Pct Ferrite Research Articles

Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades.

Effect of Tempering on the Microstructure and Tensile Properties of a Martensitic Medium-Mn Lightweight Steel

In the present study a Fe-8.8Mn-5.1Al-0.31C (wt pct) medium-Mn lightweight steel with martensitic matrix was newly designed, and the effect of tempering on its microstructure and tensile properties was investigated. When the medium-Mn lightweight steel specimen was quenched from annealing temperature of 1373 K (1100 °C), it revealed a triple-phase microstructure consisting of 64.6 pct fresh martensite, 20.2 pct ferrite, and 15.2 pct retained austenite. Tempering the as-quenched specimen at temperatures less than 573 K (300 °C) induced the precipitation of thin cementite or κ-carbide platelets in tempered martensite. With the increasing tempering temperature, the type of precipitate changed from cementite to κ-carbide, and thickening of platelets with the longer interspacing was observed by means of transmission electron microscopy and atom probe tomography. While the as-quenched specimen revealed high strain-hardening rate (SHR) and poor elongation ( 16.8 pct) mainly due to gradual transformation-induced plasticity in retained austenite stabilized by tempering. In particular, the 373 K (100 °C)-tempered specimen with tempered martensite embedded with thin cementite platelets revealed ultrahigh strengths (yield strength of 945 MPa and ultimate tensile strength of ~ 1.56 GPa) with moderate elongation of ~ 16.8 pct.

Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings

Six different composition steels were designed based on CD3MWCuN, a common industrial alloy formulated to have a microstructure consisting of 50 pct ferrite and 50 pct austenite. The steels were designed to have ± 10 pct Cr, Ni, and Mo over the nominal composition of CD3MWCuN and were obtained by remelting a keel bar of the base alloy and adjusting the composition. The + compositions variations were achieved simply by adding additional Cr, Ni, or Mo, while the − variations were obtained by adding additional amounts of the remaining major elements to the nominal composition of the keel bar. In this way it was possible to produce a series of alloys where the Cr/Ni equivalent ratio, Creq/Nieq, as calculated using the equations first employed by Schoffler varied from ≈ 1.35 to 1.65. A series of homogenization temperatures ranging from 1000 °C to 1250 °C were used to produce a uniform starting microstructure and the alloys were cooled at rates ranging from 500 °C to 1000 °C/hour from these homogenization temperatures. Air-cooled and water-quenched samples were examined as well. Based on observations of the microstructure a generalized ferrite prediction diagram was developed that takes into account homogenization temperature, cooling rate, and chromium-to-nickel equivalence ratio (Creq/Nieq) to predict the amount of ferrite that can be expected in the alloy. This diagram represents the first major evaluation of ferrite content as a function of homogenization temperature and cooling rate specifically for cast duplex alloys.

Microstructural Evolution of an Al-Alloyed Duplex Stainless Steel During Tensile Deformation Between 77 K and 473 K (−196 °C and 200 °C)

Tensile deformation behavior of an Al-alloyed Fe-17Cr-6Mn-4Al-3Ni-0.45C (mass pct) duplex stainless steel containing approximately 20 vol pct ferrite was studied in the temperature range from 77 K to 473 K (−196 °C to 200 °C). While the elongation exhibited a maximum near room temperature, the yield strength continuously increased at lower tensile test temperatures. According to the microstructural examinations, the twinning-induced plasticity and the dislocation cell formation were the dominant deformation mechanisms in the austenite and ferrite, respectively. Reduction of the tensile ductility at T < 273 K (0 °C) was attributed to the ready material decohesion at the ferrite/austenite boundaries. Tensile testing at 473 K (200 °C) was associated with the serrated flow which was ascribed to the Portevin–Le Chatelier effect. Due to a rise in the stacking fault energy of austenite, the occurrence of mechanical twinning was impeded at higher tensile test temperatures. Furthermore, the evolution of microstructural constituents at room temperature was studied by interrupted tensile tests. The deformation in the austenite phase started with the formation of Taylor lattices followed by mechanical twinning at higher strains/stresses. In the ferrite phase, on the other hand, the formation of dislocation cells, cell refinement, and microbands formation occurred in sequence during deformation. Microhardness evolution of ferrite and austenite in the interrupted tensile test specimens implied a higher strain-hardening rate for the austenite as it clearly became the harder phase at higher tensile strain levels.

