T.N. Baker

Complex heterogeneous precipitation in titanium-niobium microalloyed Al-killed HSLA steels - I. (Ti,Nb)(C,N) particles

Abstract Precipitation in Ti–Nb Al-killed microalloyed HSLA steels (Ti/N weight ratio from 4.4 to 1) was investigated in both the as-rolled and the normalised conditions using analytical electron microscopy including parallel electron energy loss spectroscopy (PEELS). An extensive formation of heterogeneously nucleated complex (Ti,Nb)(C,N) particles down to 10 nm in size was observed. The core of such a complex particle is based on TiN and has a spherical, cubic or cruciform shape. The N/(Ti+Nb) atomic ratio in the core is similar to the average value in the steel whereas the Nb/Ti ratio is much smaller than the average value and not proportional to it. Many of the cores have caps in the form of epitaxial overgrowths based on NbC. Their composition changes from Nb(C,N) to (Nb,Ti)C as the N/Ti ratio decreases. The formation of these complex particles and their detailed morphology are controlled by the processing conditions as well as the overall composition.

Processes, microstructure and properties of vanadium microalloyed steels

Abstract Vanadium as an important alloying element in steels was initially associated with the properties achieved following tempering. Interest in the microstructure was stimulated by the advent of transmission electron microscopes with a resolution of ∼1 nm together with selected area electron diffraction techniques. A second timely development was that of controlled rolling, particularly of plate and sheet products. The scope of this review will include the historical background on quenched and tempered vanadium steels, precipitation during isothermal aging, conventional controlled rolling and during thin slab direct charging and the development of strength and toughness in vanadium microalloyed steels. The characterisation of microstructure, in particular the methods for the analysis of the chemical composition of precipitates has progressed since the availability of X-ray energy dispersive analysis in the 1970s, and the role played by electron energy loss spectroscopy in providing quantitative analysis of carbon and nitrogen in vanadium microalloyed steels will be presented. There are still many topics involving vanadium microalloyed steels that are controversial. These include the nucleation sequence of homogeneous precipitates of vanadium carbonitride and whether this occurs coherently, the composition of the vanadium precipitates, the nucleation mechanism for interphase precipitation, the importance of strain induced precipitation in austenite of vanadium carbonitride, the contributions of both interphase precipitation and random precipitation in ferrite to the yield strength, and the role of the process route parameters in developing properties. These topics will be considered in this paper which concentrates on hot rolled vanadium microalloyed steels placed in the context of pertinent research on other alloys.

Analysis of the phases developed by laser nitriding Ti6Al4V alloys

Laser nitriding Ti-6Al-4V (Ti64) alloys produces complex microstructures in the solidified melt pool developed during the processing. The microstructures contain mainly titanium nitrides and the {alpha} (or {alpha}{prime}) titanium phase in the matrix. Difficulties have been encountered in identifying the phases and examining the level of residual stress induced by the processing, because of the rapid cooling rate and the nitrogen concentration introduced during the laser gas alloying which have a combined influence in determining the lattice parameters of the phases. The present work has investigated the microstructures formed mainly in a Ti64 alloy under pure argon and various nitrogen concentration environments during the processing, and also to a lesser extent commercial purity titanium (under 20 vol.% N only) studied for comparison purposes, using metallography, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). A 5 kW continuous CO{sub 2} laser with a spinning beam was used for the processing. As a result of this analysis, the identification of the TiN{sub 0.3} phase formed in a Ti64 alloy was confirmed, and the influence of the nitrogen concentration was elucidated. This is considered to be helpful for a better understanding of the laser processing process.

Effect of morphology of martensite–austenite phase on fracture of weld heat affected zone in vanadium and niobium microalloyed steels

Abstract In multipass welding, the intercritically reheated coarse grained heat affected zone (HAZ) demonstrates the worst toughness in the welded joint, since it contains a high carbon martensite with some retained austenite, known as M–A phase, which is brittle and associated with the high cooling rates following welding. The purpose of the present work was to explore those aspects of the morphology of the M–A phase which determined the ease or otherwise of crack development in welded vanadium and niobium high strength low alloy steels. Four steels were subjected to heat treatment to simulate the microstructure of an intercritically reheated coarse grained HAZ. The toughness of the simulated intercritically reheated coarse grained HAZ was assessed using both Charpy and CTOD tests. Microstructural features were characterised by scanning and transmission electron microscopy and optical microscopy. Fractographic examination of the Charpy and CTOD specimens were carried out to understand the micromechanism of fracture under different microstructural and test conditions. Evidence of both cracking and debonding of M–A phase and carbides was found, and many of the cracks appeared to develop by linking up of voids resulting from debonding. The importance of the dihedral angle 2θ in determining the interfacial energy of the two main morphologies of the M–A phase, blocky and elongated stringer particles, was considered. While both carbides and inclusions were observed, these features appear to have a minor role in determining the degree of toughness of the steels.

Study of the surface layer formed by the laser processing of Ti–6Al–4V alloy in a dilute nitrogen environment

Optical microscopy, scanning electron microscopy (SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and a hardness tester were used to understand the microstructure and to characterise the phases formed in a Ti–6Al–4V alloy at different tracks and depths, produced by laser nitriding under a dilute nitrogen environment with a spinning beam. The results show that the solidified melt consists of α′-Ti and TiNx. Using XRD, x was determined to be 0.75 while from XPS, x lay in a range of 0.5–0.8. The melt zone showed a range of hardness of between 500 and 800 HV.

