Showing papers in "Metal Science and Heat Treatment in 1977"

Acicular ferrite HSLA steels for line pipe

Abstract: An updated state-of-the-art review has been made of the new-generation high-strength low-alloy steels known as the acicular ferrite Mn-Mo-Nb steels. After individual effects of the various alloying elements were reviewed from the results of earlier research and published data, the importance of processing variables in commercial production was stressed. The most important benefits that can be realized pertain to the upgrading of impact toughness properties by the use of: 1) lower slab reheating temperatures, e.g., 1080–1150°C, 2) large total reductions, e.g., 65% below 900°C, and 3) low finish-rolling temperatures, e.g., 740–780°C. The strength properties are relatively insensitive to variations in hot-rolling schedules because there is a good balance of the three main strengthening mechanisms: grain refinement, dislocation substructure, and precipitation strengthening by Nb(C, N).

Structure and properties of nitrided binary Fe-Al, Fe-V, and Fe-Ti alloys

Abstract: 1. The formation of phases with nitrogen in the diffusion layer of binary alloys of iron with aluminum, vanadium, and titanium after nitriding at 500° for 1h occurs in the conformity with the Fe-N phase diagram. 2. With increasing amounts of aluminum in the alloy the amount of e phase decreases, while the amount of γ′ phase increases and it extends into the grains in the form of whiskers. 3. Aluminum has a slight effect on the hardness of the α solid solution; titanium and vanadium substantially increase the hardness after nitriding at 500°. 4. Sectioning and electron microscopic and electron diffraction analysis of the diffusion layer on nitrided Fe-Al alloys showed that the surface consists of e phase, with γ′ phase alloyed with aluminum beneath it. The sublayer contains two excess nitride phases-Fe4N and Fe16N2. 5. Besides e and γ′ phases on the surface of nitrided Fe-Al alloys, we observed separate crystals of Al2O3. 6. No precipitates of stable hcp AlN were observed on the surface of Fe-Al alloys after nitriding. 7. The diffusion sublayer of Fe-V and Fe-Ti alloys nitrided at 500° contains a second nitride phase-Fe16N2 — that is isomorphous with the matrix, in addition to precipitates of γ′ phase. The effect of diffusion scattering is noticeable on the matrix reflections in the form of streaks, indicating the formation of coherent GP zones.

Ductile and brittle fracture of steel

Abstract: Ductile and brittle fracture are distinguished by the condition of the fracture surface. A typical example of brittle fracture is the absence of traces of plastic deformation on the fracture surface. For all metals, especially bcc and hcp and even fcc metals (with the exception of copper and nickel), a change in the fracture mechanisms is observed (the threshold of cold brittleness) which is a characteristic sensitive to structure and composition. The transition to the brittle condition is an attribute of the crystal lattice and is probably due to an increase in the percentage of covalent bonds with decreasing temperatures.

Resistance to fracture of weldable carbon structural steel with ferrite — Pearlite and Widmanstatten structures

Abstract: 1. The resistance to fracture of steel St3 determined from the ductile-brittle transition temperature depends on structural factors — the shape and quantity of ferrite precipitates, grain size, and distribution of structural components; the static characteristics (σb, σT, δ, ψ) are practically the same for samples with different microstructures. 2. The susceptibility of steel St3 to cold brittleness is affected by the evenness of the microstructure and the mutual distributions of the structural components. The transition temperature rises with increasing grain size, the extent of the Widmanstatten structure, and the unevenness in the distribution of the structural components. The loop-shaped microstructure with fine grains is more susceptible to brittleness than the even coarse-grained structure. 3. The susceptibility to brittleness is highest for samples with a coarse-grained loop-shaped structure due to the effect of both heterogeneity and grain size. 4. The work of crack propagation depends on the quantity of ferrite in the structure: The larger the amount of ferrite and, consequently, the higher the concentration of carbon in the quasieutectoid, the lower the value ofap. The work of crack initiationai varies little with changes in the structure within the limits investigated. 5. For the preparation of structures with stress concentrators such as welded structures it is best to use steels with a network and an even fine-grained structure and also Widmanstatten structure containing no more than 45–50% ferrite.

