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Fundamentals of metal and steel, heat treatment and material strengthening

Alloy & Metal. Fundamentals of metal and steel, heat treatment and material strengthening . Metallurgy & Metallurgists. Dictionary Metallurgy = the science that explains methods of refining & extracting metals from their ores & preparing them Materials Today Magazine

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Fundamentals of metal and steel, heat treatment and material strengthening

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  1. Alloy & Metal Fundamentals of metal and steel, heat treatment and material strengthening

  2. Metallurgy & Metallurgists • Dictionary • Metallurgy = the science that explains methods of refining & extracting metals from their ores & preparing them • Materials Today Magazine • Metallurgy = the science that explains the properties, behavior & internal structure of metals. • Metallurgies • Scientists in metallurgy that probe deeply inside the internal structure of metal to learn what it looks like.

  3. Terms to know !!! • Elements = is a pure substance made up of just one kind of materials • Metal = is an element that has metallic properties, i.e. heat & electrical conductor • Compound = is a material that is composed of two or more elements that are chemically joined • Mixture = is a materials composed of 2 or more elements or compounds mixed together, but not chemically joined • Solution = is a special kind of mixture. When 2 materials combine & become a solution, one of two will become the “dictator” & the other one will become quiet & submissive. • Solid solution = is a solution in which both solvent & solute are solids. Both the “dictator” & the dissolved material are solids

  4. Alloy • Alloy = When 2 or more metals are dissolved together in a solid solution • Steel = alloy of Fe & C • Bronze = alloy of Cu & Sn • Brass + alloy of Cu & Zn

  5. Taxonomy of Metals

  6. Types of metal alloys • Groups of metal alloys: • Ferrous alloy (iron is the prime constituent). • Nonferrous alloys. • Steels: Iron-carbon alloys that may contain appreciable concentrations of other alloying elements. Carbon content is normally less than 1.0 wt%. • Cast irons: Ferrous alloys with carbon contents above 2.14 wt% (usually 3.0-4.5 wt% C).

  7. Classification for various ferrous alloys METAL ALLOYS Non-ferrous Ferrous Wrought iron Cast iron Steels Ductile (nodular) iron Special alloy cast iron Gray iron White iron Malleable iron Low alloy High alloy Low carbon Medium carbon High carbon High strength, low alloy Heat treatable Tool Tool Stainless Plain Plain Plain

  8. STEELS • Steel is an alloy or solid solution, dictator = Iron, dissolved mater. = C • Most widely used materials in the world • High strength, machined & formed easily • Steel are iron-carbon alloys that may contain appreciable concentrations of other alloying elements • Mechanical sensitive to the content of C < 1.0 wt.% • Thousands of alloys that have different compositions and/ or heat treatments.

  9. STEELS • Commonly classified according to C concentration • Low CS • Medium CS • High CS • Subclasses; according to the concentration of other allying elements • Plain CS • contain only residua; concentrations of impurities other than C & a little Mn • Alloy steels • alloying elements are added in specific concentration

  10. Steels

  11. Composition of Steels • Steel is a mater. Composed primary of iron. • > 90% iron, many steels contain > 99% iron • All steel contain 2nd element = C • % C range just above 0% ~ approx. 2.0%, many steels contain 0.15 ~ 1.0%.

  12. Effect of C in steel • Steel with the least C are more flexible & ductile, not strong. • C content increases, so do strength, hardness & brittleness • In making steel, the iron dissolves the C, when there is too much carbon for the iron to “digest” the alloy is no longer called steel.

