Applied Science in the Casting of Metals

516 Pages·1970·16.876 MB·English

by  K. Strauss (Eds.)

APPLIED SCIENCE IN
THE CASTING OF
METALS
Edited by
K. STRAUSS
Foseco International Ltd., Birmingham
PERGAMON PRESS
Oxford - New York • Toronto
Sydney • Braunschweig
Pergamon Press Ltd., Headington Hill Hall, Oxford
Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523
Pergamon of Canada Ltd., 207 Queen's Quay West, Toronto 1
Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay,
N.S.W. 2011, Australia
Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig
Copyright © 1970
Foseco International Limited
All Rights Reserved. No part of this publication may be re­
produced, stored in a retrieval system, or transmitted, in any
form or by any means, electronic, mechanical, photocopying,
recording or otherwise, without the prior permission of
Pergamon Press Ltd.
First edition 1970
Library of Congress Catalog Card No. 78-104787
PRINTED IN GREAT BRITAIN BY NEILL & CO. LTD., EDINBURGH
08 015711 4
INTRODUCTION
"APPLIED science in the casting of metals" is a comprehensive title implying
coverage of all the processes applicable to metallurgical operations. However,
this book is limited mainly to those processes in the development of which this
company has played a significant and pioneering part and a few explanatory
words as to how this title has been selected may be desirable.
The Foseco organization has been engaged in the research and development
of chemical products for the metallurgical industry since 1932.
A great deal of the fundamental work of investigation on which these products
are based has been carried out in the Foseco laboratories and the resulting
materials are available and widely used throughout the world. Most of the
research work which preceded these products has never been published in books
or technical journals.
The majority of this research work concerned the chemical treatment of
molten metals and alloys such as the oxidizing flux degassing technique for alloys
of copper and nickel, the removal of hydrogen from aluminium and its alloys
with hexa-chloro-ethane and other degassing agents and the grain refinement of
aluminium and magnesium alloys.
Since 1945, however, the basis of our activities has broadened and considerable
time and effort has been devoted to the development and practical application
of exothermic feeding aids for castings and ingots. Coatings for sand moulds
and cores, as well as gravity and pressure dies, for all metals, have also been
developed alongside sand additives for overcoming the incidence of sand defects
in castings.
Exothermic feeding was in its infancy in 1945 and the role played by fluorides
and other compounds in the heat generating reaction was not realized until
much research work in our laboratories clarified the chemistry of this rather
involved process. An extract from the report dealing with this investigation is
published here for the first time.
Considerable work has also been done on the inoculation of grey cast irons
and the production of high tensile and nodular graphite cast irons. In particular,
progress has been made in solving the problem of fading by applying inoculation
directly to the metal in the mould cavity.
vii
Viii APPLIED SCIENCE IN THE CASTING OF METALS
These points will explain the choice of title and why, for instance, "Chemistry
Applied to Metal Castings" would have been too restrictive.
Some information is included on the continuous casting of steel, vacuum
degassing of steel and the spray process of steel manufacture. This is not because
we claim any significant contribution to these developments but in order to
give an up to date survey of the technology now applied in the steel industry.
The short introductory chapter on steel-making by Professor W. Austin serves
a similar purpose.
We have tried to avoid the use of proprietary names of products, but in some
cases this was not possible, especially in the chapters dealing with exothermic
and insulating materials. The reason for this is, of course, that most of the tests
which we are now publishing were carried out with our own products and
results with similar competitive compounds are very rarely available. The
results are typical for these products only and that is why we use the names
under which they are known in the trade.
As apart from one or two exceptions, each chapter has been written by a
different author, there are obviously variations in style and presentation and
also slight overlapping in the contents of some chapters (especially in those
sections dealing with the feeding of castings). It was felt preferable to print
each author's contribution intact, even if this meant a certain amount of
repetition, thus avoiding constant references to previous or foregoing chapters.
All the chapters with the exception of that on corebinders, have been written
by members of our organization, who have considerable knowledge in their
own field. The chapter on corebinders was contributed by Messrs. E. Parkes,
G. Westwood and R. Grigg of Fordath Ltd., a company with which the Foseco
organization has very close ties.
