The influence of hot-working conditions on the structure and mechanical properties of forged products of microalloyed steel

Purpose: Results of the effect of thermomechanical processing conditions on the microstructure, mechanical properties and crack resistance are included in the present work. Conditions of forging with the method of thermo-mechanical treatment were developed basing on the analysis of precipitation kinetics of MX interstitial phases in a solid solution, plastometric examinations and investigations of the kinetics of undercooled austenite phase transformations. Design/methodology/approach: Light microscopy and transmission electron microscopy techniques were used to reveal the microstructure of samples obtained as a result of the thermomechanical forging. Mechanical properties and hardness tests as well as resistance to cracking using Charpy V samples at room and lowered temperature were carried out. Findings: Applied thermo-mechanical treatment allows obtaining fine-grained microstructure of austenite during hot-working and production of forged parts, which acquire advantageous set of mechanical properties and guaranteed crack resistance after controlled cooling from finishing plastic deformation temperature and successive tempering. Forgings produced with the method of thermo-mechanical treatment, consecutively subjected to tempering in the temperature range from 550 to 650°C, reveal the values of YS_0.2 equal from 994 to 939 MPa, UTS from 1084 to 993 MPa, KV ^-40 from 77 to 83 J and hardness ranging from 360 to 310 HBW. Research limitations/implications: Executed analyses of mechanical properties, crack resistance and hardness in quenched and tempered state revealed full usability of elaborated microalloyed steel for production of forged machine parts with high strength and crack resistance, also at decreased temperature with the method of thermo-mechanical treatment. Practical implications: The applied thermomechanical forging conditions can be useful for elaboration of an industrial forging technology for selected forged elements of microalloyed steels with high strength and guaranteed crack resistance. Originality/value: Thermomechanical forging conditions for a new-developed microalloyed steel were established.