Structural transformations and distribution of elements in steels during high-speed carbonitriding

Conclusion Induction heating of components at a high rate and feeding to the heating zone a dosed portion of active liquid provides preparation on them of diffusion layers with a controlled, sufficiently high content of carbon and nitrogen corresponding to that recommended. No redistribution of chromium and nickel in steel 40KhN was detected.The best results are obtained after high-temperature impregnation of components, interrupted cooling to the austenite decomposition temperature by a pearlitic mechanism, and repeated hardening.Physicotechnical Institute, Academy of Sciences of the Belorussian SSR, Minsk Automobile Factory, Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. I, pp. 33–36, January, 1987.     

Nitriding of chromium in nitrogen gas at high temperatures

Parabolic rate constants of the reaction of chromium with nitrogen gas under oxygen-free conditions have been determined over a range of temperature (1000–1250°C) and nitrogen pressure (0.265–101.33 kPa). The growth rate of the subnitride was measured by a thermogravimetric technique using a single specimen. Wagner's oxidation theory is used to calculate the self-diffusivity and intrinsic diffusivity of nitrogen in the subnitride from a theoretical analysis of the parabolic rate constant. The calculated diffusivities varied with the composition of the subnitride, having minimum values at intermediate compositions of the nonstoichiometric chromium nitride Cr₂N.     

Phase transformations during tempering of quenched Fe−N alloys

Conclusions It was found that tempering processes are similar in quenched steel and nitrided iron — in the first stage of tempering (20–180°) the martensite with nitrogen transforms, with formation of metastable F₁₆N₂ and temper martensite; in the second stage (180–300°) the retained austenite decomposes and the Fe₁₆N₂ Fe₄N transformation occurs; in the third stage (300–550°) the number of lattice defects decreases and the Fe₄N particles coalesce. After quenching and tempering at 500–600° the alloy consists of a ferrite—nitride mixture of the type of temper sorbite in carbon steel.Kiev Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 28–30, March, 1974.     

Characteristic Features of the Kinetics of Decomposition of Supercooled Austenite of Alloy Steels in the Pearlite Range

The kinetics of decomposition of supercooled austenite of medium-carbon Cr – Ni – Mo steels in the pearlite range is studied by dilatometric and metallographic methods with computer processing of the data. It is shown that the rate of the decomposition of supercooled austenite in the pearlite range oscillates with a low amplitude. The changes in the exponent in the Avrami and Ostin – Rikket equations that describe the decomposition of supercooled austenite are associated with simultaneous processes of austenitic transformation whose kinetics differs substantially in local regions of the metal, which is mainly connected with the polymodal nature of the grain structure of the austenite.     

Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels

Abstract

Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr₂N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr₂N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr₂N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr₂N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

Effect of alloying elements on austenite transformation kinetics in chromium-nickel rolling mill cast irons

Conclusion Tungsten both with small and large amounts of supercooling increases austenite stability, but niobium with small amounts of supercooling (550–450°C) decreases it, and with large amounts (350°C and below) it increases its stability. On alloying chromium-nickel cast iron with cobalt austenite isothermal decomposition moves to the left, and alloying with boron accelerates the austenite decomposition process in the temperature range 550–500°C with an increase in its stability in the range 500–200°C.Dnepropetrovsk Metallurgical Institute. Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 58–60, July, 1984.     

Structure and properties of low-carbon nitrogen-containing augstenite-ferrite corrosion-resistant steels

The concentration of elements in existing austenite-ferrite steels varies in a rather narrow range. This is associated with the necessity of ensuring an optimum austenite/ferrite proportion in the structure ( 40–60%), which is attained by adding ferrite- and austenite-forming elements in specified amounts. The article concerns the structure, mechanical properties, and corrosion resistance of low-carbon austenite-ferrite steels with 18–25% Cr, 2–6% Mo at –7% Ni and 0.15% N and the choice of the most suitable limiting concentrations of chromium and molybdenum.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 10–15, September, 1995.     