Our thanks are due to all the authors who have contributed one or more
chapters to this book. Mr. J. L. Francis has not only written several of the
chapters, he has greatly assisted in the critical reading and editing of most of
the manuscripts.
Mr. M. L. A. Kanssen and Mrs. L. Bourn of our Publicity Department have
drawn or redrawn many of the diagrams and sketches. Mrs. R. Bassett of Foseco
(F.S.) Ltd. and particularly Mrs. B. M. Muldoon have assisted in proofreading
and the latter has also helped in compiling the index. To all of these our grateful
thanks.
CHAPTER 1
MANUFACTURE OF IRON AND STEEL
G. WESLEY AUSTIN
THE TREATMENT OF LIQUID IRON AND STEEL
The iron and steel industry is a traditional one with a history dating back
from ca. 1500 B.C.; it developed rapidly from 1856, in which year Bessemer
announced his pneumatic steel-making process based on molten pig iron.
Historically a malleable iron or steel was produced in the mushy state direct
from iron ore and charcoal in a small shallow furnace. The product was ham­
mered to consolidate the grains of iron or steel and to force out the slag. Such a
process can be regarded as a direct process. Later a pig iron containing up to 6 %
of elements other than iron was smelted in a taller furnace, provided with an air
blast. These furnaces were the forerunners of the modern blast furnaces. The
product from these furnaces was used for iron castings, or made into wrought
iron by a puddling process and used as such, or the bars were carburized and
then melted in a crucible, to an homogeneous crucible steel.
After 1856 the pig iron was converted into steel in a Bessemer, or later a
Thomas converter in which an air blast was blown through the molten iron to
remove the unwanted elements. The invention and use of regenerative gas-
fired furnaces followed, also that of electrically heated furnaces. These enabled
pig iron and scrap in various proportions to be melted and worked into steel.
The main production of steel, throughout the world, thus became mainly an
indirect process via pig iron; this route remains the standard route today.
Iron constitutes about 5 % of the accessible part of the earth's crust. Geolo­
gical processes have resulted in concentration of the iron-forming ore deposits
of various richnesses up to 70% iron. Much lower concentrations, down to
25%, have been and are still worked. The present world trend is to use rich
deposits, transporting them in very large ships thousands of miles across oceans
to the countries where they are smelted.
The ores are mainly oxides of iron and are acid in nature. A reducing agent is
necessary to remove the oxygen and it is usually coke. Because the gangue is
mainly acid and refractory, a basic flux, usually lime is used in smelting, forming
with the acid gangue a complex fusible silicate, the slag.
The location and size of steel-producing plants in a country is altering.
Formerly small smelters were built on or near the ore fields, or the supplies of
coking coal, or the industrial centres. Now new plants are built mostly on the
coast with port facilities for very large ships. The size of the individual plants
is growing rapidly. There are now over thirty plants in the free world each with
a production exceeding two million tons of steel per year.
1
2 APPLIED SCIENCE IN THE CASTING OF METALS
The figures for the production of iron and steel are very large. They are of a
different order from those of all other metals. They give in general an indication
for each country, of the economic power of that country. Figures for the steel
production of selected countries are given in Table 1. It can be seen from the
table that the world production of steel is still increasing. The rise is more rapid
in those countries which are not yet fully industrialized. The manpower em­
ployed in the industry represents a not unimportant proportion of the working
population, being in 1966, 575,547 in the U.S.A., 312,513 in Japan, and 284,640
in the U.K.
TABLE 1. Crude Steel Production—the World and Selected Countries
Country (000 tons) 1955 1960 1965 1966
Total C.E.C.A. 52,777 73,076 85,991 85,105
United Kingdom 20,017 24,694 27,483 24,704
United States 108,647 91,920 122,490 124,700
Japan 9,408 22,138 41,161 47,769
U.S.S.R. 45,271 65,292 91,000 96,900
World 270,000 333,500 446,100 459,500
Percentage share of some countries in world steel production:
C.E.C.A. 19-5 221 19 3 18-6
United Kingdom 7-6 7-5 6-2 5-4
United States 40-2 27-8 27-4 27-1
Japan 3-5 6-7 9-2 10-4
U.S.S.R. 16-8 19 8 20-4 21-1
World 1000 1000 1000 1000
Molten pig iron is produced in the blast furnace. The furnace outline is shown
diagrammatically in Fig. 1. The principal reaction, temperature and streams of
reagents and gases are also shown. It is a strong vertical refractory-lined, almost
cylindrical furnace. Ore, coke and flux are fed in at the top through an arrange­
ment of gastight doors. The air blast is heated in stoves up to 1200°C, and may
be enriched with oxygen. Hydrocarbons may be injected at a level above the
air inlets.
