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)
· 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 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.
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
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.
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.
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 (<12%) Martensitic TYPES OF STAINLESS STEELS
1. Ferritic - When chromium is added an increase in temperature range is
seen by which ferrite is the stable structure. That is, ferrite is at all temperatures below solidification. Has low amount of carbon to
chromium ratio; therefore hardening by heat-treating is not done. Used for trim moldings and decorative applications and
where buffing to a mirror finish is important.
Advantages:
· Readily weldable (no martensite can form at the welds because chromium retards the bcc martensite.
· Cheapest of the stainless steels. · Magnetic
· Easiest of all to machine.
Disadvantages :
· Poor ductility
2. Martensitic –
High amount of carbon to chromium ratio therefore can be heat treated. More corrosive resistant than ferrite, but still corrosive. This material can be austenitic (see next type) at high temperatures. At the high temperature, carbon can be dissolved in the fcc austenite, which in turn is quenched to form a bcc martenitic structure. So the steel is austenitized, quenched, then stress relief tempered.
Advantages:
· Increase in strength
· more corrosive resistant than Ferritic · ability to hold an “edge”
· good for impact
· good up to 300 ksi when hardened.
Disadvantages: May be susceptible to red rust when annealed for
machining or fabrication. Cost 1 ½ times more than the
Ferritic stainless steels.
3. Austenitic stainless steels - Best corrosive resistance, but
hardenable only by cold working. Not heat treatable, but cold
workable.With both nickel and chromium, the fcc austenite is stabilized at room temperature to produce a stainless steel. Advantages:
· Best corrosive resistance,
· Highest of all for strength at high temperatures, · Best of all for ductility at low temperatures. · Nonmagnetic,
· Highly resistant to chemical corrosion (except one), mirror polish,
· Attractive appearance.
· Formability is outstanding characteristic of the fcc. · Strengthen drastically when cold worked
Disadvantages:
· Corrosive in hydrochloric acid and other halide acids and salts.
· Most expensive of three.
Water Quench Cold Rolled
Yield Strength 38ksi 117ksi
Tensile Strength 90 ksi 140 ksi
TOOL STEELS
These are high carbon steel alloys that have been designed to provide wear resistance and toughness combined with high strength.
Water Hardened tool steel - (W grade)high carbon plain carbon steels
Advantages:
· Account for a large percentage of all the tool steels · Least expensive.
Disadvantages
· Usually the parts are quite small
· Not used in severe usage or elevated temperatures.
· Because their hardenability is low, they should be used only for thin sections.
· They are brittle, especially at their higher hardness.
· Prolonged exposure to temperatures over 300F usually results in undesired softening.
Typical uses depending on the carbon content
· 0.60-0.75% carbon: medium hardness with good toughness and shock resistance. Examples: machine parts, chisels, setscrew · 0.75-0.90%- forging dies, hammers, sledges
· 0.90-1.1% - general purpose tooling - good wear resistance and toughness. Examples of drills, cutters, shear blades, heavy duty cutting edges.
· 1.10-1.30% extremely hard, but little toughness. Examples are small drills, lathe tools, razor blades, and other light duty applications.
Cold worked tool steels (O for Oil, A for Air, D for diffused) these contain certain
alloys to help hardenability without severe quenching. Advantages
· for larger parts because the quench is not as severe · better dimensional stability
· cracking tendency is reduced Disadvantage
· generally require annealing treatment before they can be machined. After machining, they are hardened and tempered and can retain full hardness at temperatures up to 800F
Typical Uses
Shock resisting Tool Steel (S)
Advantages
· Low carbon content for toughness, but the alloys have carbide for good abrasion resistance, hardenability, and hot-work.
Typical Uses
· Hot and cold impact use
High-Speed Tool Steel (T for tungsten based and M for molybdenum
based) Advantages
· Can be used for red-hot (1400F) applications · Good shock resistance
· Good abrasion resistance
Disadvantage Typical Uses
· wide variety of cutting applications
Hot-Worked Tool Steels (H)
Advantages
· Strength at elevated temperatures · Hardness at elevated temperatures Plastic Mold Tool Steels - (P)
CAST IRONS
Iron carbon with more than 2.11% carbon experience the eutectic reaction during cooling and are known as cast irons. Class 80-50 means tensile strength is 80ksi and yield is 50ksi
Advantages include in part
· Low liquidus temperature · Readily cast
· Inexpensive · High applications
1. GRAY IRON - is the least expensive and the most common variety. Typical ranges of carbon are 2.5% to 4% . with 91-94% iron elongation is around 1% elongation in 2"The microstructure has micoflakes of graphite dispersed in a
matrix of ferrite. Flakes have no strength so they act as voids in the
structure. The pointed ends of the flakes act as notches and crack initiation sites. Therefore the material is very brittle and extremely low in ductility. Generally sold by class (20, 30, 40 up to 80 relating to its tensile or ultimate strength)
Applications
· include large machinery parts with intricate shapes. Characteristics of Gray Iron
· Bhn= 150
· E = 10 to 20 E6 psi · Tensile is 20-60 ksi · Abrasion wear is poor · Corrosion is poor
· Weldability is poor but can be welded. Oxyacetylene torch or lectric arc, but because so brittle preheat and cool slowly · Machinability is good
· Castability is excellent · Low cost
2. MALLEABLE IRON - cooling rate is increased. Irregular spheroidal graphite particles in ferrite or pearlite matrix. Applications are axle housings, pipe fittings, brake drums.
Typical Designation
Characteristics of Malleable Iron
· Elongations range from 10 to 25% in 2" · Bhn is 110-150
· modulus is 24E6
· tensile stress is 50-56 ksi · wear resistance is poor · corrosion is poor
· welding is not done - heat of the weld would ruin the malleable properties, only a long term annealing will restore them. Brazing can be done at 1700F so it is a preferred method of repair.
· machinability is fair · castability is good
· more shock resistant than ductile iron
3. DUCTILE IRON - add magnesium (but only 1 pound per ton!!!) Spheroidal graphite particles in ferrite or pearlite matrix. Applications include valves, pump bodies, crankshafts, gears. Characteristics of Ductile Iron
· Can be heat treated
· Elongation from 10 to 25% · Bhn 140-300
· Modulus = 24E6 psi · Tensile 65-150 ksi · Wear is poor · Corrosion is poor
· Weldability is poor, but can be welded with nickel and iron electrodes.
· Machinability if fair to excellent · Castability is good to excellent.
