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At around 19:05 on January 14, 2004, a serious accident involving the lid of an oven in the heat treatment workshop of Jiangxi Ganjiang Machinery Factory occurred; as a result, the oven lid moved upward by 800 mm and became severely tilted； The mounting base is damaged ； A large amount of harmful gases in the furnace were released explosively in an instant ； Dozens of windows in the factory building were shattered by the shock ； There were many oil spots splattered on the walls and on the electrical control panels… Fortunately, no one was there at the time; otherwise, the consequences would have been disastrous. After the accident, the duty personnel found that the temperature recorded by the automatic temperature recorder on the electrical control cabinet was 250°C. The power supply to the control cabinet and the switch for the exhaust fan were in the on position, the fan inside the furnace cover was still rotating, and there were no abnormalities with the parts subjected to backfire inside the furnace (steel helmets). Course of the accident: The heat treatment workshop in this factory was tasked with carrying out heat treatment on over ten thousand steel helmets. Due to the urgency of the task, tight deadlines, and high quality requirements, the workshop had to learn from experience as it went along, as there were no complete standard operating procedures available. To meet the requirements for strength and hardness of steel helmets, the heat treatment workshop first quenches them in an oil furnace; after hardening and oil filtration, they are then tempered in a shaft-type tempering electric furnace. After multiple trials, the preliminary heat treatment process met the product quality requirements. On January 14, 2004, in an effort to meet a deadline, the workshop quenched over 60 helmets in an oil furnace; without first thoroughly removing the oil remaining on the surface of the parts, they placed these helmets in baskets and sent them into a vertical tempering furnace for tempering treatment. To improve heat treatment efficiency and save space in the quenching and tempering furnaces, the workshop stacked two steel helmets together as a set for tempering, which resulted in a stamping accident. Analysis of the accident cause: 1. Insufficient time was spent on oil filtration after quenching in the oil furnace. The initial procedural guidelines required that any residual engine oil on the surface of the quenched parts be completely removed before they could be placed in a basket and sent to the tempering furnace for further treatment; however, no specific time limit was set for this oil-filtration process. In order to save time, the operators filtered the oil for only 40 minutes before putting the parts into the vertical tempering furnace for tempering. This is a major cause of the accident. 2. Two parts are stacked together to form a group; the oil on their mating surfaces cannot be completely removed. In the process of developing heat treatment methods for steel helmets, in order to ensure the quality of this heat treatment, the parts must be heated and cooled evenly during quenching and tempering. Additionally, to reduce the space occupied by the parts in the furnace and improve heat treatment efficiency, two parts are stacked together to form one group. As a result, when the parts are quenched in hot oil, the diluted engine oil can easily seep into the gaps between the two parts and adhere to their surfaces, making it impossible to remove it completely. This is the root cause of the accident. 3. The tempering temperature is higher than the flash point of the oil. The tempering temperature is set at 250°C, which is above the oil’s flash point of 165°C; when the mixture of oil vapor and air reaches the combustion range, the oil will burn rapidly, and the large amount of smoke generated will expand quickly, posing a high risk of cover explosion accidents. This is one of the main causes of the accident. 4. The exhaust hole on the furnace lid is too small. The exhaust hole on the lid of the shaft-type tempering furnace has a diameter of φ30 cm; this hole is suitable only for the tempering of aluminum alloy wheels, and not for the tempering of helmet-shaped parts that have a lot of surface oil contamination. The small exhaust vents cause the concentration of oil vapor and air mixture to reach the combustion range, which is also one of the key reasons for accidents. 5. There were no operators on site, which violates the safety operating procedures. Article 8 of the factory’s “Safety Operating Procedures for Well-Type Tempering Ovens” states: “When using an electric oven, one must not leave the post and should monitor its operation at all times.” When the cover collision accident occurred, the operator was not on site and failed to detect in time the abnormal situation of black smoke filling the entire workshop. This was the key cause of the accident. Improvement measures: The basic condition for a flashover accident is that the concentration of the mixture of engine oil vapor and air inside the furnace reaches the combustion range; only when the furnace temperature rises to the flash point of 165°C can spontaneous ignition occur. 1. Improvement of the tempering process procedure: (1) In the original tempering process procedure, two helmet-shaped parts were stacked on top of each other; this has been changed to stacking individual helmet-shaped parts, in order to reduce the amount of quenching oil that gets trapped between the two parts. Although the space utilization within the furnace has decreased, safe production can still be achieved. (2) Before tempering, the oil filtration time should be extended, and a strict filtration time should be determined based on seasonal changes. To ensure that all the oil remaining on the surface of the parts after quenching in oil is completely removed. During winter, when temperatures are low, the oil filtration time should be appropriately extended. After improving the process procedures, it is necessary to strictly follow them and control the entry of quenching oil into the tempering furnace. This is a key measure to prevent lid-pushing accidents. 2. Increase the exhaust volume of the furnace lid. After parts are quenched in an oil furnace, if no cleaning process is employed, quenching oil will inevitably adhere to the surface of the parts. Under the high temperatures in the tempering furnace, once the concentration of the gas mixture formed by oil vapor and air reaches the flash point temperature of the oil, ignition occurs inevitably, leading to a cover explosion accident. Therefore, appropriately increasing the diameter of the exhaust holes in the tempering furnace is one of the important measures to prevent lid-pushing accidents.