Influence of the aging time on the microstructure and electrochemical behaviour of a 15-5PH ultra-high strength stainless steel
Introduction
Precipitation hardening martensitic steels have outstanding properties attributed to the precipitation of fine and densely distributed intermetallic and carbide precipitates in the matrix [1,2]. 15-5PH (precipitation hardening) ultra-high strength stainless steel is a type of precipitation hardened martensitic steels that is widely used in marine facilities, aircraft components, and nuclear plants due to its combination of high strength and good toughness and relatively good corrosion resistance [3,4]. The basic microstructure of 15-5PH martensitic stainless steel during application is a martensitic phase with Cu precipitations after aging treatments. The size, shape, and distribution characteristics of these precipitations are determined by the heat treatment process.
Many studies have been conducted on the microstructure of 15-5PH martensitic stainless steel with various heat treatments [[5], [6], [7], [8], [9]]. The changes in precipitations, intermetallic phases, and carbides have significant and complex effects on the mechanical properties of this type of stainless steel. The formation of such precipitates also has harmful effects on the corrosion resistance of precipitation hardening stainless steel. For example, the precipitation of a massive amount of M23C6 carbides was reported to block the formation of new passive film in a 13 wt.% Cr martensitic stainless steel [10]. A chromium-depleted zone formed around the carbides, which are susceptible to localized corrosion [11]. Moreover, galvanic corrosion may also occur in regions where precipitations and the matrix come into contact [12]. In general, the corrosion resistance of stainless steel is related to the passive film formed on the surface [[13], [14], [15]]. The passivity of stainless steel is related to its microstructure [[16], [17], [18]]. Therefore, it is important to study the effect of the microstructure on the electrochemical and passive behaviour of precipitation hardening martensitic stainless steel and to identify the associated corrosion mechanisms.
To our knowledge, there have been no studies conducted on the effect of the microstructure on the passive properties and corrosion behaviour of 15-5PH grade stainless steel. The present work is an attempt to obtain extensive knowledge about the microstructure, electrochemical behaviour and their relationship for 15-5PH martensitic stainless steel in a neutral chloride solution. Furthermore, the compositions of the passive films with various microstructures were determined. The microstructures of the 15-5PH martensitic stainless steel were characterized using transmission electron microscopy (TEM) and atom probe tomography (APT) analysis. The electrochemical behaviour of the passive film was observed using potentiodynamic polarization curves, electrochemical impedance spectra, and Mott-Schottky measurements. The passive films were analysed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
Section snippets
Material and heat treatments
The composition of the 15-5PH ultra-high strength martensitic stainless steel used in this work is listed in Table 1. It was melted from an ingot of 50 kg in a vacuum induction furnace and a consumable-electrode vacuum furnace. The ingot was homogenized at 1200 °C for 12 h and forged into a 20 mm-thick steel plate. Square samples (10 mm × 10 mm) with a thickness of approximately 2 mm were cut from the steel plate. To remove the deformation stress, the steel plate was subjected to solution
Microstructure and precipitation
Fig. 1 shows the surface microhardness results with different aging times at 500 °C. It was obvious that the martensitic stainless steel had a low Vickers microhardness value (∼328 VHN) after solution annealing at 1000 °C for 1 h. The microhardness increased with an increase in the aging time. A remarkable microhardness was found as the aging time increased to 240 min, and a maximum microhardness level of ∼480 VHN was achieved. The microhardness value was slightly reduced when the aging time
Conclusions
The microstructure of 15-5 PH martensitic stainless steel was significantly changed by various aging times. Disc-shaped and ellipse-shaped isolated Cu-rich clusters formed after 15 min. Moreover, after 240 min, the nano-sized (Cu, Nb)-rich carbides were observed, and the decrease of Cr content around these carbides could deteriorate the pitting corrosion resistance. Electrochemical studies indicated that the pitting corrosion increased with increasing aging time and became more severe after
Acknowledgements
This work was supported by the National Key Research and Development Program of China (No. 2017YFB0702300) and the National Natural Science Foundation of China (No. 51671029).
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QiangYu contributed equally to manuscript as the first author.