Showing papers on "Bainite published in 1973"

The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics

Abstract: Fe-Pt alloys near the composition Fe3Pt transform from fee austenite to bcc martensite at near ambient temperatures. The effect of austenite ordering in depressing theMs temperature has been reported previously, but more importantly the present work shows that ordering leads to a reversible martensitic transformation. The characteristics of this reversible transformation have been investigated by optical metallography, cinematography, and electrical resistivity measurements. It is concluded that in austenite ordered to an appropriate degree, the transformation to martensite possesses all of the characteristics of a thermoelastic martensite transformation. This transformation in ordered Fe~25 at. pct Pt alloys is the first thermoelastic martensite transformation reported for an iron-base alloy. The present experiments indicate that martensite “nuclei” are not destroyed by the transformation, and are reactivated on each cooling cycle at approximately the same temperature.

Lengthening kinetics of ferrite and bainite sideplates

Abstract: The rate of lengthening of ferrite and bainite sideplates and the radius of curvature of the plate edges were measured as a function of reaction temperature in three Fe-C alloys. These data were analyzed on the basis of an equation due to Trivedi. The analysis proved that the mobility of the sideplate edges is limited. The interfacial energy of these edges is of the order of 200 erg/cm2. Most of the supersaturation is used to drive the diffusion of carbon in austenite; comparatively little is accounted for by capillarity and by the finite mobility of the interface. On the basis of both the present results and of published micro-structural observations, it was concluded that ferrite and bainite sideplates lengthen by a ledge mechanism.

The self-accommodating character of the β 1 copper-aluminum martensite

Abstract: The phenomenological martensite theory is applied to the β1 to β1 martensitic transformation in Cu-Al. Crystallographic and morphological aspects of the resulting martensitic microstructure are discussed and verified with X-ray pole figures, which are combined with a single surface trace analysis based on the use of polarized light. From the analysis of the orientation of any single martensite plate related to that of one or more neighboring plates, it can be proved that the martensite microstructure in each former β-grain is composed of at most six self-accommodating martensite plate groups, each of which consists of four different martensite plate variants.

The Crystallography and Nucleation of Pearlite

Abstract: By choice of a suitable iron—manganese—carbon alloy it has been possible to study pearlite nodules growing in austenite, without the austenite transforming on cooling to room temperature. Thin foil electron microscopy has been used to examine the orientation relations between cementite, ferrite and austenite as well as morphological aspects of the transformation. It is shown that one of the classical ferrite—cementite orientation relations found in pearlite (Pitsch—Petch) arises when the pearlite colonies nucleate on ‘clean’ austenite grain boundaries. The other familiar relation (Bagaryatski) arises when the colonies nucleate on pre-existing hyper-eutectoid cementite layers at the austenite grain boundaries. Some observations are made on the mode of nucleation of the pearlite nodules.

A ductile iron and method of making it

Abstract: For a ductile iron of the type which has been heated, after casting, to its austenization temperature and austenized and subsequently heat-treated isothermally by quenching in a hot bath to start a bainite reaction which is continued until a desired fraction of the austenite have formed into bainite, improved properties are achieved by adding as alloying elements molybdenium 0.10 - 0.26 % and magnanese 0.3 - 1.4 % by weight and preferably also an additional alloying element which promotes the formation of a pearlite micro-structure during casting and, consequently, accelerates the austenization, said additional element consisting of nickel in an amount less than 2.5 % by weight, and tin and/or copper. Preferably said iron contains molybdenium 0.15 - 0.22 % by weight and less than 0.2 % by weight of tin and/or less than 1.0 % by weight of copper.

Method for the case carburizing of steel

Abstract: In the case carburizing of low carbon (<0.4%C) steels, the hardness of the case and toughness of the core are both improved by following a prescribed heat-treating sequence. After the article is carburized in contact with a carbon containing substance, it is quenched to below about 900*F and maintained within the bainite region for a time sufficient to transform retained austenite. The article is then austenitized by rapidly heating to a temperature above the A3 of the core, after which it is quenched and tempered in a conventional manner. This procedure provides significant economies by permitting carburization at temperatures well in excess of 1700*F, while employing high carbon potential, carburizing agents.

