A method for judging the reheat crack sensitivity of coarse grain area

Abstract

The present invention discloses a method for judging the reheat crack sensitivity of a coarse grain zone, and belongs to the technical field of welding evaluation. First, a thermal simulation experiment is performed on a low alloy heat resistant steel sample according to the actual thermal cycle process of low alloy heat resistant steel welding to obtain a sample with a coarse grain zone structure, and then a creep experiment is performed on the sample, and a preset stress is applied to the sample under constant temperature and constant strain conditions until the sample breaks, and the creep strain value is calculated according to the stress relaxation curve during the experiment, and finally, the relationship between the creep strain and the cross-sectional shrinkage rate is established in combination with the evaluation index of the short-time creep rupture test, and then an evaluation criterion for evaluating the reheat crack sensitivity of the coarse grain zone of low alloy heat resistant steel welding is established. The present invention solves the problem that the cross-sectional area measurement of the sample is inaccurate after the evaluation experiment of the reheat crack sensitivity of the coarse grain zone of low alloy heat resistant steel is interrupted, which leads to a large error in the calculation of the cross-sectional shrinkage rate and inaccurate evaluation of the reheat crack sensitivity of the coarse grain zone.

Description

Technical Field

[0001] The invention belongs to the technical field of welding joint evaluation, and in particular relates to a method for judging the reheat crack sensitivity of a coarse grain zone. Background Art

[0002] Alloy steel with a total alloying element mass fraction of less than 5% is generally called low-alloy heat-resistant steel, and its alloy series include: C-Mo, C-Cr-Mo, C-Cr-Mo-V-Nb, C-Mo-V, C-Cr-Mo-V, C-Mn-Mo-V, C-Mn-Ni-Mo and C-Cr-Mo-WV-Ti-B. In recent years, with the advent of low-alloy Mo steel, Cr-Mo steel, and Cr-Mo-V steel, low-alloy heat-resistant steel has been widely used in boilers, pressure vessels, and chemical equipment due to its excellent weldability, corrosion resistance, wear resistance, and formability. Despite this, precipitation-strengthened low-alloy heat-resistant steel welded joints have a significant tendency to reheat cracks when used for a long time under high temperature and high pressure, and they are mainly generated in the coarse-grained area of ​​the heat-affected zone. Therefore, there is an urgent need for a method for evaluating the reheat crack sensitivity of the coarse-grained area.

[0003] At present, the experimental methods involved in the evaluation of reheat crack sensitivity of low-alloy heat-resistant steel welded joints include the pin-type reheat crack test, the H-shaped restraint test and the simulated coarse-grained zone short-time creep fracture test. The shape and size of the test specimens used in the pin-type reheat crack test and the test device are the same as those of the cold crack pin test, except that an electric furnace for heating is installed at the welding pin. This pin-type reheat crack test is a stress release test under constant load tensile conditions, which can be used to obtain stable quantitative data. It is mainly used to study the influence of various components or various parameters on reheat crack sensitivity. Its disadvantage is that it cannot be close to actual production. The H-shaped restraint test is a test method for detecting hot cracks and reheat cracks in welds. There is a certain gap in the design of the specimen. Changing the size of the gap can change the degree of restraint. It is a self-restrained specimen. It can be seen that the H-shaped restraint test is mainly aimed at the detection of hot cracks and reheat cracks in welds, and is not good at evaluating the reheat crack sensitivity of the heat-affected zone. The simulated coarse grain area short-term creep fracture test includes two parts: the preparation of the simulated coarse grain area and the high temperature constant strain rate stretching. After the sample is broken, the cross-sectional shrinkage of the sample is calculated to determine the reheat crack sensitivity of the material. This method is affected by the operator's level and the error of data processing when calculating the cross-sectional shrinkage of the sample. The calculation result of the cross-sectional shrinkage is inaccurate, which often affects the evaluation result of the reheat crack sensitivity of the material. Summary of the invention

[0004] In order to solve the defects existing in the above-mentioned prior art, the purpose of the present invention is to provide a method for evaluating the reheating crack sensitivity of the coarse grain zone, which solves the problem of inaccurate measurement of the cross-sectional area of ​​the sample after the reheating crack sensitivity evaluation experiment of the coarse grain zone of low-alloy heat-resistant steel is interrupted, thereby resulting in large errors in the calculation of the cross-sectional shrinkage rate and inaccurate evaluation of the reheating crack sensitivity of the coarse grain zone.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for judging the reheat crack sensitivity of a coarse grain region comprises:

[0007] S1: low alloy heat-resistant steel specimen required for machining thermal simulation experiment;

[0008] S2: According to the actual thermal cycle process of low alloy heat resistant steel welding, the thermal simulation test parameters are set, and the thermal simulation test is carried out on the low alloy heat resistant steel sample to obtain the sample with coarse grain structure;

[0009] S3: On a creep testing machine, heat the sample obtained in S2 to the test temperature without an external load and keep it warm;

[0010] S4: applying a preset stress to the sample obtained in S3 under constant temperature and constant strain conditions until the sample breaks, and calculating the creep strain value according to the stress relaxation curve during the experiment;

[0011] S5: Combined with the evaluation indicators of short-time creep rupture test, the relationship between creep strain and section reduction rate is established, and then the evaluation criteria for evaluating the reheating crack sensitivity of the coarse-grained zone of low-alloy heat-resistant steel welding are established.

