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

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; szczepan@metal.agh.edu.pl 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/).

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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 of of processing processing powder powder metallurgy metallurgy Fe-0.85Mo-0.65Si-1.4C Fe-0.85Mo-0.65Si-1.4C steel steelare 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 of specimensh ~ 11 mm of andh ~ diameter11 mm andd ~ diameter18 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 hot a stream air at ~130 of hotC. air The at heat ~130 treatment°C. The h diagrameat treatment and microstructure diagram and microstructureof 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 of of height height hh ~~ 11 11 mm mm and and diameter diameter dd 77 mm for thermo thermo-mechanical-mechanical compression − onon a a Bähr Bähr MDS MDS 830 plastometer plastometer-dilatometer-dilatometer at strain rates of 0.001,0.001, 0.01,0.01, 0.1,0.1, andand 11 ss−11 at ◦ 775775 °C.C. This This temperature temperature corresponds corresponds to to the the austenitization austenitization end end temperature. temperature. Changes Changes in in diameter were measured with a laser dilatometer and are presented as a function of temperature in Figure2.

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Metals 2021, 11, 328 3 of 12 diameter were measured with a laser dilatometer and are presented as a function of temperature in Figure 2.

Figure 2. Dilatometric curve curve obtained obtained during during heating heating and and soaking soaking a aspecimen specimen to to the the deformation deformation temperature onon aa BährBähr MDSMDS 830830 machine.machine.

The measurementmeasurement ofof thethe strain strain rate rate sensitivity sensitivity (SRS) (SRS m-value) m-value relevant relevant to to conditions conditions of superplasticityof superplasticity [25 [25]] was was calculated calculated by: by:

σσ1 σ lnln 1 ∂ ∂σ∼ σσ2 2 m = . = . (1) m = ≅ ε (1) ∂ε∂ε̇ ln . 1ε̇1 lnε2 ε̇2 . . where σσ11and andσ 2σ2stresses stresses at at strain strain rate rateε1 εanḋ1 andε2, respectively.ε̇2 , respectively. As a resultresult ofof testing,testing, thethe densitydensity ofof specimensspecimens waswas increasedincreased toto aboveabove 7.757.75 g/cmg/cm3.. Specimens,Specimens, deformed toto totaltotal strainstrain εε ~ 0.85, were cut in halves and on their cross-sectionscross-sections microstructuremicrostructure and and hardness hardness investigations investigations were were carried carried out. For out. these, For a Leica these, DM4000M a Leica lightDM4000M microscope light microscope and Hitachi-3500N and Hitachi scanning-3500N electronscanning microscope electron microscope were used. were Metallo- used. graphicMetallographic specimens specimens were etched were by etched 4% Picral. by 4% Investigations Picral. Investigations of microstructural of microstructural parameters wereparameters carried were out using carried ImageJ out using program. ImageJ Generally, program. 5 areas Generally from individual, 5 areas from microstructures individual . −1 ofmicrostructures 5 µm × 5 µm wereof 5 μm selected × 5 μm for were the analysis,selected andfor the for analysis,ε = 0.001 and s , for due ε tȯ = the0.001 much s−1, due larger to precipitates,the much larger a larger precipitates, area 10 µ am larger× 10 areaµm was 10 μm analyzed. × 10 μm was analyzed. Additionally,Additionally, halveshalves of o specimensf specimens were were fractured fractured by bending by bending to investigate to investigate fractogra- frac- phytography using using a Hitachi-3500N a Hitachi-3500N microscope. microscope. Vickers hardness testing was on a ZwickZwick tester on randomlyrandomly selected parts of the cross-sectioncross-section with the indenter loadload 98.198.1 N.N. 3. Results 3. Results 3.1. Stress-Strain Relationships 3.1. Stress-Strain Relationships Stress-strain curves, Figure3a, indicate that the flow stress increases significantly with Stress-strain curves, Figure 3a, indicate that the flow stress increases significantly increasing strain rate. It also affects the peak strain εp corresponding to the maximum with increasing strain rate. It also affects the peak strain εp corresponding to the maxi- stress σp; the higher the strain rate, the greater the strain at peak stress. The subsequent decreasemum stress in stressσp; the is associatedhigher the withstrain dynamic rate, the recrystallization greater the strain (DRX). at peak Critical stress. strain The above subsequent decrease in stress is associated with dynamic recrystallization (DRX). Critical which the dynamic recrystallization process began, εr, was determined on the basis of Refs.strain [ 26above,27]. Relationswhich the of dynamic strain rate recrystallization to peak and critical process strains began are, shown εr, was in determined Figure3b. The on criticalthe basis strain of Ref durings. [26,2 deformation7]. Relations of of UHCS strain with rate highto pea densityk and ofcritical dislocations strains are accumulated shown in insideFigure the3b. martensiteThe critical was strain significantly during deformation smaller than of forUHCS the as-sinteredwith high density state [23 of]. dislocations accumulated inside the martensite was significantly smaller than for the as-sintered state [23].