The high hardness of alloyed ferrite after nitriding

Abstract: 1. Nitriding at temperatures above 550° leads to diffusional redistribution of aluminum atoms, which combine with the γ′ phase on the surface. In contrast to aluminum, transition d metals (titanium, vanadium, chromium) are evenly distributed in the α solid solution with nitrogen. 2. Aluminum does not increase the solubility of nitrogen in ferrite and does not form AlN. A study of the processes occurring in the diffusion zone of Fe−Al alloys in relation to the aluminum concentration and nitriding temperature and time showed that aluminum is not a nitride-forming element in ferrite. 3. The hardness of the nitrided case on ferrite with aluminum increases only in the surface zone of γ′ phase, and the hardness of the α solid solution hardly increases, since matrix precipitates of Fe4N and Fe16N2 increase the hardness of ferrite negligibly. 4. Alloys with a high aluminum concentration (6.85%) have a highly developed polygonal structure in the original condition that to a considerable extent aceelerates the diffusion of nitrogen, which is essentially transcrystalline in the given case. Transformations occur in separate local volumes of the diffusion layer in the process of nitriding, especially those that are alloyed — the α → γ′ transformation after 1 h and the γ′ → e transformation after 3 h. 5. In contrast to aluminum, transition d metals substantially increase the solubility of nitrogen in the α phase and sharply increase the microhardness and reduce the case depth. The effectiveness of the increase in the solubility of nitrogen diminishes in the following order: titanium → vanadium → chromium. 6. The hardening of ferrite alloyed with titanium, vanadium, and chromium is due to the precipitation of nitrides of the alloying elements of the maximum stoichiometric composition with a B1 lattice from the α solid solution supersaturated with nitrogen. The microhardness is highest after nitriding at 550° at the stage of complete coherence of nitrides of the alloying elements and the α matrix. The hardness of ferrite decreases sharply when the coherence is completely disrupted and the nitrides become independent phases.

Kinetics of crack growth in maraging and medium-alloy steels during low-cycle impact fatigue tests

Abstract: 1. The duration of the period of crack nucleation in steels N18K9M5T and 30KhN2MFA varies little with the magnitude of cyclic stresses. 2. A period of slow crack growth is observed in steels 30KhN2MFA and N18K9M5T at σmax=165 kgf/mm2 and is characterized by fatigue microbands in the fracture-ductile in the maraging steel, brittle in the medium-carbon steel. With increasing crack length (period of accelerated crack growth) the area occupied by microbands in the fracture decreases and the percentage of dimpled fracture increases. 3. At high cyclic stresses (σmax=260 and 360 kgf/mm2) the period of slow crack growth is absent in steels 30KhN2MFA and N18K9M5T. 4. The rate of crack growth determined by the difference of electrical potentials is in satisfactory agreement with the width of fatigue microbands, particularly in the period of slow crack growth. In the period of accelerated crack growth it is somewhat higher than the width of microbands. 5. The high regularity of fatigue microbands in the fractures of steel N18K9M5T indicates greater evenness of the properties determining the resistance to fracture at high cyclic impact loads than for steel 30KhN·2MFA.

Effect of manganese on the transformation of austenite in white chromium cast irons

Abstract: 1. With increasing carbon concentrations the rate and extent of the isothermal transformation of austenite increase in Cr-Mo and Cr-Mn cast irons. 2. The temperature range in which isothermal transformation of austenite occurs becomes broader with increasing carbon concentrations, but the temperature of minimal stability of austenite decreases. 3. With increasing carbon concentrations the initial martensitic transformation temperature decreases greatly in Cr-Mo cast irons.

Martensitic transformation in iron

Abstract: 1. The formation of martensite in iron (0.03% C) occurs at high temperatures (700–650°) and obeys well-known [5] rules established for the formation of martensite in steels—in particular, the initial martensitic transformation temperature is independent of the austenite grain size and the cooling rate. 2. The growth rate of martensite crystals is low (∼3 μ/sec for iron and 2 μ/sec for low-carbon steel) and does not depend on the temperature. 3. Martensite formation is not a thermally activated process—the low rate at which martensite is formed at high temperatures is due to the fact that the growth of martensite platelets occurs in a narrow range in which relaxation processes occur rapidly. Evidently the occurrence and relaxation of stresses is the principal factor determining the mechanism and rate of the martensitic transformation.

