Steel Types

STEEL TYPES

//steeltypes

Plain-Carbon Although called plane carbon actually the iron and carbon alloy contains manganese, phosphorus, sulfur, and silicon. Its strength is primarily a function of its carbon content, increasing with carbon amount. The ductility of plain carbon steels decreases as the carbon content increases. Some disadvantages of plain carbon steel are as follows: Disadvantages of Plain Carbon · The hardenability is low. · The physical properties (Loss of strength and embrittlement) are decreased by both high and low temps · Subject to corrosion in most environments Low Carbon Steel Has less than 0.3% carbon. Usually ferrite and pearlite, and the material is generally used as it comes from the hot forming or cold forming processes. Lacks hardenability because carbon content helps this. Advantages · Posses good formability · Posses good weldability: best of all metals : Note: as carbon % increases there is a tendency for the metal to harden and crack. · Lowest cost and should be considered first · Rated at 55-60% machinability (soft and drags which builds up heat on the tool. AISI (American Institute of Iron and Steel AISI rating compares ability to machine with 100% basis. Considers turning, reaming threading drilling, etc. Ex Al=260 and stainless steel is 60)

Typical Uses · 0.1%-0.2%: chain, stampings, rivets, nails, wire, pipe, and where very soft, plastic steel is needed. · 0.2-0.3%: structural steels, machine parts, soft and tough steels. Use for case hardened machine parts and screws. Medium Carbon Steel - have between .3 and .8% carbon. Special Advantages · Machinability is 60-70%; therefore cut slightly better than low carbon steels. Both hot and cold rolled steels machine better when annealed. Less machinable than high carbon steel since that is very hard steel. [When welding, there may be some martensite when extreme rapid cooling. So preheat (500-600F) and postheat at 100-1200F will help remove brittle structure.] · Good toughness and ductility. Enough carbon to be quenched to form martensite and bainite (if the section size is small) 1

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A goodbalance of properties can be found. That is optimum carbon level where high toughness and ductility (of the low carbon steels) is compromised with the strength and hardness of the increased carbon.

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Extremely popular and have numerous applications. Fair formability Responds to heat treatment but is often used in the natural condition.

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0.3-0.4: lead screws, gears, worms, spindles, shafts, and machine parts.

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0.4-0.5: crankshafts, gears, axles, mandrels, tool shanks, and heat-treated machine parts. 0.6-0.7: called “low carbon tool steel” and is used where a keen edge is not necessary, but where shock strength is wanted. Drop hammers dies, set screws, screwdrivers, and arbors. 0.7-0.8: tough and hard steel. Anvil faces, band saws, hammers, wrenches, cable wire, etc.

Typical Uses

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High Carbon Steels - over 0.8% carbon and less than 2.11% carbon Disadvantages · · ·

Toughness and formability and hardenability are quite low. Not recommended for welding. Usually joined by brazing with low temperature silver alloy making it possible to repair or fabricate tool-steel parts without affecting their heat treated condition.

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Hardness is high Wear resistence is high Quench cracking is often a problem with severe quenching Fair formability

Advantages

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0.8-0.9: punches for metal, rock drills, shear blades, cold chisels, rivet sets, and many hand tools. 0.9-1.0: used for hardness and high tensile strength, springs, cutting tools, press tools, and striking dies. 1.0-1.1: drills, taps, milling cutters, knives 1.1-1.2: drills, taps, knives, cold cutting dies, wood working tools 1.2-1.3: files, reamers, knives, tools for cutting wood and brass 1.3-1.4 used where a keen cutting edge is necessary, razors, saws, and where wear resistance is important 2

like boring and finishing tools. ALLOY STEELS These are usually heat treated to develop specific properties ; quenched and tempered. The difference between plain carbon and alloy steels is ambiguous. Both contain carbon, manganese, and silicon. Copper and boron are additives to both classes. Usually high allow steels contain more than 1.65% manganese, 0.6% silicon, and 0,6% copper. In addition there are specified amounts of chromium, nickel, molybdenum, vanadium, tungsten, cobalt, and boron. EFFECTS OF ALLOYING ELEMENTS Usually only a small amount of alloying element are added to steels (usually less than 5%). Mostly the purpose is to improve the hardenability and strength corrosion resistance, stability at high/low temperatures, control grain size Manganese - increases ductility, hardenability, high strain hardening capacity, slightly strengthens, excellent wear resistance Sulfur - if carefully proportioned can add machinability without imparting embrittlement. Nickel - increases toughness and impact resistance, good properties at low temperatures. With other alloys imparts excellent corrosion resistance. With certain alloys it has a small thermal expansion and used for sensitive measuring devices. Increase strength with little loss of ductility. Chromium - if added in large enough amounts can impart corrosion resistance and heat resistance and wear resistance and hardenability. Otherwise for amounts (less than 2.11%) it is used to slightly increase hardenability and strength. Molybdenum - improves hardenability and increases strength primarily under dynamic and high temperature conditions. Extremely stable at elevated temperatures. It helps to retains fine grain sizes which provides strength and creep resistance at elevated temperatures. Molybdenum carbides are used in hot work tool steels and forging dies to impart hardness even at red heat. Vanadium - like molybdenum, forms strong carbides at elevated temperatures. Also limits grain size. Tungsten- used in tool steels to maintain their hardness at elevated temperatures. Copper - increases the corrosion resistence. Limits have to be 3

controlled or it’ll sacrifice surface quality and hot-working behavior. Silicon - increases strength without limiting grain size* Used to promote large grain sizes used in magnetic applications. Used in spring steels. Boron - very important hardenability agent being several hundred times better than nickel, molybdenum and chromium. Used more for low carbon steels. Also improve machinability and cold forming. *Limits on grain size can effectively increase strength properties like elastic limit, yield point, and impact strength (toughness) with little loss of ductility. CLASSIFICATIONS AISI - American Iron and Steel Institute - general SAE - Society of Automotive Engineers - cars ASTM American Society for Testing Materials base specs on specific applications Many low carbon and structural steels AISI use a four digit number. The first is the class of alloy specified. 1XXX Carbon steels 2XXX Nickel chromium 3XXX Moybdenum 4XXX Chromium..............etc 2nd number designates the subgroup of the alloy Last two numbers designate the amount of carbon in 0.01%; therefore a 1080 steel has 0.8% carbon.

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STAINLESS STEELS- contain sufficient amounts of chromium that they are NOT considered low alloy steels. These require reduced cutting speeds (approx ½), finer feeds, lighter cuts, and sharp carbide tooling. Because the thermal conductivity of stainless steel is 1/3 that of carbon steel, the heat is held in a localized area when welding. This tends to localize the stress. The metal that is hot wants to expand but is hemmed in by the cold adjacent metal. When the material cools and contracts, the cooler metal does not move, with the result that cracks form during the solidification. Methods of dealing with these problems are available. The corrosion resistance is imparted by the formation of a strong adherent chromium oxide on the surface of the metal. Good resistance to corrosive media encountered in the chemical industry can be obtained by the addition of 4 to 6% chromium to low carbon steel. AISI classes these with a three digit number for Stainless 200 series = chromium, nickel, manganese (structure is austenitic) 300 series = chromium and nickel (structure is austenitic) 400 series = chromium only (Structure is ferritic or martensitic) 500 series = low chromium (