Showing papers on "Bainite published in 1976"

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

Abstract: The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, Ms =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, Ms = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory.

Martensite crystal lattice, mechanism of austenite-martensite transformation and behavior of carbon atoms in martensite

Abstract: Results concerning the crystal lattice of freshly formed martensite with abnormally low and abnormally high axial ratioc/a as well as the results of neutron diffraction studies of the positions of carbon atoms are reviewed. Mechanisms of the austenite to martensite transformation are considered, which can explain the formation of martensite with differentc/a for the same carbon content in the initial austenite. Occurrence of (011)m transformation twins explains both a lowering ofc/a and an orthorhombic distortion of the martensite lattice. It follows from analysis of the experimental results that the well known dependence of martensite lattice parameters and axial ratio on the carbon content relate to a partly disordered distribution of carbon atoms between three sublattices of the octahedral interstitial sites (ois). Using new values of the concentration coefficients of linear expansion of the martensite lattice, calculated for the case of carbon atoms distribution only in a single sublattice of the ois, leads to some corrections of the previous temperature and concentration dependences of order-disorder processes in martensite. Phenomena such as the reversible change of axial ratio due to the redistribution of carbon atoms between their normal positions and “traps” in irradiated martensite are described.

Electron-microscope studies of carbide decay during contact fatigue in ball bearings

Abstract: Structural changes in ball-bearing steels caused by rolling contact fatigue have been studied. Attention is focused on changes that occur in the various carbide constituents. Large spherical primary cementite particles gradually break up and disappear during the fatigue process. At the same time, fine carbide constituents present in either a tempered martensite or lower bainite matrix also disappear. The decay process is described, the transformation products identified, and the mechanisms discussed.

Bainite transformation in deformed austenite

Abstract: In this work the effect of deformation of metastable austenite on its subsequent isothermal decomposition in bainite regions was investigated in three chromium steels with varying carbon contents. Four different types of treatments were used and the effect of carbon content was studied. Results of the kinetic measurements and of structural changes showed that the most important feature of these treatments was formation of narrow ferrite strips along the slip bands in austenite, which preceded formation of bainite of the usual morphology. A marked dependency of this ferrite strips formation on temperature suggests that the over-all transformation would strongly depend on deformation and recovery processes in austenite and on their mutual relation. A new qualitative model of the bainite transformation in deformed austenite is presented.

Ferrite morphologies and carbide precipitation in a Cr-Mo-V creep-resisting steel

Abstract: Ferrite morphologies and carbide precipitate dispersions in a ½Cr-Mo-V creep-resisting steel have been studied after isothermal transformation at a number of subcritical temperatures in the ferrite and bainite ranges, and also after rapid quenching to room temperature to produce martensite. It has been found that VC precipitates by the interphase mechanism during direct transformation to ferrite, and towards the latter stages of the transformation, some fibres of Mo2C also form. Bainite morphologies ranged from granular at the higher transformation temperatures to lath-like at lower temperatures. Tempering of bainite and martensite at 700°C resulted in VC precipitation on dislocations and the interparticle spacing tended to decrease as the initial transformation temperature in the bainite range was increased. However, the best resistance to overageing during tempering was provided by ferrite produced by direct transformation at 700°C. These differences in dispersion stability are explained in term...

Method of producing a high strength steel having uniform elongation

Abstract: A steel product having a combination of high strength and formability (as measured by percent tensile uniform elongation) is produced by austenitizing a steel consisting essentially of from 0.04 to 0.17% carbon, 0.8 to 2.0% manganese, up to 1.0% silicon, up to 0.12% vanadium, up to 0.1% columbium, up to an effective amount of titanium to form titanium carbonitrides, 0.001 to 0.025% nitrogen, balance essentially iron and then cooling at a rate of no more than about 70° F/sec. to about 850° F and at a rate of more than about 10° F/sec. to transform the freshly formed austenite to a microstructure of from 10 to 35% by volume of martensite and/or lower bainite (MLB), balance essentially proeutectoid ferrite. Slower cooling rates may be employed to obtain the desired microstructure if a restricted chemical composition is used. The heat-treated steel product is characterized by an ultimate tensile strength of 80,000 p.s.i. minimum and a uniform elongation of 16% minimum.

