Study On The Fatigue Properties And Quality Inspection Of U71Mn Steel After Laser Austempering

Aiming at improving the technique of rail reinforcing and optimizing the laser quenching process,research works have been carried out in terms of four aspects in this paper,including:development of new laser quenching process for rails,analysis of the bending properties of the austempered samples,the rolling contact fatigue tests and the quantitative evaluation of the laser quenching effect by color recognition.An laser austempering process for enhancing of rails with induction heating was developed.The hardness of the sample can be regulated in a relatively wide range.The induction temperature of the typical process was 350℃,and the hardness reached 50HRC with structures of mixed martensite and bainite.The induction temperature of another process was 400℃,and the hardness reached 44.5 HRC with structures of bainite.The results of the three-point bending tests showed that,compared with the sample with a hardness of HRC64 of the existing process,the ultimate bending strength（UBS）,the fracture strain and the fracture work of the sample with a hardness of 50HRC is increased by 2.1 times,6.1 times and 12.7 times respectively.For the sample with a hardness of 44.5HRC,the above parameter is increased by 2.1 times,8.8 times and 19.5times respectively.The results of the rolling contact fatigue tests showed that the disk sample of the origin process with a hardness 64HRC presented as the best wear resistance.Necerthless,some fatigue cracks were appeard around the hardened spot.The wear resistances of the sample with structures of mixed martensite and bainite was better（2.91 times than rail substrate）than other disc samples without surface fatigue cracks.The roundness analysis indicated that the irregular deformation of the sample surface after the rolling contact fatigue test can be changed into a regular mode due to laser quenching.The relationship between the effects of laser quenching and the color characteristic values of the sample surface was found based on HSB color model.XRD and XPS analysis revealed that the color variation of the heated sample is directly related to the constituent variation of the iron oxide.The content of the Fe³⁺oxides can be decreased and the content of the Fe²⁺oxides can be increased as the heating temperature increasing.A technique for quantitative evaluating the substrate zone,the heat-affected zone,the solid phase transformation zone and the melt zone of of the rail steel sample has been developed based on the setting threshold parameters.This evaluation method provides an innovative solution for the quantitative assessment of the quality of on-line laser quenching of the rail.The optimization of the laser quenching process and the proposed image detection method are of great significance for balancing the fatigue and wear resistance of the rail material,extending the life of the rail,and reducing the cost of railway transportation.