Presentation on theme: "Chapter 12 – Steel Products"— Presentation transcript:
1Chapter 12 – Steel Products Key: carbon content:Steel – alloy consisting mostly of iron with a little carbon (0.05% % by weight)Also have:Iron = iron-carbon alloy with less than 0.005% carbon.Cast iron = carbon content between 2.1% - 4.0%Wrought iron – contains 1 – 3% by weight of slag in the form of particles elongated in one direction – more rust resistant than steel and welds betterIron – not usally used as an engineering material because of its low strength. Cast iron extremely hard to machine!
2Brief History:Iron age (12th century BC) (mostly wrought iron) – weapons made with inefficient smelting methods. The best weapons? When iron combined with carbon!Became more common after more efficient production methods were devised in the 17th century.With invention of the Bessemer process in the mid-19th century, steel became relatively inexpensive, easily mass-produced and high quality.Blast Furnace then Bessemer FurnaceThe Bessemer process is a method of steel production named for British inventor Sir Henry Bessemer. During the Bessemer process, iron workers inject air into molten steel to remove carbon and impurities. After the Bessemer process was introduced in the 1850s, steel refining and production increased dramatically. Modern steel manufacturing uses a similar technique, but the process has been refined over the years to create high-quality steel with very few impurities.Steel manufacturers use a special furnace, known as a Bessemer converter, to produce steel using this technique. They place iron in the furnace and melt it to produce a molten liquid, then use a high-powered blower to pass air through the liquified iron. As the air passes through, oxygen molecules within the air interact with minerals and carbon molecules in the metal. As the air exits the molten iron, it brings the carbon and other particles with it in the form of gas or slag. The remaining iron can then be poured into molds to form steel objects.Using the Bessemer process, manufacturers were able to produce better quality steel than was previously possible. The resulting steel was stronger and more durable, allowing larger and longer-lasting structures to be built. This process also helped manufacturers produce steel more quickly, and at a lower cost than with previous techniques. For more than a century, the Bessemer process became the most popular method of mass steel production, and much of Bessemer's techniques live on in modern industry.Low cost method for removing carbon and impurities
3The “abc’s” of Steel Making: Raw Material:Carbon in the form of cokeIron ore (Fe2O3)Limestone (CaCO3)Air (lots of it!!)
4The “abc’s” of Steel Making: CokeSolid residue product from the destructive distillation of coal.About 80 to 95% C.Made by heating black coal in small ovens at 300 C for 24 hours in a coke plant.
5The “abc’s” of Steel Making: The iron oreConsists of oxides in nature of iron and oxygenPrimarily magnetite (Fe3O4) or hematite (Fe2O3)The blast furnace basically separates the iron from the oxygen in a reduction processMined primarily in Australia, Brazil and Canada.
6The “abc’s” of Steel Making: The limestoneActs as a flux – converts impurities in the ore into a fuseable slag
7The “abc’s” of Steel Making: AirPreheated by fuel gas from the coke ovens to about 1000 C.Delivered to the blast furnace at 6,000 m3/minPasses through furnace and burns the coke to produce heat required and also generates the carbon monoxide.
8The “abc’s” of Steel Making: Typical blast furnace:1.6 tons of iron ore0.18 tons of limestone0.6 tons of coke2 -3 tons of preheated air
9The “abc’s” of Steel Making: Step 1 – The Blast Furnace:Stands 300 feet tallDesigned to run continuously for years before being relined.Heat generated by burning coke in the preheated air.Coke acts as reducing agent and changes to carbon monoxide (the reducing agent) which removes the oxygen from the iron oxide.
10The “abc’s” of Steel Making: Step 1 – The Blast Furnace:Two important chemical reactions:Oxidation of the carbon from coke:Reduction of iron ore:
11The “abc’s” of Steel Making: Step 1 – The Blast Furnace:Four primary zones – the bottom zone (zone 4) reaches temperature of 1800 C – this is where iron is tapped off.The top zone (zone 1) – where coke is burned and moisture driven off.Zone 2 – slag coagulates and is removed.
