crack length (△ a) was advanced about 3mm, and the laser beam was irradiated. The fatigue test results are shown in FIG. At this time, the absorption energy density Q of the laser beam irradiation material T-15 is 5.67 (Wsec / sec).

In FIG. 21, plots are plotted on the same ΔK-da / dN diagram to compare the crack propagation rates of the laser beam irradiator T-15 with the deterioration and recovery materials. It can be observed that the crack propagation speed of the laser beam irradiator T-15 is significantly delayed compared to the crack propagation speed of the thermal and recovery materials. It is thought to have a delaying effect.

From the fatigue crack propagation test results, when irradiating an appropriate laser beam to a material degraded by long-term use, the fatigue crack propagation speed is delayed due to the improvement of hardness value and the compressive residual stress, and the degradation of P concentration in grain boundaries is caused. Suggests that treatment is possible.

In order to improve the fatigue strength of 2¼Cr-1Mo steel deteriorated material, the hardness test, residual stress measurement, metal structure observation, and fatigue crack propagation test using laser beam irradiation method have the following effects.

TABLE 4

1) The area of 3 to 5 (W · sec / W) of energy density during laser beam irradiation of 2¼Cr-1Mo steel was shown as the maximum hardness region, and the fatigue crack propagation speed was delayed the most. The hardness value of the base material was about 220 Hv, and the hardness value was increased by about 2.5 times to 450-500 Hv, depending on the laser beam irradiation conditions.

2) As a result of histological observation, deterioration and recovery materials used for a long time exhibit the shape of bainite and ferrite, whereas when irradiated with a high-temperature laser beam, the surface hardened layer may become martensite phase transformation. The fatigue life was prolonged due to the combined effect of the occurrence of compressive residual stress and the diffusion of impurities segregated at the grain boundaries due to long-term use by high temperature heat sources.

Claims (1)

By irradiating a laser to the steel of the base material for pressure vessels which is easy to deteriorate, it causes the martensite phase transformation which is a surface hardening layer as a heat source of the irradiated laser, and generate | occur | produces the compression residual stress by the said phase transformation in a steel, A method of increasing the fatigue strength of steel for pressure vessels, wherein the impurities segregated at the grain boundaries of steel are diffused by the high temperature heat source of the laser irradiation, thereby increasing the fatigue life of the steel.

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