Research On Preheat Treatment And Uniformization Of Microstructure And Properties Of Ball Screw Raw Materials

At present the domestic ball screw is prone to generate fatigue crack, poor hot and cold processing workability, which finally result in bad dimensional stability and short service life. In this paper, preheat treatment of quenching and tempering was carried out on the spheroidizing annealing phase ball screw, with the designed orthogonal experiments to optimize the parameters of heat treatment. The characteristic of the heat treatment was described by quantitative metallography. In the end, the uniform microstructure was obtained, which was conductive to improve the processing abilities and product performance of the ball screw.The specimens of different status ball screw were detected by steel GCrl5, which showed that there existed a small amount of lamellar pearlite, large carbide particles and non-homogeneous distribution of carbide in spheroidizing annealing phase ball screw. The distribution of transition layer hardness was irregular and depth of hardened layer was not ideal for surface quenching phase ball screw. There existed banded structure in finished phase ball screw. The microstructure of the raw material was not uniform without the preheat treatment.The behaviors of ball screw in different isothermal spheroidizing annealing phase were investigated by GCr15 steel with SEM, micro hardness detection, etc. Through quantitative metallography method, the morphology and distribution of the carbide was investigated under different heat treatment conditions. It showed that the hardness was in the most uniform distribution and the effect of spheroidizing annealing was the best when the bearing steel GCr15 bearing steel was austenitized at 790℃ and speed of cooling at 20℃/h, at which condition the standard deviation of the average carbide particle diameter was minimum, the ratio of the major axis and minor axis was less than 2.1, the particle roundness was more than 0.17 and the percentage of carbide in particle diameter range from 0.5 to 1.1 μm was approximately 50%. This method could assess the quality level of spheroidizing annealing bearing steel GCr15 quantitatively and provide some guide significance for actual production.According to the divorced eutectoid transformation principle, the design and validation test was carried out for fast spheroidizing annealing process. It showed that the same spheroidizing effect of bearing steel GCr15 could also be reached under which condition the austenitizing temperature was between 790℃ and 830℃, time duration of heat preservation lasted for 10 minutes, and cooling speed was under 250℃/h. In comparison with isothermal spheroidizing annealing, the whole process time was shorted by more than half and the energy consumption was reduced.Combined with the factory production process, through the orthogonal experimental design to optimized the pre-heat treatment process about Spheroidizing annealing and tempering treatment. Through experiment and calculation prove that the biggest effect of the preheat treatment is austenitizing temperature, and the second is the tempering temperature. The optimum preheating treatment process is 2 cycle cyclic spheroidizing annealing、 austenitizing at 820℃、keep the temperature 50 minutes and tempering at 600℃ and keep the temperature 240 minutes.