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(a) (b)

FigureFigure 3. 3. FlowFlow curves curves (a ()a )and and influence inﬂuence(a) of of strain strain rate rate on on value value of ofpeak peak strain strain and and critical critical(b) strain strain below below which which was wasthe be- the ginning of dynamic recrystalization (DRX) (b). beginningFigure of 3. dynamic Flow curves recrystalization (a) and influence (DRX) of strain (b). rate on value of peak strain and critical strain below which was the be- ginning of dynamic recrystalization (DRX) (b). 3.2. Microstructure 3.2. Microstructure 3.2.The Microstructure microstructure after deformation with strain rates 0.001, 0.01, 0.1, and 1 s−1 is The microstructure after deformation with strain rates 0.001, 0.01, 0.1, and 1 s−1 is presentedThe in m Figureicrostructure 4. after deformation with strain rates 0.001, 0.01, 0.1, and 1 s−1 is presented in Figure4. presented in Figure 4.

(a) (b) (a) (b)

Figure 4. Cont. (c) (d)

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(e) (f)

(g) (h)

−1 −1 −1 FigureFigure 4. Microstructure 4. Microstructure of Fe-0,85Mo-0.65Si-1.4C of Fe-0,85Mo-0.65Si-1.4C steel steel after after testing testing with with strain strain raterate (a,b)) 0.001 0.001 s s−, 1(c,(,dc), d0.01) 0.01 s , s(−e,1f,) (0.1e,f s) 0.1, s−1, and (g,h) 1 s−1. Light microscopy (LM) for (a,c,e,g) and SEM for (b,d,f,h). and (g,h) 1 s−1. Light microscopy (LM) for (a,c,e,g) and SEM for (b,d,f,h). The initial martensitic microstructure was transformed during testing to fully ferritic withThe spheroidal initial martensitic cementite, whereas microstructure in the as was-sintered transformed state it caused during only testing partial tosphe- fully ferritic with spheroidal cementite, whereas in the as-sintered state it caused only partial roidization of Fe3C [21]. Different strain rates produced different sizes and shapes of the spheroidizationFe3C precipitates of ( FeFigures3C[21 5].–7 Different and Table strain1). Specimens rates produced deformed different at the lowest sizes strain and rate shapes of theare Fe 3 characterizedC precipitates by (Figures the largest5– 7 average and Table surface1). Specimens area 0.27 µm deformed2 of flat sections at the lowest of Fe3C strain 2 rateprecipitates. are characterized Testing at by high theer largest strain averagerates 0.01, surface 0.1 and area 1 s−1 0.27 resultedµm inof smaller ﬂat sections precipi- of Fe3C precipitates.tates with Testingan average at highersurface strain area 0.08, rates 0.05 0.01,, and 0.1 0.05 and µm 1 s−2,1 respectively.resulted in smallerLonger defor- precipitates withmation an average time at strain surface rate area of 0.001 0.08, s−1 0.05, affect anded the 0.05 coagulationµm2, respectively. during which Longer large particles deformation timegrew at strainat the expense rate of 0.001of the ssmaller,−1 affected which the also coagulation affected the during shape of which the particles. large particles Precip- grew at theitates expense of Fe3C offormed the smaller, during deformation which also affectedwith the thelowest shape strain of rate the 0.001 particles. s−1 had Precipitates shapes of which most differed from circular, f = 0.69, when f = 1 is related to the round−1 shape. The Fe3C formed during deformation with the lowest strain rate 0.001 s had shapes which shape factor for higher strain rates 0.1 and 1 s−1 was f = 0.77. most differed from circular, f = 0.69, when f = 1 is related to the round shape. The shape factor for higher strain rates 0.1 and 1 s−1 was f = 0.77. Analysis of regions with ﬂat sections of Fe3C precipitates showed that at the strain rate of 0.001 s−1, the largest fraction, 65%, consisted of carbides with surface areas up to 0.16 µm2. A large fraction, over 9%, were precipitates with areas exceeding 0.8 µm2. 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. For strain rates of 0.1 and 1 s−1, surface areas up to 0.4 µm2 had a 58% and 51% share, and small fractions of particles with areas greater than 0.22 µm2. Due to the short deformation times, no coagulation process occurred.

