Accelerated Spheroidization of Cementite in Sintered Ultrahigh Carbon Steel by Warm Deformation

metals Article Accelerated Spheroidization of Cementite in Sintered Ultrahigh Carbon Steel by Warm Deformation Piotr Nikiel 1,* , Stefan Szczepanik 1 and Grzegorz Korpała 2 1 Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Av. Mickiewicza 30, 30-059 Kraków, Poland; [email protected] 2 Institut für Metallformung, Technische Universität Bergakademie Freiberg, Bernhard-Von-Cotta Str. 4, 09-599 Freiberg, Germany; [email protected] * Correspondence: [email protected]; Tel.: +48-12-617-38-46 Abstract: Evolution of microstructure and hardness in quenched ultrahigh carbon steel Fe-0.85Mo- 0.6Si-1.4C by warm compression on a Bähr plastometer-dilatometer at 775 ◦C and at 0.001 to 1 s−1 strain rate range is reported. The material was prepared via powder metallurgy: cold pressing and liquid phase sintering. Independent of strain rate, the initial martenstic microstructure was transformed to ferrite and spheroidized cementite. Strain rate had an effect on size and shape of spheroidized Fe3C precipitates: the higher the strain rate, the smaller the precipitates. Morphology of the spheroidized carbides influenced hardness, with the highest hardness, 362 HV10, for strain rate 1 s−1 and the lowest, 295 HV10, for the lowest strain rate 0.001 s−1. Resultant microstructure and ambient temperature mechanical properties were comparable to those of the material that had undergone a fully spheroidizing treatment with increased time and energy consumption, indicating that it can be dispensed with in industrial processing. All our results are consistent with the Hall– Petch relation developed for spheroidized steels. Citation: Nikiel, P.; Szczepanik, S.; Korpała, G. Accelerated Keywords: ultrahigh carbon sintered steel; warm working; accelerated spheroidization; microstruc- Spheroidization of Cementite in ture; hardness Sintered Ultrahigh Carbon Steel by Warm Deformation. Metals 2021, 11, 328. https://doi.org/10.3390/ met11020328 1. Introduction The equilibrium microstructure of ultrahigh carbon steels, i.e., with C in the range of Academic Editor: Jose Torralba 1.0–2.1%, comprises pearlite and a grain boundary cementite network, which results in low ductility [1]. Plasticity can be achieved, e.g., by spheroidizing annealing at a temperature Received: 22 December 2020 Accepted: 10 February 2021 close to A1 [2]. Methods that increase strength and plasticity and cause grain refinement Published: 13 February 2021 and spheroidization of cementite include warm working [3–9], combined hot and warm working [1,10], cold or warm working combined with heat treatment [11], and combined Publisher’s Note: MDPI stays neutral heat treatment [12]. Ultrahigh carbon steels (UHCS) with fine microstructure of ferrite with regard to jurisdictional claims in with spheroidized cementite can have high ambient-temperature strength, hardness and published maps and institutional affil- ductility, and excellent high-temperature formability, even via superplasticity [1,10–20]. iations. Superplastic forming would be extremely advantageous for powder metallurgy technology, which has the advantage of being a near net shape manufacturing process. Sinter forging, warm forging of powder preforms, is particularly employed to manufacture near fully dense automotive gear parts such as helical pinion gears and connecting rods. Pow- der metallurgy processing of Fe-0.85Mo-0.65Si-1.4C steel was developed at the University Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of Bradford [14–16]. The specimens were slowly cooled from the sintering temperature, ◦ ◦ This article is an open access article austenitized at 950 C for 1 h, then quenched into a warm fan assisted oven at ~130 C, ◦ distributed under the terms and followed by air cooling and refrigeration, then spheroidized at 750 C and slow cooled to conditions of the Creative Commons room temperature. Attribution (CC BY) license (https:// Spheroidizing annealing after warm working of steel promotes faster and enhanced creativecommons.org/licenses/by/ cementite spheroidization. The higher the warm deformation, the higher the degree of 4.0/). Metals 2021, 11, 328. https://doi.org/10.3390/met11020328 https://www.mdpi.com/journal/metals Metals 2021, 11, x FOR PEER REVIEW 2 of 12 Metals 2021, 11, 328 2 of 12 cementite spheroidization. The higher the warm deformation, the higher the degree of spheroidization after annealing. Warm deformation leads also to ferrite grain size re- spheroidization after annealing. Warm deformation leads also to ferrite grain size refinement after annealing. Grain refinement takes place via a continuous recrystallization finement after annealing. Grain refinement takes place via a continuous recrystallization process, which is