Development of a quenching-partitioning process chain for forging components

Abstract

The aim is to realize a Q&P (Quenching and Partitioning) process for a hot forged component made of low-alloyed advanced high-strength steel (AHSS) 42MnSiCr. One advantage of this steel is the low alloy concept which is cost-effective. After forging, the component is cooled down to room temperature with a subsequent heat treatment to achieve the characteristic microstructure with martensite and retained austenite. The material is annealed and then quenched to just above the martensite finish temperature (MF-temperature). Hence, in the martensitic matrix about 10 to 15% retained austenite is included. Finally, the Q&Ped material is artificially aged at 250 °C to support the diffusion process of carbon from the over-saturated martensite into the austenite. Thereby, mechanical properties of 2000 MPa for tensile strength with fracture strains of 10% can be achieved. This paper provides details of the process and material behavior for a reduction of the process chain. The goal is to develop a technology for the quenching and partitioning treatment of forged components by using the thermal energy from forging. Ideally, the quenching step should be performed in the forming dies just above the MF-temperature with additional holding on the temperature level. The majority of forged parts have different cross sections. Therefore, the cooling conditions are inhomogeneous in each cross section of the components. This cooling behavior was analyzed in laboratory tests with a forged part. Furthermore, the heat transfer coefficients were determined for different cooling media (water, air). The cooling technology was experimentally and numerically simulated in a first step for the conventional process chain (forging, cooling to room temperature, austenitisation, quenching, artificial ageing) and correlated with the microstructural evolution in combination with the component’s mechanical properties.