Welding residual stresses in ferritic power plant steels

Abstract

Many of the degradation mechanisms relevant to power plant components can be exacerbated by stresses that reside within the material. Good design or structural integrity assessments require therefore, an accounting of residual stresses, which often are introduced during welding. To do this it is necessary to characterise the stresses, but this may not be possible in thick components using non-destructive methods. These difficulties, and a paucity of relevant engineering data, have led to an increasing emphasis on the development and validation of suitable modelling tools. Advances are prominent in the estimation of welding residual stresses in austenitic stainless steels. The progress has been less convincing in the case of ferritic alloys, largely due to the complexities associated with the solid state phase transformations that occur in multipass welding. We review here the metallurgical issues that arise in ferritic steel welds, relate these to the difficulties in calculating residual stresses, and highlight some stimulating areas for future research.

Numerical Predictions for the Thermal History, Microstructure and Hardness Distributions at the HAZ during Welding of Low Alloy Steels

“…3 kg m − . The thermal conductivity and specific heat for individual phases reported in the literature depend of the authors, although follows similar trends [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][24][25][26][27][28][29][30][31][32][33] . The reason for such lack is due to the possibility of different features of the phases formed depending on the thermal history.…”

“…The available data 22 can be fitted as shown in Equation 6. However, a common approach is to assume constant emissivity during welding simulations when Equations 2 and 3 are used to dynamically impose the volumetric heat source due to the welding arc, where the heat source parameters includes the net heat transferred to the workpiece at high temperatures [5][6][7][8][9][10][11][12][13][17][18][19] . In this model, the Equation 6, obtained by regression of the data presented by Paloposki and Liedgust 22 , is used for imposing the radiative cooling boundary conditions additionally to the double ellipsoid heat flux.…”

“…Equation 6 predicts constant emissivity for temperature below 300 o C and above 750 o C. This behavior is due to the range of measured data for low-alloyed steels, which is assumed to have similar behavior to the steel considered in this study. A clear shortcoming of this approach is that effects related to the surface parameters and environment temperature and composition are not considered and assumed negligible during the cooling by radiation and natural convection [5][6][7][8][9][10][11][12][13][17][18][19] . These shortcomings are usually overcame by adjusting the heat source parameters, shown in Table 2, which is dominant for the high temperature region (above 750 o C), meanwhile, the radiation cooling effects for low temperature (below 200 o C ) are negligible compared with the convection contribution.…”

“…Several works have been carried out and highlighted the importance of taking into account the phase transformations effects and properties evaluation developed during welding [2][3][4][5][6][7][8][9][10][11][12][13] . Simultaneously, the numerical methods have been a fundamental tool for supporting qualitative and quantitative analysis of these stresses, as well as on the prediction of the resultant microstructure and mechanical properties of the weldment.…”

Formation of welding residual stresses in low transformation temperature (LTT) materials

“…In the bcc lattice (R e ⇑), by contrast, shrinkage compensation is lower involving comparably high stress gradients. Francis et al [10] reported that displacive transformation from austenite into martensite gives higher volume change compared to reconstructive transformation of ferrite or pearlite due to a dilatation normal and a large shear strain component parallel to the habit plane in case of displasive transformation. Also, displacive transformation at low temperatures favours crystollagraphic variant selection due to higher acting stresses at these temperatures, leading to a transformation texture and anisotropic dimensional changes from what larger stress reductions can be expected.…”