Modelling of precipitation kinetics and age hardening of Fe–1–2Ni–6Mn maraging type alloy

Abstract

The precipitation hardening effect occurring in Fe – 12Ni –6Mn maraging steel has been quantified in the present work. The model obtained can be applied to the entire aging period. Precipitate growth and precipitation fraction as functions of time and temperature have also been quantified. Time – temperature –precipitation diagrams with respect to precipitate size and fraction are proposed. The precipitation fraction at peak hardness has been estimated, together with the precipitate size. Results show that the precipitation process is far from complete when peak hardness is reached. At the lower temperature of 400 ° C, aging produces precipitates at a higher fraction and of larger size than those produced at 450 and 500 ° C, which explains why aging at 400 ° C gives a stronger hardening effect. MST/5022     

Precipitation of second phases in aged Ni rich NiTiRe shape memory alloy

Abstract

This study investigated the effect of aging on the structure and precipitation of second phases of Ni₅₂Ti_47·7Re_0·3 shape memory alloys. The alloy was solutionised at 1000°C for 24 h before aging at various temperatures ranging from 300 to 600°C for 3 h. The matrix phase in both solutionised and aged specimens was martensite. Ti₂Ni phase was also present in the microstructure of both solutionised and aged specimens and its volume fraction decreased as the aging temperature increased. Ni₄Ti₃ phase began in appearance by increasing aging temperature to 400°C. Ni₄Ti₃ precipitates had lenticular and non-geometry shapes. Aging at 600°C led to precipitation of Ni₃Ti phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio increased and/or Ni content decreased in the matrix with increasing in aging temperature.     

Quantification of precipitate fraction in maraging steels by X-ray diffraction analysis

Abstract

Quantification of precipitate fraction, a critical step in modelling precipitation hardening, is difficult to carry out for maraging steels because of the small precipitate size and its low volume fraction. A method is proposed in the present work to estimate the precipitate fraction in an 18 wt-%Ni C250 maraging grade based on simple X-ray diffraction (XRD) analysis. The estimation gives reasonable values of precipitate fraction, which increases when aging proceeds. The precipitate formed at 427°C may be of a type that has not been reported before. The method proposed may be applicable to other precipitate hardened alloys. The amount of reverted austenite formed was also estimated when applicable.     

Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100?000?h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M₂₃C₆, η (Cr₃Ni₂SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.     

Precipitation hardening of Cu – Ti – Zr alloys

Abstract

The influence of age hardening temperature and time on the hardness, tensile properties, electrical conductivity, and microstructure of Cu – 4Ti – 0.1Zr and Cu – 3Ti – 0.1Zr alloys has been investigated. The resulting microstructure of these alloys suggests that zirconium addition prohibited the formation of compositional modulations in the solution treated condition. These alloys exhibited maximum hardness and strength on peak aging at 450°C for 24 h by the formation of a coherent and metastable Cu₄Ti phase (β') in modulated structure while overaging occurred by the formation of equilibrium phase β-Cu₃Ti. The electrical conductivity of both the alloys increased moderately on aging. Unlike in an earlier study of binary Cu – Ti and some ternary Cu – Ti – X alloys, overaging did not cause any discontinuous precipitation in the Cu – Ti – Zr alloys investigated. Modulated structure formed on peak aging persisted on prolonged aging at 450°C for 80 h or at 500°C for 8 h.     

Effect of predeformation on microstructure and mechanical properties of Al–Cu–Li–Zr alloy containing Sc

Abstract

The effects of prior cold deformation on the microstructures and the room temperature mechanical properties of an Al–3·5Cu–1·5Li–0·22(Sc + Zr) alloy have been observed by using TEM and tensile test at room temperature. The results show that the alloy has the character of aging hardening, and the major phase of precipitation and strengthening is T₁ phase. The result also show that prior cold deformation leads to more dispersive and uniform distribution of T₁ precipitations. It accelerates aging response, causes earlier aging peak occurrence, and enhances strength greatly. However, the plasticity of the alloy is declined with prior cold deformation. In contrast, excessive prior cold deformation causes coarsening and heterogeneous distribution of T₁ phase. It also reduces the strength of the alloy, therefore, influences the composite properties of the alloy. The favourable prior cold deformation is about 3·5% under the experimental condition.     

Precipitation hardening owing to δ′ Al3Li in low activation Al–3 Li–12Si–3Mg alloy

Abstract

The precipitation hardening behaviour owing to ordered δ′ Al₃Li in an Al–3Li–12Si–3Mg (wt-%) based alloy was investigated during aging between room temperature and 523 K after quenching from between 723 and 823 K. The induced radioactivity level of this alloy is very low, suitable for fusion reactor structures. Effects of aging temperature, solutionising temperature, and pretreatments on precipitation hardening were examined in detail. Tensile properties were also examined in the as quenched condition as a function of testing temperature between room temperature and 673 K. Yield stress exhibited a positive temperature dependence between room temperature and 523 K and decreased abruptly above 523k with increasing test temperature. The precipitation behaviour of δ′ and the strengthening mechanisms owing to δ′ were discussed.

