Properties and heat treatments of air-hardening ductile forging steels

• Eigenschaften und Wärmebehandlung von lufthärtenden duktilen Schmiedestählen

Gramlich, Alexander Roald Michael; Bleck, Wolfgang (Thesis advisor); Zoch, Hans-Werner (Thesis advisor); Krupp, Ulrich (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, Kumulative Dissertation

Abstract

Air-hardening ductile (AHD) forging steels are a recently developed class of materials that combine the mechanical properties of quenched and tempered steels with cost-efficient processing after forging. Due to the shortened process chain, these steels show an enormous cost and energy saving potential, but the insufficient impact toughness has prevented their industrial application so far. The aim of this study is to improve the mechanical properties of AHD steels with a special focus on the impact toughness, while the cyclic strength is alsoconsidered. Within this scope, different alloying concepts as well as post-quenching heat treatments which increase the impact toughness were developed and tested on a laboratory scale. Different concentrations of boron, molybdenum, titanium, vanadium and aluminium where added to a Fe-4Mn-0.5Si-0.17C-0.035Nb base alloy. The alloy design and the determination of the heat treatment parameters were accompanied by thermodynamic equilibrium calculations. A particular focus was set on the prevention of manganese embrittlement by adding boron and molybdenum, as well as the formation of fine austenite grains during intercritical heat treatment. The resulting microstructures and mechanical properties of the six alloys were investigated by a wide range of characterization methods, including high-resolution methodologies such as atom probe tomography and synchrotron X-ray diffraction. It was shown that the addition of boron and molybdenum increased the impact toughness, although the effectiveness of each element varied depending on the heat treatment conditions. The impact toughness was furthermore significantly increased by the introduction of a globular metastable austenitic phase. In the air-hardened condition, an addition of aluminium and the resulting in-situ tempering during the air-cooling proved to be the most effective strategy. In addition, the developed materials exhibit a significantly higher cyclic strength than the reference materials investigated, which is mainly due to the precipitation morphology of the different materials. In order to improve the process design, new empirical formulae for calculating the martensite start temperature (Ms) and the critical cooling rate were also determined on the basis of a comprehensive data analysis. Contrary to the existing ones, the new formulae are based on data from high alloyed steels with manganese concentrations up to 10 % and the boron concentration is considered. The most promising material achieves a yield strength of 930 MPa, a tensile strength of 1340 MPa, a uniform elongation of 4.1 %, and an impact toughness of 49 J in the air-hardened condition. Compared to the reference material 42CrMo4 (tempered condition), the cyclic strength was increased by 47 %. Thus, via a combined material, process and geometry optimization, the AHD steels offer the possibility to save energy and CO2 emission both by shortening the heat treatment and by lightweighting the components.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Engineering of Metals and Department of Ferrous Metallurgy [522110]