The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

The martensite transformation in FeNiC alloys

The effect of austenite defect structure upon the sub-zero martensite burst transformation temperature in FeNiC has been investigated using a combination of optical and electron microscopy, differential scanning calorimetry and microhardness testing. In the absence of a change in composition or dislocation density, the martensite start transformation temperature (M_s) was found to be determined by the grain size of the austenite. Above a grain size of 150 μm, M_s was found to be independent of grain size, but below 150 μm, the transformation temperature was strongly depressed by up to approximately 50 K at a grain size of 10 μm. For any given grain size, an increase in the dislocation density from that typical of a fully recrystallised specimen, i.e. approximately 10¹⁰ lines m⁻², to that of approximately 10¹⁵ lines m⁻² raised M_s by approximately 15 K. The depression of M_s and reduction in the initial burst size of the transformation with decreasing grain size was found to be related to the observation that a fine grain size results in a heterogeneous transformation restricted to a few small pockets of grains. The depression of M_s in the fine grained alloy is consistent with a segregation of active martensite nuclei into a few small grains, a suppression of the autocatalytic stimulation of martensite plates between adjacent grains, and a possible reduction in the number of martensite nuclei.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

The effect of prior deformation on the strength and annealing of reverted austenite

The strength, annealing behavior, and microstructure of reverted austenite has been measured in an Fe-31 pct Ni-0.03 pct C alloy that was plastically deformed in the martensitic state prior to the reversion to austenite. Mechanical properties of reverted austenite (e.g., austenite formed by the reverse martensite shear transformation) were measured as a function of the amount of prior deformation, heating and cooling rates to the reversion temperature, austenitizing temperature and time, repetitive cycling from martensite to reverted austenite, and prereversion heat treatments. The results showed that 80 pet prior deformation increases the yield strength of reverted austenite about 30 pct. Along with this strengthening, the dislocation configuration changes from a plate-like fine structure with a random array of tangled dislocations in reverted samples without prior deformation to a equiaxed fine structure with a high density of tangled dislocations within the fine structure in samples with 80 pct deformation prior to reversion. Although smaller amounts of prior deformation (20 pct) have only a small effect on the strength of the reverted austenite, this amount of prior deformation significantly increases the driving force for recrystallization. The results are explained on the basis that the prior deformation and the reversion process produce separate components to the strength and annealing behavior. E. GOLD, formerly with the Aeronutronic Division, Philco-Ford Corporation, Newport Beach, Calif.     

The rapid heat treatment of steel

The technique of austenitizing steel by heating rapidly through the Ac₁ to Ac₃ range, limiting the maximum temperature to the minimum required for complete austenitization and quenching immediately is discussed. Rapid austenitizing refines the austenite grain size if the initial microstructure is a fine aggregate, such as martensite or tempered martensite. Additional grain refinement usually results from two or more cycles and most steels hardenable by heat treatment become ultrafine grained and hence exhibit increased strength and toughness. Rapid austenitizing can also be applied, particularly to high-carbon steels, to develop a unique microstructure comprised of a uniform dispersion of very small carbide particles in an ultrafine grained martensitic matrix. This paper is based on a presentation made at a symposium on Altering the Time Cycle of Heat Treatment, held at the Philadelphia meeting of The Metallurgical Society of AIME, October 14, 1969, under the sponsorship of the IMD Heat Treatment Committee.     

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

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -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. T. MAKI, Formerly with the University of Illinois     

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

The Effect of an Inert Oxide Particle Dispersion on the Morphology of Martensite in Fe-27Ni-0.025C Alloys

A series of dispersion strengthened Fe-Ni alloys has been prepared by powder metallurgical techniques. This series was designed to permit evaluation of the relative effects of M_s temperature, chemical driving force, and austenite yield strength on resultant martensite morphology without altering matrix chemistry. Using a carefully selected lath-forming Fe-27Ni-.025C base alloy, incremental additions of an inert oxide dispersion resulted in a decrease in M_s temperature, an increase in the thermodynamic driving force at M_s, and an increase in the austenite yield strength at M_s to values beyond those previously associated with the lath-to-plate morphology transition. As the M_s temperature dropped below about 0 °C, martensite morphology shifted from lath to an intermediate “twinned lath” to plate, while holding constant both matrix chemistry and thermal history. Previous correlations of thermodynamic driving force and austenite yield strength with martensite morphology have been shown to break down. It is concluded that the observed transition from lath to plate martensite in the present alloy series was induced primarily by the depression of M_s temperature into the plate-forming temperature regime of the Fe-Ni system.     

The improvement of cryogenic mechanical properties of Fe-12 Mn and Fe-8 Mn alloy steels through thermal/mechanical treatments

An investigation has been made to improve the low temperature mechanical properties of Fe-8Mn and Fe-12Mn-0.2 Ti alloy steels. A reversion annealing heat treatment in the two-phase (α+ γ) region following cold working has been identified as an effective treatment. In an Fe-12Mn-0.2Ti alloy a promising combination of low temperature (-196°C) fracture toughness and yield strength was obtained by this method. The improvement of properties was attributed to the refinement of grain size and to the introduction of a uniform distribution of retained austenite (γ). It was also shown that an Fe-8Mn steel could be grain-refined by a purely thermal treatment because of its dislocated α martensitic structure and absence of ε martensite. As a result, a significant reduction of ductile to brittle transition temperature was obtained. formerly with the Lawrence Berkeley Laboratory, University of California.     

