How can the chain break failure improvement process be without an energy-saving medium frequency induction heating machine?

The chain of the power plant slag scooper is composed of O-shaped link and U-shaped hook. The workpiece material is SCM420 steel, which is equivalent to domestic 20CrMo steel. The chain drives the scraper to move and discharge slag in the water tank during operation. It bears a large tensile force during operation and a smaller force when no-load. Therefore, the parts are periodically subjected to tensile and tensile stress, and at the same time, the chain is subjected to the corrosion of ash and slag water. The technical requirement of the chain parts of the slag scooper is 7000 hours. During production, it was found that the chain parts of a certain slag scooper suffered early fracture failure after only 2800 hours of operation. The U-shaped hook broke and the link cracked. Tests have proven that such defects can be effectively avoided by strictly controlling the carburizing process and using an energy-saving medium frequency induction heating machine for heat treatment.

Technical requirements stipulate that the minimum breaking load of chain hooks and links in service is 300kN, the hardness is 52-62HRC, and the depth of the carburized hardened layer should be ≥0.5mm. The inspection found that the fracture location of the workpiece was located at the dangerous cross section of the chain hook and link. Chemical composition analysis shows that the manganese content in the workpiece is relatively high. The hardness inspection found that the surface hardness of the workpiece was lower than the required technical indicators. The fracture inspection found that the fracture of the workpiece was observed to be composed of a fiber area, a rapidly expanding radial tearing prism shape, and a shear lip at the final fracture point; some failed workpiece fractures were relatively flat and beach-shaped, with typical fatigue fracture morphology characteristics. The instantaneous fracture zone has a metallic luster. The chain link fracture is a rapid overload fracture, and the fracture morphology is the same as the first type of fracture. Microscopic analysis of the fracture surface shows that about 1/3 of the area of the fatigue fracture surface is the beach-like fatigue source area and expansion area, and the rest is the rapid fracture fracture area. Under the scanning electron microscope, typical fatigue stripe bands and fatigue crack propagation morphology characteristics can be observed. It was observed that the fatigue source originates from the surface of the workpiece. This is because the carburized layer of the workpiece is extremely thin or even not infiltrated with carbon, and the surface strength is very low. It is easy to crack under the action of alternating stress of pulling and pulling for a long time. Many inclusions were found at the fatigue source, and energy spectrum analysis showed that the inclusions contained a large amount of calcium, which indicated that ash and slag corrosion had penetrated deep into the cracks. When the crack expands beyond the critical value, the stress on the remaining cross-section exceeds the tensile strength of the material, and the workpiece undergoes instantaneous fracture failure, with the fracture showing a radial pattern. Metallographic analysis shows that the core structure of the workpiece is low carbon martensite + a small amount of ferrite, and the surface is high carbon martensite structure; however, no carburized layer structure is observed in the chain hook structure, and the core structure is low carbon martensite. Tensite + troostite + a small amount of ferrite, the surface and center of the workpiece are unqualified, and there is no carburized layer, which causes the strength, hardness and toughness of the workpiece to deteriorate sharply and decrease. Scanning electron microscopy was used to observe numerous micropores and microcracks in the heart tissue. This shows that under the action of alternating stress for a long time, the weak parts of the internal structure of the workpiece have been damaged, micropores have appeared in the martensite, and microcracks have formed under fatigue alternating stress. Since the carburized layer of the chain hook is extremely thin or even non-existent, its strength is greatly reduced, and numerous cracks are observed on its surface under an electron microscope. These defects show that when the workpiece is subjected to periodic tensile stress, cracks will first initiate on the surface of the workpiece; too thin a carburized layer or lack of carburization on the workpiece is the main cause of fatigue cracking of chain parts.

Based on the above analysis, the process improvements are proposed as follows:

(1) Strictly control the carburizing process and carburized layer quality of the slag scooping machine chain parts to ensure that the depth of the carburized hardened layer is ≥ 0.5mm and ensure that the strength and hardness of the carburized layer of the workpiece meet the technical requirements.

(2) When using an energy-saving medium frequency induction heating machine for quenching heat treatment, the quenching process and quenching quality should be strictly controlled to prevent the occurrence of troostite structure in the workpiece, which will reduce the strength and toughness of the chain links.

It has been proved by many practices that after the slag scooping machine chain adopted the above-mentioned improved process, no early fracture failure was found, and it met the working performance requirements. The product quality is excellent, the production runs normally, and it is suitable for large-scale production. The most important thing is to use The lifespan has also been greatly improved.