The state of the surface layer is of special importance in connection with the use of nitriding as the final operation in the manufacturing cycle for the preparation of reliable heavily loaded components. In this work the effect of surface plastic deformation (SPD) on the structure and contact durability of nitrided steel 16Kh2N3MFBAYu-Sh (VKS-7) was investigated.It is known that the use of surface plastic deformation as the final operation in the manufacturing cycle increases the contact durability of ground carburized and nitrided components [1, 2] due to the creation of a favorable stressed state in the surface layer. In addition, the effect of machining on the structural state of the surface layer prior to nitriding is important for nitrided components which are not ground. For example, the operation of tooth cutting forms a work-hardened layer 120-150 /zm thick on the surface of machined parts [3].In this work* the structure and properties of steel 16Kh2N3mFBAYu-Sh (0.18% C, 1.85% Cr, 2.9% Ni, 0.44% Mo, 0.13% V, 0.14% Nb, 0.2% Cu, 0.49% Mn, 0.33% Si, 0.01% Ti, 0.01% W, 0.04% AI, 0.014% Ce, 0.018% N, 0.07% S, 0.011% P) after SPD, and also after SPD subsequent ion-nitriding, were investigated.The SPD of specimens for microstructural analysis was carried out using a hard-alloy cutting tool with a blunt cutting edge (radius of the cutting edge = 1.5 ram) without removing a cutting (Fig. 1). In this operation the following working conditions were simulated: cutting speed n = 250 rpm, depth of cut t = 0.3 mm, feed rate Sf = 0.02 mm/turn. Subsequent ion nitriding of specimens for structural investigation was carried out in chemically pure nitrogen in a one-step process at 500°C for 24 h.Specimens for the contact durability tests were hydrojet shot blasted before nitriding. The shot material was ball-bearing steel, particle diameter 0.8-1.2 mm, exit velocity 8 m/sec, shot consumption 10 + 1 kg/min, conveying medium -transformer oil, pressure 0.45 + 0.05 N/ram 2. The kinetic energy of the shot arriving on a unit of treated surface area was calculated according to the equation:where Esp is the specific kinetic energy imparted by the shot to a unit of surface per unit time and r is time for hardening a unit of surface on which the shot falls under the effective angles 45-90 °. The energy E was varied from 50 to 100 kJ/m 2, T from 2 to 4 min.Specimens for the contact durability tests were nitrided by a two-step process: (1) 500°C for 24 h, (2) 540°C for 48 h.
