Conceptual DesignsConceptual Designs

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Materail SelectionMaterail 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 SelectionMaterial 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 SteelsAlloy 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 SteelsAlloy 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 SteelsAlloy 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 (200-300 ksi)

• Good weldability

• Very expensive

Cast IronsCast 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 IronsCast Irons

• Gray cast iron additional properties– Excellent metal-on-metal wear resistance

– Good corrosion resistance in all media

• Ductile iron (ASTM 536, grade 5 [60-40-18])– 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 AlloysAluminum 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)