High stress nodular iron gears and method of making same

Claim 2

A process for making a through hardened, compressively surface stressed nodular iron gear capable of withstanding applied tooth root tensile stress excursions of at least about 60,000 psi without fatigue failure, said process comprising the sequential steps of: casting a gear blank from nodular iron; annealing the blank to yield a substantially homogeneous matrix microstructure upon cooling; cooling the blank to room temperature; machining teeth into the blank; heating the machined casting in a nonoxidizing atmosphere to a temperature of about 1600° F. to about 1700° F. to austenitize the matrix and dissolve sufficient uncombined carbon to provide an austenitic phase comprising about 0.7% to about 1.1% dissolved carbon; quenching the casting to an isothermal transformation temperature of above about 425° F. and below about 650° F. and at such a rate as to avoid the nose of the time-temperature transformation curve of the nodular iron so as to preclude the formation of significant amounts of pearlite, feathery bainite or martensite prior to and during subsequent isothermal transformation at said quenching temperature; maintaining the casting at said isothermal transformation temperature for a time sufficient to convert said austenitic phase to at least 50% acicular bainite; cooling said casting from said isothermal transformation temperature to room temperature; and shot peening the surface of the gear at the roots of the gear's teeth to impart to said surface a residual compressive stress of at least about 40,000 psi and provide a residual compressive stress of about 10,000 psi at a depth of about 0.006 inch to about 0.015 inch beneath the surface.

Claim 3

A process for making a through hardened, compressively surface stressed nodular iron gear capable of withstanding applied tooth root tensile stress excursions of at least about 60,000 psi without fatigue failure, said process comprising the sequential steps of: casting a gear blank from nodular iron; heating the blank for a time and at a temperature sufficient to dissolve combined carbon and yield a substantially homogeneous matrix microstructure containing at least about 65% ferrite upon cooling; cooling the blank to room temperature; machining teeth into the blank; heating the machined casting in a nonoxidizing atmosphere to a temperature of about 1600° F. to about 1700° F. to austenitize the matrix and dissolve sufficient uncombined carbon to provide an austenitic phase comprising about 0.7% to about 1.1% dissolved carbon; quenching the casting to an isothermal transformation temperature of above about 425° F. and below about 650° F. and at such a rate as to avoid the nose of the time-temperature transformation curve of the nodular iron so as to preclude the formation of significant amounts of pearlite, feathery bainite or martensite prior to and during subsequent isothermal transformation at said quenching temperature; maintaining the casting at said isothermal transformation temperature for a time sufficient to convert said austenitic phase to at least 50% acicular bainite; cooling said casting from said isothermal transformation temperature to room temperature; and shot peening the surface of the gear at the roots of the gear's teeth to impart to said surface a residual compressive stress of at least about 40,000 psi and provide a residual compressive stress of about 10,000 psi at a depth of about 0.006 inch to about 0.015 inch beneath the surface.

Claim 4

A process for making a through hardened, compressively surface stressed nodular iron gear capable of withstanding applied tooth root tensile stress excursions of at least about 60,000 psi without fatigue failure, said process comprising the sequential steps of: casting a gear blank from nodular iron; heating the blank for a time and at a temperature sufficient to dissolve combined carbon and yield a substantially homogeneous matrix microstructure containing at least about 65% ferrite upon cooling; cooling the blank to room temperature; machining teeth into the blank; heating the machined casting in a nonoxidizing atmosphere to a temperature of about 1600° F. to about 1700° F. to austenitize the matrix and dissolve sufficient uncombined carbon to provide an austenitic phase comprising about 0.7% to about 1.1% dissolved carbon; quenching the casting to an isothermal transformation temperature of above about 425° F. and below about 650° F. and at such a rate as to avoid the nose of the time-temperature transformation curve of the nodular iron so as to preclude the formation of significant amounts of pearlite, feathery bainite or martensite prior to and during subsequent isothermal transformation at said quenching temperature; maintaining the casting at said isothermal transformation temperature for a time sufficient to convert said austenitic phase to at least 50% acicular bainite; cooling said casting from said isothermal transformation temperature to room temperature; and shot peening the surface of the gear at the roots of the gear's teeth to impart to said surface a residual compressive stress of at least about 40,000 psi and provide a residual compressive stress of about 10,000 psi at a depth of about 0.006 inch to about 0.015 inch beneath the surface.

