Precipitation Of Excess Phases Research Articles

Hot Ductility, Homogeneity of the Composition, Structure, and Properties of High-Strength Microalloyed Steels: A Critical Review

High-strength microalloyed steels are widely used in various branches of technology and industry due to the simultaneous combination of high indicators of strength, ductility, fatigue, corrosion resistance, and other service properties. This is achieved due to the reasonable choice of the optimal chemical composition and parameters of temperature-deformation treatment of steel that provide a synergistic effect on the dispersed microstructure and characteristics of excess phase precipitates, which control the achievement of these difficult-to-combine properties of rolled products. Additionally, the improvement of the level and stability of these properties, as well as the prevention of the occurrence of defects, is largely determined by the indicators of the homogeneity of the composition, structure by volume and manufacturability of the metal, and primarily hot ductility, which are controlled by the presence of precipitation of excess phases, including microalloying elements. In accordance with the circumstances noted, in the present review, a generalization, systematization, and analysis of the results of the studies are conducted on the effect of phase precipitates on the hot ductility and homogeneity of composition and structure, depending on the chemical composition and parameters of the temperature-deformation treatment of steel.

Effect of Excess Phase Precipitation on Strengthening of Structural Steels Prepared by Hot Stamping

The contribution of nano-sized precipitates to hardening metal products prepared by hot stamping from microalloyed niobium and titanium steels is evaluated. In order to calculate the characteristics of carbide precipitates (quantity, size), kinetics are modeled for their precipitation from austenite. It is shown that the increase in yield strength due to microalloying steel with niobium is greater than with titanium, and it increases with increase in their content. With a concentration of 0.1% each of Nb and Ti, the contribution to hardening steel with 0.3% C reaches 186 and 129 MPa, respectively.

Features of Low-Alloy Chromium Corrosion-Resistant Steel Alloyed with Nitrogen Structure Formation

Chromium stainless steels alloyed with nitrogen are studied by experiment. Electron microscopic study of excess phase precipitation makes it possible to identify the types of particles formed at different stages of processing and to assess the degree of their effect on properties. It is established that an increase in the strength of steel tempered at 400°C may be connected with formation during tempering of segregations of various atoms at dislocations with subsequent formation of particles coherent with the matrix, and subsequently losing coherence.

Effect of Hot Rolling Parameters on Excess Phase Precipitation in Two-Phase Ferritic-Martensitic Steels

Results are given for studying the effect of hot rolling parameters on morphological features of the structure and precipitation in hot rolled products destined for further cold rolling and heat treatment in a continuous annealing line. Temperatures for the end of rolling and coiling are varied with hot rolling of a laboratory melt. Depending on the combination of temperatures for the end of rolling and coiling temperature, the amount of precipitates and their preferred location vary.

Mechanisms for Improving Chemical and Structural Homogeneity of Hot-Rolled Product for Objects Prepared by Hot Stamping

Redistribution of components forming the structure and properties during hot rolling of microalloyed steels, similar in chemical composition to hardened steels during hot stamping, of an industrial melt containing carbon from 0.098 to 0.219%, is studied in detail. It is established that a key production parameter controlling the intensity of occurrence of these processes is the temperature at the start of rolling in the finishing group of stands, i.e., T6. An increase in T6 leads to significant enrichment with respect to carbon content, and a finer structure due to the formation of niobium carbonitride precipitates in surface layers compared with the rolled product axial zone. An increase in carbon content and a reduction in the concentration of niobium and other microalloying components within steel reduces the intensity of development of these processes. It has been shown by experiment that significant metal chemical and structural inhomogeneity forming in the continuous billet casting stage may be avoided or significantly reduced during hot rolling on the basis of controlling excess phase precipitation. This leads to a marked increase in the level of production and service properties of both hot-rolled product and metal objects prepared from it by hot stamping.

Effect of Composition, Structural State, and Manufacturing Technology on Service Properties of High-Strength Low-Carbon Steel Main Bimetal Layer

The effect of composition, structural state, excess phase precipitation, and manufacturing technology parameters on service property indices of high-strength, low-carbon steel of a main bimetal layer is studied in detail. For the low-carbon, high-strength steels developed primary importance is attached to strengthening mechanisms, connected with formation of different size carbonitride precipitates with different fineness, and secondary importance applies to the presence of strong structural components. A simultaneous improvement in strength properties and preparation of good steel weldability may be achieved by some increase in carbon content using a balanced microalloying system with a reduction in concentration of manganese, chromium, nickel, and copper, and effective alloying with boron. It is shown that the possibility of achieving high strength properties for the low-carbon steel developed with relatively slow cooling rates creates favorable conditions for obtaining a clad rolled product with a good and stable set of mechanical and other service properties, and quality indices.

