Showing papers on "Bainite published in 1985"

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

Abstract: The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Mechanical properties of 0. 40 Pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite

Abstract: A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS) and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases. Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT, irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape, and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams.

The stability of precipitated austenite and the toughness of 9Ni steel

Abstract: A correlation was confirmed between the good low temperature Charpy toughness of 9Ni steel and the stability of its precipitated austenite against the martensitic transformation. Changes in the microstructure during isothermal tempering were studied in detail. The austenite/martensite interface is originally quite coherent over ∼100 A distances. With further tempering, however, the dislocation structure at the austenite/martensite interface changes, and this change may be related to the increased instability of the austenite particles. The reduction in austenite carbon concentration does not seem large enough to account for the large reduction in austenite stability with tempering time. The strains inherent to the transformation of austenite particles create dislocation structures in the tempered martensite. The large deterioration of the Charpy toughness of overtempered material is attributed, in part, to these dislocation structures.

Effect of accelerated cooling after controlled rolling on the hydrogen induced cracking resistance of line pipe steel.

Abstract: The effect o f accelerated cooling after controlled rolling on the hydrogen induced cracking (HIC) resistance of line pipe steels produced from continuously cast slabs was examined, and the relation between HIC resistance and the microstructure in the segregated zone at the mid-thickness of plate was clarified.The optimization of accelerated cooling conditions reduced the volume fraction of high carbon martensite or upper bainite formed in the segregated zone, and this resulted in significant improvement of HIC resistance.The above result can be explained by the model that the high cooling rate as well as the optimized start and stop temperatures in accelerated cooling suppress the rejection of carbon from the non-segregated area to segregated zone during the austenite to ferrite transformation causing the more uniform profile of carbon distribution in the through-thickness direction of plate.

The effect of heat treatment, mechanical deformation, and alloying element additions on the rate of bainite formation in austempered ductile irons

Abstract: Several Alloys of Ductile Cast Iron containing various amounts of manganese, molybdenum, and nickel were austempered in the temperature range 316° to 427 °C. The rate and morphology of ferrite platelet formation (bainite reaction) were studied by optical metallography, x-ray diffraction, and hardness measurements. Austenitizing temperature, austempering temperature, and deformation by rolling were used as variables to control the kinetics of ferrite formation, stage I of the austempering reaction.

Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels

Abstract: In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength low alloy steels was suggested by the authors which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40 pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for various test temperatures have been compared with those for the new heat treatment reported previously and the conventional 1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2 pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the modified law of mixtures and so forth.

Microstructural changes in 1Cr-0.5Mo steel after 20 years of service

Abstract: Metallographie studies have been conducted on 1Cr-0.5Mo steel “taken from a pressure vessel which had been in service for 20 years in a hydrogenous environment at 524 °C. The original microstructure of the steel, reproduced by reheat treatment of the exposed material, consisted of proeutectoid ferrite and tempered bainite, the carbides being mainly cementite. The service exposure caused precipitation of needle-like M2C carbides in the ferritic regions and M7C3 carbides in the vicinity of the original cementite particles. Chromium and molybdenum moved from solid solution to the carbides during the service exposure with 72 pct and 32 pct of the total chromium and molybdenum contents, respectively, remaining in solid solution after service for 20 years.

High strength low carbon steels, steel articles thereof and method for manufacturing the steels

Abstract: High strength low carbon steels having good ultraworkability which comprises 0.01 - 0.3 wt% of C, below 1.5 wt% of Si, 0.3 - 2.5 wt% of Mn and the balance of iron and inevitable impurities. In the steel, a low temperature transformation product phase consisting of acicular martensite, bainite or a mixed structure thereof is uniformly dispersed in a ferrite phase in an amount by volume of 15 - 40%. Wire articles of these steels and methods for making the steels are also disclosed.

