Carbonitriding at 700°C with subsequent hardening of the surface layer

Conclusion Carbonitriding at 700°C in a mixture of 70–30% of ammonia and 30–70% of endogas (or natural gas) is recommended for the production of a diffusion layer with a good set of operational properties (for example, with high wear resistance).Moreover, the use of a gaseous atmosphere based on commercial nitrogen (50–90%) with ammonia, natural-gas, oxygen, or carbon dioxide additives is promising for carbonitriding at 700°C. Here, this medium should not contain more 0.5–1.0% of O₂ and 3% of CO₂. The nitrogen-based gaseous atmospheres ensure the attainment of quality diffusion layers with decreased risk of explosion and saving of energy resources.Moscow Automobile Traffic and Highway Construction Institute. Nongovernmental Production Union "All-Union Scientific-Research and Experimental Design Institute for Trade Machine Construction. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 32–36, May, 1987.     

Instrument for measuring and controlling the carbon potential of products of incomplete combustion in fluidized beds

Conclusion The instrument permits continuous measurement of the carbon potential of the atmosphere resulting from incomplete combustion of hydrocarbons in a fluidized bed. The accuracy of the measurements is ±0.02% C and the limits of measurement are 0.1–1.0% C. The operating range of the instrument is from 830 to 1000°C. It can be used as a primary instrument for automatic control of the carbon potential by changing the fuel-air ratio.Urals Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 28–31, November, 1971.     

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.     

Kinetics of saturation of steel by nitrogen and carbon in high-temperature carbonitriding with high nitrogen potential

In order to increase the bending strength, resistance to contact fatigue, wear resistance, and heat resistance of machine parts with a minimal change in their dimensions, currently the technology of high-temperature carbonitriding with high nitrogen potential is used in a number of domestic machine manufacturing plants. In this paper, we have theoretically and experimentally analyzed the effect of different carbonitriding conditions on the kinetics of diffusion saturation of the steel by nitrogen and carbon.State Technical University, St. Petersburg; State Pedagogical University, Volograd. Translated from Metallovedenie i Termiceskaya Obrabotka Metallov, No. 4, pp. 2–5, April, 1994.     

Selective Oxidation and Sub-surface Phase Transformations in Chromium-bearing Austenitic Steels

A series of Fe–15Cr–(2–3)Mo–(0.7–2.5)C (compositions in weight percent) steels was oxidised at 850°C and P_{O_2} = 5.8 × 10^–20 atm, where iron oxide is unstable. All grew external Cr₂O₃ scales according to parabolic kinetics. Depletion of chromium from alloy subsurface regions led to dissolution of chromium-rich carbides if the original alloy carbon level was less than 1.2%. Simultaneous decarburisation caused a transformation of the original austenitic or austenoferritic structure into single-phase ferrite, stabilised by the molybdenum. Diffusion analysis of the concentration profiles within this transformed zone led to satisfactory agreement with the known diffusion coefficient for chromium in ferrite. At high carbon levels, decarburisation was slow, resulting in low chromium concentrations at the internal alloy–carbide interfaces. In these cases, the carbide dissolution did not proceed and chromia scaling rates were slowed.     

Effect of strontium on the grain refining efficiency of Mg–3Al alloy refined by carbon inoculation

The effect of Sr on the grain refining efficiency of the Mg–3Al alloy refined by carbon inoculation has been investigated in the present study. A significant grain refinement was obtained for the Mg–3Al alloy treated with either 0.2% C or 0.2% Sr. The Al–C–O particles were found in the sample refined by 0.2% C, and the element O should come from reaction between Al₄C₃ nuclei of Mg grains and water during the process of sample preparation. The grain size of the sample refined by carbon inoculation was further decreased after the combined addition of Sr. The grain size decreased with increasing Sr content. Much higher refining efficiency was obtained when the Sr addition was increased to 0.5%. Sr is an effective element to improve the grain refining efficiency for the Mg–Al alloys refined by carbon inoculation. The number of Al₄C₃ particles in the sample refined by the combination of carbon and Sr was more than that in the sample refined by only carbon. No Al–C–O–Sr-rich particles were obviously found in the sample refined by the combination of carbon and a little (<0.5%) Sr addition.     