MST/1100     

Effect of long term aging on mechanical properties and microstructure of nickel base weld

Abstract

The mechanical properties of a Ni base weld have been examined after long term aging in the temperature range 600–900°C. Impact testing revealed a substantial decrease in toughness after heat treatment at 600 and 700°C. In particular, after aging at 700°C there was a marked loss of ductility, which was associated with elongated particles appearing in the fracture surfaces. The concomitant microstructural changes occurring have been investigated using analytical electron microscopy. In material heat treated at 600°C for 10000 h, seven phases were identified: M₂₃C₆, MC, η, γ′, γ″, G, and δ. At 700°C, the following phases were distinguished after 3000 h: M₂₃C₆, MC, and η. After longer testing times G, γ′, and γ″, were formed. Whereas G, γ′, and γ″ formed intragranular needles, η formed intragranular laths or plates at 700°C. The minimum in ductility observed in material aged at 700°C can be explained in terms of copious intragranular precipitation of γ′, γ″, and η.

MST/1131     

Precipitates,zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract

Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn₂, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.     

Modeling of precipitation hardening in Mg-based alloys

This work deals with the development of Mg-based alloys with enhanced properties at elevated temperatures. This is achieved by precipitation of binary phases such as MgZn₂ and Mg₂Sn during the aging of these alloys. The aim of the present work is to develop and calibrate a model for precipitation hardening in Mg-based alloys, as different types of precipitates form simultaneously. The modified Langer-Schwartz approach, while taking into account nucleation, growth and coarsening of the new phase precipitations, was used for the analysis of precipitates’ evolution and precipitation hardening during aging of Mg-based alloys. Two strengthening mechanisms associated with particle-dislocation interaction (shearing and bypassing) were considered to be operating simultaneously due to particle size-distribution. Parameters of the model, R _N i and k _{σ i} , were found by fitting of calculated densities and average sizes of precipitates with ones estimated from experiments. The effective diffusion coefficients of phase formation processes, which determine the strengthening kinetics, were estimated from the hardness maximum positions on the aging curves.     

Development of high strength in Ni–42Cr–7·7Al (at.-%) alloy

Abstract

An investigation has been made of the aging of supersaturated nickel based solid solution in a Ni–42Cr–7·7 Al (at.-%) alloy. Solution treatment of the alloy in the range 1150–1300°Cfollowed by quenching and aging at ~550–750°C produces high values of hardness and strength, e.g. 835 HV on aging at 600°C. This hardening results from a discontinuous precipitation reaction producing a fine scale lamellar structure consisting of nickel based and chromium based solid solutions; interlamellar spacings of ~60 nm were obtained. The rate of precipitation is substantially greater than that in a binary Ni–42 at.-%Cr alloy.

MST/736     

A new 1.9 GPa maraging stainless steel strengthened by multiple precipitating species

A new ultra-high strength maraging stainless steel with composition of 13Cr–13Co–4.5Ni–3.5Mo–0.5Ti (at.%) has been developed. It was demonstrated that the ultimate tensile strength of the steel could reach 1.9 GPa with reasonable ductility. This breakthrough was achieved by a combined strengthening of three different species of precipitates. The evolution of precipitates with respect of size, morphology and chemical composition during aging at 500 °C was characterized using atom probe tomography (APT) and transmission electron microscopy (TEM). The precipitates were identified to be η-phase Ni₃(Ti, Al) phase, Mo-rich R′ phase and Cr-rich α′ phase, developing out of the precursor clusters, Ni–Ti–Al-rich cluster, Mo-rich cluster and Cr-rich cluster, separately. The segregation of Mo and Cr atoms at the precipitate/matrix interfaces was detected and is considered to impede the coarsening of η-phase. Based on the characterizations, the precipitation process of these phases and their effect on mechanical properties were analyzed.     

Aging behaviour of cobalt free chromium containing maraging steels

Abstract

This paper reports an investigation of the aging behaviour of two Co free Cr containing maraging steels (Fe–1·0Si–11·2Cr–1·3Mo–9·1Ni–1·2Al–1·0Ti and Fe–0·8Si–17·2Cr–6·1Ni–0·4Al–0·9Ti, all at.-%), using hardness measurements, electron microscopy of replicas and thin foils, atom probe field ion microscopy (APFIM), and thermochemical calculations. Two different families of intermetallic phases (Ti₆Si₇Ni₁₆G phase and η Ni₃Ti) have been found to contribute to age hardening. The composition and morphology of these precipitates were studied in deformed and undeformed alloys after aging at 420–570°C for various times. In addition, reverted austenite has been found in the aged structure. Results obtained using APFIM are compared with equilibrium thermodynamic calculations and previous APFIM studies of conventional Cr free low Al and Si maraging steels having higher Mo contents.

