Lasers as a heat source have been widely applied in industrial processes [1
]. Currently, they are used for welding, cutting, remelting, and various processes with surface modification of materials [3
]. Laser sources are characterized by: high energy density, high efficiency, large temperature gradients, high repeatability of the process, and formation of a narrow heat affected zone (HAZ). Many authors report beneficial changes in the properties of various materials subjected to laser treatment in such sectors as: medical, energy, ocean engineering, etc. [7
]. The area of application of various technological laser solutions includes processing of many types of metals and their alloys, ceramics, polymers, and composites [1
], including underwater local dry welding and wet welding [12
]. Limiting the disadvantageous effects of technological use of lasers in some applications, e.g., the small width of weld, can be obtained by combining it with other heat sources to create hybrid processes. The most popular solutions include laser-metal active gas arc welding (MAG) and laser- tungsten inert gas arc welding (TIG) processes [14
Often, high-alloy steels, especially stainless steels, are subjected to laser treatment. Stainless steels occupy an irreplaceable position in many branches of industry [17
]. These steels are classified, among others according to the criteria of chemical composition and structure into: ferritic, martensitic, austenitic, and duplex (ferritic-austenitic) steels [18
]. Austenitic steels are the most popular group of high-alloy stainless steels. The content of chromium (above 18%) and nickel (minimum 8%) provides the structure stability and the corrosion resistance [21
]. In order to improve the properties of these steels, alloying elements: molybdenum, titanium, and niobium are introduced. The last two are excellent stabilizers and reduce the risk of intergranular corrosion. Austenitic steels are characterized by high corrosion resistance, good strength, and plastic parameters, hence their wide application. Improvement of mechanical properties can be achieved through the use of cold forming, and increase in corrosion resistance through the use of heat treatment. The group of austenitic stainless steels is considered well weldable by using many processes and maintaining an appropriate technological regime that includes all treatments related to the welding process, e.g., the prefabrication of the elements to be welded by using appropriate tools, the use of the consumables with an appropriate chemical composition, the control of the heat input and others. 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
], regulations and standards, e.g., EN 1011–3.
Duplex ferritic-austenitic steels are becoming increasingly popular due to their specific properties. They combine the advantages of other groups of high-alloy steels and the most important of them are high strength with good plasticity and corrosion resistance [25
]. The two-phase structure is obtained by appropriate selection of the ratio of ferrite-forming elements (mainly Cr—21–28%) and austenite-forming elements (Ni—1.5–8%, N—0.05–0.3%). Depending on the content of these elements, duplex steels are divided into lean duplex, duplex, super duplex, and hyper duplex. Lean duplex steels represent a reasonable compromise between the need to provide the appropriate strength properties, desired corrosion resistance, and the requirement of low material costs [28
]. Compared to austenitic stainless steels, they have higher strength parameters besides plasticity and higher resistance to stress corrosion. Steels from this group are characterized by good weldability, provided that the technological regime is maintained: the control of the value of heat input, the use of appropriate welding consumables dedicated to welded grade, the dilution rate, the suitable dimensions of the welding groove, the use of the appropriate type of shielding gas on the face and root side, maintaining high purity of welded components and other [29
Duplex steels are sensitive to structural changes resulting from the welding thermal cycle. This can reduce the mechanical properties of the joints as well as their corrosion resistance. High cooling rate of duplex steel joints may result in higher ferrite content in HAZ and in the weld [31
]. According to ASTM E562 standard, no more than 70% of ferrite is recommended, and according to Norskom M-601 it should be in the range of 30–70%.
Despite the fact that duplex steels are considered sensitive to high cooling rate, they can be successfully welded in conditions of large negative temperature gradients, e.g., under water and using concentrated heat sources [32
]. In addition, a significant problem during welding is the risk of chromium nitride precipitation, as well as the occurrence of micro–areas depleted in Cr and Ni. Guidelines for welding duplex steels can be found in scientific reports [17
], regulations and standards, e.g., EN 1011–3, Annex C.
