Why do shafts sometimes have low hardness, soft bands, or soft spots during high-frequency quenching? What measures should be taken?

Camshaft is an important component widely used in engines such as automobiles, construction machinery, tractors, etc. It forms a pair of friction pairs with the tappet, and its main function is to ensure the timed opening and closing of the air valve. Due to the high-speed operation of the engine and the impact and wear of the valve lifters, in addition to bearing certain bending and torsional loads during operation, good strength and surface wear resistance are also required. The selection of camshaft materials mainly depends on their working conditions and usage conditions in the engine. At present, there are many types of materials and processes for manufacturing camshafts, which can be divided into two categories: steel and cast iron.

Camshaft quenching induction coils are generally divided into two types: profile sensors and circular sensors. Profile sensors have high requirements for workpiece positioning accuracy, and multiple cams located in different planes cannot enter the induction coil continuously in sequence. Each quenched cam needs to be disassembled once, making it difficult to ensure good contact at the joint of the induction coil each time. The stability of product quality is poor and the production efficiency is low, so it is rarely used in mass production. The successful application of circular sensors on the continuous quenching of camshafts in small engines has been proven to be unsuitable by the production practices of several previous collaborating units. The main problem that occurs is that there is a large temperature difference between the peach tip and the base circle during the heating of the cam. While ensuring that the base circle meets the technical requirements, the hardened layer depth of the peach tip after quenching will inevitably exceed the technical requirements of the drawing, and it is easy to cause cracking of the cam peach tip or soft bands at the cam lift, resulting in a low first pass rate.