Intermediate temperature decomposition of supercooled high nitrogen austenite

Abstract

The mechanism of decomposition transformation of Fe–N the austenite system has been investigated. An improved process of austenitic nitriding, achieved by applying controlled nitrogen potential theory, allowed high nitrogen austenite samples with a uniform nitrogen concentration to be produced. The key point of this gas nitriding process is to keep the atmosphere at very low nitrogen potential. As a result, the nitride layer on the surface of the pure iron foil was reduced and pure iron ferrite was thoroughly nitrided, forming high N austenite (γ-Fe[N]) that is thermally stable at room temperature. The nitrogen concentration of this austenite was determined as 9·32 at.-%, which is almost the maximum value achievable in Fe–N austenite.     

Phase transformations in large forgings

Conclusions To study the kinetics of phase transformations and develop a heat treatment to prevent shattercracks in large forgings it is expedient to use diagrams of the isothermal and athermal decomposition of supercooled austenite plotted with use of double austenitizing of the samples — first at the forgoing temperature and then at a temperature of Ac₃+30–50°.Scientific-Research and Planning-Technological Institute of Machine Construction, Kramatorsk. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 52–54, August, 1981.     

Oxidation of a Three-Phase Cu–45Ni–30Cr Alloy at 700–800°C Under 1 atm O2

The oxidation of a ternary Cu–Ni–Cr alloy containing approximately 45 wt.% Ni and 30 wt.% Cr has been studied in 1 atm O₂ at 700–800°C. The alloy contains a mixture of three phases: the one with the largest copper and lowest chromium content forms the matrix, the one with an intermediate chromium content has a rather large volume fraction and forms large islands, while the phase richest in chromium forms isolated particles dispersed in the other two phases. At variance with another Cu–Ni–Cr ternary three-phase alloy containing only 20 wt.% Cr and 20 wt.% Ni, which formed complex scales containing mixtures of the oxides of the various components and double oxides, plus an irregular region composed of a mixture of alloy and oxides, the present alloy is able to form protective, external chromia scales. A similar result could be obtained with alloys containing about 20 wt.% Cr, but composed of either a single phase (Cu–60Ni–20Cr) or of a mixture of two phases (Cu–40Ni–20Cr). The need for a larger chromium content for producing chromia scales for three-phase as compared to two-phase Cu–Ni–Cr alloys is attributed to the limitations of the diffusion of the alloy components in the metal substrate imposed by their multiphase nature.     

Formation of Phases upon the Mechanosynthesis and Subsequent Annealing of Samples of Cementite Composition Alloyed with Chromium and Nickel

Using X-ray, Mössbauer, and magnetic measurements, the formation of phases has been investigated upon mechanosynthesis in a ball planetary mill and upon the subsequent annealing of samples of the cementite composition (Fe_0.95–уСr_0.05Ni _y )₇₅C₂₅, where у = 0–0.20, which contains two alloying elements (chromium and nickel). It has been shown that, in the mechanosynthesis process, cementite alloyed with chromium and a small amount of nickel and an amorphous phase alloyed with chromium and nickel have been formed. Upon heating above 300°С, the amorphous phase is crystallized into nickel-enriched cementite. In the process of annealing at higher temperatures, the most nickel-rich cementite decomposes with the formation of austenite. As a consequence, after annealing at medium temperatures, the composition of the alloys contains cementite alloyed mainly with chromium and some amount of alloyed austenite, which can be found in ferromagnetic or paramagnetic states depending on the Ni content. Annealing at 800°С bring about the complete or partial decomposition of cementite contained in the alloys. The intensity of the decomposition has been determined by the nickel content in the samples.     

Interaction of Mn-Cu solid solutions with nitrogen in the range of 780–860° C

One of the characteristic features of Mn-Cu binary alloys nitrided by gaseous nitrogen is that one of the two constitutive elements (Mn) can form nitrides, while the other one (Cu) does not give any stable compound with nitrogen. The only mixed manganese-copper nitride is the CuMn ₃ N compound. The reaction kinetics with nitrogen are very slow and there is no internal nitriding. For alloys containing less than 20 at.% Mn, nitrogen reacts very little. The nitride scale formed on the alloys of greater Mn concentrations is a mixed nitride whose formula is Cu _1–x Mn _3+x N. The techniques of examination used are SEM, EMA, and GDS analysis.     

