Conceptual Designs

Most Commonly used Metals Plain carbon steels –Available in all structural forms, thin sheets, coated or uncoated, cold formed, hot rolled, and free machining. –Use 1006 to 1020 for parts requiring severe forming or welding. Bolts, nails, automobile body panels, brackets, appliance housing, machine bases. –Use 1030 to 1060 for parts requiring higher strength w/o heat treatment. Use only if welding is not required. –Use 1040 to 1060 for parts requiring surface hardening by flame or induction methods. –Use 1080 to 1095 for small parts requiring through hardening. Springs, hammers.

Materail Selection Plain carbon steels have poor hardenability. Cold finished steels (1006 to 1050) can come in various forms (hard to skin rolled). They can be strengthened up to 100% with cold working. Cold finished parts are stronger, have better surface finish, are more dimensionally accurate. But, they have less ductility, cost more, are dimensionally less stable in machining, and loose their strength if welded. High sulfur steels are ideal for machining (AISI 1112) as the chips break up easily. High sulfur content, however, causes weld cracking.

Material Selection Alloy steels –AISI 4340 or 4140 (Through hardening grades): Have excellent strength and toughness. Available in most forms in hot rolled form. Designed for high hardenability. Best suited to high stress machine parts - gears, cams, shafting, piston rods, bolts, dies, keys, bending rolls. They can be finish machined and can be welded with special precautions. –AISI 8620 or 9310 (carburizing grades): These steels are designed for case hardening requirements. Deep cases can be created and hardened using carburizing. Used when high wear resistance is required. Suitable for gears, racks, cams, and sliding surfaces.

Alloy Steels High Strength Low Alloy Steels (HSLA) –Used for Structural applications –Stronger than plain carbon steels –Not heat treatable –Corrosion resistant (not as much as stainless steel) –Good weldability (low carbon content) –Good formability –Applications: structural forms, bars, plates. ASTM A36 (most popular), A242

Alloy Steels Ultra-strength steels –High strength and reasonable toughness. –Used for heavily loaded machine parts. –4340 and 4140 are most common Harder to weld but machinable –Maraging steels

Alloy Steels 18% nickel maraging steel –Maraging steels are carbon free iron-nickel alloys with additions of cobalt, molybdenum, titanium and aluminium. The term maraging is derived from the strengthening mechanism, which is transforming the alloy to martensite with subsequent age hardening. Very high strength ( ksi) Good weldability Very expensive

Cast Irons Grey cast iron (ASTM A 48 grades 20-60) –Easy to cast (excellent fluidity) –Low cost –Low toughness, brittle, no shock resistance –High compressive strength –Not appropriate for parts highly stressed in tension. –Low to medium tensile strength –Excellent machinability –Superb vibration damping property –Not easily weldable –Low stiffness (30 to 60% of steel)

Cast Irons Gray cast iron additional properties –Excellent metal-on-metal wear resistance –Good corrosion resistance in all media Ductile iron (ASTM 536, grade 5 [ ]) –Easy to cast/low cost. –Much better toughness than gray iron. –Stiffness is about 70% of steel –Yield strengths range from 40 o 90 ksi –Similar to gray cast iron in other properties

Aluminum Alloys General properties –Lightness: 1/3 of steel –Good thermal and electrical conductivity –High strength/weight ratio –Weldable (most alloys) –Atmospheric corrosion resistance (will not rust) –Excellent machinability –Can be die cast –Good formability –Non-magnetic –Non-toxic (pots and pans - 25% of all usage)