Time Temperature Precipitation Research Articles

The Kinetics of Static Recrystallization in Microalloyed Hypereutectoid Steels

Compression tests were conducted in order to study the static recrystallization kinetics of hot deformed austenite in hypereutectoid steels containing 1 % carbon with different levels of vanadium and silicon. Tests were performed over a temperature range of 875 to 1 100°C using strain rates of 0.01, 0.1 and 1 s -1 . Graphs of the recrystallized fraction versus time were used to quantify the kinetics of the strain-induced precipitation and generate the precipitation temperature time diagrams for the three steels. A kinetic model for static recrystallization is proposed which takes the V and Si concentrations into account.

Model for static recrystallisation critical temperature in microalloyed steels

By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start Ps and end Pf curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the model's prediction, comparing experimental values with calculated values, gives a correlation index of ∼0.9.

Influence of strain rate on recrystallisation– precipitation interaction in V, Nb, and V–Ti microalloyed steels

A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this sense, it was found that an increase in the strain rate led to a drop in the curve nose temperature TN and a reduction in the time necessary for precipitation to finalise t0·95 , as well as an increase in the recrystallisation rate.

Recrystallization and precipitation behavior during the hot deformation of Nb-Ti microalloyed steels

Constant strain rate compression tests at the temperature 900∼1100 °C and the strain rate 10−3∼20 s−1 and the interpass time 300∼1000 s of double stage interrupted tests were carried out for a microalloyed steel containing 0.024 % Nb, 0.016 % Ti, 1.39 % Mn and 0.15%C, to investigate its dynamic (DRX), static (SRX) and metadynamic recrystallization (MDRX) and precipitation behavior. Softening curves were determined by the offset method and back extrapolation method. It was found that the SRX and MDRX process have different Avrami exponent n and apparent activation energy Q. For the former n=1, Q=359 kJ/mol, while for the later n=0.57, Q=259 kJ/mol. Influences of temperature, prestrain and strain rate on the kinetics of SRX process were discussed and a mathematical model was furtherly constructed. Finally, the precipitation-time-temperature (PTT) curve was determined by the stress relaxation tests, and the influence of precipitation on the SRX process was also discussed. Direct microstructural evidence for precipitation during and after deformation was obtained by transmission electron microscopy.

Effect of nitrogen on σ transformation in duplex stainless steel weld metal

Duplex stainless steels were gas tungsten arc welded without filler material in various Ar–N2 mixed gas atmospheres. Weld metals having different nitrogen contents were annealed at 900–1300 K to examine the single effect of nitrogen on the σ transformation. Nitrogen content of the weld metal did not influence the nose temperature and the time for σ initiation in time–temperature–precipitation diagrams for σ, but did decrease the σ content after annealing for long times. These results were correlated with the nitrogen content and microstructure of the weld metals. The effect of σ on the fracture toughness of the weld metal was evaluated by the small punch (SP) test. A small amount of σ in the weld metal led to a marked decrease in the SP energy.

Effect of cold rolling on the precipitation behavior of δ phase in INCONEL 718

Systematic research has been undertaken on the effect of cold rolling on the precipitation kinetics of δ phase in INCONEL 718. Above 910 °C, cold rolling promotes the precipitation of δ phase. Below 910 °C, the precipitation of δ phase is still preceded by the γ″ precipitation in cold-rolled INCONEL 718. Cold rolling promotes not only the precipitation of γ″ phase but also the γ″ → δ transformation. The relationship between the weight percentage of δ phase and aging time follows the Avrami equation. Below 910 °C, as cold rolling reduction and temperature increase, the time exponent (n) decreases, whereas the rate of δ precipitation increases. The apparent activation energy of δ precipitation varies in the range of 1113 to 577 kJ/mol for 25 to 65% cold-rolled INCONEL 718 and decreases as cold rolling reduction increases. Precipitation-time-temperature (PTT) diagrams have been determined for the four cold-rolled INCONEL 718. The noses of the PTT curves are located at about 910 °C. These curves are shifted significantly to longer times as cold rolling reductions decrease.