In industrial applications (for example in: power generation, nuclear, petrochemical, aerospace, and shipbuilding sector) there is often a need to perform various variants of dissimilar joints [28
]. A particular group of dissimilar joints are connections between steels from different groups of stainless steels. Of these, joints of the austenitic steel type with ferritic, martensitic and duplex steel grades are most often made. For joining stainless steels with other metals in addition to laser welding also other methods can be used: metal active gas arc welding (MAG), tungsten inert gas arc welding (TIG), shielded metal arc welding, arc stud welding, friction welding, friction stir welding, diffusion welding, explosive welding, and furnace brazing. For welding different grades of austenitic and duplex steels, many variants of technologies were used, which led to obtain joints with various morphology, mechanical properties, and corrosion resistance [37
Welding of dissimilar stainless steel joints can be done between two main types of filler metals: austenitic (e.g., 309L) or duplex (e.g., 2209). Vincente et al. stated that the higher corrosion resistance of joints made by the MAG process is favored by the use of duplex steel consumable [45
]. Similarly, Rahmani et al. recommend using duplex instead of austenitic consumable when using the TIG process [46
]. However, among the various austenitic consumables, the most beneficial, both in terms of strength properties, as well as morphology and corrosion resistance, is the use of 309L [47
] or super austenitic 904L filler metal [49
]. The literature also describes attempts to weld such a material combination without filler material: using different variants of the TIG process [50
] or concentrated welding sources [33
]. Ridha Mohammed et al. stated that the mechanical properties of the duplex-austenitic steel fiber laser joints were better compared with the base materials (BM) because of the small HAZ [52
]. In the same work, the authors found the presence in welded joints the regions with a completely austenitic solidification mode which were susceptible to solidification cracking. An important observation is that when laser welding is used for super duplex and austenitic steels, the ferrite/austenite phase balance is not significantly changed by different heat input values [39
]. Existence of an unmixed zone that originated from each base material was stated by Chun et al. [53
]. The approximately 50:50 ferrite/austenite microstructural balance can be reached with solution annealing in the range of 1050–1100 °C, and it is especially important in autogenous welds [33
]. Saravanan et al. stated that the higher microhardness of the weld zone is attributed by the formation of finer and uniform grains following high cooling rate [54
Although researches on laser welding of dissimilar stainless steels have been reported, the study on structure and properties of laser beam welded austenitic/lean duplex steel joints is still relatively rarely mentioned. Therefore, the present work aims to show the ability of making 316L austenitic stainless steel–2304 lean duplex stainless steel dissimilar joints using the autogenous fiber laser welding process.
2. Materials and Methods
The test pieces were a flat plates with thickness of 8 mm made of 316L austenitic stainless steel (1.4404, UNS S31603) and 2304 lean duplex stainless steel (1.4362, UNS S32304) in delivery condition. Both materials were after heat treatment: 316L steel after solution annealing from 1050 °C and 2304 steel was hot rolled and solution annealed. The chemical composition of the used materials in accordance with the inspection certificate (spectral analysis) and the requirements of the EN 10088-2:2014 standard (minimum and maximum wt. %) are given in Table 1
. The mechanical properties of the materials in accordance with the requirements of the EN 10088–2–2014 standard are presented in Table 2
. Chromium and nickel equivalents (Creq and Nieq) were calculated according to [55
A high power continuous wave ytterbium fiber laser IPG Photonics YLS–6000 (IPG Photonic, Oxford, MS, USA) with a maximum power of 6 kW and a wavelength of 1070 nm was used (Figure 1
). The joints were made at 6 kW laser output power. The laser beam was delivered through the optical feeding fiber (transmitting fiber) of 300 µm core diameter. A welding head having a focal length of 250 mm and a focus collimator lens 150 mm was used. Focusing position was set on the surface of the plate. Welding speed was set to 25 mm/s (1.5 m/min). Welding process was autogenous—without filler metal. Shielding gas was supplied by a gas nozzle mounted at laser head to avoid the oxidation of weld beads. The flow rate of shielding gas—argon 5.0 (I1 in accordance with ISO 14175)—was set at 16 L/min. Test pieces were welded without using a ceramic backing or forming gas.
In order to identify and then eliminate any troubles with welding process the first stage of research consisted of making preliminary test joints. Initial joints were made to determine the values of significant variables, such as focal length, laser power and welding speed. The parameters of laser welding for austenitic stainless steel and lean duplex stainless steel differ, therefore, after initial experiments with similar materials, parameters matching both grades were selected. Before welding, the elements were cleaned with abrasive paper and degreased with acetone. Preliminary tests on stainless steels showed the presence of spatter if the surface was not cleaned properly just before welding.
