Endothermic Atmosphere Research Articles

Production of powders from cast iron swarf and some features of the manufacture of parts from them. II. Some features of the manufacture of P/M parts and their structure and properties

Increasing the amount of cast iron powder in such a charge and raising the sintering temperature of compacts result in the formation in the alloy structure of a coarse cementite network among the grains and a fine cementite network along the slip lines. Consequently, what is needed to increase the strength of the synthesized alloys is either a low rate of cooling in sintering or additional heat treatment, designed to prevent the presence of structurally free cementite in the structure of the finished alloys. The optimum sintering temperatures for the iron-cast iron materials in an endothermic gas atmosphere, ensuring the highest strength properties in finished P/M parts, is 1200°C. Alloys containing 10 and 20% cast iron can be recommended for construction applications, and others, for the manufacture of parts operating under conditions of friction and increased wear without dynamic loads. These include the components of the pump unit of the rotary compressor of a domestic air conditioner.

Effect of feeding method for industrial gases during high-temperature carbonitriding of articles

1. Impregnation of surface layers with nitrogen during high-temperature carbonitriding in an endothermic atmosphere depends on the input method and the rate of ammonia to the charge of articles being impregnated. The greatest content of residual ammonia, and consequently the content of nitrogen in the carbonitrided layer, is observed with input of ammonia through an injector and with an increase in the rate of its input to the surface of articles being impregnated from 180 to 250 m/sec. 2. The injector system developed for feeding industrial gases into the furnace space of a muffleless unit provides a reduction in scatter of layer thickness, content of carbon and nitrogen at the surface, and it makes it possible with an ammonia input of 10–12% of the gas carrier to obtain impregnation of surface layers with nitrogen to a depth of 0.05–0.6 mm up to 0.8–1.0 and to 0.3–0.5% respectively, and also to increase by 30–40% the rate of carbonitrided layer growth.

Austenite recrystallization in Nb/V microalloyed steels

Two Nb microalloyed steels containing 0.05 and 0.20 wt pct V were used in this study. The chemistries of these steels are shown in Table 1. The steels were vacuum induction melted as 136 kg ingots and hot rolled to a thickness of 1.0 cm. Specimens 2.5 cm x 7.6 cm were cut from the plates, retaining the original rolling direction, and thermocouple holes were drilled into the longitudinal faces of these specimens at mid-thickness. The specimens were reheated for 1 hour in an endothermic atmosphere at 1288/sup 0/C. Upon removal of the specimens from the reheat furnace, a sheathed thermocouple was inserted into the drilled hole. The temperature of the specimens was then monitored during air cooling until the desired rolling temperature of 954/sup 0/C was reached, at which time the specimens were deformed 50 pct in a rolling mill at an average strain rate of 10.2 s/sup -1/. Immediately after rolling, the specimens were transferred to a molten chloride salt bath and held for 10, 100, 1000, or 10,000 seconds at 843/sup 0/C, 954/sup 0/C, or 1066/sup 0/C, followed by brine quenching.

Application of natural gas to thermochemical treatment of steel parts

1. Carburizing and carbonitriding endothermic atmospheres with additions of natural gas and ammonia produce the optimum amounts of carbon and nitrogen in the diffusion layer, thus improving the wear and fatigue resistance of the steels studied by 30% as compared to thermochemical treatment by the application of liquid carburizers. 2. Nitriding in a complex atmosphere (70% natural gas+30% ammonia) instead of a pure ammonia atmosphere shortens the duration of the process by 20% and causes complete disappearance of the high-nitrogen component on the surface.

Structure and phase composition of aluminized radiant tubes used in an endothermic atmosphere

Structure and phase composition of aluminized radiant tubes used in an endothermic atmosphere

Nitriding of steel parts for machine construction in an endothermic atmosphere

To ensure the required characteristics of the diffusion layer in heat-resistant steels, it is recommended to subject them to two-stage nitriding: saturation at 950°C for 2 h in endogas with an addition of 12% natural gas and 4% ammonia, lowering the temperature to 900°C, holding for 5 h in endogas with an addition of 6% natural gas and 4% ammonia; exchange of gas 6 times.

Efficiency of resistance alloy heaters in furnace with carbon-containing atmospheres

The study was carried out on specimens and heaters of the most widely used Soviet resistance alloys whose nominal compositions are given in atmosphere with different carbon potentials, i.e., from 0 (air) to 1.3-1.5% C, in the temperature range 950-I150°C for periods up to 2000 h. The carbon-containing gas used was endogas obtained by partial combustion of propane--butane mixtures with air (with a coefficient for air consumption of a = 0.3) in a gas preparation unit EN-16 M02 (produced by the Chadyr--Lugansk Electrical Equipment Factory). The composition of the endothermic atmosphere obtained was 23% CO, up to 1.01% CO2, ~30% H2, up to 1.5% CH~, balance N2, dew point --16 to +II°C. Atmospheric moisture was maintained automatically by means of a moisture gauge IV 4-2, an actuator, and an NIIV regulator by changing the gas and air ratio in the original mixture. Carbon potential of the furnace atmosphere was monitored by weighing a specimen of 0.07-0.1 mm thick foil with a certain configuration of steel 08kp after soaking in the furnace for 1 h. Carbon potential was determined from the weight increase and the amount of endogas or propane was altered as necessary. Atmosphere composition was periodically monitored with a chromatograph Gazokhrom3301.

