Howard J. Stone

High-temperature strain field measurement using digital image correlation:

A method is presented for obtaining good images of sample surfaces at high temperatures, suitable for strain measurement, by digital image correlation (DIC) without the use of surface markers or speckles. This is accomplished by suppressing black-body radiation through the use of filters and blue illumination. Using only relatively low levels of illumination the method is demonstrated to be capable of providing accurate DIC measurements up to 1100 °C, and the potential to monitor strains to 1400 °C is identified. The capability of the method is demonstrated by measuring the Youngs modulus and coefficient of thermal expansion of a nickel-base superalloy at temperatures from ambient to 1000 °C; two parameters that are well established in the literature and that require high strain sensitivity for their reliable determination.

Synchrotron X-ray studies of austenite and bainitic ferrite

High-resolution synchrotron X-ray diffraction has been used to conduct in situ studies of the temporal evolution of phases during the isothermal growth of bainite. Two populations of austenitic material were identified: one corresponding to the initial austenite and the other to the carbon-enriched austenite associated with the bainitic ferrite. The observed lattice parameters and the asymmetry of the peaks from the residual austenite have been interpreted in terms of the carbon partitioning due to the transformation. The results are contrasted with an earlier study in which the austenite unit cell appeared to split into two distinct densities prior to the onset of transformation.

Intergranular and interphase microstresses

Recent advances in the characterisation and modelling of microstresses developed between grains and phases in a polycrystal are reviewed. The evolution of microstresses in materials needs to be understood and acknowledged when interpreting diffraction-based measurements of residual stress fields. Measurements of the associated microstrains can be used to test self-consistent models for polycrystalline deformation and texture evolution. Microstress development in face-centred cubic metals is now well understood, and progress is being made for other crystal systems and for two phase materials.

On the generation of microstrains during the plastic deformation of waspaloy

Abstract Neutron diffraction has been used to characterize the development of microstrains during the plastic deformation of the polycrystalline nickel-base alloy, Waspaloy. Two types of experiment were performed: (i) in situ tensile testing on the diffractometer to determine the response parallel and perpendicular to the loading direction and (ii) measurement of the orientation dependence of the microstrain accumulation using a Eulerian cradle. Large residual microstrains are shown to develop. Along the loading direction, these are typically tensile in the γ ′ phase and compressive in the γ phase; however, the values are sensitive to both the orientation of the diffracting crystallite and the amount of plastic deformation of the material. The behaviour is due to the differential deformation between grains in differing orientations (intergranular microstresses) and the two phases (interphase microstresses). It is shown that at low bulk plastic strains, intergranular microstresses develop rapidly, whilst at larger plastic strains the microstresses arising from interphase interactions become dominant. These effects have implications for the determination of residual stresses using diffraction-based techniques and these are discussed.

The Effects of Filler Metal Transformation Temperature on Residual Stresses in a High Strength Steel Weld

Residual stress in the vicinity of a weld can have a large influence on structural integrity. Here the extent to which the martensite-start temperature of the weld filler metal can be adjusted to engineer the residual stress distribution in a bainitic-martensitic steel weld was investigated. Three single-pass groove welds were deposited by manual-metal-arc welding on 12 mm thick steel plates using filler metals designed to have different martensite-start temperatures. Their longitudinal, transverse, and normal residual stress distributions were then characterized across the weld cross section by neutron diffraction. It was found that tensile stresses along the welding direction can be reduced or even replaced with compressive stresses if the transformation temperature is lowered sufficiently. The results are interpreted in the context of designing better welding consumables. [DOI: 10.1115/1.3122036]

Design of weld fillers for mitigation of residual stresses in ferritic and austenitic steel welds

Abstract Residual stresses that arise as a result of welding can cause distortion, and also have significant implications for structural integrity. Martensitic filler metals with low transformation temperatures can efficiently reduce the residual stresses generated during welding, because the strains associated with the transformation compensate for thermal contraction strains during cooling. However, it is vital that a low weld transformation temperature is not obtained at the expense of other important material properties. This article outlines the alloy design process used to develop appropriate low transformation temperature filler materials for the mitigation of residual stresses in both low alloy ferritic and austenitic stainless steel welds. Residual stresses in single pass, 6 mm bead in groove welds, on 12 mm thick plates, have been measured and compared against those obtained with commercially available conventional austenitic and ferritic filler materials. The filler metals developed here exceeded requirements in terms of weld mechanical properties, while significantly reducing the maximum residual stress in the weld and heat affected zone.