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(a) (b) (a) (b)

(c) (d) (c) (d) −−11 FigureFigure 5. 5.Surface Surfacearea areaof of ﬂatflat sections sections of of Fe Fe33CC afterafter deformationdeformation withwith strainstrain rate:rate: ((aa)) 0.001,0.001, ((bb)) 0.01,0.01, ((cc)) 0.1,0.1, ( (dd)) 1 1 s s .. Figure 5. Surface area of flat sections of Fe3C after deformation with strain rate: (a) 0.001, (b) 0.01, (c) 0.1, (d) 1 s−1.

Figure 6. Mean surface area of Fe3C precipitates as a function of time of deformation. FigureFigure 6. 6. MeanMean surface surface area area of of Fe Fe3C3C precipitates precipitates as as a afunction function of of time time of of deformation. deformation.

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(a) (b)

Figure 7. Distribution of the surface area of a flat section of Fe3C precipitates after deformation at 775 °C◦ with a strain rate: Figure 7. Distribution of the surface area of a ﬂat section of Fe3C precipitates after deformation at 775 C with a strain rate: −1 (a) 0.001, ( b) 0.01, (c) 0.1, (d) 1 s−.1 .

Table 1. Characteristic of selected parameters of Fe3C particles. Table 1. Characteristic of selected parameters of Fe3C particles. Number of Average Surface Area A, Average Feret Diameter D, 1 1 Shape Factor No. Strain Rate, s−1 Number of AnalyzedStrain Particles, - Average Surface Average Feret Shape Factor No. Analyzed µm2 µm (Circularity) f, - Rate, s−1 Area A, μm2 Diameter D, μm (Circularity) f, - 1 0.001 130Particles, - 0.27 0.71 0.69 2 0.011 0.001 344 130 0.080.27 0.360.71 0.69 0.73 3 0.1 626 0.05 0.33 0.77 4 12 0.01 655 344 0.050.08 0.350.36 0.73 0.77 3 1 f =0.1 4π A/P2 where626 A—area and P—perimeter0.05 of particles. 0.33 0.77 4 1 655 0.05 0.35 0.77 1 2 3.3.f = 4πA/P Fractography where A—area and P—perimeter of particles. Fracture surfaces of specimens that were broken after testing are shown in Figure8. Analysis of regions with flat sections of Fe3C precipitates showed that at the strain The fracture surface of a specimen deformed at strain rate 1 s−1 (Figure8c,d) had rate of 0.001 s−1, the largest fraction, 65%, consisted of carbides with surface areas up to very small dimples, about 1–2 µm in size, and for strain rate 0.001 s−1, the fracture area 0.16 μm2. A large fraction, over 9%, were precipitates with areas exceeding 0.8 μm2. For contained ﬂat surfaces and larger dimples. These features are typical of ductile rupture. 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. For strain rates of 0.1 and 1 s−1, surface areas up to 0.4 μm2 had a 58% and 51% share, and small fractions of particles with areas greater than 0.22 μm2. Due to the short deformation times, no coagulation process occurred.

3.3. Fractography Fracture surfaces of specimens that were broken after testing are shown in Figure 8.

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(a) (b)

Figure 8. Fracture s surfacesurfaces of ambient ambient temperature temperature deformed specimens which had been previously warm tested at strain −1 rate 0.001 ( a,b) and 1 s− 1(c(,cd,d).).

