Effect of heat treatment on microstructure and hardness of a worn rail repaired using laser powder deposition

Abstract

• Laser powder deposition is a highly recommended technique to repair worn rails. • 304L deposition steel gives a standard, martensite-free microstructure. • Heat treating the repaired rail increases hardness and eliminates the micropores. • Preheating may mitigate the developed microcracks at the rail-deposition interface. The frequent replacement of worn rails on tracks brings an immense economic burden on the railroad industry, and also causes significant interruptions to railroad operation. Restoration of worn rails via laser powder deposition (LPD) can considerably reduce the associated maintenance costs. This study was focused on the use of LPD to repair the worn profile of a standard U.S. rail. The microstructure of the 304L stainless steel deposits with a minimum hardness of 85 HRB was composed of austenite, δ-ferrite, and sigma. Micropores were dispersed throughout the deposit, and microcracks were found at the rail-deposition interface. The pearlitic rail substrate showed a moderate hardness of 94 HRB. The fine-grain, pearlitic-ferritic heat affected zone had the maximum hardness of 96 HRB, which was still below the minimum required hardness of 97 HRB for a typical rail. To increase the hardness to or above 97 HRB and to mitigate the microstructural defects, the as-repaired rail went through a heat treatment process. The average hardness of the as-treated rail was increased significantly, i.e., to 103 HRB. Besides, the porous and coarse-grain deposition materials were transformed into an impermeable and fine-grain microstructure. However, heat treatment intensified the microcracks at the rail-deposition interface and also led to the formation of martensite and augmentation of the micropores in the parent rail. Isolation of the base rail during heat treatment and preheating were suggested as solutions for the problematic results. The LPD process ultimately was found to be a promising technique for repairing rails.