Influence of Quenching and Partitioning Parameters on Phase Transformations and Mechanical Properties of Medium Manganese Steel for Press-Hardening Application

Abstract

It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.

Influence of Press‐Hardening in Combination with Quenching and Partitioning Treatment on the Microstructure of Medium Manganese Steel

“…Analogous to previous investigation, [ 27 ] it was also confirmed by the new evaluation method that an extension of the partitioning time has only a relatively slight effect on the phase fractions (Figure 8).…”

“…Here, it must be noted that the PH + QP states have a high phase fraction of FM, which is well above the critical amount (<20%) to allow a good combination of strength and ductility determined in previous studies. [ 27,57,58 ] Since in the present PH + QP investigations yield strength, tensile strength and also total elongation are improved with decreasing T Q , optimal T Q should be lower than the here applied ones. However, since in previous tests, the same temperatures have given good results for the investigated material, [ 27 ] it therefore stands to reason that the actual, real T Q set during press‐hardening was significantly higher than the measured target T Q and hence less TM and more FM is formed.…”

“…However, unlike Q&P heat treatments of a material with similar chemical compositions performed in salt baths in a previous study, [ 27 ] cf. Figure 15 a, the good combination of strength and ductility could not yet be achieved in the PH + QP experiments.…”

“…[ 25,26 ] In previous investigations, it has already been shown that not only the RA content but also the amount of FM strongly influences the mechanical properties. [ 27,28 ] However, since FM and TM each have a body‐centered cubic (bcc) lattice structure (of different degree of tetragonal distortion), it is difficult to quantify the phase fractions. One approach to distinguish bcc phases is using electron backscatter diffraction (EBSD).…”

Mathematical and Numerical Simulation of the Thermally Hardened Coated Machine Surfaces

“…Good selection of the control scheme of machine tools hardening process is one of the influential factors for the achievement of the desired results that suit the operational requirements and environment, through the identification and manipulation of the various variables that interfere with the rapid quench of heated steel or metal alloys to the temperature of austenitization, that varies according to their content of carbon, which turns them to the martensite phase (Blankart et al, 2021). Initially, the mechanisms of the austenization process are sophisticated due to its occurrence from diverse primary microstructures, like the cold recrystallized ferrite, tempered martensite, or a mix of cementite or pearlite and ferrite, in addition, alloying elements apply their impact in several ways at diverse stages of conversion (Weia et al, 2013).…”