Experience of Mastering Plate Production of Strength Category Sawl 450 for Deep-Water Pipes at the Vyksa Metallurgical Plant 5000 Mill

39 mm wall thickness for submarine pipelines, is mastered under industrial conditions at the 5000 mill of Vyksa Metallurgical Plant (VMZ). The effect of chemical composition and steel manufacturing technology on features of microstructure formation (including its morphological features and excess phase precipitation) is studied. This makes it possible to formulate the optimum ferrite-bainite microstructure after TMT and provide the required strength level and cold resistance for plates, including manufacture of double length workpieces.

Effect of heat treatment on mechanical properties and structure of high-strength pipe steels with a ferrite-bainite structure*

The effect of heat treatment on mechanical properties and microstructure of high-strength low-alloy ferrite-bainite pipe steels is studied. Specimens are heated in a laboratory furnace up to a temperature in the range 100–850°C. Dependences for the change in mechanical properties on heating temperature and the main features of steel structure are determined. TEM is used to study excess phase precipitation, including Nb and V carbonitrides, during heating. Unfavorable temperature ranges are determined for heating pipe steels with a ferrite-bainite structure. The results obtained may be used in industrial production and during development of new technology for thermomechanical treatment.

Dissolution and Precipitation of Excess Phases in High-Nitrogen Steels

Cast structure of ferric, austenitic and dual-phase high-nitrogen alloys and steels saturated with nitrogen at pressures of up to 3.2 MPa and solidified at cooling rates of 2 to 4•104 K/s has been investigated. Increasing the nitrogen concentration in the alloy and decreasing the cooling rate results in smaller nitrogen content in α and γ solutions and in larger content of crystallization nitrides.

Dissolution and precipitation of excess phases and the nitrogen distribution between a solid solution and nitrides in a corrosion-resistant steel

The dissolution and precipitation of nitrides are experimentally studied during the solidification and heat treatment of high-nitrogen austenitic steels. The majority of nitrogen is found to be in nitrides even upon rapid solidification, and the dissolution of these nitrides requires high temperatures and long-term holding. The nitrogen content has to be optimized according to the required steel structure, namely, austenite or austenite + excess phases. A high nitrogen content in a steel is not always effective to provide its high strength.

Resistance of weld joints of chrome?manganese?nickel steels to embrittlement at increased temperatures

1. Embrittlement of the weld joints of the investigated chrome—manganese—nickel steels in the 350–500°C range is caused primarily by processes occurring in the δ-ferrite, 475° brittleness or precipitation of excess phases at the interphase boundaries and within the δ-ferrite grains. The level of plasticity is determined not by the quantily of δ-ferrite in the structure but by the density and quantity of excess phase precipitates in it. 2. Aging processes occur in the weld zone at lower temperatures than in the base metal. For weld joints of the experimental steels 05Kh18G2N5T and 05Kh18G10N5M3F the maximum operating temperature for long service is 300°C and for 05Kh18G10N5M3AF steel below 300°C.

Precipitation of excess phases in austenite under the influence of hot deformation and recrystallization

1. Hot deformation of austenite in steels alloyed with vanadium and titanium, with subsequent recrys-tallization in the process of continuous cooling, stimulates precipitation of excess carbide phases containing vanadium and titanium. 2. Precipitation of dispersed phases that are resistant to coalescence in the process of recrystallization of hot worked austenite slows down secondary recrystallization. The grain size of the steel after cooling decreases monotonically with increasing degrees of hot deformation, while for unalloyed steel or steels alloyed with weak carbide-forming elements the variation of the grain size with the degree of hot deformation takes the form of a curve with a low point.

Kinetics of the precipitation and the structure of dispersed phases in hot-worked austenite

1. We investigated the kinetics of precipitation and the characteristic changes in the structure of dispersed phases in steel of the Kh18N10T type in the process of postdeformation holding. It was found that the dispersed phases consist of titanium carbonitride, carbide of the M6C type, and intermetallic χ and σ phases. 2. Deformation promotes formation of particles of excess phase of small size and accelerates the precipitation process: The holding time for intensive precipitation is shorter in deformed than in undeformed austenite. 3. Precipitation of excess phases inhibits not only recrystallization but also softening in the prerecrystallization but also softening in the prerecrystallization stage; there is even a slight increase in the hardness of austenite — precipitation hardening. 4. Oriented acicular precipitates inhibit all stages of recrystallization much more than equiaxed precipitates of rounded form. 5. We determined the effect of titanium as a strong carbide-forming element on the kinetics of recrystallization: The beginning of recrystallization in the steel with titanium is delayed more than 30 times at 900° and 15 times at 1000°.

Thermal brittleness of maraging steel 03Kh11N10M2T

1. To prevent precipitation of excess phase in the grain boundaries and a sharp reduction of the ductility and toughness of steel 03Kh11N10M2T the steel should be cooled at a rate of at least 2·103 deg/h. 2. To correct an original overheated structure of martensitic steels 03Kh11N10M2T, EP679, and EP699 it is necessary that they be quenched three times from 980–1000° (rapid heating to these temperatureds). 3. Triple quenching substantially improves the ductility and toughness of embrittled steel.

