engleski

Influence of weapon barrel superheat on material microstructure and hardness

The paper presents the procedures and the results of the experiment which researches the influence of weapon barrel superheat on the microstructure and hardness of weapon barrel material. The barrel superheat appears when the allowed fire rate is crossed. The fire rate defines the allowed rounds number in the determined interval of firing time. The temperature of the powder gases in the barrel reaches up to 3880 K, and the maximum pressure of the powder gases up to 6000 bar. The surface layers of the barrel bore may be heated over the temperature of 500 °C to 1000 °C for that. Therefore the test samples were tempered at 600 °C, 700°C, 800 °C and 900 °C, over a period of one hour and air cooled. The test samples have been made of the barrel material taken from a 20 mm antiaircraft gun. The analysis shows that the barrel material is heat treated steel 34CrMo4, with the homogeneous structure of the tempered martensite. The sample which was tempered at 600°C preserved the martensite structure after the air cooling, with the surface hardnes of 384 HV0, 2. By heating the steel at 700°C, the martensite structure was also preserved, but the measured hardness was 298 HV0, 2. The hardness is smaller because the annealing temperature is higher. By heating the steel at 800°C, the uniform small-grained structure was obtained. The measured hardness of 396 HV0, 2 shows that besides bainite probably martensite appeared in the microstructure of the barrel steel. The sample tempered at 900°C and air cooled has slightly bigger grains in its microstructure. The measured hardness was 408 HV0, 2, so the bainite may appear with the martensite. The test results shows that spontaneous quenching of the bore surface layers may be caused by the turbulent, cool air. In case of bigger barrel depth, cooling-rate of material would be slower, so a rough-grained ferrite-pearlite structure may appear. The steel with a rough-grained structure has very low toughness and is brittle and breakable. The test has shown that the determined fire rate must not be exceeded by weapon firing, because the barrel superheat changes significantly the microstructure and mechanical quality of the weapon barrel material and therefore, its wear resistance decreases.

weapon barrel; superheat; microstructure; hardness

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