mechanical properties of AISI 6150 steel€¦ · 870 °C, 45 min is the best quenching condition for the AISI 6150 steel. In the present work, all t he samples were austenitized at

J. Cent. South Univ. (2013) 20: 866−870 DOI: 10.1007/s11771­013­1559­y

Effect of tempering temperature on microstructure and mechanical properties of AISI 6150 steel

LI Hong­ying(李红英) 1 , HU Ji­dong (胡继东) 1 , LI Jun(李俊) 2 , CHEN Guang(陈广) 1 , SUN Xiong­jie(苏雄杰) 1

1. School of Materials Science and Engineering, Central South University, Changsha 410083, China; 2. Research Institute, Baoshan Iron & Steel Co. Ltd, Shanghai 201900, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2013

Abstract: Effect of tempering temperature on the microstructure and mechanical properties of AISI 6150 steel was investigated. All samples were austenitized at 870 °C for 45 min followed by oil quenching, and then tempered at temperatures between 200 and 600 °C for 60 min. The results show that the microstructure of tempered sample at 200 °C mainly consists of tempered martensite. With increasing the tempered temperature, the martensite transforms to the ferrite and carbides. The ultimate tensile strength, the hardness and the retained austenite decrease with increasing tempered temperature, and 0.2% yield strength increases when the temperature increases from 200 to 300 °C and then decreases with increasing the temperature, but the elongation and impact energy increase with increasing the tempering temperature.

Key words: tempering temperature; AISI 6150 steel; microstructure; mechanical property

Foundation item: Project(2011BAE13B03) supported by the National Key Technology R&D Program of China Received date: 2012−09−24; Accepted date: 2013−01−17 Corresponding author: LI Hong­ying, Professor, PhD; Tel/Fax: +86−731−88836328; E­mail: [email protected]

1 Introduction

AISI 6150 steel is a fine grained, highly abrasion­resistant chromium­vanadium alloy steel, which has high strength, high fatigue strength and large hardenability. In recent years, AISI 6150 steel has been commonly used in many industries, particularly the automotive industry [1−2]. However, some industries demand higher mechanical properties and longer service life of the materials. In order to improve the mechanical properties of AISI 6150 steel, some modifications to the steel must be required. Considerable efforts have been directed toward improving the mechanical properties by compositional modifications and various heat­treatment techniques. The small addition of Cr, V, Ni and Mn elements has improved its mechanical properties. Then, the technique of heat­treatment should be employed to improve the mechanical properties of AISI 6150 steel.

Generally, quenching and tempering are well­established means to produce strengthening in steel, while at the same time retaining or even increasing its impact toughness. This is mainly due to the martensitic structure produced by quenching and the subsequent precipitation of the fine carbides during the tempering process [3−4]. However, as large internal stresses associated with the martensitic transformation cause the material to be lack of ductility, the martensite steel is

rarely used in non­tempered condition. Tempering can increase both the ductility and toughness, which are essential for enhancing impact energy absorption. And tempered martensite lath structure also provides best dynamic strength in steel [5]. In order to explore and understand the effect of heat treatment processes on the microstructure and mechanical properties of the materials, many researchers have done a lot of work over the past few years [6−11]. SAYED et al [6] have studied the effect of the tempering temperature on the microstructure and mechanical properties of dual phase steels, and the results showed that tempering at temperatures lower than 300 °C for 1 h was suitable for attaining optimum strength and ductility. QIN et al [7] have researched the properties of 0Cr16Ni5Mo stainless steels which were normalized at 1 000 °C followed by tempering in the temperature range of 525−625 °C. The results showed that the samples tempered at 550 and 600 °C for 2 h had an excellent combination of tensile strength, elongation, impact energy, hardness and corrosion resistance. SALEMI et al [8] have explored the effect of tempering temperature on the mechanical properties and fracture morphology of a NiCrMoV steel, and it was found that the tensile strength decreased and the ductility increased with the increasing of tempering temperature.

However, very limited efforts have been directed to the effects of the heat treatment on the AISI 6150 steel.

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Therefore, in order to find out the effect of tempering temperature on the microstructure and mechanical properties of the steel, in the present work, the tensile properties, hardness, impact toughness and microstructures of tempered samples at different temperatures were studied.

2 Experimental

The chemical composition of medium­carbon AISI 6150 steel used in the present work is given in Table 1. Before this work, we had conducted a series of experiments at quenching temperatures of 800−1 000 °C for various holding time of 15−120 min, and found that 870 °C, 45 min is the best quenching condition for the AISI 6150 steel. In the present work, all the samples were austenitized at 870 °C for 45 min, followed by oil quenching to produce the martensite, and then tempered at 200−600 °C with the internal of 50 °C. Compared with the previous works conducted, it was found that tempering time of 60 min was the best for the steel.

Table 1 Chemical compositions of AISI 6150 steel (mass fraction, %) C Cr V Si Mn Ni P S Fe 0.53 1.02 0.15 0.23 0.83 0.03 0.01 0.01 Bal.

