The S195 ductile iron camshaft is heat treated using a high-frequency induction heating machine. Analysis of the causes of crack defects and process improvement

The agricultural S195 diesel engine QT600-3 ductile iron camshaft is an important part of the diesel engine, and a high-frequency induction heating machine is often used for heat treatment. During production, it was found that cracks occurred on the camshaft row and air intake surface, causing an average of 12% of the workpieces to be scrapped. Preliminary analysis believes that the key to the formation of cracks is due to improper heat treatment process, which formed coarse high-carbon flake martensite and retained austenite. Caused by excessive body, insufficient tempering and other reasons.

Macroscopic inspection revealed that micro-turtle-shaped cracks were found on the cam arc and cut surface, and strip cracks appeared on the base circle of the workpiece. The depth of the crack was about 0.005-3.5mm, and the direction of the crack was parallel or at a certain angle to the grinding direction of the grinding wheel. Microscopic morphology analysis found that most of the surface cracks started from the graphite balls, forming transgranular or grain boundary cracks. The crack roots were wide and tapered to the tail end. The measurement of retained austenite shows that the retained austenite in the cam exhaust part is too high, with a volume fraction of 20.38%. Observation of the metallographic structure revealed that the quenched martensite in the inlet and exhaust parts of the workpiece was coarse, microcracks occurred between the martensite needles in some areas, and the amount of retained austenite was high. At the same time, tempered martensite, pearlite, etc. appeared in the workpiece. Tempering uneven structures such as tempered sorbite and tempered troostite, and large phosphorus eutectic and carbide defect structures were found in the failed workpiece.

Analysis shows that the austenite grains of the workpiece are coarse due to excessive heating temperature, and coarse high-carbon martensite is formed after quenching. High-carbon coarse martensite is highly brittle, and microcracks are easily generated when the martensite collides with each other during growth. The structural stress, thermal stress and grinding stress generated during grinding of the workpiece are superimposed. When the superimposed stress is greater than the tensile strength of the workpiece, cracks will occur and expand. On the other hand, if there is too much retained austenite in the camshaft, if the tempering is insufficient, the retained austenite will undergo phase transformation and transform into martensite under the action of grinding heat and grinding stress. The volume expansion of martensite will produce a structure. Stress, this additional stress can also easily cause grinding cracks. If tempering is insufficient or tempering is not timely, cracks in the workpiece will increase. When the workpiece placement time is extended from 5 minutes to 24 hours, cracks increase by about 10%-30%. The camshaft material is ductile iron. In ductile iron, the Si content is high and the C content is low near the graphite nodules. The P and Mn contents are high away from the graphite. The Ms point is high in the area with high Si content, so martensite transformation occurs first. The Ms point is low in the area with high P and Mn content, so martensite transformation occurs later, forming a complex stress field between the two micro areas. In addition, during cooling, the plasticity of ductile iron is low at 100-200°C. Under complex stress, when the superimposed stress exceeds the tensile strength of the material, it will cause cracks in the workpiece. The cracks will start from the graphite and expand along the graphite boundary. Therefore, the cooling time of high-frequency induction heating surface quenching should be controlled at the moment when the surface layer of the workpiece completes martensitic transformation without cracking, that is, it should not be quenched. When the surface temperature reaches 250-300°C, water spray cooling should be stopped. At this time, the workpiece will use the quenching waste heat to self-temper without causing cracks and cracks in the workpiece.

To sum up, the improved process to prevent cracks in workpieces is as follows:

(1) Improve sensor size. The inner diameter of the induction coil is changed from 53mm to 50.5mm. The heating temperature of the three cams of the workpiece is controlled by changing the magnetic flux, so that the three are close to uniform temperature, so as to obtain fine martensite needles and a small amount of residual austenite structure. At the same time, the height of the induction coil is appropriately reduced from 28mm to 23mm to improve the efficiency of the sensor and save power.

(2) Control heating power. The output power of the high-frequency induction heating machine was changed from 78kW and heating for 13 seconds to 62kW and heating for 9 seconds. After quenching, the martensite needles at the three cams are small and there is less residual austenite.

(3) Control the water spray time. The water spray time selected for the test is 3.8-4.Os, and the spray stop temperature is maintained at 300°C for 1 minute. The performance of the workpiece after quenching is qualified.

(4) Improve the tempering process. A high-frequency induction heating machine is used for tempering heat treatment at a temperature of 395-420°C. The structure after tempering is tempered troostite, tempered sorbite and graphite.

This article briefly introduces the high-frequency improvement process of S195 ductile iron camshaft. Many manufacturers use the above-mentioned improvement process for heat treatment. The quality of the workpieces produced is good and meets the work needs. The scrap rate is reduced to less than 1.5%, and the economic benefits are very significant.