PSI - Issue 71

G. Narasinga Rao et al. / Procedia Structural Integrity 71 (2025) 317–324

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Comprising a precipitation-hardening type of stainless steel, this 17-4 PH material typically has a Cr content ranging from 15 to 17.5 percent by weight, along with 3 – 5 percent of nickel by weight, which is frequently observed to exhibit a martensitic crystalline configuration and may be enhanced in toughness by the integration of precipitates via an aging process (Eskandari, Lashgari et al. 2022). Samples of 17-4 PH steel treated with a solution and aged at 460 °C for 2h and 480 °C for 4h demonstrated minimal signs of sensitization. However, when the samples were aged at temperatures above 495°C, they exhibited a significant increase in sensitization, suggesting a higher risk of corrosion between the grains. Upon examining the microstructure of these samples, small, spherical niobium carbides were detected in the solutionized and aged specimens, because 0.28 wt.% Nb was added. However, these carbides do not prevent sensitization at high temperatures. No improvement in properties was noted within the temperature range of 600-650 °C, and it was observed that the formation of austenite was evident in the samples subjected to aging temperatures exceeding 595°C (Viswanathan, Nayar et al. 1989). Many studies have investigated the well 17-4 PH SS to different types of corrosion, including surface corrosion, localized corrosion, hydrogen-induced cracking, and stress-assisted corrosion. These studies found that 17-4 PH SS can be affected by both general and localized corrosion in aqueous environments (Karaminezhaad, Sharafi et al. 2006). When 17-4 PH SS was heated from 480 °C to 550 °C, its resistance to pitting corrosion increased. However, when the heating temperature was increased to 620°C, the resistance decreased. This change is due to the amount of reversed austenite, shape and location of precipitates, and percentage of martensite in the steel after heating (Kosasang, Wongkaewmoon et al. 2021). Intergranular corrosion, often referred to as sensitization, is a significant issue in 17-4 PH steel. Chromium carbides are formed and are mostly concentrated around the grain boundaries by diffusion, which is why they occur. This leads to Cr depletion in the surrounding narrow regions, making these areas anodic when exposed to a corrosive medium. Consequently, the ability to withstand corrosion is severely compromised by the precipitation of chromium carbides in steel (Bühler, Gerlach et al. 2003) . This study used atom probe tomography (APT) to examine the formation of precipitates form in 17-4 PH SS heated to 480 °C and 590 °C. After heating to 480 °C for 2 h, CrN/NbN compounds were formed at the locations where defects were present in the material. These compounds then became the starting points for the growth of other precipitates rich in Cu and Nb. After heating for 260 h at 480°C, small areas of chromium-rich precipitates, approximately 3 nm in size, were observed. After 120 h at this temperature, Ni-, Mn-, and Si-rich areas were observed. When steel was heated to 590 °C, no special types of steel phases were observed (Wang, Li et al. 2018) . Previous studies on the formation of secondary phases and the corrosion behavior of 17-4PH SS after solution treatment and aging are limited in number. To bridge this gap, a comprehensive study was conducted to explore the effects of different solutionizing and aging stages. This included solution treatment (ST) at 1040 °C for one hour, followed by water quenching and aging at 480°C for varying durations of 1, 4, 8, and 32 h. This study aimed to better understand the microstructural changes and their influence on the microhardness and corrosion resistance (CR) of 17-4 PH SS in a 3.5 wt.% Nacl solution. 2. Experimental Stainless steel, 17-4 PH (UNSS17400) was provided by the Sunrise Steel Center, Mumbai, India. The material was a rolled sheet with dimensions of 305 mm × 305 mm × 5 mm. The composition of the as-received (AR) material was evaluated using optical emission spectroscopy (Spectromaxx LMM07, CCD), and the results are detailed in Table 1. Samples were extracted from the sheet, each with dimensions of 20 mm × 20 mm × 5 mm. The samples underwent solution treatment (ST) at 1040 °C for 1 h, followed by immediate quenching in water. During this solution treatment, the existing precipitates were dissolved, resulting in the formation of a martensitic structure that was highly saturated with Cu and Cr (Murayama, Hono et al. 1999). The solution-treated samples were subjected to four different aging heat treatments, designated as ST+ aging for 1h, ST+Aging for 4h, ST+Aging for 8h, and ST+Aging for 32h, afterward, the samples were subsequently cooled to room temperature by air cooling. A summary of the heat treatments is presented in Table 2, and the heat-treatment profile is shown in Fig. 1. The specimens were mounted in epoxy resin and polished using coarse and fine-graded emery papers, followed by cloth polishing using alumina and diamond paste (1 µm). Subsequently, they were etched using Marble's reagent to reveal the microstructural changes resulting from the aging heat treatments.

Table 1: Chemical composition of the as-received 17-4PH SS (in wt.%).

Cr

Ni

Cu

Mn Si

Nb Mo

C V

P

S

Fe

15.39 8 4.211 3.518 0.65 0.458 0.19 0.223 0.043 0.074 0.021 0.035 Bal.