Showing papers on "Bainite published in 1972"

A debate on the bainite reaction

Abstract: The authors debate three topics central to the controversies which have enveloped the bainite reaction ever since it was first recognized as a distinctive mode of austenite decomposition. These include: “what is bainite?”, “the growth mechanism of the ferritic component of bainite”, and “the sources of bainitic carbide precipitation.” RFH concludes that bainite is the product of a shear transformation. Individual bainite plates are suggested to grow substantially more rapidly than volume diffusion-control allows, but a constraint such as the build-up of volume strain energy limits the extent of their growth. This mechanism of growth ensures extensive supersaturation of bainitic ferrite with respect to carbon. Whether or not carbides precipitate in association with bainite plates and whether the carbide is cementite ore, however, is a complex question in competitive reaction kinetics. New experimental evidence is presented to demonstrate thate carbide precipitated in lower bainite dissolves upon heating above the kinetic-B stemperature in an alloy steel containing 1.5 pct Si. This result is taken to support the existence of the metastable eutectoid reactionγ ⇌ α + e atca 350°C. HIA and KRK define bainite as the product of a nonlamellar eutectoid reaction. On this view, carbide precipitation thus plays an essential, rather than an ancillary role. Development of the Widmanstatten morphology by the ferritic component of bainite is shown to be inessential to the classification of a eutectoid structure as bainite. When this morphology is present, however, it is concluded to grow by the ledge mechanism, without the participation of shear, at rates of the order of or less than those allowed by volume diffusion-control. New experimental evidence is presented to show that the lengthening and thickening kinetics of individual plates within sheaves of upper bainite are consistent with this description. The results of a new calculation indicate that the initial carbon content of bainite plates lies between theα/α + Fe3C) and the extrapolatedα/(α+ γ) phase boundaries, in agreement with expectation from the ledge mechanism of growth.

The extent and nature of the strength-differential effect in steels

Abstract: Tensile and compressive stress-strain curves were obtained for several types of microstructures in a variety of steels. The strength-differential effect, previously found in martensitic structures, was present in lower, intermediate, and upper bainite and in Widmanstatten ferritepearlite as well as in ultrafine-grained martensite. An equiaxed ferrite-pear lite structure showed no strength differential. The strength differential in martensite increased as test temperature was decreased below room temperature. In several series of tests, the same specimen design was used in tension and in compression to eliminate possible strength variations due to variations in specimen preparation. Several theories which have been proposed for the strength-differential effect are discussed with respect to the present results, and it is shown that most of the previous suggestions are invalid.

Ferrite and bainite in alloy steels

Abstract: The addition of alloying elements even in small concentrations can alter the properties and structure of ferrite and bainite. The various morphologies of ferrite-carbide aggregates are surveyed including alloy pearlite, fibrous carbide eutectoids and precipitation of fine alloy carbides atγ-α interfaces. Modern ideas on the morphology and growth kinetics of ferrite and upper and lower bainite are also summarized. Using this information, an attempt is made to rationalize subcritical transformations of austenite in low alloy steels. Basic factors influencing the strength of alloy ferrites are discussed, leading to an examination of structure-mechanical property relationships in ferrite and bainite. Finally the exploitation of the ferrite and bainite reactions to produce useful alloy steels by direct transformation of austenite is explored.

The Morphology of Strain-Induced Martensite and Thermally Transformed Martensite in Fe–Ni–C Alloys

Abstract: The Ms temperature was remarkably depressed in these alloys with decrease in austenitizing temperature. Therefore, the relation between formation temperature and morphology of thermally transformed martensite was clearly determined in the same alloy by utilizing this phenomenon (thermal stabilization of austenite). The morphology of thermally transformed martensite also varied with the formation temperature even in the same alloy. Three types of the martensite were also observed in the same morphologies, and they were similarly formed in the same temperature ranges as the strain-induced martensites, respectively. The main factor determining the morphology of martensite (both the strain-induced and thermally transformed martensites) in Fe-Ni-C alloys was considered to be the formation temperature. (Received January 31, 1972)

The development of martensitic microstructure and microcracking in an Fe-1.86C alloy

Abstract: The development of the martensitic microstructure in a 1.86 wt pct C steel has been followed by quantitative metallographic measurements over the transformation range of 0.12 to 0.50 fraction transformed (f). The transformation kinetics are described by the equationf = 1 − exp [−0.008 (M s − Tq)] where Ms and Tq are the martensite start and the quenching temperatures respectively. Fullman’s analysis shows that the average volume per martensite plate decreases by almost an order of magnitude over the transformation range studied, but this decrease is less than that predicted by the Fisher analysis for partitioning of austenite by successive generations of martensite. Microcracking increases with increasingf up to 0.3, but does not increase forf above 0.3 where transformation proceeds by the nucleation of large numbers of small martensite plates. These observations indicate that a critical size of martensite plate is necessary to cause microcracking.

