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 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.

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

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

Taxonomy of Metals

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).

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

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.

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

Steels

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%.

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.

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

Classification of Steel Steel Numerical Name Key Alloys 10XX 11XX 13XX C only C only (free cutting) Mn 23XX 25XX 31XX Ni Ni-Cr 33XX 303XX 40XX Mo 41XX 43XX 44XX Cr-Mo Ni-Cr-Mo Mn-Mo 46XX 47XX 48XX Ni-Mo 50XX 51XX 501XXX Cr 515XX 521XX 514XX 61XX 81XX Cr-V 86XX 87XX 88XX 92XX 93XX 94XX Si-Mn Ni-Cr-Mo-Mn 98XX XXBXX XXLXX B Pb 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

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 Steel Name Appox % C Alloys present in Larger amount than normal case 1020 0.20 Only C 1118 0.18 1340 0.40 Mn 2340 Ni 3140 Ni & Cr 4024 0.24 Mo 4320 Ni, Cr & Mo 5135 0.35 Cr 6150 0.50 Cr & V 8622 0. 22 9255 0.55 Si & Mn

Effect of Alloys Greater strength – C, Mn & Ni added Steel Alloy Effect on Steel C Hardness-strength-wear Cr Corrosion resistance-Hardenability Pb Machinabiliy Mn Strength – Hardenability – More response To Heat Treat Al Deoxidation Ni Toughness – strength Si Deoxidation – Hardenability W High temp. strength – wear Mo High temp. strength – Hardenability S Ti Elimination of C precipitation V Fine grain – Toughness B Hardenability Cu Corrosion resistance – strength Columbium P Strengt Tellurium Machinability Co Hardness-wear 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

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

Sometimes called plain carbon steel 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

Sometimes called plain carbon steel 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

High alloy steel Tool steel Stainless steel 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 Category Description W Water Hardening O Oil Hardening A Air Hardening D Oil or air Hardening S Shock resistance H Hot working M High speed (Mo) T High speed (W) L Special purpose F P Mold making

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

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

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)

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

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)

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

Austenitic Stainless 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)

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

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

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? 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.

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

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

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

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

How Steel & Cast Iron Differ ? Example How Steel & Cast Iron Differ ? Steel Cast Iron Iron with C still in solution Iron which some of the C has precipitate out & appears as flakes C content; 1.6 ~ 2.0% C content; 2.0 ~ 6.0%C Ductile compare to C. iron Brittle compare to steel High strength Poor Strength Hard to machine Easy to machine Hard to control casting Easy to cast Low damping capacity Good Damping Capacity

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 Elements Wt.% Fe balance C 0.06 ~ 0.08 Si 0.10 ~ 0.16 Mn 0.02 ~ 0.05 S 0.01 P 0.06 ~ 0.07

Nonferrous alloys

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

Example What are some of the important properties of unalloyed copper that make it an important industrial metal? Solution: Properties of unalloyed copper, which are important to industrial applications, include high thermal and electrical conductivity, good corrosion resistance, ease of fabrication, medium tensile strength, controllable annealing properties, and general soldering and joining characteristics.

Al & its alloys Unalloyed Al; low density (2.7 gcm-3), lightweight, high electrical & conductivity, workability, ductile & low cost. Moderate melting point (660oC) Resistance to corrosion in most natural environments due to formation oxide film that form on its surface. Non-toxic, used food container & packaging Mechanical properties can enhanced by cold work & alloying Alloying elements; Cu, Mg, Si, Mn & Zn Al alloy are classified as either cast or wrought

Al & its alloys Chemical composition is designated by 4-digit number indicates the principle impurities & in some cases, the purity level Application of Al alloys; aircraft structure parts, beverage cans, Food/chemical handling, storage equipments, bus bodies, automotive parts (engine blocks, pistons & intake manifolds). To be used as eng. materials for transportation to reduce fuel assumption because its specific strength, which is quantified by the TS-specific gravity ratio. Its TS is inferior to a more dense material (such as steel), on weight basis it will able to sustain a larger load. A new generation Al-Li alloys applied in aircraft & aerospace industries has low densities (~ 2.5gcm-3), high specific moduli, excellent fatigue, low temp. toughness.

Example What are some of the properties that make aluminum an extremely useful engineering material? Solution: Aluminum is an extremely useful engineering material due to its low density (2.70 g/cm3), good corrosion resistance, good strength when alloyed, high thermal & electrical conductivities and low cost. What are some of the properties that make aluminum to be high prospect for transportation material. To be used as eng. materials for transportation to reduce fuel assumption because its specific strength, which is quantified by the TS-specific gravity ratio. Its TS is inferior to a more dense material (such as steel), on weight basis it will able to sustain a larger load.

Mg & its alloys Lightweight metal, low density = 1.74gcm-3. Moderate melting point (651oC). Applications requiring a low density metal (aircraft, aerospace & missile) Soft, low elastic modulus. Difficult to cast because molten state burn in air Low strength, poor resistance to creep, fatigue & wear. At RT, Mg & its alloy are difficult to deform. Chemically unstable; susceptible to corrosion in marine environments. Mg alloys are classified as either cast or wrought. Alloying elements; Al, Zn, Mn & some rare earth elements.

