The brake hub is an important part of the oilfield mining machinery and equipment skid-mounted drilling rig, and its material is ZG35SiMn steel. The brake hub is subjected to great friction during operation and often overheats and becomes red. Generally, pouring tap water is used for cooling. During production, it was discovered that a certain brake hub suffered cracks, cracks, and failure after only 3 months of operation. The original heat treatment process of the brake hub is normalizing + medium frequency induction heating quenching and self-tempering. The metallographic structure inspection found that the metallographic structure 10mm below the surface of the workpiece is massive ferrite + tempered troostite. The ferrite is distributed in a mesh or semi-mesh shape, and the structure in local areas is coarse. The structure in the center is iron. There are ten small amounts of inclusions in the body + pearlite; the distribution of ferrite and pearlite is uneven, large pieces of ferrite appear, and there are a small amount of concentrated sulfides. Observation found that the structure of the inspection area was coarse, and obvious long-axis grain Widmanstatten structure appeared, which increased the brittleness of the area and decreased the plasticity, toughness and strength of the workpiece. This indicated that the normalization was insufficient and the as-cast structure was not fully transformed. Examination of the crack morphology of the workpiece revealed that the crack extended inward from the surface of the brake hub, expanded in a linear transgranular manner, and bifurcated in multiple places; the crack tip developed along the inclusion defect, but no signs of oxidation and decarburization were seen.
Analysis shows that when the workpiece is braked, its dry friction causes the friction force to be very large, often causing heat and redness, and the local temperature to be higher than 800aC. During work, pouring tap water on the workpiece to cool it will cause the workpiece to be quenched and undergo a phase change. The hardness of the surface layer will increase, the internal stress will increase, and the cold brittleness will increase. Because the workpiece is repeatedly acted upon by tangential forces, cracks initiate in the area of massive carbides and inclusions on the surface layer of the workpiece. Under the action of alternating shear stress and impact force, the cracks develop and cause the workpiece to break.
From the process analysis, most of the workpiece structure is in the cast state, with coarse structure, uneven grains, inclusions, Widmanstatten structure and micro cracks. The Widmanstatten structure has poor strength, toughness and plasticity, and is highly brittle. Coupled with the coarse structure and inclusions, the workpiece has low strength, toughness and poor plasticity. It is easy to form stress concentration and cause cracks to expand. Under the action of shear stress and impact force, the workpiece is damaged. fracture. The process defect is that on the one hand, the normalizing heating and holding time is not enough, so that most of the as-cast structure and defects are retained; on the other hand, the uniformity of the furnace temperature is poor and the structure is uneven. The fracture and damage of the brake hub is the result of the combined effect of these two reasons.
Based on the above analysis, the process improvement measures to prevent workpiece breakage are proposed as follows:
(1) The furnace for heating castings should be stable and neat so that the workpiece can be heated evenly and prevent deformation of the workpiece during heating.
(2) The heating time of the workpiece is not less than 4 hours, and the temperature is maintained at 600-65yC for 1-2 hours. The holding time depends on the wall thickness of the workpiece. Due to the low actual temperature and uneven temperature of the furnace, the original normalizing temperature of 850 -9°C was increased to 980 - 1080°C. At this time, the actual temperature of the workpiece conforms to the process temperature. The workpiece is heated with a slight positive pressure, and is cooled to 850°C after insulation. Air-cooled when out of the oven below ℃.
After the brake hub adopts the above process improvement measures, the performance of the workpiece is significantly improved, the as-cast structure is eliminated, the mechanical properties of the workpiece meet the technical requirements, the workpiece structure is fine and uniform, no cracks and fracture defects occur, the workpiece quality is good, and the production runs normally.
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GS-ZP-1200
