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

Piotr Nikiel,Grzegorz Korpała,Stefan Szczepanik

doi:10.3390/met11020328

Piotr Nikiel, Grzegorz Korpała + Show 1 more

Open Access

https://doi.org/10.3390/met11020328

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Journal: Metals	Publication Date: Feb 13, 2021
Citations: 4	License type: CC BY 4.0

Affiliation: TU Bergakademie Freiberg, AGH University of Krakow

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.

Highlights

The equilibrium microstructure of ultrahigh carbon steels, i.e., with C in the range of 1.0–2.1%, comprises pearlite and a grain boundary cementite network, which results in low ductility [1]
Methods that increase strength and plasticity and cause grain refinement and spheroidization of cementite include warm working [3,4,5,6,7,8,9], combined hot and warm working [1,10], cold or warm working combined with heat treatment [11], and combined heat treatment [12]
Superplastic forming would be extremely advantageous for powder metallurgy technology, which has the advantage of being a near net shape manufacturing process

Summary

Introduction

The equilibrium microstructure of ultrahigh carbon steels, i.e., with C in the range of 1.0–2.1%, comprises pearlite and a grain boundary cementite network, which results in low ductility [1]. Ultrahigh carbon steels (UHCS) with fine microstructure of ferrite with spheroidized cementite can have high ambient-temperature strength, hardness and ductility, and excellent high-temperature formability, even via superplasticity [1,10,11,12,13,14,15,16,17,18,19,20]. For strain rate of 0.01 s−1, 65% were precipitates with surface areas up to 0.4 μm2, and about 7% with areas exceeding 22 μm2.

Results

Conclusion