Processes, microstructure and properties of vanadium microalloyed steels

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 ~1nm 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

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

In multipass welding, the intercritically reheated coarse grained heat affected zone demonstrates the worst toughness in 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 heat affected zone (IC CG HAZ). The toughness of the simulated IC CG 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

Surface engineering of Ti-6Al-4V by nitriding and powder alloying using a CW CO2 laser

A comparison has been made of the laser processing of Ti-6Al-4V (IMI-318) alloy by (a) nitriding under a dilute atmosphere, (b) preplaced 6 µm SiC powder and (c) a combination of (a) and (b). At least six laser tracks were overlapped. Microhardness maps allowed details of the variation in hardness with both melt depth and track number to be determined. The microstructure was characterised and related to the processing parameters, melt track dimensions, hardness and roughness data. It was found that the preheat generated due to the overlapping, influenced the individual track dimensions, microstructure and properties, which were also affected by the laser energy density and the nitrogen concentration in the nitriding atmosphere used in processing. Process (b) was shown to produce the smoothest surfaces, with Ra values <2μm, whilst (c) gave the highest Ra, value, 8.6 μm. These results are considered together with a wide range of data in the literature on laser processed Ti-6Al-4V

Incorporation of 3 μm SiCp into Titanium surfaces using a 2.8 kW laser beam of 186 and 373 MJ m-2 energy densities in a nitrogen environment

The formation of composite layers using a 2.8 kW laser beam of 186 and 373 MJ m−2 energy densities, on commercial purity titanium surfaces preplaced with 3 μm size, 1-4 vol.% SiCp powder in a 100% nitrogen environment, produced gold colour tracks. The tracks gave reflective surfaces after glazing at an energy density of 373 MJ m−2 and dull or a mixture of dull and shiny surfaces at 186 MJ m−2 energy density. Surface cracks were visible in tracks containing 1 and 2 vol.% SiCp, but none were observed in the 4 vol.% SiCp tracks glazed at both energy densities. In the track cross sections, vertical cracks were seen in the 373 MJ m−2 tracks but it was absent in 186 MJm−2 tracks. The SiCp particles completely dissolved in all the tracks processed in this investigation producing a complex and inhomogeneous microstructure of dendrites and needle particles. At the half way of the melt depth from the surface, the dendrites were larger and densely populated, especially after glazing at 373 MJ m−2. The hardness measurement of the MMC layer recorded a wide range of hardness values which gave loops in the hardness profiles. Hardness values ranging from 700 to 1000 Hv were observed up to a melt depth of 1 mm in many tracks and the maximum surface hardness of 2250 Hv was measured in the track containing 1 vol.% SiCp and glazed at 373 MJ m−2. The surface hardness developed 5.6-15 times the base hardness (150 Hv) depending on the dendrite population. The 3 μm size SiCp produced MMC layers 1.5-2 times greater than those previously observed with 6 μm SiCp. The large surface area for an equivalent volume fraction of the three micron carbide particles is considered to have a high laser coupling action and hence absorbed more heat energy to produce deeper melt depth compared to those produced using the 6 μm SiCp

XRD and XPS studies of surface MMC layers developed by laser alloying Ti6Al4V using a combination of a dilute nitrogen environment and SiC powder

Using a continuous-wave CO2 laser, surface engineering of a Ti-6Al-4V alloy through a combined treatment of laser nitriding and SiC preplacement was undertaken. Under spinning laser beam conditions, a surface alloyed / metal matrix composite (MMC) layer over 300μm in depth and 24mm wide was produced in the alloy by the overlapping of 12 tracks. Microstructural and chemical changes were studied as a function of (a) depth in the laser formed composite layer and (b) of the track position. Using X- ray diffraction (XRD) and X-ray photospectrographic (XPS) techniques, it was shown that the composite layer contained a complex microstructure which changed with depth. At the surface, a non-stoichiometric, cubic TiNx solid solution ( possibly a carbonitride) containing C and Si , where x ≈ 0.65-0.8, was prominent, but was replaced by α′-Ti with increasing depth to 300μm. TiC phase was also identified, and the presence of TiN0.3 and Ti5Si3 phases considered a distinct possibility.

