A method for forming welded assembly. The method includes: providing a gear workpiece; forming a joining section on the gear workpiece that defines a weld interface; applying a material to the outside surface of the gear workpiece including the joining section; cutting a plurality of teeth in the material-covered gear workpiece, each of the plurality of teeth having first and second flanks and a root, wherein the material does not reside on any one of the first flanks, the second flanks and the roots; heat treating the gear workpiece to harden the teeth; and finishing the heat-treated gear workpiece in a finishing process to form a finished ring gear, the finishing process comprising at least one of a turning operation and a lapping operation; wherein the material is disposed over the weld interface on the finished ring gear. A welded assembly is also provided.

A method for forming welded assembly. The method includes: providing a gear workpiece; forming a joining section on the gear workpiece that defines a weld interface; applying a material to the outside surface of the gear workpiece including the joining section; cutting a plurality of teeth in the material-covered gear workpiece, each of the plurality of teeth having first and second flanks and a root, wherein the material does not reside on any one of the first flanks, the second flanks and the roots; heat treating the gear workpiece to harden the teeth; and finishing the heat-treated gear workpiece in a finishing process to form a finished ring gear, the finishing process comprising at least one of a turning operation and a lapping operation; wherein the material is disposed over the weld interface on the finished ring gear. A welded assembly is also provided.

A ductile iron composition including, by weight: about 3.4% to about 4.0% Si; about 3.0% to about 3.5% C; about 0.5% to about 1.0% Cr; about 0.02% to about 0.05% Mo; up to about 0.01% S; up to about 0.5% Mn; and balance iron and incidental impurities. The composition has a a ferritic body center cubic microstructure and has a graphite nodule density of greater than 100 per mm.sup.2. A method for forming a ductile iron composition is also disclosed.

A ductile iron composition including, by weight: about 3.4% to about 4.0% Si; about 3.0% to about 3.5% C; about 0.5% to about 1.0% Cr; about 0.02% to about 0.05% Mo; up to about 0.01% S; up to about 0.5% Mn; and balance iron and incidental impurities. The composition has a a ferritic body center cubic microstructure and has a graphite nodule density of greater than 100 per mm.sup.2. A method for forming a ductile iron composition is also disclosed.

A differential assembly is disclosed herein. The differential assembly includes a ring gear and a case. The ring gear and case may comprise dissimilar materials. The ring gear may include a ferrous material and may be overmolded by the case. The case may include a non-ferrous material. Teeth may be formed in the ring gear. The teeth may be hardened through induction heating.

A differential assembly is disclosed herein. The differential assembly includes a ring gear and a case. The ring gear and case may comprise dissimilar materials. The ring gear may include a ferrous material and may be overmolded by the case. The case may include a non-ferrous material. Teeth may be formed in the ring gear. The teeth may be hardened through induction heating.

A method for selective carburization of an article using low pressure induction carburization includes: providing or procuring an article including a surface; subjecting the article to directed induction heating, wherein a first portion of the surface of the article is inductively heated to a temperature that exceeds a carburizing temperature, while a second portion of the surface of the article remains at a temperature below the carburizing temperature; and simultaneously with subjecting the article to directed induction heating, subjecting the article to low pressure carburization, thereby selectively carburizing the first portion of the surface of the article while not carburizing the second portion of the surface of the article.

A method for selective carburization of an article using low pressure induction carburization includes: providing or procuring an article including a surface; subjecting the article to directed induction heating, wherein a first portion of the surface of the article is inductively heated to a temperature that exceeds a carburizing temperature, while a second portion of the surface of the article remains at a temperature below the carburizing temperature; and simultaneously with subjecting the article to directed induction heating, subjecting the article to low pressure carburization, thereby selectively carburizing the first portion of the surface of the article while not carburizing the second portion of the surface of the article.

A method of treating a surface of a gear for a strain wave reduction gear mechanism. The method includes: taking a gear for a strain wave reduction gear mechanism as a workpiece, the gear is formed from a machine structural steel containing at least 0.2% carbon and being subjected to heat treatment after having been machined; performing a first process in which carbide particles are ejected against a surface of the workpiece so as to remove machining marks on the surface of the workpiece and so as to cause elemental carbon in the carbide particles to diffuse and permeate into the surface of the gear; and after the first process, performing a second process in which spherical particles are ejected against a surface of the workpiece for increasing an internal compressive residual stress of the gear surface by a magnitude of at least 50 MPa.

A nitrided steel part excellent in pitting resistance and bending fatigue characteristic enabling reduction of size and decrease of weight of parts or enabling demand for high load capacities to be met, using as a material a steel material containing, by mass %, C: 0.05 to 0.25%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less, S: 0.003 to 0.05%, Cr: over 0.5 to 2.0%, Al: 0.01 to 0.05%, and N: 0.003 to 0.025%, having a balance of Fe and impurities, having formed on the steel surface a compound larger of a thickness 3 m or less containing iron, nitrogen, and carbon and a hardened layer formed below the compound layer, and having an effective hardened layer depth of 160 to 410 m.