碳素工具钢是含碳量为0.65%-1.35%的优质高碳钢,其热处理后的硬度可达()。
A. 60-64HRC
B. 70HRC以上
C. 50-55HRC
D. 200HBS以上
在滑梁险槽的河心石梁尾部常有旺盛的()下延,上行船可利用,以提高航速。
A. 夹堰水
B. 旺水
C. 回流
D. 斜流
Required lifesaving equipment on existing vessels may becontinued in use on the vessel if().
A. kept on board no more than 2 years
B. inspected and serviced every 6 months
C. destroyed if more than 5 years old
D. maintained in good and serviceable conditio