Showing papers on "Bainite published in 1989"

Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet

Abstract: The effect of phosphorus and silicon on the formation of retained austenite has been investigated in a low-carbon steel cold rolled, intercritically annealed, and isothermally held in a temperature range of bainitic transformation followed by air cooling. The steel sheet containing phosphorus after final heat-treatment consisted of ferrite, retained austenite, and bainite or martensite. Phosphorus, especially in the presence of silicon, in steel was useful to assist the formation of retained austenite. Mechanical properties, such as tensile strength, uniform elongation, and the combination of tensile strength/ductility, were improved when phosphorus was increased up to 0.07 pct in 0.5 pct Si steel. This could be attributed to the strain-induced transformation of retained austenite during tensile deformation. Furthermore, two types of retained austenite were observed in P-containing steel. One is larger than about 1 μm in size and usually exists adjacent to bainite; the other one is of submicron size and usually exists in a ferrite matrix. High phosphorus content promotes the formation of stable (small size) austenites which are considered to be stabilized mainly by their small size effect and have a different formation mechanism from the coarser retained austenite in the lower P steels. The retained austenites of submicron size showed mechanical stability even after 10 pct deformation, suggesting that these small austenites have little effect on ductility. The 0.07 pct P-0.5 pct Si-1.5 pct Mn-0.12 pct C steel showed a high strength of 730 MPa and a total elongation of 36 pct.

The microstructure of chromium-tungsten steels

Abstract: Chromium-tungsten steels are being developed to replace the Cr-Mo steels for fusion-reactor applications. Eight experimental steels were produced and examined by optical and electron microscopy. Chromium concentrations of 2.25, 5, 9 and 12 pct were used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten and vanadium, three other 2.25Cr steels were produced as follows: an alloy with 2 pct W and 0 pct V and alloys with 0 and 1 pct W and 0.25 pct V. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta also was studied. For all alloys, carbon was maintained at 0.1 pct. Two pct tungsten was required in the 2.25Cr steels to produce 100 pct bainite (no polygonal ferrite). The 5Cr and 9Cr steels were 100 pct martensite, but the 12Cr steel contained about 25 pct delta-ferrite. Precipitate morphology and precipitate types varied, depending on the chromium content. For the 2.25Cr steels, M3C and M7C3 were the primary precipitates; for the 9Cr and 12Cr steels, M23C6 was the primary precipitate. The 5Cr steel contained M7C3 and M23C6. All of the steels with vanadium also contained MC.

Theoretical analysis of changes in cementite composition during tempering of bainite

Abstract: An attempt has been made to model the kinetics of changes of composition in cementite during the aging of creep resistant low alloy steels of the type used in power plant. In the model a finite difference method is used for calculating the rate at which alloying elements redistribute between ferrite and cementite, subject to the thermodynamic constraints which determine the equilibrium compositions of these phases. It has also been possible to derive approximate analytical solutions which give good physical insight into the factors controlling the approach to equilibrium. Particle size has been found to have a strong influence on the rate at which the cementite composition is changed. A theoretical basis for the time dependence of cementite composition is also provided. The method is easily adapted to multiple or anisothermal heat treatments. Further work is suggested to account for simultaneous coarsening and changes in composition and also to allow for the formation of alloy carbides at long ag...

Lower acicular ferrite

Abstract: An experiment has been designed to confirm that the mechanism of growth of acicular ferrite in steel welds is similar to that of bainite in ordinary steels. On the hypothesis that the growth mechanisms are identical, it was expected that if the carbon concentration of a weld is increased sufficiently, then for similar welding conditions, the microstructure should undergo a transition from acicular ferrite tolower acicular ferrite, in which the plates of acicular ferrite should contain cementite particles of the sort normally associated with lower bainite in wrought steels. An experimental weld of unusually high carbon concentration was prepared, and metallography confirmed the existence of lower acicular ferrite, supporting the conclusion that acicular ferrite is simply an intragranularly nucleated bainite.

Growth rate data on bainite in alloy steels

Abstract: On the basis of a theoretical analysis of published experimental data on the lengthening rate of sheaves of bainite in alloy steels, it is concluded that they lengthen at rates much faster than expected from paraequilibrium carbon diffusion controlled growth. The results are interpreted in terms of the mechanism of the bainite reaction and in the context of any solute atom–transformation interface interactions.MST/980

The microstructure of submerged arc-weld deposits for high-strength steels

Abstract: Recent theoretical work on the design of unusually high-strength steel weld deposits is compared with microstructural studies of an experimental, multipass submerged arc weld deposit. The as-deposited microstructure is found to consist of a mixture of acicular ferrite, bainite and low-carbon martensite. Owing to its low carbon concentration, the structure is highly resistant to tempering, and because the alloy becomes austenitic at a relatively low temperature, the effect of reheating due to the deposition of several layers is found to be minimal. The overall microstructure of the multirun weld thus turns out to be fairly homogeneous, differing little from the as-deposited microstructure.

