Study On Optimization Of Heat Treatment Process Of LZ50 Steel Used For The Axles Of Railway Freight Car

In today’s rapid development of china’s railway,high speed and heavy load has become an inevitable trend in the future development of railway wagons.As a key component of railway wagons,axle’s force situation has become more complicated in the high speed and heavy load conditions.The axle’s quality and performance is directly related to the vehicle driving safety.Thereby increasing the level of railway vehicle axle technology is an important symbol of railway equipment modernization,and it’s also an important technical supporting point on the development of china’s high-speed railway and heavy load technology.LZ50 steel axle heat treatment has been employed for many years,the national railway vehicle manufacturers have been exploring a new axle manufacturing process for a long time,and they have achieved some results.However,due to different equipment,materials,production methods of the manufacturers,the production process parameters used are not the uniform,the relevant literature and research findings generally gave a certain heat treatment parameters range,but lack the research of normalizing cooling.The project made an inductive analysis to the problem of LZ50 steel axle after normalizing process in the company.Through the comparative tests of different heat treatment process to the LZ50 axle steel,this thesis researched the relationship between different processes,the cooling rate and the axle microstructure and mechanical properties of the final heat treatment microstructure.On this basis,the original heat treatment process on the axles was optimized.This paper verified through a small box furnace at first,then it tested by hanging axle heat treatment production line for axle physical conduct.Focusing on the effect relationship between LZ50 axle steel performance and organizational structure under the different process parameters and normalizing cooling rate,,the paper researched different parts of the microstructure and properties in the axle as the supplementary study,and it explored differences properties and microstructure in different parts of the axle.The results show that the optimum heat treatment process for LZ50 steel axles is austenitizing normalizing at 870 ℃ and 810 ℃ respectively,and then tempering at 520 ℃.During normalizing,there is a direct influence of the cooling rate on the mechanical properties and microstructure of LZ50 steel.The optimal cooling rate is 14℃-16℃/min.The optimized process elevate 29MPa to the tensile strength of the LZ50 steel axle,the yield strength is enhanced 12 MPa,elongation is upgraded 2.6%,percentage reduction of area is upgraded 5.8%and the grain size reaches the level 6.5-7.5.