Nitrogen solubility and aluminum nitride precipitation in liquid iron, Fe-Cr, Fe-Cr-Ni, and Fe-Cr-Ni-Mo alloys

The nitrogen solubility and aluminum nitride formation in liquid Fe-Al, Fe-Cr-Al, Fe-18 pct Cr-8 pct Ni-Al and Fe-18 pct Cr-8 pct Ni-Mo-Al alloys were measured by the Sieverts' method. The temperature range extended from 1823 to 2073 K, and the aluminum contents from 1.01 to 3.85 wt pct Al. Increasing aluminum content increases the nitrogen solubility. The effect of molybdenum additions was determined for 2, 4 and 8 wt pct Mo levels. The first and second order effects of chromium, nickel, molybdenum and aluminum on the activity coefficient of nitrogen in iron were determined. The first and second order effects of chromium, nickel and molybdenum on the activity coefficient of aluminum also were determined. The nitride precipitates were identified as stoichiometric aluminum nitride, AIN, by X-ray diffraction analysis. The lattice spacing was in good agreement with the ASTM standard patterns for AIN in both higher and lower Al content solutions. The solubility product of AIN increases with increasing aluminum concentration and with temperature in liquid iron and the iron alloys studied. However, the magnitudes of the solubility products of AIN in those alloys are different because of the effects of chromium and nickel additions. Additions of molybdenum show little effect on the solubility product of AIN. The standard free energy of formation of AIN in liquid iron is: δG‡ = -245,990 + 107.59 \T J/g-molAIN, based on the standard state of the infinitely dilute solution in liquid iron for aluminum and nitrogen, referred to a hypothetical one wt pct solution, and on the pure compound for A1N.     

Nitrogen solubility and aluminum nitride precipitation in liquid iron alloys containing nickel and aluminum

The solubility of nitrogen in liquid iron-base Fe-Ni-Al alloys has been measured up to the solubility limit for formation of aluminum nitride using the Sieverts’ method. Measurements were conducted over the temperature range from 1843 to 2023 K and aluminum concentration range from 1.5 to 3.0 wt pct Al. The effect of nickel additions was determined at 2, 5 and 10 wt pct Ni. The cross interaction parameter describing the effect of nickel and aluminum on the activity coefficient of nitrogen in iron was determined. The first and second order effects of nickel on the activity coefficient of aluminum also were determined. The solubility product of aluminum nitride increases with increasing aluminum content and increasing temperature. Addition of nickel decreases the solubility products of aluminum nitride in lower aluminum content alloys. However, the effect of the cross interaction terme _Al ^NiAl becomes significant with increasing aluminum content and compensates for the effects of the first and second order nickel-nitrogen and nickelaluminum interaction terms. Therefore the effect of nickel additions show little effect on the solubility products of aluminum nitride in higher aluminum alloys.     

Nitrogen solution and titanium nitride precipitation in liquid Fe-Cr-Ni alloys

The solubility of nitrogen in liquid Fe-Cr, Fe-Ni, Ni-Cr, and Fe-Cr-Ni alloys up to 20 wt pct Ni and 40 wt pct Cr was measured by the Sieverts’ method. The first and second order interactions in iron between nitrogen and chromium, and nitrogen and nickel were determined. Chromium increases the nitrogen solubility at lower chromium concentrations but the second order interaction term which is of the opposite sign becomes significant at higher chromium levels and compensates partly for the effect of the first order interaction term. Nickel decreases the nitrogen solubility in iron. Titanium nitride formation in liquid Fe-Cr, Fe-Ni, and Fe-Cr-Ni alloys also was investigated. The first and second order interactions between titanium and chromium or nickel were determined at 1600°C. Chromium increases the solubility product of TiN, principally by decreasing the activity of nitrogen in the melt. Nickel decreases the solubility product of TiN by increasing the activities of nitrogen and titanium.     

