This group includes the original grades of stainless steels ever produced. Levels of Cr greater than 11% must be present to make a steel stainless. Alloys in this group start at 11.5% Cr and go up with relatively minor additions of other elements such as Ni and Mo. The basic structure is tempered martensite. A characteristic of this group is that very high strengths can be reached with adjustments in heat treatment. This higher strength is at the sacrifice of lower ductility.
As with other stainless steels, balancing of the chemical composition is necessary to achieve full material potential. These alloys must be heat treated to improve ductility and/or strength. Similar to carbon steels and low alloy steels, melting of these alloys require attention to the problem that they are gas pickup prone. They will pick up H from anything wet in the charge and they will pick up N from the atmosphere if held liquid in the furnace. Their melting is further complicated by the absorption of O from the atmosphere and requires exacting additions of Al to offset this tendency. Castability of this group of alloys is somewhat impaired because of very poor fluidity characteristics. These alloys require precautions to avoid cracking. Thermal processing in the finishing department requires pre-heat of the parts.
The machinability of this group of alloys is considered as good when the machining is performed on softened castings. It is not uncommon to find practices where rough machining is performed on softened castings, then parts are hardened by heat treating and finish machining is done by grinding.
Applications for this group of materials include parts where the basic corrosion resistance of Cr alone is sufficient. Because of the high strength and hardnesses easily attainable from alloys in this group they are widely used in pumps and impellers. Another example of the use of these alloy’s high hardness would be that the finest cutlery knives and surgical tools would be produced in grades from this group.