Welding of new generation stainless steels (assessment of the different welding parameters) and microstructural characterization and correlation with corrosion and mechanical properties
The applications of the oil industry for stainless steels present severe service conditions, such as an aggressive environment (containing chlorides and sulfides), high pressures and temperatures, especially in offshore applications. Oil well exploitation in increasingly severe conditions demands the use of new generation stainless steels such as duplex, super duplex and super martensitic 13Cr steels, to name a few. These steels are used because of their good mechanical properties and high corrosion resistance. However, welding of those steels is still a challenge once this procedure may cause the formation of deleterious phases, turning those regions into preferential sites for corrosive attack and mechanical failure. Besides, because of recent development, the best filler metals are not yet well established.
Thus, there is a huge demand for applied research on the welding of those steels. Research on the welding of those steels involving a variation of the main parameters (heat input, interpass temperature, the chemical composition of the shielding gas, use of purge gas) and of the filler metal chemical composition will be performed aiming to obtain the best properties of the welded structures. The evaluation of the welded joints will be achieved by the complete microstructural characterization, mechanical properties determination, and corrosion tests. The microstructural characterization will be carried out using optical microscopy and scanning electron microscopy (using EDS and EBSD detectors for the identification of the phases).
The microstructural characterization will involve, as an important step, quantification of deleterious phases through image processing and analysis methods. The department of Metallurgical and Materials Engineering (PEMM) also has a transmission electron microscope for more specific analyses. The characterization of the mechanical properties will be done through Charpy impact tests and macro and microhardness tests. Corrosion tests will involve ASTM G48 standard test (widely used for comparison purposes), EPR tests, anodic polarization, and electrochemical impedance. Depending on the kind of steel, a study will be carried out aiming to improve post welding heat treatments (temperature and time). Research to evaluate the influence of post-welding surface treatments on corrosion performance of welded joints of those new generation stainless steels will also be proposed.
LNDC structure will allow performing union welding operations using orbital TIG, conventional TIG, SMAW, SAW, FCAW and MIG/MAG. Among the activities to be performed there is also the execution of residual stress analyses in welded joints resulting from different welding procedures.
Evaluation of cladding of structural steels with corrosion-resistant alloys
Extreme conditions of oil and gas processing (as in separator vessels) demand the use of materials with good mechanical and corrosion resistance. A current solution is to use structural steels cladded with noble corrosion resistant alloys (at the region of contact with aggressive media). Structures completely manufactured with those noble alloys will result in high costs and decreased mechanical strength.
LNDC proposes researches to evaluate structural steels cladding with noble corrosion-resistant alloys such as Inconel 625. With this purpose, several welding methods will be evaluated such as SAW, FCAW, electro slag and TIG (for smaller regions). Microstructural analyses using optical microscopy and scanning electron microscopy will be performed to characterize the welded joints.
Besides, Charpy impact tests will be carried out and hardness and microhardness profiles will be obtained. As an important step, corrosion tests will be done, such as: bending, crevice corrosion, sulfide stress cracking and pitting corrosion.