Effect of Tempering Time on Microstructure, Mechanical, and Electrochemical Properties of Quenched–Partitioned–Tempered Advanced High Strength Steel (AHSS)

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PAPER Effect of tempering time on microstructure, mechanical, and electrochemical properties of quenched–partitioned–tempered Advanced High Strength Steel (AHSS)

To cite this article: Aqil Inam et al 2019 Mater. Res. Express 6 126509

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PAPER Effect of tempering time on microstructure, mechanical, and RECEIVED 8 July 2019 electrochemical properties of quenched–partitioned–tempered REVISED 9 October 2019 Advanced High Strength Steel (AHSS) ACCEPTED FOR PUBLICATION 30 October 2019 Aqil Inam1 , Yasim Imtiaz1, Muhammad Arslan Hafeez1 , Salman Munir1, Zeeshan Ali1, PUBLISHED 13 November 2019 Muhammad Ishtiaq1, Muhammad Haseeb Hassan1, Adnan Maqbool2 and Waseem Haider3 1 Department of Metallurgy & Materials Engineering, CEET, University of the Punjab, Lahore, Pakistan 2 Department of Metallurgical & Materials Engineering, University of Engineering and Technology, Lahore, Pakistan 3 School of Engineering and Technology, Central Michigan University, Mount Pleasant, MI 48858, United States of America E-mail: [email protected]

Keywords: triplex microstructure, martensite, bainite, quenching–partitioning–tempering, corrosion

Abstract The quenched–partitioned–tempered (QPT) steel is one of the novel steel in third–generation advanced high strength steels. Herein, an attempt has been made to investigate the microstructure, mechanical, and electrochemical properties of QPT steel as a function of tempering time. Microstructural analysis reveals that a high volume fraction of martensite with low fractions of retained austenite and bainite were achieved after 30 s of tempering. Mechanical tests validated that this triplex composite microstructure resulted in an 18% improvement in Vickers hardness, 73% in tensile strength (Rm) with little reduction of 12% elongation (εT) in comparison to non–heat treated steel sample. Tempering for 90 s increased the volume fractions of stable retained austenite, bainite and decreased the fractions of martensite by carbon diffusion to austenite. Further increase in tempering time stabilized retained austenite at an expense of bainite and martensite phases, resulting in reduced hardness and improved strength and elongation. Electrochemical analysis of the QPT heat–treated samples performed in 3.5% NaCl solution showed that the high corrosion rate was yielded from the steel sample tempered for 30 s, while other samples tempered for 90 to 150 s showed much lower corrosion rate than non–heat–treated steel sample.

1. Introduction

In the last few years, the demand for lightweight, cost–effective, crash-resistant, energy-efficient and environment–friendly materials has rapidly increased in automotive the industry [1]. In the automotive industry, structural components of vehicles i.e. side sills, A–pillars, B–pillars, and front cross member require high strength and high toughness [2]. To fulfill these requirements, Speer et al proposed a novel heat–treatment process known as quenching and partitioning process (Q and P) in 2003 to develop advanced high strength steels (AHSS) [3, 4]. During the partitioning stage of this process, excess carbon of supersaturated martensite diffuses out and moves to the retained austenite phase. This retained austenite stabilizes during cooling to room temperature and provides high toughness while the martensite provides high strength [5, 6]. In Q and P steels it is mandatory to keep a high level of Si to suppress the carbide formation during heat- treatment process [7–9]. Hence, the excess carbon ejected from the martensite during the partitioning and tempering process could diffuse to retained austenite to stabilize it. Due to this fact, precipitation strengthening mechanism is not involved in Q and P process [10]. To achieve further improvement in retained austenite fractions and corresponding mechanical properties by taking the benefit of precipitation strengthening mechanism, quenching–partitioning–tempering process (QPT) was proposed by Hsu et al in 2007 [11, 12]. For the QPT process, Nb and Mo are also added to improve the strength of steel [1].

Table 1. Chemical composition (wt%) of experimental steel developed.

C Si Mn Mo V Nb Fe

0.41 1.52 2.01 0.21 0.18 0.05 Bal.

