Click here to load reader

Chapter (1) CUTTING TOOL MATERIALS. TOPICS : Introduction Carbon and medium alloy steels High speed steels Cast-cobalt alloys Carbides Coated tools Alumina-based

  • View
    219

  • Download
    1

Embed Size (px)

Text of Chapter (1) CUTTING TOOL MATERIALS. TOPICS : Introduction Carbon and medium alloy steels High speed...

  • Chapter (1)

    CUTTING TOOL MATERIALS

  • TOPICS :Introduction Carbon and medium alloy steelsHigh speed steelsCast-cobalt alloysCarbidesCoated toolsAlumina-based ceramicsCubic Boron NitrideSilicon Nitride based ceramicsDiamond Whisker-reinforced tool materials

  • Introduction: Characteristics of cutting tool :Hardness (resistance to wear)Hot hardness (capacity to retain hardness at high temperaturesToughness (resistance to impact forces on tool in interrupted operations)Chemical stability or inertness (to avoid adverse reactions)

  • Cutting tool materials Carbon & medium alloy steels High speed steels Cast-cobalt alloysCarbidesCoated toolsAlumina-based ceramicsCubic boron nitrideSilicon-nitride-base ceramics DiamondWhisker-reinforced materials

  • Carbon and Medium alloy steels : Oldest of tool materialsUsed for drills taps, broaches, reamersInexpensive, easily shaped, sharpenedNo sufficient wear resistanceLimited to hand tools and low cutting speed operation. (Red hardness temp.: 200 C)

    High speed steels (HSS)Hardened to various depths Good wear resistance Suitable for high positive rake angle tools

  • Two basic types of HSSMolybdenum (M-series)Tungsten (T-series)

    M-series (6-6-4-2): Contains 6% molybdenum, 6% tungsten, 4% chromium, 2% vanadium & cobaltHigher, abrasion resistanceH.S.S. are majorly made of M-series

    T-series (18-4-1): Contains 18 % tungsten, 4% chromium, 1% vanadium & cobaltundergoes less distortion during heat treating

  • H.S.S. available in wrought, cast & sintered (Powder metallurgy)

    Coated for better performance

    Subjected to surface treatments such as case-hardening for improved hardness and wear resistance or steam treatment at elevated temperatures

    High speed steels (Red hardness temp.: 650 C)

  • Cast-Cobalt alloysCommonly known as stellite toolsComposition ranges 38% - 53 % cobalt30%- 33% chromium10%-20%tungstenGood wear resistance ( higher hardness)Less tough than high-speed steels and sensitive to impact forcesLess suitable than high-speed steels for interrupted cutting operationsContinuous roughing cuts relatively high g=feeds & speedsFinishing cuts are at lower feed and depth of cut

  • Carbides :

    (Hot hardness temp.: 1000 C)

    These carbides are also known as cemented or sintered carbidesHigh elastic modulus, thermal conductivityLow thermal expansion

    2-groups of carbides used for machining operationstungsten carbidetitanium carbide

  • Tungsten CarbideComposite material consisting of tungsten-carbide particles bonded together Alternate name is cemented carbides

    Manufactured with powder metallurgy techniques

    Particles 1-5 m in size are pressed & sintered to desired shape in a H2 atmosphere furnace at 1550C Amount of cobalt present affects properties of carbide toolsAs cobalt content increases strength, hardness & wear resistance increases

  • Titanium carbide Titanium carbide has higher wear resistance than tungsten carbide

    Nickel-Molybdenum alloy as matrix Tic suitable for machining hard materials

    Steels & cast irons

    Speeds higher than those for tungsten carbide

  • Cutting tool materials HSS alloying

    ElementProperties

    TungstenIncreases hot hardnessHard carbides formed, improving abrasion resistance

    MolybdenumIncreases hot hardness Hard carbides formed, improving abrasion resistance

    ChromiumDepth hardenability during heat treatHard carbides formed, improving abrasion resistanceSome corrosion resistance

    VanadiumCombines with carbon for wear resistanceRetards grain growth for better toughness

    CobaltIncreases hot hardness, toughness

    CarbonHardening elementForms carbides

  • Inserts

  • InsertsIndividual cutting tool with severed cutting pointsClamped on tool shanks with locking mechanismsInserts also brazed to the tools Clamping is preferred method for securing an insertCarbide Inserts available in various shapes-Square, Triangle, Diamond and roundStrength depends on the shape Inserts honed, chamfered or produced with negative land to improve edge strength

  • Insert AttachmentFig : Methods of attaching inserts to toolholders : (a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.

