Numerical Simulation And Experiment Research On Subthermal Quenching Heat Treatment Process Of Large Diameter CNG Cylinder

The large CNG cylinders endure high pressure and work in fatigue conditions of unloading and loading for a long time. On account of its characteristics of big size and high-mass, the large diameter and thick wall gas cylinder’s heat treatment is often very complicated. Quenching is an important part in heat treatment process and plays a key role in the final quality of work. The numerical simulation technology can simulate the heat treatment process accurately, and forecast heat treatment quality, provides a way of effective study method for the cylinder heat treatment process formulation and implementation.First of all, through the method of consulting documents, this paper obtained the high temperature mechanical properties of 30 CrMo steel and the specific heat of different microstructures at different temperatures etc. Both the parameters of Leblond module and Koistinen-Marburger module were computed through the TTT and CCT curves. The accurate dates obtained in the experiments provide reliable reference parameters and input parameters.Secondly, the mathematical model of cylinder quenching process was established based on the consideration of the relationship of temperature-microstructure-stress and the nonlinear of thermal physical properties and the latent heat of phase change and the transformation plasticity in the heat treatment process.Then, the geometry model and finite element model were built by using software of CAD and CAE. Afterwards, the traditional quenching and subthermal quenching process were simulated through making use of element software SYSWELD and combing actual production, and analyzed the temperature field, microstructure field and stress field of the special nodes in cylinder.Finally, for the issues of sufficient strength but lack of toughness in the cylinders in use process, according to the characteristics of subthermal quenching, the new Heat Treatment Process, namely “SQ-T”, “Q-SQ-T” and “Q-T-SQ-T” were proposed. Themechanical properties, microstructure and chemical element distribution of 30 CrMo steel after heat treatment were proved by tensile test, impact test, SEM, hardness test, and EDS.The results show that, both ferrite and acicular martensite would be gotten from quenched and high temperature tempered 30 CrMo steel after subthermal quenching and high temperature tempering, which performance the better mechanical properties.