Improvement of Transpassive Intergranular Corrosion Resistance of 304 Austenitic Stainless Steel by Thermomechanical Processing for Twin-induced Grain Boundary Engineering

Abstract

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent passive intergranular corrosion such as sensitization of austenitic stainless steels, but the effect of GBE on transpassive corrosion has not been clarified. In the present study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into type 304 austenitic stainless steels containing different phosphorus concentrations. The resulting steels showed much higher resistance to transpassive intergranular corrosion during the Coriou test, in comparison with the as-received ones. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular deterioration during GBE.KEY WORDS: grain boundary engineering; coincidence site lattice; thermomechanical processing; austenitic stainless steel; intergranular corrosion.of CSL boundaries into 316 austenitic stainless steel and consequently decreased the susceptibility to intergranular corrosion significantly.Intergranular corrosion of stainless steels can be divided into passive and transpassive corrosion, depending on the electrode potential region of the environment. 19,20) The previous studies have demonstrated that GBE is effective for preventing intergranular corrosion at passive potentials by sensitization due to intergranular carbide precipitation in 304 and 316 austenitic stainless steels. 11,15,16) Meanwhile, intergranular corrosion possibly occurs for stainless steel due to the preferential dissolution of phosphate and/or phosphorus segregated at grain boundaries under corrosion environment at transpassive potentials.21-27) GBE may probably be also effective to suppress the transpassive intergranular corrosion of austenitic stainless steels. The present study aimed to examine the effect of GBE on the transpassive intergranular corrosion of type 304 austenitic stainless steels containing different concentrations of phosphorus. Experimental ProceduresThe chemical compositions (mass%) of two 304 austenitic stainless steels used in the present study are shown in Table 1. One of them was an ordinary commercial 304 stainless steel containing 0.027 % phosphorus, termed "high-P". The other one was specially prepared to contain a low phosphorus concentration of 0.001 %, termed "low-P". The as-received 304 steels were solution heat treated at 1 323 K for 0.5 h, and termed base materials (BMs). The BMs, 6ϫ10ϫ40 mm 3 in size, were thermomechanically processed by a one-step, pre-strain plus annealing process at the similar parameters to the previous study, 11) i.e., cold-rolling by 5 % reduction in thickness and subsequent annealing at 1 220 K f...

Microstructure evolution and constitutive analysis of nuclear grade AISI-316H austenitic stainless steel during thermal deformation

“…Moreover, the impact of twinning on DRX and microstructure evolution in ASS at a high strain rate has also been reported in earlier studies [17]. In addition, the CSL boundaries are found to be effective in preventing passive intergranular corrosion [70], improving the resistance of grain coarsening [71], reducing the propagation rate of fatigue crack [72], increasing ductility and uniform elongation [73].…”

Potential of grain boundary engineering to suppress welding degradations of austenitic stainless steels

“…38 The suppression of transpassive intergranular corrosion by GBE could be also explained by the disruption of the connectivity of random boundaries, at which phosphate and/or phosphorus segregate(s) preferentially, by CSL boundaries with a lower concentration of segregant. 18 intergranular chromium carbide precipitation and corrosion than random boundaries in 304 austenitic stainless steel. [6][7][8] This suggests that GBE has a high potential to inhibit the weld decay caused for a short time exposure to the sensitisation temperature range in welding thermal cycles.…”

“…37 A Coriou test 37 of 304 austenitic stainless steels containing different concentrations of phosphorus has shown that GBE is also effective to suppress the transpassive intergranular corrosion. 18 In passive corrosion, a well distributed CSL boundaries in the grain boundary network create a discontinuous chain of chromium depletion and can arrest the percolation of intergranular corrosion from the surface. 14 An analytical transmission electron microscopic observation has demonstrated that the chromium depletion at a CSL segment introduced by twin emission into a random boundary was significantly suppressed compared with that of the original random boundary during sensitisation.…”

“…[10][11][12] Grain boundary engineered materials which are characterised by high frequencies of CSL boundaries are resistant to intergranular degradations of materials. [13][14][15][16][17][18][19][20] Randle has reviewed various GBE investigations for optimisation of grain boundary character distribution (GBCD) in materials. 21 Many of the grain boundary engineered materials (GBEMs) demonstrated a reduction of negative phenomena, such as cracking, 13 intergranular corrosion, 14,15 and stress corrosion cracking, 16 as well as improvement of ductility.…”

“…15 This paper introduces GBE of austenitic stainless steels by the single step thermomechanical processing and discusses the potential of GBE to suppress intergranular degradations during welding by reviewing our recent studies. [14][15][16][17][18][19][20] Thermomechanical processing for GBE The single step GBE processing achieved very high frequencies of CSL boundaries in austenitic stainless steels. 14,15 As an example, the optimisation during the single step GBE processing with 304 austenitic stainless steel is shown as follows.…”