VALVE MAGAZINE Spring 2025

Figure 4: Brittle fracture of a17-4 PH stem in H1100 condition, likely caused by inconsistent heat treatment and exposure to a sour environment. required, they are recommended for prolonged high-tem perature applications like ebullated bed process valves. Duplex stainless steels (2205, superduplex 2507) offer high strength and corrosion resistance but require precise heat treatment to prevent intermetallic phase formation, such as sigma phase, which reduces toughness and corro sion resistance (Figure 6). Grade 91 (modified 9Cr-1Mo), a creep-strength-en hanced ferritic (CSEF) steel widely used in power plants and refineries has two types: Type 1 (normalized and tempered) and Type 2 (with stricter tramp element control such as tin and antimony, for better long-term

Figure 5: Transversal cracking of Stellite 6 seat hardfacing in combined cycle power plant.

creep performance). Cast Grade 91 lacks these classi fications and has lower creep resistance than wrought versions. Commercially pure titanium (Grades 1–4) excels in seawater and chemical processing but lacks strength for high-pressure use. Ti-6Al-4V offers higher strength, while Grade 12 (Ti-0.3Mo-0.8Ni) resists crevice corrosion in chemical processing and mining applications. Like titanium, Monel alloys, primarily nickel-copper, are also extensively used in marine environments due to their outstanding resistance to seawater corrosion and hydrofluoric acid. Monel 400 is common in offshore and

Figure 6: Typical phase transformations of super duplex stainless steels during continuous cooling [R. Francis, et al., Corrosion, 2016].

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