|
Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60. Carbon Steels Types: Plain-Carbon
Low Carbon Steel
Medium Carbon Steel
High Carbon Steels Plain-CarbonAlthough 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 asfollows: 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 SteelHas 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: ascarbon % 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 sectionsize is small) _ 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. _ Extremely popular and have numerous applications. _ Fair formability _ Responds to heat treatment but is often used in the natural condition. Typical Uses _ 0.3-0.4: lead screws, gears, worms, spindles, shafts, and machine parts. _ 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. High Carbon Steelsover 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. Advantages _ Hardness is high _ Wear resistence is high _ Quench cracking is often a problem with severe quenching _ Fair formability Uses: _ 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 Designation | Carbon steels are designated by distinct AISI (American Iron and Steel Institute) four-digit numbers. The first two digits indicate the grades of the steels, while the last two digits give the nominal carbon content of the alloy in hundredths of a percent. Here is an example: | | XX | :0.xx% average carbon content | | AISI | 10 | 60 | | | | | | 10 | :Nonresulfurized grades | | 11 | :Resulfurized grades | | 12 | :Resulfurized and rephosphorized grades | | 15 | :Nonsulfurized grades; max Mn content > 1% |
If a letter L or B shows up between the second and third digits of an AISI number, it means that this grade is either a Leaded steel or a Boron steel; Sometimes a suffix H is attached to a AISI number to indicate that the steel has been produced to prescribed hardenability limits. Examples are: | Leaded steels | :AISI 12L14, AISI 12L15... | | Boron steels | :AISI 15B48H... | | H-steels | :AISI 1038H, AISI 15B48H... |
|
|
