Molybdenum steels

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

Moist iodine vapor rapidly corrodes metals, including most stainless steels. The initial process is the formation of corrosion centers where small amounts of metal iodide are formed which deHquesce, and the corrosion then takes place electrochemically (41,42). Only titanium and molybdenum steels are unattacked by iodine (42,43). The corrosion of molten iodine has also been studied. 

R. S. Archer, J. Z. Briggs, and C. M. Loeb, Molybdenum Steels, Irons, Mlloys, Climax Molybdenum Co., New York, 1948. 

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

Molybdfln-saure, /. molybdic acid, -stahl, m. molybdenum steel. 

Stainless steels, which contain high percentages of alloying elements, e.g. 18% chromium, 8% nickel and 3% molybdenum. Steels of this type are practically non-corrodible in appropriate circumstances. 

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. 

Al. R. D. Merrick and A. R. Ciuffreda, Hydrogen Attack of Carbon-0.5-Molybdenum Steels, 1982 Proceedings, Refining Department, Volume 61, American Petroleum Institute, Washington, D.C., pp. 101-114. 

E. A. Sticha, Tubular Stress-Rupture Testing of Chromium-Molybdenum Steels with High-Pressure Hydrogen, Journal of Basic Engineering, December 1969, Volume 91, American Society of Mechanical Engineers, New York, pp. 590-592. 

Upon prolonged exposure to temperatures above 875°F, the carbide phase of carbon-molybdenum steel may be converted to graphite. Permissible, but not recommended for prolonged use above 875 F. 

Nickel-chromium-molybdenum steel 86XX and 87XX... 

Manganese-nickel-chromium-molybdenum steel 94XX... 

Of the common alloying elements in steel, molybdenum is the niiisl effective in increasing creep—rupture strength, and the carbon— molybdenum steels generally have more than twice the creep—rupture strength of plain carbon steel at the same temperature. The most commonly used steels for high temperature service contain from 0.5 to 1.5% molybdenum. 

Molybdenum. Steels with molybdenum are usually less susceptible to temper brittleness. Molybdenum has a major effect on increasing hardenahilily and a noiahlc effect on increasing the high-temperature tensile and creep strengths of alloy steels, that is. Ihe steels have less tendency toward deformation under stress at elevated temperatures. 

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