However, modeling of multi-pass welding has been rarely carried out due to the ... (heat input and number/position of weld passes), steel chemistry and, for the ...
APPLICATION OF METALLURGICAL MODELING TO MULTI-PASS GIRTH WELDING OF SEAMLESS LINEPIPES E. Anelli, M.C. Cesile, P.E. Di Nunzio Centro Sviluppo Materiali S.p.A. - Rome, Italy G. Cumino Dalmine S.p.A. - Tenaris Group - Dalmine (BG), Italy M. Tivelli, A. Izquierdo Tamsa - Tenaris Tenaris Group - Veracruz, Mexico
Abstract The thermo-microstructural model TENWELD© 2002, able to predict the hardness and microstructure in the heat affected zone (HAZ) and weld metal of joints, has been validated through specific laboratory experiments as well as industrial trials. A sensitivity analysis has been carried out to identify the effect of chemical composition and main welding parameters on the hardness values in the HAZ of Gas Metal Arc Welding (GMAW), Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW) joints of seamless quenched and tempered (Q&T) linepipes. Maps of hardness and fraction of microstructural constituents at the end of the welding process have been calculated over the whole joint, in order to take into account the influence of the bevel geometry. The hardness is predicted with a standard deviation (σ) of 16 HV which is within the typical experimental scatter. The examples discussed show that the microstructural maps permit to quantify the extension of the local brittle zones associated with the formation of high-carbon martensite islands in the coarse-grained HAZ.
Introduction Recent developments in offshore welding technologies point out new challenges for linepipe suppliers, such as better controlled heat affected zone (HAZ) properties, whilst welding faster, and with consistent performance. The availability of a system, capable of running on a personal computer, which models the microstructures and hence the properties which arise in the HAZ of microalloyed steels as a result of multi-pass arc welds and high energy density welding processes is attractive to design the steel chemistry and assess the influence of any variation in a given welding procedure for a wide range of weldments. Sophisticated models have been developed for the prediction of various phenomena during welding, such as fluid motion, solidification, temperature distribution and residual stresses. However, modeling of multi-pass welding has been rarely carried out due to the difficulties in predicting the complex series of microstructural changes which depends on welding parameters (heat input and number/position of weld passes), steel chemistry and, for the outer regions of the HAZ, the original steel microstructure. Recently, Tenaris Group together with Centro Sviluppo Materiali has set-up a special project on field weldability of seamless quenched and tempered (Q&T) linepipes for offshore applications. In addition to a systematic assessment of the properties of girth welds produced by current welding techniques, i.e. Pulsed Gas Metal Arc Welding (PGMAW), Shielded Metal Arc Welding (SMAW)
and Gas Tungsten Arc Welding (GTAW), a thermal-microstructural model for multi-pass welding has been developed to estimate microstructure and hardness in the HAZ of girth welds of seamless Q&T pipes [1]. The model is based on metallurgical principles to make the predictions relevant to a wide range of steels and welding conditions. It uses thermodynamic and kinetic algorithms to predict microstructural development as a series of contours around the weld bead of a multi-pass joint. In this paper the most recent improvements to the thermo-microstructural model (TENWELD© 2002), developed for predicting the hardness and microstructure in the HAZ of pipes with wall thickness in the range from 6 to 50 mm, in low-C (0.05-0.18 mass% C) steels microalloyed with V, V+Nb, V+Nb+Ti, are presented and application examples are discussed together with a sensitivity analysis. General Structure of the Model TENWELD© 2002 In order to provide an universally applicable model, three main aspects were investigated and described by suitable algorithms: • • •
thermal profile arising from the welding process; microstructural features developed in a given steel as a result of the thermal cycle; hardness of the final microstructures.
Realistic assumptions were made for simplification concerning thermal flow, weld geometry, and metallurgy in order to maintain flexibility and accuracy, avoiding the use of finite element methods, which take a long time for running and high level workstations. Different modules, each one describing an elementary process, are managed by a main interface to simulate the thermal and microstructural changes occurring at a given point. Usually, when considering multi-pass welding, each point in the HAZ undergoes a succession of thermal cycles with temperature peaks which increase first as the welding runs come closer, then decrease. Depending on the peak temperature (TP) associated to each pass, different cases can be identified. Focusing on the last pass at which a complete austenitization is obtained at relatively high temperature (TP>>Ac3), one has to consider at first the growth of the austenite grains (Coarse Grained HAZ, CGHAZ) and the subsequent cooling stage leading to the decomposition of austenite to produce low temperature constituents. Then, subsequent passes can be as follows: - subcritical (TP