Vanadium microalloyed steel for thin slab casting and direct rolling

Vanadium microalloyed steels with high yield strength (»600 MPa), good toughness and ductility have been successfully produced in commercial thin slab casting plants employing direct rolling after casting. Because of the high solubility of VN and VC, most of the vanadium is likely to remain in solution during casting, equalisation and rolling. While some vanadium is precipitated in austenite as cuboids and pins the grain boundaries, a major fraction is available for dispersion strengthening of ferrite. Despite a coarse as-cast grain size, significant grain refinement can be achieved by repeated recrystallisation during hot rolling. Consequently, a fine and uniform ferrite grain structure is produced in the final strip. Increasing the V and N levels increases dispersion strengthening which occurs together with a finer ferrite grain size. The addition of titanium to a vanadium containing steel, decreases the yield strength due to the formation of V-Ti(N) particles in austenite during both casting and equalisation These large particles reduced the amount of V and N available for subsequent precipitation of fine (~5nm) V rich dispersion strengthening particles in ferrite

Laser surface modification of Ti alloys

The laser surface engineering of titanium alloys has been developed over the past 30 years to produce a modified layer up to 1mm depth, thicker than alternative techniques. CW C02 lasers have been the main lasers used for both surface cladding and alloying. Much of the early work was based on laser nitriding forming titanium nitrides throughout the molten pool. Subsequent alloying developments have included the incorporation of carbides, nitrides, oxides and silicides, and also intermetallics and rare earths, added as powders. Laser processing can now tailor surfaces with superior tribological and erosion resistant properties compared to the untreated titanium alloys

Single sided single pass submerged arc welding of austenitic stainless steel

The weld metal produced from a series of high productivity welds of 316LN austenitic stainless steel plate was examined to evaluate the effects of the use of a higher heat input process (> 2.5kJ/mm).This high heat input process was aimed at maximising single sided weld metal penetration in a single pass using simple square edge preparations and minimising time consuming handling operations. The evaluation was undertaken by correlating the local microstructure with the local toughness and microhardness of the cap, middle and root of the weld. It was established that the intermetallic phases / carbides present did not appear to have a significantly adverse effect on either corrosion or toughness. The phases observed and confirmed by the use of SAED were predominantly chi (χ) with some sigma( σ). No identifications were made of M23C6 which was observed in other studies of 316LN welds. A series of impact tests with variations in the notch positions showed that the thickness of the delta ferrite had an effect on the weld metal toughness. As a result of this work it was established that similar volume fractions of delta ferrite did not necessarily produce similar levels of weld metal toughness, but ferrite thickness did appear to have a contributory effect. Welding of 316LN stainless steel with a single sided single pass submerged arc welding process was satisfactorily undertaken up to 20mm plate thickness without preheat or post weld heat treatment. The ability to achieve this resulted in significant economic savings within the process for ship panel production combined with satisfactory weld metal properties

Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging: Part I - Processing parameters, mechanical properties and microstructure

A study simulating thin slab continuous casting followed by direct charging into an equalisation furnace has been undertaken based on six low carbon (0.06wt-%) vanadium microalloyed steels. Mechanical and impact test data showed properties were similar or better than those obtained from similar microalloyed conventional thick cast as rolled slabs. The dispersion plus dislocation strengthening was estimated to be in the range 80-250MPa.A detailed TEM/EELS analysis of the dispersion sized sub-15nm particles showed that in all the steels, they were essentially nitrides with little crystalline carbon detected. In the Steels V-Nb, V-Ti and V-Nb-Ti, mixed transition metal nitrides were present. Modelling of equilibrium precipitates in these steels, based on a modified version of ChemSage, predicted that only vanadium rich nitrides would precipitate in austenite but that the C/N ratio would increase through the two phase field and in ferrite. The experimental analytical data clearly points to the thin slab direct charging process, which has substantially higher cooling rates than conventional casting, nucleating non-equilibrium particles in ferrite which are close to stoichiometric nitrides. These did not coarsen during the final stages of processing, but retained their highly stable average size of ~7nm resulting in substantial dispersion strengthening. The results are considered in conjunction with pertinent published literature

Effect of strain level on the evolution of microstructure in a recently developed AD730 nickel based superalloy during hot forging

Design and control of microstructure of engineering parts made from nickel based superalloys with superior mechanical properties for high temperature applications, require the parts to be subjected to certain thermo-mechanical processing during forging. This often includes sequential straining and annealing at elevated temperatures followed by subsequent aging heat treatments at lower temperatures. In this study, the effect of strain magnitude on the evolution of microstructure during hot forging of a recently developed AD730 nickel based superalloy has been investigated. Microstructural heterogeneity was observed in a forged material manifested in a form of large non-recrystallized grains within the recrystallized matrix that is observed to be dependent on the level of deformation (i.e. strain magnitude). Analyses of microstructure indicated significant reduction in the fraction of low-angle grain boundaries and sub-structures with an increase in the applied strain, suggesting higher fraction of recrystallization with higher levels of strains. It was concluded that the lower strain levels were insufficient to provide enough driving force for complete recrystallization throughout the entire microstructure of the forged material