The effect of Si,Zr, Al and Mo on the structure and strength of Ti martensite

Abstract: The structure and strength of martensite in “near α” titanium alloys have been studied in the composition range (wt %) up to 10% Zr, 6%, Al, 1/2% Mo, 2.4% Si. [0001], 1/3 〈11¯20〉 dislocations, 1/3 〈10¯10〉 stacking faults and approximately {10¯11} twin related martensite plates are found to be common features of the martensite. Martensite “midribs” consist either of finely transformed material between martensite plates, or regions of low dislocation density within martensite plates.

The effect of plastic deformation on the martensite-to-austenite transition in an iron-nickel alloy

Abstract: The effect of plastic deformation introduced by rolling at room temperature on the austenite start temperature of an Fe-30.3 wt pct Ni-0.005 wt pct C alloy has been determined. The austenite start temperature increases monotonically with deformation. Microhardness measurements show that the austenite start temperature increases with the yield strength of the martensite. The temperature at which martensite reversal initiates is not affected by the amount of martensite present, and, therefore, is not dependent on the martensite plate size. It is suggested that the reverse martensite transformation initiates at the martensite-austenite interface and is controlled by interface propagation.

Some rules for phase transformations in 000N18K9M5T steel

Abstract: 1. At the normally used rates of heating the α»γ transformation occurs in the aged martensitic matrix, which together with phase work hardening from the α»γ transformation causes the formation of austenite with an increased hardness. With an increase in the austenitizing temperature the strength of the austenite drops, approaching the strength of the austenite in unaged steels. 2. The formation of a large quantity of austenite, up to 30%, after short heating in the α»γ transformation does not reduce the strength of N18K9M5T steel. The loss of strength with long holds in the α »γ region may be explained by overaging of the α-matrix. It is also possible that there is a loss of strength in the austenite. 3. If in the α»γ transformation the steel does not lose strength, the presence in the structure of a large quantity of austenite does not increase the impact strength. 4. The form of the austenite particles and their distribution in the α-matrix is determined by the quantity of austenite and does not depend upon the temperature and holding time at which this quantity is obtained. 5. The increase in the strength of the austenite after heating to temperatures close to the Ac3 (720–820°C) is inherited by the martensite, preserved after low temperature aging up to 500°C, but disappears after high temperature aging above 530°C.

Low carbon high strength steel

Abstract: LOW CARBON STEELS HAVING HIGH STRENGTH, E.G. FOR STRUCTURAL PLATE, COMPRISE MOLYBDENUM AND/OR BORON AS AN INGREDIENT COATING TO AVOID FERRITE-PEARLITE TRANFORMATION AND A SECOND INGREDIENT, IN ECONOMICAL AMOUNT, OF ELEMENTS FROM THE CLASS OF MANGANESE, NICKEL, CHROMIUM ABND COPPER COATING TO RAISE THE LEVELS OF BOTH STRENGTH AND TOUGHNESS SILULTANEOUSLY WHILE AFFORDING TRANSFORMATION IN THE BAINITE REGION, SUCH TRANSFORMATION BEING THEREBY PREFERABLY INITIATED AT A LOW TEMPERATURE, SO THAT UPON COOLING THE PRODUCT DIRECTLY FROM A HOT ROLLING OPERATION, E.G. BY AIR COOLING, THE STEEL TRANSFORMS TO A MICROSTRUCTURE EXHIVITING A PLATE-LIKE MORPHOLOGY AND HAVING YIELD STRENGTH OF AT LEAST ABOUT 95 K.S.I. AND GOOD IMPACT TOUGHNESS, NEED FOR HEAT TREATMENT, SUCH AS IN TEMPERING AFTER QUENCHING, IS OBVIATED. PROPERTIES CAN BE FURTHER ENHANCED BY A MINOR AMOUNT OF A THIRD INGREDIENT, FOR EXAMPLE COLUMBIUM, WHICH COOPERATES IN ATAINING DESIRED RESULTS.