The hot blast on entering the furnace burns the coke, also heated in its
descent against the upward streaming hot gases, in a narrow zone to carbon
dioxide: C + 0 -► C0 . This gas is almost immediately reduced by the excess
2 2
incandescent coke present to carbon monoxide: C0 + C -> 2CO, the usual
2
gas producer reaction. The lower iron oxides are reduced by carbon, by direct
reduction: FeO + C -> Fe + CO. The oxides of manganese, phosphorus and
silicon more stable than those of iron are also reduced by direct reduction by
carbon: the proportions reduced are about two-thirds of the manganese, all
the phosphorus, and the silicon depending on the temperature and the acidity
MANUFACTURE OF IRON AND STEEL 3
of the slag. The sulphur in the iron depends largely on the total sulphur content
of the charge, the basicity of the slag and the temperature.
The hot gases, mainly carbon monoxide and nitrogen, pass upwards, carrying
out as far as carbon monoxide is concerned in counter-current, and at a tem­
perature which is falling as the gases ascend, the partial reduction of the ore by
indirect reduction: Fe 0 + CO -► 2FeO + C0 , and also: Fe 0 + 3CO -+
2 3 2 2 3
Fe + 3C0 . Overall the picture is that of the ore being gradually reduced
2 2
-+Exhaust combustible gas.
C0(27'/*J CO2(13'/.) N(A) to heat the blast etc.
- Prepared ore charge, coke fuel & flux.
3Fe2 03 * CO » 2Fe304 * C02
\ Fe3 04 +C0 ► 3FeO+C02
90ft
FeO+C- *Fe + CO
MnO+C —— *Mn+CO
P2 05+ 5C - H2 + 5 CO
1S i02 + 2CH-+ySdir oc+a r2bConOs
Hot air blast ca.1000°C
C+02 +C02 —*C02+C — *2C0
rj= +Slag
{ Si02. T CaO, JAl203MgO.Mn0.etc.
1350°C I— -+Pig iron -93 V. Fe
FIG. 1. The iron-making blast furnace.
with rising temperature in passing from the top of the furnace to the hearth.
The coke consumption is reduced as the proportion of the indirect reduction
increases: i.e. as the proportion of carbon dioxide in the waste gas increases.
The ore is prepared by crushing, sizing and sintering, to give a feed of uniform
size and increased reducibility. Using rich ores, fully prepared, hydrocarbon
injection, high-blast temperatures and very large furnaces (5000 tons/day) a
coke consumption of half a ton of coke, per ton of pig iron made, may be
obtained. The average good practice coke consumption is some 25 % above this
figure.
4 APPLIED SCIENCE IN THE CASTING OF METALS
The slag composition is very roughly one-third silica, one-third lime and one-
third alumina and magnesia. The more basic the slag, i.e. the higher the lime,
the more firmly it holds sulphur and so the lower the sulphur in the iron. But
this higher melting point slag necessitates a higher coke consumption, so the
sulphur of the iron is often allowed to rise to one-quarter of 1 % and is removed
from the iron by a special process outside the furnaces. The slag is used for many
purposes, road-making, cement, slag wool, granulated fillers, etc.
The irons may be classified summarily into non-phosphoric (hematite) in
which the phosphorus content is below 0-06%, and phosphoric, in which the
phosphorus is higher, often considerably so, being up to 2%. The No. 1, or
highest foundry grade grey iron, contains about 3 % silicon and the carbon is
mainly graphitic and it is comparatively soft: at \\% of silicon (the old forge
iron), the carbon is about equally graphitic and combined: below 1 % silicon
the carbon is mainly combined and the fracture is white. The latter iron is that
used in the main for the basic steel-making process.