奥氏体化温度对奥氏体晶粒度及第二相固溶的影响

利用光学显微镜等研究了 SA5 1 6Gr60N 容器钢在不同奥氏体化温度下奥氏体晶粒尺寸的长大规律以及微合金元素 Nb、Ti、V 的固溶规律.研究结果表明,随着奥氏体化温度的升高,微合金元素 Nb、Ti、V 的固溶量逐渐增加;990~1 050 ℃时,原始奥氏体晶粒尺寸增加缓慢,晶粒细小均匀;1 070 ℃时晶粒出现异常长大现象,随后部分奥氏体晶粒急剧长大,不均匀性越来越明显;1 1 70~1 210 ℃时,奥氏体晶粒尺寸均匀化.     

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

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105?C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -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.     

The effect of austenite ordering on the martensite transformation in Fe-Pt alloys near the composition Fe3Pt: I. Morphology and transformation characteristics

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 theM _s 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. D. P. DUNNE, formerly with the University of Illinois at Urbana-Champaign, Urbana, 111. 61801     

Tempered martensite embrittlement in phosphorus doped steels

In this paper the effect of phosphorus on tempered martensite embrittlement of Ni−Cr steels is reported. It is shown that the measured degree of embrittlement depends on the phosphorus concentration, test temperature, grain size, and austenitizing temperature. Although reducing the prior austenite grain size tends to reduce the observed embrittlement, this can be offset by the fact that the low austenitizing temperatures used to produce the fine grain size cause an increased amount of impurity segregation. It is further shown that bulk phosphorus concentrations below 100 wppm may be required to avoid embrittlement of this type in ultra-high strength steels.     

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.     

The effect of austenite ordering on the transformation temperature,transformation hysteresis,and thermoelastic behavior in Fe- Pt alloys

The change and transition process in transformation kinetics from a nonthermoelastic to a thermoelastic type accompanying an increase in parent phase order in Fe-Pt alloys near the stoichiometric composition Fe₃Pt has been investigated, using Fe-23, 24 and 25 at. pct Pt alloys. The thermal hysteresis,M _s temperature, martensite tetragonality and transformation volume change have been measured for specimens with various degrees of order, and correlations among these factors are discussed. The results indicate that the martensite tetragonality, or equivalently the.degree of order of the parent phase, is not the dominant factor which dictates a thermoelastic transformation. TheM _s tempera-ture appears to play an important role in the transformation kinetics, and must be lower than a certain value to obtain a thermoelastic transformation in Fe-Pt alloys. formerly Research Assistant at the University of Illinois at Urbana-Champaign, Urbana, IL     

The effect of austenite prestrain above the Md temperature on the martensitic transformation in Fe-Ni-Cr-C alloys

The effect of austenite prestrain above theM _d temperature on the structure and transformation kinetics of the martensitic transformation observed on cooling was determined for a series of Fe-Ni-Cr-C alloys. The alloys exhibited a shift in martensite morphology in the nondeformed state from twinned plate to lath while theM _s temperature, carbon content, and austenite grain size were constant. The transformation behavior was observed over the temperature range 0 to -196°C as a function of tensile prestrains performed above theM _d temperature. A range of prestrains from 5 pct to 45 pct was investigated. It is concluded that the response of a given alloy to austenite prestrain above theM _d temperature can be correlated with the morphology of the martensite observed in the nondeformed, as-quenched state. For the range of prestrains investigated, the transformation of austenite to lath martensite is much more susceptible to stabilization by austenite prestrain above theM _d temperature than is the transformation of austenite to plate martensite.     

奥氏体化温度对51CrV4钢淬火组织和性能的影响

 为了找出51CrV4钢最佳的奥氏体化温度和最佳的综合力学性能，研究了奥氏体化温度对51CrV4钢淬火组织和性能的影响。试验结果表明，随着奥氏体化温度的升高，奥氏体晶粒逐渐长大，淬火后组织硬度呈先增大后减少的趋势，经460 ℃回火后的强度先增大后减小；当奥氏体化温度为880 ℃时，奥氏体晶粒细小均匀，得到的马氏体组织致密，强度和硬度均达到最大值；当奥氏体化温度达到910 ℃时，奥氏体晶粒粗大，而且试验钢出现明显的脱碳现象，强度、硬度和塑性明显下降。研究表明，在实现完全奥氏体化前提下，为保证晶粒均匀且不出现脱碳现象，51CrV4钢获得良好性能的最佳淬火温度为880 ℃。     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

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

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. The authors were formerly Research Assistant and Professor, respectively at Lehigh University, Bethlehem, PA.