Isothermal martensite transformation in a 1.80 Wt Pct C steel

Abstract: The linear products formed isothermally at 373 K in a 1.80 wt pct C steel (Ms = 346 K) were examined by means of transmission electron microscopy. They were first reported as “black line products” by Greninger and Troiano. The isothermal product was of a thin plate with about 0.5 μm width, and it contained {112}b transformation twins and revealed a habit plane of {3 15 10}f. The orientation relationship between austenite and product was close to the Nishiyama relationship. These crystallographic data were in good agreement with those calculated by the phenomenological theory of martensitic transformation. Consequently, the product was determined not to be lower bainite, but isothermal martensite. The black color of isothermal martensites resulted from the fact that they were easily etched by the precipitates of epsilon carbide formed during the isothermal holding.

The effect of segregation on the formation of austempered ductile iron

Abstract: Alloy segregation at the solidification cell boundaries can have a very important influence upon the kinetics of the bainite reaction. The two stages usually observed in a homogeneous matrix after isothermal holding in the range of temperatures 350 to 450 °C can be described as follows: 1 -γ→ (α) + (γ); 2-(α) + (γ) → α + silicocarbides. These reactions are functions of the elements in solution in the matrix. For this reason, segregated elements affect very sensitively the evolution of the two basic reactions. The solidification cell boundaries are often not completely transformed to bainite, which explains the presence of martensite observed in these areas. The influence of the size of the solidification cells has also been studied. The austenitization conditions are also very important. At the opposite extreme, the bainitic reaction of the matrix located along the graphite interface evolves more quickly than that observed in the matrix or at the cell boundaries.

Formation of austenite from a ferrite-pearlite microstructure during intercritical annealing

Abstract: The formation of austenite from a coarse grained ferrite-pearlite microstructure during intercritical annealing was studied. According to the variation of the microhardness values in ferrite and austenite with intercritical annealing time, austenite formation was classified into four stages: (a) austenite growth into pearlite at a slower rate than the dissolution rate of pearlite, (b) subsequent growth of austenite into pearlite and formation of thin film type austenite at ferrite grain boundaries, (c) growth of austenite into ferrite, (d) equilibrium of ferrite and austenite. In particular, plate-like austenite was observed. The experimental results indicated that the growth process of this type austenite was controlled by carbon diffusion in austenite.

Heating and cooling transformation diagrams for the rapid heat treatment of two alloy steels

Abstract: In rapid heat treatments, such as laser and induction hardening, a thin surface layer of austenite is formed on a part during heating After cooling for only a few seconds, this layer transforms into martensite, pearlite, bainite, and/or proeutectoid ferrite/cementite, depending on the cooling rate, dissolved carbon content, homogeneity of the dissolved carbon, austenite grain size, and steel composition In this work, transformation diagrams suitable for rapid heat treatments were determined by dilatometry for two alloy steels, one hypoeutectoid and one hypereutectoid A short austenitizing time after fast heating was used Models for computer calculations of phase transformations with the use of isothermal diagrams are also discussed Calculations of continuous heating diagrams from the isothermal heating diagrams gave good agreement in comparison with the observed diagrams

Production of hot rolled steel products having high strength and high toughness

Abstract: PURPOSE: To obtain the titled steel products without requiring a tempering treatment entailing hardening and tempering after reheating by subjecting a specifically composed steel to quick cooling under prescribed conditions then to direct hardening after hot rolling and subjecting the steel to self tempering under prescribed conditions by the heat internally retained therein. CONSTITUTION: The ingot contg., by weight, 0.20W0.60% C, 0.10W0.35% Si, 0.30W1.80% Mn, and if necessary ≥1 kinds among ≤1.50% Cr, ≤0.30% Mo and ≤0.005% S and consisting of the balance iron is hot rolled. The steel is quickly cooled from A 3 WA 3 +150°C down to the temp. above 50°C and below the M s point after the end of said rolling to provide the internally retained heat in such a manner that the surface temp. attains 100W600°C to effect the self tempering. The rolled steel products, more particularly, rolled steel bars, which consist essentially of the tempered martensite structure in the surface layer part and tempered martensite and bainite or supercooled ferrite pearlite structure in the central part and are much improved in the strength and toughness are thus obtd. COPYRIGHT: (C)1987,JPO&Japio