To assess the quality of the welded joints, non-destructive tests (NDT) were performed: visual testing (VT)—according to the EN ISO 17637 standard and penetrant testing (PT)—according to the EN ISO 571–1 standard. Tensile and bending tests were conducted at ambient temperature of 20 °C using Instron 1195 (INSTRON, Norwood, MA, USA) universal testing machine in accordance with EN ISO 6892–1 and EN ISO 4136 standards. For bending test according to EN ISO 15614–11 bend former with a diameter of ϕ = 27 mm was used (the bend former diameter for materials with elongation above 25% is four times the specimen thickness). The acceptance criterion was defined as a bend angle α = 180°. Specimens in the face and root areas were slightly grinded to remove notches before performing tensile and three-point bending tests.
In order to prevent changes in the material structure under the influence of temperature during cutting, the specimens for metallographic examinations (according to EN ISO 17639) were cut mechanically in a direction transverse to the welding axis at the cutting machine with intensive cooling. Then it was grinded on abrasive papers with gradation 600–2400 and polishing on a polishing cloth using an aqueous suspension of 3 µm diamonds. This preparation of the specimen was followed by two-stage etching. The first stage was etching to reveal the austenite grain boundaries for which a mixture of 50 mL of boiling water, 3 mL of HNO3, and 1 mL of HF was used. The etching was performed by a two-minute immersion of specimens in an 80 °C temperature solution. Then the specimen was thoroughly rinsed and cooled under water. The next stage was etching in Beraha-type reagent (85 mL of water, 15 mL of HCl, and 1 g K2S2O5). Etching was carried out by about 1-3 min immersion in 20 °C temperature solution (until the corresponding colored structure was obtained on the specimen). At the end the specimen was thoroughly rinsed and dried with a stream of compressed air. Macroscopic metallographic observations were performed using DSLR Nikon d7000 with Tamron 90 mm f/2.8 macro lens (Nikon Corporation, Tokyo, Japan), while metallographic microscopy tests were done on a light microscope (LM) Olympus BX51 (Olympus, Tokyo, Japan). It offers imaging in a bright field, dark field and polarized light. The tests were also carried out using the scanning electron microscope JOEL JSM-7800F (SEM) with the EDAX adapter (Japan Electronics Corporation, Tokyo, Japan) enabling EDS analysis.
Microhardness measurements—HV0.2 were carried out using FM-800 tester with a load of F = 1.9614 N (Future-Tech, Tokyo, Japan).
The ferrite number was determined using a Fischer Feritscope FMP30 for both base materials and weld (Helmut Fischer GmbH Institut für Elektronik und Messtechnik 71069 Sindelfingen, Germany). The Feritscope was calibrated on calibration standards prior to measurements. The ferrite content measurements were carried out in accordance with ISO 17655, at 6 measuring points for each place of measure.
Tests regarding dissimilar laser welding of butt joints made of 316L austenitic stainless steel and 2304 lean duplex stainless steel showed that the use of IPG YLS–6000 fiber laser with a maximum power of 6 kW allowed to obtain sound butt joints meeting the requirements of quality level B in accordance with EN ISO 13919–1 standard. During welding, the only imperfection detected by VT and PT was the underfilling of the face, while microscopic examinations did not show the presence of any other welding imperfections. Due to the very low (as for laser welding) welding speed, it was possible to achieve full penetration of 8 mm thick plates with one-sided course of the laser beam.
Obtained joints have good strength properties (higher than the minimum values required by the standard), which were confirmed in the static tensile tests (average Rm = 600 MPa). All specimens fractured in 316L austenitic stainless steel BM. The bending tests showed that plastic properties of base materials were not deteriorated and confirmed the absence of welding imperfections.
The microstructure of the dissimilar welded joint has a better austenite to ferrite ratio (closer to 50:50), compared to the microstructure observed on similar duplex steel welded joints [56
]. This is caused by mixing of 316L steel and 2304 steel as well as by lower thermal conductivity of austenitic steel, which extends the cooling process and the time for austenite formation. Within the weld face, the weld width is greater, which affects a different amount of heat to be dissipated by the base material than in the case of weld root. The differences in thermal cycles, in these areas, cause changes in weld microhardness in the face and in the root of the weld. The microhardness of the autogenous weld metal (average 254 HV0.2) is higher than microhardness of the 2304 lean duplex stainless steel (average 233 HV0.2). The solidification microstructures of laser welds are generally consistent with the prediction from the Schaeffler diagram.
Based on the results of present study, autogenous fiber laser welding of 316L austenitic and 2304 lean duplex stainless steels without using a ceramic backing can be recommended as a suitable procedure for sound welded joints.