Showing papers on "Bainite published in 1975"

The Volume Expansion Accompanying the Martensite Transformation in Iron-Carbon Alloys.

Abstract: A dilatometric investigation was conducted to determine the effect of carbon on the volume expansion accompanying the martensite transformation in iron-carbon alloys. It was found that the volume expansion at theM s temperature varies from 2.0 pct at 0.19 wt pct carbon to 3.1 pet at 1.01 pct carbon, largely due to the effect of carbon on lowering the temperature at which the transformation occurs. Also of importance is the solid solution effect of carbon on altering the lattice parameters of both the austenite and martensite phases at theMs.

Effect of microstructure on the temper embrittlement of Cr-Mo-V steels

Abstract: The effect of transformation product on the temper embrittlement susceptibility of a Cr-Mo-V steel doped with P and Sn has been investigated at different strength levels Results show that at low strength levels (< 10R C ) embrittlement susceptibilities of tempered bainite and ferrite-pearlite structures are comparable to each other, but lower relative to tempered martensite The lower susceptibility of tempered bainite relative to tempered martensite obtains up to about 40R C , above which the susceptibilities for the two structures are similar Variation of embrittlement susceptibility with microstructure is completely consistent with the degree of grain boundary segregation of P and Sn The segregation is smaller in bainite than in martensite at a given strength level and increases with increasing strength level for a given structure When compared at the same degree of embrittlement (ie, same shift in FATT), the amount of grain boundary segregate and the extent of intergranular fracture are lower in tempered bainite compared to martensite indicating that embrittlement of interfaces other than prior austenite boundaries might be an important factor in the embrittlement of bainite structures

The effect of quench rate on the properties and morphology of ferrous martensite

Abstract: The effect of high quench rate on theMs temperature, percent transformed, martensite morphology and austenite hardness has been studied for several Fe-Ni-C steels. For these steels the quench rate was varied only in the austenite region. TheMs temperature was found to increase with increased quench rate for both high- and low carbon steels while the percent transformation increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness decreased slightly with increasing quench rate while the martensite morphology changed from lath to plate. Parallel aligned plate structures were observed which resemble a twinned lath morphology. It was demonstrated that the actual difference between this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes were compared to the measured changes in martensite properties in order to identify the mechanism of the morphology shift. It was concluded that for these alloys the morphology was controlled by the austenite shear mode.

On the precipitation of epsilon-carbide in lower bainite

Abstract: AISI 4340 and 300 M steels were investigated. The orientation relation of (100)/sub ..cap alpha../ //(1120)/sub e/ carbide was observed in both lower bainite and tempered lath martensite, supporting the view that in lower bainite, carbides precipitate directly from supersaturated ferrite. (DLC)

Martensitic alloy conditioning

Abstract: A two-stage process for conditioning an annealed martensitic alloy of titanium and nickel to improve its service life and provide enhanced elongation activity under high operating stress. In the first stage of the process, the alloy is maintained under a tensile stress sufficient to strain it beyond its plastic yield point while it is repeatedly thermally cycled in a primary temperature range between a lower temperature limit below the temperature at which conversion of martensite to austenite commences on heating and an upper temperature limit at least about equal to the temperature at which essentially all the martensite is converted to austenite on heating. In the second stage of the process, the alloy is maintained at a tensile stress sufficient to strain it beyond its plastic yield point while it is repeatedly thermally cycled in a secondary temperature range between a lower temperature limit equal to or higher than the temperature at which conversion of martensite to austenite commences on heating and an upper temperature limit equal to or lower than the temperature at which conversion of austenite to martensite commences on cooling. A novel product having enhanced service life and elongation activity is obtained.

A study of retained austenite in a fine-grained Fe-12Ni-0.25Ti alloy

Abstract: The behavior of retained austenite in a fine-grained Fe-12Ni-025Ti cryogenic alloy system was investigated When the alloy was held at a temperature within the two phase (α + γ) field austenite reversion occurred by a diffusion-controlled process The redistribution of solute elements appeared to control the stability of the reverted austenite The microstructural appearance of the retained austenite was examined using transmission electron microscopy A preferential distribution of the austenite phase along martensite lath boundaries was observed A precipitate-correlated austenite was also found to occur The beneficial effect of introducing retained austenite appeared in the improvement in the tensile elongation as well as in the Charpy impact toughness at low temperatures The retained austenite in this system did not improveK IC at −196°C On the contrary, samples containing retained austenite showed increased susceptibility to unstable crack propagation in low temperature fracture toughness tests

