Defect analysis of heat treatment of quenched and tempered steel connecting rod

  1. Effect of chemical composition on mechanical properties

Although the steel of the same brand will appear in production, some chemical composition elements are the upper limit, especially the carbon element is the upper limit; Some chemical elements are the lower limit, especially carbon. Although they are all within the specified range of the same brand of steel, due to different heats when smelting the brand, it is necessary to re test the chemical composition of the steel, revise the heat treatment process parameters according to the upper and lower limit data of the chemical composition, and improve the accuracy grade of the control instrument to ensure that the mechanical properties are qualified.

  1. Effect of heat treatment on impact toughness( α k) Impact

Impact toughness after quenching and tempering( α k) The value always fails to go up, because the content of impurity element “P” exceeds the standard, “P” is an element liable to quenching and cracking. When the content of “P” is ≥ 0.020“ α The value of “K” decreases sharply, which is easy to lead to early brittle fracture of products and even quality accidents. There are many steel mills in China. Due to equipment, smelting technology and other reasons, the same grade of steel is produced, but the content of harmful element “P” is controlled at different levels.

  1. Effect of quenching temperature and cooling medium on mechanical properties

The optimum quenching temperature is recommended for all steel grades. However, the mechanical properties in actual production are different, which are closely related to the original structure of the material. If there are V, Ti and other alloy elements, the grain size is fine and the structure is dense, and the austenite grain is not easy to grow up during heating, the upper quenching temperature can be increased to increase the hardenability and hardenability of the steel. On the contrary, if the coarse grain structural steel contains Mn element, the lower quenching temperature should be used or appropriately reduced to prevent austenite grain coarsening, increase brittleness and reduce strength and toughness.

  1. Effect of high temperature tempering temperature and cooling medium on mechanical properties

After quenching and tempering at high temperature, the quenched martensite is transformed into tempered sorbite with excellent comprehensive mechanical properties and the quenching internal stress is fully eliminated. For the steel that can obtain the full hardened layer and the full hardened layer, the required tempering temperature is found on the tempering performance curve of the steel according to the design performance and technical conditions, which is usually between 500-650 ℃, but in the actual production, it is affected by the following factors: the chemical composition of the steel and the cooling rate.

  1. Influence of improper operation on mechanical properties during construction

Quenched and tempered workpieces are generally in large batches. When the furnace loading is too large, too dense, or too close to the heater and furnace door, the temperature difference between the bottom, inner and outer layers of workpieces is large, and the temperature is uneven, resulting in large differences in the performance of workpieces of the same furnace after quenching and tempering, resulting in unqualified product performance.

  1. Effect of microstructure on mechanical properties of quenched and tempered steel

Uniform and fine tempered sorbite structure is obtained by quenching and tempering treatment, which has the best comprehensive mechanical properties.

  1. Effect of quenching soft spot on mechanical properties

After quenching, the local small area hardness of the workpiece is insufficient or not hardened, and the microstructure is low-carbon quenched martensite + nodular troostite or pearlite, or even ferrite, resulting in insufficient mechanical properties and becoming the fatigue crack source and early wear center.