0. Carbide Grade Design
What they need to know: In this lesson we are going to learn the design aspects of Carbide. How its made, what makes it tough and wear resistant.
Why it’s important to them: Carbide manufacturers produce various microstructures and compositions in carbide to gain the desired balance of hot hardness, toughness, and wear resistance.
Examples/stories:

1. Cutting Tool Materials
Superhards (PCD, PCBN)
SPEED Thermal
Deformation
Resistance
Ceramics
Cermets
Coated Carbides
What they need to know: Proper tool material selection requires the consideration of several key variables. These variables include the type of workpiece material, production requirements, surface finish and tolerances, the desired cost of the part, and the desired tool cost per workpiece machined.
Why it’s important to them: All tool materials have an appropriate application. The goal of a tool engineer is to recognize the properties that each tool material offers to select the correct tool for a specific application. This class will focus on the different types of cutting tool materials and their characteristics. You will learn how each material helps you choose the best cutting tool for a particular application.
Examples/stories:
Co-HSS
HSS
FEED, DOC, Interruptions (Fracture Resistance)

2. Cutting Material Market Share
Adv. Mtls
$.8 B ==> $1.8 B
CAGR = 9.3%
Total Market
$8.6 B => $12.1B
CAGR = 3.5%

3. Cutting Tool Development – A Systems Approach
Substrate
Deformation resistance, fracture resistance, thermal & mechanical shock resistance
Coating
Wear resistance, lubricity, reduced frictional heat, surface finish
Macro-geometry
Chip control, cutting efficiency
Micro-geometry (Edge Preparation)
Cutting force, surface finish
What they need to know: Cutting tools have 4 primary considerations when they are developed. They are Substrate, Coatings, Macro and Micro Geometries.
Why it’s important to them: One of the most important characteristics of a cutting tool is the type of tool material. Most people in the metal cutting industry tend to think of tool materials as separate categories without considering other options. In fact, all the various tool materials can be ranked or categorized according to the unique balance of hardness, toughness, and wear resistance, which enable the tool to effectively resist heat, abrasion, and fracture. Primarily, the properties of a tool material directly result from its chemical composition and structure.
Examples/stories:

4. Basic Substrate Elements
W Tungsten
C Carbon
WC Tungsten Carbide
Co Cobalt
Hard
Binder
What they need to know: Cemented tungsten carbides, contain a class of hard, wear-resistant refractory metals in which the hard metals are bound together or cemented by a soft and ductile metal binder. The binder is usually cobalt or nickel.
Why it’s important to them: Carbide manufacturers produce various microstructures and compositions in carbide to gain the desired balance of hot hardness, toughness, and wear resistance. In particular, the grain size of the tungsten carbide particles directly impacts these properties. Larger grains offer improved toughness, and smaller grains provide better wear resistance.
Examples/stories:

5. WC Co Basic Substrate Elements Cemented Carbide
(Hard carbide particles bound or cemented by soft binder such as cobalt) WC
The liquid phase sintering employed in the manufacture of cemented carbides melts the cobalt binder that draws the hard WC particles together resulting in full density. Co

6. Straight carbides WC-Co
Cemented Carbides
What they need to know: There are three basic groups of tungsten carbides.
Straight , Complex , and Cobalt-Enriched grades.
Why it’s important to them: Straight grades contain tungsten carbide with a cobalt binder and are designed for high-abrasion workpiece materials such as cast iron, nonferrous alloys, and nonmetals. Due to their percentage of tungsten carbide, the straight grades of cemented tungsten carbide have the greatest resistance to flank wear.
Complex grades consist of tungsten carbide, titanium carbide (TiC), tantalum carbide (TaC), and often niobium carbide (NbC) with a cobalt binder. Complex grades are designed for materials that yield long chips, such as steels. Complex grades are produced by reducing the tungsten carbide content to allow the addition of alloying materials. This increases hot hardness in the cutting tool materials but reduces abrasive wear resistance.
Cobalt-enriched grades contain larger amounts of cobalt concentrated at the cutting tool edge. They are used for finishing as well as general-purpose and rough-machining operations. Cobalt-enriched grades maintain a higher degree of resistance to thermal deformation and exhibit a significant increase in cutting edge strength.
Examples/stories:
Straight carbides WC-Co
(Cast iron &
Non-ferrous alloy
machining)
Mixed carbides
WC-(W,Ti,Ta,Nb)C-Co
(Steel machining)

