Showing papers on "Bainite published in 1971"

The morphology of martensite in iron alloys

Abstract: Light and electron microscopy have been used to determine the main structural differences between the two major types of martensite in ferrous alloys. In the martensite that forms in dilute alloys of iron, the basic transformation unit takes the shape of a lath, and hence the term lath martensite is appropriate for identifying this morphology. Each lath is the result of a homogeneous shear, and successive shears produce a packet of parallel laths containing a high density of tangled dislocations. The other type, plate martensite, differs in the shape taken by a transformation unit and its transformation sequence is characterized by nonparallel plate formation. Investigation of a large number of binary ferrous systems shows that alloy composition and the transformation temperature influence the transition from lath to plate martensite. These two factors are discussed in terms of their possible effects on the plastic deformation mechanisms which must occur in the parent austenite and product martensite during transformation.

Effect of microstructural banding in steel

Abstract: Tensile and notch-impact properties in wrought steel containing 0.25 pct C and 1.5 pct Mn, with and without elongated inclusions and processed so as to be severely banded or virtually free of microstructural banding, are compared. A short-time, high-temperature normalizing treatment removed the banded condition. Both banding and elongated inclusions cause anisotropy in tensile ductility and impact energy. Elimination of banding is effective in reducing anisotropy in clean steel, but results in only modest improvement in steel containing numerous elongated inclusions. Eliminating microstructural banding alters austenite transformation to only a small extent but improves subsequent machining and cold forming by replacing martensite concentrated in bands with randomly dispersed small volumes of martensite in steel incompletely transformed to ferrite and pearlite or to bainite.

Martensite and retained austenite in hot-rolled, low-carbon bainitic steels

Abstract: The microstructure and occurrence of a microconstituent, consisting of martensite and retained austenite in hot-rolled plates of low-carbon bainitic steels was studied by electron microscopy and microprobe analysis. The results of the studies showed that the formation of the martensite-austenite constituent is controlled by the composition of the steel and by the cooling rate of the plates following hot-rolling. The mechanisms involved in the formation of the martensiteaustenite constituent are discussed.

A study of the peculiarities of austenite during the formation of bainite

Abstract: The formation of bainite in steel is generally accompanied by an enrichment in carbon of the adjacent austenite which can become remarkably stable as evidenced by its very slow transformation rate and its very lowMs point. This paper presents the results of a study of this residual austenite in an SAE-9262 steel. Both the carbon content and the amount of retained austenite have been determined as a function of transformation temperature. It has been shown that the carbon content of the enriched austenite passes through a maximum of 1.7 pct at a reaction temperature of 400°C. However, this remarkably high carbon content falls short of the one predicted by the Kinsman-Aaronson extrapolation of theA3 curve thus indicating that the bainitic transformation cannot be considered simply as an extension of the proeutectoid transformation. In view of the inadequacy of the standard thermodynamics theory of theBs temperature, a kinetic point of view is proposed for the definition of this temperature.

Stress- and strain-induced formation of martensite and its effects on strength and ductility of metastable austenitic stainless steels

Abstract: The effects of deformation-induced formation of martensite have been studied in metastable austenitic stainless steels. The stability of the austenite, being the critical factor in the formation of martensite, was controlled principally by varying the amounts of carbon and manganese. The formation of martensite was also affected by different test and rolling temperatures, rolling time, and various reductions in thickness. The terms “stress-induced” and “strain-induced” formation of martensite are defined. Experimental results show that low austenite stability resulted in stress-induced formation of martensite, high work-hardening rates, high tensile strengths, low “yield strengths,” and low elongation values. When the austenite was stable, plastic deformation was initiated by slip, and the work-hardening rate was too low to prevent early necking. A specific amount of strain-induced martensite led to an “optimum” work-hardening rate, resulting in high strengthand high ductility. For best results processing should be carried out aboveMd and testing betweenMd andMs. Mechanical working aboveMd had a negligible effect on the yield strength betweenMd andMs when the austenite stability was low, but its effect increased as the austenite became, more stable. Serrations appeared in the stress-strain curve when martensite was strain induced.

Martensite Transformation and Memory Effect in the NiTi Alloy

Abstract: Martensite transformation in the NiTi alloy has been studied by selected area electron diffraction. The martensites have such close packed layer structures as 4H (2\bar2) and 12 R (3\bar1) 3 . The 2H and 18R martensites are also sometimes observed. The 12R martensite appears dominantly at room temperature but the 4H martensite is formed below 0°C. They are closely connected with each other through stacking faults. The martensite transformation causes a considerable shear of layers, but, in order to release it, a new phase transformation occurs in the matrix neighbouring the martensites. By cold work, the 4H martensite and the new phase are also produced. The present results make crystallographically clear the nature of the martensite transformation in the NiTi alloy. On the basis of the results, mechanism of the memory effect in the alloy is interpreted. Origin of the memory effect in metals and alloys are also discussed.

