Mitigating the hardness drop due to divorced eutectoid transformation in pearlitic steels: Influence of the cooling rate

This master thesis investigates strategies to reduce hardness loss caused by the divorced eutectoid transformation (DET) during welding of pearlitic rail steel. The study focuses on the influence of cooling rate on DET kinetics and the resulting hardness. To this end, dilatometer experiments were conducted at three austenitization temperatures (TA): 740, 770, and 800 °C, which represent characteristic locations within the heat-affected zone (HAZ), each investigated across a range of cooling rates. Based on these experiments, continuous cooling transformation (CCT) diagrams were developed for each TA. A combination of micro- and nanohardness testing, optical and scanning electron microscopy (OM and SEM), X-ray diffraction (XRD), and particle analysis was used to characterize the resulting microstructures, phase distributions, and carbide morphologies. Additionally, high-temperature laser scanning confocal microscopy (HT-LSCM) enabled in-situ observation of grain growth and transformation dynamics. Supplementary experiments further examined the effect of extended holding times and transformation behavior at lower TA.
The results show that higher cooling rates increase the pearlite fraction, enhancing hardness. Fine pearlite with narrow lamellar spacing is key to noticeable hardness improvement, but achieving full hardness recovery requires mixed microstructures with martensite. The higher the TA, the less martensite is needed to restore hardness. At TA = 800 °C, pearlite alone can recover a large portion of the hardness loss, while at TA = 740 °C, martensite or bainite is required. Another key observation was the uneven distribution of undissolved cementite particles, which led to varying carbide spacings. These variations influence DET kinetics and result in a broad transition range rather than a sharp boundary between DET and pearlite. Supplementary tests showed that prolonged holding at TA increases carbide spacing and thereby promotes pearlite formation. However, moderately increased cooling rates are still required, as only pearlite with narrow lamellar spacing leads to a meaningful increase in hardness.