Phase transformational kinetics and hardenability of alloyed medium-carbon steels

Abstract: The phase transformational kinetics and hardenability of 0.4 pct C steels were studied as influenced by alloying elements, singly and in combination. Sixteen series of steels, each containing up to about 0.75 pct Mo, were prepared by laboratory induction air melting. The base steels contained manganese, chromium, nickel, silicon, and combinations of these elements as alloy additions. Continuous cooling transformation diagrams, hardenability diagrams, and diagrams of the effects of alloying on the beginning of bainitic transformation and the beginning of ferritic transformation were established. The essential findings of the research program are summarized in this paper to establish a better understanding of alloy interactions and their effects on structure and properties. The effects of combinations of alloying elements in delaying the bainitic and ferritic-pearlitic transformations, and in increasing the hardenability cannot be predicted from the effects of each element alone. Furthermore, in most of the multi-alloy systems studied, molybdenum has been found to enhance the effectiveness of the other alloying elements present. Continuous cooling transformation diagrams clarify these relationships and together with the hardenability data provide a basis for predicting strength levels that can be obtained for steel products of varying section size. The 1.5 pct Si-1.5 pct Mn-1.4 pct Ni-0.7 pct Cr-Mo steels exhibit the maximum suppression of bainitic transformation and are closely followed by similar steels without nickel. The wide time range of the martensitic transformation in these steels qualifies them for consideration for air-hardenable constructional components of substantial section size.

As-worked bainitic ferrous alloy and method

Abstract: THIS INVENTION RELATES TO AN AS-WORKED BIANITIC FERROUS ALLOY AND TO A NOVEL METHOD OF PROCESSING SAME TO OBTAIN OPTIMUM STRENGTH AND TOUGHNESS. MORE PARTICULARLY, THIS INVENTION IS DIRECTED TO THE HOT WORKING CYCLE OF A FERROUS ALLOY CHARACTERIZED BY AN I-T DIAGRAM OR S CURVE HAVING A DOUBLE NOSE OR A PEARLITE TRANSFORMATION KNEE OF THE BEGINNING CURVE ABOVE A BROAD BIANITIC BAY REGION. SUCH AN ALLOY IS HEATED TO AN AUSTENITIZING TEMPERATURE OF ABOUT 1500* TO 2200*F., AND SUBJECTED TO A PLURALITY OF WORKING OPERATIONS AT SUCCESSIVELY LOWER TEMPERATURES, WHERE THE FINAL WORKING OPERATION IS CONDUCTED AFTER THE BEGINNING OF THE AUSTENITE TRANSFORMATION TO BAINITE AND BEFORE THE COMPLETE TRANSFORMATION THEREOF.

Heat treatment of large forgings

Abstract: 1. The use of the similarity theory to analyze the quenching process makes it possible to obtain a generalized quenching graph for oil and water that characterizes the temperature field during cooling of forgings (machine parts) of different diameters. 2. A method was developed for determining the hardenability by means of superposing the quenching graph on the thermokinetic diagram of the steel. This method makes it possible to determine the hardened zone in forgings of different diameters with given requirements (absence of pearlitic transformation, absence of transformation in the upper bainite range, etc.). 3. To obtain high mechanical properties (high strength and low ductile-brittle transition temperature) it is necessary that the cooling time during quenching of large forgings be sufficient to lower the temperature in the center of the forging to 200–300°C. 4. To obtain high mechanical properties due to decomposition of austenite in the martensite transformation range and in the lower bainite range it is expedient to quench large forgings with unvarying section in water or through water in oil. 5. For parts varying in section (barrels and necks) the best method is water-air or sprayer cooling, permitting separate cooling of the barrel and the neck and thus permitting prolonged cooling of the barrel to the required temperature.

