Mechanical & Corrosion studies on K-TIG welded P91 Super Duplex Stainless Steel joints

In this manuscript, an attempt has been made to conduct mechanical and corrosion studies on K-TIG welded Super Duplex Stainless steels. Mechanical tests were conducted on the joints by using Universal Testing Machine, surface microhardness testing was conducted using Vickers surface microhardness testing equipment and impact tests were conducted using Izod testing equipment. Salt fog testing and potentiodynamic polarization testing was used to study the joints fabricated under different welding parameters. On conducting mechanical tests, the K-TIG welded joints exhibited 73.7% tensile strength compared to P91 parent material. Fluctuations in surface microhardness indicated that the material underwent permanent grain deformation during welding. Salt fog testing indicated an increase in corrosion resistance till a certain extent of increase in welding current. Similar effects were indentified on subjecting the joints to potentiodynamic polarization. SEM images revealed corroded regions on subjecting the joints to corrosive environment.

Keywords – K-TIG welding, P 91 super duplex stainless steel, corrosion, mechanical testing, microharndess.

Introduction

P91 super duplex stainless steel is commonly used in high-temperature and corrosive situations due to its high mechanical strength, oxidation resistance, and high-temperature creep qualities. It is an essential component in power plants, petrochemical industries, and nuclear applications. Welding P91 stainless steel presents major problems, particularly in preserving the balance of ferrite and austenite phases for excellent mechanical and corrosion resistance qualities.

Keyhole Tungsten Inert Gas (K-TIG) welding has developed as an effective technique for welding duplex and super duplex stainless steels due to its deep penetration capabilities and ability to produce defect-free welds. Several research have looked into the microstructure, mechanical characteristics, and corrosion behavior of K-TIG welded duplex and superduplex stainless steels. Cui et al. (2022) investigated the microstructure and pitting corrosion resistance of UNS S32101 duplex stainless steel welded using K-TIG and discovered that welding parameters had a substantial influence on joint corrosion performance.

Fei (2020) emphasized the significance of welding technique certification in K-TIG welding and established its effectiveness when welding high-hardness grade quenched and tempered steel and stainless steel joints.

Similarly, Fei et al. (2020) looked at the mechanical properties of deep-penetration autogenous TIG-welded dissimilar joints, emphasizing the significance of Vickers microhardness testing in determining weld integrity. Microhardness is an important component in determining the performance of K-TIG welded joints. Kavishwar et al. (2024) examined various weld cladding procedures, including K-TIG, and discovered that the welding method and post-weld treatments affected microhardness values.

Dagur et al. (2023) investigated the effects of TIG and active flux TIG welding on the microstructure and hardness of SAF 2507 super

duplex stainless steel, finding higher tensile strength and microhardness values. Furthermore, Sales et al. (2016) investigated the effect of nitrogen in the backing gas on duplex root welds and discovered a strong relationship between microhardness and corrosion resistance. Corrosion resistance is another important feature of P91 welding.

Naveen Kumar et al. (2022) proved that K-TIG welding may be utilised efficiently to weld armour steels with high mechanical characteristics and corrosion resistance. Furthermore, Fande et al. (2022) examined active TIG welding procedures and noted increases in corrosion resistance and mechanical qualities of diverse materials. Giudice et al. (2024) and Mahajan et al. (2023) found that welding settings play an important role in producing excellent corrosion performance in stainless steel welds.

Materials & Methods

The P91 plate measured 10 mm thick, 50 mm wide, and 150 mm long. To guarantee appropriate welding penetration, the connection was prepared using a single-V groove with a 60° included angle and a 2 mm root gap. Surface cleaning involved mechanical grinding to remove oxide coatings, rust, and impurities, followed by degreasing with acetone or alcohol based solvents to improve weld quality.

The ERNiCrMo-3 filler wire was considered ideal for K-TIG welding due to its superior creep resistance and mechanical qualities. 

Table 1 – Chemical properties of the base material and filler wire

K-TIG (Keyhole Tungsten Inert Gas) welding equipment worked by forming a steady plasma arc between the tungsten electrode and the workpiece, resulting in a deep keyhole that allowed for complete penetration in a single pass. The process was carried out beneath an inert gas shield, resulting in minimum oxidation and exceptional weld quality. Three samples of P91 joints were welded with changing welding currents of 400 A, 450 A, and 500 A, while other parameters remained practically constant. The plates were precisely aligned, and a backing gas was used to avoid oxidation. The welding torch was automatically controlled, resulting in homogeneous heat input. Table 2 shows the welding process parameters employed during the studies.