Development of mechanical properties of structural high-carbon low-alloy steels through modified heat treatment

Abstract: The modified heat treatment, which produces a mixed structure of martensite and lower bainite through short-term isothermal transformation at just above the martensitic transformation temperature,Mstemperature, followed by oil quenching (after conventional austenitization), has been applied to three high-carbon low-alloy steels with different levels of nickel and chromium contents at similar molybdenum levels, in which carbon was allowed to replace relatively expensive additions of nickel and chromium, for their ultra-high strength application. The significant conclusions are as follows: an ultra-high strength steel of 1900 M Pa yieldstress grade with a high toughness level can be obtained when about 60 vol % lower bainite is associated with 473 K tempered martensite of 0.60% C-1.80% Ni-0.80% Cr-0.25% Mo steel. If approximately 25 vol % lower bainite appears in 673 K tempered martensite of the steel, a 1700 M Pa yield-stress grade steel with high toughness and moderate ductility levels can be attained. However, alloying nickel is essential to some extent for development of the mechanical properties with the modified heat treatment suggested in the present work.

Reaustenitization experiments on some high-strength steel weld deposits

Abstract: The kinetics of the reverse transformation from ferrite to austenite have been investigated to provide a basis for the modelling of microstructure in multirun, steel weld deposits. The reaustenitization process has been studied for two starting microstructures, one consisting of a mixture of bainitic ferrite and austenite, and the other of acicular ferrite and austenite. Both isothermal and continuous heating experiments have been carried out under conditions where reverse transformation involves simply the growth of already existing austenite. The results are analysed theoretically using an approximate model for the growth of austenite, and an attempt is made to explain the differences in the observed reaustenitization behaviour of bainite and acicular ferrite.

Roll ring comprising a ring of cemented carbide metallurgically bonded to a cast iron body

Abstract: A roll ring for hot and/or cold rolling. The rolling track comprises one or several cemented carbide rings, which are cast into a casing made by an iron alloy. The cast alloy comprises a materially graphitic cast iron, which, after casting, contains residual austenite. This residual austenite is, at subsequent heat treatment or treatments, partly or totally transformed under volume increase to mainly bainite with the aim of reducing or totally eliminating the differential shrinkage between the cast iron and the cemented carbide, which develops as a result of cooling after casting.

Micromechanisms of Dynamic Crack Propagation in an AISI 4340 Steel

Abstract: The present work is aimed at understanding the micromechanisms of dynamic crack growth at room temperature and a low temperature (−100°C) in an AISI 4340 steel studied by Ravichandran and Clifton. For this purpose, a fractographic and metallographic examination of the specimens used by Ravichandran and Clifton was carried out. Results showed that the steel contained an appreciable amount of upper bainite (about 15%). The study suggests that dynamic crack propagation in the 4340 steel occurs in two stages, i.e. nucleation of microcracks in the upper bainite followed by propagation of these microcracks into the surrounding martensite. At low temperatures, failure of the martensite occurs by cleavage whereas, at room temperature, it occurs by shear localization, leading to microvoid growth and coalescence. At both temperatures, bainitic microcracks serve as failure initiation sites and reduce the dynamic fracture toughness of the steel.

Method for the manufacture of alloy railway rails

Abstract: An improved method for the manufacture of hardened railroad rails comprises the steps of forming a railroad rail from an alloy steel of a preselected chemical composition, force cooling the rail from a preselected cooling start temperature above about the austenite-to-­ferrite transformation temperature, terminating the force cooling when the temperature of rail reaches a preselected cooling stop temperature, and before a substantial volume fraction of the austenite in the rail head has transformed to pearlite, and holding the rail under substantially isothermal conditions until the austenite-to-pearlite transformation is complete. The chemical composition of the alloy steel is selected such that the austenite-to-pearlite reaction occurs earlier in time than the austenite-to-­bainite reaction under the substantially isothermal conditions. The use of this method makes avoidance of bainite easier to achieve, so relatively simple and inexpensive process control equipment can be utilized.

