PSI - Issue 60

K. Mariappan et al. / Procedia Structural Integrity 60 (2024) 444–455 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.2 Work hardening behaviour

Tensile work hardening behaviour of 316L(N) SS exposed to various extents of prior fatigue damage at 300, 823, 873 K is shown in Fig.7. The  vs.  plots are helpful in bringing out the various stages of tensile work hardening behaviour distinctly. As shown in Fig.7a for 0 N f specimen at 300 K, typically work hardening curve exhibits three distinct regimes: (i) an initial transient stage, where θσ decreases rapidly (TS),(ii) followed by stage-II where θσ increases gradually with σ to a maxima (stage II) and (iii) finally, stage-III where θσ decreases due to onset of dynamic recovery (stage III). Figure 7b shows the variations of  vs.  . At 300 K, all the three stages of work hardening regimes were observed in the as-received material (0 N f ), as well as in the specimens subjected to prior fatigue damage (0.05 N f , 0.1 N f , 0.3 N f and 0. 5N f ). In contrast, at 823 K and 873 K, only the specimens of as-received and fatigue damaged up to 0.05 N f exhibited all three stages of work hardening, whereas the stage II of work hardening was clearly absent in all other prior fatigued specimen (0.1 N f , 0.3 N f and 0.5N f ). These observations are in agreement with that reported by Vijayanand et al. (2011) on prior cold worked 316 L(N) SS material. The stage II athermal hardening value of  II =  /55 obtained in the present study compares favorably with that reported for similar steel in the literature [Isaac Samuel et al (2002)]. Stage II work hardening characterized by linear true stress-true plastic strain curve corresponding to constant work hardening rate, which is athermal in nature. During stage II of work hardening, the activation of cross-slip and multiple slip over longer ranges promote the formation of heterogeneous dislocation structures comprising of tangles, walls and cells, which increases with plastic strain. As reported in AISI 316L SS [Feaugas (1999)], the rapid increase in dislocation density in this stage with plastic strain, signifies the characteristic dislocations storage during stage II hardening. In the recovery dominated stage III, due to the activation of extensive cross-slip the dislocation tangles and walls disappear progressively [Feaugas (1999)]. Except for the as-received specimen, the stage III of the work hardening fall in a narrow band for the specimen with prior fatigue damage, at 823 and 873 K. From the Fig. 7b it is evident that the rate of decrease of θσ in stage III is higher at 873 than at 823 K and the rate of decrease of θσ in stage III, indicating enhanced recovery processes at elevated temperatures.

Type 316 L(N) SS - 0 N f 3x 10 -3 s -1

1000

Solid line - Voce relation Dashed line - Ludwigson relation

True stress (MPa)

300 K 823 K 873 K

100

0.001

0.01

0.1

True plastic strain (%)

Fig. 8 Constitutive description of the flow behaviour using the Ludwigson and Voce relations demonstrated on the 0 N f specimen at 300, 823 and 873 K.

3.3 Constitutive Description of the Work Hardening Behaviour Applicability of various constitutive relations, namely Hollomon, Swift, Ludwigson and Voce relations, to describe the tensile flow behaviour were examined in the light of  2 values, which quantify the goodness of fit as the sum of squares of the deviation between the experimental and the constitutive curves. The  2 values obtained for the