Table 2 – Welding Parameters

To examine their mechanical qualities, the welded joints were tested for tensile strength, surface microhardness, and impact. To ensure accuracy and reliability, each test was conducted in accordance with conventional procedures.

The welded joints were tensile tested on a Universal Testing Machine (UTM) to evaluate their ultimate tensile strength, yield strength, and elongation. Standard dog-bone-shaped specimens were prepared in accordance with ASTM E8/E8M criteria. The specimens were held in the UTM, and a controlled tensile load was applied until failure occurred. The load vs. elongation data was collected to determine the strength and ductility of the welded joints.

The Vickers microhardness test was used to determine the hardness of the weld metal, heat-affected zone (HAZ), and base metal. To create a smooth surface, the samples were polished using abrasive sheets and then finished with diamond paste. A Vickers microhardness tester was utilised, with a diamond pyramid-shaped indenter applying a constant load (usually 500 g or 1000 g) for 10-15 seconds. The diagonal length of the indentation was measured using a microscope, and the hardness value was computed. Multiple indentations were made across the weld zone, HAZ, and base metal to detect hardness changes. The IZOD impact test was used to assess the weld's resilience to abrupt impact loads. Standard notched specimens were manufactured in accordance with ASTM E23. The specimens were placed vertically in the IZOD tester, and a pendulum hammer was used to pound the notched face. The energy absorbed during fracture was measured to determine the toughness of the weld metal and HAZ. The test was carried out at room temperature to examine the performance of welded joints at various current levels.

Salt fog and potentiodynamic polarization tests were performed on the welded joints to assess their corrosion resistance. These testing helped determine the material's performance in harsh settings. Salt fog testing was used to simulate rapid corrosion in a controlled environment. The welded specimens were cleaned and degreased prior to being placed in the salt fog chamber. A 5% NaCl solution was continually atomised in the chamber at 35°C, resulting in a humid saline atmosphere.The exposure period lasted four days, with mass loss measures done every six hours. For each measurement, the samples were carefully removed, cleaned with distilled water, dried, and weighed on an analytical balance. The weight difference over time represented the corrosion rate. 10 The surface morphology of corroded samples was investigated to determine if they exhibited pitting or uniform corrosion behavior.

Potentiodynamic polarisation tests were used to assess the electrochemical corrosion behavior of the welded joints. The measurements were carried out using a three electrode electrochemical cell with a 3.5% NaCl solution as the electrolyte. The welded sample served as the working electrode, while a saturated calomel electrode (SCE) was used as the reference electrode and a platinum electrode as the counter. The experiment began by stabilizing the open circuit potential (OCP) for 30 minutes. A potentiostat applied a steadily increasing voltage (usually between -1.0 V and +1.5 V) with a scan rate of 1 mV/s. The current response was recorded to create a polarization curve, from which the corrosion potential (Ecorr) and current density (Icorr) were calculated.

Scanning Electron Microscopy (SEM) was used to study the welded joints' microstructure, weld integrity, and fracture morphology. The specimens were sectioned from the welded joints and mechanically polished with silicon carbide sheets (up to 2000 grit), then fine polished using diamond paste. To improve contrast, the samples were etched with Kalling's reagent prior to imaging.

The prepared samples were placed in the SEM chamber, and a high-energy electron beam was swept across the surface at high vacuum. Grain structure, weld flaws, and fracture features were analyzed at various magnifications using secondary electron (SE) and backscattered electron (BSE) imaging. SEM investigation revealed porosity, inclusions, and fracture propagation in the welded P91 joints.

Results & Discussions

The tensile test findings revealed that the K-TIG welded connections had 73.7% of the ultimate tensile strength of the P91 parent material. The variation in welding current has a substantial effect on the mechanical characteristics of the joints. At 400 A, the tensile strength was the lowest of the three joints, indicating insufficient heat input, which could have resulted in partial fusion or a higher fault density. Increasing the current to 450 A resulted in improved tensile characteristics, with the maximum strength and elongation, indicating appropriate heat input and weld metal fusion. However, above 500 A, a minor loss in strength was detected, most likely due to high heat input causing grain coarsening and a  possible softening impact in the heat-affected zone (HAZ). The elongation values followed a similar trend, demonstrating that high heat can reduce ductility. These findings suggest that 450 A was the optimal current for achieving outstanding mechanical characteristics. Table 3 shows the changes in tensile properties. 

Table 3 – Tensile characteristics of the joints

The Vickers hardness test findings showed that the K-TIG welded joints had 65% the hardness of the P91 parent material. The hardness values fell as the welding current rose, indicating a direct effect of heat input on the microstructure. At 400 A, the welded joints had the maximum hardness, indicating a finer grain structure due to lower heat input and negligible thermal softening in the heat-affected zone. As the current increased to 450 A, hardness decreased, most likely due to increased grain development and partial tempering effects.