Manufacture of unrefined steel plate excellent in sour-resistant property

Abstract: PURPOSE: To obtain a steel plate excellent in sour-resistant properties, strength and toughness by heating a steel of specific composition to a specific temperature and hot rolling, cooling and naturally cooling it under the specific conditions to form it into a fine bainite structure uniform in its thickness direction. CONSTITUTION: A steel is heated up to 1000W1200°C, which consists of by weight %, as the basic components 0.02W0.15 C, 0.1W0.6 Si, 0.5W1.5 Mn, ≤0.0015 P, ≤0.01 S, 0.005W0.1 Al, 0.005W0.025 Ti, and one or more kinds of the following groups ≤0.1 Nb, ≤0.005B, ≤1.0 Ni, ≤1.0 Cu, ≤1.25 Cr, ≤0.005 Ca, and the balance Fe with inevitable impurities. The steel is rolled thereafter under the draft of ≥60% at ≤850°C and at the finishing temperature of ≥Ar 3 transformation point. Next, the steel is cooled at a cooling speed of ≥30°C/sec. from the temperature of ≥Ar 3 transformation point to the temperature range of 350W550°C and then cooled naturally. In this way, a steel plate excellent in sour-resistant properties having the fine bainite structure uniform in the plate- thickness direction is obtained. COPYRIGHT: (C)1986,JPO&Japio

The mean size of plate martensite: influence of austenite grain size, partitioning, and transformation heterogeneity

Abstract: The evolution of transformation microstructure during martensite transformation is revisited. The influence of “spread” heterogeneity on current partitioning concepts is assessed. Following that, a method to infer details of martensite transformation in an average austenite grain is described. The results indicate that the mean martensite plate size may remain constant up to a significant extent of grain transformation. Moreover, it is found that the mean martensite plate volume in fine grain austenite is proportionally larger than that observed with coarse grain austenite, and these effects are ascribed to autocatalysis. Finally, a transformation model is derived which accurately describes the data up to Vv = 0.65, irrespective of austenite grain size.

Production of steel having high strength and high ductility

Abstract: PURPOSE:To obtain a heat-treated steel provided with high strength, excellent ductility and toughness in a combined austempering method of a specifically composed middle and high carbon steel by stabilizing positively residual austenite. CONSTITUTION:A middle and high carbon steel contg., by weight, 0.40-1.10% C and 0.8-2.7% Si among the elements in the steel is held at a temp. region of the Ac3 point -Ac3 point of said steel +150 deg.C to austenitize the steel. The steel is then hardened from said temp. region to a temp. region of the Ms point -M80% point of the steel. The steel is thereafter heated from the state in which >=20vol% untransformed austenite is maintained to a temp. region of 300- 450 deg.C so that the tempering of martensite and the bainite transformation of the untransformed austenite is effected. The steel is at the same time cooled to an ordinary temp. by regulating the holding time in said heating temp. region in such a way that the residual amt. of the austenite stable at an ordinary temp. attains >=5vol%. The bainite transformation is interrupted by such regulation. The steel in which the three phases, martensite, bainite and residual austenite, coexist, is thus obtd.

