CUTTING TOOL MATERIALS Chapter (1) CUTTING TOOL MATERIALS

TOPICS : Cubic Boron Nitride Introduction Silicon Nitride based ceramics Diamond Whisker-reinforced tool materials Introduction Carbon and medium alloy steels High speed steels Cast-cobalt alloys Carbides Coated tools Alumina-based ceramics

Introduction: Characteristics of cutting tool : Hardness (resistance to wear) Hot hardness (capacity to retain hardness at high temperatures Toughness (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 alloys Carbides Coated tools Alumina-based ceramics Cubic boron nitride Silicon-nitride-base ceramics Diamond Whisker-reinforced materials

Carbon and Medium alloy steels : Oldest of tool materials Used for drills taps, broaches, reamers Inexpensive, easily shaped, sharpened No sufficient wear resistance Limited 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 HSS Molybdenum (M-series) Tungsten (T-series) M-series (6-6-4-2): Contains 6% molybdenum, 6% tungsten, 4% chromium, 2% vanadium & cobalt Higher, abrasion resistance H.S.S. are majorly made of M-series  T-series (18-4-1): Contains 18 % tungsten, 4% chromium, 1% vanadium & cobalt undergoes 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 alloys Commonly known as stellite tools Composition ranges – 38% - 53 % cobalt 30%- 33% chromium 10%-20%tungsten Good wear resistance ( higher hardness) Less tough than high-speed steels and sensitive to impact forces Less suitable than high-speed steels for interrupted cutting operations Continuous roughing cuts – relatively high g=feeds & speeds Finishing cuts are at lower feed and depth of cut

Carbides : (Hot hardness temp.: 1000 C) These carbides are also known as cemented or sintered carbides High elastic modulus, thermal conductivity Low thermal expansion 2-groups of carbides used for machining operations tungsten carbide titanium carbide

Tungsten Carbide Composite material consisting of tungsten-carbide particles bonded together   Alternate name is cemented carbides Manufactured with powder metallurgy techniques Particles 1-5 μm in size are pressed & sintered to desired shape in a H2 atmosphere furnace at 1550C Amount of cobalt present affects properties of carbide tools As 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 Element Properties Tungsten Increases hot hardness Hard carbides formed, improving abrasion resistance Molybdenum Increases hot hardness Chromium Depth hardenability during heat treat Some corrosion resistance Vanadium Combines with carbon for wear resistance Retards grain growth for better toughness Cobalt Increases hot hardness, toughness Carbon Hardening element Forms carbides

Inserts

Inserts Individual cutting tool with severed cutting points Clamped on tool shanks with locking mechanisms Inserts also brazed to the tools Clamping is preferred method for securing an insert Carbide Inserts available in various shapes-Square, Triangle, Diamond and round Strength depends on the shape Inserts honed, chamfered or produced with negative land to improve edge strength

Insert Attachment Fig : 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 Strength Fig : 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 chips Controlling chip flow during machining Reducing vibration & heat generated Selection 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 & costs Longer tool life

Coating materials Titanium nitride (TiN) Titanium carbide (Tic) Titanium Carbonitride (TicN) Aluminum oxide (Al2O3) Diamond coating Thickness range: 2-15 µm (80-600 μin) Techniques used : Chemical –vapor deposition (CVD) Plasma assisted CVD Physical-vapor deposition(PVD) Medium –temperature chemical- vapor deposition(MTCVD)

Properties for Group of Materials Fig : Ranges of properties for various groups of tool materials.

Cutting tool Characteristics for coating : High hardness Chemical stability Low thermal conductivity Good bonding Little or no Porosity Titanium nitride (TiN) coating : Low friction coefficients Resistance to high temperatures Good adhesion to substrate High 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 wear Ceramics are suitable materials for tools Al2O3 (most commonly used) Multi Phase Coatings : First layer –Should bond well with substrate Outer layer – Resist wear and have low thermal conductivity Intermediate layer – Bond well & compatible with both layers Coatings of alternating multipurpose layers are also formed.

Multiphase Coatings Fig : 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 µm.

Diamond Coated tools : Use of Polycrystalline diamond as a coating Difficult to adhere diamond film to substrate Thin-film diamond coated inserts now commercially available Thin films deposited on substrate with PVD & CVD techniques Thick films obtained by growing large sheet of pure diamond Diamond coated tools particularly effective in machining non-ferrous and abrasive materials

New Coating materials : Titanium carbo nitride (TiCN) Titanium Aluminum Nitride(TiAlN) Chromium Based coatings Chromium carbide Zirconium Nitride (ZrN) Hafnium nitride (HfN) Recent developments gives nano coating & composite coating Ion Implementation : Ions placed into the surface of cutting tool No change in the dimensions of tool Nitrogen-ion Implanted carbide tools used for alloy steels & stainless steels Xeon – ion implantation of tools as under development

Alumina-Based ceramics: Cold-Pressed Into insert shapes under high pressure and sintered at high temperature High Abrasion resistance and hot hardness (1200C) Chemically stable than high speed steels & carbides So less tendency to adhere to metals Good 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 carbide cermets(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 pressure While carbide provides shock resistance CBN layer provides high resistance and cutting edge strength Cubic 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 resistance Ability to maintain sharp cutting edge Single crystal diamond of various carats used for special applications Machining copper—front precision optical mirrors for (SDI) Diamond is brittle, tool shape & sharpened is important Low rake angle used for string cutting edge

Polycrystalline-Diamond ( PCD ) Tools: Used for wire drawing of fine wires Small synthesis crystal fused by high pressure and temperature Bonded to a carbide substrate  Diamond tools can be used fir any speed Suitable for light un-interrupted finishing cuts To avoid tool fracture single crystal diamond is to be re-sharpened as it becomes dull Also used as an abrasive in grinding and polishing operations

Whisker –reinforced & Nanocrystalline tool materials New tool materials with enhanced properties : High fracture toughness Resistance to thermal shock Cutting –edge strength Hot hardness

Whiskers used as reinforcing fibers : Examples: Silicon-nitride base tools reinforced with silicon-carbide (SiC) Aluminum oxide based tools reinforced with silicon-carbide with ferrous metals makes SiC-reinforced tools Progress in nanomaterial has lead to the development of cutting tools Made of fine grained structures as (micro grain) carbides

Cutting-Tool Reconditioning When tools get worned, they are reconditioned for further use Reconditioning also involves recoating used tools with titanium nitride

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