Mechanical properties of SG-iron subjected to variable and isothermal austempering temperatures heat treatment

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Abstract

Mechanical properties of spheroidal graphite (SG)-iron subjected to variable and isothermal austempering temperatures heat treatment have been examined. Variable austempering temperature heat treatment is carried out by austenitizing at 1183 K then quenching into a salt bath held at 593 and 723 K, respectively. After quenching, the former is steadily heated to 723 K while the latter is allowed to cool progressively to 593 K. The tensile properties, impact toughness and hardness are determined and correlated with the microstructure. It has been found that, specimens quenched at 593 K and heated steadily to 723 K have higher 0.2% yield stress, ultimate tensile strength and hardness, but much less ductility than those quenched at 723 K and cooled progressively to 593 K. The elongation of specimens austempered under variable austempering temperature is significantly improved. It reached about three times of that of specimens austempered isothermally at 593 or 723 K.

Introduction

The mechanical properties of spheroidal graphite (SG) cast iron can be greatly enhanced by austempermg heat treatment [1], [2], [3], [4], [5]. These properties arise from the microstructure consisting of SG embedded in a metallic matrix. The SG size, count and distribution can be controlled through melt processing [6], [7], [8], [9], while the constitution of the metallic matrix is controlled by austempering heat treatment [10], [11], [12], [13].

Austempering is usually carried out isothermally at temperatures between 523 and 723 K, and different grades of austempered SG-iron can be obtained [5], [14]. Depending on the austempering temperature some grades of austempered SG-iron can be used for applications in which wear resistance is of primary importance. On the other hand, the other grades of austempered SG-iron are suitable for engineering applications where considerable ductility and toughness are required.

The present investigation aimed at examining an alternative heat treatment that may produce a material with a combination of mechanical properties, which cannot be obtained with a conventional austempering heat treatment. A variable austempering temperature is described. The mechanical properties is evaluated and correlated with the microstructure.

Section snippets

Experimental procedure

An SG cast iron with the chemical composition reported in Table 1 has been produced in a 50 Hz frequency induction furnace. The melt has received a spheroidization treatment, in ladle, according to ‘Sandwich’ method [15] at 1763 K, using a ‘45Fe–50Si–5Mg’ alloy. Once the spheroidization treatment has been completed the molten metal is cast into Y-shaped sand mould. The dimensions of each ingot are 40×90×210 mm3, in the parallel portion. After pouring the cast ingots are left to cool, in the

Microstructure

Fig. 5 shows the micrographs of specimens austempered isothermally at 593 and 723 K for 5.4 ks, after austenitizing at 1183 K for 3.6 ks, respectively. At austempering temperature 593 K, Fig. 5a, very fine needles of ferrite are observed with a small amount of retained austenite in between. This is because at low austempering temperature, due to high undercooling a high nucleation rate results in a large number of fine ferritic needles. This microstructure represents the lower ausferrite

Fractography

Fig. 10 shows the fracture surfaces of austempered SG-iron under different austempering heat treatment cycles followed in the present investigation. At a glance, decohesion of the SG can be seen to be the common feature in all cases. As shown in Fig. 10a, specimen austempered isothermally at 593 K for 5.4 ks, shallow dimples and cleavage fracture pattern could be observed. On the other hand, fine dimples and less areas of cleavage fracture are the characteristics of specimen austempered

Conclusions

(1) Specimens quenched from 1183 to 593 K and steadily heated to 723 K give a higher strength and hardness compared to those quenched at 723 K and steadily cooled to 593 K. However, the ductility and toughness of the later is higher than these of the latter.

(2) Specimens quenched from 1183 to 723 K and steadily cooled to 593 K give a slight increase in strength with a significant improvement in the ductility and toughness compared to those quenched at 723 K and isothermally held for 5.4 ks.

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