The valves are made of very high quality steel because they get very hot during combustion. The cool intake mixture cools the intake valve. The materials and surface coatings used on valves depend on expected temperatures. The exhaust valve is usually made of austenitic stainless steel alloy. The valve face may be hardened with a hard alloy. This is called valve hardfacing. Hard facing is applied to the teeth and gage surface by welding with a rod in the form of a uniform mixture of particles of tungsten carbide and particles of matrix metal sintered together enough to form a rigid rod with some strength, but not completely sintered; that is, the rod has some remaining porosity.