Effect of Cooling Rate on Microstructure and Mechanical Properties in the CGHAZ of Electroslag Welded Pearlitic Rail Steel

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.

Characterization and Prediction of Continuous Cooling Transformations in Rail Steels

“…Figures 8 to 10 highlight the fittings of Equations 3 and 4 to the experimental data obtained for steels A, B and C respectively. As expected 26,33,37,43 , the experimental data, as well as the fitted equations, highlighted that the higher the cooling rate, the lower the total time required for complete transformation.…”

“…During the pearlite to austenite diffusional transformation, the carbon diffusion occurs at average distances around the pearlite interlamellar spacing. As a result, the growth rate of austenite from pearlite increases as the interlamellar spacing decreases [41][42][43] . Besides that, Zhang et al 44 stated that in high carbon steels, the increase of Mn and Cr contents difficult the cementite dissolution, delaying the steel austenitizing.…”

Kinetics of Isothermal Phase Transformations in Premium and Standard Rail Steels

“…[ 27 ] The growth rate of austenite from pearlite increases as the interlamellar spacing diminishes. [ 27–29 ] It was also shown that for coarse pearlitic microstructures, pearlite–austenite transformation starts at higher temperatures. [ 30 ] In addition to that, some authors affirm that in high‐carbon steels, the Mn and Cr additions make the dissolution of cementite difficult, thus delaying the complete austenitizing of the steel.…”

Tarımsal mekanizasyonda kullanılan 30MnB5 bor çeliğinin kaynak sonrası soğutma rejiminin mekanik ve mikroyapı özelliklerine etkisi

“…Bu durumda da kaynak bölgesinin soğuması sırasındaki dönüşümler daha düşük sıcaklıklarda gerçekleşmektedir. Dönüşümün daha düşük sıcaklıklarda gerçekleşmesi yapıda büyük oranda martenzit ve beynitten oluşan bir yapı ortaya çıkartmaktadır[33,40,44,46]. Soğutma rejimine bağlı olarak soğuma hızındaki artış kaynak bölgesinde martenzit ve çok ince tane sınırı bor karbürlerin oluşmasına neden olduğu ve bu durumda 30MnB5 çeliğin IEB'lerindeki yüksek sertlik değerlerine etki ettiği ve arttırdığı düşünülmektedir.…”