Several investigations have been conducted in the QPT steels with focusing on microstructure and mechanical properties relationship [13–20] nano-precipitation and transformation induced plasticity (TRIP) [18], and austenite reversion and co–precipitation in QPT steels [21].Liet al [21] reported that nucleation of austenite occurred in the partitioning stage, while the growth of austenite took place in the tempering stage in ultra–low carbon medium manganese steel. Co–precipitation of Ni enriched particles with Cu enriched precipitates was also observed in this steel. The mechanism of co–precipitation is closely linked to the cooperative austenite reversion transformation mechanism which is controlled by the Mn movement in the microstructure. Zhou et al [13] reported that a considerable volume fraction of retained austenite (7.5%) was achieved by the QPT process which offered a signiﬁcant improvement in impact toughness and excellent combination of strength and elongation of M50 ultra–high strength steel. On the other hand, some researchers worked on hydrogen embrittlement [22, 23], strength and ductility improvement by Cu precipitates [24], toughening optimization [25, 26], fatigue behavior [27], and wear behavior [28], in QPT steel. Rong et al [24] reported that copious Cu particles of diameter 0.66–5.89 nm caused improvement in strength as well as other tensile properties. Tempering stage observed to decrease the fractions of martensite due to a continuous diffusion of carbon from martensite to retained austenite hence increases its stability. Due to this fact, a signiﬁcant improvement in the elongation of steel was observed. Gao et al [26] observed concurrent improvement in tensile strength and impact toughness in low carbon Mn–Si–Cr–Mo alloyed steel. This improvement is attributed to the triplex microstructure containing bainite, martensite, and retained austenite. The carbon diffusion from martensite to retained austenite also restricts the coalescences of bainitic plates. Investigations on electrochemical properties along with microstructure and mechanical properties of QPT treated steels are still limited and require further work. In current work, experimental steel having 2.01 wt% Mn to stabilize to austenite phase at room temperature and 0.21 wt% Mo to enhance strength was commercially produced. This steel was then subjected to QPT heat– treatment process by austenitizing at 900 °C, quenching at 230 °C, partitioning at 400 °C and tempering at 500 °C for various tempering times ranging from 30–150 s. Microstructure analysis and mechanical test results were obtained to evaluate the effect of tempering time on microstructure and mechanical properties of steel. The electrochemical analysis was performed in a 3.5% NaCl solution to analyze the variations in polarization potential and corrosion kinetics.

2. Materials and methods

2.1. Material Experimental steel of chemical composition given in table 1 was developed in a commercial induction furnace and cast into ingots followed by hot rolling into rods of 32 mm diameter. Rods were machined on computer numerical control wire cut machine into required dimensions for subsequent heat–treatment process. The samples were cleaned in an aqueous solution of Na2CO3 (10%) at 70 °C for 5 min to remove physically attached particles and other contaminations.

2.2. Quenching–partitioning–tempering process

Parameters of the QPT heat–treatment process i.e. lower austenitizing temperature (AC1), upper austenitizing temperature (AC3), bainite start temperature (Bf), bainite finish temperature (Bf), martensite start temperature (Ms) and martensite finish temperature (Mf) given in table 2 were calculated by SteCal 3.0 software ASM international 2004. The calculated value of Ms (302 °C) was verified by the Nehernberg equation (290 °C) given in equation (1) [29] which was very close to the value obtained from the SteCal 3.0 software. On the other hand, the value of BS was verified by the Kirkaldy formula given in equation (2) [30].

Ms =--498.9 300 C 33.3 Mn - 22.2 Cr - 16.7 Ni - 11.1() Si + Mo () 1

Bs =-656 57.7 C - 75 Si - 35 Mn - 15.3 Ni - 34 Cr - 41.2 Mo () 2

During QPT heat–treatment, austenitizing temperature causes the partitioning of carbon and other alloying elements between precipitates and austenite. Increasing austenitizing temperature results in the increased dissolution of precipitates and grain growth [31]. According to the aforementioned parameters, samples of

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Table 2. Calculated heat–treatment parameters for QPT process.

AC1 AC3 BS Bf MS Mf

790 860 521 401 302 −118

Figure 1. Cycle of QPT heat–treatment process applied to experimental steel.