  • Edge StrengthFig : Relative edge strength and tendency for chipping and breaking of inserts with various shapes. Strength refers to the cutting edge shown by the included angles.Fig : edge preparation of inserts to improve edge strength.

  • Chip breakers:Purpose :Eliminating long chipsControlling chip flow during machiningReducing vibration & heat generatedSelection depends on feed and depth of cut, work piece material and type of chip produced during cutting

  • Coated tools :High strength and toughness but generally abrasive and chemically reactive with tool materials

    (Hot hardness temp.: 1100 C)

    Unique Properties :Lower Friction High resistance to cracks and wear High Cutting speeds and low time & costsLonger tool life

  • Coating materialsTitanium nitride (TiN)Titanium carbide (Tic)Titanium Carbonitride (TicN)Aluminum oxide (Al2O3)Diamond coatingThickness range: 2-15 m (80-600 in)

    Techniques used :Chemical vapor deposition (CVD) Plasma assisted CVDPhysical-vapor deposition(PVD)Medium temperature chemical- vapor deposition(MTCVD)

  • Properties for Group of MaterialsFig : Ranges of properties for various groups of tool materials.

  • Cutting tool Characteristics for coating :High hardnessChemical stabilityLow thermal conductivityGood bondingLittle or no Porosity

    Titanium nitride (TiN) coating :Low friction coefficientsHigh hardnessResistance to high temperaturesGood adhesion to substrateHigh life of high speed-steel tools

    Titanium carbide (TiC) coating:Titanium carbide coatings on tungsten-carbide inserts have high flank wear resistance.

  • Ceramics :Low thermal conductivity ,resistance ,high temperature Resistance to flank wear and crater wearCeramics are suitable materials for toolsAl2O3 (most commonly used)

    Multi Phase Coatings :First layer Should bond well with substrateOuter layer Resist wear and have low thermal conductivityIntermediate layer Bond well & compatible with both layersCoatings of alternating multipurpose layers are also formed.

  • Multiphase CoatingsFig : Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers of titanium nitride. Inserts with as many as thirteen layers of coatings have been made. Coating thick nesses are typically in the range of 2 to 10 m.

  • Diamond Coated tools :Use of Polycrystalline diamond as a coatingDifficult to adhere diamond film to substrateThin-film diamond coated inserts now commercially availableThin films deposited on substrate with PVD & CVD techniquesThick films obtained by growing large sheet of pure diamondDiamond coated tools particularly effective in machining non-ferrous and abrasive materials

  • New Coating materials :Titanium carbo nitride (TiCN)Titanium Aluminum Nitride(TiAlN)Chromium Based coatingsChromium carbideZirconium Nitride (ZrN)Hafnium nitride (HfN)Recent developments gives nano coating & composite coating

    Ion Implementation :Ions placed into the surface of cutting toolNo change in the dimensions of toolNitrogen-ion Implanted carbide tools used for alloy steels & stainless steelsXeon ion implantation of tools as under development

  • Alumina-Based ceramics:Cold-Pressed Into insert shapes under high pressure and sintered at high temperatureHigh Abrasion resistance and hot hardness (1200C)Chemically stable than high speed steels & carbidesSo less tendency to adhere to metalsGood surface finish obtained in cutting cast iron and steels Negative rake-angle preferred to avoid chipping due to poor tensile strength

    Cermets, Black or Hot- Pressed :70% aluminum oxide & 30 % titanium carbidecermets(ceramics & metal)Cermets contain molybdenum carbide, niobium carbide and tantalum carbide.

  • Cubic boron Nitride ( CBN ) :Made by bonding (0.5-1.0 mm) Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under pressureWhile carbide provides shock resistance CBN layer provides high resistance and cutting edge strengthCubic boron nitride tools are made in small sizes without substrate

    Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

  • Silicon-Nitride based ceramics (SiN)They consists various addition of Aluminum Oxide ythrium oxide, titanium carbide

    SiN have toughness, hot hardened & good thermal shock resistance

    SiN base material is Silicon

    High thermal & shock resistance

    Recommended for machining cast iron and nickel based super alloys at intermediate cutting speeds

  • Diamond :Hardest known substance Low friction, high wear resistanceAbility to maintain sharp cutting edgeSingle crystal diamond of various carats used for special applicationsMachining copperfront precision optical mirrors for (SDI)Diamond is brittle, tool shape & sharpened is importantLow rake angle used for string cutting edge

  • Polycrystalline-Diamond ( PCD ) Tools: Used for wire drawing of fine wiresSmall synthesis crystal fused by high pressure and temperatureBonded to a carbide substrateDiamond tools can be used fir any speed Suitable for light

Search related