Showing papers on "Austenite published in 1973"

Fatigue crack propagation in martensitic and austenitic steels

Abstract: Fatigue crack growth rates were measured in an annealed and in an aged maraging steel and in three different austenitic steels Microhardness measurements were used to determine the plane strain plastic zone sizes as a function of ΔK and to evaluate the cyclic flow stress of the material near the crack tip The presence of a reversed cyclic plastic zone within the monotonic plastic zone was confirmed The two maraging steels work soften near the tip of the crack while the three austenitic steels work harden The fatigue crack growth rates of the maraging steels are independent of the monotonic yield stress and are typical of the growth rates of steels with a bcc crystal structure The crack growth rates in the stainless steels are an order of magnitude lower than for maraging steels for ΔK< 30 ksi √in The excellent fatigue crack growth resistance of austenitic stainless steels is related to the de-formation induced phase transformations taking place in the plastic zone and to the low stacking fault energy of the alloys

Diffusional growth limitation and hardenability

Abstract: It has long been recognized that the effects of alloying elements on the hardenability of steels is related directly to their effects on the nucleation and growth kinetics of proeutectoid and eutectoid austenite decomposition products. In the present paper, theoretical and experimental studies of proeutectoid-ferrite and pearlite growth are reviewed for systems of the form Fe-C-X, where X is a substitutional alloying element such as Co, Cr, Mn, Mo, Ni, Si, and so forth. Of principle interest is the limitation which an alloying element X imposes on the corresponding diffusional growth kinetics. Although the agreement between theory and experiment is reasonable for most systems, there remain areas of considerable controversy (e.g., the role of interface diffusion, whether or not local equilibrium is maintained at interfaces and solute segregation to interfaces).

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.

Thermodynamics of carbon in nickel, iron-nickel and iron-chromium-nickel alloys

Abstract: Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH4-H2 gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.

Tracer diffusion of59Fe and51Cr in Fe-17 Wt Pet Cr-12 Wt Pet Ni austenitic alloy

Abstract: The volume and grain-boundary diffusion of59Fe and51Cr have been studied in an austenitic iron alloy containing 17 wt pct Cr and 12 wt pct Ni. The diffusivities in this alloy of these two tracers and63Ni are compared with their diffusivities in pure iron and in other austenitic stainless steels. For volume diffusion at any particular temperature in the present alloy, Cr is the most rapid while Ni is the slowest, and all three tracers diffuse slower than that reported for pure iron or for other austenitic stainless steels. For grain-boundary transport, Fe diffuses most rapidly above 850°C and Ni diffuses most rapidly below that temperature. The activation energies for both volume and grain-bounary diffusion obey the relationshipQ Ni

Stress-induced γ→ α martensitic transformation in two carbon stainless steels. Application to trip steels

Abstract: The influence of the temperature θαof a prestraining of austenite above Mdon the subsequent stress-induced γ→ α’ transformation in the(M s, Md) range is examined in two carbon stainless steels. It is shown that the yield stress, which is controlled by the transformation, increases with θαat given testing temperature and amount of prestraining. This behavior is related to the influence of θαon the nature and arrangement of the defects present in austenite after the prestraining: planar defects(i.e., stacking faults, twins, e platelets) predominate if θαis close to Mdwhereas undissociated dislocation cells are only to be observed if θif higher. This is consistent with the strong increase of the intrinsic stacking fault energy of the austenite, as inferred from measurements using the node method on a hot stage microscope. In addition, the ability of plane defects to propagate under stress is shown to be lower after a prestraining at higher θα, which is attributed to a segregation of impurity atoms on dislocations. It is concluded that the nucleation stress of the γ→ α’ transformation is the stress necessary to allow planar defects to propagate in the prestrained austenite.

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 distribution of chromium between ferrite and austenite and the thermodynamics of the α/γ equilibrium in the Fe-Cr and Fe-Mn Systems

Abstract: New measurements of the α/γ equilibrium in the Fe-Cr system are presented and all the relevant experimental information now available, including thermodynamic data, is evaluated using a regular solution model. The two-phase field is calculated in good agreement with most of the experimental measurements. The point of minimum is obtained at 1119 K and 7.0 at. pct Cr (6.6 wt pct Cr) and the maximum solubility in austenite is 11.9 at. pct Cr (11.2 wt pct Cr). The thermodynamic quantities describing the effect of chromium on the relative stability of ferrite and austenite are similar to those for manganese. The comparison is based upon a new evaluation of the Fe-Mn system. In particular, low chromium contents are found to have a stabilizing effect on austenite up to a temperature of 1675 K.