Figure 2. Drawing of the tensile sample showing dimensions (mm) prepared according to the ASTM E8 standard.

200 mm length, 13 mm diameter were austenitized at 900 °C above AC3 for 30 min as per diameter to fully transform previous phases into austenite phase followed by quenching between MS and Mf at 230 °C for 30 s to partially transform austenite into martensite. Samples were then partitioned at 400 °C for 30 s and ﬁnally tempered at 500 °C for various tempering times,30, 90, 110, 130 and 150 s similar to the work of Zhang et al [2]. The schematic of the applied QPT heat-treatment cycles is given in ﬁgure 1.

2.3. Characterization Heat–treated experimental steel samples of size 103 mm were hot mounted and metallographically prepared by grinding on SiC papers of various grades, polishing on nylon and velvet clothes using diamond pastes on Ecomet 250 grinder/polisher. Polished samples were then etched in a 5% nital solution for 5 s. Microstructural analysis was carried out on the Leica brand (model 15000 M) light optical microscope at 1000x magnification and on the FEI brand (model Inspect S50) scanning electron microscope (SEM). Hardness testing of QPT heat–treated steel samples was performed under the load of 1000 g using a diamond indenter on Shimadzu HMV brandVickers hardness tester. Five tests were performed for each sample and finalized values were calculated by taking the mean of five readings. For tensile testing, samples were machined according to the drawing (ASTM E8 standard) given in figure 2. Tensile testing was carried out on the Hualong brand (model WDW 1000 C) universal testing machine. Samples of dimensions 10 mm3 were soldered with single conductor copper wire followed by cold mounting in polyester resin by exposing 10 mm2 surface to the electrolyte for electrochemical analysis. Samples were then

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Figure 3. Microstructural features of non–heat–treated steel sample; (a) optical micrograph, (b) SEM micrograph and QPT heat– treated steel sample tempered for 30 s; (c) optical micrograph, (d) SEM micrograph.

metallographically prepared by grinding and polishing by the aforementioned method. Variations in polarization potential and corrosion kinetics of QPT heat–treated steel samples were analyzed in 3.5% NaCl solution at ambient temperature by Potentio-dynamic polarization technique on Gamry (model 500 P) potentiostat. Saturated calomel electrode was used as a reference electrode, graphite as an auxiliary electrode and heat–treated samples as working electrodes in three–electrode cell system.

3. Results and discussion

3.1. Microstructure evolution Austenitizing at 900 °C for 30 min dissolves the previous phases into austenite which decomposes into packets and blocks of lath martensite and retained austenite after quenching between MS and Mf. The subsequent partitioning process diffuses the excess carbon of martensite into retained austenite to stabilize and increase its volume fraction [19]. The final tempering process carried out between BS and Bf causes the formation of bainite phase, precipitation of the carbides, and further stabilization of retained austenite [18]. Microstructural features of QPT heat–treated experimental steel under various tempering times are illustrated in figures 3–5. In non–heat treated form, micrographs of experimental steel exhibited pearlite (P) phase in the matrix of ferrite (F) phases as illustrated in figures 3(a)–(b). Tempering at 500 °C for 30 s after quenching and partitioning processes resulted in triplex microstructure comprising of retained austenite (RA), martensite (M) and smaller fractions of bainite (B) phases as revealed in

ﬁgures 3(c)–(d). Quenching of steel as between MS and Mf partially transformed the austenite into martensite phase and kept the remaining austenite as retained austenite in the microstructure. Short partitioning time of 30 s diffused the excess carbon of martensite to retained austenite and further stabilized this phase. Tempering between BS and Bf caused the formation of the bainite phase. So, the overall QPT process yields a triplex microstructure at this stage. Increasing tempering time to 90 s caused excess martensitic carbon to diffuse to retained austenite and bainite, resulting in the increased volume fraction of stable retained austenite and bainite with a decreased volume fraction of martensite. Compared to other tempering times, 90 s of tempering caused the formation of lath shaped bainite phase of maximum length as revealed in ﬁgures 4(a)–(b) similar to the work of Wang et al [19]. With the further increase of tempering time to 110 s, the volume fraction of the bainite phase also began to decrease due to the diffusion of carbon into retained austenite. Micrographs of 110 s tempering comprised of

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Figure 4. QPT heat–treated samples showing triplex microstructure containing martensite, bainite and retained austenite phases of steel tempered for 90 s; (a) optical micrograph, (b) SEM micrograph and triplex microstructure of steel tempered for 110 s; (c) optical micrograph, (d) SEM micrograph.