Mg & its alloys Mg alloys have replaced engineering plastic that have comparable densities inasmuch as the Mg materials are stiffer, more recyclable, & less costly to produce. Application of Mg alloys; hand held devices (chain saws, power tools, hedge clippers) Automobile ( steering wheels & columns, seat frames, transmission case) Audio-video-computer-communications equipment ( laptop computers, camcorders, TV sets, cellular telephones)

Example What advantages do magnesium alloys have as engineering materials? Solution: As engineering materials, the primary advantage of magnesium alloys is their lightness; magnesium has the low density value of 1.74 g/cm3. What are some of the properties that make a Mg can be replaces plastic as an engineering materials ? Mg alloys can replaces engineering plastic because it has a comparable densities, stiffer, more recyclable, & less costly to produce.

Ti & its alloys Pure Ti; relatively light metal (density = 4.54 gcm-3), high melting point (1668oC), high elastic modulus & high strength. Ti alloys; extremely strong, high TS, spesific strength, highly ductile, easily forged & machined. Corrosion resistance to many chemical environments. Limitation; chemical reactivity with other materials at elevated temp. Expensive because it is difficult to extract to pure state from it compound. Combine to Al for aircraft structural parts application. Application; airplane structures, jet engine, space vehicles, gas turbine engine casings, jet engine components (compressor disks, plates & hubs) surgical implants & petroleum & chemical industries.

Example Why are titanium and its alloys of special engineering importance for aerospace applications? Solution: Titanium and its alloys are of special engineering importance for aerospace applications because of their high strength-to-weight ratios. Why is titanium metal so expensive? Titanium is very expensive because it is difficult to extract in the pure state from its compounds.

Refractory Metals metals with exceptionally high melting points; above 2450oC Metal Melting Point, oC Density, gcm-3 Cost RM/ Ib Niobium (columbium) (Nb) 2468 8.57 192 ~ 210 Tantalum (Ta) 2996 16.6 780 ~ 840 Molybdenum (Mo) 2620 10.22 210 ~ 228 Tungsten 3380 19.3 450

Refractory Metals Group in periodic Table Group VB Group VIB Metal Elements Nb, Ta Mo, W Tensile strength at elevated temperature Low Elastic moduli Less High Solid solubility for interstitial elements (C,O,H,N) Electronic Configuration Stable Creep Strength Ductile-to-brittle fraction transition-temperature behaviour (DBTT) Below room temp (easy fabricate) Near OR above room temp

Refractory Metals DBTT Depends on grain size, impurity content amount of prior cold work The Higher Creep Strengths of Mo & W are attributed to their Elastic moduli, Low diffusivities

Example Define a refractory metals. Name the metal elements that are considered to be refractory elements? Solution: Refractory metals are metals with exceptionally high melting points; above 2450oC. Refractory elements i) Niobium (Nb) ii) Tantalum (Ta) iii) Molybdenum (Mo) iv) Tungsten (W)

Superalloys Superalloys have superlative combinations of properties. Most are used in aircraft turbine components Must withstand exposure to severely oxidizing environments & high temperature These materials are classified according to the predominantly metal in alloys;- Co, Ni or Fe. Other alloying elements; refractory metals (Nb, Mo, W & Ta), Cr & Ti. Other application; nuclear reactors & petrochemical equipment.

Noble metals The noble metals are a group of 8 elements that have some physical characteristics in common. Expensive & superior or notable (noble) in properties. Examples; Silver (Ag), gold (Au), Platinum (Pt), Palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) & osmium (Os). Ag, Au & Pt are used as jewelry Alloys of Ag & Au employed as dental restoration materials & IC electrical contacts. Pt used for laboratory equipment, catalyst & thermocouples.

Miscellaneous nonferrous alloys Nickel & its alloys (eg. Monel) – highly corrosion resistance. Used in pumps, valves and other components that are in contact with some acid & petroleum solutions. Lead (Pb) & Tin (Sn) and its alloys – mechanically soft & weak. Low melting temperature, quite resistance to many corrosion environment. Many common solders are lead-tin alloys, Application of lead alloys – x-ray shields & storage batteries Application of tin alloys – thin coating on the inside of plain CS cans (tin cans) used for food containers.

Miscellaneous nonferrous alloys Zn – soft & low melting temperature, reactive with several materials, susceptible to corrosion Zn applications;- thin coating on CS roofing Zn alloys applications;- padlocks, plumbing fixtures, automotive parts (door handles & grilles) & office equipments. Zirconium & its alloys are ductile, resistance to corrosion in superheated water, transparent to thermal neutrons Application of Zr alloys – Cladding for uranium fuel in water-cooled nuclear reactors. Heat exchangers, reactor vessels & piping systems for the chemical-processing & nuclear industries.