Effect of heating in endothermic atmospheres on the mechanical properties of steel 18Kh2N4VA

i. L.N. Murotskii, "Use of forced gas circulation in low-temperature heat-treating furnaces to improve the quality of heating," in~ Progressive Methods using Natural Gas for the Quality Heating of Metal [in Russian], Energiya, Moscow (1977), p. 301. 2. A.A. Volgunin, "Determination of additional heat supplied and quantity of circulating gases in tempering furnaces of units with the recirculation of combustion products," in: Research and Development at the Gorki Truck Plant [in Russian], Volgo-Vyatskoe Knizhnoe Izdatel'stvo, Gorki (1976), p. 81.

Experience in the use of endogenerators and the use of new catalysts

1. To produce an endothermic atmosphere for gas carburizing and carbonitriding it is desirable to use straight-flow type endogenerators with a retort inside diameter of 160–200 mm in the center of which are placed 50–70-diameter tubes. 2. The catalyst must be located in zones of good heating. 3. To avoid the formation of carbon soot, the outlet from the endogenerator must be cooled intensely.

Failure of radiant tubes during operation in an endothermic atmosphere

1. To increase the resistance of radiant tubes it is necessary to exclude the formation of soot inside the tube during combustion of city gas and also precipitation of soot in the furnace atmosphere. This can be ensured only by automatic control of the carbon potential in the process of chemicothermal treatment and careful regulation of combustion conditions. 2. Burning of soot shortens the service life of radiant tubes and thus should be reconsidered.

Powdered intermetallic coatings of the Ni?Al system

Powdered NiAl applied by flame coating on radiant tubes to increase the scale resistance is not recommended as a protective coating in endothermic atmosphere at 900–1000°.

Kinetics of carburizing in an endothermal atmosphere

1. On the basis of experimental data on the kinetics of carburizing at 800, 850, and 950°C we determined the temperature dependence of the coefficient of the rate of surface reactions during carburizing in an endothermal atmosphere. 2. Alloying with chromium and tungsten increases, while silicon lowers, the rate of surface reactions. The effect of silicon, tungsten, and chromium on the rate of surface reactions was calculated by means of a parameter depending only on the nature of the alloying element. 3. The quantitative dependences obtained in investigating the kinetics of carburizing of foil were confirmed in experiments with massive samples of steels U10 and ShKh15 with a decarburized layer. Comparison of the carbon content in the surface of the steel determined from tests and calculated from the diffusion equation with the rate of surface reactions taken into account showed good agreement of the results.

Structure and phase composition of surface zones of carburized and carbonitrided layers

1. Mossbauer spectroscopy makes it possible to detect the following iron-containing phases in the surface layers of carburized and carbonitrided samples: martensite, austenite, cementite, and iron oxides. 2. The quantities of these phases depend on the conditions of the chemicothermal treatment and can be used to judge the quality of the treatment with the proper correction factor. 3. The layer nearest the surface is most sensitive to the quality of the chemicothermal treatment (a layer ∼0.1 μ deep) in which the spectra of iron oxides are observed along with those of compounds characteristic of the Fe-C system. 4. With chemicothermal treatment in an endothermal atmosphere, a high concentration of iron oxides is observed in surface zones as thick as 1 μ. The higher the concentration of alloying elements with a large affinity for oxygen in the solid solution, the larger the total concentration of oxides. 5. No oxides are observed on the surface of samples carburized in an atmosphere of methane at a pressure of 10−3 mm Hg.

Restoration carburizing of steel ShKh15 in endothermal atmosphere

1. It was shown that it is possible to program the distribution of carbon concentrations in high-temperature restoration of ShKh15 in a controlled endothermal atmosphere. 2. In designing the restoration process one must take into consideration the change in the surface layer that occurs during decarburizing (grain growth and internal oxidation of alloying elements).

Thermodynamic activity of carbon in recarburizing

1. We determined the numerical values of the interaction parameters of Si, Al, Cr, Ti, V, Mo, W, Mn, and Ni, characterizing the effect of alloying elements on the thermodynamic activity of carbon for recarburizing and carburizing in an endothermal atmosphere at 950°. 2. A quantitative relationship (7) was established for the change in the thermodynamic activity of carbon in austenite under the influence of complex alloying, making it possible to calculate the carbon potential for recarburizing and carburizing of alloy steels. 3. The applicability of formula (7) was determined for carburizing of impurity-free alloys of the Fe−Cr−Ni, Fe−Ni−Si, Fe−Cr−Ni−Si, and Fe−W−Mo systems and steels 45, 45G2, 60, 60S2, U10, and ShKh15 after recarburizing. 4. Knowing the activity coefficient of carbon in complex-alloyed austenite, one can determine the necessary carbon potential of the atmosphere for a given alloy steel, using the gas-metal equilibrium diagrams for carbon steels.

