the part of the head (= top part) of a hammer that is opposite the flat end and can be of various shapes

The ball peen hammer is used in metalworking. 圆头锤在金属加工时使用。

a cross peen hammer 斩口锤

to hit metal with a hammer to make it flatter

I found that the axle ends were peened, so the wheels couldn't easily be removed. 我发现车轴的两端被砸平了，所以车轮不容易拆下来。