Chapter (1)

CUTTING TOOL MATERIALS

TOPICS :

Introduction Carbon and medium

alloy steels High speed steels Cast-cobalt alloys Carbides Coated tools Alumina-based

ceramics

Cubic Boron Nitride Silicon Nitride

based ceramics Diamond Whisker-reinforced

tool materials

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% chromium10%-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 1550C

  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 alloyingElement Properties

Tungsten Increases hot hardnessHard carbides formed, improving abrasion resistance

Molybdenum Increases hot hardness Hard carbides formed, improving abrasion resistance

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

Vanadium Combines with carbon for wear resistanceRetards grain growth for better toughness

Cobalt Increases hot hardness, toughness

Carbon Hardening elementForms carbides

Cutting tool materials – HSS alloyingElement Properties

Tungsten Increases hot hardnessHard carbides formed, improving abrasion resistance

Molybdenum Increases hot hardness Hard carbides formed, improving abrasion resistance

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

Vanadium Combines with carbon for wear resistanceRetards grain growth for better toughness

Cobalt Increases hot hardness, toughness

Carbon Hardening elementForms 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 High hardness 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 (1200C) 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

Web site:

http://www.staff.zu.edu.eg/awafa/