Acoustic Emission During Phase Transformation in Steel

Abstract: Acoustic emission was monitored during phase transformations that occur during cooling in a wide variety of steels. Acoustic emission was generated during the formation of martensite but not during the formation of ferrite, bainite, or pearlite. This observation is consistent with the rapid, diffusionless, shear-like nature of martensite formation and the slow, diffusion-controlled growth of ferrite, bainite, or pearlite. The martensite start temperatures, and the temperature range of martensite formation determined by acoustic emission were in good agreement with those determined by metallographic or dilatometric methods. The intensity of acoustic emission generated during martensite formation decreased markedly as the carbon content of the steel decreased, becoming nearly undetectable in a maraging steel. This decrease in intensity correlates with a morphological change from large plate-shaped martensite units to smaller lath-shaped martensite units as the carbon content of the steel is decreased.

Fe-33%Ni合金のマルテンサイト→オーステナイト逆変態温度に及ぼす前処理の影響

Abstract: The effect of several kinds of pre-treatments on the temperature of diffusionless reverse transformation from martensite to austenite was investigated on an Fe-33%Ni alloy. The main results obtained are as follows.(1) The T0 temperature (=(Ms+As)/2), at which the chemical free energies of austenite and martensite are equal in the alloy, was constant even if the Ms temperature was varied with different austenitizing treatments.(2) The As temperature (starting temperature of reverse transformation to austenite) increased markedly with deformation of martensite. It became clear that the reverse transformation was suppressed with deformation of martensite. However, the Af temperature (finishing temperature of reverse transformation) hardly changed with deformation.(3) Reverse transformation of the ausformed martensite was hardly affected by the degree of ausforming.

Strength and Ductility of Cr-Mo-V Steels in Creep at Elevated Temperatures

Abstract: A state-of-the-art survey of literature pertaining to low alloy Cr-Mo-V steels has been completed with a view to elucidate the effects of composition, heat treatment, and microstructure on the creep strength and ductility of the steels. It appears that minor amounts of alloy additions such as boron, titanium, and cerium and impurity elements phosphorus, sulfur, tin, antimony, aluminum, and copper may affect the creep strength or ductility or both of the steels. Higher austenitizing and lower tempering temperatures lead to improved strength at the expense of rupture ductility. An upper bainite microstructure is associated with the highest creep strength and the lowest ductility, for temperatures up to 1050°F (565°C) and for times of at least up to 10,000 h. In bainite-ferrite aggregates, creep and rupture strengths increase in proportion to the amount of bainite, and the difference in strength between the various structures is maintained at least up to 10,000 h at 1070°F (575°C). Stress rupture strengths in general increase linearly with room temperature tensile strength for temperatures up to 1000°F (538°C) and times up to 10,000 h. Variation of rupture strength and minimum creep rate with temperature and time can be adequately described by the Orr-Sherby-Dorn parameter. Activation energies for both creep and for rupture are determined to be about 90 kcal/mole (375 kJ/mole). Further, it is observed that ˙e × tr ⋍ 3.3 and that tt ⋍ 0.3 tr, where ˙ϵ tt, and tr are the minimum creep rate, time for transition from second- to third-stage creep, and time to rupture, respectively.

The Reversion of Martensite to Austenite in an Fe–16Cr–12Ni Alloy

Abstract: The isothermal transformation of martensite to austenite has been investigated in an Fe–16Cr–12 wt.-%Ni alloy at various temperatures below Af , using magnetic measurements and ∼tructural observations. Three stages of transformation were found to occur. The first was athermal in nature and occurred in the range ∼530–600° C (i.e. A s – A r). The second stage, observed at 540 and 550° C, was small in magnitude, and may represent austenite formation by an isothermally occurring martensitic mode of transformation. The third stage was detected in the range from∼570° C down to temperatures well below A s; it involved the transformation of martensite to austenite over a prolonged period, together with the diffusion-controlled formation of ferrite; the formation of this phase occurred more rapidly from deformation-induced martensite than from quench-induced martensite.

Intercritical-cycle annealing

Abstract: Hypoeutectoid steel is heated to an intercritical temperature to produce a microstructure of about 30-50% austenite The steel is then cooled as rapidly as feasible to a temperature below As and held at temperature to transform the austenitic portion Such transformation may occur either in the minimum of the pearlite region (˜1200° F) or in the bainite region (˜800° F)

Effect of Sb, P, Sn and B on the microstructure and creep properties of normalized and tempered 1.25 Cr−0.5 Mo Steels

Abstract: A program to study the effect of Sb, P, Sn and B on creep properties of four normalized and tempered 1.25 Cr−0.5 Mo steels at 538°C (1000°F) has been completed. Results show that even a combined addition of large amounts of Sb, P and Sn does not affect short time creep strength or ductility of the steel at 538°C (1000°F). Addition of B resulted in an increase or decrease of creep strength depending on the nature of the impurity species present, presumably due to B-impurity interactions. Regardless of the effect on creep strength, B additions caused sharp reductions in rupture ductility in all cases. Comparison of the present results on the four laboratory steels (100 pct bainite) with results of a previous study on a commercial steel (60 pct bainite + 40 pct ferrite) show that the effect of microstructure becomes negligible and rupture strength values of the various steels at 538°C (1000°F) approach each other at rupture times in excess of 104 h.

