Study on welding thermal cycle and residual stress of UNS S 32304 duplex stainless steel selected as external shield for a transport packaging of Mo-99

Thin plates of duplex stainless steel UNS S32304 were welded using the pulsed gas tungsten arc GTAW process (butt joint) without filler addition. The used shielding gas was pure argon and 98% argon plus 2% of nitrogen. The thermal cycles were acquired during welding, in regions near the melting pool. This alloy is candidate for the external clad of a cask for the transport of high activity radiopharmaceuticals substances. For the residual stress measurements in austenite phase an X-ray diffractometer was used in a Bragg-Brentano geometry with CuKα radiation (λ= 0.154 nm) and for ferrite phase was used a pseudo-parallel geometry with CrKα radiation (λ= 0.2291nm). The results of residual stress using sin methodology showed that the influence of the high welding temperature leads to compressive stresses for both phases of the duplex steels mainly in the heat-affected zone. It was observed a high temperature peak and an increase of the mean residual stress after addition of nitrogen to the argon shielding gas.


INTRODUCTION
The packaging or cask for the transport of substances that exhibits high gamma ray radioactivity requires adequate thermo-mechanical protection; mainly if tungsten alloys or depleted uranium shielded devices are used for the transport of the Mo-99 with activity above 0.6 TBq (16.2Ci) [1,2].
The increased reliability of the welding process on duplex stainless steels (DSS) for applications in the nuclear industry endorse of the choice this material as external shield for Type B packages [3,4].
The CNEN and IAEA safety standards [5,6] define key parameters for the design, project and validation tests of the transport devices for radioactive materials.The main rules established for external recipient of Type B packages are the thermal and mechanical tests.The first one consists in fully enclose the sample cask in a fire presenting average flame temperature of 800 °C for 30 minutes without loss or dispersal of the radioactive content.For the mechanical properties of the external cask material it is required an ultimate strength of 345 MPa and 189 GPa, for the Young's modulus (as minimum values accepted).For the thermal and impact protection, it is interesting that the material used for the external cask layer presents a low density of around 8100 kg.m -3 .In order to deal with this, in a study about the selection of materials for a new Type B package, Hara et al. defines that the material selected for cask´s external layer belongs to the family of stainless steels and nickel (Ni) alloys.However, the stainless steels present a significantly reduced and competitive price when compared to Ni alloys, showing advantage in their choice [1].
The modern DSS have basically ferritic-austenitic microstructure (around 50 -50 % ratio) that exhibits excellent properties combining high corrosion resistance and superior mechanical properties.
Allied to a competitive cost, the duplex stainless steel grades also have satisfactory weldability [3].
Despite the great advantages there are also some limitations generated during the exposure of the DSS to the high welding temperatures.During the welding cycle, thermal strains are induced in all regions adjacent to the welding [7].The thermal strains caused during rapid heating are accompanied by plastic upsetting.The thermal stresses resulting from these strains combine and react to produce internal forces that cause residual stress [8].In previous work, Machado et al. [9] studied the effect of the shielding gas composition on the residual stress distribution in the austenite phase of the duplex stainless steel welds.Concerning the radiation influences, Cárcel-Carrasco et al. [10] investigated the effect of low-level ionizing X-rays on the microstructural characteristics, resistance, and corrosion resistance of welded joints of AISI 304 steels by using gas tungsten arc welding (GTAW) process with AISI 316L as filler rods.It was observed that welds subjected to doses of 1000 Gy of ionizing radiation have an influence on its mechanical resistance and corrosion characteristics, and this is especially true for welds made in natural atmospheric conditions.In this way, the concern about these types of failures enhance when dealing with the transportation of radioactive products.
For each type of stress state, it is associated one or a combination of methods for the determination of the residual stresses, to get the best response from the experiments.The most usual methods are: X-ray and neutron diffraction; extensometric blind hole, cut ring, cut by section and removing layers.The X-ray diffraction is considered the most adequate nondestructive method for superficial residual stress measurements.Applied stresses, as external loads, are usually higher at the surface where the failures start over [11,12,13].
In this paper it is proposed the analysis of the thermal cycle of welding and residual stress of DSS welded plates for application in the external layer of a new container or cask, for radiopharmaceuticals transport.Temperature curves were acquired during experimentally controlled gas tungsten arc welding (GTAW) process in different positions from the joint line in order to optimize parameters and the soundness better quality of the fabricated parts.

