Time dependent ductility improvement of stainless steel SS 316 using stress relaxation Krishnaswamy Hariharan, Prakash Dubey, Jayant Jain n a Indian Institute of Technology, Hauz Khas, New Delhi 110016, India article info Article history: Received 10 June 2016 Received in revised form 15 July 2016 Accepted 17 July 2016 Available online 19 July 2016 Keywords: Stress relaxation SS316 Dislocation density Formability abstract Plastic deformation of materials experience stress relaxation with time when stopped intermittently. The relaxation alters the ductility and the strain hardening behaviour during subsequent deformation due to the change in mobile dislocation density and internal stress. The prior studies on ductility improvement due to stress relaxation have focussed on multiple relaxation in the uniform deformation zone. The effect of single relaxation on ductility improvement is not reported so far. In the present work, single relaxation tests are performed using stainless steel 316 material. The effect of strain, time and strain rate on the ductility improvement is presented. An increase in ductility is consistently observed in all the cases of stress relaxation experiment. Possible explanation on the underlying mechanisms for ductility im- provement based on the present understanding is discussed. A power law and logarithmic relation of stress vs time are modified in the present work to account for these changes. & 2016 Elsevier B.V. All rights reserved. 1. Introduction The use of servo-press technology has been reported to im- prove the room temperature formability of metallic materials through non-linear and time dependent slide motion [1]. Recently, a maximum improvement of upto 40% in deep drawability is re- ported using servo press technology [2,3]. The improved form- ability using servo-press is attributed to the stress relaxation phenomenon [2,4]. Stress relaxation is one of the commonly used transient me- chanical tests during which the deformation of the specimen in an uniaxial tensile test is abruptly stopped without unloading. The stress drops continuously with time during the holding period. The stress drop as a function of time during stress relaxation is used to characterize several metallurgical parameters [5] such as long range internal stress [6–8] and activation volume [9–12]. The information is also used to develop rate dependent constitutive models of plastic deformation [13–15]. Most of these studies used the stress relaxation experiment to extract a range of stress vs strain rate from a single tensile test. However, the associated im- provement of ductility during stress relaxation and the corre- sponding deformation mechanisms responsible for this improve- ment have been largely ignored until recently [16]. In a recent study, the contribution of stress relaxation in duc- tility improvement was evaluated using three different grades of steels [16]. It was observed that the ductility improvement due to stress relaxation is sensitive to strain, strain rate and the material. Similar observations were reported in dual phase steels [17] and commercially pure titanium [18]. In most of the studies [4,16–18], the samples were subjected to multiple stress relaxation in their plastic regime to enhance the contribution to ductility. The con- tribution of a single stress relaxation step to the ductility is not known so far. Since the relaxation is sensitive to the strain at which the experiments are performed, their contribution cannot be averaged in the plastic regime. Preliminary studies indicated that the ductility improvement by multiple relaxations is not ad- ditive. In other words, the ductility improvement due to 10 in- termittent relaxation cycles is not twice that of 5 intermittent relaxations. In the present work, systematic studies are performed using SS 316 to understand the contribution of single relaxation to the overall ductility. The experimental results follow similar trend of reported results in their sensitivity to strain and strain rate. It is observed that a single relaxation cycle is capable of significant contribution to the ductility of materials. The stress drop during relaxation is modelled using a power law and logarithmic model. Both the models are modified with additional parameters to ac- count for the variation of mobile dislocation density and internal stress during relaxation. 2. Experiment The procedure for stress relaxation test involves intermittent Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2016.07.074 0921-5093/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: jayantj@iitd.ac.in (J. Jain). Materials Science & Engineering A 673 (2016) 250–256