{'fraction_non_alphanumeric': 0.018689581095596132, 'fraction_numerical': 0.005800214822771214, 'mean_word_length': 5.089601046435579, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Features of residual stresses in duplex stainless steel butt welds

Duplex stainless steel finds increasing use as an alternative to austenitic stainless steel, particularly where chloride or sulphide stress corrosion cracking is of primary concern, due to the excellent combination of strength and corrosion resistance. During welding, duplex stainless steel does not create the same magnitude or distribution of weld-induced residual stresses as those in welded austenitic stainless steel due to the different physical and mechanical properties between them. In this work, an experimental study on the residual stresses in butt-welded duplex stainless steel is performed utilizing the layering technique to investigate the characteristics of residual stresses in the weldment. Three-dimensional thermos-mechanical-metallurgical finite element analysis is also performed to confirm the residual stress measurements.


Introduction
Duplex stainless steels are finding increased application in the chemical, oil and gas industries, petrochemical process plants, the pulp and paper industry, pollution control equipment, transportation and for general engineering thanks to their outstanding corrosion resistance and mechanical properties. Steel structures in engineering practice are generally fabricated by welding. During welding, residual stresses are unavoidably produced in the weld region and its vicinity. The existence of weld-induced residual stresses can be harmful to the structural integrity and the service behavior of the welded structure. Therefore, accurate assessment of the residual stresses would be very helpful to ensure the sound design and safety of the structure. The effects of solid-state phase transformation on welding residual stresses are investigated in this study. This study aims to present a FE modeling procedure for characterizing the residual stress of a duplex stainless steel, and focuses on the effects of solid-state phase transformation. In this study, microstructural analysis and residual stress measurements of duplex stainless steel butt weld were carried out to investigate the microstructural changes in the weld region and HAZ, and to determine the full volumetric change strain associated with martensitic transformation, which is required input to the FE modelling of the phase transformation effects.

Specimen
The base material used in this study is duplex stainless steel plate with 10mm thickness. 'V' butt joint configuration was prepared as shown in figure 1. The joint was welded with five passes by gas tungsten arc (GTA) welding. Welding condition is presented in Table 1 and the mechanical properties of the base metal are given in Table 2. Table 3 shows the chemical composition of the base metal and weld metal.

Microstructure and the results of measured hardness
After completion of welding, micro-structures were analysed using an OLYMPUS PME3 optical microscope. Samples used for micro-structural analysis were cut from the weld and the HAZ. Results are shown in figure 2. Figures 2(a) and 2(b), representing the weld region and the HAZ respectively, indicate the martensitic structures. Therefore, it is clear that the weld region and HAZ heated over the austenitic temperature during welding experience martensitic transformation. And Hardness is measured at each position. As shown in the table 4, the hardness of HAZ and welds is about 20 higher than the base metal due to the phase transformation.

Test for residual stress measurement
Residual stress measurements were carried out on the two axis strain gauge with the layering technique. Measuring stresses by strain gauges using the layering technique can obtain the residual stresses on the surface of the structure to be evaluated. The detailed procedure for measuring the residual stresses on the surface of the specimen is as follows. First, strain gauges are attached on the bottom surface of the specimen. Residual stresses in the small pieces are released by cutting the specimen, and longitudinal released strain and transverse released strain are measured. Longitudinal residual stress and transverse residual stress can be obtained by Equations (1) and (2) using the measured strains.
where E is Young's modulus and  is Poisson's ratio.

FE simulation of the welding process
Numerical simulation of the thermal and thermal-mechanical process associated with residual stress evolution during welding can be extremely complex since it needs to take account of the interactions between heat transfer, metallurgical transformation and mechanical fields. Complex numerical approaches are then needed to accurately model the welding process. However, as far as welding residual stress modelling is concerned, numerical procedures can be significantly simplified from the thermo-mechanical point of view, the heat input can be seen as a volumetric or surface energy distribution, and the fluid flow effect in the molten area can be simply taken into account by increasing the thermal conductivity over the fusion temperature.

Model geometry and material properties
FE simulation of the butt welding was performed on two duplex stainless steel plates with width of 250mm, length of 500mm, thickness of 10mm as shown in figure 4. The welding arc travel direction and welding start/stop position a real so shown in the figure. The type of welding process modelled was Gas Tungsten Arc Welding (GTAW) process, and six-passes welding was used to weld the plate. The 3-D FE model using eight-nodded isoparametric solid elements is shown in figure 5. Because of the symmetry with respect to the weld centreline, one half of the model was selected as the computational domain. Figure 6 shows the physical constants at high temperatures of duplex stainless steel and temperature-dependent thermo-mechanical properties of duplex stainless steel are presented in Figure 7.

Thermal analysis
During welding, the governing partial differential equation for the 3-D transient heat conduction, with internal heat generation and considering ρ, and c as functions of temperature only, is given by the thermal equilibrium equation.
where T is the temperature, K is the thermal conductivity, c is the specific heat,  is the density and Q is the rate of moving heat generation per unit volume. According to the nature of arc welding, the heat input to the work piece can be divided into two portions. One is the heat of the welding arc, and the other is that of the melt droplets. The heat of the welding arc is modelled by a surface heat source with a Gaussian distribution, and that of the melt droplets is modelled by a volumetric heat source with uniform density. Heat flux distribution at the surface of the work piece within the arc beam radius 0 r is defined by the following equation: where () qr is the local heat flux, 0 r is the arc beam radius and 1 Q is the heat input from the welding arc and given by where  represents the arc efficiency factor, which accounts for radiative and other losses from the arc to the ambient environment, I is the arc current, U the arc voltage and 2 Q is the energy induced 6 1234567890 ''"" where p V denotes the considered weld pool volume and can be obtained by calculating the volume fraction of the elements in currently being welded zone. The heat of the welding arc was assumed to be 40% of the total heat input, and the heat of the melt droplets 60% of the total heat input. The arc efficiency factor was assumed as 0.85 for the GTA welding process used in the present analysis. The heat flux was applied during the time variation that corresponded to the approach and passing of the welding torch. As for the boundary conditions during the thermal analysis, convection and radiation were both taken into consideration and their combined effect was represented in the following

Solid-state phase transformation
Solid state phase transformation is responsible for changes in both volume and yield stress in steel experiencing thermal cycles during welding. In duplex stainless steel, martensitic transformation which occurs during rapid cooling immediately after intense welding heat input, is the phase transformation associated with the significant volumetric change and the yield stress change. The metallurgical structure of the steel is assumed to be pearlite-ferrite before any thermal cycles take place. During welding, when the steel is heated over the temperature 1 A (cementite disappearance temperature), pearlite-ferrite partly transforms into austenite, and when the temperature is higher than 3 A (  -ferrite disappearance temperature), its microstructure completely changes into austenite. During austenite transformation, the steel undergoes a reduction in volume as can be schematically shown in figure 8. In this study, depending on the peak temperature that an integration point of an element reached during heating process, the decision was made whether the point underwent the martensitic transformation or not, i.e., all points whose peak temperature was higher than the temperature were considered to undergo martensitic transformation when cooling to the onset temperature s M . When the temperature during cooling reached to the end temperature f M , the martensitic transformation was considered to be full and complete.