Pct Mn Research Articles

Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels

It is well known that a significant amount of retained austenite can be obtained in steels containing high additions (>1 pct) of Si, where bainite is the predominant microconstituent. Furthermore, retained austenite with optimum characteristics (volume fraction, composition, morphology, size, and distribution), when present in ferrite plus bainite microstructures, can potentially increase strength and ductility, such that formability and final properties are greatly improved. These beneficial properties can be obtained largely by transformation-induced plasticity (TRIP). In this work, the effect of a microalloy addition (0.035 pct Nb) in a 0.22 pct C-1.55 pct Si-1.55 pct Mn TRIP steel was investigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing the effects of prior austenite grain size, austenite recrystallization temperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same steel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the state of Nb in austenite. Promoting Nb(CN) precipitation by the change in TMP conditions resulted in a decrease in the V RA . These findings are rationalized by considering the effects of changes in the TMP conditions on the subsequent transformation characteristics of the parent austenite.

Effect of the Ti/N ratio on the hardenability and mechanical properties of a quenched-and-tempered C-Mn-B steel

Ten experimental 0.18 pct C-1.2 pct Mn- 0.002 pct B steels with various Ti/N ratios were evaluated in this study. The hardenability of these steels was first determined using Jominy tests. Slab sections were then rolled to produce 12.5-mm-thick plates, and subsequently quenched and tempered for mechanical property evaluation. The volume fraction of coarse (greater than 1 µm) TiN particles was measured in all steels using quantitative metallographic techniques. Scanning transmission electron microscopy was used to investigate fine precipitates, and scanning electron microscopy was used to examine the fracture surface of Charpy specimens. The results show that a complete boron (B) hardenability effect is obtained with Ti/N ratios ≥2.9, a value slightly below the stoichiometric Ti/N ratio of 3.4. Any excess Ti, above that which combines with N, provides an additional increase in hardenability on quenching (effect of Ti in solution) and an increase in strength on tempering (Ti (C,N) precipitation). Steels with a higher (Ti)(N) product develop a higher volume fraction of coarse TiN particles during solidification. These coarse TiN particles result in reduced toughness levels of the heat-treated plates evaluated in the present study.

Decagonal quasicrystal and related crystalline phases in Mn−Ga alloys with 52 to 63 a/o Ga

A decagonal quasicrystal (DQC) and six related intermetallic phases with large unit cells have been found in binary Mn−Ga alloys with 52 to 63 at. pct Ga by means of transmission electron microscopy (TEM). As does the Al−Mn DQC, the Ga−Mn DQC also has a periodicity of 1.25 nm along its tenfold axis. However, its Mn content, determined by electron microprobe X-ray analysis (about 45 to 50 at. pct Mn), is much higher than that of the Al−Mn DQC (about 20 to 30 at. pct Mn). The compositions of the intermetallic phases are about 53, 56, 58, and 62 at. pct Ga, corresponding respectively to the unknown structures of MnGa (50.7 to 53.4 at. pct Ga), Mn5Ga6 (55 at pct Ga), Mn5Ga7 (57.9 at. pct Ga), and Mn3Ga5 (62.9 at. pct Ga) given in the binary Mn−Ga phase diagram (Metals Hand-book, T.B. Massalski, J.L. Murray, L.H. Benneft, and H. Baker, eds., ASM, Metals Park, OH, 1986, vol. 2, p. 1144). Their lattice types have been determined by selected area electron diffraction. The ferromagnetic Mn3Ga5 is tetragonal, a=1.25 nm and c=2.50 nm; Mn5Ga7 is orthorhombic, a=4.57 nm, b=1.25 nm, and c=1.44 nm; Mn5Ga6 has two different but closely related orthorhombic unit cells, a=1.26 nm, b=1.25 nm, and c=1.48 nm as well as a=0.77 nm, b=1.25 nm, and c=2.36 nm; MnGa also has two different and related unit cells, one orthorhombic with a=2.04 nm, b=1.25 nm, and c=1.48 nm and the other monoclinic with a=2.59 nm, b=1.25 nm, c=1.15 nm, and β≈=110 deg. All these orthorhombic phases have b=1.25 nm, being the same as the periodicity along the tenfold axis of the Ga−Mn and Al−Mn DQCs. Moreover, all these six intermetallic phases give electron diffraction patterns displaying a pseudo-tenfold distribution of strong diffraction spots and are considered to be crystalline approximants of the Ga−Mn DQC.

