Abstract: The steel boiler tubes that burst have been detected and analyzed. The test results show that the temperature and pressure of the blocked parts rise sharply due to poor circulation when the boiler tubes are working, making the part of the tube overheat and the structure grows. The boiler tube cannot withstand excessive pressure, causing the boiler tube to burst.
 
A newly installed boiler tube in a thermal power plant burst after using for only 5 to 6 hours. The boiler tube is made from 20 and has a size of 80 × 5.5mm. When the boiler tube is working, there is water in the tube. The combustion chamber is heated by gas outside the tube wall and the tube should be at medium temperature and medium state under a normal condition.
 
1. Detection and Analysis
Macro observation
Macroscopic observation of the burst boiler tube is performed. The length of the crack is about 150mm. Several intermittent longitudinal shallow cracks can be seen on the outer wall of the tube near the crack. Figure 1 is the physical appearance. The source of the crack should be located at the part with the largest radial opening of the pipe body.
 
1.2 Sampling and detection
Carry out chemical composition analysis of the boiler tube. Take samples for metallographic analysis at the fracture source and the uncracked tube matrix, and number them as sample 1 and sample 2, and take samples for mechanical performance testing from the tube near the crack.
 

Figure 2 Metallograph of microstructure
 
1.2.1 Analysis for chemical composition
Analysis results of the chemical composition of the boiler tube are shown in Table 1. The chemical composition of the boiler tube is qualified after testing and analysis.
 
Table 1 Analysis results of chemical composition

 
1.2.2 Metallographic inspection
Grind the transverse and longitudinal inspection surfaces of metallographic samples 1 and 2; analyze the inclusions after the longitudinal inspection surfaces are polished; the test results of non-metallic inclusions on the fracture source of sample 1 are 0.5 for sulfides, 0.5 for oxides and 0.5 for silicate. The detection results of non-metallic inclusions on the substrate of sample 2 are grade 0.5 for sulfide, grade 0.5 for oxide, and grade 1.5 for silicate. After the transverse surface is polished, it is corroded with 4% nitric acid alcohol reagent for structural inspection. The structure of the fracture source of sample 1 is martensite plus a small amount of ferrite structure. The grain size is grade 6.5 (Figure 2a). The microstructures at 10mm and 15mm away from the cracking site are ferrite plus bainite plus martensite, Figures 2b and 2c; the matrix microstructure of sample 2 is ferrite and pearlite. The grain size is grade 11 (Figure 2d).
 

(a) Structure at the crack (b) Structure at about 10mm away from the crack (c) Structure at about 15mm away from the crack (d) Matrix structure of the sample at the uncracked site

 
Figure 2 Metallographic structure
 
1.2.3 Mechanical performance tests
Take mechanical performance test samples from the pipe body near the crack. The results are shown in Table 2. According to the mechanical test results, the yield strength fluctuates greatly and the strength value is generally high.
 
Table 2 Test results of mechanical properties of the cracked boiler tube

 
2. Analysis of the result
2.1 The analysis of the material
From the analysis of the chemical composition of the boiler tube and the detection results of metallographic inclusions, the chemical composition of the boiler tube meets the requirements of the supply standard GB/T699-1999. The detection of non-metallic inclusions in the cracked part and the matrix part of the boiler tubes are within the allowable range.
 
2.2 Metallographic structure and performance analysis
The analysis of the metallographic structure of the cracked part and the matrix part of the boiler tube shows that there are obvious differences in the structure and grain size of different parts. The structure of the cracked part is martensite plus a small amount of ferrite grains. The grain size is grade 6.5. The structure of the part which is 10mm and 15mm away from the cracking site is ferrite plus bainite plus martensite structure. The matrix structure is ferrite and pearlite. The grain size is grade 11. It reflects that the tissue at the boiler tube burst site is abnormal. The on-site operating condition of the boiler tube is that water flows through the pipeline of the boiler tube during work, and the outside of the tube wall is a gas-heated combustion chamber, which is baked at high temperature. During normal operation, the boiler tube takes away the heat through the cooling water in the tube. Once the cooling water flow inside the tube is blocked, it will cause a sharp increase in the partial temperature and pressure of the tube body. It is prone to bursting. and the boiler tube has just been installed and operated for 5 to 6 hours. When the new boiler is just running, the pipeline is prone to blockage and other poor circulation. At the time of the on-site investigation, the pressure of the boiler tube did suddenly increase, which indicates that there is a blockage in the tube. Because of the blockage in the pipeline, the temperature and pressure are too high in the partial pipe. When the boiler tube bursts, a large amount of cooling water come out to quench it, resulting in martensite at the burst part. The structure near martensitic structure also changed. The corresponding mechanical performance test results also reflected that the strength near this part was obviously higher.
 
3. Conclusion
From the comprehensive analysis of the inspection results, the boiler tube is in operation due to poor circulation, and the temperature and pressure of the blocked part rise sharply, resulting in the partial overheating of the tube body and the grain growth of the tissue, which is difficult to withstand the high pressure, resulting in a burst of the boiler tube.