Process improvement of crankshafts using high-frequency induction annealing equipment

The crankshaft material of agricultural diesel engines is made of ductile iron. The crankshaft is subjected to bending, compression, tensile and torsional alternating stress and fatigue stress during operation. The workpiece is required to have high strength, good toughness, high wear resistance and high fatigue resistance. Technical requirements: crankshaft hardness is 40-48HRC. The heat treatment process is: 890℃×45min heating for austenitization, (260-270)℃×60min isothermal treatment and then air cooling quenching. During production, it was found that several crankshafts had cracks at the connecting rod journals. Some were individually distributed in straight lines, and some were network cracks or branched cracks. The cracks varied in depth. Deep cracks penetrated the thickness of the workpiece, and the cracks were about 0.6 in depression. mm, causing the workpiece to be scrapped.

The hardness of the crankshaft was tested to be 49-50HRC, which is higher than the process requirement. Metallographic observation found that the workpiece was well spheroidized, and the graphite was in the form of balls or lumps, with a small amount of flakes or thick flakes, and the spheroidization rate was level 2. Check the depth of grinding cracks, the shallowest part is 0.2-0.4mm, and the deepest part is 0.5-2.0mm. Metallographic analysis of the structure at the crack of the workpiece. After isothermal quenching of the workpiece, the structure at the crack is lower bainite + retained austenite + a small amount of martensite. Most of it is white structure. At the same time, there are nitride inclusions and small graphite distribution. Along the crack The grain boundaries extend along the inclusion two-phase in the white tissue area, or extend inwards transgranularly in the lower bainite and a small amount of martensite areas.

Analysis shows that the isothermal temperature of the crankshaft is relatively low and the hardness is relatively high, resulting in high internal stress in the workpiece. Excessive grinding amount of the workpiece, blunt grinding wheel and improper cooling process will cause grinding network, branching and linear cracks in the workpiece. The occurrence of grinding cracks is closely related to excessive internal stress caused by structural defects in the workpiece. When the structural stress, thermal stress and machining stress of the workpiece exceed the tensile strength of the workpiece, cracks will occur and expand rapidly as the stress increases.

There are many inclusion defects in the crankshaft structure, and there is phosphorus eutectic segregation in the white structure area, which further increases the structural stress at the grain boundaries and within the grains, causing stress concentration. The areas of grain boundary inclusions and phosphorus eutectic segregation in the workpiece are high stress areas, which are also areas of stress concentration and strength weakness in the workpiece. Cracks first occur here and propagate along grain boundaries and defects. Uneven cooling of the workpiece also promotes crack expansion.

In order to prevent cracks and fractures in the workpiece, high-frequency induction annealing equipment should be used to perform the following process improvement measures on the crankshaft:

(1) In order to reduce the stress of the workpiece, stress relief annealing is added.

(2) Adjust the isothermal treatment temperature. The isothermal temperature of the crankshaft is relatively low, the hardness is relatively high, and the internal stress increases. It is advisable to use the upper limit isothermal temperature or slightly higher to ensure that the hardness of the workpiece is within the required technical indicators.

(3) Use small feed amount for grinding to reduce grinding stress and avoid grinding cracks.

(4) Improve the quality of crankshaft castings, reduce impurities, prevent oxidation of molten iron, and avoid defects such as nitride inclusions and phosphorus eutectic segregation.

After taking the above process improvement measures, the crankshaft production was normal, no cracks occurred, and the quality was excellent.