Production of steel sheet having high-ductility high-strength multiple structure

Abstract: PURPOSE:To obtain a steel sheet having a multiple structure which is excellent in tensile strength, ductility and spot weldability by subjecting a high Si-Mn steel slab contg. Si and Mn at specific ratios to hot rolling, then to a heat treatment under specific conditions including continuous annealing. CONSTITUTION:The steel slab contg., by weight %, 0.12-0.30% C, 1.5-3.0% Si, 1.1-2.4% Mn, having the relation 3.7-Si =70 deg.C/sec cooling rate and is held for 1-5min, following which the sheet is cooled down to room temp. at >=2 deg.C/sec cooling rate. The steel sheet having the multiple structure consisting of martensite, bainite, ferrite and residual austenite is produced.

Roll ring, comprising cemented carbide and cast iron, and method for manufacture of the same

Abstract: The present invention discloses a roll ring for hot and/or cold rolling. The rolling track comprises one or several cemented carbide rings, which are cast into a casing made by an iron alloy. The cast alloy comprisin'g a materially graphitic cast iron, which after the casting contains residual austenite. This residual austenite is at subsequent heat treatment or treatments partly or totally transformed under volume increase to mainly bainite with the aim of reducing or totally eliminating the differential shrin­kage between the cast iron and the cemented carbide as a result from cooling after the casting.

High strength hot rolled steel plate excellent in workability, and its production

Abstract: PURPOSE:To manufacture a high-strength hot-rolled steel plate having high ductility and excellent in workability, by subjecting a steel containing specific percentages of C, Si, and Mn to hot finish rolling under specific conditions, to cooling, and then to winding. CONSTITUTION:A steel having a composition consisting of, by weight, 0.15-0.4% C, 0.5-2.0% Si, 0.5-2.0% Mn, and the balance iron with inevitable impurities is subjected to hot finish rolling at >=80% total draft, and the reduction-end temp. is regulated to Ar3+ or -30 deg.C. Subsequently, cooling is applied at =40 deg.C/sec cooling rate, followed by winding at 350-500 deg.C. By this method, the steel plate constituted of a structure in which the ratio of the space factor of polygonal ferrite VPF(%) to the average grain size of polygonal ferrite dPF(mu), VPF/dPF, is regulated to >=7 and residual austenite is contained >=5% by volume ratio and which is composed of ferrite, bainite, and residual austenite can be obtained.

Alloy steel for very low temperature use

Abstract: PURPOSE:To manufacture a steel free from deterioration in toughness due to temper brittleness while obviating the necessity of particular heat treatment, by providing a specific composition in which Si and Mn contents are reduced and deterioration in hardenability due to the above reduction or increase in thickness is compensated by the addition of other alloy components. CONSTITUTION:This alloy steel has a composition consisting of, by weight, 0.02-0.15% C, <=0.10% Si, <0.10% Mn, 7.0-11.0% Ni, 0.20-2.0% Cr, 0.1-0.5% Mo, 0.01-0.035% Al, and the balance Fe and containing, if necessary, 0.1-0.5% Cu or 0.01-0.06% V and is composed of martensitic and lower bainite structures. This alloy steel is capable of reducing tempered brittleness sensitivity even if cooling at the time of tempering or heat treatment after welding is applied by means of slow cooling, and accordingly, this alloy steel can maintain high toughness even when applied to a thick-walled part or a large-sized structural member. Further, this alloy steel can be manufactured without requiring particular manufacturing stages, such as direct hardening after hot rolling.

Effect of the haz microstructure on the fracture toughness of offshore microalloyed structural steels

Abstract: Microstructural characterisation is necessary to understand the combined effects of high heat input welding energies and chemical composition on the Heat Affected Zone (HAZ) toughness of High Strength Low Alloy (HSLA) steels. The paper establishes both the deleterious influences of Martensite - Austenite - Carbide (MAC) constituent and of proeutectoid ferrite, and the favourable effect of acicular ferrite, compared with lath upper bainite, on toughness. The acicular ferrite arises from a heterogeneous nucleation on intragranular TiN precipitates.

High toughness cast steel

Abstract: PURPOSE:To manufacture a cast steel having high machinability and toughness in the cast steel having a bainitic matrix structure contg. specific ratios of C, Si, Mn, P and S by specifying the various conditions of spheroidal graphite, the amt. of bainite and the amt. of residual austenite. CONSTITUTION:The cast steel contg., by weight, 1.0-1.8% C, 1.0-3% Si, =60%; and 90-50% bainite and 10-50% residual austenite are regulated. Specific amounts of Mo, Ni, Cu, V, Ca and Mg are furthermore added to said cast steel at need. By this method, the material for cast iron having high toughness and machinability and in which spheroidal graphite is deposited can be obtd.