[0012] Preferably, in S1, during machining, the surface temperature of the low alloy heat-resistant steel sample is controlled to be ≤100°C throughout the entire process.

[0013] Preferably, in S1, the low alloy heat-resistant steel sample is a rod-shaped sample.

[0014] Preferably, in S2, the thermal simulation experiment parameters include heating rate, peak temperature and cooling rate.

[0015] Preferably, in S2, the thermal simulation experiment uses a thermocouple to measure the thermal cycle of the characteristic points of the temperature field or uses a finite element simulation method to obtain the temperature field of the welding process.

[0016] Preferably, in S3, the experimental temperature is determined according to the service environment of the low alloy heat-resistant steel.

[0017] More preferably, the heating time is 2 minutes, the experimental temperature is 500-750° C., and the insulation time is 10 minutes.

[0018] Preferably, in S4, the temperature and strain of the sample remain unchanged during the experiment, and the stress changes with time. The curve recording the change of stress with time is the stress relaxation curve.

[0019] Preferably, in S4, the creep strain value is calculated according to the following formula:

[0020]

[0021] In the formula, ε c is the creep strain; E is the elastic modulus; is the creep stress change rate.

[0022] Preferably, in S5, for the experimental results of the same sample, the creep strain is calculated using the data recorded during the creep experiment, and the cross-sectional shrinkage rate of the sample is calculated using the cross-sectional area of ​​the fracture position before and after the sample is broken, thereby establishing a relationship between the creep strain and the cross-sectional shrinkage rate of the sample.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects:

[0024] The method for judging the reheat crack sensitivity of the coarse grain zone disclosed in the present invention is based on the combination of experimental measurement and numerical calculation. By calculating the creep strain and combining the evaluation index of the short-time creep rupture test, the relationship between the creep strain and the cross-sectional shrinkage rate is established, and further the evaluation criteria for evaluating the reheat crack sensitivity of the coarse grain zone of low alloy heat-resistant steel welding are established. This is mainly because the creep strain and the cross-sectional shrinkage rate are both performance indicators that reflect the plastic deformation ability of the coarse grain zone sample under high temperature external load conditions. The measurement of the cross-sectional shrinkage rate of the sample is greatly affected by human factors, while the creep strain is measured by the instrument and has higher accuracy. At the same time, the present invention does not rely on the experience and skill level of the operator, and solves the problem that the cross-sectional area measurement of the sample is inaccurate after the reheat crack sensitivity evaluation experiment of the coarse grain zone of low alloy heat-resistant steel is interrupted, resulting in large errors in the calculation of the cross-sectional shrinkage rate and inaccurate evaluation of the reheat crack sensitivity of the coarse grain zone.

[0025] Furthermore, in S1, during machining, the surface temperature of the low alloy heat-resistant steel sample is controlled to be ≤100°C throughout the entire process, which can prevent the sample surface temperature from being too high during machining, causing phase change in the sample surface structure, and having an adverse effect on the sample performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the method flow of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be further described in detail below in conjunction with the accompanying drawings, which are intended to explain the present invention rather than to limit it.

[0028] like Figure 1The method for judging the reheat crack sensitivity of the coarse grain zone of the present invention comprises:

[0029] In the first step, technicians use machining to take samples along the longitudinal direction of the steel pipe or steel rod. The total length of the processed rod-shaped specimen is 150mm, the length of the test section is 10mm, the diameter of the test section is φ6mm, the clamping section is M10mm, the transition angle is 45°, and the surface temperature of the specimen does not exceed 100℃ during the machining process. The surface of the processed specimen is smooth and burr-free;

[0030] The second step is to calculate the welding heat input Q = ηUI / v according to the actual welding parameters of the low-alloy heat-resistant steel and measure or simulate the thermal cycle of the coarse-grained area of ​​the actual joint by finite element method. According to the thermal cycle, the corresponding heating rate, peak temperature and cooling rate are set on the thermal simulation test machine to simulate the heating process of the sample during welding, and finally obtain the sample of the coarse-grained area organization of the low-alloy heat-resistant steel;

[0031] The third step is to install the sample of the coarse-grained area of ​​low-alloy heat-resistant steel on the creep testing machine. Under the condition of no external load, the sample is heated to the test temperature (500-750°C) within 2 minutes. The specific experimental temperature is determined according to the actual service conditions, and then kept at this temperature for 10 minutes.

