Internal Nitridation Research Articles

Effects of nitriding at low nitrogen partial pressures on yield and fatigue in some stainless steels

AbstractNitriding stainless steel at nitrogen partial pressures from 0·25 to 0·95 atm and temperatures from 980 to 1160 C resulted in selective internal nitridation of the strong nitride-forming elements, Ti and Cr. Titanium nitride ahrays formed in Tibearing steels, but chromium nitride only for nitriding conditions helmr the Cr2N Solvus. A nitrided case could, therefore, consist of up to three zones, one of nitrogen in solid solution, one of precipitated TiN, and one of Cr2N or mixed rhromium-titanium nitride, depending on nitriding conditions and composition. The case was the shallmrer the greater the intervention of internal nitridation in the diffusion process. Swiace hardness, strength, and fatigue endurance were usefully increased by nitriding. The strengthening was wholly attributed to nitrogen in solid solution, the precipitates being too coarse to contribute. Thus, tensile andfatigue strengthening were directly related to case depth, which, in turn, was optimized by nitriding Ti-free steels with...

Formation of diffusion layers during gas nitriding

1. In the process of two-stage nitriding a layer with a dense zone of e carbonitrides is formed, which ensures dimensional stability of machine parts and good operating characteristics. Grinding can be replaced with light polishing. 2. The properties are best for the nitrided case with a thin (several μm) dense layer of e carbonitrides. The presence of a dense layer accelerates internal nitriding despite the low nitrogen potential of the atmosphere in the second stage of nitriding. 3. To obtain a nitrided layer of smaller thickness (parts of machines not subjected to overhaul) one can use brief gas nitriding with a given nitrogen potential for each steel.

Effects of internal oxidation and nitridation on the magnetic properties of non-oriented electrical steels

The quantitative effects of internal oxidation and nitridation on the core loss and a-c permeability of several silicon-aluminum non-oriented electrical steels are described. Zones of internal oxidation have a large and relatively constant effect on the magnetic properties of alloys containing a wide range of silicon concentrations. For 0.64 mm sheet composed of alloys containing 0.5 to 2.5 percent silicon, oxide subscales increase the 1.5 T, 60 Hz core loss from 0.05 to 0.07 W/kg per μm of subscale thickness. Zones of internal nitridation in aluminum-bearing alloys have a different influence depending on the size and distribution of the nitride precipitates. Zones of internal nitridation in which the individual precipitates have linear dimensions from 0.1 to 1 μm increase the 1.5 T, 60 Hz core loss of 0.64 mm sheet by 0.015 to 0.07 W/kg. Subscales composed of coarse nitrides having maximum linear dimensions from 2 to 7 μm do not increase the core loss significantly.

Mo-Ti合金の内部窒化

Mo-Ti合金の内部窒化

Calculation of hardening of the diffusion layer during nitriding of steel

1. We calculated the hardening of the nitrided layer on steel by use of the Mott-Nabarro theory for coherent nitride precipitates and by the Orowan theory for noncoherent precipitates. 2. The existing nitriding technique does not make it possible to obtain the level of hardening of the diffusion layer that is theoretically possible. 3. The methods considered can be used as the basis for optimizing the geometry and morphology of hardening nitride phases precipitated in zones of internal nitriding and also to calculate the composition of Nitralloys.

Morphology and chemistry of internally nitrided Fe-3at.% Mo

Internal nitriding of Fe-3at.% Mo produces a sequence of precipitated phases of different morphology and chemistry which were identified through a combined use of the field-ion microscope, transmission electron microscope and FIM-atomprobe. The Fe, Mo and N contents of the precipitate continue to change until the final equilibrium phase is reached while the morphology of the precipitate changes from thin coherent platelets (zones) through semi-coherent plates to incoherent spheres. The thin platelets coarsen at 600°C the rate of which appears to be diffusion-controlled. The modified Lifshitz-Wagner theory applied to the initial coarsening rate gives reasonable values for the interfacial energies. An apparent stabilization of the platelet diameters at a later stage of aging was found to be due to the formation of the equilibrium phase.

