PSI - Issue 14

S Usha Rani et al. / Procedia Structural Integrity 14 (2019) 142–149 Author name / Structural Integrity Procedia 00 (2018) 000–000

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2.2. Method The ERW tubes were produced using hot rolled strips made of 26MnB5 steel. The required dimension of tubes with outer diameter 25mm and thickness of 3.6mm were produced through appropriate cold drawing and annealing sequence. The tubes were bent to the shape using rotary draw bending. This bent profile was maintained for all the fatigue tests and standardized as the test coupon for the fatigue study in this work as shown in Fig. 2. These bent tubes were hardened above Ac3 temperature followed by quenching in a sealed quench furnace. The DoE based on Grey based Taguchi method was performed using MINITAB software. Two parameters (tempering temperature and time) and three levels were given as input that resulted with L9 OA (Orthogonal Array). The tempering was done in a box type heat treatment furnace at temperature of 400°C ,450°C and 500°C for 15, 30 and 60 minutes. After tempering, the microstructures and mechanical properties of each heat-treated sample was studied. In this work, the tensile test was conducted on Shimadzu 250KN universal tensile testing equipment. Microstructural study was carried by Zeiss optical microscope with image analyzer software and Joel IT100 scanning electron microscope. Bulk hardness and micro hardness were studied using Rockwell hardness tester and Shimadzu micro Vickers hardness tester at 200gm load.

Fig. 2. Schematic drawing of bent tube used for fatigue testing

3. Results and discussion 3.1. Hardness testing

The Micro Vickers hardness (HV) were obtained after each stage of the process sequence. Table 2 shows the micro hardness values of the tube at base metal (BM), weld zone (WZ) and heat affected zone (HAZ) regions under different condition. The weld center had the maximum hardness of 547 HV and after annealing the hardness dropped to 189 HV which is almost nearer to base metal hardness and thus the homogenization of weld and base metal was achieved in the annealing stage of ERW tubes prior to cold drawing to achieve the required dimension of the tube.

Table 2: Micro Vickers hardness after each process sequence. Location BM (HV) WZ (HV)

HAZ (HV)

Condition ERW (54 x 4.6 mm)

199

547

281

Annealed

183

170

168

3.2. Microstructural analysis The BM consist of ferrite pearlite microstructure. The ERW tubes consist of martensite and dual phase in WZ and HAZ respectively. The microstructure after ERW welding, annealing and cold drawing were obtained using Clemex microscope as shown in Fig. 3. The picral was used as etchant to observe the flow lines as shown in Fig, 3 (a). The weld flow line pattern shows a pronounced hourglass pattern. It is observed that after annealing, the