Materials Research Express 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 View the article online for updates and enhancements. This content was downloaded from IP address 141.209.33.91 on 04/02/2020 at 20:48 Mater. Res. Express 6 (2019) 126509 https://doi.org/10.1088/2053-1591/ab52b7 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]. © 2019 IOP Publishing Ltd Mater. Res. Express 6 (2019) 126509 A Inam et al 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 2 Mater. Res. Express 6 (2019) 126509 A Inam et al 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