  13. Effect of C in steel: Microstructure In steel, iron dissolves the C In gray cast iron, the C precipitates out as C flakes In ductile cast iron, the C precipitates out as a small round nodules

  14. Classification of Steel • 4 numbers/ digits • 1st 2 digits refer to the alloy content • Eg; • 5147 steel, ’51’ = steel has a lot of Cr • 2517 steel, ’25’ = amount of Ni • 1040 steel, ’10’ = very little alloy content except C

  15. Steel Numbering System • Last 2 (or 3 in 5 digits case) digits refer to % of C in steel. • Eg; • 1040 steel, ’40’ = 0.40% C

  16. Effect of Alloys • Greater strength – C, Mn & Ni added • Corrosion Resistance – Cr or Cu added • Better machinability – Pb & S added • Physical properties at high temp. – W or Mo are recommended

  17. STEELS Low alloy High alloy Less expensive Less alloy content Few special properties More expensive More alloy content Special properties

  18. Overview Sometimes called plain carbon steel Low alloy steel Low carbon steel Medium carbon steel High carbon steel • 0.05 ~ 0.35% C • Comparatively less strength • Comparatively less Hardness • Easy Machining & Forming • Least Expensive • Largest quantity Produced • 0.35 ~ 0. 50% C • Hard & strong after heat treating • More expensive than Low CS • 0. 50 ~ 1.0% C • High strength & hardness • Hard & strong after heat treating • More expensive than Low & medium CS

  19. Application of Low Alloy Steel Low alloy steel Sometimes called plain carbon steel Low carbon steel Medium carbon steel High carbon steel • Fence wire • Auto bodies • Galvanized sheets • Storage tanks • Large pipe • Various parts in building, bridges & ships • Wheels • Axles • Crankshafts • Gear • Tools • Dies • Knives • Railroad wheels • High strength materials application

  20. Is a grade of steel which one or more alloying elements have been added in larger amounts to give it special properties that ordinary cannot obtained with CS High alloy steel Tool steel Stainless steel Widely used Used as cutting tools, mould & dies Machine parts Extremely good corrosion resistance Expensive than CS Harder to cut & machine High Cr and/or Ni

  21. High Alloy Steel Stainless steel Tool Steel Ferritic Martensitic Austenitic Precipitation hardening

  22. Stainless Steel • Excellent corrosion resistance in many environment due to Cr content (>11~ 12% Cr) • Corrosion resistance enhanced by Ni & Mo • Cr forms a surface oxide that protects the underlying Fe-Cr alloy from corroding. To produce the protective oxide, the SS must be exposed to oxidizing agents • SS are divided into 3 classes based on the microstructure phase constituent • Ferritic • Martensitic • Austenitic

  23. Ferritic Stainless Steel • FSS are essentially Fe-Cr binary alloy containing about 12 ~ 30% Cr • Called ferritic bcause their structure remains mostly ferritic (BCC, α iron type) at normal heat treatment conditions. • Relatively low cost • Mainly used as general construction materials • The present of the carbides in this steel reduces its corrosion resistance to some extent • Considered non-heat-treatable because they are all single phase, α iron type alloys whose crystal structure does not change under normal heat-treatment conditions. • Eg; • 430 SS (general-purpose, non-hardenable uses, range hood, restaurant equipment) • 446 SS (High-temp. application, heater, combustion chambers)

  24. Type 430 (ferritic) SS strip annealed at 788oC. The structure consists of a ferrite matrix equiaxed grain & dispersed carbide particles.

  25. Martensitic Stainless Steel • MSS are essentially Fe-Cr alloys containing 12 ~ 17 % Cr with sufficient C (0.15 ~ 1.0 %). • Produced from quenching from the austenitic phase region • Called martensitic because they are capable of developing a martensitic structure from austenitic condition by quenching heat treatment. • Can be adjusted to optimize strength & hardness but corrosion resistance is relatively poor compared to the ferritic & austenitic steel • High hardness due to hard martensitic matrix & the presence of a large concentration of primary carbides. • Considered as heat-treatable because the carbon content is sufficient for the formation of a martensitic structure by austenitizing and quenching processes. • E.g.; • 410 SS ( General purpose, heat-treatable machine parts, pump shafts, valves) • 440A SS (Cultery, bearing, surgical tools) • 440C SS (Balls bearing, valve parts)

  26. Type 440 (martensitic) SS hardened by autenitizing at 1010oC & air cooled. Structure consists of primary carbides in martensite matrix.

