Influencing factors and countermeasures of low temperature impact toughness of steel forgings

Influencing factors and countermeasures of low temperature impact toughness of steel forgings
09MnNiD steel is on the basis of carbon manganese steel added 0.45% ~ 0.85% nickel element, so as to improve the low temperature impact toughness of steel, steel in the low temperature range of -45 ~ -70℃ still maintain high strength and low temperature toughness. Therefore, it has been widely used in the parts of low temperature pressure equipment.

Heat treatment is an indispensable key process in the forging production. Reasonable and correct heat treatment can give the forging excellent comprehensive mechanical properties and ensure the safety and reliability of the product under service conditions. In the production process of 09MnNiD steel forgings, the performance and heat treatment of some products need to be outsourced. In the process of mechanical property testing of the forgings returned to the factory, the low-temperature impact absorption work sometimes appears to be lower than the standard value, and its value fluctuates greatly. The low-temperature impact toughness of the products is not qualified, which affects the timely delivery of the products.
The manufacturing of forgings is a complex system engineering. The quality of forgings is closely related to the smelting of steel used in forgings, the heating of billet, forging forming, post-forging heat treatment, rough processing, performance heat treatment, physical and chemical testing, finishing and other links. Therefore, to produce high quality 09MnNiD steel forgings, the quality control of the following processes should be strengthened:

(1) Improve the purity of steel for forging, reduce the non-metallic inclusions in steel, and strictly control the content of harmful elements such As P, S, Sn, Sb and AS in steel.

(2) The billet should have sufficient forging ratio during forging to fully compact and penetrate the internal metal, break the dendrite structure and non-metallic inclusions in the steel, and improve its distribution form. Increase the density of the metal inside the forging and reduce the internal defects of the forging.

(3) 09MnNiD steel forgings should be subjected to critical inter-heat treatment between the temperature of Ac1 ~ Ac3 after forging, so as to change the grain size, microstructure, second phase size and distribution of the steel used for forging, and improve the plasticity toughness and cold and brittle resistance of the forgings after tempering treatment.

(4) When the workpiece is heated through the temperature range of Ac1 ~ Ac3 during quenching and quenching, it needs to be heated rapidly according to the power of the heat treatment furnace to increase the superheat of the steel during the phase transition, improve the nucleation rate of austenite and achieve the purpose of refining austenite grains.

(5) quenching cooling tank should have sufficient capacity, and need to be equipped with strong stirring and cooling water circulation system. When the workpiece is quenched out of the oven, the workpiece is quickly quenched into the water for full cooling, so that the organization of the forging is fully transformed, and the quenching transition product is more fine and uniform.

(6) Timely loading and tempering the workpiece after quenching. 09MnNiD steel forging tempering temperature between 670 ~ 690℃, tempering holding time according to (1.8 ~ 2.2) min/mm calculation to determine. Tempering heat preservation after the oven to adopt air cooling.
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