Effect of grain size on self -healing behaviour of sensitized S304HCu stainless steel

• Published in: Materials chemistry and physics 2018-03, Vol.207, p.203-211
• Main Authors: Rajesh Kannan, P., Muthupandi, V., Devakumaran, K., Sridivya, C., Arthi, E.
• Format: Article
• Language: English
• Subjects: Activation; Ageing; Chemical precipitation; Chromium; Chromium carbide; Creep strength; DLEPR; DOS; Effects; Electric power generation; Grain boundaries; Grain size; Grain structure; Healing; Matrix; Oxalic acid; Oxidation resistance; Power plants; Precipitates; Reaction kinetics; S304HCu; Stainless steel; Stainless steels
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2017.12.012

S304HCu stainless steel is extensively used for superheater and reheater applications in power plants due to its superior creep strength and oxidation resistance which are derived from the coherently dispersed precipitates formed in austenite matrix. In addition to the beneficial precipitates, chromium carbide precipitates can also form. Formation of chromium carbide precipitates during service will lead to sensitization. In this investigation, S304HCu samples were solutionized at 1100 °C and 1200 °C for 1 h, and further aged at 750 °C for upto 400 h to introduce sensitization and subsequent healing. The influence of ageing time and grain size on the degree of sensitization (DOS) was evaluated by using oxalic acid and double loop electrochemical potentiodynamic reactivation (DLEPR) tests. DOS initially increased up to 10 h of ageing and subsequently was reduced due to healing and attained a steady value at 50 h. Faster and higher DOS was observed in coarse grain structure than in the fine grain structure. However, healing was largely delayed in coarse grained structure. The difference in solute and excess vacancy concentration and shifting of the nose in TTP curve were attributed to the difference in the kinetics of sensitization and healing. The extent of the attack on the grain boundary was analyzed using AFM. [Display omitted] •Coarse grains accelerate sensitization and retard the healing rate.•Diffusion mechanisms favouring sensitization and healing are identified.•Oxalic acid test is unable to detect desensitisation.•DLEPR test is superior to oxalic acid test to evaluate DOS and healing.