MATERIALS AND METHODS
DSS sheet specimens were produced with dimensions 72 x 72 x 1.8 mm 3 and subsequently buttwelded autogenously by the GTAW process in the Welding Laboratory of the Federal University of Espírito Santo (LabSolda/DEM/UFES).Table 1 shows the chemical composition of the UNS S32304 duplex stainless steel used in the experimental work.The alloy's chemical composition was certified by the commercial supplier Aperam Inox América do Sul S/A.
For welding process, pulsed current and direct polarities were used with automatic drive systems.
The samples were fixed in order to reproduce the conventional welding process, as shown in Fig. 1.
A batch of samples was welded with commercial purity argon as shielding gas and another batch was welded using a mixture of argon plus 2% of nitrogen.The gas flows rate on the welds in both cases were 10 L.min -1 .The AWS Class EWTh-2 electrode was kept negative being located at 2 mm at a 90° angle to the plates, according to direct polarity welding GTAW process.For the pulsed current, the square wave was kept balancing background and pulse currents with equal times of 0.9 s.In Table 2, it's shown the welding and thermal cycle parameter.
Regarding the parameters such as voltage (U), pulsed and background currents (Ip, Ib), pulsed and background times (tp, tb), the welding speed (v) along with 0.6 arc efficiency for the pulsed GTA welding, it was possible to calculate the welding heat input (H) per mm using Eq. 1 [8].The welding heat input shown in Table 2 was determined using the average voltage, current and the average weld speed. ( The temperature was measured and recorded using K-type thermocouples attached to a data acquisition system.Four thermocouples were positioned and fixed to the sample plate's surface (Fig. 2), using high voltage capacitive discharge generator at different distances, along transversal and longitudinal lines of the weld bead.The thermocouples were located at 3.0 mm, 3.5 mm, 4.0 mm and 5.0 mm from the joint line.The signals from the thermocouples were acquired in a multi-channel universal data acquisition system (DAQ) amplifier using MX boards -PT1000, at each channel, for room temperature automatic conditioning.The measured total error limit at 300 K room temperature is ± 1 K and the temperature drift (K-type) was used K/10K ratio where the uncertainty was ≤ ± 0.5.
For the residual stress measurement, the X-ray diffraction method was used performing the multiple exposure technique, according to SAE HS-784 standard [14].This method allows obtaining a more precise determination of ψ (Psi) that represent the angle between specimen surface normal (Ns) and the normal diffracting planes.Strain ε values are recorded for different sample tilt angle (ψ) at constant azimuth angle ϕ.Strain ε vs sin 2 ψ is plotted to estimate the stress values.Measurements were performed in transverse axis of the plate crossing the weld bead.The location of the residual stress measurements were solidified zone (SZ), heat-affected zone (HAZ) and base metal (BM) for both samples.The distances for each measurement are shown in Table 3.
Table 3: All positions for residual stress measurement.For the residual stress measurements in the austenite phase, plane (420), a Rigaku X-ray diffractometer was used in a Bragg-Brentano geometry and CuKα radiation (= 0.154 nm).And then, to investigate the residual stress in the ferrite phase, plane (211), a Rigaku X-ray diffractometer was used in a pseudo-parallel geometry and CrKα radiation (= 0.229 nm).

Phase
The sample was set at position = 0 ° and, in some cases,  = 90 ° in the direction of rolling.The residual stresses were calculated by the sin 2  method.For the determination of position 2 of the analyzed plane, the localization method was used according to intensity using the mathematical function Pearson7A [15] and the graphical program FityK [16].The choice of the function was based on the literature and on the correlations obtained when comparing the results with other functions.For this analysis, the elastic constants are shown in Table 4.

Thermal Cycle
Fig. 2 shows the welded plates after the pulsed GTAW process along with the positions of the thermocouples.It was observed that the welding line width for the pure Ar as shielding gas flux is less (around 10 mm, see Fig. 2 (a)) than the welded plate with Ar+2%N2 shielding gas flux (12 mm, in Fig. 2 (b)).In addition, the welding process using Ar+2%N2 also presented a visible thermal damage in welding zone.
The temperature distributions for the four thermocouples were plotted for both specimens in Fig. 3.The thermal cycle for sample Ar+2%N2 as shown in Fig. 3 (b), a temperature peak near 736 °C is observed for thermocouple TK1 (3.0 mm distant from the melting pool) being about 25 °C higher than the obtained on pure argon sample considering the same region.Y. C. Lin et al. [18], also shown a peak temperature of a thermal cycle increased with increasing nitrogen content more heat is carried into shielding gas.Thus, increasing the nitrogen content is carried more heat into the workpiece and increases the weld metal area.In addition, according to the studies by K. H. Tseng and C. Chou [8], using pulsed GTAW process a greater amplitude ratio can reduce the temperature difference between the fusion zone and unaffected base metal in welding and therefore the welding residual stress can be reduced.