Crystallography of the martensite transformation in Fe−Al−C alloys

Abstract: Crystallographic measurements on highly tetragonal bct martensite in two Fe−Al−C steels have been made and the results have been interpreted in terms of the phenomenological crystallographic theory of martensite formation. Agreement between theory and experiment was found to be excellent. Of particular interest is the high martensite tetragonality (c/a→1.14) in aluminum-steels compared to others. Because of this the principal distortions of the Bain deformation are substantially smaller and consequently the magnitudes of the shape and lattice invariant deformations are reduced considerably. Measurements of the dilatation parameter δ indicate that the shape deformation is not significantly different from an invariant plane strain.

AUTHOR'S REPLY TO THE DISCUSSION OF: STRUCTURE AND MECHANICAL PROPERTIES OF TEMPERED MARTENSITE AND LOWER BAINITE IN Fe-Ni-Mn-C STEELS

Abstract: The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni−Fe and 0.25 C-3 Mn−Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn−Fe and 0.25 C-7 Mn−Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features.

Structure and mechanical properties of tempered martensite and lower bainite in Fe−Ni−Mn−C steels

Abstract: The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni−Fe and 0.25 C-3 Mn−Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn−Fe and 0.25 C-7 Mn−Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features.

Influence of austenite and martensite strength on martensite morphology

Abstract: The martensite morphology and austenite flow strength have been determined in a variety of ferrous alloys chosen so that the austenites were paramagnetic, ferromagnetic, substitutional strengthened, and interstitial strengthened. It is demonstrated that two of the most important variables in determining the habit plane (and thus morphology) of martensite in a given alloy are the resistances to dislocation motion in austenite and in ferrite (i. e., martensite). In the wide variety of alloys where martensite with a {259}γ habit plane was observed, the austenite flow strength atMs is greater than 30,000 psi. At lower austenite strengths, either {225}γ or {111}γ habit planes are found depending on the resistance to dislocation motion in ferrite. Thus, {225} martensites are not always found as part of the spectrum between {111} and {259} martensites but only in the cases (e. g., interstitial strengthening) where ferrite is preferentially strengthened relative to austenite. All of the observations are consistent with the idea that the habit plane observed in a given alloy is the one involving the minimum plastic work for the lattice invariant shear.

低合金鋼溶接熱影響部の応力除去焼鈍割れの研究(第1報):sr割れの特徴および発生条件の検討

Abstract: The conditions under which Stress Relief Cracking (SR cracking) may occur and characteristics of SR cracking have been investigated with welded high tensile strength steels of tensile strength 80 kg/mm2 and low alloy steels, lCr-0.5Mo-V, Nb, or Ti steels using the bend restraint SR cracking test specimens and y groove restraint SR cracking test specimens.The following results were obtained from. this investigation.(1) SR cracking is an intercrystalline cracking which occurs in the coarsened grains region in H.A.Z. during stressrelieving heat treatment and does not occur in deposited metal and unaffected parent metal.(2) The residual stress and local stress raisers are necessary to initiate this cracking.(3) The aging in H.A.Z. during stressrelieving heat treatment, especially precipitation hardening by the secondary hardening elements, Nb, Ti, V, etc., promotes the initiation of SR cracking. Therefore, the initiation of SR cracking is suppressed when micro structures of the upper bainite, ferrite and perlite are produced in coarnsened grain region by the increase of heat input and pre-heating during welding process, and secondary hardenings are limited during stressrelieving heat treatment.

Effect of Structure on Creep Strength of a Low-Alloy Cr-Mo-V Steel

Abstract: The published data on the creep and rupture strength of low-alloy ferritic steels are critically examined and it is concluded that there is no evidence that maximum strength accrues from a fully upper bainite structure. The creep strength, over a range of stress, of samples of one cast of low-alloy Cr-Mo-V steel in various structural conditions has been examined. It is shown that there is no gain in strength (primary creep strain or secondary-creep rate) to be obtained by increasing the bainite content above ∼20% at 11 hbar and 575°C (848 K). At higher stresses, creep strength improves up to ∼60% bainite. The stress-dependence of creep is structure-sensitive and its variation indicates that at itresses <∼11 hbar material containing ∼20–30 % bainite will have a lower secondary-creep rate than fully bainitic structures.

