is usually performed on steels requiring additional heat treatment for 

 hardening, on hot formed pressure vessel heads, and when specified by the 

 applicable material specification. 



Steels having sufficient carbon can be hardened by heating above the 

 austenitizing temperature, holding at this temperature long enough for 

 transformation to occur, removing the steel from the furnace, and immediately 

 quenching it in water or oil. The resulting surface hardness depends on the 

 carbon content, section size, and quenching medium. The depth of hardening 

 depends also on alloy content and grain size. After quenching, steels are 

 tempered by heating to a specified temperature (below the transformation 

 temperature) and holding them at this temperature for a specified time, 

 usually an hour per 2.54 centimeters of thickness. This process restores 

 ductility. The particular tempering temperature used depends on the alloy 

 content, mechanical strength requirements, and end use. 



Low carbon steels are often stress relieved by heating between 593 

 Celsius (1100 Fahrenheit) and the austenite transformation temperature to 

 remove resudial stresses resulting from prior forming or welding operations. 

 Stress relieving restores ductility and toughness. It may also improve 

 fatigue life. Welds areas are often postweld heat treated locally, i.e., 

 stress relieved, using proprietary portable heating equipment. 



(c) Alloy Additions . Alloying elements are added during the 

 steelmaking process to improve mechanical properties or to improve corrosion 

 resistance. Small additions, singly, of copper, nickel, chromium, silicon, 

 and phosphorus have been shown to be effective in improving the corrosion 

 resistance of steel. The greatest improvements in corrosion resistance are 

 obtained by the addition of specific combinations of these alloying elements, 

 such as specified by ASTM Standard A690 for H-piles and sheet piles intended 

 for service in the splash zone. Other steels suitable for marine applications 

 and having improved atmospheric corrosion resistance are ASTM Standards A242, 

 A441, and A588. 



Additions of chromium and molybdenum improve the high temperature oxida- 

 tion resistance as well as improve the high temperature strength of steel. 

 High-pressure steam tubes and piping are often 1.25 percent chromium alloy 

 steel. Stainless steels meeting ASTM 400 series specifications include type 

 410, with 12 percent chromium, and type 430 with 18 percent chromium. 

 However, because of a tendency to pit, the 400 series stainless steels are 

 not recommended for marine service. 



b. Aluminum . 



(1) Alloy Strengthening . Aluminum, in high purity form, is soft 

 and ductile but does not possess enough strength for most commercial applica- 

 tions. The addition of alloying elements, either singly or in combination, 

 impart strength to the metal. Aluminum alloys can be classified into two 

 catagories: nonheat-treatable and heat- treatable. The nonheat- treatable 

 wrought alloys can be strengthened by cold working only and are usually 

 designated in the 1000, 3000, 4000, or 5000 series. The degree of cold 

 working is termed the aluminum strain hardening or temper, denoted by an "H" 

 followed by a number. 



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