Extreme diffusional activity in steel in a state of transformation

Conclusions The pretransformation state in steel is characterized by an increased diffusional activity of the iron atoms. The temperature range of this state may be utilized for plastic deformation and heat treatment in steels. The diffusional mobility of the atoms and the plasticity of the steel in the pretransformation state (at 700–720 and 780–800°C for carbon and high speed steel, respectively) are approximately the same as those at 1100–1200°C.Utilization of the pretransformation state for technological purposes permits economies in the use of energy, due to the decreased process temperatures.The narrower temperature interval for commercial processing (±3–5°C) should not be an obstacle to its use in practice, since presently existing equipment permits temperature regulation with an accuracy up to ±1°C.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 13–15, July, 1990.     

Embrittlement of ferritic chrome steels in welding

Conclusions High-temperature heating characteristic of a weld joint in the area of overheating of the heat-affected zone causes an increase in the parameters and the static distortions of the crystalline lattice of chrome steels with an increase in nitrogen content. At the same time, the inelastic effects, the Snoek and Kester peaks, increase, which is caused by the introduction of nitrogen into the crystalline lattice and its interaction with the fields of stresses and dislocations.After imitation of the structure of overheating steels contaminated with nitrogen have a high impact strength, which is an indication of the weak influence of supersaturation of the crystalline lattice on the tendency of chrome steels toward embrittlement in welding.An increase in the carbon content has a negative influence on the impact strength of chrome steels after high-temperature welding heating. In comparison with steels contaminated with nitrogen there is practically no increase in the lattice parameters of such steels. After high-temperature heating the static distortions of the lattice decrease. There is also a decrease in the peaks of internal friction caused by migration of carbon atoms along the interstices of the crystalline lattice into the zones of stresses and their interaction with the dislocations.Embrittlement of ferritic chrome steels as the result of carbide precipitates is also possible after short-time heating in the 550–850°C range, which is indicated by the decrease in the crystalline lattice parameters and also the maximum in internal friction caused by carbon atoms. In heating to 400–550°C nitrides are also formed together with carbides.The effects revealed after short-time heating make it possible to relate the mechanism of embrittlement of ferritic chrome steels in welding to strengthening of the heat-affected zone metal as the result of precipitates of finely dispersed carbonitrides.Central Scientific-Research Institute for Machine Building Technology Scientific Production Association. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 41–45, January, 1984.     

Improving the strength properties of steels by vanadium nitride case-hardening

Conclusions To attain an effective increase of the strength properties of steels 40AF and 45AF, it is necessary to lay down rules for the content of vanadium within the limits 0.08–0.12%, of nitrogen 0.012–0.018%, of residual aluminum not more than 0.015%, normalizing and hardening temperature within the limits 940–960°C, tempering temperature 570–600°C.Foundry Institute (IPL), Academy of Sciences of the Ukrainian Production Association ZIL. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 55–58, April, 1984     

Influence factors and microstructure evolution during preparation of nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders

Nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders with crystalline size of about 35 nm were synthesized at 1300 °C from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation techniques. Influence of main technological parameters was investigated by X-ray diffraction, and microstructure of the milled powders and reaction products was studied by scanning electron microscopy. The results show that the phase evolution of TiO₂ follows TiO₂ → Ti₃O₅ → Ti(C, N), and (Ti, W, Mo, V)(C, N)–Ni composite powders with higher nitrogen content and smaller crystalline size can be produced by introducing high nitrogen pressure. By contrast with high nitrogen pressure, high synthesizing temperature and long isothermal time can contribute to dissolution of W, Mo and V atoms into Ti(C, N). In addition, synthesizing temperature has a significant effect on the microstructure evolution of (Ti, W, Mo, V)(C, N)–Ni composite powders.     

Die steels for high-speed automatic hot die forging machines

Conclusions Steel 3Kh3M3F produced by ESR has the best combination of strength, hardness, and toughness, and also resistance to crazing. The life of punches made of this steel was double that of standard steel 3Kh2V8F and considerably higher than that of steels with a higher carbon content (0.4–0.5%).The life of 3Kh3M3F punches (ESR) was three to four thousand bearing races higher than that of the same steel melted in an open furnace.These data lead us to recommend that punches for high-speed water-cooled presses be manufactured from steel 3Kh3M3F (ESR) with the following chemical composition: 0.26–0.34% C, 2.8–3.3% Cr, 2.5–2.9% Mo, 0.40–0.60% V (ChMTU-1-963-70).The following heat treatment is recommended: preliminary heating in an electric furnace at 500–510°, salt bath at 850–860°, and salt bath at 1040±10°. The parts should be quenched in oil with a temperature of 120–150°. The first tempering after quenching should be conducted in a salt bath at 600° for 2 h, with cooling in air. The second tempering should be conducted in a salt bath at 560° for 2 h, with cooling in air. The hardness of the parts after heat treatment is HRC 49–51.All-Union Scientific-Research Institute of the Bearing Industry. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 20–25, November, 1973.     

