A rolling-contact shaft member, which is made of high-carbon steel and whose outer peripheral surface serves as a rolling-contact surface that rolling-contacts a mating material, includes: a carbonitrided layer having a carbon concentration of 1.1 to 1.6 wt % and a nitrogen concentration of 0.05 to 0.6 wt % in the range from the surface to the depth of 10 μm. The rolling-contact shaft member has a Vickers hardness of 700 to 840 HV at the outer peripheral surface and has a Vickers hardness of 600 HV or less in its central portion. A maximum value of an absolute value of a gradient of a change in the Vickers hardness from the outer peripheral surface to the central portion is 100 to 340 HV/mm.

A rolling-contact shaft member, which is made of high-carbon steel and whose outer peripheral surface serves as a rolling-contact surface that rolling-contacts a mating material, includes: a carbonitrided layer having a carbon concentration of 1.1 to 1.6 wt % and a nitrogen concentration of 0.05 to 0.6 wt % in the range from the surface to the depth of 10 μm. The rolling-contact shaft member has a Vickers hardness of 700 to 840 HV at the outer peripheral surface and has a Vickers hardness of 600 HV or less in its central portion. A maximum value of an absolute value of a gradient of a change in the Vickers hardness from the outer peripheral surface to the central portion is 100 to 340 HV/mm.

A method for manufacturing a shaft body by welding a plurality of shaft members together and forming the shaft body, the method including: a primary tempering step of subjecting a range in at least one of the shaft members, which is in the vicinity of an end of another shaft member side adjacent thereto, to tempering before the shaft members are welded together so that a strength of an end side of a region thereof is lower than a strength at a side which is opposite to the end of the region thereof; a welding step of welding the shaft members together after the primary tempering step; and a secondary tempering step of tempering the vicinity of a weld part between the shaft members after the welding step.

A method for manufacturing a shaft body by welding a plurality of shaft members together and forming the shaft body, the method including: a primary tempering step of subjecting a range in at least one of the shaft members, which is in the vicinity of an end of another shaft member side adjacent thereto, to tempering before the shaft members are welded together so that a strength of an end side of a region thereof is lower than a strength at a side which is opposite to the end of the region thereof; a welding step of welding the shaft members together after the primary tempering step; and a secondary tempering step of tempering the vicinity of a weld part between the shaft members after the welding step.

A nitrided steel member including an iron nitride compound layer formed on a surface of a steel member having predetermined components, wherein: in X-ray diffraction peak intensity IFe.sub.4N (111) of a (111) crystal plane of Fe.sub.4N and X-ray diffraction peak intensity IFe.sub.3N (111) of a (111) crystal plane of Fe.sub.3N, which are measured on a surface of the nitrided steel member by X-ray diffraction, an intensity ratio expressed by IFe.sub.4N (111)/{IFe.sub.4N (111)+IFe.sub.3N (111)} is 0.5 or more; Vickers hardness of the iron nitride compound layer is 900 or less, Vickers hardness of a base metal immediately under the iron nitride compound layer is 700 or more, and a difference between the Vickers hardness of the iron nitride compound layer and the Vickers hardness of the base metal is 150 or less; and a thickness of the iron nitride compound layer is 2 to 17 μm.

A nitrided steel member including an iron nitride compound layer formed on a surface of a steel member having predetermined components, wherein: in X-ray diffraction peak intensity IFe.sub.4N (111) of a (111) crystal plane of Fe.sub.4N and X-ray diffraction peak intensity IFe.sub.3N (111) of a (111) crystal plane of Fe.sub.3N, which are measured on a surface of the nitrided steel member by X-ray diffraction, an intensity ratio expressed by IFe.sub.4N (111)/{IFe.sub.4N (111)+IFe.sub.3N (111)} is 0.5 or more; Vickers hardness of the iron nitride compound layer is 900 or less, Vickers hardness of a base metal immediately under the iron nitride compound layer is 700 or more, and a difference between the Vickers hardness of the iron nitride compound layer and the Vickers hardness of the base metal is 150 or less; and a thickness of the iron nitride compound layer is 2 to 17 μm.

An apparatus for backward flow forming a material may comprise a mandrel having a headstock at a proximate end of the mandrel, the mandrel configured to rotate about an axis, a plurality of rollers disposed radially outward of the mandrel configured to exert force on the material to form a work piece at a plastic deformation zone, wherein the work piece flows from the plastic deformation zone between the plurality of rollers and the mandrel toward a distal end of the mandrel, and a catcher, coaxial to the mandrel, and removably coupled to the work piece at a traveling end of the work piece.

A non-thermal refined nitrocarburized component with excellent bending straightening and fatigue strength, includes a base metal steel material having a composition consisting of: (mass %), C: 0.35 to 0.50%; Si: 0.10 to 0.35%; Mn: 2.3 to 2.8%; S: 0.01% or less; N: 0.0030 to 0.0250%; Cu: 0 to 1.0%; Mo: 0 to 0.3%; Ni: 0 to 0.5%; Ti: 0 to 0.020%, the balance: Fe, impurities, and 3.10 £ (0.316 C+0.122).Math.(0.7 Si+1).Math.(5.1 Mn−1.12).Math.(0.364 Ni+1).Math.(2.16 Cr+1).Math.(3 Mo+1) £ 6.00. Impurities include P: 0.08% or less, Al: 0.05% or less, and Cr: less than 0.20%. In a stress concentrated region, an HV hardness 0.05 mm from a surface is 410 to 480, an HV hardness 1.0 mm from the surface is 200 or more, a compound-layer depth is 5 mm or less, and a base metal micro-structure is bainite.

A shaft part excellent in static torsional strength and torsional fatigue strength containing, by mass %, essential elements of C: 0.35 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.5 to 2.6%, P: 0.050% or less, S: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, and O: 0.003% or less, further containing optional elements, having a balance of Fe and impurities, having a chemical composition satisfying formula (1), having at least one hole at an outer circumferential surface, having a volume ratio (R1) of 4 to 20% of retained austenite at a position of a 2 mm depth from the outer circumferential surface, having a volume ratio of retained austenite at a position of a 2 mm depth from the outer circumferential surface in an axial direction of the hole and at a position of a 20 μm depth from the surface of the hole as R2, and having a reduction rate Δγ of 40% or more of retained austenite found by the formula (A): Δγ=[(R1−R2)/R1]×100: Formula (1): 15.0≤25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≤27.2 (Notations of elements in formula are contents of the elements).

A shaft part excellent in static torsional strength and torsional fatigue strength containing, by mass %, essential elements of C: 0.35 to 0.70%, Si: 0.01 to 0.40%, Mn: 0.5 to 2.6%, P: 0.050% or less, S: 0.005 to 0.020%, Al: 0.010 to 0.050%, N: 0.005 to 0.025%, and O: 0.003% or less, further containing optional elements, having a balance of Fe and impurities, having a chemical composition satisfying formula (1), having at least one hole at an outer circumferential surface, having a volume ratio (R1) of 4 to 20% of retained austenite at a position of a 2 mm depth from the outer circumferential surface, having a volume ratio of retained austenite at a position of a 2 mm depth from the outer circumferential surface in an axial direction of the hole and at a position of a 20 μm depth from the surface of the hole as R2, and having a reduction rate Δγ of 40% or more of retained austenite found by the formula (A): Δγ=[(R1−R2)/R1]×100: Formula (1): 15.0≤25.9C+6.35Mn+2.88Cr+3.09Mo+2.73Ni≤27.2 (Notations of elements in formula are contents of the elements).