With the continuous increase in speed and axle load of heavy haul trains, the deterioration of bainitic steel rails is exacerbating, posing serious safety risks to railway transportation. The prompt resolution of surface damage repair issues of bainitic steel rails is imperative. While surface additive manufacturing repair methods such as laser deposition are suitable for damages of various sizes and exhibit good bonding between the repair layer and base material, they tend to form a large amount of brittle martensite structure in the repair layer and heat-affected zone. By employing preheating treatment temperature field control methods before laser deposition, the formation of martensite during the laser deposition process can be hindered. Combined with subsequent isothermal heat treatment, a repair layer primarily consisting of bainitic structure can be achieved, potentially leading to high-quality, cost-effective, and efficient surface repair of bainitic steel rails. This study systematically investigated the microstructural evolution and mechanical properties of laser-deposited ultra-high-strength steel repair layers under different temperature field conditions (laser deposition state, preheated at 280 ℃ and 320 ℃), The focus was on analyzing the effects of different laser deposition thermal cycling processes on the relative proportions of bainite/martensite structures and substructural characteristics in laser-deposited ultra-high-strength steel, to elucidate the relationship between microstructural features and mechanical properties. These research findings have certain guiding significance for practical applications of laser deposition in heavy-haul steel rail repairs.

Utilizing laser deposition technology, a test steel repair layer is fabricated on the 30CrMnSiA medium carbon alloy steel substrate, using the compositions of the substrate and test steel detailed in Table 1. A constant-temperature preheating stage is positioned beneath the substrate (as shown in Fig. 1) to maintain the temperature of the substrate and deposited layers above the designated preheating temperatures of 280 ℃ and 320 ℃. Following the completion of each metal deposition layer, the sample is allowed to cool to the specified preheating temperature before commencing deposition of the subsequent metal layer, followed by natural cooling to room temperature. For comparative analysis, laser-deposited state samples are generated by performing laser deposition using identical process parameters. X-ray diffraction (XRD) analysis is employed to examine the phases present in the samples. The color metallographic method is utilized for statistical assessment of the proportions of bainite and martensite structures within the samples. Field emission scanning electron microscope (FESEM) is utilized along with an electron backscatter diffraction (EBSD) probe, to analyze the crystal orientation and substructure of the microstructure of the samples, with data processing performed using the AZtecCrystal software. Tensile properties of the samples are evaluated using an electronic universal testing machine.

The XRD results (Fig. 2) reveal that the samples preheated at 280 ℃ and 320 ℃ exhibit 82% and 46% higher retained austenite (RA) content, respectively, compared to laser-deposited state samples. The detailed microstructure observed through SEM (Fig. 4) indicates that the laser-deposited state samples predominantly exhibit a lath-like martensite microstructure characterized by large-sized packets. In the sample preheated at 280 ℃, the bainite microstructure had a lath-like morphology with relatively large-sized packets and abundant blocky martensite/austenite mixed microstructures positioned between the bainite packets with various orientations. In the sample preheated at 320 ℃, the bainite microstructure displays a combination of lath-like and leaf-like morphologies, with the leaf-like bainite having an average width of around 1.2 μm. Notably, the leaf-like bainite effectively segregates the original austenite grains. The inverse pole figure (IPF) mapping results (Fig. 5) reveal that in the laser-deposited state samples, numerous martensite packets can traverse the entire original austenite grains, and the average size of the packet is the largest, reaching up to 15.5 μm. For the sample preheated at 280 ℃, most of the bainite/martensite packets grew parallel throughout the entire original austenite grains, forming a relatively large-sized packet, with an average size of 13.5 μm, which is a slight reduction compared to the laser-deposited state. In contrast, for the sample preheated at 320 ℃, the original austenite grains were segmented into multiple bainite/martensite packets with different orientations. The size of the packets significantly decreased, with an average size of 8.5 μm, which is 55.0% of the size observed in the laser-deposited state. Based on the band contrast (BC) maps and distribution of BC values obtained from the EBSD results (Fig. 6), precise content of bainite and martensite in the samples at different preheating temperatures can be obtained. In the sample preheated at 280 ℃, the proportion of bainite and martensite is close, accounting for approximately 47.9% and 52.1%, respectively. In contrast, the bainite content (62.5%) in the sample preheated at 320 ℃ is significantly higher than the martensite content (37.5%). Tensile properties results (Fig. 9) show that compared to the laser-deposited state sample, the sample preheated at 280 ℃ exhibits higher elongation (8.4%, corresponding to an increase of 23.5%), and the ultimate tensile strength is slightly lower than that of the laser-deposited state sample (1491 MPa). For the sample preheated at 320 ℃, the ultimate tensile strength (1346 MPa) is approximately 10.0% lower than that of the laser-deposited samples, but elongation is significantly higher (57.4%). Particularly, the uniform elongation is more than 115% higher than that of the laser-deposited samples, indicating a better strength-ductility combination.

This study systematically investigated the microstructure and mechanical property evolution of laser-deposited ultra-high strength steel repair layers. The focus was on analyzing the influence of thermal cycling processes on the relative proportion of bainite/martensite structures, substructure characteristics, and mechanical properties in three different conditions: direct laser deposition, preheating at 280 ℃, and preheating at 320 ℃. The main conclusions are as follows: (1) The microstructure of the laser-deposited experimental steel mainly consists of martensite with a small amount of retained austenite (volume fraction is 5.65%). However, the microstructure of the experimental steel subjected to preheating during the laser deposition process is primarily composed of bainite and martensite dual-phase structure, with a significant increase in the content of retained austenite compared to the laser-deposited state (increase of 82% and 46% for preheating at 280 ℃ and 320 ℃, respectively). (2) In the 280 ℃ preheated experimental steel, the bainite morphology is mainly lath-like, with relatively large-sized packets (13.5 μm), and abundant blocky martensite distributed between the bainite packets. As the preheating temperature increased to 320 ℃, the experimental steel showed a mixed morphology of lath-like and leaf-like bainite microstructure, with the leaf-like bainite effectively segregating the original austenite grains. The size of the bainite packet (8.5 μm) was significantly reduced, and a small amount of martensite lath was distributed between the bainite packets. (3) The as-deposited experimental steel exhibited the highest tensile strength (1501 MPa) but the lowest elongation (only 6.8%), with a fracture mode characterized by typical quasi-cleavage. After preheating treatment, the plasticity was significantly improved with only a slight decrease in tensile strength (less than 10.0%). The elongation of the experimental steel preheated at 320 ℃ increased by 57.4% compared with that of the laser-deposited state, and the fracture mode transformed into microvoid coalescence fracture.