−1 3.4. HardnessThe fracture surface of a specimen deformed at strain rate 1 s (Figure 8c,d) had very small dimples, about 1–2 μm in size, and for strain rate 0.001 s−1, the fracture area Mean hardness HV10 measured in randomly selected sites on cross-sections of de- contained flat surfaces and larger dimples. These features are typical of ductile rupture. formed specimens is reported in Table2. 3.4. Hardness Table 2. Average hardness HV10 of Fe-0.85Mo-0.65Si-1.4C steel after testing. Mean hardness HV10 measured in randomly selected sites on cross-sections of deformed specimensNo. is reported in Table Strain 2. Rate, s−1 Hardness HV10 1 0.001 295 ± 18.8 Table 2. Average2 hardness HV10 of Fe-0.85Mo-0.65Si 0.01-1.4C steel after testing. 339 ± 13.8 ± 3−1 0.1 359 6.9 No. 4Strain Rate, s 1Hardness HV10 362 ± 2.9 1 0.001 295 ± 18.8 2 0.01 339 ± 13.8 The lowest hardness of 295 HV10 was after deformation with strain rate 0.001 s−1. 3 0.1 359 ± 6.9 Hardness increased with increasing strain rate, after deformation with strain rate 1 s−1 4 1 362 ± 2.9 hardness was 362 HV10. Hardness HV2 maps are presented in Figure9. Hardness of the specimen deformed with strain rate 0.001 s−1 was smallest in the −1 centralThe zone, lowest the placehardness of the of highest 295 HV10 strain was intensity. after deformation The reverse waswith the strain case rate for specimens 0.001 s . −1 Hardnessdeformed increase with a straind with rate increasing 0.01, 0.1, strain and 1 srate−1,, where after deformation in the central with zone strain and diagonally rate 1 s hardnessthe greatest was hardness 362 HV10. was Hardness located. HV2 maps are presented in Figure 9.

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(a) (b)

Figure 9. Hardness HV2 maps of specimens after testing at strain rate: ( a) 0.001, ( b) 0.01, (c) 0.1, (d) 1 s−−1.1 .

−1 4. DiscussionHardness of the specimen deformed with strain rate 0.001 s was smallest in the central zone, the place of the highest strain intensity. The reverse was the case for speci- The ﬂow curves at 775 ◦C and strain rate of 0.001−1 s−1 exhibited strain hardening to mens deformed with a strain rate 0.01, 0.1, and 1 s−1, where in the central zone and di- pronounced stress peak and dynamic softening and recrystallization followed by steady agonally the greatest hardness was located. state deformation. Peak and critical strains for both warm tested materials are summarized in Table3. 4. Discussion Table 3. Comparative dataThe for flow warm curves testing at quenched 775 °C and and spheroidizedstrain rate of quenched 0.001−1 s Fe-0.85Mo-0.65Si-1.4C.−1 exhibited strain hardening to pronounced stress peak and dynamic softening and recrystallization followed by steady Bähr at 775 ◦C—Quenched Gleeble Quenched and Spheroidised at 750 ◦C[22] state deformation. Peak and critical strains for both warm tested materials are summa- . ε εp,- rizedεc ,-in Table 3.σ p, MPa σc, MPa εp,- σp, MPa 0.001 0.16 0.04 110 97 0.05 150 0.01Table 3. 0.28Comparative data - for warm testing 243 quenched and - spheroidized quenched 0.16 Fe-0.85Mo-0.65Si-1.4C. 250 0.1 0.28 0.09 304 270 0.2 330 1 0.47Bähr at 775 0.24 °C —Quenched 542 458Gleeble Quenched 0.22 and Spheroidised at 460 750 °C [22] 훆̇, s−1 εp, - εc, - σp, MPa σc, MPa εp, - σp, MPa 0.001 0.16 0.04 Superplastic110 ﬂow has97 these fundamental requirements:0.05 the grain size of the150 material 0.01 0.28 - must be243 very small (typically- less than 10 µm), deformation0.16 temperature greater250 than about 0.1 0.28 0.09 304 270 0.2 330 0.4–0.5 Tm (where Tm is the absolute melting temperature [25]), and strain rate in the range 1 0.47 0.24 10−4–10−5422 s −1. The strain458 rate sensitivities from compression0.22 tests were: 0.17–0.30460 at 700 ◦C for Gleeble and 0.22–0.37 at 775 ◦C for Bähr. Although the strain rates were similar, the startingSuperplastic microstructures flow has were these different fundamental for those requirements of quenched: the (Bähr grain tests) size of and the quenched material mustand already be very spheroidized small (typically (Gleeble less tests)than 10 steels. μm), deformation temperature greater than about 0.4–0.5 Tm (where Tm is the absolute melting temperature [25]), and strain rate in

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Superplasticity is generally ﬁrst found in tensile tests, which were not carried out on our material due to the shape and size of the sintered specimens. Only compression and warm forging [22,24] tests were conducted, and it is quite possible that tensile superplasticity existed. For engineering applications, warm working such as forging is more relevant, and superplastic deformation did not occur in our spheroidized material [22,24], though it cannot be excluded in similar processing conditions. The quenched-only steel is compared with the fully spheroidized material of the same composition in Table4.