The oxidation behavior of the nimonic alloy PE16 in carbon dioxide at 700–800°C

The oxidation behavior of the nimonic alloy PE16 in carbon dioxide has been examined, at 700–800° C, for periods up to 10,250 hr duration. At all temperatures the oxidation kinetics were pseudoparabolic. The chromium-rich and titanium-bearing oxide scale was adherent, except at 800° C, when 10% spalled. Intergranular oxidation beneath the outer scale resulted in the formation of alumina and to a lesser depth, titanium oxide. The penetration increased parabolically with time and also with temperature, the activation energy being 50 kcal/mole. After oxidation at all temperatures the carbon profiles across the oxidized alloys were determined by nuclear microprobe analysis and indicated three distinct regions. From the gas interface carbon was picked up increasingly in the oxide scale, with a peak concentration (0.1–0.34 wt. %) at the oxide-alloy interface. The carbon level then fell sharply and to the depth of the titanium-bearing intergranular oxide the alloy was decarburized. At this juncture carbon had entered the alloy to a maximum concentration of 0.23–0.50 wt. % and a depth which increased both with temperature and exposure. Carburization is attributed to a crevice corrosion mechanism.     

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.     

The Influence of Oxide Layers on the Initiation of Carbon Deposition on Stainless Steel

This paper examines the role played by oxide layers in the deposition of carbon on two 20Cr–25Ni–Nb-stabilized austenitic steels containing either zero or 0.56 wt% Si. Selective preoxidation, at 550°C in Ar/H₂/H₂O, was used to produce regions covered by either chromia or silica or, in the silicon-free alloy, to be bare metal. Deposition was performed, also at 550°C, in a CO₂/1% CO/1000 vppm C₂H₄ gas mixture having an estimated carbon activity very much greater than unity. This gas also had an oxygen potential sufficient to form magnetite, but not nickel oxide. It was found that, even at these high carbon activities, none of the oxides formed could catalyze carbon deposition and that this occurred only when the gas had direct access to the alloy substrate. The carbon filaments formed were found, by high-resolution electron microscopy, to be solid, have a turbostratic structure, and to contain at least one nickel particle at their tips. The source of these nickel particles is the alloy substrate and a mechanism is proposed for their formation.     

Effect of commercial carbon-containing controlled atmosphere composition on the intensity of article carburization

A reduction in the duration of carbon impregnation during gas carburizing is normally achieved either by increasing the temperature of the process or creating rapid circulation of the atmosphere within the furnace. Both of these methods have their specific drawbacks and it is difficult to accomplish them in practice. A simpler and cheaper method is carbon impregnation without controlled atmospheres containing only CO and H₂. Here alongside a reduction in the duration of the process it is possible to use slow atmosphere velocities within the charge which simplifies the construction of the carburizing furnace.Moscow Automobile Building Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 24–26, October, 1994.     

The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides

To increase crack propagation resistance in cemented carbide cutting tools, it is sometimes of interest to create tough surface zones in the substrates. A way to do this is to use so-called gradient sintering in the manufacturing of the cutting tool. In this sintering process a nitrogen and titanium containing cemented carbide is sintered in a nitrogen free atmosphere. The difference in nitrogen activity between atmosphere and cutting tool during sintering will create an outward nitrogen diffusion. Due to thermodynamical coupling between nitrogen and titanium, this gives rise to an inward titanium diffusion, which creates a surface zone depleted of hard cubic carbo-nitrides, and enriched in ductile binder phase. By varying the carbon content of the material, the nitrogen activity is affected, and this in turn affects the surface zone formation.

In this report, Ti(C,N)–(Ti,W)C–WC–Co, Ti(C,N)–NbC–WC–Co, and Ti(C,N)–TaC–WC–Co cemented carbides were studied. All three materials were produced in series with varied carbon content, in order to study the effect of carbon on gradient surface zone formation.