Steel structure manufacturing is widely used in shipbuilding, boilers, lifting transportation, mining and transportation engineering and other industrial fields. In mine machinery, heavy and super heavy coal conveying equipment is mostly made of cast - welding structure. Therefore, the welding failure analysis and preventive measures of this steel structure have become a concern

1.Cause of welding failure of cast - welding structure

Generally speaking, welding failure is the fracture of the welded joint under a certain condition (such as stress, temperature, material, welding quality and actual operating conditions, etc.). Once the joints fail, the components which are closely related to each other will be separated, tearing and expanding, causing the damage of the welding structure, causing the equipment to stop and affect the normal production

The basic conditions of welding failure are as follows: first, the design of welded structure is unreasonable, for example, there are problems in the layout and design of partial or whole weld. Second，the material itself has defect, such as the plate has chemical segregation, there are porosity, porosity and cracks in the cast steel. Third, the application of welding technology is unreasonable, such as the selection of welding materials and the formulation of welding process. Fourth, the working environment and working conditions of components are poor, such as being subjected to alternating and impact loads, resulting in fatigue failure of structural materials

For joint failure analysis, we should start from two aspects: first, the root causes; two, the harm.

2.Root cause of welding failure of structural parts

Usually, the defects of the material itself, such as the in homogeneity of the chemical composition, the local micro crack, etc, cold and hot cracks, incomplete penetration, slag inclusion, blowholes and undercut caused by various reasons tec. The higher residual stresses in the near joint area during welding (including the weld stress and the heat stress zone transformation stress), as well as the embrittlement of microstructure softening and cooling at high temperature, it is the root cause of joint failure, and also provides the conditions for the brittle fracture or expansion of the joint.

3. Harmfulness of welding failure of structural parts

The scraper conveyor, loader and crusher are the key equipment of coal face, and the working conditions are complex. As the scraper conveyor not only needs to complete the transportation of coal and clean the floating coal, it also serves as the base point for the track of shearer, the traction support, hydraulic support, it is responsible for the whole process of falling, loading, transporting, supporting and controlling in the process of coal mining, The reliability of underground equipment determines the economy of efficient coal mining, so the quality of the equipment, the long life and the performance have a direct impact on the coal production

Because of the characteristics of the scraper conveyor and the transshipment machine, the strength and reliability of the connection between the middle slots are particularly important. Its structure is mostly made by cast steel and ordinary low alloy plate Q345 (16Mn), high strength plate and wear resistant plate (NM360) through welding, the cast steel is mainly composed of carbon, manganese and silicon (C-Mn-Si) series. Years of practice have proved that the welding performance of such cast steel is relatively good, but the joint failure of welded structural parts is also easy to occur in the welding parts of such structures, Once the fracture failure occurs in the underground working face, it will affect the normal operation seriously or even stop production of the whole equipment, caused great economic losses.

4.Welding failure mode

Due to the particularity of the structure, the failure modes of welding are different.

1) Due to irrational design, there is a phenomenon of excessive local rigidity and stress concentration.

2) material defects：Compared with the rolled sheet, the cast steel has the characteristics of poor toughness and low yield strength, as well as unreasonable welding process, improper application of welding standard, and incorrect selection of welding methods.

3）The welding technology level and the good or bad of the welding position. also welding inspection levels, including material inspection and weld inspection, in addition, the ambient temperature is also an important factor affecting the welding quality.

The above may be the cause of the failure of the welded joint. Once the structural parts of the above defects enter the use stage, it is difficult to avoid the occurrence of failure and damage.

In summary, to avoid welding failure for this kind of structures, simply analyzing and solving problems from one aspect is not unrealistic. It is a combination of many factors, but under certain conditions, it is the result of a certain factor playing a major role. This should be considered from many aspects such as design, technology, material and welding, so as to rationalize the factors of the above aspects as far as possible, to find the most important reasons and to take the necessary technological measures to effectively prevent them

To be continued