Before the microstructures of samples were observed by scanning electron microscopy (SEM), they were mechanically polished and etched with 4% nital solution. The phase constituents in samples were analyzed by a D/max­2500 X­ray diffractometry (XRD), which was operated at 40 kV and 40 mA with scanning

speed of 0.5 (°)/min. The volume fraction of retained austenite in the steel was determined by comparing the integrated X­ray diffraction intensities of the ferrite and austenite phases with the theoretical intensities.

The mechanical properties of the heat­treated samples were evaluated by tensile test, impact toughness test and hardness test. The tensile specimens were machined from the heat treated samples in parallel to the rolling direction. The tensile test was conducted on an Instron Universal Testing Machine (Instron 5581) at room temperature according to ISO 6892—2009. The test speed was 30 mm/min, which corresponded to a strain rate of 10 −2 s −1 . At least, five specimens were tested and average values were calculated for each condition. Key parameters obtained from stress−strain curves include 0.2% yield strength (YS), ultimate tensile strength (UTS), and percentage elongation. Standard Charpy V­notched specimens with the size of 55 mm×10 mm×10 mm was machined to find out the impact properties. At least five samples of each heat treatment process were tested and their average value was taken as the impact toughness value of plates under those conditions. The Vickers hardness of all specimens was measured under 5 N applied load for 10 s. The average hardness of one sample was reported from measurements over 10 locations.

3 Results

3.1 Microstructure The microstructure of the steel in quenched

condition is shown in Fig. 1(a), where the microstructure consists of the martensite (M). The microstructures of

Fig. 1 SEM micrographs of heat treated samples: (a) As­quenched sample; (b) Tempered at 200 °C; (c) Tempered at 400 °C; (d) Tempered at 600 °C

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samples tempered at 200−600 °C for 60 min are shown in Figs. 1(b)−(d). Tempering is a diffusion type phase transformation from the quenched martensite to the tempered martensite (M), ferrite (F) and carbides (C). The microstructure of the sample tempered at 200 °C (Fig. 1(b)) mainly consists of the lath martensite, which differs slightly from the quenched sample. Both the quenched martensite and tempered martensite are of lath shape. With the increase of the tempering temperature, the lath martensite transforms to ferrite and carbides. Once the tempering temperature increases to 400 °C, the microstructure of the tempered sample is clearly different from that of the quenched sample. Figure 1(c) shows that the microstructure of sample tempered at 400 °C consists of lath martensite, ferrite and rod­shape carbides. When the tempering temperature increases to 600 °C, the microstructure of the tempered sample mainly consists of ferrite and carbides, and the carbides change their shape from rod to spheroidal shape.

3.2 Tensile properties The tensile properties of the AISI 6150 steel

samples depend on the tempering temperature. The properties of the quenched sample are also evaluated for comparison. The variations of ultimate tensile strength (UTS), 0.2% yield strength (YS) and elongation as a function of tempering temperature are shown in Fig. 2. It can be seen that the variations of tensile properties with temperature consist of several stages: 1) With the tempering temperature increasing up to 200 °C, the UTS and elongation slightly increase, and YS slightly decreases; 2) From 200 to 300 °C, the UTS decreases by about 100 MPa, while the YS rises sharply and reaches a peak value of 1685 MPa, and the elongation increases by about 1.2%; 3) From 300 to 600 °C, the UTS decreases sharply from 1 946 to 1 115 MPa, and the YS decreases continuously from 1 685 to 1 050 MPa, while elongation rises sharply and continuously from 7.5% to 15.9%.

Fig. 2Variations of tensile properties with tempering temperature

3.3 Impact energy Impact energy gives a good indication of the energy

required to initiate and propagate a crack. The variation of impact energy with tempering temperature is shown in Fig. 3. As can be seen from Fig. 3, the impact energy of the quenched sample is 10 J. With the tempering temperature increasing up to 200 °C, the impact energy rises to 16 J. However, the impact energy changes slightly after tempering at 200−350 °C. With further increasing tempering temperature from 350 to 600 °C, the impact energy increases continuously and reaches 43 J at the tempering temperature of 600 °C.

Fig. 3Variation of impact energy with tempering temperature

3.4 Hardness Figure 4 exhibits the influence of tempering at

various temperatures for 60 min on the average value of Vickers hardness. It can be seen that the hardness of AISI 6150 steel gradually decreases from HV 608 to HV 360 with increasing the tempering temperatures in the range of 200−600 °C. When comparing the results of samples tempered in various temperature ranges, it is found that the rate of decrease of the Vickers hardness in lower tempering temperature range of 200−500 °C is higher than that in temperature range of 550−600 °C. Moreover,

Fig. 4Variation of hardness with tempering temperature

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the hardness of the steel without tempering is the highest and reaches HV 689.

