Effect of Heat Treatment on the Phase Composition and Corrosion Resistance of 321 SS Welded Joints Produced by a Defocused Laser Beam

Abstract

The effect of heat treatment of welded joints made of steel 321 on corrosion resistance, phase composition, residual stresses, and distribution of alloying elements was studied using optical microscope (OM) and scanning electron microscope (SEM), electron dispersive spectroscopy (EDS), X-ray diffraction (XRD), and intergranular corrosion testing (IGC). Samples previously obtained by the authors using defocused laser beam, which led to the formation of directionally crystallized austenite with lathy and skeletal δ-ferrite, were investigated. Based on X-ray diffraction studies in the base metal, the maximum number of peaks of various phases was presented, which decreased after exposure to the heating effect of the welding process and subsequent heat treatment. The distribution of alloying elements, in particular, Ti and Si, was significantly affected by heat treatment depending on the regimes. A spot chemical analysis showed that the nickel content differs in δ-ferrite and austenite by 1.5%–2% whereas the chromium content in these phases is not significantly different. Tests have shown that all samples have high resistance to intergranular corrosion, which can be explained by the insufficient dissolution of titanium carbides in austenite and the absence of chromium carbides formation along austenite grain boundaries, due to high cooling rates when welding by a defocused laser beam, and as a result, the high δ-ferrite content in which chromium dissolves.

Influence of Laser Beam Wobbling Parameters on Microstructure and Properties of 316L Stainless Steel Multi Passed Repaired Parts

“…The cladded metal of the first, second, and third passes differs only in the crystallization direction.The microstructure shows that the FA crystallization mode matches the one calculated by formula(3). In all three passes, crystallization is observed according to the FA mode, while in the middle of the second pass of the NHT sample there is a region (length is 600-700 µm, width is 100-150 µm, Figure10second pass) crystallized according to the AF mode, which is due to the larger volumes of the melt pool and, accordingly, lower cooling rates, especially in the central part of the second pass, which is consistent with the results of[9]. This crystallization mode can lead to the formation of crystallization cracks; however, in our case, no cracks were observed, even after mechanical tests.…”

“…In turn, the cooling rates depend on the selected type of welding [4], welding modes [5,6], and the presence of concurrent heating [3,4,7] or cooling during welding or recovery cladding [8]. Additionally, the phase composition can vary in depth of the weld seam (top, middle, and bottom parts of the seam), for example, when welding with a defocused laser beam or laser beam using the longitudinal wobbling mode [9][10][11]. That also has a significant influence on the performance properties of products.…”

“…Under the conditions of multi-pass welding, these irregularities can result in extensive zones of lack of fusion [27,28]. When the laser spot diameter is increased by defocusing due to the Gaussian distribution of energy in the spot, there is a loss of power at the edges [9,10]. In this case, a further increase in the laser irradiation power is necessary.…”

Autogenous Fiber Laser Welding of 316L Austenitic and 2304 Lean Duplex Stainless Steels

“…The main risks directly associated with the welding process are the formation of hot cracks and intergranular corrosion. General guidelines for welding this group of material can be found, among others, in many scientific reports [17,18,24], regulations and standards, e.g., EN 1011-3.…”