controlled by cementite spheroidization and coarsening [8]. Supersat- process, which is controlled by cementite spheroidization and coarsening [8]. Supersat- uration of solid solution and high density of vacancies and dislocations of quenched steel uration of solid solution and high density of vacancies and dislocations of quenched increase the speed of carbon diffusion and accelerate the spheroidizing of cementite. steel increase the speed of carbon diffusion and accelerate the spheroidizing of cementite. Crystal defects are also sites of cementite nucleation. On the other hand, these defects are Crystal defects are also sites of cementite nucleation. On the other hand, these defects constantly generated during warm deformation, providing energy for diffusion and are constantly generated during warm deformation, providing energy for diffusion and consequently acceleration of cementite coagulation [6,21]. consequently acceleration of cementite coagulation [6,21]. Achieved in spheroidized PM Fe-0.85Mo-0.65Si-1.4C steel were: density ~7.2 g/cm3, Achieved in spheroidized PM Fe-0.85Mo-0.65Si-1.4C steel were: density ~7.2 g/cm3, graingrain size size ~ ~3030 μm,µm, yield yield strength strength 410 410 MPa MPa,, and and elonga elongationtion 16% 16% [14]. [14]. Searching Searching for for condi- condi- tionstions for for superplastic behavior,behavior, twotwo types types of of experiments experiments were were subsequently subsequently carried carried out. out. In Inone, one, the the rings rings were were forged forged on aon screw a screw press p betweenress between flat plates flat plates at 700–750 at 700◦C.–750 In the°C. secondIn the secondset of experiments, set of experiments, carried carried out on out a Gleeble on a Gleeble HDV-40 HDV machine-40 machine at Technische at Technische Universität Uni- −3 −2 −1 versitätBergakademie Bergakademie Freiberg, Freiberg discs were, discs compressed were compressed at strain at rates strain of rates 10−3 ,of 10 10−2,, 1010−1,, 10 and, −1 and1 s− 1 tos ~1.15 to ~1.15 natural natural strain strain [22– [2224].–24 Superplastic]. Superplastic behavior behavior was was not observed.not observed. Grain Grain size sizedecreased decreased to ~7 toµ ~7m, μm and, and yield yield strength strength increased increased to 740 to Mpa.740 MPa. AnAn alternative wayway ofof warm warm working working the the steel steel is in is a in quenched a quenched state. state. The criticalThe critical strain strainneeded needed for transformation for transformation of microstructure of microstructure via dynamic via recrystallization dynamic recrystallization is smaller for theis smallerinitial martensite for the initial microstructure martensite than microstructure for initial pearlite than for microstructure, initial pearlite which microstructure is associated, whichwith a is high associated density ofwith dislocations a high density after quenchingof dislocations [8,21 ,after23]. Investigationquenching [8,21,23] of warm. Inves- defor- tigationmation ofof quenchedwarm deformation Fe-0.85Mo-0.65Si-1.4C, of quenched includingFe-0.85Mo the-0.65Si search-1.4C, for superplasticincluding the behavior, search foris the superplastic subject of behavior this communication., is the subject of this communication. 2.2. Materials Materials and and Methods Methods ProceduresProcedures ofof processingprocessing powder powder metallurgy metallurgy Fe-0.85Mo-0.65Si-1.4C Fe-0.85Mo-0.65Si-1.4C steel steel are described are de- scribedin detail in in detail Refs. in [14 Ref–16s.]. [14 Mix–16 of]. powdersMix of powders Hogänas Hogänas Astaloy Astaloy 85Mo, graphite,85Mo, graphite and silicon, and ◦ sicarbidelicon carbide were compacted were compacted at 600 Mpa. at 600 Liquid MPa. phase Liquid sintering phase sintering was carried was out carried at 1295 outC at to 1295produce °C to cylindrical produce specimenscylindrical ofspecimensh ~ 11 mm of andh ~ diameter11 mm andd ~ diameter 18 mm and d ~ density 18 mm above and 3 ◦ density7.4 g/cm above. Heat 7.4 treatmentg/cm3. Heat comprised treatment austenitizing comprised austenitizing at 970 C and at quenching970 °C and byquenching a stream ◦ byof hota stream air at ~130of hotC. air The at heat~130 treatment°C. The h diagrameat treatment and microstructure diagram and microstructure of martensite and of martensiteretained austenite and retained after quenchingaustenite after are presentedquenching in are Figure presented1. in Figure 1. (a) (b) FigureFigure 1. 1. HeatHeat treatment treatment diagram diagram ( (aa)) and and microstructure microstructure of of quenched quenched steel steel ( (b). TheThe quenched quenched specimens specimens were were Electrical Electrical Discharge Discharge Machining Machining ( (EDM)EDM) machined to cycylinderslinders

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