Abstract: The leakage at the four tubes such as the water-cooled wall, superheater tubes, reheater tubes, and economizer tubes in a power station boiler is one of the important factors that cause the unplanned shutdown of the unit. In order to deeply analyze the leakage of the butt welding seam of the reheater tube of one of the four boiler tubes, a macro inspection, analysis of the material composition, hardness test, metallographic structure analysis and scanning electron microscopy analysis were carried out on a leaking tube. The results show that the cracks originated in the coarse-grained zone near the fusion line of the welding seam, with typical intergranular cracking properties. Combined with the welding characteristics of the steel welding process, the analysis shows that the cracks are welded reheat cracks. By setting the preheating temperature of welding reasonably, optimizing the welding process, and improving the stress state of the service pipes, it is helpful to reduce the risk of cracks at the welding seam, reduce the leakage rate of the four boiler tubes, and improve the safety and reliability of the unit.
Introduction
The four-pipe leakage of power station boilers is one of the most important factors that cause unplanned shutdowns of thermal power generating units. According to a group's statistics, four-pipe leakages accounted for 51% of unplanned shutdowns of units, among which leakage caused by cracks of welding seams accounted for 26.8%, becoming the main factor affecting the safe and stable operation of the unit and ensuring the safety of power supply. Therefore, it is particularly urgent to analyze the causes of the leakage of the four boiler tubes and take corresponding prevention and control measures. The following analyzes the causes of cracking of welding seams and leakage of the boiler reheater in a certain power plant after service, and proposes preventive control measures. A power plant boiler is a subcritical intermediate primary reheating natural circulation steam drum furnace, and the furnace type is HG1025/17.4-YM28. Single furnace with balanced ventilation, four corners tangential combustion, and solid slag discharge. The main parameters of the boiler are as follows: The maximum evaporation capacity is 1025t/h; outlet steam temperature of the superheater is 541°C; outlet pressure of the superheater 17.40MPa; inlet or outlet temperature of reheated steam is 330/541°C; inlet or outlet pressure of reheated steam is 3.917/3.737MPa, and the water supply temperature is 282.3℃. The boiler was put into operation in the first half year of 2006 and has worked for 91,000 hours when it leaked in 2019. The leak occurred in the clamping tube of the platen reheater, and the burst position was located at the 41-meter-elevation weld of the entrance section of the holding tube of the 12th panel on the right side of the furnace. Macro inspection, hardness test, and metallographic analysis were carried out to analyze the cause of the cracking of the platen reheater butt welding.
1. Testing
1.1 Macro inspection and geometric measurement
The material of the reheater tube is 12Cr1MoVG, and the specification is 63mm × 4mm. A crack with the circumference of about 81 mm near the fusion line of the pipe's outer wall of base metal's welding seams at the upper side, accounting for about two fifths of the entire pipe's circumference. The length of the cracks on the inner wall of the pipe is about 76mm, and the morphology of the crack is shown in Figures 1 and 2. It can be judged that the crack originated from the outer wall and expanded to the inner wall in the circumferential direction. Cut along the longitudinal direction of the tube sample, and the crack is generated near the fusion line of the outer wall, being perpendicular to the surface of the tube sample and extending along the wall thickness direction.
Figure 1 The schematic diagram of the appearance of the tube
Figure 2 The crack morphology on the inner wall of the pipe sample
The wall thickness and outer diameter of the pipe sample were measured with a vernier caliper. The data showed that the wall thickness of the pipe sample on both sides of the welding seam was not thinned, and the outer diameter of the pipe sample was not significantly thickened.
1.2 Analyses of the chemical composition
The analysis of the chemical composition was performed on the welding seam with leakage and base metal samples. The results are shown in Table 1. The analysis results show that the chemical composition meets the technical requirements of GB/T 5310-2017 Seamless Steel Pipes for High Pressure Boilers.
Table 1 The chemical composition of bursted welding seams and base metal wt%
| Composition |
C |
Si |
Mn |
Cr |
Mo |
V |
P |
S |
| Upper base materials |
0.147 |
0.234 |
0.596 |
1.05 |
0.27 |
0.198 |
0.015 |
0.001 |
| Welding seams |
0.081 |
0.320 |
0.626 |
1.20 |
0.35 |
0.193 |
0.012 |
0.004 |
| Lower base materials |
0.123 |
0.238 |
0.558 |
1.05 |
0.27 |
0.176 |
0.011 |
0.001 |
GB/T 5310-2017
12Cr1 MoVG |
From 0.08 to 0.15 |
From 0.17 to
0.37 |
From 0.40 to
0.70
|
From 0.90 to
1.20
|
From 0.25 to 0.35 |
From 0.15 to
0.30
|
Being less than or equal to
0.025 |
Being less than or equal to
0.01 |
1.3 The Brinell hardness test
The welding seam and base metal of the leaking pipe are sampled and processed for a sample for the hardness test. According to the requirements of Metallic Material Brinell Hardness Test Part 1: Test Method GB/T231.1, the UH250 Brinell hardness tester is used for testing. The test results are shown in Table 3. It can be seen from the table that the hardness value of the upper and lower sides of the base material of the leaking pipe and that of the welding seam are in accordance with the Technical Supervision Regulations for Metals in Thermal Power Plants DL/T 438. The base metal hardness of 12Cr1MoVG is required to be between 135 and 195HB, and the hardness value of the welding seam is not 100HB greater than that of the base metal and less than 270HB.
Table 2 Testing results of Brinell hardness
| Performance |
Measurement results |
Average values |
| Upper base materials |
157 |
166 |
163 |
162 |
| Welding seams |
227 |
228 |
224 |
226 |
| Coarse grain area |
166 |
172 |
169 |
169 |
| Fine grain areas |
178 |
173 |
180 |
177 |
| Lower base materials |
162 |
159 |
161 |
161 |
1.4 The tensile test
Cut different parts of base materials of the leaking pipe, and then process them as 3 pieces of longitudinal arc tensile specimens with full wall thickness, using a CMT5105 microcomputer-controlled electronic universal testing machine. Perform the tensile test based on the Metallic Material Tensile Test Part 1 Room Temperature Testing Method GB/T 228.1 and the testing results are shown in Table 3. The tensile performance test results show that the tensile of the tested sample meets the technical requirements of the Seamless Steel Pipes for High Pressure Boilers GB/T 5310.
Table 3 Tensile testing results
| Items |
Lower yield strength
ReL(MPa) |
Tensile strength Rm
(MPa) |
Elongation after fracture A (%) |
| Sample 1 |
345 |
494 |
24.0 |
| Sample 2 |
351 |
514 |
23.5 |
| Sample 3 |
343 |
501 |
27.5 |
| GB/T 5310-2017 |
Being greater than or equal to 255 |
From 470 to 640 |
Being greater than or equal to 21 |