Nitrogen solubility in liquid Fe-V and Fe-Cr-Ni-V alloys

Nitrogen solubility in liquid Fe, Fe-V, Fe-Cr-V, Fe-Ni-V and Fe-18 pct Cr-8 pet Ni-V alloys has been measured using the Sieverts’ method for vanadium contents up to 15 wt pct and over the temperature range from 1775 to 2040 K. Nitrogen solution obeyed Sieverts’ law for all alloys investigated. Nitride formation was observed in Fe-13 pet V, Fe-15 pet V and Fe-18 pet Cr-8 pet Ni-10 pet V alloys at lower temperatures. The nitrogen solubility increases with increasing vanadium content and for a given composition decreases with increasing temperature. In Fe-V alloys, the nitrogen solubility at 1 atm N₂ pressure is 0.72 wt pet at 1863 K and 15 pct V. The heat and entropy of solution of nitrogen in Fe-V alloys were determined as functions of vanadium content. The first and second order interaction parameters were determined as functions of temperature as: $$e_N^V = \frac{{ - 463.6}}{T} + 0.148 and e_N^{VV} = \frac{{17.72}}{T} - 0.0069$$ The effects of alloying elements on the activity coefficient of nitrogen were measured in Fe-5 pet and 10 pet Cr-V, Fe-5 pet and 10 pet Ni-V and Fe-18 pet Cr-8 pct Ni-V alloys. In Fe-18 pet Cr-8 pet Ni-10 pet V, the nitrogen solubility at 1 atm N₂ pressure is 0.97 wt pet at 1873 K. The second order cross interaction parameters, e _N ^Cr,V and e _N ^Ni,V , were determined at 1873 K as 0.00129 and ? 0.00038 respectively.     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at 1873 K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logJn, 1873K = 0 to −1.4 as, log(wt pct N)_T = (-247/T-1.22)-(4780/T-1.51) (logf N, 1873K)- (1760/T-0.91) (logfN,1873K)².     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at {dy1873} K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logf_N, 1873K = 0 to -1.4 as, log (wt pct N)_T = (-247/T - 1.22) - (4780JT - 1.51) (logf_n, 1873K)-(1760/T -0.91) (logf_N,{dy1873}K )².     

Effect of titanium on the nitrogen solubility in complex liquid Fe−Cr−Ni alloys

The effects of dilute additions of titanium up to 0.20 wt pct on the solubility of nitrogen in two complex Fe−Cr−Ni alloys were examined over the temperature range 1450 to 1600°C. Sieverts' law was obeyed by all titanium-bearing alloys up to some nitrogen pressure below one atmosphere. ‘Breaks’ in each solubility plot were observed that corresponded to the formation of titanium nitride. Titanium additions were observed to lower the nitrogen solubility in each group of alloys. This effect is opposite to that previously observed in pure iron. Calculated values of the solubility product (pct Ti) (pct N) for TiN formation in each alloy increased with rising melt temperature.     

The formation of AIN crystals in an Fe-Mn-Al-C alloy during the nitriding process

We observed the formation of AIN crystals in an Fe-Mn-Al-C alloy during a nitriding process. The composition of the alloy was Fe-30.4 wt pct Mn-8.7 wt pct Al-1.0 wt pct C. The nitriding process consisted of heating the Fe-Mn-Al alloy in pure nitrogen at 1000 °C for 1 hour. During nitriding, AIN crystals formed in the regions near the surface layer of the alloy. The aluminum nitride formed along specifically preferred orientations of the metal matrix. We discovered that the formation of the secondary arms of the AIN crystals was related to the direction of flux of the nitrogen, and we proposed a growth mechanism for the formation of secondary arms on the primary AIN plates. As the supply of nitrogen continued from the surface, the AIN Widmanstätten side plates formed the secondary arms growing on the leeward side of the primary AIN plates.     

Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys

The thermodynamics of titanium in Fe-Cr alloys and of inclusion formation in Fe-Cr-N-Ti alloys was investigated. A metal-nitride-gas equilibration technique was used to measure the activity of titanium. The equilibrium titanium content of the metal that is in equilibrium with pure solid titanium nitride and nitrogen gas at 1 atm was determined. The activity coefficients of titanium it(f_Ti) relative to 1 wt pct standard state in Fe were calculated for Fe-Cr alloys from the experimental results. The first-order interaction coefficient between titanium and chromium, e _Ti ^Cr , was determined to be 0.024 at 1873 K. The solubility of nitrogen in Fe-Cr alloys was measured and was found to increase with chromium content, which is in agreement with previous work. Thermodynamic calculations were made in order to predict under what conditions titanium nitride will form in 409 stainless steel and was compared with inclusions found in plant samples. The inclusion stability diagrams for 304 stainless steel and Fe-18 pct Cr and Fe-9 pct Cr alloys were computed.     

Nitrogen solubility in liquid Fe-Ta,Fe-Cr-Ta,Fe-Ni-Ta,and Fe-Cr-Ni-Ta alloys

The nitrogen solubility in liquid Fe-Ta, Fe-Cr-Ta, Fe-Ni-Ta, and Fe-18 pet Cr-8 pet Ni-Ta alloys was measured using the Sieverts’ method. The experiments covered the temperature range from 1782 to 2031 K, and tantalum contents from 2.0 to 20.0 wt pct Ta. Nitrogen solution obeyed Sieverts’ law and no nitride precipitation was observed in this concentration range. Tantalum increases the nitrogen solubility and the heat of solution of nitrogen is more negative at higher tantalum contents in these alloys. The excess enthalpy and entropy of solution of nitrogen were determined. The first and second order interaction parameters between nitrogen and tantalum were determined as a function of temperature, e _N ^Ta = -101.7/T + 0.018 and e _N ^TaTa = -3.27/T + 0.0022. The effects of alloying elements on the activity coefficient of nitrogen were measured and the second order cross-interaction parameters between nitrogen and Ta with Cr and Ni were determined at 1873 K as e _N ^CrTa = 0.00052 and e _N ^NiTa = 0.00045.     

Nitrogen solubility and nitride formation in austenitic Fe-Ti alloys

The equilibrium nitrogen solubility and nitride formation in austenitic Fe and Fe-Ti alloys were measured in the temperature range from 1273 to 1563 K. Specimens 0.5 mm thick were equilibrated with four different nitrogen-argon gas mixtures containing 1 pct hydrogen. The nitrogen solubility in austenitic iron obeys Sieverts' law. The equilibrium nitrogen content was determined to be log (wt pct N)_γ-Fe, P_{N2=1 atm} = (539 ± 17)/T − (2.00 ± 0.01). The precipitated titanium nitride was identified as cubic TiN, and the solubility product was determined to be log(wt pct Ti) (wt pct N) = −14,400/T + 4.94.     

Corrosion Behavior of High Nitrogen Nickel-Free Fe-16Cr-Mn-Mo-N Stainless Steels

The purpose of the current study is to develop austenitic nickel-free stainless steels with lower chromium content and higher manganese and nitrogen contents. In order to prevent nickel-induced skin allergy, cobalt, manganese, and nitrogen were used to substitute nickel in the designed steel. Our results demonstrated that manganese content greater than 14 wt pct results in a structure that is in full austenite phase. The manganese content appears to increase the solubility of nitrogen; however, a lower corrosion potential was found in steel with high manganese content. Molybdenum appears to be able to increase the pitting potential. The effects of Cr, Mn, Mo, and N on corrosion behavior of Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were evaluated with potentiodynamic tests and XPS surface analysis. The results reveal that anodic current and pits formation of the Fe-16Cr-2Co-Mn-Mo-N high nitrogen stainless steels were smaller than those of lower manganese and nitrogen content stainless steel.     