  27. AusteniticStainless Steel • Austenitic steel are essentially Fe-Cr-Ni ternary alloys containing about 16~25% Cr & 7~20% Ni. • Called austenitic since their structure remains austenitic (FCC, γ iron type) at all normal heat-treating temperatures. • Better corrosion resistance than ferritic & martensitic SS because the carbides can be retained in solid solution by rapid cooling. • E.g.; • 301 SS (High work hardening rate alloy, structural applications) • 304 SS (Chemical & food processing equipment) • 304L SS (Low carbon for welding, chemical tank) • 321 SS (Stabilized for welding, process equipment, pressure vessels) • 347 SS (Stabilized for welding, tank cars for chemicals)

  28. Type 340 (austenitic) SS hardened strip annealed 5 min at 1065oC and air cooled. Structure consists of equiaxed austenite grains.

  29. Example • What are the 3 basic types of stainless steels? • What is the basic composition of ferritic stainless steels & Why are ferritic stainless steels considered non-heat-treatable? • What is the basic composition of martensitic stainless steels and why are these steels heattreatable? • What are some applications for ferritic and martensitic stainless steels? Solution; Refer your lecture note

  30. Example What makes it possible for an austenitic stainless steel to have an austenitic structure at room temperature? Solution; Austenitic stainless steel can retain its FCC structure at room temperature due to the presence of nickel, at 7 to 20 weight percent, which stabilizes the austenitic Fe structure. What makes austenitic stainless steels that are cooled slowly through the 870 to 600ºC range become susceptible to intergranular corrosion? Solution; When slowly cooled through 870 to 600ºC, some austenitic stainless steels become susceptible to intergranular corrosion because chromium-containing carbides precipitate at the grain boundaries.

  31. CAST IRON Special alloy cast iron Special properties Gray iron more common Ductile (nodular) iron Higher quality White iron most brittle Malleable iron Higher quality

  32. Cast Irons • Iron-Carbon alloys of 2.0 ~ 6.0%C • Typical composition: 2.0-4.0%C,0.5-3.0% Si, less than 1.0% Mn and less than 0.2% S. • Si-substitutes partially for C and promotes formation of graphite as the carbon rich component instead Fe3C.

  33. Example What are the cast irons? What is their basic range of composition? Solution: Cast irons are a family of ferrous alloys intended to be cast into a desired shape rather than worked in the solid state. These alloys typically contain 2 to 4 percent C and 1 to 3 percent Si. Additional alloying elements may also be present to control or vary specific properties.

  34. Example What are some of the properties of cast irons that make them important engineering materials? What are some of their applications? Cast irons are easily melted and highly fluid and do not form undesirable surface films or shrink excessively; consequently, they make excellent casting irons. They also possess a wide range of strength and hardness values and can be alloyed to produce superior wear, abrasion, and wear resistance. In general, they are easy to machine. Their applications include engine cylinder blocks and gear boxes, connecting rods, valve and pump casings, gears, rollers, and pinions.

  35. Gray Cast Iron • Fe-C-Si alloys • Composes of: 2.5-4.0%C, 1.0-3.0%Si and 0.4-1.0% Mn. • Gray cast iron contain large amount of C in the form of graphite flakes. • Microstructure: 3-D graphite flakes formed during eutectic reaction. They have pointed edges to act as voids and crack initiation sites.

  36. Gray Cast Iron • Properties: • Hard & brittle • Relatively poor TS because graphite flakes in the structure • excellent compressive strength, • excellent machinability, • good resistance to adhesive wear (self lubrication due to graphite flakes), • outstanding damping capacity ( graphite flakes absorb transmitted energy), • good corrosion resistance and it has good fluidity needed for casting operations. • Easy to cast • It is widely used, especially for large equipment parts subjected to compressive loads and vibrations. • Eg; brake disc, cylinder blocks, cylinder heads, clutch plates, heavy gear boxes and diesel engine castings