Transformation Temperatures and Welding Residual Stresses in Ferritic Steels

Residual stress in the vicinity of a weld can have a large influence on structural integrity. Here the extent to which the martensite-start temperature of the weld filler metal can be adjusted to mitigate residual stress distributions in ferritic steel welds has been investigated. Three single-pass groove welds were deposited by manual-metal-arc welding on 12mm thick steel plates using filler metals designed to have different martensite-start temperatures. Their residual stress distributions were then characterised by neutron diffraction. It was found that a lower transformation temperature leads to a potentially less harmful stress distribution in and near the fusion zone. The experimental method is reported and the results are interpreted in the context of designing better welding consumables.Copyright

Study of cation order-disorder in MgAl2O4 spinel by in situ neutron diffraction up to 1600 K and 3.2 GPa

Abstract The temperature-dependence of the cation distribution in synthetic spinel (MgAl2O4) was investigated using in situ time-of-flight neutron powder diffraction at ISIS, the pulsed-neutron source at the Rutherford Appleton Laboratory. Neutron diffraction patterns of stoichiometric MgAl2O4 were collected on heating from room temperature to ∼1600 K at pressures of ∼2.6 GPa. The cation distribution was determined directly from site occupancies obtained by Rietveld refinement. The equilibrium non-convergent ordering was analyzed using the OʼNeill-Navrotsky (1983) thermodynamic model, which fits the observed behavior well over the temperature range of the measurements. Fitting the data between 790 and 1600 K yields α = 31(6) kJ/mol and β = -20(13) kJ/mol in the expression for the free energy of ordering. The high-pressure temperature-dependent behavior, as compared to equivalent ambient-pressure behavior, demonstrates that disordering occurs to a much greater extent in MgAl2O4 at high pressure and that pressure favors disordering toward the inverse structure.

A two phase elastic–plastic self-consistent model for the accumulation of microstrains in Waspaloy

Abstract A two phase self-consistent scheme for the accumulation of elastic lattice strains in the nickel-base superalloy Waspaloy is presented. The microstructure is idealised as a set of randomly orientated anisotropic grains which are assumed to be spherical and embedded within a homogenous effective medium which is assigned the properties of the bulk. Each grain is modelled as a medium with the properties of identically aligned cubic γ -Ni with a spherical inclusion of γ ′-Ni 3 Al. The γ ′ is treated as an elastic anisotropic solid, and the γ is modelled as an elastic–plastic single crystal according to the Taylor–Bishop–Hill plasticity theory. The diffraction elastic constants and microstrains accumulated are compared with those found experimentally using neutron diffraction, as previously reported by Stone et al . The predictions are found to be adequate, and in particular the shift of load, as reflected in the microstrain, from the matrix to the precipitate between 3 and 10% plastic strain is reproduced. The implications of these results for the development and use of Reitveld-derived schemes for the measurement of bulk residual stress at spallation sources are discussed.

Effect of interpass temperature on residual stresses in multipass welds produced using low transformation temperature filler alloy

Abstract Weld filler alloys that exploit transformation plasticity through low austenite to martensite transformation temperatures offer an effective method of reducing residual stresses in strong steel welds. However, in multipass welds, the heat input from later weld passes may be insufficient to retransform prior welding passes, leading to the accumulation of thermally induced strains and elevated residual stresses. In this work, the residual stress distributions produced around arc welds fabricated with a martensitic weld filler alloy that transforms at a low temperature have been studied as a function of the number of passes deposited and the interpass temperature. It is found that when the interpass temperature is above the transformation temperature of the weld metal, the entire multipass weld transforms as a single entity, thus permitting the optimum exploitation of the transformation plasticity. In contrast, the deposition of new metal with a relatively low interpass temperature leads to increased residual stresses in the underlying layers, reducing or eliminating the beneficial stress states previously created.