Cemented Tungsten Carbides. Production, Properties, and Testing

406 Pages·1998·6.174 MB·English

by  Gopal S. Upadhyaya (Auth.)

Preface
Cemented carbides or hard metals are among the most critical mate-
rials in engineeriinngd ustries. They provtihdee majority of turning tools and
milling cutters and mining tools, apart from wear resistant structural parts.
The first book in the English language in this area was the translation of the
famous DaWihl's book, Handbuch der Hartmetalle (in German), published
by Her Majesty's Stationery Office, London, in 1955. The second book
entitled Cemented Carbides by Swarzkopf and Kieffer (Macmillan, New
York, 1960) was also the translation of a German book. An introductory
book entitled Principles of TungsteCna rbide Engineering (B. E. Vaughan)
was published in 1972 by Society of Carbide and Tool Engineers, BridgeviUe
(U. S. A.), but was meant for the users of carbide tools, rather than the
makers. At the same time a Russian book originally published in 1970,
Sintered Metal Carbides authored by N. Romanova et al., was translated
into English by MIR Publishers, Moscow. This was written as a textbook
for vocational schools. The book by K. .J .A Brookes, Cemented Carbides
for Engineers and Tool Users (1983), was adapted from his Worm
Directory and Handbook of Hard Metals and published by International
Carbide Data .U( K.). No doubt the book is very informative, but it does not
cover scientific concepts which lead to the enormous growth in new
compositions. The latest book entitled Cutting Tools (R. Edwards), pub-
lished bTyh e Institute of Materials, London (1993), is again more useful for
application engineers, and does not include materials science aspects. The
author of the present book for quite some time felt the need for a book on
tungsten carbide based hard metals ni order to cover the processing,
microstructure, and properties for the following groups of readers:
viii
Preface ix
(cid:12)9 Senior metallurgical/mechanical/materials engineering
students
(cid:12)9 Engineers and supervisors in cemented carbide industries
(cid:12)9 Researchers engaged in development of cemented carbides
(cid:12)9 Application engineers for cemented carbides.
As the audience is quite varied, the book was intentionally planned to
be written in a style so that readers may be selective in their approach.
The present book is divided into eighteen chapters which are written
in such a manner that readers with some basic knowledge of cemented
carbide can pick up any chapter independently. The introduction in Ch. 1
covers some of the historical aspects in development of cemented carbides.
Chapter 2 describes relevant crystal structure and phase equilibria including
the role of alternate binders in influencing WC-Co phase equilibria. Chapter
3 describes production of metals (tungsten, cobalt) and various refractory
carbides, the primary being tungsten carbide. In "Consolidation of Ce-
mented Carbides" (Ch. 4), technological aspects including sintering param-
eters have been described, while the next chapter covers "Sintering Behavior
of Cemented Carbides" from a fundamental view point. Microstructural
aspects of cemented carbides including their quantitative treatment are
essential to understanding the resultant sintered properties of this group of
materials. Chapter 6 discusses those ni detail. "Mechanical Behavior of
Cemented Carbides" (Ch. 7) describes the basic deformation features
including major mechanical properties. Subsequent chapters describe
magnetic properties (Ch. 8), wear and erosion (Ch. 9), thermal shock
resistance (Ch. )01 and corrosion and oxidation (Ch. )11 of cemented
carbides. Joining of sintered tools with other substrates has still not lost its
significance, even after the advent ofindexable inserts. Chapter 21 describes
some of its scientific and technological aspects. Chapter 31 covers "Testing
and Quality Control" which is important not only to production engineers,
but also to any dedicated experimentalist. The next chapter on "Classifica-
tion and Applications of Cemented Carbides" is to acquaint engineers and
researchers with the complexity of parameters which one confronts ni real
life situations. A feel of these aspects si essential in tailoring the composition
and processing of the products. Chapter 51 describes coatings (single or
multiple), which have greatly enhanced the life of indexable inserts. The
interface of coating and substrate, ot~en ignored so far, needs to be drawn to
a much deeper investigation. Fine grained cemented carbides although
discovered quite some time back, have, of late, emerged as premium
x Preface
products and have found their unique position. Keeping this in mind, their
treatment has been covered in a separate chapter (Ch. 16). Functionally
graded cemented carbides are just emerging, but attractive. More and more
cemented carbide scraps of different grades need to be efficiently sorted and
reclaimed. Apart from this, toxicity is a major problem, hitherto not paid
serious attention. The last two chapters cover those aspects.
