Study on the Relationship between Mechanical Properties and Microstructure of 4130X Steel for Tube Trailer Cylinders after High-Temperature Exposure

The gas cylinders for tube trailers are constructed with SAE 4130X steel. They have high strength and toughness characteristics, but the microstructure of cylinder material may change after a fire accident, which may lead to a change of the mechanical properties. Therefore, to evaluate the tube trailer cylinder safety after a fire accident, it is necessary to study the material microstructural and mechanical properties changes and to further to propose an effective evaluation method. In this work, the microstructure of a SAE 4130X steel re-heated by fire in temperature was characterized by electron backscattered scanning diffraction (EBSD). The experimental results showed that the 4130X steel had obvious microstructure changes and the impact toughness decreased sharply after 750 °C. It was analyzed that because the 750 °C was higher than the austenite starting formation temperature of the 4130X steel, the ferrite in the material undergoes austenite reversion and decomposition during the natural cooling. Pearlite was formed resulting in a decrease in strength and toughness. Below the austenite starting formation temperature, recrystallization occurred with increasing temperature, and the decrease in interfacial density resulted in a decrease in tensile strength, and the yield strength decrease and the impact toughness increase due to the decrease in dislocation density and sub-block boundaries density.