Quenching and Tempering Process of Grade 12.9 Fastener Steel

Quenching and Tempering Process of Grade 12.9 Fastener Steel

The automotive industry is moving toward high performance and light weight. Advances in the automotive industry have placed higher demands on bolts for coupling, namely high design stress and light weight. High-strength bolt has become one of the iconic parts of automotive quality and cost. For example, engine bolts, the original materials have been difficult to meet the requirements of the new engine high stress. The main raw material for the production of fasteners is cold heading steel wire rods. High-strength and high value-added cold heading steel is an important research and development direction of material production enterprises. The rational quenching and tempering treatment process of high-strength fasteners is also one of the important contents of metallurgical industry research. Quenching and tempering is a key process in the production of high strength fasteners. After quenching and tempering, the fasteners have good strength and toughness, and have excellent comprehensive mechanical properties. In the quenching and tempering process, the quenching and tempering process parameters will affect the overall mechanical properties of the final product. SCM440 steel is high-strength fasteners steel, which requires strict heat treatment parameters.

The production process of high-strength fasteners includes wire rod, spheroidizing annealing, pickling, phosphating, saponification, drawing, cold heading, rolling, surface inspection, quenching and tempering, inspection and storage. It can be seen from the production process that the fastener is deformed before the spheroidized structure. SCM440 steel has good plasticity and can achieve high reduction of area. In the case of the same quenching process, the higher the tempering temperature, the lower the tensile strength and the yield strength, and the yield ratio of quenching at 850°C reaches 0.9, which is higher than 870°C. The purpose of tempering is to eliminate the internal stress generated during quenching and improve the comprehensive performance index of steel. As the tempering temperature increases, the tensile strength and yield strength of SCM440 steel decrease, and the area shrinkage rate shows an increasing trend. Microstructure determines the mechanical properties, and the size, morphology, and distribution of carbides also affect the mechanical properties of steel.

The suitable quenching and tempering process is 850 °C for 12.9 grade fasteners made of SCM440 steel. The holding time is 80 minutes, then quenching, and 480°C for 3 hours tempering. The tensile strength, yield strength and end face shrinkage after quenching and tempering treatment were 1242 MPa, 1163 MPa, and 55%, respectively, which met the technical requirements of 12.9 high-strength fasteners. The quenching temperature has an effect on the grain size of the prior austenite. If the quenching temperature is too high, the grain is coarsened, resulting in a decrease in the yield ratio. Tempering treatment is an important factor affecting the mechanical properties after quenching and tempering. The higher the tempering temperature, the easier the carbide grows and the lower the tensile strength.