Since a large number of references including very recent ones from
various sources were used in writing the book, the bibliographic significance
is also attached. The author did limit the scope only to bulk cemented
tungsten carbides, and topics like hard facing have been omitted. Titanium
carbide/carbonitride based cermets occupy their own distinctive, although
limited, position, but are beyond the scope of this book. All attempts, made
from time to time, to replace tungsten carbide by other materials have not
succeeded and cemented tungsten carbides are going to remain unchallenged
for many years to come.
Finally a word about the terminology. The author believes the term
cemented carbide si more scientific with good clarity, but for historical
reasons he is not in favor of discarding altogether the term hard metal. For
this reason, both terms are freely used ni the text without any bias. This is
also the practice in the current literature. It is hoped that the readers will not
be detracted by this position of the author.
The authorw ould like ttoh anvka rious organizations which permitted
reproduction of illustrations from their literature. Special thanks are due to
his two able past research students ni the field of cemented carbides, Dr. .S
K. Bhaumik and .D Banerjee, who assisted ni writing the first draft of some
of the chapters. The author is indebted to his friend Dr. Henri Pastor of
CERMeP, Grenoble, an authority on cemented carbides, for giving timely
support ni various ways. His special gratitude is to late Academician G. .V
Samsonov, under whose able guidance he learned the basics of refractory
carbides.
It is a matter of utmost pleasure that the book is published on the
occasion of 001 years at~er the synthesis of tungsten carbide by the ingenious
French chemist Moissan.
Kanpur, India Gopal .S Upadhyaya
August 1997
1
Introduction
The advent of cemented carbides or hardmetals began with the idea
of replacing costly diamond wire drawing dies for tungsten filaments. This
group of sintered materials (WC-Co) had outstanding properties of high
levels of hardness and wear resistance. The history of cemented carbides
began in Germany during the first World War, when K. Schroter of Osram
Studiengesellschaft succeeded in producing the alloy, probably following
a proposal by Dr. Franz Skaupy of the German Incandescent Gaslight
Joint-Stock Company. Kolaska has given the full details of the historical
perspectives. 11[ Table 1 summarizes some of the developments in ce-
mented tungsten carbides. 13[]2[
After the application of cemented carbides in drawing dies, attention
was drawn to better cutting tools and finally to a variety of wear parts and
machine components. Cemented carbides (hard metals) were introduced
to the market by Fried, Krupp of Germany in 1927 under the name Widia
(wie Diamant~like diamond) which survives today. This consisted of 6%
cobalt binder in WC. Although major discoveries were made in Germany,
many of the later developments took place in USA, Austria, Sweden,
Japan, and other countries. Presently, the German flagship Krupp Widia is
the Widia division of US Company Cincinnati Milacron. Swedish head-
quartered Sandvik Corporation, is now by far the largest carbide producer
in the world. In the former Soviet Union (now CIS), the progress in
Cemented Tungsten Carbides
cemented carbide production was also quite significant. There were
innovative developments in saving the scarce tungsten by introducing
other transition metal hard carbides.
Table 1. Developments of Cemented Tungsten Carbides MT
1923-25 WC-Co
13-9291 WC-TiC-Co
13-0391 WC-TaC(VC,NbC)-Co
8391 WC-Cr3C 2 -Co
1948-70 Sub-micron WC-Co
1956 W C-TiC-T a (Nb)C-C 3 r 2C -Co
1959 WC-TiC-HfC-Co
1965-75 Hot isostatic pressing (HIP)
1965-78 TiC, TiN, Ti(C,N), HfC, HfN and AI203 CVD coatings on WC-base hardmetal
1968-69 WC-TiC-Ta(Nb)C-HfC-Co
1968-69 WC-TiC-Nb(Ta)C-HfC-Co
17-9691 Thermochemical surface hardening
1974-77 Polycrystalline diamond on WC-base hardmetal
1973-78 Multi-carbide, carbonitride/nitride and multiple carbide/carbonitride/nitride/
oxide coatings
1891 Many thin coatings with A1ON (aluminium oxynitride)layers
1983-92 Sinter-HIP
1992-95 Plasma CVD diamond coating
1993-95 Coating complex carbonitrides
4991 Fine-grain WC/Co agglomerates in tougher WC/Co matrix
1994 Nanocrystalline cemented carbides
*All dates in this table are approximations.
