Materials Lab Procedures
Materials Lab Procedures MSE 300
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This 17 page Class Notes was uploaded by Eudora Blick on Monday October 26, 2015. The Class Notes belongs to MSE 300 at University of Tennessee - Knoxville taught by Philip Rack in Fall. Since its upload, it has received 29 views. For similar materials see /class/229812/mse-300-university-of-tennessee-knoxville in Materials Science Engineering at University of Tennessee - Knoxville.
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Date Created: 10/26/15
Surface Treatments Applications Biomedical biocompatible coatings on implants drug coatings for sustained release Mechamcal I Tribological friction and wear tool steels implants I Fatigue minimize surface defects add compressive stress I Hardness Corrosion I Protective coatings for harsh environments catalytic converters electrochemical cells Thermal modify thermal coefficient of expansion to minimize stresses for thermal cycling Electronic dielectric barriers electrical grounds Optical re ective coatings or antire ective coatings tinted glass solar cells Many many more Example Fatigue Crack initiation and propagation I Crack initiation at the sites of stress concentration microcracks scratches indents interior comers dislocation slip steps etc Quality of surface is important I Crack propagation gt Stage 1 initial slow propagation along l crystal planes with high resolved shear stress Involves just a few grains and has at fracture surface gt Stage ll faster propagation perpendicular to the applied stress Crack grows by repetitive blunting and sharpening process at crack tip Rough in fracture surface I Crack eventually reaches critical dimension and propagates very rapidly Factors that affect fatigue life I Magnitude of stress mean amplitude I Quality of the surface scratches sharp transitions and edges Solutions gt Polishing removes machining aws etc gt Introducing compressive stresses compensate for applied tensile stresses into thin surface layer by Shot Peening ring small shot into surface to be treated High tech solution ion implantation laser peening gt Case Hardening create C or N rich outer layer in steels by atomic diffusion from the surface Makes harder outer layer and also introduces compressive stresses gt Optimizing geometry avoid internal comers notches etc Factors that affect fatigue life environmental effects Thermal Fatigue Thermal cycling causes expansion and contraction hence thermal stress if component is restrained Solutions gt eliminate restraint by design gt use materials with low thermal expansion coef cients Corrosion fatigue Chemical reactions induce pits which act as stress raisers Corrosion also enhances crack propagation Solutions gt decrease corrosiveness of medium if possible gt add protective surface coating gt add residual compressive stresses Surface Hardening Thermochemical treatments to harden surface of part carbon nitrogen Also called case hardening May or may not require quenching Interior remains tough and strong Carburizing Lowcarbon steel is heated in a carbonrich environment 7 Pack carburizing packing parts in charcoal or coke makes thick layer 0025 0150 in 7 Gas carburizing use of propane or other gas in a closed furnace makes thin layer 0005 0030 in 7 Liquid carburizing molten salt bath containing sodium cyanide barium chloride thickness between other two methods Followed by quenching hardness about HRC 60 Nitri ding Nitrogen diffused into surface of special alloy steels aluminum or chromium Nitride compounds precipitate out 7 Gas nitriding heat in ammonia 7 Liquid nitriding dip in molten cyanide bath Case thicknesses between 0001 and 0020 in with hardness up to HRC 70 Other Case Hardening Carbonotriding use both carbon and nitrogen 0 Chroming pack or dip in chromiumrich material adds heat and wear resistance Boronizing improves abrasion resistance coefficient of friction Heat Treatment Methods Furnaces 7 Fuel red parts exposed to combustion products 7 Electric 7 Batch or continuous 7 Vacuum prevents oxidation of surface 7 Salt bath 7 Fluidized bed particles suspended by gas ow improves heat transfer Surface Hardening Methndq Target area Previously namened areas comam gt Ll Flame Induction heating Highfrequency HF resistance heating Electron or laser beam heating Curmm dlmunn Induction Reasons to Surface Harden 39 Increase wear resistance Increase surface strenght for load carrying crush resistance Induce suitable residual and compressive stresses Improve fatigue life Impact resistance Methods to Surface Harden Heat Treatment 7 Induction 7 Flame 7 Laser 7 Light 7 Electron beam Case Hardening 7 Carburizing 7 Cyaniding 7 Carbonitriding 7 Nitriding Flame Hardening Hammad mm mm gm Quench wulm mm mm Flam hardymaul Hal mam Lama mm mm mm hirdemng mum mm m a mum Pack Carbun39zjng Hm m urbmlzing mmparamm above me nansmmmon mmpelalure 5 memes gasket Wm cunuaHed vemmg Seana sme cuminquot Heat Treatment Procedure typically for medium to high carbon steels 7 Heat