Figure 5. QPT heat–treated samples showing triplex microstructure containing martensite, bainite and retained austenite phases of steel tempered for 130 s; (a) optical micrograph, (b) SEM micrograph and triplex microstructure of steel tempered for 150 s; (c) optical micrograph, (d) SEM micrograph.

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Table 3. Mechanical properties of QPT heat–treated experimental steel.

Sample ID Rm (MPa) RP0.2 (MPa) εT (%) RA (%) Hardness (VHN)

As received 813 774 17 37 516 QP and T–30 s 1410 1380 15 33 610 QP and T–90 s 1130 1113 26 49 504 QP and T–110 s 1310 1317 19 41 538 QP and T–130 s 1258 1126 22 45 509 QP and T–150 s 1195 1133 25 47 469 reduced fractions of martensite, bainite and significantly increased volume fraction of retained austenite as revealed in figures 4(c)–(d). This tempering time also caused precipitation of unstable carbides. Tempering period of 130 and 150 s transformed unstable carbides into stable carbides (cementite). This tempering time also reduced volume fractions of martensite and bainite and increased the volume fraction of retained austenite to the maximum level compared to other tempering times as illustrated in figure 5. This is because prolonged tampering process caused maximum diffusion of carbon from martensite to retained austenite [29].

3.2. Mechanical properties Tensile properties of experimental steel were observed to be highly sensitive to tempering time in the QPT heat– treatment process. Variations in tensile properties including tensile strength (Rm), yield strength (RP0.2), elongation (εT) and reduction in area (RA) with varying tempering time are presented in table 3. Experimental steel in non–heat–treated form exhibited lower values of Rm (813 MPa),RP0.2 (774 MPa) and sufficiently high values of Vickers hardness (516 HV),εT (17%) and RA (37%) attributed to the presence of softer and ductile phases such as ferrite and pearlite in the microstructure. Various strengthening mechanisms such as solid solution strengthening [32, 33], grain boundary strengthening [34, 35], point defect strengthening, precipitation strengthening, and martensitic strengthening have been reported [36–39]. In this work, two major strengthening mechanisms including martensitic strengthening and precipitation strengthening occurred and significantly improved the strength. While improved fractions of stable retained austenite provided high values of εT and RA. Tempering at 500 °C for 30 s significantly improved the Vickers hardness (610 HV),Rm(1410 MPa) and

RP0.2 (1380 MPa) at a little expense of εT (15%) and RA (33%) compared to non–heat–treated steel. This improvement is attributed to the phase transformation from ferrite–pearlite to martensite, retained austenite and a lower fraction of bainite. As mentioned earlier, tempering after the Q and T process caused carbon diffusion from martensite to other phases. Therefore, tempering for 90 s provided a relatively long time for carbon diffusion, stabilized retained austenite, and slightly improved volume fraction of bainite. This transformation resulted in a reduction in Vickers hardness (504 HV),Rm(1130 MPa) and RP0.2 (1113 MPa) but remarkably increased the εT (26%) and RA (49%) compared to 30 s tempered sample. Further increase in tempering time to 110 s provided an optimum combination of mechanical properties. As the optimum values of hardness (538 HV),Rm(1310 MPa) and RP0.2 (1317 MPa), εT (19%), and RA (41%) were achieved at this tempering temperature. Tempering for 130 and 150 s made the material ductile, reduced the values of hardness, strengths, and improved the elongation compared to 110 s tempered sample. The reason behind this fact is the maximum diffusion of carbon from martensite and bainite to retained austenite.

Rm×εT factor was also calculated for all samples by multiplying the value of Rm with the εT. Variations in Rm×εT factor with varying tempering time in the QPT heat–treatment process are plotted in ﬁgure 6.