The effect of internal stable nitride and oxide dispersion on the high temperature oxidation of Fe [sbnd]Cr alloys

The behaviour of Fe Cr alloys containing either a dispersed nitride or oxide phase is examined. Fe-12, 14 or 18% Cr containing 0.5 or 1 wt% Ti were used. The oxide dispersion was introduced by an internal oxidation treatment using a 50/50 Cr/Cr 2O 3 mixture in a sealed quartz capsule at 1100 or 1200°C. Internal nitrides were produced by nitridation in either an 80/20 or a 90/10 H 2/N 2 mixture at 1150°C. Subsequent oxidation tests of the treated alloys were carried out isothermally at 900, 1000 and 1100°C, or by thermal cycling (3 h cycles) at 1100°C. The dispersed nitride phase is not as effective in improving the oxidation resistance as corresponding dispersed oxides. Considerable particle coarsening is observed and the nitrides tend to dissociate and are converted to oxide near to the alloy/scale interface.

Hardening of alloy TsM2A by internal nitriding

Hardening of alloy TsM2A by internal nitriding

Nitride-Hardening of an Austenitic Stainless Steel Containing Titanium

AbstractThe internal nitriding of Fe : 20 wt.-% Cr, 25 wt.-% Ni, 1·5 wt.-% Ti has been investigated at 900–1150° C using nitrogen pressures of 0·1–1·0 atm. The conditions under which Cr2N and CrN are formed and then subsequently reduced in hydrogen are described. The precipitated phase previously reported as TiN is identified as (Ti, Cr)N in which the chromium content decreases with (1) increasing nitriding temperature, (2) decreasing nitrogen pressure, and (3) hydrogen reduction. The particle size of the precipitated (Ti,Cr)N increases linearly with distance from the surface and the precipitate density is inversely proportional to the cube of this distance. The particle size also increases slowly with decreasing nitrogen potential and markedly with increasing temperature. For the experimental conditions used, the nitriding rates are in accordance with the kinetics deduced from internal nitriding or oxidation theory. The nitrided alloy shows significant improvements in high-temperature tensile properties ...

The nitridation of chromium and Cr-Ti alloys

Reaction zones and growth kinetics were studied after exposing high-purity chromium and alloys containing 0.5, 3.0, and 5.0 wt pct Ti to 1 atm of nitrogen between 1000° and 1400°C. Outer layers of Cr2N and regions of internal nitridation, containing dispersed TiN particles, grew in a parabolic manner. An exact solution of Maak’s simplified analysis for internal oxidation provided calculations of nitrogen diffusion in the internal-nitride zone of each alloy. Extrapolation gave the relationshipD = 9.6 × 10-3 exp (-28,500/RT) cm2 sec-1 for nitrogen diffusion in high-purity chromium. Increasing titanium to 5.0 wt. pct gaveD = 2.5 × 10-3 exp (-24,000/RT) cm2 sec-1.

Mechanisms of growth and loss of coherency of HfN particles in molybdenum

Internal nitriding of a Mo-1 at.% Hf solid-solution alloy produces a homogeneous dispersion of coherent plate-like HfN precipitates oriented on {001} planes of the Mo matrix. During nitriding growth of the HfN particles occurs principally at the advancing nitriding front by Ostwald ripening, which produces an increase in average particle size with increasing depth below the external surface. The high nitrogen activity in the alloy behind the nitriding front effectively inhibits dissociation and growth of the precipitates. The plate-like HfN particles grow in diameter in a coherent manner. Thickening of the particles occurs by the nucleation and growth of coherent ledges on the planar surfaces. This thickening process produces an increase in the coherency strain which is relaxed by the formation of intrinsic dislocation loops within the particles by the coalescense of vacancies. The large HfN particles developed by this growth mechanism retain substantial coherency strain. A severe particlecoarsening mechanism is operative at high temperatures in regions of the nitrided alloy containing a high density of small fully coherent precipitates. The coarsening occurs by the migration of grain boundaries which sweep up the small fully coherent precipitates, the components of which then feed large semicoherent particles growing behind the boundary.

