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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martensite structure gets homogenized in Fig. 3 (b) and the width of welded region is reduced after cold drawing as observed in Fig. 3 (c).

a

b

c

Fig.3. Microstructure in (a) As ERW condition; (b) Annealing condition; (c) Cold Drawn condition

The bent tubes were through hardened and quenched in a sealed quench furnace. The microstructure of hardened tube consists of lath martensite as shown in Fig. 4. The hardened tube has hardness of 50 HRC (Rockwell hardness, C-Scale) and 415 HV. The hardening was followed by tempering at varied temperature such as 400°C, 450°C and 500°C for 15, 30 and 60 minutes.

Fig. 4. Microstructure in as hardened condition

After each trial microstructure analysis were performed at each heat-treated condition. The Fig. 5 shows the microstructure after each combination of tempering temperature and time. When the hardened tube was tempered at 400°C for 15, 30 and 60 minutes, there was a drop in the hardness at 15 and 60 minutes, except for 30 minutes which is evident from the microstructure shown in Fig. 5 (a to c). During tempering, the carbides tend to precipitate out of matrix as the recovery initiates when sufficient temperature is attained. As the temperature is increased to 450°C, the recovery of martensite was gradually increased as shown from Fig. 5 (d to f). When further increased to 500°C, the recovery is increased further as shown in Fig. 5 (g to i) and is also accompanied with change in substructure which will soften the structure gradually (Defa et al. (2012)).