7. Composition / Grain Size vs. Properties
5 - 12% Cobalt and 1-5 µm carbide grain size
Fine grained
(1 µm)
Coarse grained
(5 µm) 1
5 µm 5
12%
5 - 12% Cobalt
WC grain size
The machining performance of carbide cutting tool is closely related to its properties, which, in turn, are determined by their composition and microstructure.
Increasing the cobalt content or WC grain size increases the toughness of the tool material at the expense of deformation resistance. This tradeoff between toughness and deformation resistance is an important characteristic of cemented carbide tool material.

8. Hardness Determines resistance to abrasive wear
Higher hardness at higher abrasion resistance
Depends on:
Composition (primarily WC content)
Microstructure (WC grain size)
Measured by Rockwell or Vicker’s method
Hardness is an important mechanical property of a tool material and gives a measure of abrasive wear resistance. Abrasion resistance increases with increasing hardness.
Hardness is determined by composition (amount of hard WC) and microstructure (WC grain size).
In cemented carbides, hardness is measured by either the Rockwell A-scale diamond cone indentation test (HRA) or by the Vickers diamond pyramid indentation test (HV). Both tests are performed on a finely ground, lapped or polished planar surface placed at right angles to the indentor axis. The Rockwell A test employs a load of 60 kg, whereas a range of loads can be used in the Vickers test.

9. High Temperature Hardness
Hardness decreases steadily with increasing temperatures
Micro hardness based on 1kg load
Grades with medium WC grain size
A: WC-3Co B: WC-6Co C: WC-12(Ti,Ta,Nb)C-8Co D: WC-2TaC-12Co
% of Cobalt
A true measure of the resistance of the tool to plastic deformation in metalcutting operations can only be obtained by measuring hardness at elevated temperatures. Measurements of hardness over a wide range of temperatures are therefore valuable for cutting tool development.
The figure shows hot hardness data for a number of cemented carbide tool materials. In these examples hardness steadily decreases with increasing temperatures with no crossover from one material to another. However, some cemented carbide compositions may show a smaller rate of decrease with increasing temperature resulting in crossover.

10. Carbide inserts get their strength for their thickness
Compressive Strength
Measure of deformation resistance
Compressive strength of carbide is higher than most other materials
Like hardness, compressive strength decreases with increasing cobalt, increasing grain size, or increasing temperatures
One of the unique properties of cemented carbides is their high compressive strength. Uniaxial compression tests can be performed on straight cylindrical samples or on cylinders having reduced diameters in the middle to localize fracture.
Typical values of compressive strength of cemented carbides range from 3.5 to 7.0 GPa (0.5 to 1.0x106 psi).
Compressive strength decreases as %Co, WC grain size, or temperatures increase.
Carbide inserts get their strength for their thickness

11. Transverse Rupture Strength
TRS measures the bending fracture strength of carbides
TRS = f (composition, microstructure, porosity)
Excellent quality control tool
The most common method of determining the fracture strength of cemented carbides is the transverse rupture test. In this test, a rectangular bar is placed across two sintered carbide support cylinders, and a gradually increasing load is applied. Transverse rupture strength is determined from the dimensions of the test bar, the distance between the supports, and the fracture load.
TRS is a function of composition, microstructure, porosity and other surface defects in the specimen. The latter can introduce a large scatter in the experimental data. Nevertheless, it is an excellent quality control test.