[0032] The fourth step is to apply a certain stress to the sample under constant temperature and constant strain conditions until the sample breaks, and record the stress relaxation curve during the experiment;

[0033] Step 5: Calculate the creep strain value based on the stress relaxation curve data

[0034] The sixth step is to establish the relationship between creep strain and section reduction rate by combining the evaluation indexes of the short-time creep rupture test as shown in Table 1, and further establish the evaluation criteria for evaluating the reheating crack sensitivity of the coarse-grained zone of low-alloy heat-resistant steel welding.

[0035] Table 1

[0036]

[0037] The present invention will be further explained below with a specific embodiment:

[0038] This embodiment uses 15Cr1Mo1V low alloy heat-resistant steel as the research object, and uses machining means to process thermal simulation specimens with a length of 150mm, a test section length of 10mm, a test section diameter of φ6mm, and a clamping section of M10mm and φ10mm. The heat-affected zone of manual arc welding is simulated, and the corresponding heat inputs are 2.77kJ / cm, 3.02kJ / cm, 3.28kJ / cm, 3.53kJ / cm and 3.78kJ / cm (see Table 2) according to the welding voltage of 20V, the welding speed of 20m / h, and the welding current of 110A, 120A, 130A, 140A and 150A, respectively. The heating rate, peak temperature and cooling rate of thermal simulation are set. The obtained specimens are subjected to creep experiments, and the creep strain is calculated according to the experimental results as shown in Table 3. Combined with the evaluation index of short-time creep rupture test, the relationship between the critical creep strain and the section shrinkage rate under the five welding heat inputs is obtained, as shown in Table 3. Then, the creep strain evaluation standard for evaluating the reheating crack sensitivity of the coarse-grained zone in the heat-affected zone of low-alloy heat-resistant steel is obtained, as shown in Table 4.

[0039] Table 2

[0040]

[0041] Table 3

[0042]

[0043]

[0044] Table 4

[0045]

[0046] It should be noted that the above is only a part of the implementation of the present invention, and equivalent changes made to the system described in the present invention are all included in the protection scope of the present invention. Those skilled in the art of the present invention can replace the specific examples described in a similar manner, as long as they do not deviate from the structure of the present invention or exceed the scope defined by the claims, they all belong to the protection scope of the present invention.

Claims

1. A method for judging the reheat crack sensitivity of the coarse grain zone, characterized in that: include: S1: Low alloy heat-resistant steel specimen required for machining thermal simulation experiment; during machining, the surface temperature of the low alloy heat-resistant steel specimen is controlled to be ≤100℃ throughout the whole process; S2: According to the actual thermal cycle process of low alloy heat resistant steel welding, the thermal simulation test parameters are set, and the thermal simulation test is carried out on the low alloy heat resistant steel sample to obtain the sample with coarse grain structure; S3: On a creep testing machine, heat the sample obtained in S2 to the test temperature without an external load and keep it warm; S4: Apply a preset stress to the sample obtained in S3 under constant temperature and constant strain conditions until the sample breaks. Calculate the creep strain value based on the stress relaxation curve during the experiment. The creep strain value is calculated according to the following formula: In the formula, is the creep strain; E is the elastic modulus; is the creep stress change rate; S5: Combined with the evaluation indicators of the short-time creep rupture test, the relationship between creep strain and cross-sectional reduction rate is established, and then the evaluation criteria for evaluating the reheating crack sensitivity of the coarse-grained zone of low-alloy heat-resistant steel welding are established; for the experimental results of the same sample, the creep strain is calculated using the data recorded during the creep experiment process, and the cross-sectional reduction rate of the sample is calculated using the cross-sectional area of ​​the fracture position before and after the sample is pulled apart, so as to establish the relationship between the creep strain and cross-sectional reduction rate of the sample.

2. The method for judging the reheat crack sensitivity of the coarse grain zone according to claim 1, characterized in that: In S1, the low alloy heat-resistant steel specimen is a rod-shaped specimen.

3. The method for judging the reheat crack sensitivity of the coarse grain zone according to claim 1, characterized in that: In S2, the thermal simulation experiment parameters include heating rate, peak temperature and cooling rate.

4. The method for judging the reheat crack sensitivity of the coarse grain region according to claim 1, characterized in that: In S2, the thermal simulation experiment uses thermocouples to measure the thermal cycle of the characteristic points of the temperature field or uses the finite element simulation method to obtain the temperature field of the welding process.

5. The method for judging the reheat crack sensitivity of the coarse grain zone according to claim 1, characterized in that: In S3, the experimental temperature is determined according to the service environment of the low alloy heat-resistant steel.

6. The method for judging the reheat crack sensitivity of the coarse grain zone according to claim 3, characterized in that: The heating time is 2 minutes, the experimental temperature is 500-750°C, and the holding time is 10 minutes.

7. The method for judging the reheat crack sensitivity of the coarse grain zone according to claim 1, characterized in that: In S4, the temperature and strain of the sample remain unchanged during the experiment, and the stress changes with time. The curve recording the change of stress with time is the stress relaxation curve.

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