Microhardness and Microstructure of Fiber Laser Welded S960 and S700 Steels

Ö. Ekinci ISSN 2587-1943 MICROHARDNESS AND MICROSTRUCTURE OF FIBER LASER WELDED S960 AND S700 STEELS Ö. Ekinci 1, Z. Balalan 2 Original scientific paper In this study, keyhole laser bead-on-plate welding of 6 mm thick high strength low alloy (HSLA) S960 steel plateand keyhole laser butt welding of 13 mm thick (HSLA) S700 steel plates were performed by using 16 kW fiber laser machine. Microhardness measurements and microstructural study on the fusion zone, heat affected zone and base material were carried out for different welding parameters that are used for welding of S960 and S700 steels. Furthermore, X-Ray Diffraction (XRD) patterns of the welded zone was performed. In laser bead on plate welding joints of S960 steels, the average microhardness value of fusion zone (FZ) is approximately 60 HV higher than that of base metal for all the welding specimens, while the microhardness value of FZ in laser butt welded joints of S700 steel is approximately 30 HV. On the other hand, microstructures of FZ consists of martensite phases, whereas both base material S960 and S700 consist of tempered martensite and strip-like martensite. Keywords: laser welding, microhardness, microstructure, S960, S700 steels FİBER LAZER KAYNAKLI S960 VE S700 ÇELİKLERİNİN MİKROYAPI VE MİKROSERTLİKLERİ Bu çalışmada, 16 kWfiber lazer kaynak makinesi kullanılarak 6 mm kalınlığındaki yüksek dayanımlı düşük alaşımlı S960 çelik plaka yüzeyine kaynak ve 13 mm kalınlıktaki S700 çelik plakaların alın kaynağı yapılmıştır. Farklı kaynak parametreleri kullanılarak kaynak edilen S700 ve S900 çeliklerinin kaynak bölgesinin ısı tesiri altında kalan bölgesinin (ITAB) ve ana malzemenin mikroyapı ve mikrosertlik değerleri tespit edilmiştir. Ayrıca, XRD testiyle kaynak dikişindeki fazlar elde edilmiştir. S960 çeliğinin kaynaklarında (ITAB)’da kalan bölgenin sertliği yaklaşık 60 HV ve bu kaynak işlemlerinde (ITAB)’da sertlik değerinin ana malzemenin sertlik değerinden daha yüksek çıkmıştır. S700 çeliğinin alın kaynağında ITAB’ın yaklaşık 30 HV değeri elde edilmiştir. Diğer taraftan hem S960 hem de S700 çeliğinin kaynağında ITAB’ın martenzit, temperlenmiş martenzit ve şerit şeklinde fazlardan meydana gelmiştir. Anahtar Kelimeler: Lazer kaynağı, mikrosertlik, mikroyapı, S960, S700 çelikleri 1 Introduction High strength low alloy (HSLA) steels have been used in a wide range of applications for years because of their great strength, toughness, weldability and strength weight ratio. For instance, these HSLA steels are used as construction elements in the shipbuilding, offshore industries, pressure vessels, the automotive industry and oil transportation pipes (Oñoro et al., 1997; Yan, et al., 2009) The use of HSLA steels as construction elements make lighter and more slender components possible and lowers setup costs without losing structural integrity (Shi and Han, 2008; Takasawa et al., 2012). Welding is a fundamental fabrication method in joining HSLA steels (Ghosh et al., 2010). Welding HSLA steels with traditional arc welding techniques produces large heat inputs in the welding zone, which leads heat affected zone (HAZ) to become soft and thus strength of welding zone decreases (Viano et al., 2000; Zhang et al., 2012). This is an important issue for high strength HSLA steels, however, this issue might be overcome via rapid water cooling. On the other hand, laser welding can be a good alternative for welding HSLA steels, which is a non-contact welding technique, tenders great welding speeds, low distortion, strong and tough joints due to its large power density and low heat inputs (Esfahani et al., 2015; Hao et al., 2015). In this study, keyhole laser welding of S960 and S700 HSLA steels was successfully carried out without using filler wire. In order to determine the weldability of these HSLA steels, the microstructure, XRD and microhardness tests were performed and assessed. International Journal of Innovative Engineering Applications 3, 1(2019), 15-21 2 Materials and Method In this study, 6 mm thick HSLA steel plate (S960, Tata Steel, y,min is 960 MPa) was used for keyhole fiber laser bead on plate welding. The chemical composition of S960 base material is given in Table 1. Additionally, 13 mm thick HSLA steel plate (S700, Tata Steel, y,min is 700 MPa) was used for keyhole fiber laser butt welding. The chemical composition of S700 base material is given in Table 2. A 16 kW continuous wave fiber laser (IPG YLS-16000) was utilized for keyhole welding operations. Experimental setup for the keyhole fiber laser welding is shown in Figure 2. During welding operations, argon gas was employed for shielding the top and back surfaces of the workpieces so as to protect the molten metal from the surrounding atmosphere. Keyhole laser welding experiments were performed by applying a variety of welding speeds and laser powers. On the basis of previous published studies (Guo et al., 2017) with single pass laser welding of HSLA steels, bead on plate single pass laser weldings of 6 mm thick S960 steel was performed at welding speeds of 0.9 m min, 1.05 m min, 1.2 m min and 1.5 m min with laser powers of 4 kW, 5 kW and 6 kW, respectively. Moreover, single pass laser butt weldings of 13 mm thick S700 steel was performed at welding speeds of 0.5 m min and 0.72 m min with laser powers of 11.5 kW and 13 kW. Welding parameters are presented for S960 and S700 steels in Table 3 and Table 4. After keyhole laser welding operations, welded parts were cut from welded areas to reveal weld cross sections. The weld cross sections were ground, polished and then etched with a solution of 3% Nital for about 15 s. Weld cross sections of S960 and also S700 steel produced with different 15 Microhardness and microstructure of fiber laser welded s960 and s700 steels 3 welding parameters are given in Figure 3 and 4. In order to examine the microstructure of the welds, JEOL JMS 6510 scanning electron microscope (SEM) was used. For crystal phase identification in the fusion zone, XRD analysis was conducted by Rigaku Ultima IV X-Ray Difractometer machine using Cu target (λ = 1.544 Å). Micro-hardness across the welded joint was measured using a load of 100 g with a Vickers micro-hardness machine (Emco Test DuraScan). Hardness determination was implemented in the base metal, fusion zone and heat affected zone. Results and discussion Macrostructures The weld bead of the keyhole laser bead on plate welding of S960 and butt welding of S700 HSLA steel with weld cross section presented in Figure 2. Porosity was observed in weld zone of S5 and S6 of S700 steel while undercut was observed in the weld bead of S3 and S4 of S960 steel. Weld sagging due to excessive viscosity at the bottom of S1specimen of S960 steel was also observed. On the other hand, undercut, weld sagging or porosity were not observed in the weld zone of S2 of S960 steel. Macrostructure shows that, weld bead width of S5 of S700 steel is noticeably wider than that of S6 of S700 steel be (...truncated)