Steel Microstructure Effect on Mechanical Properties and Corrosion Behavior of High Strength Low Carbon Steel

Different thermomechanical treatments were applied to a high strength low carbon steel with a novel chemical composition. As a result, three different microstructures were produced with dissimilar mechanical and corrosion properties. Subsequently, a tempering heat treatment was applied to redistribute the phases in the steel. Microstructure A with 56 pct martensite and 32 pct bainite presented high strength but medium ductility; microstructure C with 95 pct ferrite and 3 pct martensite/austenite resulted in low strength and high ductility, and finally microstructure B with 98 pct bainite and 2 pct martensite/austenite resulted in high strength and ductility. Alternatively the corrosion behavior obtained by polarization curves was characterized in 0.1 M H2SO4, 3 M H2SO4, 3.5 wt pct NaCl, and NS4 solutions resulting in similar magnitudes, while the corrosion behavior acquired by electrochemical impedance spectroscopy had slightly differences in 3 M H2SO4.

Microstructures and Mechanical Properties of a New As-Hot-Rolled High-Strength DP Steel Subjected to Different Cooling Schedules

Controlled rolling followed by accelerated cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon dual-phase steel. The objective of the study described here was to explore the effect of cooling schedule, such as air cooling temperature and coiling temperature, on the final microstructure and mechanical properties of dual-phase steels. Furthermore, the precipitation behavior and yield ratio are discussed. The study demonstrates that it is possible to obtain tensile strength and elongation of 780 MPa and 22 pct, respectively, at the two cooling schedules investigated. The microstructure consists of 90 pct ferrite and 10 pct martensite when subjected to moderate air cooling and low temperature coiling, such that the yield ratio is a low 0.69. The microstructure consists of 75 pct ferrite and 25 pct granular bainite with a high yield ratio of 0.84 when the steel is directly cooled to the coiling temperature. Compared to the conventional dual-phase steels, the high yield strength is attributed to precipitation hardening induced by nanoscale TiC particles and solid solution strengthening by high Si content. The interphase precipitates form at a suitable ledge mobility, and the row spacing changes with the rate of ferrite transformation. There are different orientations of the rows in the same grain because of the different growth directions of the ferrite grain boundaries, and the interface of the two colonies is devoid of precipitates because of the competitive mechanisms of the two orientations.

Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.

Paraequilibrium Carburization of Duplex and Ferritic Stainless Steels

AISI 301 and E-BRITE stainless steels were subjected to low-temperature (743 K) carburization experiments using a commercial technology developed for carburization of 316 austenitic stainless steels. The AISI 301 steel contained ~40 vol pct ferrite before carburization but had a fully austenitic hardened case, ~20-μm thick, and a surface carbon concentration of ~8 at. pct after treatment; this “colossal” paraequilibrium carbon supersaturation caused an increase in lattice parameter of ~3 pct. The E-BRITE also developed a hardened case, 12- to 18-μm thick, but underwent a more modest (~0.3 pct) increase in lattice parameter; the surface carbon concentration was ~10 at. pct. While the hardened case on the AISI 301 stainless steel appeared to be single-phase austenite, evidence for carbide formation was apparent in X-ray diffractometer (XRD) scans of the E-BRITE. Paraequilibrium phase diagrams were calculated for both AISI 301 and E-BRITE stainless steels using a CALPHAD compound energy-based interstitial solid solution model. In the low-temperature regime of interest, and based upon measured paraequilibrium carbon solubilities, more negative Cr-carbon interaction parameters for austenite than those in the current CALPHAD data base may be appropriate. A sensitivity analysis involving Cr-carbon interaction parameters for ferrite found a strong dependence of carbon solubility on relatively small changes in the magnitude of these parameters.