Claim 5

A process for making a through hardened, compressively surface nodular iron gear capable of withstanding applied tooth root tensile stress excursions of at least about 73,000 psi without fatigue failure, said process comprising the sequential steps of: casting a gear blank from nodular iron; heating the blank to above the eutectoid temperature for a time sufficient to austenitize the blank, dissolve any nonpearlitic cemetite and homogenize the microstructure of the matrix; slowly cooling the blank down through the eutectoid temperature at a rate less than about 100° F. per hour to yield a homogeneous matrix comprising about 65% by weight or more ferrite; machining teeth into the blank; heating the machined casting in a non-oxidizing atmosphere to a temperature of about 1650° F. to austenitize the matrix and dissolve sufficient uncombined carbon to provide an austenitic phase comprising about 0.8% to about 1.1% dissolved carbon; quenching the casting to an isothermal transformation temperature of about 455° F. and at a rate of at least about 66° F. per second so as to avoid the nose of the time-temperature transformation curve of the nodular iron and thereby preclude the formation of significant amounts of pearlite, feathery bainite or martensite prior to and during subsequent isothermal transformation at said quenching temperature; maintaining the casting at said isothermal transformation temperature for a time sufficient to convert said austenitic phase to at least about 65% acicular bainite; cooling said casting from said isothermal transformation temperature to room temperature; and shot peening the surface of the gear teeth to impart to said surface at the roots of the teeth a residual compressive stress of at least about 40,000 psi and less than about 85,000 psi and provide a residual compressive stress of about 10,000 psi at a depth of about 0.008 inch to about 0.013 inch beneath the tooth root surface.

Claim 6

A process for making a through hardened, compressively surface nodular iron gear capable of withstanding applied tooth root tensile stress excursions of at least about 73,000 psi without fatigue failure, said process comprising the sequential steps of: casting a gear blank from nodular iron; heating the blank to above the eutectoid temperature for a time sufficient to austenitize the blank, dissolve any nonpearlitic cemetite and homogenize the microstructure of the matrix; slowly cooling the blank down through the eutectoid temperature at a rate less than about 100° F. per hour to yield a homogeneous matrix comprising about 65% by weight or more ferrite; machining teeth into the blank; heating the machined casting in a non-oxidizing atmosphere to a temperature of about 1650° F. to austenitize the matrix and dissolve sufficient uncombined carbon to provide an austenitic phase comprising about 0.8% to about 1.1% dissolved carbon; quenching the casting to an isothermal transformation temperature of about 455° F. and at a rate of at least about 66° F. per second so as to avoid the nose of the time-temperature transformation curve of the nodular iron and thereby preclude the formation of significant amounts of pearlite, feathery bainite or martensite prior to and during subsequent isothermal transformation at said quenching temperature; maintaining the casting at said isothermal transformation temperature for a time sufficient to convert said austenitic phase to at least about 65% acicular bainite; cooling said casting from said isothermal transformation temperature to room temperature; and shot peening the surface of the gear teeth to impart to said surface at the roots of the teeth a residual compressive stress of at least about 40,000 psi and less than about 85,000 psi and provide a residual compressive stress of about 10,000 psi at a depth of about 0.008 inch to about 0.013 inch beneath the tooth root surface.

Claim 7

A through hardened, high stress gear capable of withstanding occasional applied tooth root tensile stress excursions of at least about 60,000 psi, said gear comprising a base portion and a plurality of teeth integral with and extending from said base portion, said teeth each comprising a central core and a skin cold worked by impingement on said core at least at the tooth's root where it joins the base, said base, core and skin comprising nodular iron the matrix of which comprises at least about 50% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.7% to about 1.1% by weight and a Vickers hardness between about 577 and 720, and said skin having a residual compressive stress of at least about 40,000 psi at its surface and about 10,000 psi at a depth between about 0.006 inch and 0.015 inch below said surface.

