【精品】XX材料体系

精品精品 XX XX材料体系材料体系 美国材料体系及相关By Johnny 一 相关组织美国有多家学会 协会从事钢铁标准化工作 涉及钢 铁材料标准的标准化机构 主要有AISI 美国钢铁学会 ACI 美国合金铸造学会 ANSI 美国国家标准学会 ASTM 美国材料与试验协会 SAE 美国汽车工程师协会 ASME 美国机械工程师协会 AWS 美国焊接学会AMS 航天材料规格 美国航空工业最常用的一种材料 由SAE制定 MIL 美国军用标准QQ 美国联邦政府标准RWMA 美国电阻焊接机制造商协会标准1975年起美国又采用了铝业协会 A A 铜业发展协会 CDA 表示方法 美国材料与试验协会 ASTM 和美国机动车工程师还共同研究制定 了 金属和合金统一数字编号系统 UNS系统 UNS是金属与合金 牌号统一数字体系的简称 它是由ASTM E507和SAE J1086等技术标准推荐使用的 General Propertiesof SteelsThe followingtable liststhe typicalproperties of steels atroom temperature 25癈 The wideranges ofultimate tensilestrength yield strength and hardnessare largelydue todifferentheat treatmentconditions Properties CarbonSteels AlloySteels Stainless Steels ToolSteels Density 1000kg m3 7 857 857 75 8 17 72 8 0Elastic Modulus GPa 190 210190 210190 210190 210Poisson s Ratio0 27 0 30 27 0 30 27 0 30 27 0 3Thermal Expansion 10 6 K 11 16 69 0 159 0 20 79 4 15 1Melting Point 癈 1371 1454Thermal Conductivity W m K 24 3 65 226 48 611 2 36 719 9 48 3Specific Heat J kg K 450 2081452 1499420 500Electrical Resistivity 10 9W m 130 1250210 125175 7 1020Tensile Strength MPa 276 1882758 1882515 827640 2000Yield Strength MPa 186 758366 1793207 552380 440Percent Elongation 10 324 3112 405 25Hardness Brinell3000kg 86 388149 627137 595210 620 二 ASTM标准钢铁牌号表示方法简介美国材料与试验协会 ASTM 标准广泛用于钢铁材料 它的特点是能够代表标准制定部门 钢铁 企业和用户三方协商一致的意见 因此被广泛使用 2 1The Familiesof Stainless Steels Stainlesssteels areiron based alloyscontaining aminimum of about10 5 chromium this formsa protectiveself healing oxidefilm which is the reasonwhy thisgroup of steels has their characteristic stainlessness or corrosion resistance The abilityof theoxide layerto healitself meansthat the steel iscorrosion resistant no matterhow muchof thesurface is removed This is not the case whencarbon orlow alloy steels areprotected fromcorrosion bymetallic coatings such aszinc orcadmium orby organioatingssuch as paint Although allstainless steelsdepend onthe presenceof chromium other alloyingelements are often addedto enhancetheir properties The categorisationof stainless steels isunusual amongstmetals inthat it is basedupon thenature of their metallurgicalstructure the termsused denotethe arrangementof theatoms whichmake upthe grainsof the steel and which can beobserved when a polishedsection through a pieceof thematerial isviewed at high magnificationthroughamicroscope Depending uponthe exactchemical position of the steel themicrostructure may be madeup of the stablephases austeniteor ferrite a duplex mix of these two the phasemartensite createdwhen somesteels arerapidly quenchedfrom ahigh temperature or astructure hardened by precipitatedmicro constituents The relationshipbetween the different families is asshown inFigure1 A broadbrush parisonof theproperties of thedifferentfamiliesisgiven inTable1 2 1 1Austenitic StainlessSteels This group containsat least16 chromium and6 nickel the basicgrade304is referredto as18 8 and rangethrough to the highalloy or super austenitics such as904L and6 molybdenum grades Additional elementscan beadded such as molybdenum titanium orcopper to modifyor improvetheir properties making themsuitable formany criticalapplications involvinghigh temperatureas wellas corrosionresistance Thisgroupofsteels is alsosuitable forcryogenic applicationsbecause theeffect of the nickelcontent inmaking the steel austeniticavoids theproblems ofbrittleness atlow temperatures which is a characteristicof othertypes ofsteel The relationshipbetween thevarious austeniticgrades isshown inFigures2 2 1 2Ferritic StainlessSteels These are plainchromium 10 5to18 grades suchas Grade430and409 Their moderatecorrosionresistanceand poorfabrication properties are improvedin thehigher alloyedgrades suchas Grades434and444and in the proprietarygrade3CR12 The relationshipbetween thevarious ferriticgrades isshown inFigure3 2 1 3Martensitic StainlessSteels Martensiticstainless steels are alsobased onthe additionof chromiumasthemajor alloyingelement butwith ahigher carbon and generallylower chromium content e g 12 in Grade410and416 than the ferritic types Grade431has achromiumcontentof about16 but themicrostructure isstill martensitedespite thishigh chromiumlevel becausethis gradealso contains2 nickel The relationshipbetween thevarious martensiticgrades isshown inFigure4 2 1 4Duplex StainlessSteels Duplexstainless steelssuchas2304and2205 these designationsindicate positionsof23 chromium 4 nickel and22 chromium 5 nickel butboth gradescontain furtherminor alloyingadditions have microstructuresprising amixture of austenite andferrite Duplex ferritic austenitic steelsbine someof