Introduction 3
Modifications of Schroter's compositions by replacing a portion of
tungsten carbide with other carbides (in particular those of titanium,
tantalum) led to a major discovery, that such additions were essential for
cutting steel at speeds which provide economic advantages.
Schwarzkopf'sESl discovery that solid solutions of more than one carbide
are superior to individual carbides was the starting point of the develop-
ment of multicarbide cutting tools for the high speed machining of steel.
Simultaneously with the beginning of tungsten carbide based ce-
mented carbides, the attention of scientists was drawn towards tungsten
free alloys (TiC based cermets), mainly driven by the concern over sup-
plies of raw materials. The industrial commercial culmination of such
products came around 1960 after research by Humenik and Moskowitz at
Ford's Dearborn Research Laboratories, where nickel-molybdenum bonded
titanium carbide hard metals were developed. Such material, no doubt,
found increased technical interest due to the advantage of special applica-
tions, but, unlike WC-Co, they lacked the universality of their applications.
As more and more attempts were made to replace WC-based cemented
carbides, a still better appreciation of this group of hard metals was
realised. This clearly suggests the preponderance of tungsten carbide
among all carbides in the development of hard metals. The 14Erohtua
elsewhere in his monograph describes in detail what makes tungsten
carbide so special.
Historically mostc utting tools were of brazed construction. As labor
costs rose, it became cheaper to replace a precision clamped insert rather
than regrind a brazed tool. Further, insert life could be extended by adding
more cutting edges or comers to one or both sides of an insert up to six on
a triangle or eight on a square (indexable inserts). Nowadays moulded-in
chip breakers are getting more common, which are difficult to duplicate
after regrinding.
A milestone in the development of indexable inserts was the advent
of chemical vapor deposition (CVD) of TiC over WC-Co in the early
1960's. It was triggered in Switzerland by the development of a wear
resistant finish for steel watch cases, which was much cheaper than the
solid carbide scratch proof cases. Such coated cutting tools offered
improvements in cutting speeds and life and also extended the use of
indexable inserts into new areas. Physical vapor deposition and modified
CVD processes for coating soon came into the picture. The attributes of
both CVD and PVD were combined in the latter process (Plasma CVD).
The latest technique, however, is CVD of hard metal with pure diamond or
Cemented Tungsten Carbides
a 'diamond-like' amorphous coming. More than a decade has passed since
Matsumoto et 15t.la showed, by the hot filament method, the first techno-
logical approach to a simple diamond synthesis process, i.e., low pressure
diamonds. The industrial fabrication of such diamond-coated cemented
carbides was announced in the autumn of 1994 at the Automotive Tool
Exhibition Fair in Chicago by five companies. Although the coating
adhesion might not yet satisfy the stringent demands, it seems to be
reasonably good for quite a number of applications.
