Chromium-Molybdenum Hydrogen-Resistant Steels

Another important group of steels for high-pressure components is low-alloy weldable steels containing up to 4% chromium and up to 1% molybdenum. 

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Both industry experience and research work indicate that postweld heat treatment (PWHT) of chromium-molybdenum steels in hydrogen service improves resistance to high temperature hydrogen attack. The PWHT stabilizes alloy carbides. This reduces the amount of carbon available to combine with hydrogen, thus improving high temperature hydrogen attack resistance. 

The addition of carbide stabilizers to steel reduces the tendency toward internal Assuring. Elements such as chromium, molybdenum, tungsten, vanadium, titanium, and niobium reduce the number of nucleation sites by forming more stable alloy carbides which resist breakdown by hydrogen and, therefore, decrease the propensity to form methane.9 The solid-line curves in Figure 1 reflect the increased resistance to internal attack when molybdenum and chromium are present. 

Chrome-moly steels have % to 9% chromium and either % or 1% molybdenum. Chromium increases scaling resistance and molybdenum improves or provides elevated temperature strength. Carbon content is kept low to maintain weldability. Chrome-moly steels are widely used for piping and pressure vessels operating up to 1000°F in environments such as steam or hydrogen and are available in many product forms. The two most popular alloys are 1 chrome- moly and 2 chrome-1 moly. 

Alloys other than those shown in Figure 1 are also suitable for resisting high temperature hydrogen attack. These include modifled carbon steels and low alloy steels to which carbide stabilizing elements (molybdenum, chromium, vanadium, titanium, or niobium) have been added. European alloys and heat-treating practices have been summarized by Class.11 Austenitic stainless steels are resistant to decarburization even at temperatures above 1000°F (538°C).12... 

Tempered steel is used for the construction of all these pressure reactors, alloyed with chromium and molybdenum to make them resistant to hydrogen under pressure, so that no decarburization, embrittlement and grain boundary splitting occurs. These pressure reactors are more and more being constructed out of several layers. In this way the total reactor wall is built up of a multitude of individual layers, which are wound round a central tube. The advantage of such a multiple layer construction, as against pressure chambers with a plate-girder construction, is that the central tube is subject to chemical demands and the adjacent layers... 

Alloying steels for improved corrosion resistance in chemical and hydrocarbon plants is based on chromium and molybdenum additions. The lowest of these alloys, lCr-l/2Mo and l-l/4Cr-l/2Mo, are often used above 800°F (425°C). Low-aUoy Cr-Mo steels (with 5% or greater Cr) are resistant to high-temperature sulfidic corrosion. However, the Cr-Mo alloys find their most critical use in high-temperature, high-pressure hydrogen service. The most commonly used alloys are the... 

Carbide-forming alloying elements, such as chromium and molybdenum, increase the resistance of steel to hydrogen attack. 

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