Powder Metallurgy

146 Pages·1965·3.014 MB·English

by  S. A. Tsukerman (Auth.)

POWDER
METALLURGY
BY
S. A. TSUKERMAN
TRANSLATED FROM THE RUSSIAN
BY
R O B E RT E. H U NT
AND
H. S. H. M A S S EY
TRANSLATION EDITED BY
A. R. E N T W I S LE
Lecturer in Metallurgy,
University of Sheffield
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Copyright © 1965
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First edition 1965
Library of Congress Catalog Card No. 64-19363
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INTRODUCTION
POWDER metallurgy—a young but extremely important branch
of modern technology—concerns the production of metal powders
and of the various articles made from them. Powder metallurgy is
sometimes called metal-ceramics, because the processes used in the
manufacture of articles from powders resemble those used in
ceramic production, but this does not mean to say that metallo-
ceramic materials are necessarily obtained by a combination of
metals and ceramics.
The essence of powder metallurgy is as follows: a mixture,
composed of specially selected and prepared powders, is compressed
in dies under pressures of 10-100 kg/mm2. The half-finished object
obtained (the pressing) has a strength which, although insufficient
for the article to be used, permits transportation to the next techno-
logical operation. The final mechanical strength of the material
is achieved only as a result of a high temperature treatment—sinter-
ing, which is conducted at a temperature below the melting point
of the basic metal which goes into the mixture (66-75 per cent of
melting point). This basic technological scheme contains a number
of variations which are examined under the appropriate chapter
headings.
The development of powder metallurgy is due to its great advan-
tages over other methods in certain applications. The powder
metallurgy process has provided a practical solution to the problem
of producing refractory metals, which have now become the basis
of making heat-resistant materials and cutting tools of extreme
hardness. It is impossible on an industrial scale to melt such mater-
ials and produce articles from them by normal methods, because it
is difficult to select a furnace lining which would not melt at high
temperatures (for example, the melting point of tungsten is 3400°C)
or which would not react with the fused metal or compound.
Powder metallurgy alone enables alloys to be made from metals
insoluble in each other, which because of liquation cannot be
melted by ordinary methods. This is the case if the melting points
of the metals constituting the alloy are very different, for example
vii
viii INTRODUCTION
tungsten and copper (3400°C and 1083°C), iron and lead (1535°C
and 327°C) etc.
It is also possible to produce materials which contain non-
metallic as well as metallic components, and also materials and
articles composed of two (bimetals) or several layers of different
metals. One of the interesting applications of powder metallurgy
is the incorporation of non-metallic components into a metal base,
something which cannot be accomplished by other means.
A special advantage is the possibility of obtaining porous materials
with a controlled porosity, which is impossible to achieve by melting
and casting.
Modern technology is inconceivable without powder metallurgy
products, the various fields of application of which expand every
year (Fig. 1). Thus metal parts for electric lamps and radio valves
are made from the powders of refractory metals—tungsten, molyb-
denum and tantalum. Modern cutting tools made from hard alloys,
obtained by means of powder metallurgy, have caused a real revolu-
tion in metal cutting processes, and in mining; the speed of machin-
ing metals has increased by ten times. In industry various anti-
friction materials as well as porous bearings, filters and many other
articles are successfully being used. Powder metallurgy enables
articles to be made from both the heaviest metals (tungsten, uran-
ium) and the lightest (beryllium, porous aluminium). From metal
powders we can produce materials with a wide range of properties.
With powdered iron, we can, depending on the technology, obtain
materials with mechanical properties which correspond to cast
iron, bronze or even lead.
Besides this, powder metallurgy makes it possible to obtain
metals of high purity (uncontaminated by material from the furnace
lining or by deoxidizers), unlike the metals produced by con-
ventional processes.
Another essential advantage of powder metallurgy is its ability
to provide finished parts without machining. This superiority is
especially evident in mass production, because here a number of
economic advantages are gained; an increase in output, a lower-
ing of labour costs, the release of significant numbers of machine
tools, the absence of loss of metal in machining scrap (which make
up from 20 to 80 per cent of the weight of metal in other processes).
Thus, for example, in the manufacture of gears by powder
metallurgy, the technological process consists of simple and labour-
INTRODUCTION ix
FIG. 1. Range of uses of powder metallurgy products.
saving operations; obtaining the powder pressing, sintering and
sizing. In the case of the manufacture of such a gear by casting and
machining, the blanks made of cast iron undergo machining pro-
cesses which consist of drilling the hole, turning to external and
internal diameter, machining the key way and teeth, and finishing;
X INTRODUCTION
moreover the last two operations are extremely labour-consuming.
A diagram is shown in Fig. 2 comparing the technology of the
manufacture of the gear by the usual method of machining and by
that of powder metallurgy. The manufacture of 1000 gears by the
usual method requires about 30 hr work by a skilled worker, whilst
manufacture by powder metallurgy takes only 10 hr work in all,
by a semi-skilled worker.
The drawbacks of powder metallurgy, which render its use
difficult and restricted, ought to be mentioned alongside its advan-
tages. Among the fundamental drawbacks must be placed the high
cost of metal powders (especially so since in the manufacture of
articles from metal powders, the impurities contained in them go
over into the article, and therefore it is necessary to use powders of
the highest purity), and the lack of simple methods of obtaining
alloy powders—of steels, bronzes, brasses, etc. Articles made out
