A mould for one piece casting of a tool, which has a working component (17) of steel and a body (18) of grey iron has a first mould cavity section (4) for the steel and a second mould cavity section (5) for the grey iron, with an interconnection zone therebetween. A dividing plane (3) between the sections (4, 5) is planar and horizontal and located at the interconnection zone. A duct (12) leads from the first section (4) to an accommodation space (11) for possible surplus of steel. In a method for one piece casting of a tool, which has a working component (18) of steel and a body (18) of grey iron with an interconnection zone therebetween, the steel is cast in a first mould cavity section (4) and the grey iron in a second mould cavity section (5). A dividing plane (3) between the mould cavity sections is planar and horizontal. An accommodation space (11) for surplus of steel is provided to permit steel to flow from the first mould cavity section (4) at the level of the dividing plane (3) into the accommodation space.