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C.

Fcc/Hcp martensitic transformation in the Fe-Mn system: Experimental study and thermodynamic analysis of phase stability

A new experimental study ofA s andM s in the Fe-Mn system has been performed by using two complementary experimental techniques,viz., dilatometry and electrical resistivity measurements, which are applied to the whole composition range where the transformation can be detected,i.e., between 10 and 30 pct Mn. We used theA s andM s temperatures as input information in an analysis based on thermodynamic models for the Gibbs energy of the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. In these models, the magnetic contribution to Gibbs energy is accounted for, which allows us to study, by calculation, the influence of the entropy of magnetic ordering upon the relative stability of the phases. The picture of magnetic effects upon the fcc/hcp transformation that emerges from our work is as follows. At low Mn contents, the martensitic transformation temperatures are larger than the Neel temperature of the fcc phase, and bothA s andM s decrease linearly with increasing Mn. This encourages an extrapolation to zero Mn content, and we use that to critically discuss the available information on the fcc/hcp equilibrium temperature for Fe at atmospheric pressure. At sufficiently large Mn contents, we haveM s <T N y , which implies that the fcc phase orders antiferromagnetically before transforming to the hcp phase. Since hcp remains paramagnetic down to lower temperatures, the ordering reaction in fcc leads to a relative stabilization of this phase, which is reflected in a drastic, nonlinear decrease ofM s.

Effects of laser-shock processing on the microstructure and surface mechanical properties of hadfield manganese steel

The effects of laser-shock processing (LSP) on the microstructure, hardness, and residual stress of Hadfield manganese (1 pct C and 14 pct Mn) steels were studied. Laser-shock processing was performed using a Nd: glass phosphate laser with 600 ps pulse width and up to 120 J/pulse energy at power density above 1012 W/cm2. The effects of cold rolling and shot peening were also studied for comparison. Laser-shock processing caused extensive formation of e hexagonal close-packed (hep) martensite (35 vol pct), producing up to a 130 pct increase of surface hardness. The surface hardness increase was 40 to 60 pct for the shot-peened specimen and about 60 pct for the cold-rolled specimen. The LSP strengthening effect on Hadfield steel was attributed to the combined effects of the partial dislocation/stacking fault arrays and the grain refinement due to the presence of the e-hcp martensite. For the cold-rolled and shot-peened specimens, the strengthening was a result of e-hcp martensite and twins with dislocation effects, respectively. Shot peening resulted in a relatively higher compressive residual stress throughout the specimen than LSP.

Gravity segregation of complex intermetallic compounds in liquid aluminum-silicon alloys

Primary crystals of intermetallics that are rich in iron, manganese, and chromium form at temperatures above the liquidus, and because their density is higher than that of liquid alumi-num, they cause gravity segregation in the melt. Segregation may occur either in the mold at slow cooling rates or in the bulk liquid in furnaces or ladles. The kinetics of settling of these intermetallic compounds in a melt of Al-12.5 pct Si having 1.2 pct Fe, 0.3 pct Mn, and 0.1 pct Cr has been studied. Sedimentation was investigated at 630 ‡C for settling times of 30, 90, and 180 minutes in an electric resistance furnace. The effect of settling time and height of melt on the volume percent, number, and size of intermetallic compounds was studied by image anal-ysis. The volume percent of intermetallics increases with distance from the melt surface. Both the number of particles and the average size increase during sedimentation. The rate of settling varies with position in the melt due to depletion of intermetallics near the surface and an increase near the bottom. The settling velocities obtained experimentally were compared with terminal velocities calculated by Stokes’ law. Good agreement was generally found. The settling speed of intermetallics reaches the terminal velocity at very short times and very close to the liquid surface. Stokes’ law is therefore applicable to virtually all locations within the melt.