Electrical resistivity and microstructural changes accompanying the isothermal decomposition of austenite in eutectoid steel

Abstract: Isothermal transformation diagrams derived by optical metallography and by electrical resistance changes were compared. Incubation periods and the onset of the various transformations, as determined by these two methods, were in close agreement. However, the 100 pct transformation times that were determined by optical metallography always preceded those determined by the resistance method. The time differences between these data increased with decreasing temperature. This may be accounted for in terms of composition adjustments which are not possible to observe metallographically. The acceleration of the onset of the lower bainite reaction at temperatures approaching theM s temperature was also observed.

Phase transformational kinetics and hardenability of low-carbon, boron-treated steels

Abstract: The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.

An investigation of metallurgical factors which affect the fracture toughness of ultra high strength steels

Abstract: The relationship between microstructure, heat treatment and room temperature fracture toughness has been determined for the low alloy ultra-high strength steels 4130, 4330, 4340, 4140 and 300-M. Optical metallography, microprobe analysis, and scanning electron microscopy were used to characterize the structure and morphology, while both Charpy V-notch impact tests and plane strain fracture toughness tests were used to determine the fracture properties. The normal commercial heat treatment resulted in the formation of some bainite in all the alloys. MnS inclusions on prior austenite grain boundaries were found to initiate cracks during loading. By increasing the austenitizing temperature to l200 C, the fracture toughness could be increased by at least 60%. For some alloys increasing the severity of the quench in conjunction with the higher austenitizing temperatures resulted in further increases in the fracture toughness, and the elimination of any observable upper bainite. There was no correlation between the Charpy impact test results and the fracture toughness results. The alloys 4130, 4140, 4340 all showed a severe intergranular embrittlement when austenitized at high temperatures and tempered above 200 C, while the alloys 4330 and 300-M exhibited no drop in toughness for the same heat treatment conditions. The as-quenched tensile specimens had a very low 'micro' yield strength which rapidly increased to the level of the 'macro' yield strength when tempered.

The Crystallography of the Bainite in a Low-carbon Low-alloy High Strength Steel

Abstract: The bainite that jorms in a low-carbol1 low-alloy steel at the temperatures as low as the Ms point exhibits the

Correlation of coercive force to microstructure in cyclic martensite → austenite transformations in an fe-ni-co alloy

Abstract: Microstructural changes which occur in an Fe-29 Ni-18 Co alloy during cyclic martensite austenite transformations have been monitored by coercive force measurements and transmission electron microscopy. Cycling was performed between liquid nitrogen and temperatures between 400°C (just aboveAs) and 800°C (about 100°C above Af). For all temperatures belowAf, there was no measurable change in the value ofHc following immersion in liquid nitrogen; changes inHc occurred only in a subsequent high temperature cycle. This stabilization of the reversed austenite was attributed to transformation-induced strains. On the other hand, for temperatures of 700° and 800°C and any given cycle,Hc following liquid nitrogen immersion indicated formation of new martensite. At 800°C, transformation-induced strains were sufficient to promote recrystallization of the reversed austenite. An irrational martensitic habit plane for the nucleation of the reversed austenite was identified.

