Surface-hardened Layer Properties of Newly Developed Case-hardening Steel

Abstract

The effects of fine-particle peening conditions on the surface-hardened layer properties of newly developed case-hardening steel, i.e., transformation-induced plasticity-aided steel with a chemical composition of 0.2% C, 1.5% Si, 1.5% Mn, 1.0% Cr, 0.2% Mo, and 0.05% Nb (mass%) were investigated for the fabrication of automotive drivetrain components. The surface roughness decreased with decreasing archeight of fine-particle peening after vacuum carburization. A white layer developed on the surface of the steel peened at arc-heights greater than 0.41 mm (N). The maximum Vickers hardness and maximum compressive residual stress increased with increasing arc-height in the steel. These values were higher than those of commercial case-hardening steels. The increased volume fraction and expansion strain of the strain-induced martensite increased the hardness and compressive residual stress in the surfacehardened layer of the steel, although severe plastic deformation made a substantial contribution to enhancing the surface-hardened layer properties.

Effects of Vacuum-Carburizing Conditions on Surface-Hardened Layer Properties of Transformation-Induced Plasticity-Aided Martensitic Steel

“…With increasing carbon potential, the maximum volume fraction of the retained austenite, the maximum hardness, and the maximum compressive residual stress all increased, although the maximum harness under carbon potentials of 0.70 mass % to 0.80 mass % was nearly the same. These maximum values are slightly higher than those of the SNCM420 steel [19]. The maximum volume fraction of strain-induced martensite of the steels subjected to FPP was obtained on the surface or at the subsurface region under all carbon potentials tested.…”

“…The maximum retained austenite fraction, hardness, and compressive residual stress were strongly dependent on the carbon potential of vacuumcarburization except for the maximum residual stress. The maximum values of retained austenite fraction and Vickers hardness were slightly higher in the TM steel than in the SNCM420 steel [19]. Figure 4 shows the carbon potential dependences of the maximum retained austenite fraction, hardness, and compressive residual stress.…”

“…The volume fractions of the retained austenite and the MA-like phase increased with increasing carbon potential in the TM steel. The TM steel was found to possess a larger amount of retained austenite and a smaller amount of MA-like phase than the SNCM420 steel [19]. …”

“…Higher fatigue strengths of TM steel may be achieved by gas-or vacuum-carburizing with shot peening and/or FPP [14][15][16][17][18]. To date, there have only been a few reports on the microstructure and the fatigue strength of carburized TM steel subjected to FPP [19,20]. However, no investigation has been conducted into the effects of vacuum-carburization conditions on the surface-hardened layer properties of TM steel.…”

An Overview of Fatigue Strength of Case-Hardening TRIP-Aided Martensitic Steels

“…Sugimoto et al [60][61][62] have reported that high fatigue strength and low notch-sensitivity for fatigue of the TM steel are achieved by fine-particle peening and/or vacuum carburization because of small surface roughness and the improved surface-hardened layer properties. In addition, they showed that further increase in fatigue limit of the steels is achieved by using of multi process of vacuum carburization and fine-particle peening which increases the hardness and residual stress in the surface hardened layer [63][64][65].…”