3.3. Electrochemical properties Electrochemical characteristics of QPT heat–treated experimental steel were investigated as a function of tempering time in 3.5% NaCl solution by potentiodynamic polarization technique. Corrosion rates were observed to be highly sensitive to tempering time in the QPT heat–treatment process. Variations in corrosion rate with tempering times are plotted in ﬁgure 7, while corresponding potentiodynamic polarization curves are illustrated in ﬁgure 8. The observed trend was asymmetrical in which non–heat–treated sample exhibited a moderate value of corrosion rate of 5.518 mpy. This is because in non–heat–treated form experimental steel contained ferrite– pearlite phases. It has been reported that the ferrite phase is highly susceptible to corrosion attack and increases the corrosion rate of steel [40]. Variations in carbon concentration in different phases i.e. ferrite–pearlite and bainite create microstructural inhomogeneity and result in galvanic or bi–metallic corrosion cell formation. Ferrite is an active phase so it acts

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Figure 6. Plot of variations in Rm×εT factor of QPT experimental steel as a function of tempering time.

Figure 7. Plot of variations in corrosion rate of QPT heat–treated experimental steel in 3.5% NaCl solution as a function of tempering time.

as an anode while pearlite as cathode [41]. So, the dissolution of steel begins with the dissolution of the ferrite phase by the following reactions: Fe+ Fe2+- 2e Fe+ Fe3+- 3e On cathodic site, as 3.5% NaCl electrolyte is a neutral solution, so the cathodic reaction is [42]: -- 2HO22++ 2e H 2OH So, the overall reaction will be +-2 Fe+ OH Fe() OH 2 or +-3 Fe+ OH Fe() OH 3 But there may be a possibility of ferric chloride or ferrous chloride due to the presence of excessive chloride ions in the solution [40]. +-3 Fe+ Cl FeCl3

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Figure 8. Potentio–dynamic polarization curves of QPT heat–treated experimental steel in 3.5% NaCl solution. or +-2 Fe+ Cl FeCl2 The pearlite is a laminar microstructure of ferrite and cementite, so it formed a localized galvanic cell between ferrite and cementite. Similarly, bainite is a non–laminar mixture of ferrite and cementite. The ferrite phase becomes the anode and the cementite phase act as cathode resulting in a further increase in the dissolution of iron from the plain carbon steel [43]. The maximum corrosion rate was resulted by sample tempered for 30 s, attributed to the smaller grain size of the martensite phase formed by the quenching process and availability of a greater number of attacking sites. On the other hand, sample tempered for 90, 110, and 150 s provided much lower corrosion rates. These lower corrosion rates are attributed to the diffusion of carbon from martensite and bainite to retained austenite. Supersaturated martensite underwent a greater corrosion attack. With the diffusion of carbon from martensite, carbon depleted martensite formed which underwent much less corrosion attack and resulted in a much lower corrosion rate.

4. Conclusions

Microstructural, mechanical and electrochemical properties of QPTexperimental steel were investigated as a function of tempering time. Following were the extracted conclusions;

Tempering for 30 s caused the formation of a high volume fraction of martensite with low fractions of retained austenite and bainite. An increase in tempering time caused diffusion of carbon from both martensite and bainite to retained austenite. Fully stabilized retained austenite with volume fractions of martensite and bainite were obtained at 110 s of tempering. Tempering for 30 s signiﬁcantly improved (18%) Vickers hardness, (73%) Rm at a little reduction (12%) of

εT compared to non–heat–treated steel. Increase in tempering time, offered reduced hardness, improved strength, and improved elongation compared to the non–heat–treated steel. An optimum combination of mechanical properties was achieved at 110 s of tempering. The highest corrosion rate was resulted by a sample which was tempered for 30 s. While all the other samples resulted in a much lower corrosion rate compared to non–heat–treated steel sample. Tempering period of 110 s resulted in lowest corrosion rate of experimental steel after QPT heat–treatment.

ORCID iDs

Aqil Inam https://orcid.org/0000-0001-6112-6131 Muhammad Arslan Hafeez https://orcid.org/0000-0001-7337-7595

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Waseem Haider https://orcid.org/0000-0003-4235-3560

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