Effect of adsorption of alloying elements on the morphology of precipitates in Ni-Cr-Nb alloys

1. T-T-T diagrams were plotted for Ni-Cr-Nb alloys (multicomponent commercial alloy 70NKhBMYu and the basic ternary alloy N70Kh20B9) that show the change in the morphology of second phases precipitated during aging. 2. The aging conditions were determined for obtaining a lamellar structure of discontinuous decomposition throughout the bulk of alloys 70NKhBMYu (930–900°C for 1–2 h) and N70Kh20B9 (880–750°C for 1–8 h). 3. The structure and morphology of second phases precipitated during aging of Ni-Cr-Nb alloys are affected by the character of adsorption of the element added. 4. The mechanism of the effect of aluminum, tungsten, and molybdenum in Ni-Cr-Nb alloys on the morphological sequence of precipitation of excess phases is correlated with the effect of these elements on the stability of dispersed phases and the surface energy of the interphase boundaries.

Lithoidal fracture of high-carbon steels

1. The effect of carbon on the susceptibility of steel to the formation of primary and resistant lithoidal fracture is not the same in all cases. The directional effect and the intensity of the effect depend on the quantity and composition of the carbide phase and its solubility in austenite. Increasing amounts of carbon in the steel favor local melting during overheating, especially in the grain boundaries, intersections of grains, and in places rich in low-melting impurities (phosphorus) and elements lowering the melting point of iron, and also around nonmetallic inclusions of the MS type. With satisfactory fracture (fine crystalline, cryptolithoidal) of such steel the most important properties are irreversibly low. 2. It was found that the large enrichment in phosphorus (50–60 times) of energetically weak zones of the matrix (boundaries and joints of grains, boundaries between the matrix and nonmetallic inclusions) occurs only with their melting. 3. Overheating of alloy steel Kh2N4V to temperatures leading to the first portions of liquid steel induces substantial enrichment in phosphorus, chromium, and tungsten. Cooling leads to precipitation of excess phase of complex composition (white phase). The precipitation of this phase predetermines the resistance of the pseudograins to rectification. 4. Stable lithoidal fracture is due to precipitation in boundary areas on the surface of prior austenite grains of compounds difficult to dissolve in ordinary heat treatments that include nitrides, special carbides, carbonitrides, intermetallic compounds, and other excess phases.

Electron microscopic study of precipitation of excess phases in deformed alloy 36NKhTYu

1. The solid solution of alloy 36NKhTYu with precipitates of metastable finely dispersed γ′ phase [(NiFe)3 (TiAl)] having a fcc lattice begins to decompose in the process of deformation — preferentially in the grain boundaries at e=50% and in the bulk of the grains at e=70%. 2. The decomposition of the solid solution in the alloy deformed at the tempering temperature (600–650°) is characterized by intermittent precipitation in the grain boundaries and bulk of the grains of metastable γ′ particles, and by continuous precipitation of finely dispersed γ′ particles in the bulk of the matrix at 700–750°. 3. The formation of stable intermetallic η phase, leading to weakening, begins after tempering at 700° for 2 h.

Heat resistance of Cr?Ni and Cr?Ni?Mo steels with boron additions

1. At temperatures below the equcohesive temperature, i.e., with transcrystalline fracture, the long-term strength of Cr−Ni and Cr−Ni−Mo steels increases continuously with increasing boron additions up to 0.005%. At temperatures above the equicohesive temperature, where fracture is intergranular, the long-term strength of the steels is highest at boron concentrations of 0.001–0.0018%. 2. The following mechanisms of the increase in the long-term strength with small boron additions were established: a) small boron additions increase the strength of the grain boundaries, which is manifest at temperatures above the equicohesive temperature; b) small boron additions slow down the diffusion processes of precipitation of excess phases in the boundaries and within the grains during creep; c) additional strengthening of the grain boundaries evidently occurs at concentrations of 0.004–0.005% B due to precipitation of dispersed borides.

Phase transformations in steel 1Kh12N2VMF during aging, and mechanical properties

1. Aging of steel 1Kh12N2VMF at 200–400°C for up to 5000 h induces precipitation of cementite-Fe3C. 2. At 475°C a sequence of transformations occurs with increasing aging time: (Fe, Cr)3C→(Cr, Fe)2C →(Cr, Fe, Mo, W, V)23C6. 3. (Cr. Fe)2C is formed at 525–570°C, which with increasing aging time is replaced by (Cr, Fe, Mo, W, V)23C6; Fe2W is also precipitated at these temperatures. 4. At higher temperatures up to 760°C (Cr, Fe, Mo, W, V)23C6, VC, and Fe2W are formed. At temperatures above 600°C the amount of these phases decreases with increasing temperatures and aging times due to the higher solubility in α-iron. 5. The changes in the mechanical properties of the steel during aging are determined by diffusion processes in the precipitation of excess phases and by redistribution of alloying elements between phases and the solid solution.

Characteristics of the eutectoid transformation and precipitation of excess phases in silicon steels

Characteristics of the eutectoid transformation and precipitation of excess phases in silicon steels