Diagrama de Recristalización-Precipitación-Tiempo-Temperatura (RPTT) de un acero microaleado al vanadio

A method developed in the CENIM is described which allows to study the interaction recrystallization-induced precipitation by the deformation of vanadium microalloyed steel. By means of torsion test and applying the back extrapolation method, has been determined the recristallized fraction to diferent temperatures. When the precipitation begins, the recristallized fraction is separated of the Avrami's equation and this allows to know the instant the precipitation begins (Ps) and the instant the precipitation finishes (Pf). Thereby can be traced the Recrystallization- Precipitation-Time-Temperature (PTT) diagrams, which graphically show the interaction Recrystallization-Precipitation and simultaneously allows the determination of the static recrystallization critical temperature (SCRT). The mentioned temperature represents the limit between the two phases, before and after of the precipitation.

Determination of precipitation–time–temperature (PTT) diagrams for Nb, Ti or V micro-alloyed steels

A method is described, developed in CENIM-CSIC, for studying the kinetics of strain induced precipitation in micro-alloyed steels. Using torsion tests, the statically recrystallized fraction has been determined for three Nb, V and Ti micro-alloyed steels at different temperatures and strains. When precipitation starts, the recrystallized fraction deviates from Avrami’s equation, making it possible to identify the moment at which precipitation starts (Ps) and finishes (Pf). In this way precipitation–time–temperature(PTT) curves can be drawn, showing the precipitation kinetics in graph form.

Influence of microalloy type and content on induced precipitation kinetics in microalloyed steels

Using torsion tests, and applying the back extrapolation method, the statically recrystallized fraction has been determined for low carbon microalloyed steels with different V, Nb and Ti contents, at different temperatures and an equivalent strain of 0.20%. At temperatures below the static recrystallization critical temperature (SRCT), or the temperature at which strain induced precipitation commences, recrystallized fraction against time curves show a plateau caused by precipitations. The formation of the plateau makes it possible to evaluate precipitation-time-temperature (PTT) diagrams, and thus to know the precipitation kinetics. The PTT diagrams show that an increase in the microalloy content raises SRCT and reduces incubation time. For similar Nb, V and Ti contents, Nb is the element which raises SRCT the most. Der statische Rekristallisierungsanteil mikrolegierter Stähle mit verschiedenen Gehalten an Vanadium, Niob und Titan wurde durch Torsionsversuche bei unterschiedlichen Temperaturen ermittelt. Hieraus lassen sich Rekristallisierungsanteil-Zeit-Kurven erstellen, die bei Temperaturen unterhalb der kritischen Rekristallisierungtemperatur (SRCT) ein Plateau zeigen. Das Auftreten dieser Zone mit konstantem Rekristallisierungsanteil ermöglicht es, Auscheidung-Zeit-Temperatur-(PTT)-Diagramme aufzustellen und die Auscheidungskinetik kennenzulernen. Die PTT-Diagramme zeigen, daß die SRCT mit steigendem Gehalt an Mikrolegierungselementen ansteigt und die Inkubationszeit abnimmt. Auf jeden Fall bewirkt Niob einen stärkeren Anstieg der SRCT als Vanadium oder Titan

Influence of Strain on Induced Precipitation Kinetics in Microalloyed Steels.

Using torsion tests and applying the back extrapolation method, the statically recrystallized fraction of low carbon microalloyed steels containing vanadium, niobium and titanium, respectively, has been determined for different temperatures and strains. From the recrystallized fraction against time curves it is possible to draw precipitation-time-temperature (PTT) diagrams. These diagrams show that the strain does not affect precipitation kinetics in an independent manner, but that this influence is related with the microalloy content. The greater the strain applied, the shorter the incubation time of induced precipitation, but this influence diminishes as the microalloy content increases. It is also demonstrated that the incubation time is practically independent of the nature of the metal microalloy (V, Ti, Nb). In this sense, new expressions are proposed to relate the incubation and precipitation end times with the strain and the microalloy content. A model is also established for precipitation kinetics at the curve nose temperature.

