Effect of austenite grain size on the structure and contact durability of nitrided steel

Abstract

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...