thefeatures ofeach class they areresistant tostress corrosioncracking albeit notquite asresistant astheferritic steels their toughnessis superior to that of theferriticsteelsbut inferior to that of theaustenitic steels and theirstrength isgreater thanthatof the annealed austenitic steels by afactor ofabout two In additionthe duplexsteels havegeneral corrosionresistances equalto orbetter than304and316 and ingeneral theirpitting corrosionresistances aresuperiorto316 They sufferreduced toughnessbelow about 50 C andafter exposureabove300 C so areonly usedbetween thesetemperatures The relationshipbetween thevarious duplexgrades isshown inFigures5 2 1 5Precipitation HardeningStainlessSteelsThesearechromium andnickel containingsteels that can developvery hightensile strengths The mostmon gradein thisgroup is 17 4PH also known as Grade630 with theposition of17 chromium 4 nickel 4 copper and0 3 niobium The great advantage of these steelsis that they can be suppliedin the solution treated condition In thiscondition the steel isjust machineable Following machining forming etc the steelcan be hardened by a single fairly lowtemperature ageing heat treatmentwhich causesno distortionoftheponent 2 1 6Characteristics OfStainlessSteelsThe characteristicsofthe broad groupof stainless steels can be viewedas paredto themore familiarplain carbon mild steels As ageneralisation thestainlesssteelshave Higher workhardening rate Higher ductility Higher strengthand hardness Higher hotstrength Higher corrosionresistance Higher cryogenictoughness Lower magicresponse austenitic only Must retaincorrosion resistantsurface in the finishedproduct These propertiesapply particularlyto theaustenitic familyand tovarying degreesto othergrades andfamilies These propertieshave implicationsfor thelikely fieldsof applicationfor stainlesssteels but alsoinfluence thechoice offabrication methodsand equipment Table1 Part A Comparative Propertiesof stainlesssteel families Alloy GroupMagic Response1Work HardeningRateCorrosion Resistance2Hardenable AusteniticGenerally NoVery HighHigh ByCold WorkDuplex Yes Medium Very High NoFerritic YesMedium MediumNo MartensiticYesMedium Medium Quench Temper PrecipitationHardening YesMediumMediumAge Harden1 Attraction ofsteel to a mag Note somegrades canbe attractedto amag ifcold worked 2 Varies significantlywithin betweengrades withineach groupe g free machininggrades havelower corrosionresistance those gradeshigher inmolybdenum havehigher resistance Table1 Part B Comparative Propertiesof stainlesssteel families Alloy GroupDuctility HighTemperature ResistanceLow TemperatureResistance3Weldability AusteniticVery HighVery HighVery HighVeryHighDuplex MediumLow Medium High FerriticMediumHighLow LowMartensitic Low Low LowLow PrecipitationHardening MediumLowLowHigh3 Measured bytoughness orductility atsub zero temperatures Austenitic gradesretain ductilityto cryogenictemperatures 三 steel MaterialProperties andHeat treatmentOverview3 1Objectives ofHeat TreatmentsHeat Treatmentis thecontrolled heatingand coolingof metalsto altertheir physicaland mechanical properties withoutchanging theproduct shape Heat treatmentis sometimesdone inadvertentlydue tomanufacturing processesthat eitherheat orcool themetal suchas weldingor forming Heat Treatmentis oftenassociated withincreasing thestrength ofmaterial but it can alsobe usedto altercertain manufacturabilityobjectives suchas improvemachining improve formability restore ductilityafter acold workingoperation Thus it is avery enablingmanufacturing processthatcannot onlyhelp othermanufacturing process but canalso improveproduct performanceby increasingstrength orother desirablecharacteristics Steels areparticularly suitablefor heat treatment since theyrespond wellto heat treatment and the mercialuse ofsteels exceedsthatofany othermaterial Steels are heat treatedfor oneofthefollowing reasons 3 1 1Softening Softeningis doo reducestrength orhardness remove residual stresses improve toughnesss restore ductility refine grainsize orchange theelectromagic propertiesofthesteel Restoring ductility or removingresidualstressesis anecessary operationwhenalarge amountof coldworking is to beperformed suchasin acold rolling operationor wiredrawing Annealing full Process spheroidizing normalizing andtempering austempering martempering arethe principalways bywhich steel is softened 3 1 1 1Full annealingThe processof slowlyraising the temperature about50 C 90 F above theAustenitic temperatureline A3or lineACM in the case of Hypoeutectoidsteels steels with0 77 Carbon It is held at this temperature for