To increase comprehensive properties of cylinder steel, high-pressure cylinder steel 30CrMo-M was developed on the basis of steel 30CrMo. Tests and researches were made on factors which influence heat treatment properties, such as chemical composition, quenching temperature, and tempering temperature. The results indicated that the 30CrMo-M steel has stable chemical composition, its strength rises with increase of carbon equivalent, and thus it has better relativity. When concentration of quenching liquid is 5%, yield ratio of the steel can be guaranteed to be low, thus safety of cylinder is improved. Tempered between 570°C~622°C, the strength of the steel is increased and plasticity rises with increase of temperature. When tempered at 570°C, inner wall structure of cylinder contains tempering martensite, the strength is high but the plasticity is low. When tempered between 610°C~615°C, temper brittleness happens very easily and toughness is on the low side. The best heat treatment technology is: quenched at 930°C with temperature holding for 40 minutes and quenching liquid concentration as 5%, tempered at 580°C with temperature kept for 90 minutes. The microstructure treated by this technology is tempered sorbite with higher strength, plasticity and toughness.
The rotary expanding is one of the best pipe rolling processes for tube trailer cylinders. The thesis briefly introduced the manufacturing technology features and key equipment of rotary expanding mill of Tianjin Pipe Corporation. The indexes of technical requirements were put forward according to relative cylinder standards and test results were given by a third party. Material performance assessment was conducted by China Standardization Committee on Boilers and Pressure Vessels. The results show that the technical indexes of cylinder pipe including chemical composition, nonmetallic inclusion grade, grain size, microstructure feature, surface quality, mechanical performance and bending test can meet the requirements with an international advanced level.
In order to find out the effects of tempering process on properties of axle steel, the thesis analyzes the change trend of properties of LZ20Mn2 axle steel pipe with variation of tempering temperature. And SEM was used to observe and analyze the fracture. The results show that the strength and hardness of LZ20Mn2 axle steel drop gradually and its toughness and plasticity are increased when tempering temperature goes up. Change of material properties trends to slow down after tempering at 570 °C. From point view of toughness, the technology for the best quality LZ20Mn2 axle pipe steel is the one with quenching at 880 °C with temperature holding for 30min, and tempering at 530 °C with 90min holding temperature. After heat treatment, the microstructure is fine grain size tempered sorbite structure and has good plasticity and toughness, so it can meet the requirement of both high strength and high toughness for axle.
The crack growth rate test of XCQ16 and XCQ16-1 axle steels were introduced using MTS material test machine. The test data of the crack propagation rate of axle steels has been analyzed by Paris formula. The results showed the crack propagation rate test of XCQ16 was faster than XCQ16-1, and the fatigue residual of XCQ16-1 life is longer. The correlated coefficient of the experimental data indicates that the result of this paper is comparatively ideal and has reference value to appraising the life span.
To ensure a smooth production, some stability indicators, including mean (μ), standard deviation (σ), standard deviation coefficient (V), as well as the probability (P) of variable values that fall within the scope of the standard were introduced to evaluate and to control the chemical composition and mechanical properties of seamless gas cylinder 30CrMo-M. Stability levels were defined according to the enterprise performance appraisal system. By using SPSS and MATLAB software, a regression equation was established that evolved chemical composition, tensile strength, yield strength and elongation. Tensile strength and yield strength are positive correlated with C, Si, Mn, and elongation is just the opposite. According to the stability indicators and regression equations, the entire mechanical properties meet the standard requirements by reducing Mn, Si content by 0.02%, while maintaining C content constant. According to the stability indicators, the best heat treatment system, quenching at 930 ~ 935°C for 40 minutes and tempering at 580 ~ 595 °C for 90 minutes, was established.
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