Abstract
The effect of cooling rate, ranging from 6 to 1 °C/s, on microstructure and mechanical properties in the coarse-grained heat affected zone (CGHAZ) of electroslag welded pearlitic rail steel has been investigated by using confocal scanning laser microcopy (CSLM) and Gleeble 3500 thermo-mechanical simulator. During heating, the formed austenite was inhomogeneous with fractions of untransformed ferrite, which has influenced the pearlite transformation during cooling by providing additional nucleation sites to pearlite. During cooling, at 6 °C/s, the microstructure was composed of martensite and bainite with little pearlite. From 4 to 1 °C/s, microstructures were completely pearlite. Lowering the cooling rate of the CGHAZ from 4 to 1 °C/s increased the pearlite start temperature and reduced the pearlite growth rate. Meanwhile, this increase in pearlite start temperature enlarged the pearlite interlamellar spacing. Alternatively, increasing pearlite interlamellar spacing in the CGHAZ by lowering the cooling rate from 6 to 1 °C/s reduced the hardness and tensile strength, whereas toughness was found unaffected by the pearlite interlamellar spacing. It has been found that a cooling rate of 4 °C/s leads to the formation of pearlite with fine interlamellar spacing of 117 nm in the CGHAZ of electroslag welded pearlitic rail steel where hardness is 425 HV, tensile strength is 1077 MPa, and toughness is 9.1 J.
Highlights
Continuous welded pearlitic rails (CWR) are widely used in railroad applications where thermite welding and flash butt welding are the joining techniques [1,2,3,4,5]
The proposed thermal cycles were chosen based on experimentally measured thermal cycle in the coarse-grained heat affected zone (CGHAZ) during electroslag welding of pearlitic rail steel
It has been observed that, even at the peak temperature of 1302 ◦ C there are some traces of parent austenite grain boundaries, as shown in Figure 3f, which are due to the presence of untransformed ferrite
Summary
Continuous welded pearlitic rails (CWR) are widely used in railroad applications where thermite welding and flash butt welding are the joining techniques [1,2,3,4,5]. Electroslag welding is an alternative welding process where by changing the welding parameters a wide range of cooling rates can be achieved, leading to a variety of microstructures and mechanical properties. Micenko and Li infer that in the welding of pearlitic rail steel an ideal microstructure in the CGHAZ would be pearlite with fine interlamellar spacing, which should be the consequence of a specific cooling rate [9]. This requires a fundamental and comprehensive understanding of microstructure formation and pearlite interlamellar spacing that occur in the CGHAZ under different cooling rates
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