Although fine grained WC-Co (WC grain size below 1 ktm) ce-
mented carbides were introduced as far back as in 1946, more systematic
studies have recently been done. The alloys tend to partially replace high
speed steels for applications requiting high levels of hardness and wear
resistance. There is a strong tendency towards using even finer grades of
cemented carbides including nano crystalline grade. For a detailed histori-
cal perspective on this subject, one may refer to an excellent account by
]6[.sggifpS
Of late, new developments have been noticed in which fineg rain low
cobalt agglomerates habveee n embedded a in much tougher mixture of higher
cobalt, medium grain size carbide. Such product combines the wear resistance
of the f'me particles with the strength and shock-resistance of the 13t.xirtam
Coming to the application sector, about 67% of total hardmetal goes
into metal cutting tools. Mining, oil drilling, and tunneling industries
which take about 13%. The share by wood working and construction
industries are 11% and 9% respectively (Fig. .)1 Over a span of 21 years,
the demand for cemented carbide cutting tools has almost doubled. On a
global basis, about 2300 tonnes of hard metal were used for various tools
making the value on the order of 11 million D. Marks. It is interesting to
note that until the middle of the 1980' s, the share of high speed steels was
higher than that of cemented carbides. However, of late, the situation is
reversed with cemented carbides having 50% of the total market. High
speed steels have 45%, ceramics about 4%, and %1 for polycrystalline
diamond (PCD) and cubic boron nitride 17t.)NBC(
Although cemented carbide production is now 70 years old, the
secrecy of its details is still attached to individual multinational firms who
claim to compete with one another, sometimes overzealously. Brookes MT
correctly had a dig on some producers who are not shy about citing pseudo-
scientific properties in their advertising campaign. It is perhaps because of
this secrecy that the unique ISO grading ofc emented carbides was framed
so that no manufacturer' s ISO grade is exactly same as another' .s It merely
Introduction 5
gives the user an idea where the grade should perform in the hardness and
toughness scale.
8000
6000
:::.'.:
E
O
(cid:12)9 4000
s:
2000
~working dna
scitlalP
1979 1982 1986 1989 1991
Figure 1. Share of cemented carbide applications in various industries.
In summary, it can be undoubtedly said that the significance of
cemented carbides still remains undiminished. The advent of other super-
hard materials could still not dilute the impetus of the basic and applied
investigations on tungsten carbide based cemented carbides. The emphasis
is now more on techno-economic gains and a still longer tool life coupled
with efficient methods for reclamation.
In conclusion, one may be reminded of the comments of Schr6ter[9]
as far back as 1934, which are still relevant today:
During the past few years industrial inventions have resulted
less and less from inspiration, a sudden fortunate idea, or
even chance. New industrial methods are usually discovered
only after the expenditure of great effort and the application
of every investigative faculty. Hard metal carbide is no
child of chance.
Cemented Tungsten Carbides
REFERENCES
.1 Kolaska, H., Powder Met. Int., Vol. 25, p. 113 (1992)
.2 Exner, H. E., int. Metals Reviews, No. 4, .p 941 (1979)
.3 Brookes, K. J. A., Metal Powder Report, Vol. 50, No. ,21 .p 22 (1995)
4. Upadhyaya, G. S., Nature and Properties of Refractory Carbides (in Press)
.5 Matsumoto, S., Sato, Y., Tustisimi, N., and Setaka, N., J. Mat. Sc., Vol. ,71
p. 3106 (1982)
.6 Spriggs, G. E., Int. .J of Refractory Metals and Hard Materials, Vol. ,31
p. 142 (1995)
.7 Kolaska, H., and Grewe, H., Powder Metallurgy of Hard Metals, Lecture
Series, European Powder Metallurgy Association, Shrewsbury, p. 6/1 (1995)
.8 Schwarzkopf, P., Deutsche Edelstahl Werke AG, German Patent 720502
(Patented 1929, issued 1942)
9. Gurland, J., and Knox, J. D., Tungsten and Refractory Metals 3, (A. Bose,
and R. J. Dowding, eds.), Metal Powder Industries Federation, Princeton,
.p 219 (1996)
2
C r y s t a l S t r u c t u r e and
P h a s e E q u i l i b r i a
The present chapter describes crystal structures of various phases
involved in cemented carbides. In addition, relevant binary and ternary
phase diagrams are also discussed.
1.0 CRYSTAL STRUCTURE
I.I Tungsten Carbide
In the W-C system, three individual carbides are established" W2C , WC
and ct-WCl_x. The modification ct-WEC was studied by Lander and Gemer. ]1[
A hexagonal type CdI 2 structure with lattice periods a = 2.992, c = 4.721A,
c/a = 1.578 and three atoms in a unit cell was established. The associated
structure of this type was confirmed by electron diffraction studies.t2]
Parthe and Sadagopant3] used the neutron diffraction method to
establish the structure of WC (group P612-Dah). t The interatomic dis-
tances in the structure of rhombic WEC are presented in Table .1 Yvon et
al. ]4[ used neutron diffraction to determine the position of carbon atoms in
WEC and established that the ordered structures of carbides quenched from