Growth kinetics of bainite plates and widmanstÄtten needles in the Β ′ phase of a Ag-45at. pct Cd alloy

Abstract: The morphology of bainite plates and widmanstAtten needles formed in the Β′ phase of a Ag-45 at. pet Cd alloy at temperatures 160 to 300‡C was studied by optical and scanning electron microscopy. Both precipitate forms were similar in appearance to precipitates reported for Cu-Zn alloys. The growth kinetics of both bainite plates and widmanstAtten needles were measured by interrupted annealing and scanning electron microscopy. Using the bainite thickening kinetics measured at 160, 200 and 240‡C, the Frank-Zener model for growth of planar precipitates, and supersaturation data obtained from the Ag-Cd metastable phase diagram enabled the effective chemical diffusivities, Deff, to be calculated for the three transformation temperatures. The results were in good agreement with the expected diffusivities. The lengthening kinetics of bainite plates at 160‡C and of widmanstAtten needles at 240‡C were analyzed using Trivedi’s model for diffusion-controlled growth. Deff obtained from the lengthening kinetics of the needles was in good agreement with the Deff value obtained from the thickening kinetics of the plates, indicating that widmanstAtten needles lengthened and bainite plates thickened at a rate controlled by volume diffusion. Bainite plates lengthened only in the early stage of growth and at a rate approximately 180 times larger than that permitted by volume diffusion. Possible reasons for such behavior were discussed.

Producing rolled steel products

Abstract: In the rolling mill the temperature of the product is reduced to between Ac3 and Ac3 + 150° C before it enters the finishing stands. The product emerging from the finishing stands is quenched to provide a surface layer of martensite and/or bainite. Subsequently the heat of the core of the product is allowed to raise the temperature of the surface layer to between 450° and 750° C, so that the quenched layer is tempered.

The effect of the first and second stages of tempering on microcracking in martensite of an Fe-1.22 C alloy

Abstract: The effect of tempering on microcracking in the plate martensite of an Fe-1.22 C alloy was investigated by isothermal heat treatments in the temperature range between 180 and 225°C. The second stage of tempering, followed by X-ray measurement of retained austenite, was confirmed to depend upon the diffusion of C in austenite, and the transformation product was found to consist of very closely spaced cementite lamellae in ferrite. Microcracking, despite the volume expansion that accompanies the transformation of the retained austenite, decreased only slightly with time during the second stage. The major decrease in microcracking occurred during the first stage, a result attributed to the plastic deformation that accompanies the dimensional changes caused by the reduction of the lattice tetragonality of the high carbon martensite in the first stage. Metallographic observations of surface relief and etching effects associated with martensite plates provided evidence of the first-stage plastic flow.

Transitions between stress-induced martensites in Cu Al Ni alloys

Abstract: Stress-induced martensitic transformations have been studied in β1 Cu Al Ni single crystals at temperatures aboveM s . Close toM s γ′ martensite is formed, well aboveM s β 1 ′ martensite forms, whilst in an intermediate temperature range β1′ martensite initially forms and then transforms to γ′ on continued stressing and particularly on unloading, γ′ martensite is also formed when the stress-induced β1′ is cooled below a critical temperature. The γ′ martensite has a (101) twinned structure. The morphological and crystallographic aspects of the γ1′→ γ′ transition are discussed in detail. The two twin variants have different habit planes with respect to the β1′ phase, one being (201)γ′ and the other (001)γ′. A thermodynamic argument is presented to explain the γ1′→ γ′ transition, taking into account the relative stabilities of the β1′ and γ′ phases with respect to the β1, and the relative value for the critical driving force to nucleate the stress-induced β1′ and γ′ structures from the β1 phase

Martensite plate arrangements in 01 tool steel

Abstract: In a commercial 01 tool steel austenitized at 1200°C, martensite plates form in small clusters within austenite which has been partitioned by grain boundaries and preexisting martensite plates. But the surface of martensite plate mid-planes increases only about one fourth as rapidly and the plates are about six times as long as would be expected from random formation in austenite randomly partitioned by preexisting plates. Martensite plates maintain a constant thickness until their radius is restricted to approximately the distance which is tip affected, about 10 μm. The plate thickness to radius ratio is not constant.

Steel member for reinforcing rubber compositions and method of making same

Abstract: A steel member, essentially rectangular in cross-section, having an aspect ratio greater than 2 having an adhesive coating on the surface thereof, wherein the reinforcing member is characterized by a tensile strength greater than 200 ksi and a microstructure of tempered martensite, bainite and mixtures thereof. A method is disclosed for making a steel member comprising the steps of slitting in coil form steel stock containing more than about 0.40% carbon, continuously heat treating the stock so as to obtain a microstructure of tempered martensite, bainite or mixtures thereof and then applying an adhesive coating to the stock.

Isothermal studies of bainitic and martensitic transformations in some low alloy steels

Abstract: The transformation kinetics of the upper and lower bainitic reactions were determined for four commercial quenched and tempered steels, AISI 4130, AISI 4140, D6AC and AMS 6416 (300-M). Their chemical compositions are given. The kinetics of the bainite reaction below the martensite start temperature (M/sub s/) was determined.