Estimation of the Temperature-Dependent Nitrogen Solubility in Stainless Fe-Cr-Mn-Ni-Si-C Steel Melts During Processing

The influence of chemical composition, temperature, and pressure on the nitrogen solubility of various high alloy stainless steel grades, namely Fe-14Cr-(0.17-7.77)Mn-6Ni-0.5Si-0.03C [wt pct], Fe-15Cr-3Mn-4Ni-0.5Si-0.1C [wt pct], and Fe-19Cr-3Mn-4Ni-0.5Si-0.15C [wt pct], was studied in the melt. The temperature-dependent N-solubility was determined using an empirical approach proposed by Wada and Pehlke. The thus calculated N-concentrations overestimate the actual N-solubility of all the studied Fe-Cr-Mn-Ni-Si-C steel melts at a given temperature and pressure. Consequently, the calculation model has to be modified by Si and C because both elements are not recognized in the original equation. The addition of the 1st and 2nd order interaction parameters for Si and C to the model by Wada and Pehlke allows a precise estimation of the temperature-dependent nitrogen solubility in the liquid steel bath, and fits very well with the measured nitrogen concentrations during processing of the steels. Moreover, the N-solubility enhancing effect of Cr- and Mn-additions has been demonstrated.     

Thermodynamics of carbon in nickel,iron-nickel and iron-chromium-nickel alloys

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH₄-H₂ gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.     

The effect of nitrogen on stacking fault energy of Fe-Ni-Cr-Mn steels

The effect of nitrogen content on stacking fault energy (SFE) has been measured in a series of Fe-21Cr-6Ni-9Mn alloys. Stacking fault energies were determined from node measurements using weak beam imaging techniques in transmission electron microscopy. Nitrogen additions lower the SFE from 53 mJ/m² at 0.21 wt pct to 33 mJ/m² at 0.24 wt pct. Further increases to 0.52 wt pct do not markedly change the SFE. Carbon and silicon had no effect on SFE in the ranges 0.010 to 0.060 wt pct C and 0.17 to 0.25 wt pct Si. The shift in SFE from 0.21 to 0.24 wt pct N is accompanied by a transition to a more planar plastic deformation mode. The sharp transition precludes the use of linear regression analysis for relating SFE to nitrogen content in this class of alloys.     

Microstructure and mechanical properties of ductile Ni3AI-type compound in Fe-(Ni,Mn)-AI-C systems rapidly quenched from melts

By the rapid quenching technique, nonequilibrium Ni₃Al-type compounds with high strength and hardness as well as large elongation have been found in Fe-Ni-Al-C and Fe-Mn-Al-C systems. This formation region is limited to about 7 to 55 wt pct Ni, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Ni-Al-C and to about 7 to 65 wt pct Mn, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Mn-Al-C. The Ni₃Al-type compound has fine grains of about 1 to 10 μm in diam. Their Vickers hardness and yield strength increase with increase in the amounts of carbon, aluminum or nickel and the highest values attain about 665 DPN and 1690 MPa for Fe-Ni-Al-C and 600 DPN and 1740 MPa for Fe-Mn-Al-C. Elongation increases with decrease in carbon or aluminum and attains about 11 pct for Fe-20 wt pct Ni-6 wt pct Al-1.2 wt pct C and 28 pct for Fe-20 wt pct Mn-8 wt pct Al-1.6 wt pct C. The good strength and ductility of the Ni₃Al-type compounds remain unchanged on tempering for 1 h until heated to about 750 K. Further, it has been found that the addition of chromium, molybdenum or cobalt is effective for the improvement of mechanical properties and thermal stability of the compounds. Thus, the use of materials containing Ni₃Al-type compounds may be attractive for fine gage high-strength wire or plate applications. Formerly Graduate Student of Tohoku University.     