  37. White Cast Iron • Fe-C-Si alloys • Composes of: 1.8-3.6%C, 0.5-1.9%Si and 0.25-0.8%Mn. • White cast iron contain large amount of iron carbide that make them hard & brittle • All of its C is in the form of iron-carbide (Fe3C). It is called white because of distinctive white fracture surface. • It is very hard and brittle (a lot of Fe3C). More brittle difficult to machine • It is used where a high wear resistance is dominant requirement (coupled hard martensite matrix and iron-carbide). • Eg; iron mills, stone breaker

  38. Malleable Cast Iron • Fe-C-Si alloys • 2.0 ~ 2.6% C, 1.1 ~ 1.6% Si • Malleable cast irons are 1st cast as white cast iron & then are heat-treated at about 940oC & held about 3~20 hrs. • The iron carbide in the white iron is decomposed into irregularly shaped nodules or graphite. • Less voids and notches. • Ferritic MCI: • Ductile, 10% EL, • High TS, 35 ksi yield strength, • 50 ksi tensile strength. • Excellent impact strength, • good corrosion resistance • good machinability.

  39. Malleable Cast Iron • Ductile iron with ferrite matrix (top) and pearlite matrix (bottom) at 500X. • Spheroidal shape of the graphite nodule is achieved in each case. • Advantageous properties of malleable cast irons are toughness, moderate strength, uniformity of structure and ease of machining and casting.

  40. Pearlitic Malleable Cast Iron • Pearlitic MCI: by rapid cooling through eutectic transformation of austenite to pearlite or martensite matrix. • Composition: 1-4% EL, 45-85 ksi yield strength, 65-105 ksi tensile strength. Not as machinable as ferritic malleable cast iron.

  41. Ductile Cast Iron • Fe-C-Si alloy • 3.0 ~ 4.0% C, 1.8 ~ 2.8% Si. • Ductile cast iron contain large amount of C in the form of graphite nodules (spheres). • Without a heat treatment by addition of ferrosilicon (MgFeSi) formation of smooth spheres (nodules) of graphite is promoted. • Properties: 2-18% EL, 40-90 ksi yield strength, 60-120 ksi tensile strength.

  42. Ductile Cast Iron • Attractive engineering material due to: good ductility, high strength, toughness, wear resistance, machinability and low melting point castability. • Applications for ductile cast irons include valve and pump casings, crankshafts, gears, rollers, pinions and slides.

  43. Example Why are ductile cast irons in general more ductile than gray cast irons? Solution Ductile cast irons are, in general, more ductile than gray cast irons because their spherical graphite nodules are surrounded by relatively ductile matrix regions which allow significant deformation without fracture. In contrast, the gray cast irons consist of an interlacing network of graphite flakes which can be fractured easily.

  44. Example Why does the graphite form spherical nodules in ductile cast irons instead of graphite flakes as in gray cast irons? Solution; Graphite forms spherical nodules in ductile cast irons because the levels of phosphorus and sulfur are reduced significantly compared to those in gray cast irons; these two alloying elements prevent the formation of nodules and thus promote the formation of graphite flakes

  45. Special alloy cast iron • Contain High % of Ni, Cu, Cr & other alloys • Ni, Cu & Cr good corrosion & chemical resistance to acids. • Greater strength & better high temperature properties • Used in cylinders, pistons, piston rings & turbine stator vanes

  46. Example How Steel & Cast Iron Differ ?

  47. Wrought Iron • Very different from cast iron • Almost pure iron, little C content • Low strength & hardness • Good corrosion resistance • Many fibrous stringers of slag are distributed throughout wrought iron

  48. Nonferrous alloys

  49. Cu & its alloys • Unalloyed Cu is so soft & ductile; difficult to machine • Highly resistant to corrosion • Unalloyed Cu cannot be hardened or by strengthened by heat heat-treating procedures • Mechanical & corrosion properties can be improved by alloying • Cold working and/or solid-solution alloying must be utilized to improved the mechanical properties • Cu alloys, e.g.; Brass, Bronze, Beryllium Cu, Cartridge brass, Cu-Ni alloy, Tin bronze, Al bronze • Application; Electrical wire, nails, valves, automotive radiator, condenser, heat exchanger components, pistons rings, bearing, gears & so on

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