JPS6128291B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for evaluating the cracking susceptibility of materials using a constant displacement test, and more particularly to a method for testing the stress relief annealing cracking susceptibility. Stress relief annealing treatment (hereinafter referred to as SR treatment) is widely used for the purpose of reducing residual stress caused by welding and improving the structure of the heat affected zone. However, C, Si, Mn, P, S, Cu, Mo,
When low-alloy steel and high-alloy steel containing Al, Ti, V, No, B, etc. are subjected to SR treatment, cracks (hereinafter referred to as SR cracks) may occur in the coarse grain region of the weld heat affected zone. There were many problems during construction.
Therefore, in order to prevent cracks that occur with such SR treatment, it is first necessary to clarify the susceptibility of steel materials to cracks. The conventional method for testing susceptibility to SR cracking is shown below. Figure 1 shows a method of applying restraining force by bending, where 1 is the base material bent into a U-shape, 2 is the weld metal, 3 is the weld heat affected zone, and 4 is the coarse grain area of the weld heat affected zone. It is a notch made in the. The open end side of the base material 1 is restrained by bolts 5, and in this state 500
SR cracking is reproduced by performing SR treatment at ~750℃. Figure 2 shows that after placing a test bead 7 on a beveled base material 6, a restraining force is applied by placing a restraining bead 8 a predetermined number of passes, and then SR treatment is performed to reproduce SR cracking. This is the way to do it. In FIGS. 3 and 4, the base material is punched into an H-shape to form a test piece 9 with a groove, a weld bead 10 is placed in the groove, and the strain due to heat shrinkage is absorbed into the test piece 9. It is a total restraint. This restraining force can be set to an arbitrary value by variously changing the shape of the test piece. This test piece is then subjected to SR treatment to reproduce SR cracking. Figure 5 shows that a compact type specimen 11 is prepared after a welding heat-affected zone coarse-grained region is imparted to the base metal, a constant strain is applied to this by applying a load in the direction of arrow A, and in that state SR SR by processing
This is a method to reproduce cracks. Here, the initial strain change due to thermal expansion of the test piece and testing machine that occurs during the SR treatment is kept constant by varying the load. The main factors involved in SR cracking include metallurgical factors such as chemical composition, structure of the weld heat affected zone, and grain size, and mechanical factors such as confinement stress and stress relaxation characteristics.These factors can be quantified. It is very important to understand this in order to evaluate the SR cracking susceptibility. However, in the prior art, there was a particular problem in quantitative evaluation of mechanical factors. That is, the first
The method shown in the figure uses bending displacement as a parameter for evaluating SR cracking, but this is a value unique to this test method. It is difficult to express this value in terms of general parameters such as stress and stress intensity factor due to the shape of the test piece. The method shown in FIG. 2 uses the amount of angular displacement as a mechanical parameter, but this value is also unique to this test piece. In addition, in this test method, by attaching a strain gauge or the like to the toe, which is generally a likely starting point for SR cracking, it is possible to evaluate using residual stress, which is a common parameter. However, there is usually an unwelded part in the weld, and SR cracking often occurs due to stress concentration in this part. Therefore, the value of residual stress at the toe cannot necessarily be used as a parameter for evaluating cracking. The methods shown in FIGS. 3 and 4 use the degree of restraint as a mechanical parameter. This value is determined only by the shape of the test piece. However, when evaluating SR cracking, the occurrence of SR cracking is greatly influenced by factors related to welding conditions such as welding heat input, cooling rate, toe shape and stress concentration, and so the degree of restraint alone is not sufficient. It is not possible to evaluate SR cracking. In this case, it is possible to use the residual stress at the toe as a parameter, but as with the method shown in FIG. 2, it is impossible to evaluate stress concentration at the unwelded part. In this way, the method of actually placing a weld bead and reproducing SR cracking in its heat-affected zone satisfies metallurgical factors such as the structure and grain size of the heat-affected zone, but it is difficult to evaluate mechanical factors. There are many difficulties. In particular, SR cracking occurs due to notches at the toe and stress concentration in unwelded areas, so a test method that can accurately determine stress concentration at the notch is required. Regarding this point, the method shown in Fig. 5 involves providing a simulated thermal history of the weld heat-affected zone coarse-grained region to the base metal, and then producing a compact type test piece.
This is a method of reproducing SR cracking by performing SR processing under a load, and it is possible to evaluate using parameters such as opening displacement and stress intensity factor that are suitable for evaluating the occurrence and propagation of cracks. be. However, in order to reproduce the stress relaxation characteristics under a constant displacement state similar to that of an actual weld using this test method, it is necessary to
The testing machine must be able to reliably eliminate thermal strain caused by thermal expansion of the test piece and testing machine during heating and holding during SR treatment. However, in order to eliminate large stress fluctuations caused by minute displacements due to thermal expansion, a very complicated control device is required, resulting in an expensive device. When quantitatively evaluating the occurrence of cracks in a material under a constant strain state, the present invention eliminates fluctuations in the amount of strain that occur due to temperature changes using the test piece itself, thereby constantly maintaining a constant strain state. This paper relates to a stress relief annealing cracking susceptibility test method for the purpose of In the present invention, a test piece 13 having a notch 12 is prepared from a test material as shown in FIG. This applies a predetermined constant amount of strain. Therefore, the amount of thermal expansion caused by changes in the temperature of the test environment or by arbitrarily changing the environmental temperature will be different from the amount of strain initially applied to the test piece 13. It is possible to easily perform a strict constant strain test without any adverse effects. In this case, there are no particular restrictions on the size and length ratio of the test piece 13. Further, the amount of strain imparted by driving the wedge 14 can be quantified by measuring the driving amount of the wedge 14 or the amount of displacement of the crack tip opening using a contact strain meter or the like. From this value, the amount of opening displacement and stress intensity factor that can optimally evaluate the occurrence and propagation of cracks can be easily determined. Furthermore, when the radius of the crack tip is increased, it becomes possible to evaluate cracking susceptibility using stress as a parameter. The present invention will be explained in more detail using an example in which SR cracking susceptibility during welding work was determined. first,
For steel materials having chemical components as shown in Table 1, the thermal history of the coarse grain region of the weld heat affected zone was imparted by high frequency heating or the like.

