Method for heat treating by induced current

A method of hardening steel workpieces in localized areas to closely controlled depths of hardening with a high degree of symmetry and uniformity comprising the steps of:

• feeding each workpiece axially to an inductor coil with a dwell at a position when the localized areas to be treated are surrounded by the coil with the spacing between the workpiece and the coil materially exceeding the spacing required for close electromagnetic coupling, the parts being positionally related for loose electromagnetic coupling in a zone having a substantially lower flux density gradient than zones closely adjacent to said coil to minimize product variations resulting from inaccurate positioning of the workpiece, slight irregularities in the coil or dimensional variations from workpiece to workpiece, such feeding occurring with loose mechanical constraint of the workpiece so that it is approximately centered with respect to the coil but is free to float to a more precisely centered position, energizing the coil during the dwells to electromagnetically precisely align and center each workpiece with respect to the coil and to establish loosely coupled induction heating of each workpiece with however a sufficiently high rate of heat generation to raise the metal in the localized areas to austenitizing temperature at locations shallower than the desired depth of hardening without heat penetration to austenitizing temperature occurring below the desired depth of hardening, and quenching the workpiece by subjecting it to coolant fluid to accomplish rapid cooling prior to the occurrence of austenitizing heat penetration below the desired depth of hardening.

A method of hardening steel workpieces in localized areas to closely controlled depths of hardening with a high degree of symmetry and uniformity, the workpieces comprising screws, bolts or fasteners, or the like having a head portion of relatively large outside diameter and a shank portion of relatively small outside diameter, the method comprising the steps of:

• providing an inductor coil having an inside diameter greater than the maximum outside diameter of the workpieces, unidirectionally feeding the workpieces including the heads thereof axially through the coil with a dwell at a position when each workpiece shank is surrounded by the coil and is substantially centered with respect to the electromagnetic field produced thereby, the spacing between said shank and coil materially exceeding the spacing required for close electromagnetic coupling, energizing the coil to accomplish loosely coupled induction heating of each workpiece shank with the coil and any intervening insulator means spaced from the shank by at least the distance by which the head of the workpiece protrudes radially beyond the shank with however a sufficiently high rate of heat generation to raise the metal in the localized areas to austenitizing temperature at locations shallower than the desired depth of hardening without heat penetration to austenitizing temperature occurring below the desired depth of hardening, and quenching the workpiece by subjecting it to coolant fluid to accomplish rapid cooling prior to the occurrence of austenitizing heat penetration below the desired depth of hardening.

A method of hardening steel workpieces in localized areas to closely controlled depths of hardening with a high degree of symmetry and uniformity, the workpieces comprising screws, bolts or fasteners, or the like having a head portion of relatively large outside diameter and a shank portion of relatively small outside diameter, the method comprising the steps of:

• providing an inductor coil having an inside diameter greater than the maximum outside diameter of the workpieces, unidirectionally feeding the workpieces including the heads thereof axially through the coil with a dwell at a position when each workpiece shank is surrounded by the coil and is spaced therefrom by an amount materially exceeding the spacing required for close electromagnetic coupling, such feeding occurring with loose mechanical constraint of the workpiece so that its shank is approximately centered with respect to the coil but is free to float to a more precisely centered position, energizing the coil during the dwells to electromagnetically precisely align and center each workpiece shank with respect to the coil and to accomplish loosely coupled induction heating of each workpiece shank with the coil and any intervening insulator means spaced from the shank by at least the distance by which the head of the workpiece protrudes radially beyond the shank with however a sufficiently high rate of heat generation to raise the metal in the localized areas to austenitizing temperature at locations shallower than the desired depth of hardening without heat penetration to austenitizing temperature occurring below the desired depth of hardening, and quenching the workpiece by subjecting it to coolant fluid to accomplish rapid cooling prior to the occurrence of austenitizing heat penetration below the desired depth of hardening.

A method of hardening steel workpieces in localized areas to closely controlled depths of hardening with a high degree of symmetry and uniformity, the workpieces comprising thread taps, cutting tools, recess driving tools or the like with a portion of the shank having alternating flutes and ridges extending lengthwise thereof, comprising the steps of:

• feeding each workpiece axially through an inductor coil with a dwell at a position where each workpiece is substantially centered with respect to the electromagnetic field produced by said coil, each workpiece in said position having the fluted and ridged shank surrounded by the coil with the spacing between the workpiece and the coil materially exceeding the spacing required for close electromagnetic coupling, the parts being positionally related for loose electromagnetic coupling in a zone having substantially lower flux density gradient than the zones closely adjacent to said coil to minimize product variations resulting from, inaccurate positioning of the workpiece, slight irregularities in the coil or dimensional variations from workpiece to workpiece, energizing the coil during the dwells to establish loosely coupled induction heating of each workpiece with however a sufficiently high rate of heat generation to raise the metal in the localized areas to austenitizing temperature at locations shallower than the desired depth of hardening without heat penetration or austenitizing temperature occurring below the desired depth of hardening, and quenching the workpiece by subjecting it to coolant fluid to accomplish rapid cooling prior to the occurrence of austenitizing heat penetration below the desired depth of hardening.

A method of hardening steel workpieces in localized areas to closely controlled depths of hardening with a high degree of symmetry and uniformity, the workpieces comprising thread taps, cutting tools, recess driving tools or the like with a portion of the shank having alternating flutes and ridges extending lengthwise thereof, comprising the steps of:

• feeding each workpiece axially through an inductor coil with a dwell at a position where the fluted and ridged shank is surrounded by the coil with the spacing between the workpiece and the coil materially exceeding the spacing required for close electromagnetic coupling, the parts being positionally related for loose electromagnetic coupling in a zone having substantially lower flux density gradient than the zones closely adjacent to said coil to minimize product variations resulting from, inaccurate positioning of the workpiece, slight irregularities in the coil and dimensional variations from workpiece to workpiece, such feeding occurring with loose mechanical constraint of the workpiece so that the fluted and ridged shank is approximately centered with respect to the coil but is free to float to a more precisely centered position, energizing the coil during the dwells to electromagnetically precisely align and center each workpiece with respect to the coil and to establish loosely coupled induction heating of each workpiece with however a sufficiently high rate of heat generation to raise the metal in the localized areas to austenitizing temperature at locations shallower than the desired depth of hardening without heat penetration to austenitizing temperature occurring below the desired depth of hardening, and quenching the workpiece by subjecting it to coolant fluid to accomplish rapid cooling prior to the occurrence of austenitizing heat penetration below the desired depth of hardening.