12. Transverse Rupture Strength
TRS increases with increasing cobalt content and decreasing grain size.
There appears to be a good correlation between TRS and milling performance of the carbide tool. During milling, the tool is subjected to tensile stresses as it leaves the cut, and a material with high transverse rupture strength should be able to resist fracture under these conditions.
Straight WC-Co Alloys

13. Measures resistance to fracture Determines intrinsic tool toughness
Fracture Toughness
Measures resistance to fracture
Determines intrinsic tool toughness
Less sensitive than TRS to
Specimen size and geometry
Surface finish
Flaws such as porosity
Depends on:
Composition (more cobalt = higher toughness)
Grain size (coarser grains = higher toughness)
Fracture toughness is another indicator of the toughness of the carbide tool. It is less sensitive than the TRS to such extrinsic factors as specimen size, geometry, surface finish, and flaws.
Fracture toughness parameter (KIc) indicates the resistance of a material to fracture in presence of a sharp crack and thus provides a better measure of the intrinsic strength of the cemented carbide than the TRS.

14. Fracture toughness increases as Co% and WC grain size increase.
Shows the test setup consisting of a notched rectangular test bar with a sharp precrack that is loaded in a four-point bend loading mode. KIc is calculated from the critical load required for catastrophic fracture of the test specimen.
KIc of cemented carbides increases with cobalt content and with WC grain size. Cubic carbides (TiC, TaC, NbC,….), on the other hand, decrease the fracture toughness.
Fracture toughness increases as Co% and WC grain size increase.

15. KIc • k E • a e.g. Thermal Shock Resistance
Required in operations like milling
No laboratory test developed yet
Various empirical parameters used
KIc • k
E • a
e.g.
where KIc = fracture toughness
k = thermal conductivity
E = Young’s modulus
a = thermal expansion coefficient
Resistance to thermal shock is an important property that determines tool performance in interrupted cutting such as milling particularly in presence of coolant.
No laboratory test can consistently predict the thermal shock resistance of a tool material. However, there are several empirical parameters that provide an indication of thermal shock resistance of tool materials.

16. Strength/Toughness (Resistance to Fracture)
Engineered Substrates: Cobalt-Enrichment
Strength (toughness) of WC-Co tools is a strong function of cobalt content.
Strength/Toughness (Resistance to Fracture)
% Cobalt
The early coated tools were prone to catastrophic fracture when applied at high feed rates or in intermittent cutting operations. Engineered substrates were therefore developed to improve the edge strength of the coated tools.
This development, pioneered by Kennametal, involved designing an insert with higher cobalt at the periphery than in the bulk.
The concept of “cobalt-enrichment” was based on the recognition that the fracture strength or toughness of cemented carbide materials increases with increasing cobalt content.

17. Cobalt - Enrichment Technology
Higher cobalt (& lower or no cubic carbides) at the insert periphery gives high edge strength while the bulk with lower Co provides deformation resistance
Applicable to a broad range of machining
KC850 was the first cobalt-enriched tool that revolutionized the metalcutting productivity of coated cemented carbide tools. However, its use was restricted to lower machining speeds.
Subsequently, second generation cobalt-enriched tools were developed for higher speed applications.
The cobalt-enrichment concept is so commonplace now that cutting tool manufacturers are using it even for high speed finishing applications to assure insert edge security.

18. Cobalt - Enriched Tool
The microstructure of KC850 comprises stratified cobalt-enrichment with almost three times cobalt at the insert periphery than in the bulk.
The insert is CVD coated with TiC-TiCN-TiN (~10 µm total coating thickness).

19. Metalcutting environment: heat (thermal deformation)
Tool Material Design
Metalcutting environment:
heat (thermal deformation)
pressure (deformation, fracture)
wear (pure abrasion, chemical wear, notching)
interrupted cuts (thermal & mechanical cycling)
The first step in tool material design is understanding the environment the tool is subjected to. This environment is complex and results in various wear processes as indicated above.

20. Substrate Property Considerations in Tool Material Design
Turning / Drilling:
High speed finishing requires deformation resistant substrate
General purpose machining requires an optimum combination of deformation resistance and edge strength
Roughing operations require bulk toughness
The next few slides discuss substrate property considerations in various machining applications.