[Table] A test piece was prepared from this material by making a block 32 mm long, 28 mm wide, and 10 mm thick with a notch 2 mm wide and 16 mm long. Here, the notch tip radius is 0.1
mm. This specimen has a thickness of 1.5-2.5mm and a width of 12mm.
The stress intensity factor shown in Table 2 was given by driving a wedge into the specimen.

[Table] These test pieces were subjected to SR treatment at 600℃ for 3 hours. As a result, SR cracking occurred in the notch layer in the specimens with a high applied stress intensity factor. Figure 7 shows the relationship between the length of this SR crack and the stress intensity factor. From this, SR cracking occurs at a specific stress intensity factor depending on the material, and the length is linearly proportional to the stress intensity factor, so it is possible to quantitatively understand the cracking susceptibility of steel materials. Ta. According to the test method of the present invention, unlike various conventionally used methods, stress relaxation characteristics under a constant strain state can be easily reproduced, and the test method is able to easily reproduce stress relaxation characteristics in a constant strain state. This makes it possible to accurately grasp the relevant factors.

[Brief explanation of the drawing]

Figures 1 to 5 show the conventional method of testing the weld cracking susceptibility of materials. Figure 1 is a front view showing a test method in which a restraining force is applied by bending, and Figure 2 Figure 3 is a perspective view showing a test method in which a restraining force is applied by placing a restraining bead after the test material is grooved and a test bead is placed. After that, a weld bead is placed on the groove part, and the stress due to the thermal contraction is restrained throughout the test piece. Figure 5 is a front view showing the crack susceptibility test method using a compact type test piece, Figure 6 is a front view showing the test piece used in the test method of the present invention, and Figure 7 is the length and length of SR cracks according to the test method of the present invention. FIG. 3 is a diagram showing the relationship with stress intensity factor. 12...notch, 13...test piece, 14...wedge.

Claims (1)

[Claims] 1. When evaluating the susceptibility of materials to cracking, a constant amount of strain is applied by driving a wedge of the same or different material into a test piece with a notch, and then a constant displacement test is performed in a specified environment. A stress relief annealing cracking susceptibility testing method characterized by: