Specimens of pure Fe and of Fe-0.8 mass % C, Fe-0.5 mass % Cr, Fe-4.0 mass % Cr, Fe-4.0 mass% Ni, and Fe-10.0 mass% Ni alloys were borided in boriding powder. A boron-compound layer developed consisting of a surface-adjacent “FeB” sublayer on top of an “Fe2B” sublayer. Layer-growth kinetics were analyzed by measuring the extent of penetration of the “FeB” and “Fe2B” sublayers as a function of boriding time and temperature in the range 1025–1275 K. Layer growth is dominated by B diffusion through “FeB/Fe2B”. This diffusion process is of strongly anisotropic nature. Consequently, ragged interfaces occur between the substrate and the boride layers. The depths of the tips of the most deeply penetrated “FeB” and “Fe2B” needles have been taken as measures for diffusion in the easy [001] diffusion directions. Assuming unidirectional B diffusion and parabolic growth, a simple model of layer growth has been given. It accounts for the specific volume difference between “FeB” and “Fe2B”. In contrast with earlier work, the model provides values for the kinetic parameters for growth of each of the phases in the boron-compound layer.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400005069

Boriding of Fe and Fe–C, Fe–Cr, and Fe–Ni alloys; Boride-layer growth kinetics

Some mechanical properties of borided AISI H13 and 304 steels

A thermodynamic analysis of the phase equilibria in the Pb-Sn-Sb ternary system was conducted. A regular solution approximation based on a two-sublattice model was adopted to describe the Gibbs energy of formation of the individual phases in the binary and ternary systems. In the case of some component binary systems, the effect of pressure also was considered. Experimental data obtained by differential thermal analysis (DTA) and electron probe microanalysis (EPMA) in the present study, along with literature data on phase boundaries and thermochemical properties, form the basis for the evaluated thermodynamic parameters used in the calculation. Calculated and experimental phase boundaries agree fairly well.

Thermodynamic study of phase equilibria in the Pb-Sn-Sb system

Boriding kinetics of H13 steel

The tribocorrosion behavior of plasma nitrided Ti–6Al–4V alloy is investigated in a 0.9 M NaCl solution at room temperature by means of a sliding wear tribometer, utilizing alumina balls as the counterface material. Tests were performed at open circuit potential and under anodic polarization and the electrochemical parameters, namely anodic current, potential and galvanic current were continuously monitored. The plasma nitriding treatments were performed at two different temperatures, 973 K and 1173 K, and the resulting microstructures and phases were characterized. At 973 K a thin TiN–Ti 2 N layer was formed while at the higher temperature this compound layer was thicker and an inner hardened sublayer was present. In the absence of such an inner layer the sliding of the alumina pins promoted the nucleation and growth of microcracks in the nitride layer, leading to quick failure of the coating. The samples treated at higher temperature showed a more uniform wear process of the nitride layer, without crack formation, and even after its removal, the hardened sublayer did not suffer from the typical delamination wear of untreated Ti–6Al–4V. The damaging or removal of the protective passive film due to rubbing leads to the formation of a galvanic couple between the anodic wear track and the outside surface. The fraction of electrochemically removed material with respect to the total wear volume was approximately 0.2 for the untreated alloy and 0.5 for the nitride layer.

Tribocorrosion behavior of plasma nitrided Ti–6Al–4V alloy in neutral NaCl solution

The variation in hardness of the phases (Fe, M)B and (Fe, M)2B (M ≡ Cr or Ni), which are the predominant components of the borided layer obtained on iron alloys, was defined and related to increase in chromium, nickel and carbon contents. It was found that chromium increases the hardness both of the borided layer as a whole and of the boride components, even though these values are systematically lower than those measured on pure borides. Carbon, which is insoluble in this type of phase, accumulates at the boride-matrix interface and, because of its modification of the boron diffusion mechanism, it indirectly increases the hardness of the borided surface. Nickel reduces slightly but systematically the hardness of the borides, in particular of the (Fe, Ni)2B phase in which it has its highest concentration.

The effect of carbon, chromium and nickel on the hardness of borided layers

The chemical and physical characteristics of ion-nitrided surface layers, obtained on α-β titanium alloys, are examined and correlated both with the working conditions adopted in the ion-nitriding process and with the alloy chemical composition. Besides the influence of the working parameters on the morphology and on the microstructures of the ion-nitrided surface layers, mainly the alloy element distributions both in surface coatings and in the substrate are analysed for five α-β titanium alloys of industrial use, and for titanium c,p. as reference, ionnitrided at various treatment temperatures. The nitriding process forms, on titanium alloy parts, high-hardness surface layers consisting of TiN (δ phase) and Ti2N (ɛ phase) nitrides and an interstitial solid solution of nitrogen in the close-packed hexagonal lattice of titanium (α phase). The presence and the extent of these phases as well as the ion-nitrided layer morphology are essentially determined by the alloy chemical composition and the working parameters. In particular a low-temperature treatment produces an extended nitrogen diffusion in the matrix beneath a thin continuous nitrided layer, while a high-temperature treatment produces prevalently a continuous nitrided surface layer. The alloy element distribution appears differentiated in the various phases and may be correlated with the different affinity of these elements with nitrogen.

Surface engineering and chemical characterization in ion nitrided titanium and titanium alloys

Surface engineering and chemical characterization in ion-nitrided titanium and titanium alloys

Surface layers having elevated hardness were produced by ion nitriding of titanium and α-β alloys of Ti-6wt.%Al-4wt.%V, Ti-4wt.%Al-2wt.%Sn4wt.%Mo and Ti-4wt.%Al-2wt.%Mn. The treatment was conducted at temperatures between 1073 and 1273 K for times of 4–32 h, using gaseous mixtures of nitrogen and hydrogen containing 20–80 vol.% nitrogen. X-ray diffraction, optical microscopy and microhardness were used to characterize the hardened surfaces. In most of the samples, this surface was composed of the phases δ (TiN type), ϵ (Ti 2 N type) and α (solid solution of nitrogen in titanium); in other cases only the δ and α phases were present. The operating parameters which influence the composition of the layer were identified and the particular crystalline texture of the ϵ phase is discussed.

C. Gianoglio

Papers

The effect of carbon, chromium and nickel on the hardness of borided layers

Surface engineering and chemical characterization in ion nitrided titanium and titanium alloys

Surface engineering and chemical characterization in ion-nitrided titanium and titanium alloys

Characterization of surface layers in ion-nitrided titanium and titanium alloys

Laser boronizing of some titanium alloys