| Summary: | 碩士 === 國立高雄大學 === 化學工程及材料工程學系碩士班 === 101 === This study is focused on the effect of heat treatment on the mechanical properties of he bolts made of SAE 4140H chromium molybdenum alloyed steel. Sample bolts were selected out from a production line what were quenched and then tempered according to similar heat treatment conditions as running manufacturing. Their mechanical properties were studied via tensile and hardness testing machines. Their microstructures in different positions on cross sectional cut of test bolts were investigated in detail.
The experimental results show that the quenched martensite in SAE 4140H varies from inside to the outer edge with different morphology and hardness. This shows that different tempering treatment may cause varied tempered martensite morphology. When samples were tempered by different temperatures microstructure will be changed from martensite to tempered martensite, and its tensile and yield strengths decrease as the tempering temperature increases. And it can be found that the position with higher hardness becomes faster temper softened after quenching. According to the experiment result of the same hardness with different tempering parameter, it shows the prolonged tempering leads to relatively narrow microstructure.
Furthermore, an observation on the test materials under the tempering temperature 530℃, 550℃ and 570℃ for 0 to 120 minutes shows that, the hardness of tested bolts decreases rapidly when tempering time is less than 30 minutes. As in this stage, the hardness variation cannot be estimated precisely.
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