Structure and mechanical properties of Fe−Cr−C−Co steels

Abstract: As part of a continuing program concerning the microstructures and mechanical properties of steels in which particular attention is given to transformation substructures, the present work is concerned with martensite and bainite in Fe−Cr−C steels with and without cobalt. Although cobalt raises theM s temperature it does not affect the extent of twinning for the same carbon level and so M s temperature alone does not control transformation substructure. Thus cobalt is not effective in retaining dislocated martensite as carbon is increased and in this regard cobalt is not beneficial to toughness. TheM s temperatures of the steels were relatively high and hence isothermal transformation yielded mixtures of bainites and tempered martensite depending on the temperature of transformation. The mechanical properties of the isothermally transformed steels were inferior to those of the tempered steels due to the interference of upper bainite or (tempered) martensite during the isothermal transformation. Thus, in the steels having highM s temperatures the twinning tempered martensitic structure had relatively better mechanical properties compared to the isothermally transformed steels. Attempts to produce desirable autotempered structures by air cooling (single heat treatments) were not successful and did not improve the mechanical properties since the structure consisted of a mixture of bainite and martensite.

Stress-induced martensite formation in Cu−Al−Ni alloys

Abstract: A study has been made of superelasticity and the strain-memory effect in Cu−Al−Ni alloys in the composition range 14 wt pct Al and 2 to 3 wt pct Ni These alloys have a bcc structure on quenching and show a low temperature martensitic transformation which is responsible for both the superelastic and strain-memory effects Tests on both single and polycrystalline specimens showed that the maximum superelasticity occurred close toA s At higher temperatures the effect gradually decreased, whilst at lower temperatures it decreased very quickly The magnitude of the effect was large in single crystal specimens (>58 pct), but small in polycrystal specimens (<15 pct) The superelastic effect was caused by stress-induced martensite (SIM) Two types of SIM were observed; thin plates of thermoelastic martensite which were always reversible, and wide plates of burst-type martensite This burst-type martensite was responsible for the major portion of SIM, and whether it was reversible or not on removal of the stress controlled the amount of superelasticity observed The strain-memory effect occurred on deformation either in the martensitic state (temperature

Continuous cooling transformation kinetics of thermomechanically worked low-carbon austenite

Abstract: Continuous cooling transformation diagrams were determined for molybdenum-boron steels containing 0.24, 0.4, and 0.66 pct Mo with 0.1 pct C, and also 0.4 pct Mo with 0.2 pct C, after thermomechanically working by compressive deformation to 12, 25, and 50 pct reduction at 830°C (1525°F), as well as for the steels in the underformed condition. In underformed specimens, higher carbon or molybdenum decreased the limiting cooling rate for the avoidance of polygonal ferrite formation. The same was true for deformed specimens, although increased deformation raised the limiting cooling rates of all compositions. The limiting cooling rate for polygonal ferrite formation increased exponentially with austenite, deformation, as measured by true strain. Thermomechanical working also raised bainite start temperatures at fast cooling rates and caused small increases in martensite start temperatures.

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

Abstract: The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveAs, with microcracks forming during theM→γ→M transformation.

The Effects of Prior Transformation and Prestrain on the Habit Planes of Acicular Iron-Nickel Martensite

Abstract: The habit planes of martensite in two iron-based alloys containing 31.7 and 32.8 wt.-% nickel have been measured by two-surface trace analysis after both prior transformation and prestrain of the austenite. The habit plane in strain-free austenite was close to {3, 10, 15}, the plane predicted by the simple crystallographic theories of martensite formation. The habit planes of plates forming after the initial plates displayed a considerable deviation from {3, 10, 15}. It has been shown that the previously reported scatter of the habit plane, which is not due to experimental error, can be attributed to the large strains introduced into the austenite by the growing martensite plate and cannot be accounted for by the current multiple-shear latticeinvariant deformation theories of the crystallography of martensite. Some evidence is presented indicating that the lattice-invariant plane-strain condition on the habit plane must be relaxed in order to explain the habit planes in strained austenite.

Multistage heat treatment

Abstract: Multistage heat treatments have been used to clarify transformation mechanisms and to provide a basis for reducing heat treating time in commercial practice and/or obtaining improved properties. The influence of stepped transformation on the kinetics of the bainite reaction is examined and the results suggest means for improving the hardenability of medium alloy steels. The application of double aging treatments to age hardening systems also is examined.