21. Milling: Substrate Property Considerations in Tool Material Design
Since milling involves thermal and mechanical cycles, the cutting tool is subjected to thermo-mechanical fatigue.
Low magnetic saturation alloys have higher resistance to crack initiation arising from thermal fatigue.
Higher cobalt contents impart resistance to mechanical fatigue.

22. Milling: Substrate Property Considerations in Tool Material Design
For finish milling, choose a substrate with high Transverse Rupture Strength (TRS).
For general purpose milling, select a tool with high cobalt and cubic carbides (dry milling) or no cubic carbides (wet milling).
Rough milling operations require straight WC-Co grades.

23. CVD-Coated Tools
CVD-coated carbides comprise ~70% of all coated WC-Co tools.
Employed in a variety of applications: turning, boring, threading, grooving, parting, and milling
Used in machining of carbon, alloy, and stainless steels, gray & ductile irons, and Ni-based alloys over a range of speeds and feeds.

24. CVD-Coated Tools with Cobalt-Enriched Substrates
These tools combine the wear resistance of hard coatings with edge-toughened substrate
Broad applications (medium to heavy roughing to semi-finishing operations)
Machining of carbon, alloy, and stainless steels, gray and ductile cast irons, and high temperature alloys
The majority of carbide cutting tools is CVD coated.
CVD coated tools are used in a wide array of machining applications and on a wide range of workpiece materials.

25. Medium-Temperature CVD Coated Tools
Microstructures of these tools show negligible eta phase at the coating-substrate interface.
Generally exhibits columnar microstructure.
Suitable for roughing to finish machining over a wide range of workpiece materials
The current CVD coated tools invariably contain TiCN layer deposited by MTCVD process because of the lack of eta phase at the coating-substrate interface. Besides, MT-TiCN possesses a good combination of wear resistance and toughness. As a result, MTCVD coatings are useful for a wide range of machining conditions and workpiece materials.

26. Lower cutting forces with sharp edged tools
PVD-Coated Tools
Advantages: Smooth, low-friction, fine-grained, and crack-free coating even over sharp edges with compressive residual stresses
Lower cutting forces with sharp edged tools
Reduced tool-tip temperatures
Finer workpiece finish
The sharp, tough PVD coated tools are particularly well suited to milling, drilling, grooving, threading, as well as profiling operations. They perform well on difficult-to-machine materials such as high-temperature alloys, titanium alloys, and austenitic stainless steels.

27. Coating Property Considerations in Tool Material Design
TiC: Applied by CVD. High hardness at lower temperatures but hardness decreases rapidly with increasing temperature. Good for abrasive wear resistance.
TiN: (CVD or PVD) Less hard than TiC. Often used as top coating layer for cosmetic appeal and used edge identification. Also used as interlayers between other coatings to refine grain size and possibly enhance adherence. Chemically more stable than TiC.

28. Coating Property Considerations in Tool Material Design
TiCN: Applied by the normal temperature or medium temperature CVD, or PVD process Intermediate in hardness between TiC and TiN. Has good balance of abrasion resistance and toughness.
Al2O3: Not as hard as Ti-based coatings at lower temperatures, but harder at higher temperatures. Has high chemical stability (excellent resistance to crater wear and notching).

29. Application of CVD Coated Tools
Turning 1045 steel with 2.5mm doc and 0.40mm/rev feed rate
(Tool life based on 0.25mm flank wear)

30. Typical workpiece materials: Carbon, alloy, and stainless steels
PVD-Coated Tools
Particularly useful in threading, grooving, parting, finish-turning, milling, and drilling operations
Typical workpiece materials:
Carbon, alloy, and stainless steels
hardened steels
high-temperature alloys
titanium alloys
The sharp, tough PVD coated tools are particularly well suited to milling, drilling, grooving, threading, as well as profiling operations. They perform well on difficult-to-machine materials such as high-temperature alloys, titanium alloys, and austenitic stainless steels.