This group includes alloys where Cr, Ni and Mo are added in modest amounts to raise the tensile and yield strengths beyond what can normally be achieved with carbon steels. Alloys with less than 8% total alloy content are considered as low alloy steels. Alloying is also necessary to maintain strengths in heavy section castings. Tensile strengths up to 150,000 psi are readily achievable. Weldability is maintained even at high strength levels.
These alloys must be heat treated to achieve their full property potential. The heat treatments require the formation of a high temperature phase (above 1400F) followed by water of oil quenching. Ductility of this quenched structure is then improved by tempering (600F - 1300F).
Similar to carbon 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. Production of these alloys as castings also require precautions to avoid cracking. Thermal processing in the finishing department requires pre-heat of the parts. It is common practice to final inspect castings in this group beyond visual inspection with non-destructive testing techniques to assure crack free parts.
Applications for materials in this group are diverse and include any application where design above normal steel strength levels is required. Weight to strength considerations often require the use of low alloy steel castings.