TG4: 3-Pass Slot Weld (Stainless Steel)
NeT Task Group 4 (TG4) was formed in 2007 to extend the progress made in NeT Task Group 1 (TG1) to a more complex problem.
The new Task Group examined residual stresses in a specimen fabricated from a AISI 316L(N) austenitic stainless steel plate containing a central slot, which was filled with three superimposed weld beads, introduced using a mechanised tungsten inert gas (TIG) weld process with AISI 316L filler metal. This specimen design is also representative of a short, shallow weld repair geometry (without an original weld), which is a configuration of practical interest.
The three-pass slot weld contained a significant volume of weld metal, which necessitated residual stress measurements in weld metal, with the corresponding challenges of composition, large grain size, crystallographic texture, and significant plastic deformation.
This configuration also necessitated simulations to explicitly model the development of mechanical properties and residual stress in multi-pass weld metal via multiple thermo-mechanical cycles. The weld design, with three superposed passes, allowed moving heat source simulation in practicable time scales both by preserving symmetry, and by limiting the number and length of individual weld beads. AISI 316L(N) is known to cyclically harden relatively quickly, so three passes were judged to be sufficient to achieve significant hardening and allow discrimination between different material constitutive models.
A number of nominally identical specimens were fabricated under closely controlled conditions, with detailed records kept of the manufacturing history, weld process parameters, transient temperatures during welding, and the resulting geometric distortions. Comprehensive stress-strain material property characterisation was then undertaken, extending to the isothermal cyclic tests necessary to calibrate the mixed isotropic-kinematic material hardening models required for accurate weld residual stress prediction. Parallel residual stress measurement and simulation round robins were performed by a large number of participants from around the world.
Fig. 2. TG4 – thermocouple readings compared to simulations
Fig. 3. TG4 – through thickness distribution of longitudinal residual stresses, measurement results shown together with a best estimate average
Residual stresses were measured using neutron and high energy synchrotron diffraction, surface X-ray diffraction, surface and deep hole drilling, the contour method, and ultrasonics. Neutron diffraction measurements were made at eight different instruments. The diffraction measurements database alone is large enough to generate reliable mean profiles, to identify clear outliers, and to establish that there is no statistically significant difference in the residual stress field in the specimens used for the non-destructive measurements. NeT Task Group 4 gives a unique insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods.
Fig. 4. TG4 – specimen instrumented with thermocouples
NeT Task Group 4 is also a unique test bed for the development and validation of weld residual stress simulation techniques in austenitic stainless steel. Its combination of extensive materials characterization, accurately characterized welding temperature transients, and reliable residual stress and distortion measurements is currently unrivalled. About thirty finite element simulations were submitted to the network over the course of the project, giving insights into the required accuracy of welding thermal solutions, the mechanical solution accuracy achievable using optimized material constitutive models, and the level of acceptable error in finite element residual stress simulation results for use in structural integrity assessments of high integrity engineering component.
Fig. 5. TG4 –transverse residual stresses along a line 5 mm below the welded plate surface, measurements and simulations
Fig. 6. TG4 – material response to cyclic loading, experimental data vs. simulations using Code Aster and ABAQUS
In summary, NeT Task Group 4 has generated a large body of materials property data, weld residual stress measurements and weld thermo-mechanical residual stress simulation results. At the moment it is arguably the most comprehensive benchmark study of its type in existence .
M C Smith, A C Smith, C Ohms and R C Wimpory, The NeT Task Group 4 residual stress measurement and analysis round robin on a three-pass slot-welded plate specimen, Int Jnl Press Vess and Piping 164, 3-21 (2018).