Phase stability and atom probe field ion microscopy of type 308 cre stainless steel weld metal

Improvement in high-temperature creep-rupture properties of type 308 stainless steel welds due to the controlled addition of boron is related to microstructural evolution during welding and thermal phase stability at creep service temperatures. The microstructure of boron-containing type 308 austenitic stainless steel welds, in the as-welded state, consisted of 8 to 10 pct ferrite in an austenite matrix. Atom probe field ion microscopy studies revealed segregation of boron and carbon to ferriteaustenite boundaries in the as-welded state; the segregation level was less than one monolayer coverage. On aging at 923 K for 100 hours, M23C6 carbides precipitated at ferrite-austenite boundaries. On further aging at 923 K for 1000 hours, the ferrite transformed into σ phase. Similar microstructural evolution was observed in a type 308 stainless steel weld without boron addition. The volume fractions of M23C6 carbides were identical in boron-containing and boron-free welds. Atom probe results from the welds with boron addition in the aged condition showed that the boron dissolved in the M23C6 carbides. However, lattice parameter analysis showed no apparent difference in the extracted carbides from the welds with and without boron. Creep property improvement due to boron addition could not be related to any change in the volume fraction of carbides. However, the results suggest that the incorporation of boron into M23C6 carbides may reduce the tendency for cavity formation along the M23C6 carbide-austenite boundaries and hence improve the resistance to creep fracture. The observed microstructural evolution in welds is consistent with thermodynamic calculations by THERMOCALC software.

The effects of microstructure, strength level, and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.

Influence of long- term aging and superimposed creep Stress on the Microstructure of 0.50Cr-0.50Mo-0.25V Steel

In an effort to understand the effects of long-term aging at high temperatures and those of the superimposed creep stresses on the microstructural variations in a 0.50Cr-0.50Mo-0.25V steel, the shoulder and gage portions of the specimens subjected to stress rupture tests at 500 °C and 540 °C have been studied by transmission electron microscopy. The optical microstructure, in the normalized and tempered condition, consists of about 95 pct ferrite with the remainder bainite. The ferrite has an extremely fine precipitation of vanadium carbide (VC) as interphase precipitate as well as in random distribution. In the bainitic area, M3C is present in the globular as well as plate-like morphology. Specimens ruptured at 500 °C and 540 °C were selected for transmission electron microscopic study. Both the interphase precipitates and randomly distrib- uted precipitates of VC coarsen considerably due to prolonged exposure at elevated tempera- tures, and ultimately fine M2C platelets nucleate and grow on either side of these precipitates, resulting in the formation of the so-called H-carbides. The superimposed creep stress has a marginal effect on the coarsening kinetics of VC but has enhanced its precipitation along dis- locations and has promoted the formation of H-carbides.

Influence of long-term aging at 520 °C and 560 °C and the superimposed creep stress on the microstructure of 1.25Cr-0.5Mo steel

In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at 520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure of the steel consists of 90 pct ferrite and 10 pct bainite, and M3C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr2N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation. On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo2C, while the superimposed creep stress has promoted the precipitation of Mo2C needles with fine globular precipitates of M23C6. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo2C needles and ellipsoidal M23C6 carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible.

Hydrogen compatibility of Femnal alloys

Five Femnal alloys with various ferrite/ austenite were prepared by mainly adjusting the carbon content. Hydrogen compatibility of these alloys was investigated. A slow extension rate tensile test in one atmosphere of hydrogen gas showed that hydrogen embrittlement (HE) increased with the content of ferrite in the alloys. The alloy with a full austenite phase was strong, ductile, and essentially immune to HE, whereas the one with full ferrite was weaker, very brittle, and suffered serious damage in hydrogen. A sustained load test was further conducted in hydrogen gas at 25°C to 75 °C for the alloys with 10, 35, and 65 pct ferrite. No subcritical crack growth occurred in the alloy with 10% ferrite. Typical three-stage environment-assisted crack growth was observed for the alloys with 35 and 65 pct ferrite. A kinetics study of the subcritical crack growth indicated that the cracking rate was basically controlled by hydrogen transport through the ferrite grains. Austenite phase could be embrittled provided sufficient hydrogen was dissolved. To reduce HE in Femnal alloys, the content of ferrite should be reduced; this can be achieved mainly by controlling the carbon content in the alloys.