Effect of dendritic segregation of manganese on the transformation of austenite in steel

Abstract: 1. We investigated the effect of manganese (0.58–2.45%) on the transformation of austenite under isothermal conditions after solidification and homogenizing annealing. 2. In the presence of manganese segregation the pearlite and bainite transformations in steel with 0.58–2.45% Mn occur over a longer time period and wider temperature range than in the homogenized steels. 3. The isothermal transformation of austenite begins in the axial sections of dendrites with a low stability due to the low concentration of manganese. Austenite later decomposes in sections between branches that are rich in manganese.

Effects of Microstructure, Composition, and Strength on the Nil Ductility Transition (NDT) Temperature of HY-80 Steel

Abstract: : Steel from 22 heats of low-carbon Ni-Cr-Mo steel, HY-80 (ASTM a543-65, and MIL-S-16216G (Ships)) was heat treated to study the effects on the drop weight nil ductility transition (NDT) temperature of (1) commercial variation in composition and inclusion content, (2) variation in microstructure such as prior austenitic grain size and the relative amount of isothermally produced ferrite or bainite in a tempered martensitic matrix, and (3) the observed variation in strength obtained after a one-hour 1150 deg F temper followed by water quench to prevent embrittlement while cooling from the tempering temperature.

Wear resistant cast iron - contg chromium and manganese having high quenchability and hardness

Abstract: Process for making a wear resistant cast iron which contains chromium and manganese. The composition of the cast iron is 1.3-2.29% C, 8-14% Cr, 0.8% Si (max), 4.01-7.0% Mn, 1.5% Mo (max), 0.08% S (max), and 0.1% P (max). The iron is cast and cooled in stages together with heat treatment, i.e. cooling to 1200 degrees C-1150 degrees C. at a rate of at least 3 degrees c. min-1, heat treating at 820 degrees C-1100 degrees C. followed by cooling to 780-650 degrees C. leaving at this temp. for 2 hrs. and then cooling to 450 degrees C.-400 degrees C., at a rate of 0.666 degrees c. min.-1 at least. The structure of the cast iron is composed of M7C3 carbides in a matrix of martensite bainite and residual austenite. The cast iron has high hardness accompanied with high quenchability and good wear resistance e.g. HRc = 56-59 after heat treatment, is useful as structural parts in the crushing and grinding of ores where high wear resistance is needed.

Formation of abnormal structures in pipe of steel 12Kh1MF

Abstract: 1 The tendency of steel 12Kh1MF to form abnormal structures (with a low percentage of temper sorbite) is due to the structure of the steel after normalization and increases with decreasing amounts and sizes of bainite sections and also with decreasing ferrite grain sizes 2 Raising the normalization temperature and holding time and lowering the tempering temperature and holding time reduce the probability that abnormal structures will be formed 3 The method of cold deformation affects the kinetics of the transformation of austenite in steel 12Kh1MF In drawn pipe the amount of bainite after normalization as well as the amount of temper sorbite is larger than in rolled pipe, and thus the strength characteristics are higher To obtain the same structure and properties in drawn and rolled pipe requires heat treatment conditions that differ 4 The formation of abnormal structures in pipe of steel 12Kh1MF is also affected by the chemical composition and melting procedure

Properties in Creep of Low Alloy Steels in Relation to their Microstructure

Abstract: The creep rupture strength of low alloy steels, 1/2Mo, 1Cr-1/2Mo, 21/4Cr-1Mo, 5Cr-1/2 Mo, 9Cr-1Mo was studied in relation to their microstructure.The results obtained are as follows;(1) The intersection of the stress-rupture time curves was observed in a steel with different heat treatment. This was observed also in 1Cr-1/2Mo and 21/4Cr-1Mo steels.(2) High Cr steels (5Cr-1/2Mo, 9Cr-1Mo) show weaker creep strength than low Cr steels (1/2Mo, LCr-1/2Mo, 21/4Cr-1Mo).(3) The Cr content in the residues does not increase by tempering. As for the amount of Mo in the residues, low Cr steels show remarkable increase in Mo content but high Cr steels show slight increase in Mo content.(4) Microscopical observation revealed that N. T. treated specimen contained finer precipitates in the ferrite than Ann. treated specimen. Tempered bainite which was observed in N. T. treated 21/4Cr-1Mo steel was apt to form coarse carbides.In fine the problems with the creep rupture strength of commercial low alloy steels will more clearly be boiled down to alternative between the following two questions:(1) whether or not the low alloy steels contain in their microstructure either bainite or martensite, or (2) whether or not the ferrite is adulterated with carbide particles.