Kinetics of oxidation of carbon in liquid iron-carbon-silicon-manganese-sulfur alloys by carbon dioxide in nitrogen

The oxidation of carbon with the simultaneous oxidation of silicon, manganese, and iron of liquid alloys by carbon dioxide in nitrogen and the absorption of oxygen by the alloys from the gas were studied using 1-g liquid iron droplets levitated in a stream of the gas at 1575 °C to 1715 °C. Oxidation of carbon was favored over oxidation of silicon and manganese when cast iron (3.35 pct C, 2.0 pct Si, 0.36 pct Mn, and 0.05 pct S) reacted with CO2/N2 gas at 1635 °C. An increase in the flow rate of CO2/N2 gas increased the decarburization rate of cast iron. The rate of carbon oxidation by this gas mixture was found to be independent of temperature and alloying element concentrations (in the range of silicon = 0 to 2.0 pct manganese = 0 to 0.36 pct and sulfur = 0 to 0.5 pct) within the temperature range of the present study. Based on the results of a kinetic analysis, diffusion of CO2 in the boundary layer of the gas phase was found to be the rate-limiting step for the reactions during the earlier period of the reaction when the contents of carbon, silicon, and manganese are higher. However, the limiting step changed to diffusion of the elements in the metal phase during the middle period of the reaction and then to the diffusion of CO in the gas phase during the later period of the reaction when the content of the elements in the metal were relatively low. For the simultaneous oxidation reactions of several elements in the metal, however, the diffusion of CO2 in the gas phase is the primary limiting step of the reaction rate for the oxidation of carbon during the later period of reaction.

Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M5C2 carbides, although some were M3C. The M5C2 carbide, also known as the Hagg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M5C2 rods appeared to nucleate heterogeneously on M2C carbides and persist in ferrite regions from which the needlelike M2C carbides had disappeared. This suggests that the M5C2 carbide is more stable thermodynamically than M2C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.

Simulation of the hot rolling and accelerated cooling of a C-Mn ferrite-bainite strip steel

By means of torsion testing, the microstructures and mechanical properties produced in a 0.14 Pct C-1.18 Pct Mn steel were investigated over a wide range of hot-rolling conditions, cooling rates, and simulated coiling temperatures. The austenite grain size present before accelerated cooling was varied from 10 to 150 μm by applying strains of 0 to 0.8 at temperatures of 850 °C to 1050 °C. Two cooling rates, 55 °C/s and 90 °C/s, were used. Cooling was interrupted at temperatures ranging from 550 °C to 300 °C. Optical microscopy and transmission electron microscopy (TEM) were employed to investigate the microstructures. The mechanical properties were studied by means of tensile testing. When a fine austenite grain size was present before cooling and a high cooling rate (90 °C/s) was used, the microstructure was composed of ferrite plus bainite and a mixture of ferrite and cementite, which may have formed by an interphase mechanism. The use of a lower cooling rate (55 °C/s) led to the presence of ferrite and fine pearlite. In both cases, the cooling interruption temperature and the amount of prior strain had little influence on the mechanical properties. Reheating at 1050 °C, which led to the presence of very coarse austenite, resulted in a stronger influence of the interruption temperature. A method developed at Institut de Recherche Siderurgique (IRSID, St. Germain-en-Laye, France) for deducing the Continuous-Cooling-Transformation (CCT) diagrams from the cooling data was adapted to the present apparatus and used successfully to interpret the observed influence of the process parameters.

Behavior of steels near the incipient melting temperature

A new method of incipient melting temperature (IMT) detection has been developed in which a constant-strain-rate tensile deformation is applied to a specimen whose temperature is simultaneously increasing. The IMT is determined in a single test, and any phase transformations before the IMT will also be detected by the effects on the stress vs strain behavior in the same experiment. By means of such tests, the incipient melting behavior of a series of steels with carbon levels from 0.031 to 0.45 wt pct was examined. For the steels containing 0.08 to 0.097 pct C and about 1.5 pct Mn, it was found that incipient melting occurs in the two-phase (γ + δ) region in the temperature range from 1470 °C to 1480 °C and is significantly influenced by microalloying elements. In the ultralow-carbon steel (0.031 pct C), the IMT is in the single-phase δ region, and for the medium-carbon steel containing 0.42 pct C (hyperperitectic) it is in the γ single phase.

