Effect of drilling parameters on the hole surface integrity of low alloy steel for nuclear power during BTA deep hole drilling

Boring trepanning association (BTA) deep hole drilling is widely used in machining tube sheet of steam generator. In order to get a better service integrity, the surface quality after machining is required to be higher. In this paper, the effect mechanism of BTA deep hole drilling on the integrity and quality of the machined surface layer of low alloy steel SA508Gr.3Cl.2 for nuclear power is investigated. The results show that the gradient microstructure can be obtained by BTA drilling on the surface of the inner hole, including the recrystallized layer with grain refinement and the plastic deformation layer with high-density sub-crystal structure and grain distortion. With the increase of drilling speed and feed rate, the thickness of deformation layer increases. During the BTA deep hole drilling, the proportion of low-angle grain boundaries (LAGBs) increases with the increasing depth from the machined surface. The increase of drilling speed leads to the increase of recrystallization degree and the proportion of LAGBs in the machined surface. The effect of feed rate on the proportion of LAGBs is opposite. The machined surface is characterized by regular peak and valley, and there are typical surface defects mainly involving feed marks, surface tearing, and plowing grooves. With the increase of drilling speed, the surface roughness will decrease. The effect of feed rate on surface roughness is obviously lower than that of drilling speed. With the increase of drilling speed, and feed rate, the depth of hardened layer increases gradually, which is caused by dislocation strengthening and fine grain-strengthening effect during BTA drilling process. Higher drilling speed is recommended in forming a better machined surface with a strengthening layer of a certain thickness.