Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels

The effects of Cr and/or Mo additions and tempering temperatures on mechanical properties in relation to the microstructural evolution during tempering were investigated in induction-tempered steels. The additions of Cr and/or Mo result in the finer distribution of cementite particles due to the decrease in the coarsening rates of cementite particles above tempering temperature of 400°C, while their influence is less effective at low tempering temperatures. Accordingly, the increments of tensile strength and yield strength by the addition of alloying elements become more pronounced at high temperatures above 400°C. The occurrence of maximum peak of yield strength at 400°C would be related to further precipitation of the cementite at low temperatures, and the subsequent spheroidization and coarsening process of the cementite at high temperatures. The addition of alloying elements does not change the minima in Charpy impact values, related to tempered martensite embrittlement, since alloying elements do not have an influence on the decomposition of retained austenite and the formation of the cementite at boundaries. The strain-hardening exponent, n, decreases up to 400°C and then continuously increases with tempering temperature. This abrupt increase of n at 300°C is related to the transformation of retained austenite during straining in induction-tempered steels.     

On coarsening of cementite during tempering of martensitic steels

ABSTRACT

The coarsening of cementite in a martensitic Fe–1C–1Cr (wt-%) alloy upon tempering at 700°C is investigated. When considering that the main location of cementite is at grain boundaries, classical coarsening theory can accurately predict the mean size evolution, while the predicted size distribution evolution disagrees with the experimentally observed log-normal distribution maintained throughout the whole tempering (5000?h). We conclude that classical theory of coarsening, as given by Lifshitz–Slyozov–Wagner and included in the Langer–Schwartz Kampmann–Wagner numerical approach for modelling precipitation reactions, is not fully adequate to simulate coarsening of cementite for tempering in practice.     

Kinetic Model of TiN Particle Dissolution and Coarsening during Welding Thermal Cycle

With carbon extraction replica technique,electron diffraction and EDAX analysis,second particle size distribution in two Ti microalloyed steels and their heat-affected zones were investigated.The results show that the particles in the Ti microalloyed steels are TiN particles,and the TiN particles in the steel with lower Ti/N ratio exhibit smaller size and lower dissolution and coarsening rate and extent.Based on the investigation results,kinetic models for TiN particle dissolution and coarsening during welding thermal cycle were developed.The predicted values calculated by using the models are in good agreement with the experimental ones.     

Dissolution and coarsening of large niobium carbonitrides in a microalloyed steel

Observations and measurements of the kinetics of coarsening and dissolution of large cuboidal niobium carbonitrides during solution treatments of a high nitrogen niobium microalloyed steel are reported. At temperatures between 1473 and 1573 K a competitive coarsening and dissolution process was established where the larger niobium carbonitrides grew at the expense of the smaller, or employing niobium and nitrogen which remained in solid solution. In this temperature range growth or dissolution rates and critical sizes could be determined from the analysis of the evolution of particle size distribution. At higher temperatures (1623–1723 K), only a dissolution process existed, where the dissolution rates as a function of particle size was found to increase with increasing temperature.     

Estimation of the mean size of the cementite particles by hardness measurement in plain carbon steel

The relationship between the Vicker's hardness VHN and the mean size of the coarsening cementite particles, ¯d in plain carbon steel during tempering at high temperature has been deduced and corroborated by experimental data analysis. It is found that VHN varies linearly with 1/¯d. Hardened plain carbon steel with 0.68, 0.56 and 0.34% carbon, has been tempered at 973 to 861 K as a function of time for the investigation. A master plot of VHN against ¯d(1–V _v)/V _v has been drawn which provides the mean particle diameter of the cementite particles as a function of hardness, irrespective of time and temperature of tempering and carbon content of the steel.     

Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering

Utilizing the hardness measurement, an attempt is made to study the tempering kinetics of DM hot-work die steels, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the increasing of tempering temperature will result in precipitation and coarsening of the intra-lath spherical carbides. Meanwhile, it is significantly observed the dissolution of needle-like Mo2C carbides with increasing of tempering temperatures. The growth mechanism of carbides is coarsening of large spheroids after complete and impingement precipitation on dislocations. In addition, the activation energy ΔH of DM steels during additional tempering between 620 °C and 700 °C is slightly higher than that of the diffusion of usual alloying elements (e.g. Cr, V, Mo and W) in ferrite, which may be attributed to a higher Mn content in DM steels that dissolves into carbides and retardsthe coarsening of carbides at high temperature.     