Strengthening of Cr−Ni steels with unstable austenite

Abstract: 1. Deformation of austenite without the formation of martensite causes an increase of strength by 0.7–0.9 kg/mm2 for each 1% deformation. The highest yield and ultimate strengths obtained for steel Kh18-N9 (30% deformation) do not exceed 45 and 85 kg/mm2 respectively (original values 25 and 60 kg/mm2). 2. The defects in deformed austenite are inherited by the martensite formed during subsequent cooling to low temperatures at degrees of deformation up to 8–10%, corresponding to the initial formation of cellular structure. The determining factor in strengthening at these degrees of deformation is the stabilization of austenite, i.e., the small tendency to form martensite. Deformation by rolling stabilizes austenite at small and large degrees of deformation. 3. The martensitic transformation in steel Kh16N6 (cooled to −196°) causes an increase in the amount of martensite from 10 to 70%, leading to an increase of the ultimate strength by 30 kg/mm2 and yield strength by 55 kg/mm2, i.e., 0.5 and 1 kg/mm2 respectively for each 1% martensite formed. 4. Up to 4–6% deformation by elongation and up to 40% deformation by rolling of steel Kh16N6 with a primarily martensitic structure leads to the formation of additional martensite (to 20–25%) and to an increase of the ultimate strength by 10 kg/mm2 (elongation) and 40 kg/mm2 (rolling), and increase of the yield strength by 80–90 kg/mm2. Calculations of the increase in yield strength from the amount of strain martensite on the basis of the section of the curve where there is almost no increase in the amount of martensite gave the following results: 10–13 kg/mm2 with deformation by elongation and 1–2 kg/mm2 for each 1% deformation by rolling. The same values of the strength can be obtained with 3–4% deformation by elongation and 20% deformation by rolling. 5. The martensitic transformation and the subsequent strain hardening of martensite are the determining factors in the high strength of Cr-Ni steels with unstable austenite.

Effect of the rate of cooling from the austenitizing temperature on the heat resistance of boiler steels 12Kh1MF and 15Kh1M1F

Abstract: 1. With increasing cooling rates from austenitizing temperature the long-term strength increases for chromium-molybdenum-vanadium steels. Maximum cooling rates of 50 deg/min for steel 15Kh1M1F and 150 deg/min for steel 12Kh1MF ensure a structure with 100% bainite. 2. The long-term ductility of steels 12Kh1MF and 15Kh1M1F decreases continuously with increasing cooling rates. 3. The best combination of long-term strength and long-term ductility is obtained with a ferritic-bainitic structure. The amount of structurally free ferrite should not exceed 70% for steel 12Kh1MF and 30% for steel 15Kh1M1F.

Resistance of tool steels to plastic deformation

Abstract: 1. The elastic limit of tool steels quenched to a high hardness depends on the composition of martensite. Raising the carbon and silicon concentrations in martensite is accompanied by an increase of microstresses and reduction of the elastic limit. Alloying with chromium, molybdenum, and tungsten, increasing the binding force, increases the elastic limit of steels with a martensitic structure. 2. The grain size has no effect on the elastic limit. Retained austenite lowers the resistance to plastic deformation. 3. Excess carbides (12–13%) do not change the elastic limit. The secondary carbides precipitated during tempering increase the elastic limit, while coalescence of the carbides lowers it. 4. Bainite has a low elastic limit due to the presence of retained austenite and large internal stresses. Tempering after isothermal quenching, lowering the internal stresses, increases the elastic limit. 5. A high elastic limit can be obtained in steel with the following composition: 0.56–0.62% C, 4–5% Cr, 0.8–1.2% Mo, 0.3–0.5% Si, 0.2–0.3% V (6Kh5MF type).

High toughness alloy ateel with improved weldability

Abstract: : In accordance with the present invention, an alloy steel is provided which exhibits a high Charpy V-notch shelf energy and a low transition temperature. Its asquenched microstructure contains significant amounts of bainite thereby allowing a substantially lower carbon content. The combination of low carbon content along with moderate alloy content results in a low tendency toward HAZ cracking and a superior steel for producing various types of structural weldments for use as submarine hulls, ship hulls, tanks, etc.

Morphology and crystallography of martensite in Fe-23% Ni-0·38% C alloy induced by plastic deformation of austenite

Abstract: A tensile deformation at the temperatures between Ms and Md induces in Fe-23% Ni-0·38% C alloy the so-called butterfly martensite, whose morphology is different from that of thermally transformed martensite. The amount of these butterflies increases gradually with deformation and contributes to the transformation — induced plasticity. The difference in morphology corresponds with a different structure, substructure and crystallography and it is caused by the change of transformation mechanism.