What is the induction heat treatment of leaf springs?

1. Leaf spring steel and manufacturing process route

The cross-sectional area (length*width) of a leaf spring is generally much larger than that of a circular cross-section coil spring. In order to be quenched and obtain good comprehensive mechanical properties, low-alloy hot-rolled spring flat steel is often used. The main series include silicon-manganese steel, chromium-manganese steel, boron spring steel and multi-component micro-compounded elastic steel. The quality of leaf springs is closely related to the raw materials. It also depends on production equipment, technology and testing methods to ensure it. Quenching and tempering are key processes.

2. Quenching and tempering of leaf springs

Due to the different steel types and specifications used for leaf springs, the quenching and tempering process parameters are also different. In actual production, it is necessary to consider the specific heat treatment equipment, energy and process flow, etc., to determine the furnace temperature of the heating furnace and the operating speed of the leaf spring, thereby determining the heating time and actual temperature. In a professional manufacturer of leaf springs with an annual output of 5,000 tons, there are two series of heating furnaces: one is a continuous production heating furnace, and the other is a periodic operating heating furnace. The former mostly uses hydraulic stepping heating furnaces. The advantages of its equipment are that the cycle and stepping distance are adjustable, and the furnace temperature distribution is relatively uniform. For the production of small batches and multiple varieties of leaf springs, periodic operating furnaces should be used. If possible, it is recommended to use protective atmosphere heating to reduce oxidation and decarburization of the leaf spring surface.

Quenching into form is an important process in the production of heat treatment for leaf springs. That is, the heated leaf spring is placed on the quenching machine to be pressed into shape, and then cooled in oil. There are two commonly used quenching machines: one is a mechanical quenching machine, which has the advantages of large clamping force, automatic quenching and swinging into the oil, reliable operation, and convenient maintenance; the disadvantages are high noise and slow oil entry. Not suitable for use in continuous production. The other is a hydraulic vertical oil-injection quenching machine. Its clamping and swinging actions are each undertaken by a hydraulic cylinder, and can be controlled by a microcomputer or conventional electrical. In order to apply the forming quenching of short variable cross-section leaf springs, a reverse bow flexible chain forming quenching machine has recently been designed and manufactured. The quenching medium can circulate and flow, which can improve the quenching quality. The leaf springs have good shape and are suitable for mass production. A production line with multiple quenching machines installed in the quenching tank uses a plate conveyor chain. The workpiece is placed on the quenching machine for clamping, cooled by oil, rocked, and automatically unloaded. The plate spring is then sent out of the quenching tank by the conveyor chain. This can reduce the labor intensity of operators and enhance production continuity. Water-soluble quenching media can also be used instead of quenching oil to avoid problems such as fuel ignition and excessive oil contamination that pollutes the environment. There are also two ways to temper leaf springs: continuous tempering furnace and periodic tempering furnace. Both should be equipped with thermal circulation devices to ensure uniform furnace temperature. In the former, the tempering temperature of the leaf spring is selected by the required hardness value, generally 450-500 degrees Celsius. When tempering quickly, the furnace temperature can be increased appropriately to avoid undesirable phenomena such as insufficient tempering. The latter uses a car-bottom tempering furnace, which has a large charge, a thermal circulation device, and a longer leaf spring tempering time. Due to its uniform furnace temperature distribution, it effectively ensures the tempering quality of the leaf spring, is easy to operate, and has low production costs.

3. Defects and prevention of heat treatment of leaf springs

(1) The hardness of the quenched leaf spring is insufficient or too high. The main reason is that the heating temperature of the leaf spring is too low or too high, or caused by insufficient cooling, unevenness, or improper tempering process.

(2) Overheating or overburning. Overheating of the leaf spring can be remedied by quenching and tempering. If it is overheated, it can only be scrapped.

(3) The surface of the spring plate is severely oxidized and decarburized. The above two defects are caused by the furnace temperature being too high and the holding time being too long.

(4) The deformation of the leaf spring does not meet the technical requirements (such as side bending, etc.).