Kinetic parameters during the tempering of low-alloy steel through the non-isothermal dilatometry

The kinetic study of the tempering reactions of a low-alloy steel (AISI 1050) was carried out through non-isothermal dilatometry. The kinetic parameters of the first and third stages on tempering (here referred to as processes I and II) are calculated by procedures which assume that the nucleation and growth reactions obey a Kolmogorov–Johnson–Mehl–Avrami (KJMA) kinetic model. The recipes to obtain the kinetic parameters (E, K ₀, n) of the reactions on tempering are presented. The first stage of tempering is characterized by the growth of the transition carbide nuclei formed during the quenching, n = 1 (site saturation situation). This stage is controlled by the pipe diffusion of the iron atoms. The third stage of tempering is characterized by the cementite nucleation on dislocations due to the gradual dissolution of the transition carbide, n = 0.66. The cementite growth is controlled by diffusion of the iron atoms through dislocations and in the matrix.     

Accelerated spheroidization of hypoeutectoid steel by the decomposition of supercooled austenite

A thermal cycling treatment was used to produce a spheroidized structure of hypoeutectoid steel from direct decomposition of supercooled austenite. Scanning electron microscopy and quantitative metallography were employed to study the changing microstructure during the thermal cycling treatment. A conventional spheroidizing annealing was also investigated for comparison. It has been shown that the thermal cycling treatment results in a structure of cementite spheroidites homogeneously distributed in a ferrite matrix within a very short processing time, and the coarsening of cementite particles is controlled by the coupled diffusion of both carbon and iron atoms.     

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r³ vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r² vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.     

Hydrogen-induced embrittlement of nickel-chromium-molybdenum containing HSLA steels

ABSTRACT

Several advanced nickel-chromium-molybdenum high strength lowalloy steels newly developed by our research team exhibit excellent mechanical strength, toughness and hardenability. However, the phenomenon of hydrogen-induced embrittlement will easily occur for these high strength steels. In this research, the hydrogeninduced embrittlement of 8625-Modified steel (8625M steel) was studied. Experimental results show that the dominant hydrogen trapping site of the 8625M steel is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogens trapped in the dislocations are diffusible. The as-quenched 8625M steel has the highest dislocation density and accordingly the highesthydrogen content after hydrogen charging. This makes the asquenched 8625M steel exhibit severe hydrogen embrittlement. After tempering at 200°C and 300°C, the dislocation density drops, and hence these tempered specimens have lower ultimate tensile strength loss. After 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again, being ascribed to the formation of needlelike and film-like cementite which will weaken the strength of martensite. After 500°C tempering, the 8625M steel has the lowest dislocation density, and the inter-lath cementite become discontinuous and spheroidal, making the 500°C tempered specimen have the lowest ultimate tensile strength loss and the highest elongation after hydrogen charging in this study.     

Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30?min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400?nm and the cementite lamella were fully spheroidised with the particle diameter of ～150?nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300?nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.     

Mathematical modelling of microsegregation in eutectic and peritectic binary alloys

Abstract

A mathematical model of microsegregation for eutectic and peritectic binary alloys was implemented using a finite volume method to solve the differential equations for mass transport. In this model simple ideas are used to handle phase boundaries and coarsening without the need to employ node jumping schemes or any transformation of variables to fix the domain size. Some model results were compared with available analytical solutions, revealing excellent agreement, which proved the approach useful to solve dissolution and diffusion coupled problems as well as microsegregation ones. Furthermore, good agreement was observed between the model results and measurements of eutectic volume fractions published previously for an Al–Cu alloy. The model was also capable of showing some important features of a typical peritectic transformation. Some instability was observed during model calculations, but it was easily handled by a time step refining technique.     

Modelling and characterisation of Mo2C precipitation and cementite dissolution during tempering of Fe–C–Mo martensitic steel

Abstract

The precipitation and Ostwald ripening behaviour of needle shaped Mo₂C particles during the tempering of a ternary Fe-C-Mo martensitic steel have been characterised and modelled, taking account of local equilibrium, the capillarity effect, and the simultaneous enrichment and dissolution of cementite. Particles of Mo₂C are represented as paraboloids of revolution, with the tip radius chosen to yield the maximum lengthening rate. Transmission electron microscopy has been used to validate the theory; measurements of the average length, volume fraction, and number density of particles showed good agreement with experimental observations.     

A Criterion for the Change from Fast to Slow Regime of Cementite Dissolution in Fe-C-Mn Steels

The present study clarifies the role of Mn in cementite on the driving force of cementite dissolution and the growth of austenite. From an experimental study, the effects of manganese composition and temperature on the cementite dissolution were shown. From a theoretical analysis based on thermodynamic and kinetics considerations, a criterion for the change from fast to slow regime of cementite dissolution was proposed. This criterion is in good agreement with the experimental results. It can be easily calculated and can define the composition and temperature ranges where the cementite dissolution is slow or fast.     

