Martin Strangwood

Intergranular Corrosion and Stress Corrosion Cracking of Sensitised AA5182

AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.

The massive transformation in TiAl alloys: Mechanistic observations

Abstract The massive α → γm transformation, as observed using analytical transmission electron microscopy, in Ti49Al, Ti48Al2Nb2Mn, Ti55Al25Ta and Ti50Al20Ta alloys is described. Conventional solution heating and quenching experiments have been combined with the more rapid quenching possible using electron beam melting in order to provide further insight into the early stages of the transformation in these alloys. It is shown that the γ develops first at grain boundaries as lamellae in one of the grains and that these lamellae intersect and spread into the adjacent grain in a massive manner. Consequently, there is no orientation relationship between the massive gamma (γm) and the grain being consumed whereas there is the expected relation between the γm and the first grain which is inherited from the lamellae. It is further shown that the γm grows as an f.c.c. phase after initially growing with the L10 structure. Furthermore, it is shown that the massive f.c.c. phase then orders to the L10 structure producing APDB-like defects which are actually thin 90° domains separating adjacent domains that have the same orientation yet are out of phase. The advancing γm interface tends to facet parallel either to one of its four {111} planes or to the basal plane in the grain being consumed by impinging on existing γ lamellae. Thin microtwins and α2 platelets then form in the γm presumably due, respectively, to transformation stresses and supersaturation of the γm with titanium for alloys containing ∼48% Al; indeed, there is a local depletion in aluminium across the α2 platelets as determined using fine probe microanalysis.

Cleavage initiation in Ti–V–N and V–N microalloyed ferritic–pearlitic forging steels

The effects of silicon (0.53 and 1.05 wt.%) and titanium (<0.002 and 0.022 wt.%) on microstructure and mechanical properties of vanadium microalloyed medium carbon steels heat treated after rolling to simulate the thermal cycle of hot forging have been determined using room temperature tensile tests, impact tests, optical microscopy and scanning electron microscopy (SEM). Silicon was found to increase strength values whilst titanium had a strong refining action on prior austenite grain size. Room temperature Charpy ‘U’ notch impact energies were all on the lower shelf; ductile–brittle transition temperatures, determined from fracture appearance in Hounsfield impact tests, ranged from 100 to 145°C, scaling with material strength. Initiation in the Charpy tests was by microcracking of coarse (Ti,V)(C,N)-containing single or multi-phase inclusions except in the low strength, titanium-free case when the absence of a completely continuous grain boundary ferrite layer allowed matrix microstructure initiation by interfacing pearlite colonies to occur.

Preliminary study of the inhomogeneous precipitate distributions in Nb-microalloyed plate steels

Three as-thermomechanically controlled rolled (TMCR) microalloyed steels containing Nb levels between 0.023 and 0.057 wt% have been characterised in terms of phase balance and ferrite grain size distribution (optical microscopy and image analysis). In addition, transmission electron microscopy (TEM) has been carried out on carbon extraction replicas from sub-surface and mid-thickness positions to determine fine precipitate size distribution and areal number density in ferrite and pearlite. Precipitate distributions were also determined after simulated re-heating schedules. The ferrite grain size distributions have been related to precipitate distributions and rolling schedules, whilst the development of precipitate distributions has been considered based on solidification, reheating and deformation behaviour.

Fracture behaviour in medium-carbon Ti–V–N and V–N microalloyed ferritic-pearlitic and bainitic forging steels with enhanced machinability

Abstract The microstructure, tensile and impact behaviour of medium-carbon Ti–V–N and V–N microalloyed resulphurised forging steels with and without Cr and Ca additions have been determined before and after forging up to 75% reduction. Titanium additions refined the prior austenite grain size resulting in ductile failure becoming fully transgranular rather than mixed inter- and transgranular. However, the titanium additions also resulted in the presence of coarse (Ti,V)(C,N) particles which acted as cleavage initiation sites reducing toughness by increasing the fracture appearance transition temperature. Ductile fracture mode variation did not affect upper shelf energy, which was determined by inclusion volume fraction (reduced by Ca additions) and, to a lesser degree, by the inclusion size and shape (modified by Ca addition and forging). The addition of Cr increased the hardenability of the steels resulting in the presence of bainite in the otherwise ferrite and pearlite microstructure. A small fraction of bainite (7%) was found to improve the strength levels however a large fraction (83%) was less effective resulting in an increase in the tensile strength but a decrease in the proof strength. The amount of bainite did not affect the fracture properties of the steels with the other microstructural features of inclusion volume fraction and type having a dominant effect.