A CsCl-type phase in electrodeposited AlMn alloys

Since the work of Goedecke and Koester it has been known that the central part of the Al-Mn phase diagram is occupied at room temperature by the [gamma][sub 2] phase, while at high temperature two other phases, [gamma] and [gamma][sub 1], have been reported in the same compositional range. A more recent study has confirmed the existence of these phases. The low-temperature phase has a rhombohedrally distorted [gamma]-brass lattice of the Al[sub 8]Cr[sub 5]-type. The structures of the high-temperature phases have not been determined since they could not be retained in alloys quenched to room temperature due to their high transformation rates. Only recently, the structure of the high-temperature [gamma]-phase was studied in-situ by Ellner using x-ray diffraction. According to this study, [gamma] has a bcc structure (W-type) with a lattice parameter of 0.3063 nm. The structure of the second high-temperature [gamma]1 phase has not been determined directly due to technical problems associated with its higher formation temperature; however, it has been suggested to be of the cubic [gamma]-brass type. The previous study of electrodeposited Al-Mn alloys with compositions ranging up to 50 atomic pct Mn revealed a phase exhibiting an x-ray diffraction pattern which was consistent with a CsCl-typemore » structure with a lattice parameter of 0.299 nm. The compositions of the alloys containing this phase ranged from 36 to 43 atomic pct Mn. At lower Mn concentrations, the electrodeposits contained the cubic [gamma]-brass phase described earlier while at higher Mn concentrations, metastable [tau] (CuAu-type) was observed. In this paper the authors confirm the presence of a CsCl-type phase in electrodeposited Al-Mn alloys and associate it with the [gamma]-phase reported by Ellner.« less

Chemical composition and structural identification of eutectic carbide in 1 pct Mn ductile iron

Manganese, which may be used in ductile cast iron as a potent hardenability promoter, segregates in the intercellular region. This segregation becomes more severe as a consequence of poor inoculation, low cooling rate, or increasing of nominal Mn content in the alloy. In severely Mn-segregated regions, Mn eutectic carbide may be formed, which has a deteriorating effect on the mechanical properties of casting. In this study, a 1 Pct Mn ductile iron was used to investigate the chemical composition and crystal structure of the Mn eutectic carbide by electron microscopy (scanning and transmission), X-ray, and an electron probe microanalyzer (EPMA). Transmission electron microscope (TEM) and X-ray studies show that the crystal structure of carbide is orthorhombic with lattice parameters ofa = 14.825,b = 11.415, andc = 8.880 (A). The concentrations of Mn, Si, and Cr in carbide, analyzed by scanning electron microscope-energy-dispersive X-ray spectrometer (SEM-EDX) and transmission electron microscope-energy-dispersive X-ray spectrometer (TEM-EDX), were 5.0 to 7.0, 0.5 to 2.8 and 1.5 to 2.2 (wt Pct), respectively. The ratio of Fe plus other metal atoms to C was calculated from EPMA experiments to be 2.5-2.9. It was shown that by diminishing Mn segregation, precipitated eutectic carbide can be reduced. It is expected that this can be achieved by reducing nominal content of Mn or by increasing nodule count.

Effects of austenite grain size and cooling rate on Widmanstätten ferrite formation in low-alloy steels

Deformation dilatometry is used to simulate the hot rolling of 0.20 pct C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.06 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 µm. In general, the volume fraction of WidmanstAtten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size having the more significant effect. The Nb steel has the lowest transformation temperature range and the greatest propensity for WidmanstAtten ferrite formation, while the amount of WidmanstAtten ferrite is minimized in the Ti steel (as a result of intragranular nucleation of polygonal ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a coarse WidmanstAtten structure. With a sufficiently fine prior austenite grain size (e.g., ≤30 µm), significant amounts of WidmanstAtten structure can be avoided, even in a Nb-alloyed steel.

Effects of Widmanstätten ferrite on the mechanical properties of a 0.2 pct C-0.7 pct Mn steel

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925 °C or 1150 °C to produce nominal austenite grain sizes of 60 and 200 µm, resspectively. The plates were then cooled at rates in the range of about 2 °C/min to 400 °C/min to produce mixed polygonal ferrite/Widmanstatten ferrite/pearlite microstructures. The percentage of WidmanstAtten structure (a Widmanstatten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and WidmanstAtten structure. Both yield and tensile strength increase with an increase in the volume fraction of WidmanstAtten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/WidmanstAtten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.

