Chromium-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. 

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. 

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. 

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. 

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

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

Hydrogen gas producers in the United States and Europe have accumulated decades of experience with transmission pipelines fabricated from carbon steels and currently operate over 1500 km of pipeline [13,49]. Large-diameter piping systems (>2.5cm) used for local distribution of hydrogen gas in large-scale industrial operations also make extensive use of low-alloy steels, such as the chromium-molybdenum steels, particularly for service at elevated temperature. Pipelines and piping fabricated from low-alloy and carbon steels have functioned safely and reliably in hydrogen gas service. 

Welding Problems With Cr-Mo Pipe. Do not use low-chromium steel pipe unless you are willing to pay for more careful welding and post-weld heat treatments. At a given hardness, low-alloy chromium-molybdenum steels have somewhat more ductility than carbon steels. However, because they air harden so much, they usually require post-weld heat treatments to toughen the weld metal and heat-affected zone. This heat treatment complicates field welding. 

The oxidation resistance of stainless steel alloys with different compositions is compared to the oxidation resistance of carbon steel in Fig. 11.6 [4]. Carbon steels perform satisfactorily at temperatures below 500 °C. Chromium-molybdenum steels doped with up to 2% silicon have low corrosion rates up to 700 °C. Ferritic stainless and martensitic steels have superior oxidation resistance when compared to carbon and Cr-Mo steels. The corrosion rate of the alloy drastically decreased (increase in the resistance to oxidation) upon increasing the chromium content from 8% to 25%. 

It is reported that, for instance, commercial cermet insert (ISO IC-30) showed a great advantage of lower wear than conventional WC-based cemented carbide insert (ISO K-25) with TiCN coating in the cutting of chromium molybdenum steel (AISI 4140) for the speed range up to 250 m/min, while both tools were almost... 

Influence of Cutting Speed Equation 15 predicts that the tool temperamre increases with the increase of cutting speed. Figure 11 shows the tool temperature in turning of chromium-molybdenum steel AISI 4140 and cast iron ASTM A220 (Ueda et al. 2008). The tool temperature increases with the increase... 

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