sufficienttime forall thematerial totransform intoAustenite orAustenite Cementite asthecasemay be It is then slowlycooled at the rateofabout20 C hr 36 F hr in afurnace to about50 C 90 F into theFerrite Cementite range At thispoint itcanbe cooled in room temperature airwith naturalconvection The grainstructure hascoarse Pearlitewith ferriteor Cementite depending onwhether hypoor hypereutectoid The steelbees softand ductile 3 1 1 2Tempering aprocess donesubsequent toquench hardening Quench hardened partsareoftentoo brittle This brittlenessis causedbyapredominance ofMartensite This brittlenessisremovedby tempering Tempering resultsin adesired binationof hardness ductility toughness strength and structuralstability Tempering is not tobe confusedwith temperson rolledstock these tempersare anindication ofthe degreeof coldwork performed The mechanismof temperingdepends onthesteelandthetempering temperature The prevalentMartensite isa somewhatunstable structure When heated the Carbonatoms diffusefrom Martensiteto forma carbideprecipitate andthe concurrentformation ofFerrite andCementite which isthe stableform Tool steelsfor example lose about2to4points ofhardness onthe RockwellC scale Even thougha littlestrength issacrificed toughness as measuredby impactstrength is increasedsubstantially Springs andsuch partsneed tobe muchtougher these aretempered to a muchlower hardness Tempering isdone immediatelyafter quenchhardening When thesteel coolstoabout40 C 104 F after quenching itisready tobe tempered The partis reheated toa temperature of150to400 C 302to752 F In thisregion asofter andtougher structureTroostite isformed Alternatively thesteelcanbe heatedtoatemperatureof400to700 C 752to1292 F that resultsin asofter structureknownasSorbite This hasless strengththan Troostitebut moreductility andtoughness The heatingfor temperingis bestdone byimmersing the parts inoil for temperingupto350 C 662 F and thenheating theoil withthe partsto theappropriate temperature Heating ina bathalso ensuresthattheentire parthas the same temperatureand willundergo thesame tempering For temperaturesabove350 C 662 F itisbest touse abath ofnitrate salts The saltbaths canbeheatedupto625 C 1157 F Regardless ofthe bath gradual heatingis importantto avoidcracking thesteel After reachingthe desiredtemperature the partsare heldat thattemperatureforabout2hours then removedfrom thebath andcooledinstill air 3 1 2Hardening3 1 3Material Modification3 2One ofthe mainfactors contributingtotheutility ofsteelsisthebroadrange ofmechanicalpropertieswhichcanbe obtainedby heattreatment For example easy formabilityand goodductility maybe necessaryduring fabricationofapart Once formedvery high strength partmaybeneeded inservice Both ofthese materialproperties areachievable fromthesamematerial All steels canbesoftened tosome degreeby annealing The degreeof softeningdepends onthe chemicalpositionofthe particularsteel Annealing isachieved byheating to and holdingat a suitable temperature followed by cooling atasuitablerate Similarly steels canbehardenedor strengthened This canbe aomplishedby coldworking heat treating or anappropriate binationofthese Cold workingisthetechnique usedto strengthenboth lowcarbon lowalloyed steelsand highly alloyed austenitic stainlesssteels Only reasonablyhighstrength levels canbe attainedinthecarbon lowalloyed steels but thehighlyalloyedausteniticstainlesssteelscanbe coldworked torather highstrength levels Most steels are monlysupplied tospecified minimumstrengthlevels Heat treatingistheprimary techniquefor strengtheningthe remainderofthe steels Some monheattreatmentofsteelsare listedbelow Martensitic hardeningPearlitic transformationAustempering Agehardening Figure1Schematic ofTime TemperatureTransformation DiagramCarbon andalloy steelsare Martensitichardenedbyheating tothe Austenitizingtemperaturefollowed bycoolingattheappropriate rate One requirementfor fulltransformation toMartensite isthat coolingmust ourprior tothe nose ofthetransformation startcurve inFigure1 Cooling frequentlyours byquenching inoil orwater Some steelsare capableof Martensitictransformation whenair cooled Ms iswhen theMartensite transformationstarts Mf iswhen theMartensite transformationfinishes Martempering isa martensitictransformation wherethepartis cooledrapidly toabove theMs andheld untilthetemperaturebees uniformacross thecross section After Martensitictransformation thesteel isthen tempered Tempering consistsof reheatingthesteeltoanintermediate temperature Tempering causesmicrostructural changesinthesteel inaddition torelieving internalstresses andimproving tough