surface to austenize interior stays below austenite transition temperature 7 Cool to form surface martensite Interior is not modified Surface is in compression 7 Subsequent tempering typically done Heat treatment Characteristics 7 Hardened depth depends on Frequency for induction heating 7 effects the depth of the skin 7 Example 1000Hz 7 459mm 1000000 Hz 7 02508mm Heat flow ame 7 Surface Rc 5060 martensite or tempered martensite 7 Interior 7 Rc 1020 pearliteferrite Case Hardening Reasons Easy to control depths Good for complicated parts Mass production compatible Can use with low carbon steels cheaper and tougher Case Hardening Carburizing gas mixtures 7 C0 C02 H2 H20 and N2carrier gas 7 Reactions 2co 9 Cs c02 COH2 9 C H20 7 Control C0C02 H2 H20 ratios to carburize or decarburize C arbon Gradient yquot ham m Carbmizing TimeTemperature Plots Case depth Inches Carbunzmq nme hours Case Hardened Gear Tooth a an Apmnwlycsmudr hnmunnd anmumwmu mm mm m 2 mm mm Ix Microstructure Through Carburized Surface mm m 7 rmuunu m W m mm r g am nl wvn mzaman C New 4m Wm hmmnm 1 mm aux Microhardness Through Case zone nmuncv Hum Su acc n s a m 70 5mm an W 7 a 2 so 7777 W S y 1 m g I 20 0 nuns DOIO 006 mm 0025 0030 5 hum HMS nasu Dmanu mm m Hznre 86 Dmtnmmnan nf 12m dzpih by mcmhudnux mmy O 0 What is diffusion Diffusion is material transport by atomic motion Atoms of type A Atoms of type B Inhomogeneous materials can become homogeneous by diffusion For an active diffusion to occur the temperature should be high enough to overcome energy barriers to atomic motion Atomic Vibrations 39 Heat causes atoms to vibrate 39 Vibration amplitude increases with temperature 39 Melting occurs when vibrations are sufficient to rupture bonds 39 Vibrational frequency 1013Hz 39 Average atomic electronic energy due to thermal excitation is of order kT with a attribution around this average energy PE expE cf k Boltzmann s constant lJled39WIf or 862x10 5eVIf T Absolute temperature Kelvin What is diffusion Interdiffusion and Selfdiffusion Diffusion is material transport by atomic motion l nterdifnsion occurs in response to a concentration gradient more rigorously to a gradient in chemical potential a Diffusion of Cu atoms Cu CuNi alloy Ni Diffusion of Ni alums lt7 a oneoeooooooo DQOOQQOQDQQQ oooonmoooooo 000000600000 000090000000 000000000000 009900000000 000000000000 000800000000 000000000000 b b 100 Cu Concentration of Ni Concentration of NI Cu Positiun Position Before W a After Diffusion Mechanisms To move from lattice site to lattice site atoms need energy to break bonds with neighbors and to cause the necessary lattice distortions during motion from site to another This energy comes from atomic vibrations E1V kT Vitom migration Vacancy migration Rpf rp After Atomic migration by a mechanism of vacancy migration Materials flow the atom is opposite Interstitial diffusion depends on temperature This is generally faster than vacancy diffusion because there are many more interstitial sites than vacancy sites to jump to Requires small impurity atoms eg C H O to fit into interstices in host nge udnnoo 0 0 0 0 0 0 0 0 O O 0 0 Self diffusion motion of atoms Within a pure host also occurs Predominantly vacancy in nature difficult for atoms to fit into interstitial sites because of size Diffusion Flux The ux of diffusing atoms J is used to quantify how fast diffusion occurs The ux is de ned as either in number of atoms diffusing through unit area and per unit time e g atomsmZsecond or in terms of the mass ux mass of atoms diffusing through unit area per unit time eg kgmZsecond 7 1 dM gt xdirection Unit area A throu gh which atoms move SteadyState Diffusion Steady state diffusion the diffusion ux does not change with time Concentration pro le concentration of atomsmolecules of interest as function of position in the sample Concentration gradient dCdx Kgmquot the slope at aparticular point on concentration pro le dc 0 A e C x dx 7 A x 7 x SteadyState Diffusion Fick s first law Fick s rst law the diffusion ux along direction X is proportional to the concentration gradient J 39 D V whereDisthediffusion coef cient J flux of atoms across plane with areaA The concentration gradient is often called the drzvmgforce in diffusion but it is not a force in the mechanistic sense The minus sign in the equation means that diffusion is down the concentration gradient Diffusion Temperature Dependence I J D Diffusion coefficient is the measure of mobility of diffusing species DDnexp 7Q DU itemperatureindependent preexponential mzs Qdithe activation energy for diffusion Jmol or eVatom R 7 the gas constant 831 JmolK or 862x10395 eVatomK T7 absoluteternperature K The above equation can be rewritten as 1 Q 1 aninD0QRT 0r logDlogDn39 23R quot quot n quot quotthherefnre L quotquot L I plotting lnD versus NT or logD versus 11 Such plots are Arrhenius plots Diffusion Temperature Dependence 1 f f f f 10712 7 3 1043 10 1045 lefuswn Eoe lmem NZS 107m m i i 07 08 09 10 1 1 12 Reclpvocaf temperature IODDK Graph of log D vs lff has slop of QdZSR