The hardness dropped further at 500 A, which can be attributed to high heat input, resulting in substantial grain coarsening and softening of the weld metal and heat-affected zone. These findings demonstrate that increased welding currents reduce hardness, with 400 A producing the most hardened weld of the examined joints. The Izod impact test f indings revealed that the impact strength of the K-TIG welded joints was less than that of the P91 parent material.

The welding current affects the toughness of the weld joints, with increased heat input typically boosting impact resistance. Joint 1, welded at 400 A, had the lowest impact strength of 45 J due to its increased hardness and finer grain structure, making the material more brittle. When the welding current was increased to 450 A, the impact strength rose to 55 J, indicating improved fusion and a more balanced microstructure. However, at 500 A, the impact strength reduced somewhat to 50 J, most likely due to excessive heat input, resulting in grain coarsening and probable softening of the heat-affected zone. The P91 parent material had an impact strength of 75 J, demonstrating that, while greater welding currents increase toughness to some extent, excessive heat can have a negative impact on the overall mechanical properties of the welded joints.

The salt spray test findings showed that corrosion resistance improved as welding current rose, resulting in less mass loss over time. The P91 parent material had the lowest mass loss due to its homogeneous microstructure and natural corrosion resistance. Among the welded joints, Joint 3 (500 A) had the least mass loss, whereas Joint 1 (400 A) had the highest corrosion rate, indicating that higher heat input improved the weld corrosion resistance. Table 4 shows the results of salt fog testing.

The salt spray test findings revealed that the mass loss of the welded joints decreased as the welding current increased, indicating greater corrosion resistance at higher heat inputs. Joint 1, welded at 400 A, had the highest corrosion rate, indicating that reduced heat input resulted in a more porous weld structure, making it more prone to corrosion. Joint 2, welded at 450 A, showed less mass loss due to better fusion and a more uniform microstructure. Joint 3, welded at 500 A, had the lowest corrosion rate, demonstrating that increased heat input resulted in improved material densification, limiting corrosive attack paths.

Table 4 – Results of salt fog tests

After 36 hours, mass loss decreased across all joints as a protective oxide layer formed, slowing further corrosion. The P91 parent material lost the least amount of mass throughout, indicating that it is more resistant to corrosion than the welded joints. These findings emphasize the importance of welding parameters in determining corrosion behavior.

Table 5 – Potentiodynamic polarization test results

              Figure 1. SEM images of corroded regions

The potentiodynamic polarisation test findings showed that increasing welding current increased corrosion resistance, as evidenced by a decrease in corrosion current density (Icorr) and an increase in polarisation resistance (Rp). Joint 1 (400 A) had the highest corrosion rate, whereas Joint 3 (500 A) had the best corrosion resistance, corroborating the pattern shown in the salt spray test. Table 5 displays the results of the potentiodynamic polarization experiments. The SEM study of the corroded surfaces indicated significant differences  in microstructural degradation between the P91 parent material and the welded joints. The original P91 material has a reasonably homogenous and dense microstructure with low pitting, indicating high corrosion resistance. The oxide layer developed on the surface provided considerable resistance to subsequent corrosion attack. Figure 1 shows the SEM pictures of the corroded sections.

Joint 1, welded at 400 A, has the most extensive corrosion damage, including deep pits and surface cracks. The large density of microvoids and uneven corrosion patterns suggested that the lower welding current produced a less refined microstructure, making it more susceptible to corrosive attack. Joint 2, welded at 450 A, had less corrosion damage, shallower pits, and a more uniform oxide coating, indicating greater corrosion resistance due to better fusion and grain refinement. Joint 3, welded at 500 A, had the least corrosion damage, with fewer and smaller pits and a compact and stable oxide coating. The increased heat input at this welding current is likely to improve microstructure homogeneity, minimizing flaws that could act as corrosion initiation sites.

These findings demonstrate that increasing welding current enhances corrosion resistance, with 500 A producing the best stable microstructure among welded junctions.

Conclusions

The investigation of K-TIG welded P91 joints found that welding parameters had a substantial influence on mechanical qualities and corrosion resistance. Tensile testing revealed that the welded connections maintained 73.7% of the tensile strength of the P91 parent material. Joint 2 (450 A) had the best tensile strength, while Joint 3 (500 A) saw a little drop due to excessive heat input. Hardness tests found that the welded joints had 65% of the parent material's hardness, with greater welding currents causing a drop in hardness due to grain coarsening. Impact tests revealed that higher welding current increased impact strength, whereas excessive heat input marginally lowered toughness.

