Water Bath Quenching And Its Impact On The Microstructure And Mecanical Performance Of 70# High Carbon Steel Wire

The semi-finished materials which is used to produce the wire rope usually need the quenching treatment during the actual production process. Generally the way of quenching treatment is lead bath quenching. However, lead bath does harm to the environment and human body. Hence, the water bath quenching treatment, a new kind of energy conservation and environmental protection quenching treatment, has became a hot research subject. In this paper, we analyze the behavior of microstructure transformation of the 70 # steel, choose a sodium polyacrylic acid solution of 90 ℃and 9% as the medium of water bath quenching, use double sections of water bath quenching method to study the microstructure and property of the steel wire with different linear velocity and immersed length in the first bath, then propose the proper process of the optimized water bath quenching. We also compare the microstructure and property of the optimized water bath with the lead bath quenching, research the effect of the work hardening behavior and mechanical property of steel wire after the water ba th and lead bath quenching.Using the method of thermodilatometry, we study the continuous heating and cooling phase transformation and the behavior of the isothermal cooling phase transformation of the 70 # steel, establish the phase transformation kineti cs model of austenite and pearlite, then determine the water quenching cooling curve of 70 # steel by the simulation of finite element modeling with the temperature field during each section of double sections of water bath quenching method. Further, based on the calculate of 70 # high carbon steel’s CCT and TTT curve during the continuous cooling transformation, reveal the feasibility of the replacement of water bath quenching and guide the process design of the water bath quenching.During the water bath quenching of 70 # steel, when the immersed length in the first water channel is same, with the increasing of the linear velocity, the size of sorbite become smaller, but the space between sorbite is larger, the changing of hardness is not so obvious, the tensile strength of steel wire is reduced, and the plastic increased. Considering all sides, we selected 24 m/min speed as the best choice. At the same linear velocity, with the increasing of the division in length, the size of sorbite is almost unchanging, but the space between sorbite is decreased, the tensile strength of steel wire rised and the plasticity reduced. Through the comprehensive analysis of the microscopic structure and mechanical properties, we choose the linear velocity of 24 m/min, the wate r bath immersion length in the first phase area of 1300 mm as the best manufacturing technique.We can get sorbite by the optimized water bath quenching and lead bath quenching, and the size of sorbite after the water bath quenching is a little bigger than the lead bath quenching. The tensile strength of steel wire after water bath quenching is slightly lower than lead bath quenching, but the vickers hardness values, times of torsion and elongation are almost the same with the lead bath quenching.Based on the modified Ludwik model, we established the tensile strain hardening model of lead bath quenching and water bath quenching process for the steel. We found that both the strain hardening coefficient K and strain hardening exponent n of them are alike, which shows that water bath quenching of steel wire has good performance at cold drawing process. After the water bath quenching process, through the multi-channel water tank of cold drawing, steel wire’s tensile strength、plasticity and the number of torsions have met the mechanics performance of the 70 reverse cold-drawn steel wire and the twisting process requirement of the wire rope. After the water bath quenching process, the properties of cold drawing steel twist wire rope are equal to the lead bath quenching, so it can replace the lead bath quenching completely.