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.

Estimating the hardenability of carbon steels

Abstract: A new hardenability test for shallow-hardening steels was developed which allows the precise measurement of the hardenability of “pure” Fe-C alloys: The quantitative hardenability effect of variation in the austenite grain size of carbon steels was found to vary linearly with dγt-1/2, where dγ is the mean austenite grain diameter. Using high-purity steels, the quantitative hardenability effects of C and the common alloying elements Mn, P, S, Si, Cu, Ni, Cr, Mo, V, Ti, and Zr were determined. From these data, the hardenability of carbon steels, with and without residual elements, can be estimated from chemical composition and grain size by a new and relatively simple method.

An investigation of grain-boundary embrittlement in Fe−P, Fe−P−S, and Fe−Sb−S alloys

Abstract: Grain-boundary embrittlement of pure iron due to phosphorous, antimony and sulfur is studied using fracture appearance transition temperature measurements and Auger electron emission spectroscopy chemical analysis of fractured surfaces. Phosphorus and sulfur are found to segregate to grain boundaries in the entire ferrite range. Segregation to grain boundaries of these elements in the austenite appears to be negligible. It is shown that the segregation of these elements to grain boundaries in iron does not conform to the Gibb's equilibrium segregation model. Sulfur appears to be a more severe embrittler of iron than does phosphorous.

Intergranular Corrosion of Iron-Nickel-Chromium Alloys

Abstract: Almost since their commercial inception about one-half century ago, it has been recognized that stainless steels and related iron-nickel-chromium alloys are susceptible to a particularly insidious mode of corrosive attack, namely intergranular corrosion. Indeed, in their classic paper, Bain et al.1 stated, “One of the few shortcomings of the austenitic stainless steels … [is that] … the metal, after exposure to moderately elevated temperature in the general vicinity of 1000-1500oF, has frequently been found to be subject to a very characteristic· form of intergranular corrosion, even in environments which ordinarily have no effect upon the normal alloy.”

The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: II. Crystallography and general features

Abstract: Ordering of austenitic Fe ~ 25 at. pct Pt alloys results in a progressive change from a "normal" burst type transformation to a thermoelastic one. Experimental measurements are presented which show that despite marked changes in transformation behavior and kinetics, the transformation crystallography remains essentially unaffected by ordering. It is proposed that the thermoelastic transformation results from the effect of ordering in

Stress relaxation and mechanical equation of state in austenitic stainless steels

Abstract: Stress relaxation experiments have been performed on types 304 and 316 stainless steel at room temperature. The stress-strain rate data show that these materials exhibit the behavior of mechanical equation of state. The experimental results are discussed using the concept of internal stress.

Experimental and thermodynamic study of the equilibria between ferrite, austenite and intermediate phases in the fe-mo, fe-w, and fe-mo-w systems

Abstract: The phase equilibria between ferrite and austenite and between ferrite and the intermediateR- andµ-phases have been studied by subjecting high purity alloys of Fe-Mo, Fe-W, and Fe-Mo-W to well controlled heat treatments. The equilibrium compositions at the two-phase interfaces were measured with an electron microprobe. Some of the experimental results show reasonable agreement with previously reported values, but are believed to be of higher accuracy. The new experimental data were, together with available relevant thermodynamic information, evaluated in terms of mathematical models for the individual phases and the parameters thus obtained were then used to calculate the two-phase boundaries in the binary Fe-Mo, Fe-W, and ternary Fe-Mo-W systems. In the course of this study the free energy of formation of theR andµ phases was also evaluated.

Prediction of alloy hardenability from thermodynamic and kinetic data

Abstract: A simple algorithm for the calculation of the hardenability of low alloy eutectoid steels is presented. This involves the semiempirical prediction of concentration-dependent CCT start and pearlite velocity curves using fundamental thermodynamic and kinetic data and current theories of nucleation and growth. The combination of analytic cooling curves with these predictions and a pearlite growth model based on site saturation at grain corners leads to a good prediction of the hardenability of steel 4068, extensively examined by Jominy, and a grain-size dependence which is qualitatively correct. It is also demonstrated via Taylor and binomial expansions of the undercooling within the various kinetic expressions that the hardenability at low alloy concentrations is correctly represented by linear or quadratic addition formulas. At the same time, it is unequivocally demonstrated that Grossman type multiplication formulas are theoretically incorrect. The quadratic terms in the addition formula quantify the so-called “synergistic” effects. The most important positive terms involve an interaction between austenite and ferrite stabilizers. Austenite stabilizers, by depressing the effective temperature of nucleation and diffusional growth processes involving partition of ferrite stabilizers in pearlite, make the latter processes more sluggish. In agreement with long-standing experience, it is concluded that the strongest and most economic hardenability effects can be obtained via mixtures which include both austenite and ferrite stabilizers.