Estudio de la cinética de precipitación inducida mediante la determinación de los diagramas Precipitación-Tiempo-Temperatura (PTT) en aceros microaleados

A method developed in the CENIM is described which allows to study the induced precipitation kinetics by the deformation in microalloyed steels. By means of torsion tests the statically recristallized fraction of three microalloyed steels with Nb, V or Ti, has been determined to different temperatures and deformations. When the precipitation begins, the recrystallized fraction is separated of the Avrami's equation and this allows to know the instant the precipitation begins (Ps) and the instant the precipitation finishes (Pf). Thereby can be traced the Precipitation-Time-Temperature (PTT) diagrams which graphically show the precipitation kinetics.

The influence of niobium on the static recrystallization of hot deformed austenite and on strain induced precipitation kinetics

This work presents a study of the influence of Ti on the kinetics of static recrystallization measured at different temperatures and on the kinetics of induced precipitation. The recrystallized fraction has been measured using the method known as back extrapolation. It has been found that using this method the softened fraction and the recrystallized fraction are approximately equal. The method used has also made it possible to draw, as will be seen later, precipitation-time-temperature (PTT) diagrams, until now determined in other Nb microalloyed steels using theoretical equations, or using transmission microscopy, and discovering in this way the kinetics of the precipitation induced by the strain. The recrystallized fraction will be modeled in accordance with Avrami's equation, as its application in hot rolling is of great use.

Static Recrystallization of Hot Deformed Austenite and Induced Precipitation Kinetics in Vanadium Microalloyed Steels.

Using torsional tests and applying the back extrapolation method the kinetics of the static recrystallization of hot deformed austenite has been studied in three microalloyed steels with different vanadium contents and this has been modelled at temperatures above and below the static recrystallization critical temperature (SRCT). The SRCT has been found to be a function of the strain, the austenite grain size and naturally the microalloy contents, in this case N and V. A study has also been made of the kinetics of the precipitation induced by the strain and the precipitation-time-temperature (PTT) diagrams have been determined for the three steels. The no-recrystallization temperature (Tnr) has also been determined and the conclusion was reached that the difference between the Tnr and SRCT is mainly due to the austenite grain size, which is smaller in the determination of the Tnr due to the successive recrystallizations which occur before precipitation takes place.

Nb(C, N) Precipitation and Austenite Recrystallization in Boron-Containing High-Strength Low-Alloy Steels

The influence of boron addition, amount of deformation, and solution heat-treatment temperature on the precipitation and recrystallization behaviors of a family of high-strength low-alloy (HSLA) steels was studied. A stress relaxation technique was employed to detect the occurrence of austenite recrystallization and to determine the precipitation start(P s) and finish(P f) times. After preheating to 1100 °C or 1200 °C for 30 minutes, the specimens were cooled to test temperatures between 800 °C and 1000 °C. They were subsequently deformed to true strains of 5 or 25 pct and subjected to stress relaxation. The advent of recrystallization produced a sharp increase in relaxation rate, while the occurrence of carbonitride precipitation led to the appear- ance of a stress plateau. The results indicate that the presence of boron (1) accelerates carbo- nitride precipitation and (2) retards austenite recrystallization when present in combination with Nb. The precipitation-time-temperature (PTT) diagrams determined in this investigation are C-shaped for both the B-free as well as the B-modified Nb steels. These data were analyzed in terms of the classical theory of nucleation, on the basis of which it is demonstrated that the acceleration of the nucleation kinetics of precipitation can be attributed to the segregation of boron and of boron-vacancy complexes to dislocations and grain boundaries, as well as to the faster diffusion of Nb in the presence of boron.