Ferrous casting having high strength, high hardness, and high toughness and its manufacture

Abstract: PURPOSE:To obtain a ferrous casting excellent in castability and having high strength, high hardness, and high toughness by applying austempering treatment to a ferrous casting material having high Si content and containing specific amounts of B so as to form this material into a structure of bainite and residual austenite in which spheroidal graphite is dispersed. CONSTITUTION:A casting member having a composition consisting of, by weight, 0.5-2.5% C, 1.0-4.0% Si, <0.6% Mn, <0.05% S, 0.01-0.1% B, and the balance Fe is subjected to austempering treatment. Since a matrix structure in which flake graphite is spheroidized by the presence of B and uniformly dispersed and which is composed of bainite and residual austenite is formed, the ferrous casting member having high Si content, excellent in castability, and improved in strength, hardness, and toughness can be obtained.

Transformation processes and products for C-Mn steels during continuous cooling

Abstract: The continuous cooling transformation (CCT) diagrams for four C–Mn steels were constructed using dilatometry and metallography. All diagrams involved the formation curves of second phases and the formation curves of matrix structures. Between polygonal ferrite and martensite, acicularferrite and bainite were observed. The formation of acicularferrite was associated with a kinetic change which was an indication of the formation of second phases such as pearlite, pseudopearlite, and martensite. The pseudopearlite wasfound to form by separate precipitation of cementite and ferrite in austenite. It was thought that the transformation of martensite second phases followed a Kurdjumov–Sachs relationship. Three types of carbide, i.e. upper bainitic, lower bainitic, and Widmanstatten, were observed in continuously cooled bainite. An increase of manganese content suppressed the formation of polygonal ferrite, promoted the formation of acicular ferrite and bainite, and changed the second phase from pearlite ...

Production of high-strength hot rolled steel sheet having excellent balance of strength and ductility

Abstract: PURPOSE:To form a bainite structure in which austenite metastable at an ordinary temp. remains and to produce a high-strength hot rolled steel sheet having excellent balance of strength and ductility by specifying cooling rate and coiling temp. after hot rolling of an ordinary C-Si-Mn steel without requiring special elements such as Cr. CONSTITUTION:The steel having the compsn. contg., by weight %, 0.35-0.55% C, 1.0-2.5% Si, and 1.5-3.0% Mn, and contg. =10% austenite phase metastable at an ordinary temp. and consists of the balance bainite is obtd. and the steel sheet having the excellent ductility while having high strength is obtd.

Process for manufacturing sintered steel bodies, and bodies obtained thereby

Abstract: In a process for manufacturing sintered steel bodies, sintered steel powder is pressed and sintered, the sintered body being austenitised by heating to temperatures above 8000 degrees. After the austenitic heat treatment, the sintered body is cooled rapidly to a temperature of 280 to 450 degrees C and kept in this temperature range for between 5 and 60 minutes, a bainitic microstructure of the structure being achieved. The sintered steel body whose microstructure consists of bainite.

Hot rolled steel sheet with high strength and distinguished formability

Abstract: A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in a ratio (V PF /d PF ) of polygonal ferrite volume fraction V PF (%) to polygonal ferrite average grain size d PF (μm) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productiviity and without requiring special alloy elements.

High strength non-heat treated steel and its manufacture

Abstract: PURPOSE:To manufacture a non-heat treated steel having excellent balance of strength and toughness by regulating the structure to which suitable amounts of Cr, Mo, Nb, Ti, B, etc., are added and after subjected to forging to low C martensite or bainite or the mixed structure therebetween. CONSTITUTION:The steel having the compsn. consisting of, by weight, 0.04-0.20% C, 0.02-1.0% Si, 1.0-3.0% Mn, 0.5-3.0% Cr, 0.05-1.0% No, 0.01-0.2% Nb, 0.01-0.05% Ti, 0.0003-0.005% B, consisting, at need, of <=3.0% Ni and one or more kinds among <=0.15% S, <=0.3% Pb, <=0.01 % Ca, <=0.3% Se, <=0.3% Te and <=0.3% Bi and consisting substantially of Fe is cast. The steel is hot forged at 900-1300 deg.C and is thereafter cooled, by which the main constituent of the structure is regulated to martensite or bainite or the mixed structure therebetween. In this way, the non-heat treated steel retaining high strength and good toughness can be obtd.