Steel, strength-toughness

High strength, low alloy (HSLA) steels often contain 0.10—0.30% molybdenum. These steels exhibit toughness at low temperatures and good weldabiHty. They are used extensively for undersea pipelines (qv) transporting gas and oil from offshore weUs to pumping stations on shore, and are also used extensively in remote Arctic environments. 

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Most other rotor parts in gas turbines are individually forged. This includes compressor wheels, spacers, distance pieces, and stub shafts. All are made from quenched and tempered low-alloy steels (Cr-Mo-V or Ni-Cr-Mo-V) with the material and heat treatment optimized for the specific part. The intent is to achieve the best balance of strength, toughness with ductility, processing and nondestructive evaluation capability, particularly when it is recognized that some... 

Miscellaneous High Strength Steels. Strengths above 1400—1700 MPa (203,000—246,500 psi) are not often used or requked in steels because of difficuldes in joining, ductility, and/or toughness. For specialized uses, however, it is possible to achieve such strength (see High PRESSURE... 

Molybdenum is a hard, silvery metal with a very high melting point. It is used primarily to make alloys with other metals. An alloy is a mixture of two or more metals. The mixture has properties different from those of the individual metals. The most common alloys of molybdenum are those with steel. Molybdenum improves the strength, toughness, resistance to wear and corrosion, and ability to harden steel. 

Chromium Cr 0.6-1.2, C 0.2-0.5 Improved strength, toughness, heat resistance Improved castings, stainless steels... 

Group H covers hot work tool steels, which retain strength, toughness and resistance to softening at elevated temperatures. Group H steels are very strong and have important structural applications, such as rolls and fasteners (e.g., HI 1 and HI3). 

Alloying for improved mechanical properties can improve strength, toughness, and fatigue resistance. Such steels are normally heat treated to enhance their properties. Welding these alloys can degrade them. 

Judy, R. W., Jr. and Goode, R. J., Fracture Extension Resistance (R-Curve) Characteristics for Three High-Strength Steels, Fracture Toughness Evaluation by R-Curve Methods, ASTM Special Technical Publication 527, American Society for Testing and Materials, (1973) 48-61. 

Another important aspect of steel is heat treatment, which helps to improve hardness, strength, toughness, and wear resistance. Lowering the carbon content in the alloy steel improves the weldability, without affecting the strength. Table 9.4 gives the chemical composition of various steels used for underground pipelines. 

Spheroidal-graphite cast iron combines the strength, toughness and ductility of steel with the ease of casting of grey cast iron. Usually abbreviated to SG iron, it is also known as ductile or nodular iron. 

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