A creep technique for monitoring MnS precipitation in Si steels

A newly developed creep method is described for investigating the kinetics of manganese sulfide precipitation in two Si steels at hot working temperatures. The method was also applied to a Ti steel, in which the precipitation kinetics were previously determined using a stress relaxation technique. Prior to loading, the specimens are solution-treated for half an hour and then immediately cooled to the test temperature. A constant stress is applied to the sample by means of a computerized MTS machine, and the strain is recorded continuously during testing. The resulting creep rate is sensitive to the occurrence of precipitation; thus, the slope of the true strain-log (time) curve decreases immediately after the initiation and increases on the completion of precipitation. The precipitation-time-temperature (PTT) diagrams determined in this way on the three tested steels are of classical C shape. Because higher dislocation densities and internal stress levels are maintained, the present technique is more effective for monitoring the precipitation events occurring in both austenitic and ferritic phases than the previously developed stress relaxation method.

A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures

Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e., in the absence of precipitation) can be described by the relation σ = σ0 -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified. Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25 pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the compositions of the steels tested.

Microstructural changes in austenitic stainless steels during long-term aging

AbstractThe microstructural changes, precipitation behaviour, and mechanical properties of typical austenitic stainless steels (304 H, 316 H, 321 H, 347 H, and Tempaloy A–1) have been examined after long-term aging. The steels were aged statically in the temperature range 600–800°C for up to 50000 h. The microstructural changes were observed by optical and transmission electron microscopy, and the extracted residue was identified using X-ray analysis. Time–temperature precipitation diagrams were made for each steel. The amount of σ-phase was measured in samples aged at 700°C. The hardness and impact-value changes, and the tensile properties of aged samples were measured.MST/358

Effect of vanadium and molybdenum addition on high temperature recovery, recrystallization and precipitation behavior of niobium-based microalloyed steels

The effects of multiple microalloy additions on the dynamic recovery and recrystallization behavior of austenite were investigated in a series of 0.05% C-l.25% Mn steels. The steels contained one or more of 0.30% Mo, 0.035% Nb or 0.115% V, and a reference plain carbon steel was also tested. Constant strain rate compression tests were conducted in the temperature range 1150°–875°C, that is, above and below the relevant carbonitride solution temperatures ( T sol ). Dynamic recrystallization-time-temperature (RTT) curves were developed from the flow curves at 5.6 × 10 −3 and 3.7 × 10 −2 s −1. At the higher strain rate, only the solute retardation of recrystallization was detected for the vanadium, Nb-V, Nb-1.9 Mn and No-Mo steels. An additional delay due to the dynamic precipitation of Nb(CN) was observed in the Nb-1.25 Mn steel, where there was a break to the right in the RTT curve. These breaks were seen in the results for all the steels at the lower strain rate. They appeared about 50°–100°C below the calculated T sol for the particular carbonitride, and coincided with the intersection of the RTT and PTT (precipitation-time-temperature) curves. The solute retarding potential of each element was normalized to 0.1 at.% with a solute retarding parameter (SRP). The SRP for niobium, molybdenum and vanadium when added singly, was 63, 10 and 3% respectively. In the multiply alloyed steels, the SRP attributable to each element was smaller than when it was present alone. Possible explanations for this negative interaction are advanced. Dynamic PTT curves for the precipitation of carbonitrides are presented for the Nb-1.25 Mn, Nb-1.9 Mn, Nb-Mo, Nb-V and Nb-Mo-V steels. The addition of molybdenum, vanadium or manganese is seen to retard the dynamic precipitation of Nb(CN). These delays are explained in terms of the decrease in T sol resulting from the reduction in the carbon and nitrogen activity coefficients in the joint presence of the former elements. An empirical formulation for the effect of manganese on NbC solubility is deduced from results reported in the literature which is consistent with the present observations.