Orowan strengthening by Mo2C fibers and needle interphase precipitates in Fe-C-Mo Dual-Phase steels

Two unique morphologies of Mo/sub 2/C are attainable through isothermal heat treatments of Fe-C-Mo steels. Specifically, both a fibrous carbide structure and an interphase needle carbide structure (i.e., the needles are arranged in layers or sheets) can be produced. The monotonic behavior of an Fe-0.2C-4Mo alloy has shown that the dense carbides contribute significantly to the total yield strength in these microstructures. This study considers the strengthening of these two different carbide distributions and morphologies on the basis of Orowan strengthening. This allows for the evaluation of Orowan strengthening predictions for the two different Mo/sub 2/C precipitate distributions. Two alloys, nominally Fe-0.2C-4Mo and Fe-0.2C-2Mo were used. The alloys were encapsulated in quartz and homogenized for 4 days at 1200/sup 0/C. The final heat treatment consisted of austenization for 30 minutes at 1250/sup 0/C, and quenching into nitrate salt at 625 or 725/sup 0/C for the times specified for each alloy. This produced dual-phase microstructures consisting of 60 pct ferrite and Mo/sub 2/C surrounding equiaxed martensite (40 pct) packets.

Effect of ferrite content on the creep strength of 12Cr-2Mo-0.08C boiler tubing steels

The effect of ferrite content on the creep of a 12Cr-2Mo-0.08C, ferritemartensite, heatresisting steel was investigated. Creep tests were performed at 550, 600, and 650 °C for up to 10,000 hours on four steels containing, respectively, 51, 38, 23, and 12 pct ferrite. The variation in ferrite content was obtained through nickel additions of 0.6, 1.3, and 2.0 pct. Rupture time, minimum creep rate, elongation, and reduction in area are reported. The effect of ferrite content was found to be small but significant. At short rupture times, increasing ferrite content reduces creep strength. At long exposure times, however, increasing ferrite content increases creep strength. This effect is explained by the precipitation of Laves phase, Fe2Mo, and possibly Cr2N, in the ferrite during testing. This precipitation progressively strengthens the ferrite relative to martensite. In addition, martensite is continually weakening because of the coarsening of M23C6, the only precipitate found in the martensite. Nickel at the 2.0 pct level radically decreased creep resistance in these steels by accelerating aging of the precipitate structure in martensite.

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo, and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M23C6 carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.

Effect of Sb, P, Sn and B on the microstructure and creep properties of normalized and tempered 1.25 Cr−0.5 Mo Steels

A program to study the effect of Sb, P, Sn and B on creep properties of four normalized and tempered 1.25 Cr−0.5 Mo steels at 538°C (1000°F) has been completed. Results show that even a combined addition of large amounts of Sb, P and Sn does not affect short time creep strength or ductility of the steel at 538°C (1000°F). Addition of B resulted in an increase or decrease of creep strength depending on the nature of the impurity species present, presumably due to B-impurity interactions. Regardless of the effect on creep strength, B additions caused sharp reductions in rupture ductility in all cases. Comparison of the present results on the four laboratory steels (100 pct bainite) with results of a previous study on a commercial steel (60 pct bainite + 40 pct ferrite) show that the effect of microstructure becomes negligible and rupture strength values of the various steels at 538°C (1000°F) approach each other at rupture times in excess of 104 h.

Ferrite formation in neutron irradiated type 316 stainless steel

Magnetic measurements were made on Type 316 stainless steel specimens that had been neutron irradiated to fluences of 1.8 × 1022 neutrons/cm2 at 425°C and 3.5 × 1022 neutrons/ cm2 (E > 0.1 MeV) at 500 and 600°C. A significant increase of magnetization was observed for the irradiated specimens compared to the unirradiated specimens. The shape of the magnetization vs field curves showed that the irradiated specimens contained many small superparamagnetic particles. The magnetic particles are assumed to be the ferrite phase although other possibilities cannot be excluded. The amount and distribution of the magnetic phase varied with pre-irradiation and post-irradiation heat treatment. The maximum value of magnetization was equivalent to 3.6 pct ferrite in a specimen annealed 100 h at 760°C before irradiation and 1 h at 500°C after irradiation at 425°C.

The influence of martensite and ferrite on the properties of two-phase stainless steels having microduplex structures

The mechanical properties of a series of stainless steels ranging in composition from 16.5 pct Cr, 5.5 pct Ni to 23.9 pct Cr, 2.9 pct Ni have been determined. The series of alloys lie along an approximate 1700°F tie line with room temperature microstructures ranging from 100 pct martensite to 100 pct ferrite. Yield and tensile strengths increased directly with increasing martensite content. In alloys containing on the order of 40 to 60 pet martensite, the presence of a fine dispersion of tougher, albeit stronger, martensite was quite effective in lowering the ductile-to-brittle impact transition temperature.