Influence of process parameters on microstructures and properties of the heat - Affected zone (HAZ) and fusion zone (FZ) of the dissimilar metal welding

bstract This paper reports the changes of microstructure and properties in the fusion zone (FZ) and heat - affected zone (HAZ) in dissimilar weld between Austenitic stainless steel (304L) and low carbon steel under varying of welding parameters. The dimension of base materials was 500x250x3 (mm). The welding current ranged from 80A to 120A and travel speed were limited 100-200 mm/s and constant arc voltage of 25V. The welding joints were made by gas metal arc welding (GMAW). As the result, the microstructures and properties in three regions had significant differences and changed under vary welding process parameters. HAZ width and grain sizes in carbon steels are larger than stainless steel. Besides, tensile strength and hardness were affected by welding parameters. Weld joint at 100A, 120 mm/min would be give the highest tensile strength and hardness. Keywords: Dissimilar metal welding, welding parameters, HAZ, microstructures, OM. 1. Introduction Nowadays, dissimilar metal welds are used in many fields such as nuclear, power, oil and gas or shipbuilding industry, It was widely known that dissimilar metal joints are intrinsically an uncertainty because of difference in the physical, mechanical and metallurgical properties of the base metals to be joined. Besides, the intense heat applied during process produces both metallurgical and physical inhomogeneity in the various zones of the weld leading to differential properties of the weld section. Therefore, the investigation of microstructure, solidification, phase transformation and mechanical properties during welding process is the best way to understand the essence of welding. In other words, in order to produce high quality joints, researching all factors affecting to mechanical properties is necessary. As the previous researchers, different welding regions in arc welding are classified into three zones as fusion zone (FZ), heat - affected zone (HAZ) and base metal (BM) which are influenced by heat input rate and heat transfer factors [3]. FZ is the area between two pieces of base metal being melted and solidified. HAZ is the area of base metal which is not melted and has had its microstructure and properties altered by the heat. The change of welding parameters has different impact on weld geometry, microstructure and properties in welding zones. For instance, as the heat input and welding speed both increase, the weld pool becomes more elongated, shifting from elliptical to teardrop shaped [2]. Amuda [1] shows that, under welding conditions, the primary solidification structure of stainless steel changed from a predominantly ferrite structure to a matrix interspersed with increasing fraction of inter - dendrite martensite in the weld metal and grain boundary martensite in the HAZ. In the carbon welding, ferrite is formed under different shapes as grain boundary ferrite, polygonal ferrite or acicular ferrite. Besides, upper bainite, lower bainite or Widmanstatten can be observed [2]. Other researches also represent that grains are smaller, the mechanical welding is better. THE INTERNATIONAL CONFERENCE ON MARINE SCIENCE AND TECHNOLOGY 2016 HỘI NGHỊ QUỐC TẾ KHOA HỌC CÔNG NGHỆ HÀNG HẢI 2016 203 Although the researching about cause-effect between welding parameters and microstructure has invested, the problems concern dissimilar metal welding documents are limited. Based on previous study, the objective of this study was to find out the changes of microstructure and properties of welding between carbon steel and stainless steel under the varying of current and welding speed. 2. Experimental materials and procedure The base metals were 304L stainless steel and carbon steel using gas metal arc welding process. The dimensions of plates were 500 x 250 x 3 mm. The chemical composions of base metals, filler show in the table below. Table 2.1. The chemical compositions Alloys (%) C Mn Si S P Cr Ni Mo V SS 304L 0,09 1,54 0,49 < 0.005 0,005 18.3 7.56 0,13 0,11 Carbon steel 0,1 0,62 0,02 0,04 0,05 0,02 0,08 0.005 0,01 Filler ER 309L 0,08 0.7 0,8 < 0.005 < 0.003 19.7 11.8 0.1 0.09 The welding parameters in the proceses were current and welding speed shown in the table 2.2. The values of these factors were selected by experimental statistic datas. Table 2.2. Experimental numbers and welding parameters Parameters 1 2 3 4 5 6 7 8 9 I (A) 60 80 100 60 80 100 60 80 100 V(mm/min) 120 120 120 160 160 160 200 200 200 Affter welding processes, samples were cut to examine their microstructure, tensile strength and hardness. The microstructure was observed by OM LEICA MDS4000M. The tensile strength and the hardness were determined by WEW1000B and hardness tester ARK600, respectively. 