|Abstract: ||本研究之目的在於探討D6AC高強度低合金鋼之板材與雷射銲件,經不同溫度回火處理後,其微結構組織對材料機械性質與疲勞裂縫成長特性之影響。 實驗結果顯示:隨回火溫度升高,板材與銲件之拉伸強度降低,而延伸率與衝擊韌性則隨回火溫度升高而增高。於相同回火溫度,板材之強度高於銲件,而銲件之平均接合效率仍高達95%以上,銲件在過度回火區產生局部頸縮現象,使其延伸率遠低於板材;此外經預熱銲件在銲道形成韌性較佳之下變韌鐵與回火麻田散鐵混合組織,以致其衝擊韌性與疲勞裂縫成長特性均優於板材。在實驗.DELTA.K範圍內,回火溫度及應力比,對板材疲勞裂縫成長特性並無明顯之影響。裂縫在銲道及熱影響區成長時,其成長方向產生大幅變化,致使其疲勞裂縫成長速率下降,疲勞破斷面顯示粗糙且具穿晶與沿晶混合模式破壞特徵;而板材之破斷面則為平整,具典型脆性穿晶疲勞破斷特徵。|
In this work, the mechanical and fatigue properties of the D6AC high strength low alloy steel plate and laser welds, which undergone various heat treatment was to be investigated and its microstructure effect. Results reveal that the strength decreased, while elongation and impact values increased, with increasing the tempering temperature for base metals and weldments. At the same tempering temperature, the strength of base metals was higher than that of the weldments. However the joint efficiency of the laser welds was still in excess of 95%. Because of the local necking deformation in the overtempering zone of the welds, the elongation of the welds is lower than that of the base metals. The effect of tempering at various temperature doesn't make an obviously change in fatigue crack growth properties. The fusion zone purification and refined tough microstructure existed in the welds let the welds always show higher impact values than the base metal, corresponding to the same tempering temperature. It has been observed that crack undergoes great change in the growth direction in the weldmetal. Therefore, the crack growth rates in the weldmetal were lower than that in the base plate for the same stress intensity factor range (.DELTA.K). For the portion of the heat-affected zone undergone the same microstructural transformation as the weldmetal during welding, similar fatigue properties would be anticipated between these two region. Fatigue fractographies showed transgranular and intergranular (mixed mode) fracture occurred in the weldmetal.