Diffusivity of nitrogen in chromium subnitride “Cr2N”

The intrinsic diffusivity of nitrogen in chromium subnitride, Cr₂N, has been measured, by a thermogravimetric technique, over a range of nitrogen pressures at temperatures from 1050 to 1250°C. The diffusivity varies with the composition of the nitride. The values obtained for the diffusion coefficient are in agreement with previously reported values obtained by the author from kinetic measurements of the reaction of nitrogen with chromium. The results confirm that Wagner's theory of oxidation may be used to calculate diffusivities.     

Microstructure of chromium nitride layer on stainless steel for polymer electrolyte membrane fuel cell separator

The microstructure of the chromium nitride layer formed on 316L Stainless Steel (SS) by electroplating Cr and subsequent thermal nitriding was investigated by electron microscopy. Thermal nitriding was carried out at a nitrogen partial pressure of 100 torr at 1100 °C for 2 h. The X-ray Diffraction (XRD) patterns showed that the structure of electroplated chromium was not crystalline and that the nitrided layer was composed of only hexagonal Cr₂N phase nitride. The nitride layer showed uniform and columnar-shaped grains of chromium nitride and contained many micro-pores that may have formed due to the volume difference between the amorphous chromium and the crystalline chromium nitride. Chromium oxide was also observed in the nitride layer. It was discovered that the number of micro-pores and oxide particles needs to be decreased in order to improve the interfacial contact resistance.     

Influence of Heat Treatment on Cr18Mn18 High Nitrogen Steel Precipitating Behavior and Microstructure

The grain size and precipitate amount which are affected by heat treatment have significant impact on the properties of high nitrogen austenitic stainless steel. In this study, Cr18Mn18 high nitrogen steel sheet is employed to investigate the effects of precipitate on austenitic grain size. It can be seen that the lamella precipitates which are rich in nitrogen and chromium nucleate in the austenite grain boundary and grow inward into grain when aged at 800 ℃ through electron probe micro-analyzer. The transmission electron microscopy results demonstrate that the precipitate is Cr2N and its morphology are detected as ellipsoid-like with major axis of 100-300 nm and minor axis of 50-100 nm roughly. The experiment show that coarsen of the austenite grain is quite critical at 1000-1100 ℃. However, the samples which pre-precipitated at 800 ℃ for 240 min to obtain the most nitride precipitate exhibits much smaller grain size than the as-rolled samples after solid solution treated at 1000, 1050 and 1100 ℃ for 240 min. The results show that the nitride precipitates in the grain boundary can effectively pin the austenite grain boundary and inhibit the grain growth.     

The Diagram of Electrochemical Equilibrium of 12X18H10T Steel

Equilibrium potential–pH diagrams for the austenite of a 12X18H10T steel and possible second phases, such as titanium carbides TiC, Cr₂₃C₆-based M₂₃C₆ carbides, FeCr intermetallic compound (-phase) as well as silica and sulfide inclusions are considered. Concurrent effect of titanium, chromium, manganese, and silicon, on the chemical and electrochemical stability of stainless steel is discussed.     

Microstructural aspects of the corrosion of Alloy 800

Transmission electron microscopy studies on solution-annealed Alloy 800 revealed small (100–200 nm), spherical-shaped titanium carbide (face centered cubic structure) and large (200 nm–5 μm), faceted titanium nitride (hexagonal structure) particles randomly distributed in the austenite matrix. The volume fraction of former particles was found to be greater than that of the latter. Corrosion studies of the alloy in acidic, chlorides and acidic chloride environments at room temperature indicated that the passivity of Alloy 800 was adversely affected by the addition Cl⁻ ions. X-ray photoelectron spectroscopy revealed that the surface film formed on the alloy at the onset of passivity consisted of Cr³⁺ (as Cr₂O₃), without any Fe³⁺/Fe²⁺ or Ni²⁺. Scanning electron microscopy studies indicated initiation of pitting at large, faceted particles, not at small, spherical-shaped ones.