High Temperature Heat-affected Zone Research Articles

Microstructural characterization of simulated heat affected zone in a nitrogen-containing 2205 duplex stainless steel

In order to investigate the microstructural evolution in a nitrogen-bearing 2205 duplex stainless steels (DSS) during welding, a simulated weld thermal cycle with 5 kJ cm −1 heat input followed by exposure at 700 °C for different time intervals was performed. The microstructure of high-temperature heat affected zone (HTHAZ) developed with the thermal experience was characterized via optical metallography and transmission electron microscopy (TEM). The duplex structure with equivalent phase components was drastically destroyed by the rapid thermal cycle. In the simulated HTHAZ structure, three different morphologies of newly formed austenite were observed in the coarse-grained δ-ferrite matrix; i.e. allotriomorphic austenite, Widmanstäten autenite and intragranularly nucleated autenite. During the exposure at 700 °C, the intragranularly nucleated austenite got coarse and the Widmanstäten austenite grew progressively. TEM revealed that several variants of rod-like Cr 2N were precipitated selectively at intragranular and intergranular sites. From the analyses of diffraction patterns of TEM, Kurdjumov–Sachs orientation relationship was found to describe the interface between intragranularly nucleated autenite and δ-ferrite, while Pitch–Schrader orientation relationship to describe the disposition between hexagonal Cr 2N precipitates and δ-ferrite matrix.

Modelling creep failure in welded plates under uniaxial loading

Understanding more fully the behaviour of weldments is important since they can reduce the lifetime of structures at high temperatures, but performance factors are omitted from most existing design codes (with the exception of the high temperature fast reactor codes). Simple mathematical models which produce approximate solutions quickly are useful since they allow the design engineer to carry out rapid simulations. In this paper the ideas of continuum damage mechanics are incorporated into a model, based on the Cosserat theory of plates and a multi-axial version of Norton's creep law, which has been used recently to calculate the steady state creep strain rates in a weldment. Failure results are calculated numerically for a ferritic plate of constant thickness subject to uniaxial loading, the plate containing parent and weld metals, type IV material and a high-temperature heat-affected zone (HAZ). Results obtained for various material parameters and weldment configurations show that rupture times depend strongly on the choice of generalized stress in the damage evolution equation. The results also reveal that changes to the material strength of a region cannot be made in isolation if physically realistic results are to be obtained and, further, that early failures can occur if there are large differences in strength between the various parts of the weldment.