JPS6219275B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for preventing segregation cracking in on-site circumferential welding of line pipes, and aims to provide a method that can appropriately prevent segregation cracking in welds during on-site welding of line pipes using cellulose welding rods. be. It is clear that on-site welding is essential when forming line pipes. By the way, most on-site welding of line pipes has traditionally been carried out using a highly efficient pipe-laying method called the Stove Pipe welding method. That is, this method uses a cellulose coated welding rod with a high hydrogen content, that is, one with a hydrogen content of 30 to 50 c.c. per 100 g of weld metal, and as mentioned above, it is possible to perform highly efficient pipe laying work. Although this is possible, it is well known that cracks in the base material occur due to hydrogen, causing various troubles. Typical examples of such cracks are root cracks 12 and segregation cracks 11 as shown in Fig. 1. Root cracks 12 have been known for a long time, and the second pass after the root pass is performed by preheating and by performing a second pass after the root pass. This can be prevented by a hot pass performed while the steel is at a moderately high temperature, and by the use of low carbon equivalent steel. However, most of the latter segregation cracks 11 are caused by central segregation inherent in steel sheets manufactured by continuous casting, and this segregation cannot be prevented with the current steelmaking technology. The state of occurrence is as shown in the micrographs in Figures 2 and 3. However, from the perspective of welding operations regarding such segregation cracking, it is effective to use a low hydrogen welding rod with a hydrogen content of 4 to 5 c.c. per 100 g of weld metal and apply carbon dioxide welding to prevent this cracking. Although effective, the former results in a decrease in work efficiency, and the latter change in welding method increases the initial cost, requires retraining of the welder, and limits the applicable pipe diameter. By the way, when using cellulose welding rods, it has been confirmed in the laboratory that keeping the interlayer temperature above 150°C is effective in reducing cracking, but it is not necessary to maintain the interlayer temperature at such a temperature during actual work. This is impossible due to the work involved in removing slag from the previous layer welding, and even if it were possible, it would not be possible to completely prevent cracking. In view of the above-mentioned circumstances, the present invention has been made through detailed research in order to completely and easily prevent the segregation cracks 11 that occur when a cellulose coated arc welding rod containing high hydrogen content is used for circumferential welding of pipes. As a result, it was invented, and the tube end was
We succeeded in effectively preventing this by carrying out a specific heat treatment at 600-900℃. That is, to explain the present invention, the present inventors fixed the test specimens 1 and 1 from above with restraint bolts 3 on a serrated pedestal 2, as shown in FIG. At the same time, we performed a reproduction test using the Schnadt-Fisco test, which was conducted under the condition that the ends of each test piece 1 and 1 were also held with support bolts 4. As a result, most of the cracks described above were typical as shown in Figures 2 and 3. As seen in the example, it has become clear that the heat is generated from the boundary between the heat affected zone (HAZ) 5 and the base material matrix 6. It was also confirmed that this cracking tends to occur when the microhardness of the segregated part is high, and that it can be effectively prevented by setting the hardness of the segregated part to Hv300. Therefore, this type of cracking is a type of hydrogen delayed cracking that occurs due to the interaction between the hardenability of the segregated part and the hydrogen that has diffused from the welded part.
As can be seen from the schematic diagram of the crack generation mechanism shown in Figure 5, the micro-hardness distribution curve is different from the hardness distribution curve of the normal part in the matrix part 6. The hydrogen distribution curve is high at HAZ part 5 and the boundary line, while the hydrogen distribution curve is high at HAZ part 5.
This is thought to be due to the combination of these two factors creating harsh conditions. Also, cracks do not occur in the HAZ part 5 because it undergoes a tempering heat cycle due to the multi-layered structure.
This is because its hardness is lower than that of segregation in the protoplasm. Based on the above study results, the test material was heat treated to various heating temperatures in order to soften the hardened structure of the segregated part of the base material, and then the test material was heated according to the test conditions shown in Table 1 below.
Schadt-Fisco test was conducted.

【table】

[Table] That is, the test results are as shown in Table 2 below, and the cracking rate is significantly reduced at heating temperatures of 600 to 900°C, particularly 600 to 800°C.

[Table] The same effect can be obtained by any heating method such as gas heating, induction heating, or resistance heating.
Below 600°C, the tempering effect is insufficient in any case, and above 900°C, the hardness does not decrease due to austenitization, so favorable results cannot be obtained, and furthermore, the tempering effect is insufficient during heating and cooling. growing. The heating time is preferably within 10 minutes because if it is too long in the temperature range, there is a risk of precipitation of carbonitrides and material deterioration. Furthermore, regarding the heating region, the effects of the present invention can be fully obtained generally within a range of 50 mm or less from the tube end, including the portion that melts during welding. Also, it is the second target steel type.
In addition to the Nb-V series X70 listed in the table, it has been found that it is effective for any of the X52 to X100 steels that are generally used as line pipe steel materials, and that the same effects as described above can be obtained. It is being The heat treatment for circumferential welding of pipes was performed after welding with the aim of dissipating hydrogen and reducing hardening of the HAZ area.
There is a post-weld heat treatment called PWHT, which is a preheating operation of 50 to 250℃ with the main purpose of hydrogen dissipation, but in the present invention, the welded part of the pipe end is heated to a specific temperature of 600 to 900℃ before welding. This is different from the post-weld heat treatment and preheating operations described above, as the purpose is to soften the hardened structure of the central segregated part of the base metal before heating and lower the weld cold cracking susceptibility. According to the present invention as described above, when performing on-site welding of line pipes, it is possible to accurately prevent the occurrence of segregation cracks and achieve advantageous welding using a general and highly efficient cellulose-based high hydrogen content welding rod. This is an invention with great industrial effects.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and Fig. 1 is a cross-sectional view showing the state in which hydrogen cracking occurs due to a cellulose welding rod, and Fig. 2 shows the specific state in which segregation cracking occurs. Figure 3 is a micrograph showing the metal structure of the segregation crack at 50x magnification, Figure 4 is an explanatory diagram of the Schnadt-Fisco test, and Figure 5 is the segregation crack. FIG. 2 is a schematic explanatory diagram of the generation mechanism. In these drawings, numeral 11 indicates a segregation crack, 5 indicates a HAZ portion, and 6 indicates a steel material portion.

Claims (1)

[Claims] 1. Segregation in on-site circumferential welding of line pipes, which is characterized by heating the ends of the steel pipes to 600 to 900°C before welding when performing on-site circumferential welding of line pipes using cellulose-based welding rods containing high hydrogen content. How to prevent cracking.