Corrosion testing revealed that greater welding currents improved corrosion resistance by lowering mass loss and increasing oxide layer development. Potentiodynamic polarisation studies confirmed this, with Joint 3 (500 A)  having the lowest corrosion current density. SEM research demonstrated that lower welding currents caused severe pitting and microvoids, whereas higher welding currents produced a more uniform and stable microstructure.

Overall, increasing the welding current enhanced the mechanical and corrosion qualities of the welded joints up to 450 A, beyond which excessive heat input reduced mechanical performance while maintaining higher corrosion resistance.

References

1. Cui, S., Pang, S., Pang, D., Tian, F., & Yu, Y. (2022). The microstructure and pitting corrosion behavior of K-TIG welded joints of the UNS S32101 duplex stainless steel. Materials, 15(16), 5612.

https://doi.org/10.3390/ ma15165612

2. Fei, Z. (2020). In-depth welding procedure qualification of Keyhole Tungsten Inert Gas welded high hardness grade quenched and tempered steel and dissimilar stainless steel. University of Wollongong Theses Collection. https:// ro.uow.edu.au/theses1/936 Fei, Z., Pan, Z., Cuiuri, D., Li, H., & Huang, W. (2020). Microstructure and mechanical properties of deep penetration autogenous TIG-welded dissimilar joint between creep strength enhanced ferritic steel and austenitic stainless steel. The International Journal of Advanced Manufacturing Technology, 106(9 10), 3915–3929.

https://doi.org/10.1007/s00170-020 04921-1

3. Kavishwar, S., Bhaiswar, V., Kochhar, S., & Singh, R. (2024). Comprehensive studies on conventional and novel weld cladding techniques and their variants for enhanced structural integrity: An overview. Welding International. https://doi.org/10.1080/09507116.2024.1234567

4. Dagur, H., Kumar, R., Singh, V., Chauhan, S., & Kumar, S. (2023). Effect of TIG and activated flux TIG welding processes on weld bead geometry, microstructure, and hardness of SAF 2507 grade super duplex stainless steel joints. Engineering Research Express, 5(4), 045015.

https://doi.org/10.1088/2631-8695/acd5b2

5. Korkmaz, E., & Meran, C. (2021). Mechanical properties and microstructure characterization of GTAW of micro alloyed hot rolled ferritic XPF800 steel. Materials Science and Technology, 37(5), 552–565.

https://doi.org/10.1080/ 02670836.2021.1882824

6. Sales, A. M., Westin, E. M., & Colegrove, P. (2016). Effect of nitrogen in backing gas on duplex root weld properties of heavy-walled pipe. Welding in the World, 60(6), 1147 1158.

https://doi.org/10.1007/s40194-016-0355-1

7. Naveen Kumar, S., Balasubramanian, V., & Murugan, N. (2022). Effect of welding consumables on the ballistic performance of shielded metal arc welded dissimilar armor steel joints. Journal of Materials Engineering and Performance, 31(5), 2526–2540. https://doi.org/10.1007/ s11665-022-06591-x

8. Fande, A. W., Taiwade, R. V., & Raut, L. (2022). Development of activated tungsten inert gas welding and its current status: A review. Materials and Manufacturing Processes, 37(1), 1–18.

https://doi.org/10.1080/10426914 .2022.2035710

9. Shravan, C., Radhika, N., Deepak Kumar, N. H., & Sharma, S. (2023). A review on welding techniques: Properties, characterizations, and engineering applications. Advances in Materials and Processing Technologies, 9(3), 785–810. https://doi.org/10.1080/2374068X.2023.2178693

10. Mahajan, A., Devgan, S., & Sharma, A. (2023). Surface alteration of Cobalt-Chromium and duplex stainless steel alloys for biomedical applications: A concise review. Materials and Manufacturing Processes, 38(10), 1785 1802. https://doi.org/10.1080/10426914.2023.2247310

11. Giudice, F., Missori, S., Scolaro, C., & Sili, A. (2024). A review on fusion welding of dissimilar ferritic/austenitic steels: Processing and weld zone metallurgy. Journal of Manufacturing and Materials Processing, 8(2), 123.

https:// doi.org/10.3390/jmmp8020123

12. Zhang, L., Wu, Z., Li, Y., & Zhao, F. (2024). Mechanical properties and corrosion resistance of TC4 titanium alloy joints by plasma arc welding + gas tungsten arc welding combination welding. Journal of Materials Science, 59(3), 1458–1472.

https://doi.org/10.1007/s10853-024-08746-8