Effect of Mn/Cr microsegregation on martensite formation and mechanical degradation in flash-butt welded U75V rail steel

Flash-butt welding is the primary joining technique for continuous welded rail, yet the weld center zone (WCZ) of high-carbon U75V rail steel remains susceptible to localized microstructural degradation. This study investigates the effect of Mn/Cr microsegregation on martensite formation and mechanical property deterioration in as-welded flash-butt joints using EPMA, FESEM, EBSD, and microhardness mapping. EPMA elemental mapping reveals pronounced Mn and Cr enrichment in interdendritic regions, with segregation indices of 1.82 ± 0.11 and 4.83 ± 0.92, respectively (n = 15). Spatial overlay analysis confirms that martensite islands (area fraction 10.7 ± 2.0%, range 8.0–14.0%) are confined to Mn/Cr-enriched zones, while dendrite cores with nominal composition exhibit normal pearlite. JMatPro CCT calculations indicate that the combined Mn/Cr enrichment shifts the pearlite nose by approximately 50 °C and one order of magnitude in time, causing the segregated regions to bypass pearlite transformation and form high-carbon martensite under actual weld cooling rates. The resulting martensite islands (absolute hardness 620–710 HV) create a local hardness difference ΔHV = 280–360 against the adjacent pearlite matrix (absolute hardness 330–370 HV), reducing joint fracture efficiency to 87.5 ± 1.9% (n = 3, p < 0.001) and impact energy by 48.5% (n = 3, p < 0.01). Fracture surface EDS analysis confirms preferential crack initiation at Mn/Cr-enriched martensite locations. These findings establish the complete mechanism chain from interdendritic microsegregation through localized martensite formation to mechanical degradation, providing a microstructural basis for flash-butt welding process optimization.