Review: IPMD 14th Edition 2010-2011, 17 pages, 11248 words 

Authors: Dr. Henri Pastor, Consultant, France and Dr. Leo Prakash, WTP Materials Engineering, Germany [1]

[1] WTP Materials Engineering, Konrad Adenauerstr. 27, D-72108 Rottenburg, Germany

Cemented carbides, or hardmetals as they are alternatively called, were first developed for wear resistance applications some 80 years ago. Despite the competition from new materials, WC-Co based hardmetals are still the preferred choice in terms of cost, tool life, and productivity.

Dr. Henri Pastor and Dr. Leo Prakash review some of the recent technical developments in WC-Co materials with specific focus on ultrafine and nano powder production, sintering, binder phases, gradient and functional materials, and new processing techniques to achieve complex shapes to extremely close tolerances.

Introduction

Cemented carbides or hardmetals are a mature class of powder metallurgical liquid phase sintered composite materials consisting of at least one hard and wear resistant phase (in the majority of cases this being tungsten carbide (WC) and a ductile and softer metallic phase from the Iron group of metals (mainly cobalt and its alloys).

The invention of cemented carbides dates back to the beginning of the 20th century, and is generally attributed to Karl Schröter who disclosed his invention in 1923 in a patent application [1]. The first products were wire drawing dies and these were quickly followed by cutting tools for the machine tool industry.

Cemented carbides today form the backbone of the tool manufacturing industry with such diverse applications as machining of metallic and non metallic materials, chipless forming (wire drawing, can tooling, forging, stamping, mill rolls, powder compacting punches and dies, high pressure dies and anvils), mining (oil well drill bits, rock drill bits), industrial nozzles (sand blasting, water jet cutting, painting, glue dispensers) industrial wear parts (dental and medical tools), paper, plastic and textile knives, guide rolls, seal rings, road planing knives, tire studs, forestry tools, earth moving and consolidation tools), and for functional applications (watch cases and bracelets).

The smallest carbide part weighing a fraction of a gram is the tip of a ball point pen (produced in billions per year) and the largest parts like rolls and dies weighing upto a ton (produced in hundreds annually). The current annual consumption of cemented carbide worldwide is estimated to be in the range of 60,000 tons (see Global Market Review, also published in the IPMD 13th Edition for more information), and the high growth rate in demand is being fuelled by China.

The 100th anniversary of the discovery of tungsten deposits in China was celebrated in 2007 and China boasts of about two thirds of all currently known ore deposits of tungsten in the world.

Cemented carbides are the material of choice in all applications where wear resistance combined with toughness and strength is required at ambient and elevated temperatures. Fig. 1 shows typical properties of cemented carbides in use today. A typical microstructure of a tungsten carbide cobalt bonded cemented carbide shown in Fig. 2 consists of the hard and wear resistant WC phase embedded in a ductile metallic matrix of cobalt...............

Further sections of this article include:

• Production technology of cemented carbides
• Development trends
• Coarse (macrocrystalline) powders
• New sintering techniques
• Design of new binder phases
• Gradient materials
• Surface modification of the hardmetal substrate for CVD coatings
• Composite materials
• Processing
• Outlook

Figures and Tables:

Fig. 1 Hardness, wear resistance and Fracture toughness of straight WC-Co cemented carbides as a function of WC grain size and Cobalt content (Courtesy Sandvik Hardmaterials)
Fig. 2 Typical microstructure of a WC-Co Hardmetal (Courtesy Ceratizit)
Fig. 3 Application range of straight cemented carbide grades (Courtesy Sandvik Hardmaterials)
Fig. 4 Empirical relationship between the FSSS value of WC powders and the extrapolated Hardness values of WC polycrystals [94]
Fig. 5 FESEM micrographs of a WC 4NPO-Powder and the corresponding microstructure of a 10wt% cobalt – 1% Chromium doped hardmetal [94]
Fig. 6 Classical methods for production of WC powder
Fig. 7 WC-10wt%Co nanocomposites powder produced by spray drying of liquid sources, subsequent hydrogen reduction and carburization, which enhances the WC particle refinement and homogeneous distribution of cobalt matrix. (Courtesy Nanotech, Korea)
Fig. 8(a) SEM of ‘polyol cobalt’ powders
Fig. 8(b) Polyol cobalt powders have very narrow grain size distribution
Fig. 9 Schematic diagram for the generation of highly-oriented plate-like WC grains in W+graphite+Co alloy
Fig. 10 Selection of solid carbide end mills. (Courtesy Zhuzhou Cemented Carbide Corp)
Fig. 11 Helical milling cutter made from TiC-WC with Ta, Nb, and Mo binder. (Courtesy Kennametal)
Fig. 12 Microdrills used for the machining of plastic circuit boards (PCB’s) (Courtesy Sandvik Hard Materials)
Fig. 13 Carbide tools used in mining, oil exploration and rock drilling. (Courtesy Zhuzhou Cemented Carbide Corp)
Fig. 14 Selection of WC-Co indexable cutting tool inserts. (Courtesy Zhuzhou Cemented Carbide Corp.)
Fig. 15 Comparison between calculated WC+free binder windows and experimental results (45) on WC- Ni-Fe-Co alloys with 20% binder metal.
Fig. 16 Influence of the morphology and grain size of prealloyed FeCoNi powders on the porosity of WC DS 60 – 7,5% FeCoNi Hardmetals [94]
Figs. 17-19 Properties of Ampersint MAP M 1800 bonded hardmetal in comparison to a WC-Co
Fig. 20 Properties of Ampersint MAP M 1800 bonded hardmetal in comparison to a WC-Co
Fig. 21 Fine grain size WC with NiMoCr binder used for sealing rings in corrosive environments. (Courtesy Sandvik Hard Materials)
Fig. 22 Sintered carbide balls used in ball point pens are subject to both wear and corrosion. The tiny balls are made from WC-TiC powder with Ni-Mo-Cr binder. (Courtesy Sandvik Hard Materials
Fig. 23 CVD coated carbide cutting tool insert with highly tensile gradient peripheral zone of only 20µm. This zone has a shock absorbing effect (Courtesy Ceratizit)
Fig. 24 Drill with cut away cross section of the welded carbide head [66]
Fig. 25 Carbon Differential controlled to tailor Hardness gradient [66]
Fig. 26 Microstructural details of the composite drill bit head [66]
Fig. 27 Drawing mandrel of DP carbide
Fig. 28 Examples of popular cutting tool geometries of parting inserts produced by PIM (Horn) [96]
Fig. 29 PIM coated cemented carbide bit for a drilling application (Kennametal- Widia ) [97]. The old drill geometry is shown at the bottom left of the picture and the new coated PIM insert in the middle. This insert can be reground, precision mounted onto the steel shaft , and provides for better chip control and optimal coolant flow. The optimised tool geometry decreases cutting forces by 20%
Fig. 30 Replaceable PIM coated cemented carbide drilling insert heads (Seco) [98]
Fig. 31 Precision pressed and sintered cemented carbide parts with dimensions and achievable tolerances are shown here [2]
Fig. 32 Examples of inserts pressed on CNC presses with bores pressed perpendicular to the pressing direction (Courtesy Ceratizit) [93]
Fig. 33 Example of a die fixture for producing carbide inserts with a bore perpendicular to the pressing direction as offered by Dorst [95]. The schematic on the left shows the punches in the pressing position, whilst the picture on the right depicts the ejection position
Fig. 34 Prototype Inserts with chipbreakers perpendicualr to pressing direction (Courtesy Ceratizit) [93]
Fig. 35 Coated PIM Milling tool insert (Courtesy Seco) [98]
Fig. 36 16 micron Thick PVD coating (Courtesy Oerlikon Balzers) [100]
Fig. 37 An example of the versatility of todays PVD coatings (Courtesy Oerlikon Balzers) [100]