3. Results and discussion 3.1. Microstructure Because of using filler ER 309L in welding process, austenite (γ) and ferrite (δ) were formed in fusion zone (figure 3.1a). Generally, grain morphology in FZ is equal structure and independent of process parameters. Figure 3.1 (a) Microstructure in fusion zone, V = 160 mm/min, I = 100 A According the results of hardness test indicate that HAZ width in carbon steel is larger than stainless steel. This can be expressed by the significant difference of thermal conductivity between carbon steel and stainless steel. Figure 3.2 shows that HAZ width in stainless steel is very narrow and concentrates at fusion line. THE INTERNATIONAL CONFERENCE ON MARINE SCIENCE AND TECHNOLOGY 2016 HỘI NGHỊ QUỐC TẾ KHOA HỌC CÔNG NGHỆ HÀNG HẢI 2016 204 Figure 3.2 Microstructure in HAZ of stainless steel, I =100A, V= 200 mm/min By comparing microstructure in HAZ of carbon steel, grain size is much larger than its base metal. This can be observed by comparing grain size in figure 3.2, figure 3.4 with figure 3.3. Moreover, it depends on welding parameters. Figure 3.4 represents the change of grain size under varying of current welding. It can be observed that if the current is constant, the welding speed is slower, heated metal at the high temperature is kept for a long time, grain size is larger. Meanwhile, if we consider under constant welding speed, the increasing of current welding is cause of developing of grain size (figure 3.5). (a) Carbon steel (b) Stainless steel Figure 3.3 Microstructure of base metal . (a) (b) (c) Figure 3.4 Grain size in HAZ of carbon steel under varying of welding speed, I = 100A (a) V = 120 mm/min, (b) V = 160 mm/min, (c) V = 200 mm/min Figure 3.5 Grain size in HAZ of carbon steel under varying of welding current, V = 200 mm/min. (a) I = 60A, (b) I = 80A, (c) I = 200 A THE INTERNATIONAL CONFERENCE ON MARINE SCIENCE AND TECHNOLOGY 2016 HỘI NGHỊ QUỐC TẾ KHOA HỌC CÔNG NGHỆ HÀNG HẢI 2016 205 3.2. Mechanical properties The results of ultimate tensile strength and hardness test for dissimilar welding between 304L stainless steel and carbon steel were shown significant difference affected of current and welding speed. Figure 3.6 was shown the estimated means of tensile strength. As the results, welding parameters at 100A and 120 mm/min would be given the highest tensile strength. Figure 3.6 Tensile strength The values of hardness are represented in the figure 3.7. As can be observed from the chart, there are small gap among regions. Hardness in HAZ of carbon steel would be higher than others. Besides, the hardness of sample 3 at 100A, 120 mm/min stand at the highest level compare with others. Figure 3.7 Hardness values 4. Conclusion In this work, stainless steel and carbon steel were jointed using GMAW under varying of current and welding speed. Microstructure and mechanical properties of weld zones were investigated. From this study, following conclusions can be drawn: - The grain sizes observed in fusion zone are equal structure and independent of process parameters. Meanwhile, microstructure in HAZ of stainless steel and carbon steel has Tensile strength (KG/mm2) Welding speed (mm/min) Hardness (HV) x (mm) THE INTERNATIONAL CONFERENCE ON MARINE SCIENCE AND TECHNOLOGY 2016 HỘI NGHỊ QUỐC TẾ KHOA HỌC CÔNG NGHỆ HÀNG HẢI 2016 206 significant difference and controlled by welding parameters. HAZ width and grain sizes in carbon steels are larger than stainless steel. - Tensile strength and hardness were affected by welding parameters. Weld joint at 100A, 120 mm/min would be give the highest tensile strength and hardness. References [1]. M.O.H. Amuda and s.Mridha. Microstructural features of AISI 430 Ferritic stainless steel (FSS) weld produced under varying process parameters. international journal of mechanical and materials engineering (IJMME), Vol. 4 (2009), No. 2. 160 - 166. [2]. Sindo Kuo. Welding metallurgy. Published by John Wiley and Sons, Inc., Hoboken, New Jersey. 2003. [3]. Zakaria Boumerzoug, Chemseddine Derfouf, Thierry Baudin. Effect of welding in microstructure and mechanical properties of an industrial low carbon steel. Engineering, 2010, 502-506.