Analysis of corrosion cracking of stainless steel nozzle of boiler gas burner

1. Chemical composition analysis

In order to find out the cause of corrosion cracking of the stainless steel nozzles of the effective boiler gas burner, it is necessary to analyze the chemical composition of the nozzles with the problems. In the actual testing process, the inspectors should strictly follow the chemical testing standards and obtain the accurate chemical composition of the nozzle by using the fixed full-spectrum direct reading spectrometer. By analyzing the chemical results, we know that the chemical material of the nozzle conforms to the initial standard.

2. Mechanical performance test

In the process of detecting the stainless steel nozzle of boiler gas burner, it is necessary to carry out the mechanical property test of the stainless steel nozzle. Before the test, according to the test specifications, the test personnel to the stainless steel nozzle bend material for tensile test, its tensile strength and normal temperature yield strength meet the use standard, in the occurrence of corrosion and fracture of the bend test, the bend elongation is far lower than the normal value; After the end of the tensile test, the stainless steel nozzle elbow material did not appear necking, and the crack area showed the characteristics of brittle fracture.

3. Metallographic observation

The inspector shall grind, polish and corrode the elbow material at the cracked location in Figure 1, and then place it under the observation mirror to find the presence of coking attachment and carburizing layer in the elbow in the cracked area. By increasing the microscope magnification to improve the observation coefficient, it can be found that the coking attachment inside the elbow presents a block shape and exists in a continuous network, and the coking attachment far away from the inside of the elbow mainly exists in a broken chain, and the carburizing layer appears a shrinking trend; There are a lot of carbides in the coking attachment, as shown in FIG. 3. Through careful observation, it can be known that the more close to the inside of the elbow carbide material content is higher, the depth of its internal carburizing layer compared to the external carburizing layer thickness decreased, the number of grain boundary material. After sampling and observing the elbow with severe corrosion in FIG. 2, the outside of the elbow is first ground and polished, and then examined through a microscope. It can be seen that the grain boundary in the severely corroded area of the elbow has a great degree of oxidation, and a large number of carbides appear inside. The shape of the area near the inside of the elbow is similar and the oxidation degree is less.

4. SEM morphology observation and energy spectrum analysis

In order to get more accurate reasons for corrosion cracking of stainless steel nozzles of boiler gas burner, the inspectors should also observe the SEM morphology of the bends with corrosion conditions, so as to get accurate reasons. In the actual operation, the inspector needs to sample the elbow with serious corrosion problems first, and then treat it. After observation by electron microscope, it will be found that there are more cracks and voids in the elbow. By analyzing the carbide precipitation phase and austenite collectivity, it can be found that the permeable layer of elbow pipe is mainly composed of various elements.