Vacuum carburizing of steel 18KhGT

Abstract: 1. Raising the vacuum carburizing temperature from 960° (standard carburizing temperature for races of steel 18KhGT at State Bearing Factory No. 1) to 1040° accelerates the process by 60%. 2. Vacuum carburizing provides a clean surface and can be used as the final operation.

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

Abstract: 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.

Effect of differences in grain size on the mechanical properties of steel 18Kh2N4MA

Abstract: A difference in grain sizes has a negative effect on the ductile characteristics of steel. The ductile characteristics (T50,ap) of the steel with differing grain sizes (grades 1 and 8) are slightly better than those of the coarse-grained steel (grade 1). A difference in grain size mainly lowers the resistance to ductile (ap) and brittle (T50) fracture. The strength characteristics (σb, σ0.2), hardness (HRC), and ductility (δ, Ψ) of the quenched and tempered steel do not depend greatly on the original austenite grain size. With ductile fracture the negative effect of a difference in grain size is more evident after high-temperature tempering. A difference in grain size has the same negative effect on the ductile-brittle transition temperature (resistance to brittle fracture) after tempering at both low and high temperatures.

Texture strengthening of titanium alloys

Abstract: 1. Increasing the degree of deformation in cold and warm rolling increases the texture of α alloy VT5-1. 2. With increasing deformation of α+β alloy VT14 the quantity of basal texture, changes, reaching a peak at 70% deformation. 3. For α alloy VT5-1, σbbi/σb is equal to 1.45, while it is 1.15 for α+β alloy VT14 and 1.13 for β alloy VT15. 4. Heating to 1000–1100° in the β region weakens the basal texture of titanium alloys, leading to pyramidal and prismatic orientations. Heating in the α or α+β regions slightly reduces the intensity of the basal texture (by 10–15%). Heating to 880° for quenching leads to a greater reduction of the deformation texture than annealing at 750°. 5. The decomposition of the original β phase during aging of alloy VT14 changes the texture of the precipitated α phase to prismatic, which facilitates deformation through the thickness of the plate and lowers the strengthening factor to 0.8. 6. Rolling after heating to 800° with small deformation in each pass and changing the direction of deformation by 45° in each pass ensures a larger percentage of the basal texture in alloy VT14 than rolling without changing the direction, with larger deformation in each pass at the same total deformation.

Structure and properties of beryllium bronze after aging under load

Abstract: 1. It was established that aging of beryllium bronze in a field of elastic stresses leads to precipitation of particles of second phase with a given orientation in relation to the axis of the applied load. In this case the mutual positions of the particles of second phase change. 2. The aging process is slowed down under load as compared with ordinary aging. 3. Aging under load substantially increases the elastic limit σ0.005 (from 73 to 93 kgf/mm2) and the yield strength σ0.2 (from 107 to 117 kgf/mm2); the ultimate strength remains almost unchanged. 4. Aging of beryllium bronze under bending load leads to a substantial increase in resistance to small plastic deformations, the elastic limit increasing considerably with a small allowance for residual deformation.

Nature of the dark component in carbonitrided steels

Abstract: The so-called dark component in carbonitrided steels consists of pores filled with molecular nitrogen precipitated in the pores after some critical activity in austenite is reached. The effect of carbon and alloying elements on the formation of the dark component results from their effect on the thermodynamic activity of nitrogen.

Some characteristics of structure formation in manganese cast irons and steels

Abstract: 1. Manganese lowers the solubility of carbon in the melt, while austenite raises the eutectic equilibrium temperature and shifts the eutectic point toward lower carbon concentrations. 2. With a high manganese concentration in cast iron the shift of the eutectic point leads to a sharp change in the ratio of phases and their composition. The amount of cementite is insufficient for the formation of a dense matrix, and it is possible for the eutectic to grow as a conglomerate of phases with a predominance of austenite. 3. By refining of the conglomerate structure (by increasing the solidification rate, e.g.) in combination with heat treatment it is possible to isolate carbide inclusions in the austenite matrix from each other, which provides a combination of mechanical properties unusual for cast iron (strength, ductility, and toughness). 4. Under conditions of nonequilibrium eutectic solidification the austenite in manganese cast irons is highly supersaturated with carbon and manganese. The decomposition of supersaturated austenite at high temperatures leads to formation of a pearlite-like component consisting of platelets of cementite containing manganese and interlayers of austenite. At low temperatures (during tempering after quenching, e.g.) this austenite ages, with a precipitation hardening effect.