Table 4. Microstructure, yield strength, and hardness of Fe-0.85Mo-0.65Si-1.4C.

Ferrite Yield Processing/ Hardness State Microstructure Grain Size Strength Reference Testing HV10 µm MPa 1300 ◦C, cooled slowly to Sintered fine pearlite and grain boundary Fe C 400 * 196–325 [15,16] room temperature 3 austenitized at 970 ◦C, isothermally ferrite matrix and dispersed Quenched 1–3 715–1030 * 295–362 Current report quenched, Bähr tested at 775 ◦C spheroidal carbides ferrite matrix and dispersed room temperature deformation 30 350–410 150–230 [15,24] spheroidal carbides Spheroidized at fine-grained ferrite and Gleeble tested at 700 ◦C 6–7 769 310 [22] 750 ◦C homogeneously distributed carbides fine-grained ferrite and twice forged: 700 ◦C and 750 ◦C 6–7 769–744 - [22] homogeneously distributed carbides * estimated.

The Moon [28] relation, YS = (HB − 97)/0.27, for sintered steels was used to estimate yield strength from hardness. The Hall–Petch relation for the yield strength is:

−1/2 σy = σ0 + kyd (2)

where σy is yield strength, σ0 the friction stress, and ky the Hall–Petch strengthening coefﬁcient, was extended to spheroidized steels by Syn et al. [13]:

−1/2 −1/2 σy = 310 Ds* +460 L (3)

where L is the grain size and Ds* the inter-carbide spacing. They reported that the prediction is good, within 20%, when compared to previous experimental data. Calculations for this relation for spheroidized Fe-0.85Mo-0.65Si-1.4C are presented in Table5.

Table 5. Yield strengths of spheroidized Fe-0.85Mo-0.65Si-1.4C measured and according to Syn et al. [13] formula.

Grain Size, Inter-Carbide 310 Ds* −1/2, 460 L−1/2, Calculated Yield Yield Strength, State L, µm Spacing, Ds*, µm MPa MPa Strength, MPa MPa Quenched, spheroidised during B¨аhr testing 1–3 0.628–0.298 378–549 265–460 643–1009 715–1030 * Spheroidized at 750 ◦C 30 0.78 339 83 422 350–410 Spheroidized and forged 7 0.826–0.59 403–520 187 667 769–744 * estimated.

Additionally, it is seen that for these data the correspondence between experimental and theoretical results is equally good. The results further indicate that conventional forging should be as successful for quenched as for spheroidized material.

5. Conclusions Thermo-mechanical treatment at all strain rates investigated of quenched Fe-0.85Mo- 0.65Si-1.4C steel by warm testing at austenite start temperature 775 ◦C led to rapid spheroidization of cementite. The (compressive) strain rate sensitivity was 0.24. Metals 2021, 11, 328 11 of 12

During testing at lower strain rates of 0.001 and 0.01 s−1, coagulation of the carbides took place, which affected the size and shape of the Fe3C precipitates. After deformation with higher strain rates, the distribution of Fe3C carbides was more homogeneous and their shape was more circular. The smallest hardness, 295 HV10, was for strain rate of 0.001 s−1 and the highest, 359 and 362 HV10, were in specimens after deformation with strain rates of 0.1 and 1 s−1, when ultraﬁne cementite precipitates were formed. These and previous results on spheroidized Fe-0.85Mo-0.65Si-1.4C are consistent with the yield stress Hall–Petch relation developed by Syn et al. [13] for spheroidized UHCSs. The results indicate that conventional forging should be as successful for quenched as for spheroidized material.

Author Contributions: Conceptualization, P.N. and S.S.; methodology, S.S.; software P.N.; validation, P.N., S.S., and G.K.; formal analysis, S.S.; investigation P.N., S.S., and G.K.; data curation, P.N.; writing—original draft preparation, P.N. and S.S.; writing—review and editing, P.N. and S.S.; visual- ization, P.N.; supervision, S.S.; project administration, P.N.; funding acquisition, AGH Krakow and TUBA Freiberg. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The work described forms part of the cooperative program between the Institute of Metal Forming TU BA Freiberg, Germany, and AGH University of Science and Technology, Krakow, Poland. Authors thank Andrew Wronski (Emeritus, UOB, England) for constructive comments on the draft and S.C. Mitchell for producing test samples. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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