Steels combining toughness and machinability

Abstract: A method for strengthening of carbon and low alloy steels and steels produced thereby wherein a carbon or low alloy steel is partially austenitized to produce a ferrite-austenite mixture, the resulting ferrite-austenite mixture is quenched to an intermediate temperature to render the austenite metastable, with the quenching being at a rate sufficient to avoid transformation of the austenite to ferrite and pearlite, and working the quenched steel at a temperature up to the maximum temperature at which bainite can exist whereby the ferrite-austenite mixture is converted to a ferrite-bainite mixture having high levels of machinability, strength and toughness.

A rapid magnetometric technique to plot isothermal transformation diagrams

Abstract: A sensitive magnetic permeability method for rapid determination of isothermal transformation diagrams in steels and iron base alloys is described. The method consists of quenching the sample from an austenitizing temperature to a subcritical temperature in an isothermal bath, and holding it within the magnetic field of an inductor coil. The increase in permeability accompanying austenite decomposition increases the inductance of the coil, and this changes the resonant frequency of the circuit. An automatic continuous recording of the corresponding period provides a convenient and accurate method for following the austenite decomposition of AISI 4340 in the bainite and martensite temperature ranges. This method provides quantitative information on austenite decomposition kinetics within two seconds after the start of quenching.

Effect of high magnetic fields on the martensite transformation

Abstract: The effect of high magnetic fields up to 132 kOe on the martensite transformation has been investigated in two alloy steels, 52100 bearing steel and a type 410 stainless steel. In both cases the martensite start temperature is raised by the application of a magnetic field, and the increase inMs is linear with field. The rate of formation of martensite is not affected by the field. Numerical values for the entropy of the austenite-martensite reaction can be obtained from the experimental results, and are in reasonable agreement with previous results and with theoretical calculations.

Metallographic study of hydrogen-induced cracking in line pipe steels

Abstract: Does the presence of bainite and martensite promote hydrogen induced cracking in line pipe steels, or are other factors dominant? Some observations on two representative HSLA steels.

Production of medium or low carbn high tensile wire rod

Abstract: PURPOSE:To improve the strength, ductility, and cold forgibility of a medium or low carbon steel wire rod by hot working and cooling the wire rod under specified conditions to convert the structure to bainite.

Procedure for manufacture of steel band or strip

Abstract: A process for manufacture of a steel band or steel strip, the steel comprising spheroidal carbide particles in a ferritic base, wherein a steel, the major part of which is a carbonaceous ferritic austenite conversion product which is, structurally, at least one of lamellar perlite, granular perlite, sorbite, bainite and martensite, and which is at a temperature below the steel A cl -temperature is heated to a temperature which is above the A cl -temperature and within the ferrite metastability range, the steel is then rolled at a temperature in the said metastable temperature range, and the steel is then cooled to below the A cl -temperature, the heating, rolling and cooling taking place in a time short enough to ensure that no substantial conversion to austenite occurs in the steel.

Steel bolt mfr. - with hardened and tempered skin and unhardened core

Abstract: Steel bolt with a high elastic limit has a longitudinal structure comprising at least two coaxial zones, the outer, surface region being composed of martensite or tempered bainite and the inner zone, the core of the bolt being composed of untempered ferrite and pearlite. Between these two regions there may also be an intermediate region of untempered bainite. The bolt has better mechanical properties than previously obtd. from non-alloy steel, and the properties can be as good as those of more expensive alloy steels contg. e.g. Ni and Cr.

Steel abrasive materials

Abstract: Disclosed is a refined and alloyed martensite-free cast steel abrasive material and method of making the same The abrasive material is either shot or grit and has a composition which includes carbon in about the range of 060 to 120 percent by weight, silicon in about the range of from 030 to 150 percent by weight, sulphur in about the range of from 00 to 01 percent by weight with the remainder substantially all iron, trace elements and impurities The substantially martensite-free micro-structure is characterized by the presence of one or more of the group consisting of lower bainite, upper bainite, pearlite or ferrite and mixtures thereof, thus providing a more ductile abrasive particle of improved wear resistance

Manganese steels for maximum wear resistance - heat-treated to give a martensitic structure

Abstract: Steel of compsn. 0.9-1.4 wt.% C, 3.0-8.0 wt.% Mn, 1.0-2.5 wt.% Cr, 0.5-2.5 wt.% Mo, 0.25-2.0 wt.% Si, Balance iron and impurities, is heated at 900-1100 degrees C, cooled to 200 degrees C to obtain an austenite, then pref. heated for >1 hr. at 500-690 degrees C to transform the austenite to pearlite and bainite, then heated for >5 min. at 690-800 degrees C and cooled to 50 degrees C to transform to martenite and secondary carbides. By this method the steel can be heat-treated to a hardness of Rockwell C62, but at an intermediate stage in the heat-treatment is sufficiently soft to be easily machined.