Modelling of austenite stability in low-alloy triple-phase steels

A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels.     

The effect of reverted austenite on the mechanical properties and toughness of 12 Ni and 18 Ni (200) maraging steels

A study has been made of the effects of reverted austenite on the mechanical properties and toughness of three maraging steels. It is found that reverted austenite has no detrimental effects on the mechanical properties and toughness and even improves these properties when precipitated along martensite lath boundaries. This occurs for underaged specimens. A detrimental effect on toughness is found when reverted austenite precipitates at prior austenitic grain boundaries which occurs for over aged specimens. Overaged precipitates are also responsible for the decrease in toughness in the overaged condition. C. A. PAMPILLO, formerly with the Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, Pa. H. W. PAXTON, on leave from Carnegie-Mellon University     

The mechanical stability of precipitated austenite in 9Ni steel

The strains inherent to the martensitic transformation of austenite particles in 9Ni steel create dislocation structures in the tempered martensite. These dislocation structures were studied by the complementary techniques of X-ray line profile analysis and transmission electron microscopy. The energy required to form these dislocation structures affects the thermodynamics of the transformation. We propose that changes in these dislocation structures reduce the “mechanical stability” of the austenite particles as they grow larger during isothermal tempering.     

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low-alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple-phase high-strength high-formability low-alloy steels.     

Low‐alloy TRIP steels: a correlation between mechanical properties and the retained austenite stability

In recent years the technology of low‐alloy TRIP steels has considerably advanced. The mechanical properties are characterised by a combination of high yield strength and high uniform elongation as well as enhanced formability. In the present work an effort to correlate mechanical properties with the retained austenite stability was made. Two low‐alloy TRIP steels were investigated. The first of them represents a typical composition of the low‐alloy TRIP steels, while the other one contains aluminum as alloying element. The influence of the heat treatment on the mechanical properties and especially on the amount and stability of the retained austenite was determined. The retained austenite stability was measured with a single specimen technique, in which a tensile specimen was used to determine the M^σ_S temperature with a loading‐unloading procedure. The results showed that there is a strong influence of the stability of the retained austenite on the mechanical properties. Increased stability combined with a high amount of retained austenite, exhibited an increase in both, yield strength and uniform elongation while increased amount of retained austenite with low stability did not show the same good combination of mechanical properties. The results clearly indicate that in order to get the maximum TRIP effect, a good combination of austenite stability and amount is required.     

On the influence of interactions between phases on the mechanical stability of retained austenite in transformation-induced plasticity multiphase steels

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M _s temperature induced by carbon enrichment.     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

The morphology and substructure of martensite in maraging steels

Thin foil transmission electron microscopy, X-ray diffraction and dilatometric techniques have been used to study the martensitic γ → α transformation in three steels with nominal contents of 8 pct nickel and 0.2 pct beryllium and chromium contents of 12, 14 and 16 pct. In each case the martensite formed as laths with a habit plane close to {225}_γ. With increasing chromium content and increasing cooling rate greater numbers of the laths were observed to be internally twinned. Detailed analysis of the martensitic transformation suggested that the internally twinned laths are formed by a sequence of γ→ ε or faulted γ→ ά. The orientation relationships between the three phases γ, ε and α, determined from selected area diffraction analysis, corresponded to Kurdjumov-Sachs.     

The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed. Formerly Graduate Student, Seoul National University.     

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M^σ_s temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M^σ_S temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.     

Effects of strain states on stability of retained austenite in medium Mn steels

Based on uniaxial tensile and plane strain deformation tests,the effects of strain states on the stability of RA(retained austenite)in medium Mn steels,which were subjected to IA(intercritical annealing)and QP(quenching and partitioning)processing,were investigated.The volume fractions of RA before and after deformation were measured at different equivalent strains.The transformation behaviors of RA were also investigated.The stability of RA differed across two different transformation stages at the plane strain state:the stability was much lower in the first stage than in the second stage.For the uniaxial tension strain state,the stability of RA corresponded only to a single transformation stage.The main reason was that there were two types of transformations from RA in the medium Mn steel for the plane strain state.One type was that the martensite originated in the strain-induced stacking faults(SISF).The other type was the strain-induced directly twin martensite at a certain equivalent strain.However,for the uniaxial tension state,only the strain-induced twin martensite was observed.Dislocation lines and dislocation tangles were also observed in specimens deformed at different strain states.In addition,complex microstructures of stacking faults and lath-like phases were observed within a grain at the plane strain state.     

Correlation of isothermal bainite transformation and austenite stability in quenching and partitioning steels

The possible decomposition of metastable austenite during the partitioning process in the highend quenching and partitioning(QP)steels is somewhat neglected by most researchers.The effects of primary martensite and alloying elements including manganese,cobalt and aluminum on the isothermal decomposition of austenite during typical QP process were studied by dilatometry.The transformation kinetics was studied systematically and resulting microstructures were discussed in details.The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries.This effect can be eliminated by a flash heating which recovered dislocations.Co addition significantly promoted the bainite transformation during partitioning while Al and Mn suppressed the isothermal bainite transformation.The bainite transformation played an important role in carbon distribution during partitioning,and hence the amount and stability of austenite upon final quenching.The bainite transformation during partitioning is an important factor in optimizing the microstructure in QP steels.     