Steel member for reinforcing rubber composites and method of making same

Abstract: A steel member, essentially rectangular in cross-section, obtained by mechanically working round steel stock, having an aspect ratio greater than 2, wherein the member is characterized by a tensile strength greater than 200 KSI, a fatigue endurance limit greater than 100 KSI and a heat treated microstructure of tempered martensite, bainite or mixtures thereof. A method is disclosed for making a steel member comprising the steps of providing steel stock with a round cross-section having a pearlitic microstructure and at least 0.35 percent carbon, forming an essentially rectangular member having an aspect ratio of at least 2 and heat treating the member so as to provide a microstructure of tempered martensite, bainite or mixtures thereof.

The Thermal Stabilization of Austenite in High-Nickel Steels

Abstract: The thermal stabilization of austenite has been studied in two high-nickel steels which transformed to martensite at sub-zero temperatures; the martensite was of {259}γ habit in one alloy and predominantly of {225}y in the other. In both steels appreciable stabilization was obtained after ageing between the ambient temperature and ⊂ 90°C. The observations are considered in the light of the previously proposed theories for stabilization. It is concluded that stabilization at well below O°C is due to stress-relaxation, although as the temperature of stabilization is raised the diffusion of carbon may become an increasingly important factor. The influence of habit plane of the martensite on stabilization supports the concept that the controlling mechanism acts to inhibit nucleation of the martensite.

Oriented Nucleation in the Reverse Transformation of Fe-30 Ni

Abstract: Reverse transformation of different martensite textures has been investigated. Starting with austenite the martensite was produced by rolling, deep cooling, or by a combined treatment. In most cases, during the reverse transformation the reconstitution of the original austenite texture occurred. This is explained by oriented nucleation of residual austenite during reverse transformation.

Method of making light gage members of unalloyed low carbon steel sheets

Abstract: Thermal hardening is applied to the material of light gage cold-formed members made of unalloyed low carbon steel sheets having a carbon content not greater than 0.50 percent and a thickness not greater than 7 millimetres. Thermal hardening consists in quenching such steels from a temperature in the austenitizing range below the Ms temperature of starting martensite formation, holding the steel at such temperature until transformation is completed and then cooling it down to room temperature. By such method, an increase by 100 to 200 percent of the yield point may be obtained without distortion of the member. This seems to be due to the nose of the TTT-curve of beginning martensite formation being cut by the cooling curve so that softer crystal structures such as pearlite and bainite are formed which provide for absorbing residual stresses during martensite formation.

High-strength structural steels with a structure of low-carbon bainite subjected to thermomechanical treatment

Abstract: The development of TMT for low-pearlite and pearlite-free steels has led to the development of steels containing 0.06% C, 1.8% Mn, 0.3% Mo, and 0.05–0.09% Nb. Depending on the type of TMT, the most important parameters of which are the final rolling temperature and the total deformation at temperatures below 900°, the transformation occurs partially or completely in the intermediate region. The high dislocation density of the bainite has a substantial effect on the increase of the yield strength.

Temper embrittlement in steel for thiek-walled gun tubes

Abstract: Difference in tempering response between bainitic and martensitic microstructural constituents helps explain occasional appearance of temper embrittlement in heavy section ordnance components.

Effect of Alloying Elements on the Toughness of Bainite in Medium Carbon Low Alloy Steels

Abstract: Synopsis: The effect of C(0.25-0.45%), Ni(0-2.6%), Mo(0-0.5%) and Cr(0-2.2%) contents on the toughness of the as formed and tempered bainite was investigated in 0.4%C-0.8%Mn-1.1%Cr-0.25% Mo steels, and factors which affect the toughness of bainite were discussed. Bainite was formed in the isothermal transformation. Increase of C, Ni and Cr contents and rise of transformation temperature raised the transition temperature of bainite. The results were attributed to the increase of both fracture facet size and carbide length and the latter was more deteriorate. In the increase of Cr content, another factor might be also necessary to be taken into account for the full interpretation of the results. Addition of molybdenum lowered the transition temperature of tempered bainite mainly by the relief of the temper embrittlement.