Welding Procedures and Type IV Phenomena J. A. Francis, V. Mazur CSIRO Manufacturing and Infrastructure Technology, Adelaide, Australia H. K. D. H. Bhadeshia Materials Science and Metallurgy, University of Cambridge, United Kingdom

Background Part of a collaborative project involving CSIRO, Universities of Wollongong and Cambridge. Part of a collaborative project involving CSIRO, Universities of Wollongong and Cambridge. Failure of welds in 9-12 wt % Cr ferritic power plant steels. Failure of welds in 9-12 wt % Cr ferritic power plant steels. If implemented, these steels enable higher steam temperatures and pressures, greater thermodynamic efficiency. If implemented, these steels enable higher steam temperatures and pressures, greater thermodynamic efficiency. Aim: to develop technologies, procedures that ameliorate type IV cracking. Aim: to develop technologies, procedures that ameliorate type IV cracking.

The Type IV Problem 9–12 wt. % Cr ferritic power plant steels are usually: Austenitised between 1050 and 1080 o C. Austenitised between 1050 and 1080 o C. Transformed to martensite. Transformed to martensite. Tempered between 740 and 820 o C. Tempered between 740 and 820 o C. Post-weld heat treatment at ~ o C. BASE METAL  Normalised & Tempered  Unaffected by Welding  Heat treated ~ 750 o C WM WELD METAL, CGHAZ  Normalised & Tempered  Austenitised  Heat treated~ 750 o C FINE-GRAINED, IC-HAZ  Normalised & Tempered  Carbide Coarsening, Partial...austenitisation  Heat treated ~ 750 o C

Neural Networks in a Bayesian Framework Why neural networks? Data on type IV creep failures are available in the literature. To date, interpretation based on HAZ microstructures, mainly qualitative. Quantitative models when data available, but physical model is not. Why Bayesian? Simple models are favoured. Bayesian framework enables reliable estimates to be obtained of: i) modelling uncertainty. ii) model-perceived level of noise in the output.

Σ f Σ f Σ f Σ f Σ f y 1 x1x1 xjxj 1 Input LayerHidden LayerOutput Layer  Three-layer feed forward networks.  Activation function in second layer is a hyperbolic tangent; linear …in third layer.  Transfer functions together with weight distributions completely …define model.

The Database VariableMinimumMaximumVariableMinimumMaximum C wt. % Normalising Temp. ( o C) N Normalising Time (h)0.52 B00.003Tempering Temp. ( o C) Cr8.4512Tempering Time (h)16 Mo Heat Input (kJ/mm) V Preheat Temperature ( o C) Nb Preparation Angle (deg.)045 W02.21PWHT Temperature ( o C) Mn PWHT Time (h)0.258 Si Internal Pressure Test? (0/1)01 Cu03Test Temperature ( o C) Ni Test Duration (h) Al Rupture Stress (MPa)40150  53 type IV failures included in database.  Overambitious set of variables can limit data available for analysis.

Overfitting Problem Training involves fitting a flexible, non-linear function to a training database. Possibility of fitting noise in the training data. Model-perceived noise level in the rupture stress was 5%.

Pre-Setting the Level of Noise in the Output Model forced to stop training at noise level of 15%. Why 15% ? 1)Test and training data must be similarly noisy. 2)Significance of input variables should be insensitive to seeds used to initiate training.

Significance of Input Variables Significance of Input Variables Magnitude of bar indicates how much input explains variations in output. Small magnitude implies unimportant input or influence lost in 15 % noise. Major effects correctly recognised by the model. Preheat temperature perceived as significant, heat input as insignificant.

Preheat TemperatureHeat Input  Modelling uncertainty increases where dataset …sparsely populated.  Effect of preheat temperature is unambiguous.

Interpretation of Trends

 Type IV cracking occurs in creep-softened region between regions …that are harder in creep.  There is a mismatch in creep strain within HAZ during a cross-weld …creep test.  Mismatch induces triaxial stresses in type IV region, encouraging …localised damage. (Albert et al. (2004); Li et al. (2003))  We propose that triaxiality diminishes with wider type IV regions.  Wider HAZ’s, type IV zones, expected from higher preheat T.  Heat input less significant since far-field temperature unaffected; by …contrast, preheat changes far-field T.

Conclusions  Ameliorate type IV phenomenon using maximum preheat ….temperature consistent with phase changes and practical ….aspects. Preheat temperature significant because type IV cracking is localised; an increase leads to a widening and delocalisation of the type IV region.  The type IV rupture stress insensitive to heat input. The width of …type IV zone is more dependent of preheat, since this affects far- …field temperatures.  Therefore, productivity need not be compromised.