4 Discussion

As can be seen from Figs. 2−4, the mechanical properties of AISI 6150 steel are quite sensitive to the tempering temperature. Compared with the samples tempered at different temperatures, the strength and hardness of quenched sample are the highest, but the ductility is the lowest. This can be explained based on the phase transformation of steel during the quenching process, where the lattice structure of the steel changes immediately from a face­centered cubic to a body­centered tetragonal phase. Martensite formation is accompanied by a large amount of distortion, which rapidly increases the strength and hardness of steel. However, the internal stress generated during martensite formation causes significant reduction of the ductility and toughness.

Tempering process relieves the internal stresses across the lath boundaries by permitting local rearrangement of atoms [11]. Below the tempering temperature of 300 °C, internal stresses generated are not fully released. With complete recovery of stresses at tempering temperature of 300 °C, a rearrangement of the dislocation structure takes place, which restricts their movement and leads to an increase in 0.2% yield strength. With increasing the tempering temperature, the concentration of the tempered martensite decreases and the presence of the ferrite and carbides increases, mainly due to the diffusion of carbon atoms into cementite and the movement of dislocations by thermal assistance [12]. Therefore, the ultimate tensile strength and 0.2% yield strength decrease and the elongation increases.

AISI 6150 steels are susceptible to tempered martensite embrittlement (TME) within a specified temperature range. The phenomenon is usually characterized on a plot of impact energy as a function of tempering temperature. And it can be observed that a maximum in the ductile−brittle transition temperature corresponding to the minimum in the impact energy [13]. It is seen from Fig. 3 that the impact energy increases with increasing the tempering temperature.

Figure 5 depicts the XRD patterns of the tempered sample at 200 °C for 60 min. The peaks represent the diffraction intensity of different crystal orientations of the ferrite. The volume fraction of retained austenite is achieved by the result of Rietveld spectrum fitting of the tempered samples after XRD phase testing. Figure 6 shows the variations of retained austenite with tempering temperature. As can be seen in Fig. 6, the quenched sample contains the maximum amount of retained austenite. With the tempering temperature increasing up

to 200 °C, the volume fraction of retained austenite varies from 3.60% to 2.93%. When the tempering temperature increases from 200 to 300 °C, the volume fraction of retained austenite sharply decreases to 0.62%, mainly due to the fact that most of the retained austenites transform to tempered martensite after tempering at these temperatures. These lead to an increase in 0.2% yield strength and elongation, and a slight decrease in ultimate tensile strength. The retained austenite decreases slightly in the tempering temperature range of 300−500 °C. At tempering temperature above 550°C, the volume fraction of retained austenite is close to zero.

Fig. 5 XRD pattern of tempered sample at 200 °C

Fig. 6Variation of retained austenite with tempering temperature

According to the Hollomon­Jaffe relation of the hardness and a tempering parameter M: H=f(M)= f(T(t+lg t)) [14], it is known that the hardness is affected by the tempering temperature if the tempering time is a constant. Tempering can be considered as a phase transformation promoted by diffusion from an unstable state towards a quasi equilibrium state. Therefore, the hardness can be used to define any tempering state. As the evolutions of the tempering time and temperature are

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also controlled by diffusion mechanisms (particularly carbides precipitation and growth), a tempering kinetic law is proposed in the form of the Johnson­Mehl­Avrami type equation [15]:

v 0 0 ( ) exp( ( ) ) m H H H H D t ∞ = + − − ⋅ (1)

where H0 is the hardness after quenching, H∞ is the hardness in the tempered state, Hv is the hardness of an intermediate state between the quenched state and the tempered state, t is the tempering time, m is the ageing exponent depending on the material and the previous heat treatment, and D depends on tempering temperature and follows the Arrhenius equation:

D=D0exp[−Q/(RT)] (2)

where D0 is the pre­exponential constant, Q is the activation energy of the tempering transformation, R is the perfect gas constant (equal to 8.31 J/(K⋅mol −1 )) and T is the tempering temperature in K. According to the Eq. (1) and Eq. (2), if the activation energy of tempering transformation and tempering time remain constant during the tempering process, it can be calculated that the hardness of the AISI 6150 steel decreases with the increase of the tempering temperature. These calculated results closely agree with the measured experimental values of Fig. 4.

5 Conclusions

1) At low tempering temperature of 200 °C, the tempered microstructure mainly consists of lath martensite. With increasing the tempered temperature, the martensite becomes less and transforms to ferrite and carbides. The microstructures of the AISI 6150 steel consist of ferrite and carbides at the high tempering temperature of 600 °C.

2) Increasing the tempering temperature decreases the ultimate tensile strength and the hardness, but increases the elongation and impact energy. However, 0.2% yield strength increases when the temperature increases from 200 to 300 °C and then decreases with increasing the temperature.

3) The retained austenite sharply decreases at the tempered temperatures of 200−300 °C, and then decreases slightly in the temperature range of 300−500 °C. When the tempering temperature is higher than 550 °C, the content of retained austenite decreases to zero.

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(Edited by YANG Bing)