The reversion of martensite to austenite in certain stainless steels

Abstract: An investigation has been made of the reversion of martensite (α′) to austenite (γ) in two stainless steels (i) Fe-16 wt% Cr-12 wt% Ni (of low interstitial content) (ii) Fe-15 wt% Cr-8 1/2 wt% Ni-2 wt% Mo-0.09 wt% C. The alloys were refrigerated to produce ∼ 12 to 15% martensite (α′) and then heated for short times at various temperatures ranging from below As to above Af. With rapid heating the reversion of α′ to γ occurs largely by a shear mechanism. In the Fe-16Cr-12Ni alloy individual grains of α′ transform to grains of reversed γ of similar size and shape. In the carbon-containing alloy there is evidence of break-up of the α′ grains on reversion. An increase in the strength results from reversion and this is attributable mainly to the high dislocation density of the reversed γ.

Elastic constants of martensite

Abstract: The elastic constants of Fe−Ni−C martensite are increased by retained austenite. Also, nickel markedly decreases the elastic constants of such martensite. Therefore, to properly evaluate the effect of carbon on the elastic constants of Fe−Ni−C martensite, it is necessary to first correct for both retained austenite and nickel content. When these corrections are made, both the Young's modulus and the shear modulus of Fe−Ni−C martensites. decrease with increasing carbon content, in agreement with earlier work on Fe−C martensites. thus, an increased lattice stiffness cannot be used to explain the high strength of martensite.

Delta Ferrite and Martensite Formation in Stainless Steels From studies of 70 chill-cast stainless steel alloy types, nickel- and chromium-equivalents of 13 elements are evaluated for use in a revised Schaeffler diagram

Abstract: The effects of composi­ tion on delta ferrite and martensite formation were studied in chill-cast experimental stainless steel alloys. Nickel content was varied in each of seventy different alloy types to pro­ duce structures ranging from fully stable austenite to ones containing high percentages of delta ferrite or martensite. The nickel or chromium equivalents of Mn, Mo, Si, V, W, Ti, Cb, Ta, Al, C, N, Co and Cu were evaluated by regression analyses.

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.

Hydrogen embrittlement studies of a trip steel

Abstract: The conditions of cathodic charging, gaseous hydrogen environment, and loading for which a TRIP steel may or may not be susceptible to hydrogen embrittlement were investigated. In the austenitic state, the TRIP steel appeared to be relatively immune to hydrogen embrittlement. It was shown that it is the strain-induced martensitic phase, α′, which is embrittled. In TRIP steel single-edge-notch specimens under fixed loads in gaseous hydrogen, slow crack growth occurs when the stress intensity level exceeds a threshold level of about 25 ksi-in.1/2 and the growth rate varies approximately as the 2.5 power of the stress intensity level. The activation energy for this slow crack growth was found to be about 10,000 cal/g-atom, the approximate activation for hydrogen diffusion in martensite. Thus it was concluded that slow crack growth in TRIP steel loaded in gaseous hydrogen involves the diffusion of hydrogen through the α′ phase.

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.

The particle-to-matrix bond in dispersion-hardened austenitic and ferritic iron alloys*

Abstract: The bonding strength, or ‘work of adhesion’, W, between particle and matrix in a number of austenitic and ferritic iron alloys has been determined from electron micrographs of the solid/solid contact angle, θ, at cavities formed by cold working and annealing. The critical strain to bring about cavitation, ec, was measured by plotting the strain corresponding to a sudden fall in rate of work-hardening. In deriving an equation to predict ec, account must be taken of the dependence of the equilibrium shape of the cavity on θ. Good correlation between experimental and predicted ec, as a function of W, was found. Brittle fracture of the matrix is likely to occur if θ is very low and the volume fraction is high. A criterion for brittle fracture, based on θ, is suggested which successfully predicts brittle fracture in one of the alloys studied.

Manufacturing method of high tension, high toughness steel

Abstract: Method for obtaining a recrystallized austenite structure in steel by carrying out more than one severe reduction per pass between 1,100 DEG C and 950 DEG C in the hot rolling process of a steel comprising 0.005 - 0.15% C, less than 0.6% Si, 0.70 - 2.0% Mn and 0.01 - 0.15% Nb and containing other effective elements if necessary, with the remainder being essentially Fe, and then rolling the steel to obtain a high toughness, high tension steel plates and strips.