Manufacture of hot rolled steel stock

Abstract: PURPOSE:To improve not rolling efficiency end yield and to reduce cost, by combining particle diameter forecast contg. forecast of hardness of each microstructure and forecast of structural vol. ratio, and suitably selecting component and manufacturing condition. CONSTITUTION:In case of obtaining products by rolling conventional carbon steel at the temp. higher than Ar3 transformation temp., then cooling the plate, at first, an austenite grain diameter dgamma, and a residual stress quantity epsilon just before starting cooling are calculated from component, heating and rolling conditions. Next, vol. ratio of each structuer (ferrite, pearlite, bainite and martensite), hardness and grain diameter after end of cooling are calculated from cooling condition, dgamma, and epsilon thereafter. Further, material quality of the final hot rolled steel stock is calculated from these microstructures (vol. ratio, hardness and grain diameter of structure), and at least manufacturing conditions (component, heating condition, rolling condition and cooling condition) are controlled so that the material quality becomes the desired one. In this way, the material quality is accurately estimated and the hot rolled steel stock having accurately controlled quality is manufactured.

Non-heattreated steel for hot forging

Abstract: PURPOSE: To provide a non-heattreated steel with a high impact value by subjecting a steel containing specific amounts of C, Si, Mn, V, Nb, Al and N to hot forging and then to cooling so as to form a mixed structure of ferrite and bainite. CONSTITUTION: The steel consisting of, by weight, 0.20W0.50% C, 0.10W1.00% Si, 1.0W2.0% Mn, 0.05W0.30% V, 0.01W0.06% Nb, 0.01W0.06% sol Al, 0.01W0.03% N and the balance Fe with inevitable impurities is refined, which is cast into a steel ingot and then rolled. On subjecting to hot forging and then to air cooling, this stock is formed into the mixed structure of ferrite and bainite or a single phase structure of bainite. If necessary, ≥1 kind among 0.30W1.0% Cr, 0.3W2.0% Ni and 0.05W0.50% Mo or further proper amounts of S, Pb, Te, Bi, Ca, etc., are incorporated to the above steel. Owing to the above composition of the non-heattreated steel, deterioration in toughness in the high temp. region can be improved. COPYRIGHT: (C)1986,JPO&Japio

Production of extra fine wire

Abstract: PURPOSE: To obtain an extra fine wire having high strength without requiring a subsequent heat treatment by subjecting a steel wire having a specific compsn. and composite structure to intermediate annealing then to a heat treatment under specific conditions and drawing such wire to the extra fine wire. CONSTITUTION: The steel wire which consists of 0.01W0.30wt% C, ≤1.5% Si, 0.3W2.5% Mn and the balance Fe and unavoidable impurities and consists of needle-like martensite, bainite or the mixed structure thereof and of which the low-temp. transformation forming phase is uniformly dispersed in the ferrite phase is produced. The wire having such composite structure is drawn to an intermediate wire having 3.5W0.5mm diameter. Such intermediate wire is austenitized by quickly heating the same at 20°C/sec and is the quickly cooled by which the structure is transformed to the needl-like martensite, bainite or the composite structure thereof. The wire is thereafter hardened by heating the same to an AC 1 WAC 3 region so as to uniformly disperse the low-temp. transformation forming phase by 10W50vol% into the ferrite phase. Such intermediate wire having the composite structure is drawn to ≤150μm diameter, by which the extra wire is obtd. COPYRIGHT: (C)1987,JPO&Japio

Cast iron for rocker arm

Abstract: PURPOSE:To provide the titled cast iron having excellent resistance to wear, scuffing, etc. by consisting said iron of sepcifically composed C, Si, Cr, W, V, Mo, Co and Fe and dispersing and crystallizing carbide into the matrix consisting of structure such as austenite or the like. CONSTITUTION:A cast iron for a rocker arm is a cast iron consisting of 0.5- 4wt% C, 0.1-2.0% Si, 0.5-35% Cr, 0.05-20.0% W, 0.03-10.0% V, 0.05-15.0% Mo, 1.0-10.0% Co and the balance substantially Fe, which Fe is incorporated at >=10%. Said cast iron has the structure dispersed and crystallized with carbide in the matrix consisting of the structure contg. the phase of at least >=1 kind among austenite, martensite, bainite and pearlite. The above-mentioned cast iron is highly resistant to wear and scuffing and decreases damage on the mating material to be slided with when said cast iron is used not only for unleaded gasoline but for high lead gasoline as well.

