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The paper presents studies of the structure and properties of low-carbon alloy steels after carburizing. A comparison of four types of carburizing (batch carburization, conveyor carburization, fluidized bed carburization and ionic carburization) is made. The influence of technological modes of chemical-thermal treatment on the phase composition and amount of carbide phase in a carburized layer is considered. It is shown that the type of carburizing affects the operational properties of parts of aircraft mechanisms. The best results were shown by the details after ion vacuum carburizing. The increase in wear resistance and contact fatigue is associated with the formation of a high-quality surface layer having a higher hardness and a higher proportion of the carbide phase. Such properties cannot be obtained after conventional types of carburizing in a shaft or conveyor furnace. In this case, ionic carburizing and carburizing in a fluidized bed make it possible to halve the technological regime of carburizing in comparison with two other methods.

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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.

The austenite grain size in forgings and rolled stock used for the production of medium-size and large nitrided toothed gears is [5][6][7] depending The investigation was carried out using, as an example, the martensitic-class steel 16Kh2N3MFBAYu-Sh (VKS-7) containing 0.18% C, 0.49% Mn, 0.33% Si, 1.85% Cr, 2.9% Ni, 0.2% Cu. 0.01% Ti, 0.44% Mo, 0.13% V, 0.01% W, 0.04% AI, 0.018% N, 0.14% Nb, 0.01% Ce, 0.011% P, 0.07% S.In order to obtain various austenite grain sizes the steel was subjected to the following heat treatments: (1) Heat in salt bath at 1200°C (0.5 h), oil quench (grain size No. 5-6, d~ = 40-60 gin): (2) Normalize at 950°C (1 h) + temper at 690°C (2 11) + oil quench from 900°C (1 h) (grain size No. 8-9, dg = 15-20 #m); C3) Normalize at 950°C (1 h) + temper at 680°C (2 h) + oil quench after high-frequency heating to 950°C (1 rain) (grain size No. = 12-13. dg = 3-5 #m), Specimens treated as indicated above were subjected to a final anneal at 600°C, 2 h, and nitrided in a glow-discharge plasma using a two-step process: (1) 500°C. 20 h; (2) 540°C, 48 h.The microstructures of the nitrided layers on specimens of steel 16Kh2N3MFBAYu-Sh, revealed by chemical etching in a 5 % solution of HNO 3 in alcohol, are shown in Fig. I. The thicknesses of the nitrided zones after different heat treamaents are almost identical, and equal to 10 ~m. Filaments of -,/'-phase are seen at the grain boundaries of all specimens investigated. The depth of penetration of these along the grain boundaries (lv,) depends on the austenite grain size: l.y, = 90, 60, and 25/xm for grain sizes of No. 5-6, No. 8-9, and No. 12-13, respectively. The results of the durometric and microstructural studies are shown in Table I.Investigation of the structure of the nitrided layer by the thin-film method on the transmission electron microscope TESLA-BS-540 at an accelerating voltage of 120 kV showed that practically no coarse grain-boundary precipitates of alloying element nitrides are observed in the nitrided specimens with grain sizes of No. 12-13 and No. 8-9. Coarse precipitates of specific nitrides formed on the high-angle grain boundaries of specimens with a grain size of No. 5-6 during nitriding. This is probably connected with the pronounced decrease in the high-and low-angle grain boundary area.Fractographic studies of the nitrided layer were carried out on the scanning electron microscope TESLA BS-340 at a magnification of 3000 x. It was determined that the mechanism of fracture in the diffusion zone was the same in all casesintragranular (transcrystalline) quasibrittle cleavage, independent of the austenite grain size. Facets exhibiting weak microrelief, at whose boundaries microcracks developed in one direction, were observed on the fractured surface. The quasicleavage facets were much smaller than the austenite grain size (Table 2), and surrounded by indistinct boundaries with fracture ridges. The decrease in facet size on going from grain size No. 5-6 to No. 8-9 is particularly notable. In all cases the fracture of the center of the s...