Hydrogen-assisted stable crack growth in iron-3 wt% silicon steel

Observations of internal hydrogen cleavage in Fe3Si are reported. Hydrogen-assisted stable crack growth (H-SCG) is associated with cleavage striations of a 300 nm spacing, observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). High resolution SEM revealed finer striations, previously undetected, with a spacing of approximately 30 nm. These were parallel to the coarser striations. Scanning tunnelling microscopy (STM) also showed the fine striation spacing, and gave a striation height of approximately 15 nm. The crack front was not parallel to the striations. Transmission electron microscopy (TEM) of crack tip plastic zones showed {112} and {110} slip, with a high dislocation density (around 1014 m−2). The slip plane spacing was approximately 15–30 nm. Parallel arrays of high dislocation density were observed in the wake of the hydrogen cleavage crack. It is concluded that H-SCG in Fe3Si occurs by periodic brittle cleavage on the {001} planes. This is preceded by dislocation emission. The coarse striations are produced by crack tip blunting and the fine striations by dislocations attracted by image forces to the fracture surface after cleavage. The effects of temperature, pressure and yield strength on the kinetics of H-SCG can be predicted using a model for diffusion of hydrogen through the plastic zone.

Bauschinger effect in Nb and V alloyed line-pipe steels

Abstract The UOE process is used for cold forming of large diameter steel line-pipes. Pipe strength has been found to increase (work hardening) or decrease (Bauschinger effect) after the UOE process compared to the plate depending on the steel grade, plate and pipe processing history. The steel chemistry, through the presence of microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and presence of retained work hardening, affects the magnitude of the Bauschinger effect. In this paper the microstructures of two (Nb and Nb–V alloyed) steel plates, in terms of (Nb,V)(C,N) particle distributions and dislocation densities, have been related to the Bauschinger parameters in the as rolled and annealed initial conditions. The Bauschinger stress parameter increases with microalloy particle number density and dislocation density increase and the relative importance of the two effects is discussed.

Measurement of phase transformation in steels using electromagnetic sensors

Abstract An electromagnetic (EM) sensor, capable of detecting the formation of ferromagnetic ferrite from paramagnetic austenite below the Curie temperature, has been developed and assessed. The long term aim of this work is to develop a method for monitoring microstructure online during strip steel processing. In the present paper, the initial results of variation in trans-impedance with microstructure obtained for three different types of steel with varying carbon contents are discussed. It was found that the EM sensor can successfully detect the formation of ferrite below the Curie temperature, but trans-impedance values are affected by the presence of a decarburised ferrite ring that forms around the specimens tested in a furnace. It was also found that the trans-impedance value is monotonically (non-linearly) related to ferrite volume fraction, and depends on the morphology and distribution of the ferromagnetic phase and, hence, is influenced by the prior austenite grain size.

Effect of deformation and Nb segregation on grain size bimodality in HSLA steel

Abstract Bimodal ferrite grain sizes (mixed coarse and fine grain bands) have been observed in Nb microalloyed thermomechanically controlled rolled (TMCR) steel plates and are undesirable as they can reduce toughness. This paper examines the role of rolling deformation on the formation of bimodal grain structures in reheated continuously cast slab material with initial uniform or bimodal austenitic grain structures. The slab material contains solute rich and solute poor regions, due to interdendritic segregation, which have been shown to cause bimodal austenite grain structures during reheating within a certain temperature range. It is known that deformation in the partial recrystallisation region can result in a mixed coarse and fine grained structure. Theoretical calculations (based on the Dutta–Sellars model) and deformation experiments indicated that the segregated microalloying elements (particularly Nb) can promote bimodality during deformation by affecting the local recrystallisation kinetics over a much wider range of temperatures than for a homogeneous material.

Segregation behaviour in Nb microalloyed steels

Abstract Four continuously cast slabs from two sources were characterised by optical microscopy and scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS) in the as cast state and after normalisation close to the Ae 3 temperature. For all four slabs the composition of the majority of the slab depth (>80%) was the bulk composition and in this region microsegregation, particularly of Nb, was found to agree with predicted partition ratios from equilibrium thermodynamics. For two of the slabs, which gave final solidification as single phase austenite, equilibrium thermodynamics were also able to predict the compositional centreline macrosegregation. In the other two cases, this simple approach was not successful, which has been ascribed to solidification as a more complex phase mixture (austenite and δ-ferrite) or more complex processing during solidification (e.g. soft reduction).