The isothermal decomposition of austenite in hot-rolled microalloyed steels

The isothermal decomposition of austenite has been examined in a set of 0.1 C, 1.4 Mn steels containing small amounts of Ti, V, or Nb. The volume fraction of ferrite was measured as a function of transformation temperature and holding time, after hot rolling. Precipitation of carbonitrides, in both the austenite and the ferrite, was examined by electron microscopy of extraction replicas. The decomposition is slowest in the Nb-alloyed steel, in which the start of transformation is delayed and ferrite growth rates are much lower than in the other steels. In the V-alloyed steels, ferrite growth rates are lower than in the plain carbon or Ti alloyed steels. These results are discussed in terms of the effects of carbonitride precipitation in the austenite during high temperature deformation and in the ferrite during transformation. The roles of V and Nb in solution are also considered.     

Precipitation of TiC in thermally embrittled maraging steels

It is shown that maraging steels can be embrittled by the precipitation of TiC during slow cooling and/or intermediate annealing in the austenite temperature range. An important aspect in this embrittlement is the occurrence of lamellar precipitation of TiC at the austenite grain boundaries, generating a cellular structure of large fern leaf-like carbides. Within the austenite grains a nonuniform distribution of irregularly plate-shaped TiC particles are formed with (100) austenite habit orientation. Quenching to martensite, prior to any intermediate anneals, changes the carbide distribution upon subsequent annealing treatments into a fine dispersion of TiC particles. The embrittlement resulting from the various isothermal annealing treatments in the austenite temperature region could all be directly related to the carbide distribution in the prior austenite grain boundary region.     

Transformation from austenite in alloy steels

This paper is concerned with the direct transformation of austenite at high temperatures to form ferrite and alloy carbide dispersions. The ferrite/austenite interfaces vary from high energy random boundaries to low energy planar boundaries which grow by step propagation, while the alloy carbide morphologies include a pearlitic form, fine fibers and fine banded arrays of particles. It is shown that these morphologies are closely related to the mode of growth of the ferritic matrix. The role of various alloying elements on the carbide dispersion is examined, and the effects of other metallurgical variables on the banded dispersions are discussed, including factors which influence the dispersion stability. The mechanical properties of directly transformed alloy steels are shown to depend largely on the ferrite grain size and the state of the carbide dispersion. Micro-alloyed steels subjected to controlled rolling provide an excellent example of the achievement of high strength and toughness levels by control of these variables. The paper finally attempts to show how such benefits can be achieved in low and medium alloy steels, and in particular where resistance to creep failure at elevated temperatures is an important property.     

Precipitation of TiC in thermally embrittled maraging steels

It is shown that maraging steels can be embrittled by the precipitation of TiC during slow cooling and/or intermediate annealing in the austenite temperature range. An important aspect in this embrittlement is the occurrence of lamellar precipitation of TiC at the austenite grain boundaries, generating a cellular structure of large fern leaf-like carbides. Within the austenite grains a nonuniform distribution of irregularly plate-shaped TiC particles are formed with (100) austenite habit orientation. Quenching to martensite, prior to any intermediate anneals, changes the carbide distribution upon subsequent annealing treatments into a fine dispersion of TiC particles. The embrittlement resulting from the various isothermal annealing treatments in the austenite temperature region could all be directly related to the carbide distribution in the prior austenite grain boundary region.     

Transformation from austenite in alloy steels

This paper is concerned with the direct transformation of austenite at high temperatures to form ferrite and alloy carbide dispersions. The ferrite/austenite interfaces vary from high energy random boundaries to low energy planar boundaries which grow by step propagation, while the alloy carbide morphologies include a pearlitic form, fine fibers and fine banded arrays of particles. It is shown that these morphologies are closely related to the mode of growth of the ferritic matrix. The role of various alloying elements on the carbide dispersion is examined, and the effects of other metallurgical variables on the banded dispersions are discussed, including factors which influence the dispersion stability. The mechanical properties of directly transformed alloy steels are shown to depend largely on the ferrite grain size and the state of the carbide dispersion. Micro-alloyed steels subjected to controlled rolling provide an excellent example of the achievement of high strength and toughness levels by control of these variables. The paper finally attempts to show how such benefits can be achieved in low and medium alloy steels, and in particular where resistance to creep failure at elevated temperatures is an important property.     

Use of corrosion-resistant maraging steels in long-lived thread pieces

The effect of isothermal holdings at 350, 500, 580, 660, and 780°C during heating to various sustenitizing temperatures on the grain size, aging processes, structural and mechanical properties, and the stress-strain curves of 03Kh11N10M2T-VD and 03Kh11N8M2F-VD maraging steels with 0.002% B is studied. X-ray diffraction analysis and metallographic examination are performed, and the corrosion and fatigue characteristics of these steels are determined. At the aged state with the maximum strength, the steels exhibit no strain-hardening ability upon tension and retain a high local plastic deformation during necking (ψ ≤ 60%). Preliminary thermal-cycling treatment at 500–800°C causes grain refinement and increases the plastic properties of the steels (the uniform elongation increases to 20%). Isothermal holding during heating to the austenitizing temperature affects an elastoplastic transition at a low tensile strain.