Claim 8

A through hardened, high stress gear capable of withstanding occasional applied tooth root tensile stress excursions of at least about 60,000 psi, said gear comprising a base portion and a plurality of teeth integral with and extending from said base portion, said teeth each comprising a central core and a skin cold worked by impingement on said core at least at the tooth's root where it joins the base, said base, core and skin comprising nodular iron the matrix of which comprises at least about 50% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.7% to about 1.1% by weight and a Vickers hardness between about 577 and 720, and said skin having a residual compressive stress of at least about 40,000 psi at its surface and about 10,000 psi at a depth between about 0.006 inch and 0.015 inch below said surface.

Claim 9

A through hardened high stress gear capable of withstanding occasional applied tooth root tensile stress excursions of at least about 73,000 psi, said gear comprising a base portion and a plurality of teeth integral with and extending from said base portion at the tooth roots, said teeth each comprising a central core portion and a skin cold worked by impingement on said core, said base, core and skin comprising nodular iron the matrix of which comprises at least about 65% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.8% to about 1.0% by weight and a Vickers hardness between about 613 and 720, and said skin at said roots having a residual compressive stress of between about 40,000 psi and 85,000 psi at its surface and about 10,000 psi at a depth between about 0.008 inch and 0.013 inch below said surface.

Claim 10

A through hardened high stress gear capable of withstanding occasional applied tooth root tensile stress excursions of at least about 73,000 psi, said gear comprising a base portion and a plurality of teeth integral with and extending from said base portion at the tooth roots, said teeth each comprising a central core portion and a skin cold worked by impingement on said core, said base, core and skin comprising nodular iron the matrix of which comprises at least about 65% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.8% to about 1.0% by weight and a Vickers hardness between about 613 and 720, and said skin at said roots having a residual compressive stress of between about 40,000 psi and 85,000 psi at its surface and about 10,000 psi at a depth between about 0.008 inch and 0.013 inch below said surface.

Claim 11

A through hardened high stress automotive drive hypoid gear set comprising a ring and pinion gear, said ring gear being capable of withstanding occasional applied tooth root tensile stress excursions of at least about 90,000 psi, and said pinion gear being capable of withstanding occasional applied tooth root tensile stress excursions of at least about 73,000 psi, said gears each comprising a base portion and a plurality of teeth integral with and extending from said base portion at the tooth roots, said teeth each comprising a central core portion and a skin cold worked by impingement on said core, said base, core and skin comprising nodular iron the matrix of which comprises at least about 65% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.8% to about 1.0% by weight and a Vickers hardness between about 613 and 720, and said skin at said roots having a residual compressive stress of between about 40,000 psi and 85,000 psi at its surface and about 10,000 psi at a depth between about 0.008 inch and 0.013 inch below said surface.

Claim 12

A through hardened high stress automotive drive hypoid gear set comprising a ring and pinion gear, said ring gear being capable of withstanding occasional applied tooth root tensile stress excursions of at least about 90,000 psi, and said pinion gear being capable of withstanding occasional applied tooth root tensile stress excursions of at least about 73,000 psi, said gears each comprising a base portion and a plurality of teeth integral with and extending from said base portion at the tooth roots, said teeth each comprising a central core portion and a skin cold worked by impingement on said core, said base, core and skin comprising nodular iron the matrix of which comprises at least about 65% by weight non-feathery acicular bainite having a dissolved carbon content of about 0.8% to about 1.0% by weight and a Vickers hardness between about 613 and 720, and said skin at said roots having a residual compressive stress of between about 40,000 psi and 85,000 psi at its surface and about 10,000 psi at a depth between about 0.008 inch and 0.013 inch below said surface.