12The “abc’s” of Steel Making: Step 1 – The Blast Furnace:Products from the blast furnace:Iron transported in steel shelled ladlesPig iron (brittle w/ 4% carbon)
13Step 2: Manufacturing of Steel from Iron Two common methods:Bessemer Furnace = Ingots = molten steel poured into molds to create ingots which then go through forging press and roughing mill to create billet, bloom or slab, OR:Continuous cast – continuous process to again create a billet, bloom, slab or “as cast semis”Solidification ConcernsThe liquid steel is converted into finished shape called steel castingsOr the liquid steel is converted into suitable shapes for further processingIngots (to be machined to size, or remelted and cast)Continuous casting (to make bar, slabs, billets)Containment vessels are used in particular types of steel manufacture for pouringKnown as ladles (typically hold as much as 100 tons of molten steel)Stirring, degassing, reheating, and various injection procedures are performed to increase the cleanliness of the steelIf you have ever seen the making of gravy during thanksgiving, the turkey drippings will separate (oil and broth). It can be difficult to keep the broth (bottom) and discard the oil (floating)By extracting the metal from the bottom of the ladle, the slag and floating matter are not transferredSolidification shrinkageThere is a large discontinuity at the melting point of the density of the steel in its liquid and solid states
14Step 2 – The Bessemer converter: Used for REFINEMENT:Takes pig iron with high C content and removes C.Removes impurities such as Si and Mn (via oxides)Much smaller furnace (vs. Blast furnace)Lowered cost of steel makingPoured into molds to form ingotsIn the U.S., commercial steel production using this method stopped in It was replaced by processes such as the basic oxygen (Linz-Donawitz) process, which offered better control of final chemistry. The Bessemer process was so fast (10–20 minutes for a heat) that it allowed little time for chemical analysis or adjustment of the alloying elements in the steel. Bessemer converters did not remove phosphorus efficiently from the molten steel; as low-phosphorus ores became more expensive, conversion costs increased. The process permitted only limited amount of scrap steel to be charged, further increasing costs, especially when scrap was inexpensive. Use of electric arc furnace technology competed favourably with the Bessemer process resulting in its obsolescenceReplaced by basic oxygen process and electric arc furnace.
18Optional Step 2 (directly from blast furnace) Step 2 w/ Continuous CastingOvercomes the ingot related difficulties of:Piping and entrapped slagMore cost effectiveProcessmolten metal continuously flows from the ladle into a tundishthrough a bottomless,water-cooled moldtemp controlled water spray not fully cooledStraightened, reheated, sized, and cut-offAdvantagesCommon for Structural ShapesContinuous CasingOvercomes the ingot related difficultiespipeentrapped slagAnd structural variations along the length of the product.Continuous Casting Processmolten metal continuously flows from the ladle into a tundishTundish is a second ladle that bottom feeds the molten metal.through a bottomless water-cooled mold (usually made of copper)temp controlled water spray not fully cooled, but solidStraightened, reheated, sized, and cut-offALL non-stop Makes Billets, slabs, barsAdvantagesEliminates: piping, mold splatter, removing the ingot from the mold, further processing of ingotsReduces:energy used, costs, Scrap, oxide inclusionsimproves surfaces conditions, chemical composition
19Continuous CastingSteel is refined in the BOF or EAF and poured into a laddle. Laddle is transported to the caster facility and poured into the caster tundish. From the tundish, poured into a mold which determines final shape stays in the mold long enough to form a solid outer skin. The final caster is cut to length and usually undergoes another process for making the final shape. Note, this is a cast product so has flaws inherent to casitngs – read page 352
21Steel Types (Brief Overview) Much more detail in Chapter 14
22Cast Iron Types (remember carbon > 2%) Gray ironDuctile ironAustempered ductile ironWhite ironMalleable ironMuch more will be said about cast irons later!White Cast Iron … contains cementite.Hard and brittle, hence their use is limited to wear resistant partsVirtually unmachinable except for grindingApplications: liners for ore crushing mills and some agricultural machinery partsGray Cast Iron … iron with graphite flakesExcellent machineability, high thermal conductivity, vibration dampening properties, and good wear resistanceApplications: camshafts, small cylinder blocks, heavy duty brake drums, exhaust manifolds and clutch platesDuctile iron – does not exhibit yuelding – greater % elongation and generally higher tensiles strength – crankshafts and heavily loaded gears
23HRS vs. CRS HRS HRS Characterized by: AKA hot finishing – ingots or continuous cast shapes rolled in the “HOT” condition to a smaller shape.Since hot, grains recrystallize without material getting harder!Dislocations are annihilated (recall dislocations impede slip motion).HRS Characterized by:Extremely ductile (i.e. % elongation 20 to 30%)Moderate strength (Su approx 60 – 75 ksi for 1020)Rough surface finish – black scale left on surface.