Gaseous oxygen absorption by molten iron and some Fe-AI,Fe-Si,Fe-Ti,and Fe-V alloys

An experimental investigation of the initial rates of oxygen dissolution in molten iron and some Fe-(≤9 pct Al), Fe-(≤6 pct Si), Fe-(≤1 pct Ti) and Fe-(≤1 pct V) alloys was carried out in pure oxygen. Two experimental techniques were employed in this study: a modified constant-volume Sieverts method and a falling droplet technique. It was found that the oxidation behavior of liquid iron-based alloys in gaseous pure oxygen as a function of alloy composition was similar under conditions of the falling droplet and modified constant volume Sieverts methods. Marked declines in the oxygen absorption rates were observed for Fe-Al and Fe-Si alloys when the initial alloy compositions reached 6 wt pct Al and 3 wt pct Si in iron, respectively. This behavior indicated a change in mode of oxidation from a burning to a passive type. Fe-Ti and Fe-V alloys initially containing up to 1 wt pct solute in iron exhibited only a burning type behavior. The sudden decline in oxygen absorption rate in molten iron-aluminum and iron-silicon alloys is discussed in terms of changes in the nature of the surface oxide film with increasing amounts of alloying element in the metal.     

High-temperature,high-pressure nitrogen concentration in Fe-Cr-Mn-Ni alloys

The thermodynamic expressions used to express the nitrogen concentration in iron based alloys prepared under high pressure requires the explicit use of pressure terms. The nitrogen concentration follows Sievert's law for low alloy additions, but significant deviations are observed at higher alloy additions. This Bureau of Mines study extends these new thermodynamic results to higher pressures (200 MPa) and to a greater range of iron alloys: Fe-(0-30)Cr-(0-30)Mn-(0-30)Ni. The experimental results also show that pressure-alloy concentration effects exist.     

Retardation of intermetallic phase formation in experimental superferritic stainless steels

While superferritic stainless steels containing 29 pct chromium possess excellent resistance to corrosion, they may, under certain conditions, be embrittled by the precipitation of intermetallic phases. The extent to which the precipitation reactions can be retarded by alloying additions of aluminum and copper has been evaluated. It was found that additions of aluminum to an Fe-29 pct Cr-4 pct Mo-1.5 pct Ni base alloy suppress the precipitation of the undesirable sigma and chi intermetallic phases, but additions of up to 3 pct aluminum promote 475 ‡C embrittlement. Additions of copper slightly reduce the precipitation of sigma and chi phases under most conditions but enhance 475 ‡C embrittlement. The resistance to corrosion in 10 pct H₂SO₄ and 10 pct FeCl₃ was assessed. All the aluminum-containing alloys performed significantly better in H₂SO₄ than the base alloy; however, large additions of aluminum had a deleterious effect on the pitting resistance in FeCl₃. Additions of copper improved the resistance to FeCl₃ and lowered the rate of corrosion in the H₂SO₄ solution used.     

Influence of aluminum and chromium on the activity of oxygen in nickel based melts at 1600?C

The activity of oxygen in technically important nickel melts containing 15 wt pct cobalt and 5 wt pct molybdenum has been determined at 1600‡C for various concentrations of chromium ranging from 0 to 30 wt pct and aluminum varying from 0 to 15 wt pct. The activity of oxygen was measured by an electrochemical technique using yttria-doped thoria electrolyte cells. The results obtained are analyzed in terms of activity coefficients of oxygen as a function of aluminum and chromium contents in the melts. Clear positive deviations of the experimentally determined from the calculated activity coefficients of oxygen were found when aluminum was added to Ni-Co-Mo melts with or without chromium. From the results obtained in the range between 1 and 10 wt pct aluminum, the following equilibrium constants for the reaction 2 [Al] + 3 [O] ⇋ Al₂O₃ in the nickel based melts at 1600‡C were calculated: loga ₀ =-2/3 log [pct Al] - 3.94 for 0 pct Cr loga ₀ = -2/3 log [pct Al] - 4.21 for 10 pct Cr loga ₀ = -2/3 log [pct Al] - 4.81 for 20 pct Cr loga ₀ = -2/3 log [pct Al] - 5.06 for 30 pct Cr.