09MnNiD steel is based on carbon-manganese steel with 0.45% to 0.85% nickel added to improve the low-temperature impact toughness of the steel, allowing the steel to maintain high strength and low-temperature toughness in the low temperature range of -45 to -70°C. . Therefore, it has been widely used in low-temperature pressure equipment parts.
Heat treatment is an indispensable key process in the production process of forgings. Reasonable and correct heat treatment can give forgings excellent comprehensive mechanical properties and ensure the safety and reliability of products under service conditions. In the production process of 09MnNiD steel forgings, the performance heat treatment of some products needs to be outsourced. During the mechanical property testing process of the forgings returned to the factory, sometimes the low-temperature impact absorbed energy is lower than the standard value, and the value fluctuates greatly. The low-temperature impact toughness of the product is unqualified, which affects the timely delivery of the product.
The manufacturing of forgings is a complex systematic project. The quality of forgings is closely related to the smelting of forging steel, heating of billets, forging forming, post-forging heat treatment, rough machining, performance heat treatment, physical and chemical testing, finishing and other links. Therefore, in order to produce high-quality 09MnNiD steel forgings, the quality control of the following processes should be strengthened:
(1) Improve the purity of steel for forgings, reduce non-metallic inclusions in the steel, and strictly control the content of harmful elements such as P, S, Sn, Sb, and As in the steel.
(2) The steel billet must have a sufficient forging ratio during forging, fully compact and penetrate the internal metal, break up the dendritic structure and non-metallic inclusions in the steel, and improve its distribution shape. Increase the density of the metal inside the forging and reduce the internal defects of the forging.
(3) After forging, 09MnNiD steel forgings should be subjected to intercritical heat treatment between Ac1 and Ac3 temperatures to change the grain size, tissue morphology, second phase size and distribution of the steel for forgings, and improve the plasticity and toughness of the forgings after quenching and tempering. and cold brittle resistance.
(4) When the workpiece passes through the Ac1~Ac3 temperature range during quenching, tempering, quenching and heating, it needs to be heated rapidly according to the maximum power of the heat treatment furnace to increase the superheat of the steel during phase transformation, increase the nucleation rate of austenite, and achieve refinement. The purpose of austenite grains.
(5) The quenching cooling water tank must have sufficient capacity and must be equipped with a strong stirring and cooling water circulation system. When the workpiece is quenched and comes out of the furnace, the workpiece is quickly quenched into water for full cooling, so that the structure of the forging can be fully transformed and the quenching transformation products can be made smaller and more uniform.
(6) After the workpiece is quenched, it is tempered in the loading furnace. The tempering temperature of 09MnNiD steel forgings is between 670 and 690°C, and the tempering and holding time is calculated and determined according to (1.8 to 2.2) min/mm. After tempering and insulation, air cooling is adopted.