MST/1558     

Influence of annealing on texture and magnetic properties of 18% nickel, 2400 MN m−2grade maraging steel

Abstract

The texture incorporated during cold rolling of maraging steel persists even when the samples are annealed at temperatures close to the melting point. The texture influences the microstructural developments observed following aging, which in turn affects the magnetic properties of maraging steel. The maximum coercive field, observed after aging at about 700°C, increased from about 11 140 A m^?1 in the as rolled and aged condition to about 14 720 A m^?1 in the specimen annealed at 1000°C before aging. The corresponding saturation magnetisation values also increased from about 1·040 to about 1·300 T. The prior annealing affects the morphology of the precipitates as well as the reverted austenite.

MST/3129     

Microstructural evolution in bainite,martensite, and δ ferrite of low activation Cr–2W ferritic steels

Abstract

The microstructural evolution in (2–15)Cr–2W–0·1C (wt-%) firritic steels after quenching, tempering, and subsequent prolonged aging was investigated, using mainly transmission electron microscopy. The steels examined were low induced radioactivation ferritic steels for fusion reactor structures. With increasing Cr concentration, the matrix phase changed from bainite to martensite and a dual phase of martensite and δ ferrite. During tempering, homogeneous precipitation of fine W₂C rich carbides occurred in bainite and martensite, causing secondary hardening between 673 and 823 K. With increasing tempering temperature, dislocation density decreased and carbides had a tendency to precipitate preferentially along interfaces such as bainite or martensite subgrain boundaries. During aging at high temperature, carbides increased in size and carbide reaction from W₂C and M₆C to stable M₂₃C₆ occurred. No carbide formed in δ ferrite. The precipitation sequence of carbides was analogous to that in conventional Cr–Mo steels.

MST/1049     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Influence of carbon on aging response and tensile properties of eutectoid beta titanium alloy Ti–13Cr

Abstract

The influence of carbon addition on the aging response of quenched Ti–13Cr (wt-%) has been investigated using hardness tests, tensile tests, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that carbon refines beta grain size, leads to fine and homogenous alpha precipitation and reduces grain boundary alpha. The carbon addition accelerates the rate at which hardening occurs during aging and increases the peak hardness of the aged specimens. A significant improvement in room temperature tensile strength and ductility has also been achieved in the carbon containing alloy after aging at 500°C. The effects of carbon on the aging response appear to be attributed to dislocations introduced by carbides during quenching, elastic strain created in the matrix by carbides and gettering effect of Ti₂C carbides. The influence of each of those mechanisms has been demonstrated through experiments and the factors giving rise to the improvements in properties are also discussed in terms of the microstructural observations.     

Hexagonal martensite decomposition and phase precipitation in Ti–Cu alloys

The mechanical behavior of Ti–Cu alloys can be improved by controlling Ti₂Cu precipitation. In eutectoid alloys, such precipitation can be achieved by the decomposition of martensite in response to aging heat treatment. The purpose of this work is to discuss the evolution of precipitates during the decomposition of hexagonal martensite in Ti–Cu alloys. First, samples with near-eutectoid compositions were prepared in an arc furnace equipped with a non-consumable tungsten electrode and water-cooled copper hearth under a high purity argon atmosphere. After chemical homogenization at a temperature in the beta field, the samples were water-quenched and examined by differential scanning calorimetry and high-temperature X-ray diffraction. The results indicate that rapidly quenched near-eutectoid Ti–Cu alloys present Ti₂Cu precipitates. Regardless of the cooling rate applied, such precipitation is unavoidable. No evidence of beta phase stabilization was found in the rapidly quenched samples. Precipitation temperatures of coherent and incoherent phases of 415 °C and 550 °C, respectively, were determined from the differential scanning calorimetry measurements. Ti₂Cu precipitation was examined in situ by high temperature X-ray diffraction experiments. The total decay of martensite was found to occur above 575 °C. Vickers hardness testing of aged samples revealed a correlation between phase precipitation and hardening.     

Study of the austenite quantification by X-ray diffraction in the 18Ni-Co-Mo-Ti maraging 300 steel

In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560°C, 600°C and 650°C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560°C and 600°C. At 650°C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.     

Comparative study of precipitation effects during aging in superaustenitic and superferritic stainless steels

Abstract

The concept of designing a steel that would match high corrosion resistance coupled with improved mechanical properties when exposed at elevated temperatures has always been a challenge for a metallurgist. The present paper relates the results of microstructural analysis of two commercial superaustenitic stainless steel grades and a novel experimental grade of superferritic stainless steel. A study of the microstructural stability and attendant mechanical property evolution has been carried out, employing a variety of aging experiments. Following prolonged aging up to 1000 h in the temperature range between 650 and 950°C, microstructure evolution was determined, while the mechanical properties were preliminary assessed via hardness testing. The present study helped clarify the effect of high temperature exposure on the precipitation sequences of both superaustenitic and novel superferritic stainless steels. The heat treatments performed indicate a number of precipitation sequences of intermetallic phases taking place, such as σ phase formation, clearly specifying a time–temperature regime where care must be taken during the fabrication and use of these materials.