Microstructural characterisation of duplex 316 weld metals : the effects on mechanical and high temperature properties

Several 316 manual metal arc weld metals produced with commercial and experimental consumEbles, have been aged between 600-850oC. This resulted in the progressive dissolution of the δ-ferrite with the appearance of austenite, M_23C_6, MX and intermetallics σ, X and η phase. The amount and distribution of the secondary phases were strongly dependent on small compositional variations including C, Cr, Mo, Si and residual Nb, Ti and V. In addition, the morphologies of the M_23C_6 and intermetallics σ and X were sensitive to the ageing temperature. During the δrightarrowy transformation, the δ-ferrite becomes progressively enriched in Cr, Mo and Si and depending on the local composition of the δ-ferrite, eventually transforms to intermetallic σ or X. A model has been formulated in which the rate of δ-ferrite dissolution is controlled by solute diffusion along the δ/y boundary. The effect of ageing and the subsequent microstructural transformations on the tensile, impact and creep properties, have been discussed in terms of changes in the precipitate distribution and morphology. Although the effect of secondary phases on the tensile properties were relatively small, the precipitate distribution and morphology had a large effect on both the creep properties and the ambient impact toughness. In the creep tests, the duplex deposits of nominally the same composition exhibited very large differences in minimum creep rate, with the rutile deposits being approximately 2-3 orders of magnitude stronger than the duplex deposits. A correlation between the minimum creep rate and weld metal composition has been established for all the duplex deposits. The model implies that the variations in rupture properties are largely controlled by the residual levels of Nb, Ti and V present in the deposits. It has been shown that the formation of carbide and intermetallics is not necessarily a pre-requisite factor for the embrittlement of the weld metal. The present study indicates that the sensitivity of the weld metal to embrittlement by carbide and intermetallics is dependent on the matrix ductility or the MX particle dispersion. Low ductility fracture occurred when the δ/y or y/y grain boundaries were outlined by continuous films of carbide and intermetallic, in the presence of hardened grain interiors.