The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel

https://doi.org/10.1016/j.msea.2009.07.042Get rights and content

Abstract

4340 steel bars were austenitized at 850 °C for 1 h followed by heating at 700 °C for 90 min and quenching into a salt bath at the temperature range of 300–450 °C for 1 h to obtain dual structures with 34 vol.% fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness tests revealed that increasing the austempering temperature from 300 to 400 °C leads to a reduction in yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in dual phase steel produced by austempering at 450 °C, yield and tensile strength and hardness increased and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit decreases with increasing austempering temperature from 300 to 400 °C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400 °C, which confirms the tendency to brittle behavior.

Introduction

Due to the superior mechanical properties such as continuous yielding behavior, appropriate combination of strength–ductility and better formability and excellent surface finish in contrast to other HSLA steels of similar chemical composition, dual phase steels have been developed over the past few decades [1]. The main reason for using DP steels in high strength applications such as automobile and aerospace industries is reduced cost [2]. These steels contain a high strength phase (martensite or bainite) and a softer matrix (ferrite) [3]. Because of some problems such as local necking in heat affected zone (HAZ), low stretch-flange formability and a special production method for ferrite–martensite steels, bainite becomes a good substitution for DP steels [4].

Intercritical annealing is a conventional heat treatment to produce ferrite–bainite microstructures which contains heating in α + γ region for a given time and then cooling by an appropriate rate to a temperature above Ms (austempering temperature) for transformation of austenite to bainite [5]. Yazici et al. [6] studied low carbon steel to produce DP steels under different heat treatments. Bainite morphology is a function of austempering temperature; at temperatures above and close to Ms, lower bainite and at temperatures close and below pearlite transformation temperature, upper bainite forms [7]. Therefore, bainite morphology and its mechanical properties vary with austempering temperature. Mechanical properties of lower bainite are superior because of the finer and more uniform carbide distribution, higher internal stress, high dislocation density, amount of carbon dissolved in bainitic ferrite, and fine bainitic ferrite grains [8]. Lin [9] and Zhang et al. [10] reported that austempering at low-temperature bainite regime gives martensite/lower bainite dual microstructure with good combination of mechanical properties. Recently, Wen et al. [11] studied the effect of austempering temperature and time on microstructural and mechanical properties of a GCr18Mo steel and showed that austempering treatment dramatically increases strength–toughness to 1.5–2.5 times. Saxena et al. [12] surveyed on austempering of a medium-carbon–manganese steel by modifying austenitizing temperature and cooling rate. They showed that uniformly distributed ferrite with fine pearlite and upper bainite with 12–16% volume fraction of ferrite resulted in the best combination of tensile properties. However, few investigations have been carried out on microstructure-properties relationship of ferrite–bainite steels. In some cases, it is reported that increase in bainite content generally increases their yield ratio (ratio of yield strength to tensile strength), reduction of area and fatigue endurance limit [13], [14]. Tomita [15] studied Fe–0.1C steel to determine the effect of shape, size and distribution of martensite on tensile properties of DP steels. Tayanc et al. [16] reported the highest fatigue strength of a DP steel obtained at intercritical annealing temperature of 760 °C.

Previous work by one of the authors showed that bainite–0.34 ferrite microstructure gives the best mechanical properties [17]. In the present research, high bainite dual phase steels with 34% ferrite were produced by austempering at different temperatures. The effect of bainite morphology on mechanical properties of these steels was considered.

Section snippets

Materials and experimental procedure

A low-alloyed medium-carbon steel were used in this investigation in the form of 38 mm diameter rod. Its chemical composition is shown in Table 1 which is consistent to AISI4340 steel. Microstructure of as-received material was ferrite and pearlite (Fig. 1).

To obtain ferrite–bainite microstructures, the samples were austenitized at 850 °C for 1 h and then directly transferred to 700 °C and held for 1.5 h at this temperature. The samples then soaked and held 1 h in a salt bath with different

Ferrite–bainite microstructures

Fig. 2(a)–(d) shows SEM micrographs of produced microstructures with different bainite morphology. The darker phase is ferrite and the whiter one is bainite. From Fig. 2(a) it can be seen that the aspect ratio of bainitic ferrite blades is high which confirms the needle-like lower bainite. In some regions, parallel needle-like blades with different orientation can be seen. In Fig. 2(b) (350 °C austempering temperature), bainitic ferrite aspect ratio is smaller than that of 300 °C austempering

Conclusions

Ferrite–bainite microstructures with different bainite morphologies were obtained by appropriate heat treatment of AISI4340 steel. Morphology of bainite was studied by SEM. Hardness, tensile, impact and fatigue tests were done, and fractography studies were performed on fracture surfaces of tensile and fatigue tested specimens. From the results, the following conclusions can be drawn:

  • Bainite morphology in microstructures produced at 300 and 350 °C austempering temperatures was lower and mixed,

Acknowledgment

The authors would like to acknowledge Sharif University of Technology (SUT) for financial supports of this research.

References (29)

  • U. Liedl et al.

    Comp. Mater. Sci.

    (2002)
  • S. Sun et al.

    Mater. Sci. Eng. A

    (2002)
  • X.J. Jin et al.

    Mater. Sci. Eng. A

    (2006)
  • J. Wen et al.

    Mater. Sci. Eng. A

    (2006)
  • A. Saxena et al.

    Mater. Sci. Eng. A

    (2006)
  • M. Tayanc et al.

    Mater. Des.

    (2007)
  • S.A. Sajjadi et al.

    J. Mater. Process. Technol.

    (2007)
  • A. Salemi et al.

    Mater. Character.

    (2008)
  • A.J. Trowsdeles et al.

    Metal. Trans. A

    (1983)
  • H.K.D.H. Bhadeshia

    Bainite in Steels: Transformation, Microstructure and Properties

    (2001)
  • C.A.N. Lanzillotto et al.

    Metal. Sci.

    (1982)
  • M. Yazici et al.

    Materialprüfung

    (2003)
  • F.B. Pickering

    Physical Metallurgy, Design of Steels

    (1978)
  • X. Lin

    Mech. Eng. Mater.

    (2000)
  • Cited by (0)

    View full text