A certain type of crankshaft assembly is composed of left and right cranks, connecting rods and crank pins (as shown in Figure 1). The material of these parts is 20CrMo, and the heat treatment process is carburizing and quenching. Because the workpiece is small, the required hardening layer is shallow, and the finished product requires 0.6-0.9mm (514hv),

The surface hardness of multi-purpose furnace is 56-64hrc, the heat treatment sequence is 0.8-1.0 mm (514hv), and the surface hardness is 57-64hrc. It can be seen that the carburizing layer of these parts belongs to shallow carburizing.

The hardness after grinding should be within the required range. After the left and right cranks, connecting rods and crankpins are pressed together, they meet certain requirements for the moment of fastening, so there is a certain interference halo between the cranks and crankpins. The connecting rod and crank pin are required to be wear-resistant, but the crank is not required to be wear-resistant. It also requires a proper fit between strength and toughness. After the crank is pressed, the crank pin is subjected to the compressive stress. If the crank is subjected to the supporting stress, the strength and toughness of the crank will not match well, and the crank is easy to crack.

This paper mainly introduces the trial production process of crank heat treatment process.

Process analysis

According to the above analysis, to ensure the Quliang heat treatment project qualified, the following problems need to be solved:

(1) Whether the equipment can ensure that the hardening layer of a furnace workpiece is within the required range.

(2) Can the workpiece after heat treatment have a reasonable carbon concentration gradient (hardness gradient of hardened layer) to ensure the surface hardness after grinding.

(3) After the crank is pressed, whether the torque is qualified or not is related to the surface hardness, core hardness and hardened layer depth of the crank hole.

The main purpose of carburizing the crank in the crankshaft assembly is as follows:

First, improve the strength of the surface to ensure that the torque is qualified.

Second, after surface carburizing and hardening, the fatigue strength is improved.

Third, the surface has a reasonable hardness to facilitate assembly with other parts.

Fourth, the crank is small and has the requirement of strength and toughness. It has high requirements for hardened layer, carbon concentration gradient (hardness gradient of hardened layer), core hardness, surface hardness and retained austenite.