Research on Post-Fire Metallographic Structure and Hardness of Quenched and Tempered High Strength Steel 07MnMoVR

Abstract

Quenched and tempered high strength steel 07MnMoVR provides an excellent combination of strength and toughness potentially providing significant cost savings in petrochemical industry. Exposure to fire will subject steel to thermal conditions that may alter the material’s microstructure and properties. The extent of the fire damage and the potential reusability of the components can be evaluated by fitness-for-service (FFS) assessment after a fire event. According to API 579-1/ASME FFS-1, metallurgical investigation and mechanical testing are the chief means for the assessment of fire damage. This paper presents the details of an experimental investigation on the post-fire metallographic structure and hardness of 07MnMoVR steel. Metallographic analyses and hardness testing were performed on coupons exposed to elevated temperatures varying from 550°C to 850°C for half an hour to 8 hours and then naturally cooled in air or cooled by water. The results show that the microstructure of as-received 07MnMoVR steel consisted of tempered sorbite and bainite. With increasing heat exposure temperature, bainite disappeared gradually. The recovery and recrystallization of ferrite began to occur after heat exposure at 650°C for 5hrs. When the heat exposure temperature exceeded 750°C, the effects of duration time and cooling rate on microstructure were both significant. A linear correlation is indicated by fitting the ultimate tensile strength and hardness. Due to the drastically degradation of impact toughness of 07MnMoVR steel after heat exposure exceeded 650°C, it is suggested that the removal and testing of material samples shall be utilized to evaluate the fire damage of components, besides replication or in-situ field metallography and hardness testing. This study can provide basis data and guidelines for the fitness-for-service assessment of 07MnMoVR steel suffered from a fire event.

Thermal Response and Damage Features of Fabricated Steel Structures under Fire

“…However, as an emergency event, fire accidents can greatly affect the stability and bearing capacity of steel structures, and there are a lot of blanks in the research on the thermal response and damage features of fabricated steel structures under fire [9][10][11][12][13][14]. Thus, research efforts to fill in these blanks are of utterly importance [15][16][17][18] since they could not only contribute to enhancing the fire resistance of steel structures and lowering the probability of fire accidents [19,20], but also provide useful technical support for engineering design, construction and management, thereby promoting the application of fabricated steel structures in more projects. The attained research results could serve as both theoretical evidence and practical experience for formulating fire prevention specifications and design guidelines, ultimately dedicating to raising the fire prevention level of the entire construction industry.…”