The exploration of oil & gas fields towards more demanding environments require the continuous development of line pipe steels capable to withstand severe working conditions (high pressure, high temperature, sour environment, large deformations) maintaining good weldability in terms of hardness control in the Heat Affected Zone (HAZ). Therefore, the response to welding thermal cycles needs to be taken into account since the first steps of the product development. This paper describes a methodology to ascertain the weldability of new line pipe steels.
A matrix of welding trials, based on different welding technologies, was designed; the manufactured welds were assessed in terms of hardness and microstructure using standard and non-standard approach.
Gas Tungsten Arc Welding (GTAW) cold wire technique with anthropomorphic robot was selected as principal process, being extremely robust, accurate and repeatable and able to reproduce with good accuracy the thermal field of different arc welding operations. Furthermore, GTAW process permits to manage separately the so called “Arc Energy” (Volt*Ampere) and wire feeding: this additional degree of freedom gives the possibility to produce different combinations of Heat Inputs and Diluting Ratios. Single and multi-pass bead on plates (WT∼40mm) with no pre-heating were used to promote high restrained configurations and high residual stresses to reproduce the worst welding condition. Furthermore bead on plate have been performed by Nd:YAG laser to generate fastest cooling rates and narrowest welding pools.
The investigation activity was performed according to the API RP 2Z for what concerns the characterization of the main Heat Affected Zone both for single and multi-pass. For the individuation and investigation of the HAZ sub-zones a thermal model has been selected, validated by dedicated instrumented welding activity and used to predict the thermal cycle in selected points/zones of the HAZ (e.g. CGHAZ, FGHAZ...).
In depth metallographic/hardness analysis was performed on the welded samples using non-standard methods: low, medium and high magnification optical microscope analysis to generate a complete morphological map, suitable hardness paths with low and standard loads along the main direction of heat propagation in the bead and “tint etching” to better individuate the phase fractions of the different sub-zones of the HAZ.
Finally, proprietary and commercial predictive tools, using different input parameters (e.g. heat input, cooling rate, composition...) were used to calculate hardness and phase fractions under the experimental operative conditions. Results were compared to those obtained by the experimental activity.