Structure and properties of as-hot-rolled Fe–0·06C–1·5Mn steel: potential for line-pipe application

Abstract: An investigation has been made to develop a simple duplex steel for line-pipe application. Controlled rolling of an Fe–0.06C–1.5 Mn alloy followed by direct quenching produced a duplex ferrite–bainite structure in which the coarse upper bainite regions were uniformly distributed within a fine-grained ferrite matrix. The microstructure and mechanical properties of this duplex structure were strongly influenced by the processing variables. Decreasing the finish-rolling temperature improved both tensile and impact properties. This was due mainly to a refinement of the ferrite grain size. There was an abrupt increase in the yield strength and a small increase in the ductile–brittle transition temperature as a result of cold working (pipe forming) because of the steel's continuous yielding behaviour and high initial work-hardening rate. The mechanical properties attained in this duplex steel are attractive for low-temperature line-pipe applicaiion.MST/155

Mechanism of embrittlement in tempered martensite

Abstract: In this paper the total driving force for the decomposition of retained austenite and martensite are calculated together with the nucleation and growth characteristics of cementite in the two phases. The results demonstrate that the driving force for the decomposition of martensite is an order of magnitude less than that of austenite. However, the driving force for cementite precipitation in martensite is two orders of magnitude greater than in austenite with a much shorter incubation period. On short term tempering cementite precipitates from martensite whereas on longer term tempering decomposition of retained austenite occurs because of the increase in driving force which is enhanced by the contraction of the martensite on decomposition. It is argued that the precipitation of cementite from the austenite results in tempered martensite embrittlement, a mechanism dependent upon the two related decomposition processes. The segregation of trace impurities or the precipitation of cementite at the gr...

Reinforced steel bar having excellent low-temperature toughness and sea water resistance

Abstract: PURPOSE: To produce a reinforced steel bar for reinforced concrete having excellent sea water resistance without decrease in toughness at a cryogenic temp. by using a low-Ni steel as a starting stock and subjecting the material to hot rolling, hardening and tempering under specific conditions thereby producing the reinforced steel bar. CONSTITUTION: The billet of the low-Ni alloy steel which contains <0.17% C, <0.03% Si, <0.70% Mn, <0.015% P, <0.005% S and 2W4% Ni and in which the content of P and S is decreased as far as possible is hot rolled to a steel bar at 750W850°C finishing temp. The bar is forcibly cooled to harden the surface part and is then placed on a cooling bed to recuperate the heat in the steel bar and to temper automatically the steel bar. The reinforced steel bar which has the tempered martensite or bainite layer of ≥3mm depth in the outside circumferential part of the steel bar, has internally the structue consisting of ferrite and pearlite and has ≥40kg/mm 2 yield strength and ≥4kg-cm Carpy impact value at -150°C as well as the excellent low-temp. toughness and sea water resistance is obtained. COPYRIGHT: (C)1986,JPO&Japio

Manufacture of high strength and ductility steel material

Abstract: PURPOSE: To obtain steel material having high strength and large local elongation, by hot working steel contg. Si, Mn, Al, then heat treating the steel under a specified condition to compose it mainly of bainite structure, further soaking treating it at a specified temp. CONSTITUTION: Steel composed of, by weight 0.05W0.3% C, 0.5W2.5% Si, 3W6% Mn, 0.003W0.1% Al, and the balance Fe with inevitable impurities is hot worked, and finished at 800W1,100°C range with a complete austenite state. Next, immediately the steel is cooled at the temp. range up to 650°C at 5W100°C/sec rate to 650°C W room temp. range, to compose the main structure of bainite structure. Thereafter, it is soaking treated at 650W700°C for 2minW3 hr, then furnace cooled at about 0.3°C/min rate. The steel material has high strength and high workability, especially is superior in characteristic such as bore spreading property and bendability. COPYRIGHT: (C)1987,JPO&Japio