Carburizing of heat resistant steels in a controlled endothermic atmosphere

1. The optimal bending strength of high-alloy heat resistant steels 18Kh2N4VA, 14KhGSN2MF, 20Kh3MVF, and 20KhNV4MF after carburizing and heat treatment results from a carbon concentration of 0.9–1.1%; the optimal concentration is 0.8–0.9% for steels of the 18-25KhGT and 25KhGM types. 2. The highest contact fatigue strength of these steels is attained at a higher carbon concentration (1.3–1.4%) than for steels of the 12KhN3A type. 3. The highest heat resistance of the carburized steels investigated can be attained by increasing the surface carbon concentration to 1.4–1.5%. The heat resistance is highest for steel 20KhNV4MF, with a surface hardness of HRC 59–60 after tempering at 375–400°C. 4. In carburizing steels with a large amount of carbide-forming elements (chromium, tungsten, molybdenum, and vanadium) in a controlled endothermic atmosphere it should be kept in mind that these elements substantially increase the carbon concentration in γ-iron. With prolonged processing at elevated carbon potentials the amount of carbide phase in the surface increases greatly. Intensive carbide formation can be prevented by lowering the equilibrium concentration of carbon in γ-iron by 0.1–0.2%.

Thermodynamic calculations of the precipitation of sooty carbon in furnaces with controlled atmospheres

1. The destruction of lining materials in endothermal and carburizing (endothermal +3–5% CH4) atmospheres due to sooty carbon deposited on them occurs mainly at 400–700°. 2. The sooty carbon inducing destruction of lining materials at these temperatures is precipitated due to reactions (1), (2), and (3). 3. The destruction of lining materials due to sooty carbon deposited on them at 400–700° is more intense in a carburizing atmosphere than in an endothermal atmosphere. 4. In a carburizing atmosphere the destruction of lining materials at 950–1200° is possible (but to a lesser extent than at low temperatures) due to precipitation of sooty carbon by reaction (6). 5. The thermodynamic calculations are confirmed by experimental data obtained in a study of the durability of lining materials and also observations of commercial electric furnaces operating with carbon-containing controlled atmospheres.

Internal oxidation during carburizing

1. Microstructural, x-ray structural, and microspectral analyses indicate that internal oxidation occurs in steels 25KhGT, 25KhGM, and 25KhGNM carburized in an endothermic atmosphere. 2. The addition of 0.6% Mo to the steel does not eliminate or reduce the internal oxidation zone of chromium and manganese. 3. Internal oxidation to a depth of 0.03 mm is quantitatively observed by x-ray structural analysis in all steels investigated, although it is possible that oxides also occur at greater depths. 4. The oxides formed are primarily spinels of the types nFeO·Cr2O3 and MnO·Mn2O3. 5. With increasing cleanness of the surface the number of oxides formed and the depth at which they are formed decrease substantially. 6. Manganese and chromium do not occur together in spinels, while in some cases titanium and molybdenum oxides are encountered together with one of the two main types of oxides.

Saturation of steel in carburizing and nitrocementation

1. Comparative studies of carburizing and nitrocementation processes at 850° in a controlled endothermic atmosphere showed that the addition of ammonia in nitrocementation lowers the surface concentration of carbon somewhat (by 0.03–0.05%), increases the case depth by 20–25%, and increases the extent of the hypereutectoid zone with an elevated carbon concentration. 2. The most practicable mechanism of the effect of nitrogen in nitrocementation is the increase of the thermodynamic activity of carbon.

Internal oxidation during case-hardening of steels in endothermic atmospheres

1. Thermodynamic calculations showed that internal oxidation of titanium, manganese, silicon, and chromium is possible during carburization in an endothermic atmosphere. 2. One finds internal oxidation in the majority of alloy steels subjected to carburization in an endothermic atmosphere. The outward indication of such oxidation is the formation of a troostite network on the surface to a depth of 0.01–0.03 mm. Internal oxidation is accompanied by a sharp reduction in the carbon content of the solid solution after carburizing and hardening. 3. When a troostite network is formed at a depth of more than 0.014 mm there is a negligible reduction of the bending strength but a sharp reduction of the fatigue strength. 4. To prevent internal oxidation during carburization and the ensuing reduction in strength it is necessary to add 5–10% NH3 to the working chamber of the furnace for 10 min before completion of the process or use a steel alloyed with 0.5% Mo and the minimum chromium concentration (no more than 0.5%). 5. Internal oxidation and the troostite network can result from nitrocementation if the saturation of the surface with nitrogen is insufficient (below 0.1%). For this reason it is necessary to control not only the carbon content but also the nitrogen content of the case.