Modelling and characterisation of V4C3 precipitation and cementite dissolution during tempering of Fe-C-V martensitic steel

Abstract

The precipitation and Ostwald ripening behaviour of V₄C₃ (plate shaped) particles during the tempering of a ternary Fe - C - V martensitic steel have been characterised and modelled, taking account of local equilibrium, the capillarity effect, and simultaneous cementite enrichment and dissolution. Particles of V₄C₃ are represented as parabolic cylinders of revolution, with the tip radius chosen to yield the maximum lengthening rate. Transmission electron microscopy has been used to validate the theory; measurements of the average length, volume fraction, and number density of particles showed good agreement with experimental observations.     

Carbide dissolution in thin wall ductile iron

Abstract

This investigation focuses on the study of the dissolution by annealing of carbides present in thin walled ductile iron parts. Ledeburitic carbides are usually present in the microstructure as a consequence of the rapid solidification rate induced by the small thickness. The dissolution treatment is carried out by annealing in the austenitising temperature range. The study involves unalloyed ductile irons of different equivalent carbon values that show initial amounts of free carbides that reach 40%. The results show that even very large amounts of cementite can be rapidly and easily dissolved by annealing. The dissolution rate ranges between 2 and 9% per minute. This dissolution rate is much faster than that expected from the literature data and is attributed to the particular characteristics of thin wall castings, such as the low concentration of carbide stabilising elements and the small distance for diffusion of carbon from the cementite to the nodules. The short time required for carbide dissolution implies that carbides can be completely dissolved during the austenitisation stage of many practical heat treatment cycles.     

Theoretical analysis of changes in cementite composition during tempering of bainite

Abstract

An attempt has been made to model the kinetics of changes of composition in cementite during the aging of creep resistant low alloy steels of the type used in power plant. In the model a finite difference method is used for calculating the rate at which alloying elements redistribute between ferrite and cementite, subject to the thermodynamic constraints which determine the equilibrium compositions of these phases. It has also been possible to derive approximate analytical solutions which give good physical insight into the factors controlling the approach to equilibrium. Particle size has been found to have a strong influence on the rate at which the cementite composition is changed. A theoretical basis for the time dependence of cementite composition is also provided. The method is easily adapted to multiple or anisothermal heat treatments. Further work is suggested to account for simultaneous coarsening and changes in composition and also to allow for the formation of alloy carbides at long aging times.

MST/874     

Numerical analysis of particle transport during solidification using models based on stochastic differential equation

Abstract

In the present work, a comprehensive theoretical model is developed to describe the particle transport mechanisms in a solidifying binary melt in the presence of random thermofluidic fluctuations offered by the surrounding fluid medium. The detailed transport phenomena in the particle and bulk phases are coupled together through a stochastic formalism, capturing the physical mechanisms and consequences of complex interparticle interactions and the associated growth and/or dissolution of the crystals. The equation of the motion of the particles is modelled using the theory of stochastic differential equations. Numerical simulation study reveals the statistically randomised nature of the evolution of particle phase, which otherwise cannot be captured from a purely deterministic viewpoint. The mathematical model is also tested by comparing present numerical results with reported experimental observations; a very good agreement can be observed in this regard, thereby establishing the authenticity of the proposed formulation.     

Modelling of the agglomeration of Ni-particles in anodes of solid oxide fuel cells

The degradation of anodes of solid oxide fuel cells (SOFC), which consist of a porous metal – solid electrolyte material is described by a two particle model. The model is based on two main assumptions. Firstly, the difference in metal particle diameter is the driving force for the observed coarsening of the larger metal particle during long term annealing. Secondly, surface diffusion of metal atoms on the particle surface is the dominant diffusion mechanism. Additionally, a function was introduced which considers the limited space for the growth of the nickel particles in the cermet material. The found analytical function for the growth kinetics was compared to experimental results for the growth of nickel particles in a nickel - yttria stabilised zirconia (YSZ) anode annealed at 1000°C up to 4000 h. The model describes the time dependence of the observed particle radii in an adequate way. The resultant surface diffusion coefficients for Ni are lower than results found in literature. Possible explanations are discussed. However, the result shows that the proposed mechanism – surface diffusion of nickel atoms - is fast enough to explain the found amount of Ni agglomeration in SOFC anodes and is therefore considered to be the dominant mechanism.