A ball-peen (or ball-pein, see Spelling differences; also known in Europe and North America as ball pane) hammer is a type of peening hammer used in metalworking. It is distinguished from a point-peen hammer or chisel-peen hammer by having a spherical or hemispherical peening head. Though the process of peening has become rarer in metal fabrication, the ball-peen hammer remains useful for many tasks such as tapping punches and chisels. The ball-peen hammer should never be used to drive nails, as a claw hammer would, as the head of a ball-peen hammer is made of very tough - but relatively soft - steel, making it ideal for hammering on harder metals. Continual nail hammering with a ball-peen hammer will "dent" and deform its face. The head of a claw hammer is exactly the opposite: It is made of very hard (hence, more brittle) steel, and is designed for hammering relatively soft nails. The two hammers are not interchangeable.
The original function of the hammer was to "peen" riveted or welded material so that it will exhibit the same elastic behavior as the surrounding material. Specifically, striking the metal imparts a stress at the point of impact which results in strain-hardening of that area. Strain hardening raises the elastic limit of a material into the plastic range without affecting its ultimate strength. A strain-hardened material will not deform under the same low stresses as a non-hardened material. Most metals can be "worked" by such methods until they lose all of their ductile characteristics and become strong but brittle.
The ball-peen hammer is also used to expand and shape the free end of copper roves, light rivets and similarly, "setting" the rivet in place to complete the joint. Peening is also the method by which steel drums are formed and tuned.
A ball-peen hammer is also known as an engineer's hammer or a machinist's hammer and may be graded by the weight of the head. It is the mechanic's hammer of choice when making gaskets or driving drift pins.