Evaluation of chemical nucleation driving force for stoichiometric compounds with variable composition

Abstract

This work puts in evidence some inconsistencies in the method currently used in the literature for estimating the driving force for nucleation of stoichiometric phases with variable chemical composition, such as carbonitrides of Ti, Nb, etc. in steel. An alternative approach, properly accounting for the capillarity contribution to the Gibbs free energy change associated to the formation of the critical nucleus, is proposed. The chemical composition of the critical nucleus, which maximises the driving force for precipitation and allows the calculation of the critical radius, is found. Some examples of application are presented and the results are compared to those from the classical approach and discussed. The proposed method eliminates the contradiction encountered in previous approaches found in the literature and demonstrates that for these compounds the usual thermodynamic solution of the chemical equilibrium, which refers to flat surfaces, can not be directly extrapolated to describe the nucleation.     

Transformation characteristics of medium carbon V – Ti – N microalloyed steel for non-quenched/tempered oil well tubes

Abstract

The phase transformation points of a medium carbon V - Ti - N microalloyed steel were determined, as were the continuous cooling transformation curves of austenite heated at 1100 ° C without deformation and heated at 1200 ° C with two pass deformation. The data have been further used to analyse the real production scheme of N80 grade hot rolled non-quenched/tempered seamless tubes. The results have showed clearly that the commonly used 'in line normalisation' is not always necessary in the non-quenched/tempered production process of N80 grade hot rolled seamless oil well tubes. The above viewpoint has been further demonstrated by the microstructural examination of specimens sampled from industrial production.     

Hot dip galvanising and galvannealing of P and Mn strengthened TiNb IF steels

Abstract

Phosphorus, Mn, or Si solid solution strengthened sheet steels stabilised with Ti and Nb are known to have poor galvanising and galvannealing properties. When these steels are continuously annealed in higher dewpoint atmospheres, selective oxidation processes can occur in the subsurface region of the sheet. A laboratory simulated continuous galvannealing process was therefore used to study this effect in P and P+Mn solid solution strengthened sheet steels stabilised with Ti and Nb, by making a detailed analysis of the fundamental surface and interface processes that control the wetting of the surface by the liquid Zn, the inhibition layer formation, and the Fe-Zn reaction.     

Very strong low temperature bainite

Abstract

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10^-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Continuous separation of Fe–Al–Zn dross phase from hot dip galvanised melt using alternating magnetic field

Abstract

Experiments to continuously separate Fe–Al–Zn dross phase from hot dip galvanising zinc melt were conducted on a laboratory scale apparatus by using high frequency alternating magnetic field. Effects of processing time (t) on separation efficiency were investigated. The experimental results show that using the electromagnetic repulsive force resulting from the electrical conductivity difference between zinc melt and Fe–Al–Zn dross phase, the deleterious zinc dross particles causing surface defects of galvanising steel sheets can be continuously separated from the zinc bath under alternating magnetic field, and the separation efficiency increases with the increase in processing time. When the magnetic frequency is 17·5 kHz, the effective magnetic flux intensity is 0·1 T, the cross-section of the ceramic square pipe is 10 × 10 mm, and the processing time is 0·6–2·5 s, the separation efficiency of zinc dross varies from 43·76 to 85·71%, and the experimental results are in reasonable agreement with the theoretical results.     

Role of grain refining in hot cracking and macrosegregation in direct chill cast AA 7075 billets

Abstract

Direct chill (DC) casting experimental results are presented for an Al–Zn–Mg–Cu alloy with and without grain refining by attempting the inoculant addition either in the furnace or in the launder at two different casting speeds. Despite considerable structural refinement macro segregation remains unchanged with grain refining at a lower casting speed. Hot cracking, on the other hand, is totally prevented at this casting speed. Depending on the grain refining practice, either hot tears do not appear at all or prior cracks existing in the non-grain refined billet are completely healed. At a higher casting speed, the severity of segregation increases. The hot cracking tendency, however, has shown a distinct difference with grain refining method, with furnace-refined alloy resulting in a crack-free billet. But the billet grain refined in the launder exhibited hot cracking. Overall, these experimental results confirm the beneficiary effect of grain refining on hot cracking.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract

The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.     

Finite element modelling of tungsten inert gas welding of aluminium alloy 2024

Abstract

A finite element model to predict the evolution of stress and distortion for a bead on plate tungsten inert gas (TIG) weld in aluminium alloy 2024 plate is described. The thermal model was calibrated against thermocouple measurements, and the mechanical model was validated against direct measurements of residual strain made using synchrotron X-ray diffraction. Particular attention has been paid to the clamping arrangement, the transfer of heat between the plate and the copper backing plate, and the selection of appropriate thermal history dependent mechanical properties. The validated model has been used to determine the optimal arrangement for 'low stress no distortion' welding using laser heating applied to both sides of the joint line ahead of the TIG torch. In this manner it is predicted that the peak longitudinal tensile stresses in the weld region can be reduced to 15% of their normal values.