Dispersion strengthening austenitic stainless steels by nitriding

The internal nitridation of thin sections of austenitic Fe−Cr−Ni−Ti alloys containing up to 2 pct Ti was studied over the temperature range 1600° to 2210°F in order to develop a method of strengthening the alloys through the introduction of a dispersoid of stable titanium nitrides. The interparticle spacing (IPS) of the nitrides was found to increase linearly with depth from the external surface; the effects of various parameters on the rate of change of IPS vs depth are presented. The mechanical properties of these alloys at room and elevated temperatures were markedly improved by internally nitriding. Useful mechanical properties were obtained up to 2200°F, with typical properties at 2000°F of 10 to 20 ksi 0.2 pct offset yield strength and 15 to 25 ksi ultimate tensile strength, but section thickness was limited to about 10 mils because of the increase in IPS with depth and the long nitriding times needed for thicker material. In order to produce a small interparticle spacing in a heavier section, internally nitrided 5 mil strip was consolidated by hot roll bonding and evaluated at a 60 mil thickness by tensile and rupture testing at 2000°F. It is demonstrated that the approach taken in this work offers a feasible technique for making a high temperature alloy having useful engineering properties.

Method of coupling liquid heads with ultrasound: Huttenwerk Oberhausan AG , French patent 1, 565, 567 (2 May 1969) (applied for 14 March 1968) (in West Germany 22 March 1967)

The very high cycle fatigue properties of the Ni-based single-crystal superalloy CMSX-4 coated with an MCrAlY type coating and grit-blasted were studied at 1000 °C, 20 kHz and R = -1. A recrystallized layer thicker than the size of the larger casting pores (>50 μm) drastically reduces the fatigue life without environmental influence. The recrystallized layer and roughness introduced by grit-blasting degrades the environmental resistance of the alloy and the resultant fatigue life. The fatigue life of the coated specimens is also greatly reduced from damage accumulation in the interdiffusion zone by grain boundary cracking, oxidation and internal nitridation.

Kinetics of the internal nitridation of austenitic Fe-Cr-Ni-Ti alloys

Austenitic alloys containing up to 4 pct Ti were nitrided over the temperature range 1805° to 2210°F. The rate of internal nitridation for the simultaneous formation of both chromium and titanium nitrides was found to fit the theoretical expressions developed for internal oxidation. From the theoretical expressions the diffusion coefficients were calculated from the data, and the activation energy for diffusion of nitrogen in an 18Cr-12Ni-2Ti alloy was determined to be −47.4±0.3 kcal per mol. The presence of oxygen in the nitriding gas is shown to cause a decrease in nitriding rate due to surface limiting of nitrogen absorption; this effect of oxygen became progressively more pronounced as pct Ti was increased and temperature was decreased. Similarly, boron, carbon, silicon, and nickel were found to decrease the nitriding rate; this decrease in rate is traced to a decrease in the solubility for nitrogen in the alloy as these elements are added.

Internal nitriding of tungsten-base alloys containing hafnium

It is shown that W, W-Re, and W-Re-Mo alloys containing as much as 3 at. % Hf in solid solution are subject to internal nitriding when exposed to nitrogen gas at temperatures above about 1500°C. The hafnium nitride particles which are formed as a result of nitriding precipitate as plates on {001} matrix planes and this plate-like habit is retained up to at least 2000°C. At 1 atm N 2 pressure and temperatures between 1500 and 2000°C, the kinetics of internal nitriding are found to be controlled exclusively by the diffusion of nitrogen in the base metal or alloy matrix. The simple theoretical model developed by Wagner for the diffusion-controlled kinetics of internal oxidation is shown to provide an adequate description of the internal nitriding behavior. Permeation constants for nitrogen in W derived from the measured internal nitriding rates are found to be in good agreement with values obtained from direct steady-state permeation measurements. Using published solubility data, the diffusion coefficient for nitrogen in tungsten, D = D 0 exp ( − Q D / RT), has been evaluated from the relation P = ND, where P is the permeation constant and N the solubility at 1 atm pressure. The results yield D 0≅ 1.2 × 10 −2 cm 2/sec and Q D ≅ 32.2 kcal/mole.

Internal nitridation of copper alloys

Internal nitridation of copper alloys