Residual stress
Fig. 4 shows the plot strain ε x sin²ψ for the material as received.According to sin 2 ψ method, the strain vector ε is obtained through the relationship of distance between crystallographic planes calculated from diffraction pattern.So, it's observed that the slopes of the data are decreasing, indicating that the residual stresses have a compressive behavior, with values -263 ± 15 MPa for the austenite phase, and around of -192 ± 17 MPa for the ferrite phase.
As seen in Figure 5, a stress profile was found for the different phases of the samples.It is observed that in the sample welded only with pure argon, the solidified zone presented lower residual tensile strength, compared to the sample welded with shielding gas composed by the mixture argon and nitrogen.For both samples, there is a decrease in the stress state value from the heat-affected zone until base metal with original compressive residual stresses.It is also observed that the sample welded with the argon plus 2 % nitrogen protective gas flux has compressive stresses values starting at 8 mm from the center of the solidified zone, whereas the sample welded with pure argon protective flux only from 10 mm (Fig. 5(a)).This is can be correlated to the peak temperature using Ar + 2% N2 as shielding gas observed in Fig. 4 and each phase in the material will have a different response to an applied welding heat input.As reported by Johansson et al. [17], the typical coefficients of thermal expansion at 100°C for both ferrite and austenite phases are 10.4 x 10 -6 °C-1 and 17.2 x 10 -6 °C-1 , respectively.Thus, since the two phases may have different coefficients of thermal expansion, different thermal stresses are introduced for each phase during cooling from an elevated temperature of welding. .On the regions close to the heat source, the results of residual stresses obtained for the austenite, show an increase of the stress in relation to the value found for the sample as received.This may be related to the recrystallization/grain growth of the phase in this region [17].
According to Fig. 5, the addition of nitrogen leads to a slight increase in the tensile residual stress of the phases.In the study made by Muthupandi et al. [19], the Ar-N2 mixture has a higher ionization potential, increasing the welding energy and peak temperature.Lundin et al. [20] describe that additions of diatomic gases cause contraction in the contours of the plasma close to the anode.In addition they mention that the content of nitrogen increases the heat transferred by the arc into the workpiece.This can lead to distortion of plate during welding process and increased residual stress.
For Hsieh et al. [21], it was observed that with the addition of nitrogen, the peak temperature increases and consequently the residual stresses of the austenitic steels too.

CONCLUSION
Based on the experimental results on a UNS S32304 duplex stainless steel, the following: The measurement of residual stresses in samples measured by X-ray diffraction using the sin 2  method, indicated that the effect of the welding thermal cycle on the heterogeneous microstructure causes the development of tensile residual stress in the solidified zone and otherwise (compressive) residual stress around 8-12 mm range away from the weld joint for both samples.
Comparison on the results for both samples revealed that the stress state is more affected by mixture using Ar+2%N2 as shielding gas.For this kind of shielding gas, all temperature peaks are slightly higher than the ones observed in the samples using pure argon as shielding gas.
Finally, it is suggested an investigation on the gamma ray radiation interaction in welded samples of DSS for the application in the outer layer of casks for radiopharmaceuticals transport, as Type B packages.According to literature, the welds subjected to doses of ionizing radiation have a reduced capacity to resist corrosion and impact toughness, being especially true for welds made without an appropriated atmosphere.

Figure 1 :
Figure 1: Welding arrangement for the duplex stainless steel UNS S32304 thin plates.

First, Fig. 3 (
a) the thermocouples TK1 and TK2 (3.0 mm distant from the melting pool) were placed closer to the weld bead where the temperatures were near to 711 °C and 622 °C, respectively.Through the TK3 and TK4 thermocouples it's shown the symmetric temperature peaks in regions close to the interface of the heat-affected Zone (HAZ) to the base metal (BM) for sample welded using pure argon as shielding gas.

Figure 2 :
Figure 2: The welded plates with thermocouples location.a) the weld plate #WA using pure Ar as shielding gas and b) the sample #WAN with Ar+2%N2 as shielding gas.

Figure 3 :
Figure 3: Distribution of temperatures for samples (a) Pure Ar and (b) Ar + 2%N2.It shows the temperature variation measured with each thermocouple for the UNS 32304 duplex stainless steel, 1.8 mm long stripe, while being welded.

Figure 4 :
Figure 4: The graph ε x sin²ψ for the material as received.

Figure 5 :
Figure 5: Residual stress profiles of the austenite (a) and ferrite phases (b) of the duplex stainless steel UNS S32304 welds.The dashed line represents average residual stress (-265 ± 13 MPa) for the as-received material.

Table 1 :
Chemical composition (% mass and ppm) of the duplex stainless steel UNS S32304

Table 2 :
Welding parameters used for the pulsed GTAW process.