of metal powders possess, as a result of their porosity, an increased
tendency to oxidation, moreover oxidation may occur not only on
the surface but also throughout the whole body of the article.
Powder metallurgy products possess comparatively poor plastic
properties (impact strength, elongation). It is unprofitable to
manufacture articles in small quantities on account of the high cost
of the dies. Because of specific difficulties in pressing powders, the
size and shape of the articles produced are limited.
The role played by powder metallurgy in technology is extending
more and more. The output of powder metallurgy products at the
present time is not large—about 0*1 per cent of the world production
of metal, but this figure does not give a true idea of the importance
of powder metallurgy in technology, both from the point of view
of the quality of the materials and articles produced, and of ex-
penditure of metal on one unit of production. Thus, 1 kg of articles
made from iron powder is equivalent to 2-4 kg of cast metal (due
to the absence of loss of metal in machining scrap, and to a lower
specific gravity, etc.); 1 kg of hard alloys (for cutting or pressing
tools) replaces 10 kg of high-alloy tool steel.
INTRODUCTION xi
FIG. 2. Manufacture of a gear:
left—by machining; right—by powder metallurgy.
CHAPTER I
HISTORICAL CONSIDERATIONS
IT CAN be said that powder metallurgy is as old as the pyramids
and as new as a present-day bomber.
In antiquity articles were sometimes made by hot forging a
sintered powder mass. Daggers ornamented with gold powder
were found in the tomb of the Egyptian Pharaoh Tutankhamon,
who lived in the fourteenth century B.C. There is evidence that the
Incas, even long before the discovery of America by Columbus,
made articles by sintering the powders of precious metals. The
temple at Delhi (India), built in the fourth century A.D., is decorated
with columns weighing 6-5 tons, made by forging hot pieces of
reduced iron, because during iron-ore reduction by carbon it was
impossible to reach the fusion temperature of iron.
However, all these materials and articles were obtained not by
the modern method of sintering previously compressed powders,
but by means of hot forging a sintered powder mass.
The first industrial application of modern powder metallurgy
methods was made by the prominent Russian scientist-metallurgist
Petr Grigor'evich Sobolevskii.
At the start of the nineteenth century in Russia, attempts were
made at using metals in industry which have high melting points.
However, the furnaces existing at that time proved to be unsuitable
for smelting such metals. Therefore, one had to obtain platinum,
for instance, of which the melting point is 1773°C, by roundabout
means—by melting arsenious alloys and subsequently eliminating
arsenic during a prolonged calcination in an oxidizing atmosphere;
in this way the arsenic lowers the fusion temperature of the platinum-
bearing alloys. But it was impossible to obtain useful amounts of
platinum by these imperfect means; moreover, arsenic is extremely
injurious to the health of the workers.
At that time, rich deposits of platinum were discovered in the
Urals. The Russian mint decided to coin its money from it. The
methods of obtaining malleable platinum were still unknown, and
1
2 POWDER METALLURGY
for the speedy development of the platinum industry it was necessary
to work out a technology for obtaining it. This problem was success-
fully solved by P. G. Sobolevskii with the help of V. V. Liubarskii.
P. G. Sobolevskii was born in 1781 into the family of a professor
of medicine and botany, and passed out of the Polish Noblemen's
P. G. Sobolevskii
Army Cadet School, but by 1804 he had left the Service. Possessed
of brilliant faculties, he worked successfully at solving various
technological problems. From 1816 until the end of his career,
Sobolevskii worked in the Department of Mining; up to 1824 in the
Department of Works, and from 1825 in the St. Petersburg School
of Mining (which afterwards changed its name to the Mining
Institute). Here, with his active participation, was founded a
HISTORICAL CONSIDERATIONS 3
"Combined Laboratory for the Department of Mining and Salt-
extraction, for the School of Mining and for the Central Mining
Chemical Depot".
Facing this institution were some extremely important and
interesting tasks: (1) the testing and analysis of ores, salts and all
minerals discovered in Russia; (2) experiments relating to the
washing and cleansing of ores, and to the extraction of salts and
other metallurgical operations. In this way the laboratory became
the first research establishment to link chemistry with the mining
and metallurgy industries—the first Russian scientific research
institute for mining and metallurgy.
One of the most weighty problems before P. G. Sobolevskii and
his laboratory was that of finding a method of purification and a
production technology for malleable platinum. At first, Sobolevskii
tried the method of melting platinum with arsenic (Akhard's
method). Convinced that it was not perfect, he made an attempt at
discovering a new way of making metallic articles.
Turning away completely from smelting platinum, Sobolevskii
took purified sponge platinum, obtained by the chemical processing
of natural compounds, packed it into a mould and compressed it
with a press, then he heated (sintered) the compressed product
and pressed it again. The result of this working was that the metal
changed in appearance and solid platinum products were obtained.
This is how Sobolevskii describes his method:
Almost all malleable metals known to us, except platinum, can be
refined by melting, which is sometimes repeated several times, so that
together with the refinement proper of the metal, it receives also the
malleability peculiar to it; but the refining of platinum does not give it
the slightest malleability, owing to the impossibility of melting it under
the most fierce heat of smelting furnaces, and that is why the working
of platinum, as well as its refining, demands special methods which
differ vastly from the metallurgical processes seen in the working of
other metals.
And further on:
The experiment . . . justified our expectations and presented us with
the most simple and sure means of turning refined platinum into a
malleable state. The method is described as follows: the refined