A machine component includes a core made up of a steel for machine structural use, and a medium carbon-containing layer and a high carbon-containing layer formed of the steel for machine structural use, the medium carbon-containing layer covering the core, the high carbon-containing layer covering the medium carbon-containing layer and having a carbon concentration of 0.8-1.5%. The high carbon-containing layer is made up of a martensitic structure having carbides dispersed therein and a residual austenitic structure, wherein spheroidized carbides with an aspect ratio of 1.5 or less constitute 90% or more of a total number of the carbides, and the number of spheroidized carbides on prior austenite grain boundaries is 40% or less of the total number of the carbides.

A method for producing a machine component excellent in pitting resistance characteristics and toughness includes a carburizing step, performed on a steel material containing 0.13-0.30% C and 0.90-2.00% Cr in mass % and at least one of Si, Mn, Ni, Mo, Nb, V, Ti, B, Al, and N, balance Fe and unavoidable impurities; heating the material to 850-1030 C. to attain carbon concentration in a surface of 0.8-1.5%; cooling the material at an average rate of 5 C./sec or lower from a temperature higher than the A.sub.cm point of a surface layer to a cooling end temperature that is at least 50 C. lower than the A.sub.1 point to cause the surface layer to have a pearlite or bainite structure with dispersion; spheroidizing annealing at a temperature not higher than the A.sub.cm point at the surface layer; heating the material to not higher than the A.sub.cm point at the surface layer; and performing tempering.

Described is a method of modifying material properties of a workpiece using a high-pressure-torsion apparatus, comprising a working axis, a first anvil, a second anvil, and an annular body, comprising a first recirculating convective chiller, a second recirculating convective chiller, and a heater, positioned between the first recirculating convective chiller and the second recirculating convective chiller along the working axis. The method comprises compressing the workpiece along a central axis of the workpiece and. simultaneously with compressing the workpiece along the central axis, twisting the workpiece about the central axis. The method further comprises. while compressing the workpiece along the central axis and twisting the workpiece about the central axis, translating the annular body along the working, axis of the high-pressure-torsion apparatus, collinear with the central axis of the workpiece, and heating the workpiece with the heater.

Described is a method of modifying material properties of a workpiece using a high-pressure-torsion apparatus, comprising a working axis, a first anvil, a second anvil, and an annular body, comprising a first total-loss convective chiller, a second total-loss convective chiller, and a heater, positioned between the first total-loss convective chiller and the second total-loss convective chiller along the working axis. The method comprises compressing the workpiece along a central axis of the workpiece and, simultaneously with compressing the workpiece along the central axis, twisting the workpiece about the central axis. The method further comprises, while compressing the workpiece along the central axis and twisting the workpiece about the central axis, translating the annular body along the working axis of the high-pressure-torsion apparatus, collinear with the central axis of the workpiece, and heating the workpiece with the heater.

Described is a method of modifying material properties of a workpiece using a high-pressure-torsion apparatus, comprising a working axis, a first anvil, a second anvil, and an annular body, comprising a first conductive chiller, a second conductive chiller, and a heater, positioned between the first conductive chiller and the second conductive chiller along the working axis. The method comprises compressing the workpiece along a central axis of the workpiece and, simultaneously with compressing the workpiece alone the central axis, twisting the workpiece about the central axis. The method further comprises, while compressing the workpiece along the central axis and twisting the workpiece about the central axis, translating the annular body along the working axis of the high-pressure-torsion apparatus, collinear with the central axis of the workpiece, and heating the workpiece with the heater.

When a rod-shaped workpiece (W) having an outer peripheral surface with a circular cross section is inductively heated to a quenching temperature while being conveyed at a predetermined velocity along an axial direction of the rod-shaped workpiece (W), the rod-shaped workpiece (W) being currently conveyed is heated to a predetermined temperature equal to or lower than the quenching temperature by a first heating coil (2A), which is electrically connected to a first high-frequency power supply (3) and has a constant output. Then, the rod-shaped workpiece (W) being currently conveyed is heated so as to be maintained at the quenching temperature by a second heating coil (2B), which is electrically connected to a second high-frequency power supply (4) and has a constant output.

When a rod-shaped workpiece (W) having an outer peripheral surface with a circular cross section is inductively heated to a quenching temperature while being conveyed at a predetermined velocity along an axial direction of the rod-shaped workpiece (W), the rod-shaped workpiece (W) being currently conveyed is heated to a predetermined temperature equal to or lower than the quenching temperature by a first heating coil (2A), which is electrically connected to a first high-frequency power supply (3) and has a constant output. Then, the rod-shaped workpiece (W) being currently conveyed is heated so as to be maintained at the quenching temperature by a second heating coil (2B), which is electrically connected to a second high-frequency power supply (4) and has a constant output.

Provided is a method for manufacturing a magnetostrictive torque sensor shaft mounting a sensor portion of a magnetostrictive torque sensor. The method includes conducting heat treatment on a shaft material including chrome steel or chrome-molybdenum steel by carburizing, quenching and tempering, and conducting shot peening on the shaft material after the heat treatment at least on a position where the sensor portion is to be mounted. The shot peening is conducted by firing shot with a particle size of not less than 0.6 mm and a Rockwell hardness of not less than 60 at a jet pressure of not less than 0.4 MPa for a jet exposure time of not less than 2 minutes.

Provided is a method for manufacturing a magnetostrictive torque sensor shaft mounting a sensor portion of a magnetostrictive torque sensor. The method includes conducting heat treatment on a shaft material including chrome steel or chrome-molybdenum steel by carburizing, quenching and tempering, and conducting shot peening on the shaft material after the heat treatment at least on a position where the sensor portion is to be mounted. The shot peening is conducted by firing shot with a particle size of not less than 0.6 mm and a Rockwell hardness of not less than 60 at a jet pressure of not less than 0.4 MPa for a jet exposure time of not less than 2 minutes.

The invention provides a method for preventing cracking along the surface at the inner hole of a hollow shaft during horizontal water quenching, including: step 1: transferring the shaft from heating furnace to quenching equipment; step 2: cooling the inner hole of the shaft, while the outer circle of the shaft is in the state of air cooling, and the cooling time on the outer circle is designed based on the temperature of the shaft being not lower than its A.sub.1 temperature, therefore, the compressive stress is formed at the surface layer of the inner hole, and small tensile stress is formed at the sub-surface layer; and step 3: cooling the outer circle and the inner hole of the shaft at the same time, wherein during the cooling, the cooling intensity of the inner hole is adjusted to gradually reduce so that sub-surface layer of the inner hole still exhibits low tensile stress while martensite formed in the surface layer experiences self-tempering, which effectively prevent the formation of quenching cracks along the surface of the inner hole. The invention solves the cracking problem during the horizontal water quenching for hollow shafts with large diameter and small hole-diameter.