Effects of strain rate and temperature on deformation behaviour of IN 718 during high temperature deformation

Abstract The hot deformation characteristics of a wrought IN 718 alloy were investigated by compression testing at constant strain rates in the range of 0.1 to 5 × 10 −3 s −1 , and testing temperatures in the range of 950 to 1100 °C using a 200 ton capacity microprocessor controlled Fielding hydraulic press. Examination of the microstructures was carried out by optical microscopy and TEM. The flow stress of the compression tests showed a single peak in the flow stress-strain curves, and indicated that a dynamic recrystallization transition took place during the hot compression. The relationship between the peak stresses (σ p ) and the Zener-Hollomon parameter ( z ) can be expressed by σ p = 0.5Z 0.17 . “Necklace” microstructures were observed at testing temperatures below 1050 °C, for strain of 0.7. The fraction of recrystallized grains increased with the increasing temperature and strain, and decreasing strain rate. Fully recrystallized microstructures were observed at temperatures 1050 °C or greater, with a strain of 0.7.

Crack-free hard surfaces produced by laser nitriding of commercial purity titanium

The results of laser nitriding of commercial purity titanium (CPTi) in diluted and undiluted nitrogen environments are described in this paper in terms of surface cracking, surface hardness, hardness profile and microstructure. Surface cracking can now be totally eliminated either by nitriding in a dilute nitrogen atmosphere or by controlling the laser parameters and gas flow rate in a pure nitrogen environment. The hardness developed in a crack-free trail surface produced in a dilute nitrogen environment was less than half of the maximum hardness that can be attained using pure nitrogen. The surface hardness after glazing in a dilute environment was dependent on the nitrogen concentration in the mixture, and increased with increasing nitrogen content. The nitrided layer generated in a dilute environment possessed a fine needle-like structure, the needle size increasing with increasing nitrogen concentration in the gas mixture. In a pure nitrogen environment the nitrided layer possessed a dendritic structure of titanium nitride. The trail surfaces were found to be relatively smooth when nitrided at 50 mm s− using 2.8 kW laser power with a 5 mm defocused distance.

Effects on dynamic and metadynamic recrystallization on microstructures of wrought IN-718 due to hot deformation

Abstract The recrystallized grain size of IN-718 under hot-working conditions is related to the deformation parameters. The high temperature deformation of IN-718 in compression has been studied at temperatures of 950–1100°C over a range of constant strain rates of 0.1, 5 × 10−2 and 5 × 10−3 s−1 using a 200 tonf press. After deformation the material was water quenched. The time interval between the termination of deformation and water quenching was a critical factor in this work. It was found that the dynamically recrystallized grain sized increased with increasing temperature and decreasing strain rate under steady state conditions. Metadynamic recrystallization was observed at temperatures of 1050 °C and higher with strain rates of 0.1 and 5 × 10−2 s−1. The fraction of metadynamic recrystallization increased with increasing strain rate and temperature. An increase in metadynamic recrystallization leads to a relative increase in the recrystallized grain size. This study clarified some of the disputed areas involving the effect of strain rate on the final grain size.

Effects of nitrogen gas flow rates on the microstructure and properties of laser-nitrided IMI318 titanium alloy (Ti–4V–6Al)

Abstract Laser surface melting of IMI318 titanium alloy under a pure nitrogen environment of different gas flow rates, was carried out with a 1.4 kW laser beam, produced by a 5 kW CO2 continuous laser at 5 mm defocused distance, and 10 and 50 mm s−1 sample traverse velocities. The nitrided tracks were analysed in terms of microstructures, hardness, melt depth and surface conditions, including surface cracking. The tracks contained surface cracks, and showed gold-coloured melt surfaces, with decreasing colour intensity at lower laser energy densities and reduced gas flow rates. The microstructures consisted mainly of dendrites, the concentration of dendrites being found to increase with increasing gas flow rates and laser energy densities. A maximum surface hardness of 1480 VHN was developed after glazing in a pure nitrogen environment, the hardness being found to be related to the dendrite populations. Lower hardnesses were recorded with decreasing gas flow rates and microstructures containing mixtures of dendrites and needles.

Characteristic features of laser-nitrided surfaces of two titanium alloys

Trails of IMI115 commercial purity titanium (CP-Ti) and IMI829 alloys, glazed in a pure nitrogen environment using a 1.35 kW CO2 laser beam with different specimen speeds, have been characterized in terms of surface conditions, hardness, melt depth and microstructure. Rippling marks and porous edges were found in all the trail surfaces. Pores and cracks along the width of the trails were evident. Cracks were observed to be connected with pores, and they were found to propagate down the melt depth where they were arrested close to the heat-affected zone. The cracks were found to increase in number with increasing speed. The melt region consists of a dendritic structure of TiN, and the dendrites were more prevalent in CP-Ti than in IMI829. The melt profile was irregular in shape, and the generation of a differential capillary flow pattern as well as the precipitation of TiN in the melt are considered to be responsible for this phenomenon. A very steep hardness profile was produced in all the trails. Nitrided CP-Ti and IMI829 both had a high Vickers surface hardness of around 2000 HV. The precipitation of TiN dendrites is thought to increase the hardness of the melt in general and, in particular, the larger dendrites developed a higher hardness in CP-Ti than in IMI829.