Effect of microstructure on the corrosion and deformation behavior of a newly developed 6Mn-5Cr-1.5Cu corrosion-resistant white iron

An experimental study has been made of the effect of heat treatment on the transformation behavior of a 4.8 pct Cr white iron, alloyed with 6 pct Mn and 1.5 pct Cu, by employing optical metallography, X-ray diffractometry, and differential thermal analysis (DTA) techniques, with a view to assess the suitability of the different microstructures in resisting aqueous corrosion. The matrix microstructure in the as-cast condition, comprising pearlite + bainite/martensite, transformed to austenite on heat-treating at all the temperatures between 900 °C and 1050 °C. Increasing the soaking period at each of the heat-treating temperatures led to an increase in the volume fraction and stability of austenite. M3C was the dominant carbide present in the as-cast condition. On heat-treating, different carbides formed: M23C6 carbide was present on heat-treating at 900 °C and 950 °C; on heat-treating at 1000 °C, M7C3 formed and persisted even on heattreating at 1050 °C. The possible formation of M5C2 carbide in the as-cast and heat-treated conditions (900 °C and 950 °C) is also indicated. Dispersed carbides (DC), present in austenite up to 950 °C, mostly comprised M3C and M5C2. On stress relieving of the heat-treated samples, M7C3-type DC also formed. The hardness changes were found to be consistent with the micro-structural changes occurring on heat-treating. The as-cast state was characterized by a reasonable resistance to corrosion in 5 pct NaCl solution. On heat-treating, the corrosion resistance improved over that in the as-cast state. After 4 hours soaking, increasing the temperature from 900 °C to 1050 °C led to an improvement in corrosion resistance. However, after 10 hours soaking, corrosion resistance decreased on increasing the temperature from 900 °C to 950 °C and improved thereafter on increasing the heat-treating temperature. Deformation behavior responded to the microstructure on similar lines as the corrosion behavior. Although in an early stage of development, the composition thus developed betters the performance of 22 pct Ni containing Ni-Resist irons as far as strength and freedom from pitting and graphitic corrosion are concerned; however, the corrosion resistance is somewhat lower. In conclusion, the usefulness of the different microstructures in attaining a useful combination of corrosion resistance and deformation behavior has been assessed. The data thus generated provide definite clues to developing new materials with improved performance for resisting aqueous corrosion in marine environments.

Microstructure and ordering of L12 titanium trialuminides

The present article reports and discusses the results of the microstructural characterization of various modifications of Ll2 trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll2 trialuminide alloys Al3Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is 67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization increases about twofold, except in the Al3Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al3Ti + Cu alloy. The homogenized Al3Ti + Cr and Mn alloys have greater lattice parameters than the Al3Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll2 Al3Ti + X alloys dramatically decreases with increasing titanium content.

The crystallography of bainite in a medium-carbon steel containing Si, Mn, and Mo

A composite microstructure consisting of upper bainite laths and lower bainite plates, both carbide-free, plate (twinned) martensite, and carbon-enriched retained austenite, was produced by air-cooling a medium-carbon alloy steel (0.55 pct C, 1.35 pct Si, 0.78 pct Mn, 0.45 pct Mo) from 900 °C. Well-defined midribs and subunits were found to be associated with both the upper bainite laths and lower bainite plates, clearly showing that the two kinds of bainite growvia a sympathetic nucleation and growth process. The orientation relationship between the bainite and austenite, as determined by electron diffraction, showed that the close-packed planes in the two phases were separated by 0.5 deg and the close-packed directions were 1.9 apart. The habit plane determined from the midrib was (5 127)f, about 20 deg away from the nearly parallel close-packed planes (111)f/(011)b. The experimentally determined orientation relation-ship and habit plane (as defined by the midrib) were in good agreement with the predictions of the phenomenological theory of martensite crystallography.

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation, microstructure, and room-temperature mechanical properties

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part II. effect of testing temperature on stress-strain behavior and deformation-induced austenite transformation

Stress-strain behavior and deformation-induced transformation of retained austenite were studied for intercritically annealed and isothermally transformed Si-Mn and Ni steels as a function of testing temperature between −80 °C and 120 °C. Rapid decrease of retained austenite at small strains dominates at low-temperature testing and in microstructures containing martensite. The austenite transformation in microstructures without martensite shifts to larger strains with increasing testing temperature. The accompanying increase of strain-hardening rates at larger strains deters the onset of necking and improves ductility. The benefits of the austenite transformation lead to a peak in ductility between 20 °C and 70 °C in the Si-Mn steel and at 70 °C in the Ni steel. The peaks are dependent on the nature of the dispersed microconstituents produced in the ferrite during isothermal transformation. Higher testing temperatures enhance the mechanical stability of the austenite and result in lower ductility.