Effect of rare-earth metals on cold brittleness and temper brittleness of structural steels

Abstract: 1. Small additions of REM (0.1–0.15%) to structural steels have no essential effect on the ductile-brittle transition temperature after quenching and tempering or embrittling treatment. 2. Alloying of structural steels with large quantities of REM (0.4–0.65%) shifts the ductile-brittle transition temperature to lower temperatures and practically eliminates susceptibility to reversible temper brittleness. 3. REM does not enter into the solid solution and does not enrich the grain boundaries but is found only in nonmetallic inclusions. 4. The main reason for the substantial reduction of the ductile-brittle transition temperature of steels alloyed with REM is that REM combines harmful impurities (phosphorus and its analogues) in nonmetallic inclusions.

Determining thermal fatigue of steels for die casting of aluminum alloys

Abstract: 1. The total deformation of a sample under the influence of thermal cycling and uniaxial stress characterizes the resistance to thermal fatigue of die-casting molds. 2. Experimental determination of the deformation makes it possible to calculate the number of cycles to failure of die-casting molds due to thermal fatigue. 3. Calculating the thermal fatigue by the method proposed makes it possible to compare different steels. It is recommended that these calculations be made in selecting materials for die-casting molds. 4. Maraging steel N21M2T2B is a promising material for die-casting molds used for aluminum alloys.

Welding HSLA line pipe steels

Abstract: This paper has reviewed the various aspects of weldability as it is influenced by the base metal composition and processing in HSLA line pipe steels. The Mo-Nb steels meet these stringent weldability requirements. Consumables have been developed for both girth and seam welding the Mo-Nb steels, and the strength and toughness required for severe service can be obtained with consumables of reasonable cost and good operating characteristics.

Interaction of magnesium with gases

Abstract: 1. At 750° no protective film preventing evaporation of magnesium is formed on the surface of a molten magnesium alloy of the Mg−Al−Zn system in an atmosphere of nitrogen or N2+1–20 vol. % SO2. 2. In an atmosphere of 40–50 vol. % N2+60–50 vol. % CO2 a coating of soot is formed on the surface of the molten magnesium alloy, preventing evaporation of magnesium. 3. Sulfur dioxide slows down the oxidation of magnesium in an atmosphere of CO2, and 20 vol. % SO2 is sufficient to block defects in the MgO film. 4. In an atmosphere of CO2 with 1–10 vol. % SF6 a dense protective film is formed on the surface of magnesium, the thickness of the film changing little with the SF6 concentration. 5. The optimal quantity of SF6 in a nitrogen atmosphere is 1.5–3 vol. %; the thickest and densest protective film is formed on the surface of the molten magnesium alloy under these conditions.

Effect of cobalt on the structure and properties of steel 18Kh2N4MA

Abstract: 1. Small cobalt additions (0.2–0.5%) have almost no effect on the kinetics of phase transformations, structure, or mechanical properties of steel 18Kh2N4MA, but lower its hardenability. 2. The addition of as much as 3% Co raises the change points and reduces the amount of retained austenite in the structure; cobalt strengthens the steel and lowers the notch toughness by 1–3 kgf-m/cm2 at all testing temperatures; the value of T50 rises 20o after quenching and high-temperature tempering, but after low-temperature tempering the percentage of brittle components in the fracture increases. The hardenability decreases as much as 40% at this cobalt concentration. 3. The cobalt concentration of structural steel 18Kh2N4MA, melted with use of ferronickel containing cobalt, should not exceed 0.5%.

Temperature dependence of the hardness of diffusion coatings of Ta 2 C and TaB 2

Abstract: 1. The temperature dependence of the hardness of diffusion coatings of Ta2C and TaB2 was determined. 2. Low-temperature and high-temperature inflections were found in the variation of the hardness of Ta2C and TaB2 with temperature. 3. It was found that the hardness of Ta2C and TaB2 in relation to temperature can be described by the equation $$H = H_0 e^{\alpha - \sigma T} $$ , where the values ofa and α are constant for each stage of softening.