24HRS vs. CRS CRS CRS Characterized by: AKA cold finishing – coil of HRS rolled through a series of rolling mills AT ROOM TEMPERATURE.Since rolled at room temperature, get crystal defects called dislocations which impede motion via slip!AKA work hardeningLimit to how much you can work harden before too brittle.How reverse? Can recrystallize by annealing.CRS Characterized by:Less ductlie – almost brittle (i.e. % elongation 5 to 10%)High strength (Su approx 120 ksi for 1020)
26AISI - SAE Classification System American Iron and Steel Institute (AISI)classifies alloys by chemistry4 digit number1st number is the major alloying agent2nd number designates the subgroup alloying agentlast two numbers approximate amount of carbon (expresses in 0.01%)American Iron and Steel Institute (AISI)classifies alloys by chemistrystarted by Society of Automotive Engineers (SAE)provide standardization of steel used in the automotive industryexpanded by AISI to include all engineering materials4 digit number1st number is the major alloying agent2nd number designates the subgroup alloying agentlast two numbers approximate amount of carbonexpresses in 0.01%1080 steel would be plain carbon steel with 0.80% carbon4340 steel would be Mo-Cr-Ni alloy with 0.40% carbonRefer to table 6-2 in book
27Plain Carbon Steel vs. Alloy Steel Plain Carbon Steel (10xx)Lowest costShould be considered first in most application3 ClassificationsLow Carbon SteelMedium Carbon SteelHigh Carbon SteelPlain Carbon SteelLowest costShould be considered first in most application3 ClassificationsLow Carbon SteelLess than 0.20% CarbonGood formability and weldabilityLacks hardenability (Difficult to harden)Medium Carbon Steel0.20% to 0.50% CarbonGood toughness and ductilityPoor Hardenability (typically limited to water quench)High Carbon SteelGreater than 0.50% carbonLow formabilityHigh hardness and wear resistancePoor hardenability (quench cracking occurs)
28Plain Carbon Steel (10xx) 1018Low carbon Yield strength 55ksi1045Medium carbon Yield strength 70ksiASTM A36 or A37 – aka structural steelLow carbon Yield strength 36ksi12L14Low carbon Yield strength 70ksi1144Medium carbon Yield strength 95ksiExamples of some plain carbon steels (non alloys)1018: low carbon (.18%)easy to cold form, bend, braze, and weldMaximum hardness Rockwell 72BYield strength is 55ksi1045: medium carbon (.45%)more difficult to machine than 1018Maximum hardness Rockwell 90BYield strength is 70ksiA36: low carbon (.26%) (cold rolled)General purpose steelMaximum hardness Rockwell 68BYield strength is 36ksi12L1: low carbon (.26%) contains LEADExcellent machining (due to lead)Ductile for bending, crimping, riveting.Maximum hardness Rockwell 85B1144: medium carbon (.44%)Good heat treat capabilitiesMaximum hardness Rockwell 97BYield strength is 95ksi
29Plain Carbon Steel vs. Alloy Steel > 1.65%Mn, > 0.60% Si, or >0.60% CuMost common alloy elements:Chromium, nickel, molybdenum, vanadium, tungsten, cobalt, boron, and copper.Added in small percents (<5%)increase strength and hardenabilityAdded in large percents (>20%)improve corrosion resistance or stability at high or low tempsAlloy SteelWhat classifies a steel as an Alloy Steel> 1.65%Mn, > 0.60% Si, or >0.60% CuDefinite or minimum amount of an alloying element is specifiedMost alloying elements added to steel are < 5%to increase strength and hardenabilityMost alloying elements added to steel are > 20%to improve corrosion resistance or stability at high or low temps
30Corrosion Resistant Steel Stainless Steel10.5% < Cr < 27% = stainless steel – used for corrosion resistanceAISI assigns a 3 digit number200 and 300 … Austenitic Stainless Steel400 … Ferritic or Martensitic Stainless Steel500 … Martensitic Stainless SteelStainless Steel is corrosion resistant!!trade namedue to chromium oxide on the surface of the metalS.S. Series200 and 300 … Austeniticnonmagnetichigh formabilityvery high corrosion resistancetwice the cost of ferritic S.S.400 … Ferritic or Martensiticpoor ductility and formabilitylowest cost S.S.500 … Martensitichigh strength1.5 times the cost of ferritic S.S.
31Wear Resistant, High Strength and Tough High Carbon steels Tool SteelWear Resistant, High Strength and ToughHigh Carbon steelsModified by alloy additionsAISI-SAE ClassificationLetter & Number IdentificationWear Resistant, High Strength and ToughHigh Carbon steelsModified by alloy additionsAISI-SAE ClassificationLetter & Number Identification
32Classification Tool Steel Letters pertain to significant characteristicW,O,A,D,S,T,M,H,P,L,FE.g. A is Air-Hardening medium alloyNumbers pertain to material type1 thru 7E.g. 2 is Cold-workClassificationLetters pertain to significant characteristicW,O,A,D,S,T,M,H,P,L,FE.g. A is Air-Hardening medium alloyNumbers pertain to material type1 thru 7E.g. 2 is Cold-workAn A2 is an Air-Hardenable, Cold-worked material.Read book and look at table 6-6