The molten iron may be cast into pigs in sand beds or in casting machines,
or transported molten to the steelworks.
There are processes in use and under development using gaseous and oil fuels
for the direct production of sponge or pelletized iron, but these are based mainly
on local comparatively inexpensive supplies of gas and a suitable ore. Some
extension of these plants and processes is likely.
Scrap iron may be regarded as a very rich iron ore and may be added to the
process at any stage of the route from the blast furnace to the steel furnace.
It may also be melted and carburized to a first-class pig in a hot-blast cupola and
then converted to steel by almost any of the steel-making processes, but perhaps
most profitably by an oxygen-blown process.
TREATMENT OF MOLTEN IRON BETWEEN
BLAST FURNACE AND STEELWORKS
The molten iron for the steelworks is generally transported in refractory-
lined insulated steel ladles. The capacity of these varies from 50 tons to several
hundred tons. The very large ladles are usually torpedo shaped. The iron may be
so conveyed over long distances sometimes over the state railway systems. The
ladles of molten metal act as buffer stocks, or they may be emptied into a large
refractory-lined storage vessel, a mixer, often fired, of up to 2000 tons capacity.
Some desulphurization is usually a necessity in the case of irons made under
conditions of acid smelting and this may be carried out to some extent in the
transport by special ladles before the iron is poured into the mixer. The form­
ation of sulphides, insoluble in the molten iron, those of sodium, calcium,
magnesium and manganese, is the principle used. The commonest reagent added
to the ladle is soda, or soda + lime. This process, the Brassert process, is more
effective under reducing conditions as is the case with pig iron, 3^% carbon, but
not always so with steel. The main reaction is as follows: FeS + Na 0 + C ->
2
Fe + Na S + CO. Dispersion, agitation with intimate contact, is necessary
2
MANUFACTURE OF IRON AND STEEL 5
for maximum effect. If soda ash is employed the C0 evolved increases the
2
agitation. Blast-furnace slag should be excluded. Desulphurization below 0-04%
sulphur is difficult. The process is often carried out in two stages for better
desulphurization. It may also be carried out as a continuous process. Incidental
difficulties may arise with fuming and by serious attack on refractories.
Lime and soda (9:1) may be injected into molten iron with a carrier gas.
Excellent results have been obtained with this technique. Other reagents,
notably calcium carbide or calcium silicate may be used. (Note the presence of
the insoluble sulphide formed + the deoxidant.) Magnesium is used for the
desulphurization of iron in the production of nodular iron. It is generally used
as an alloy with nickel or in a pressurized ladle, to moderate the reactions.
The intimate contact between the reagent and metal provided by the carrier
gas technique may be obtained by mechanical means as in the shaking ladle
(Kalling and Eketorp) or by the centrifugal whisk which provides circulatory
pumping and mixing (Oesberg). The sulphur can be reduced by these means to
very low figures using lime and soda, or calcium carbide.
Desulphurization also takes place in the mixer where the iron sulphide/
manganese reaction occurs: FeS + Mn -> Fe + MnS. The manganese sulphide
separates in view of its low solubility in iron and passes into the mixer slag.
Some 30% of the sulphur may be removed in the mixer. The reaction also
occurs in the transport ladles.
Several reagents are thus available for the desulphurization of molten pig
iron and there are many methods of applying them. The use of magnesium is
expected to increase.
Some basic irons with a high silicon content are often desiliconized before
use in basic steel making. The process may be carried out by air or oxygen
blowing in an acid-lined converter, or oxygen may be lanced into the molten
iron which is contained in a ladle or rotary furnace.
THE MANUFACTURE OF STEEL
The main production of steel in the world is by indirect processes based on
pig iron, and by remelting the scrap arising in the steelworks, and also by
melting industrial scrap. The world production of crude steel is given for four
selected years in Table 1. The world output of pig iron was just over 70% of
these values so the difference is largely provided by scrap.
Given the range of chemical composition of the irons produced in the blast
furnace, and that of commoner steels, it is clear that a proportion of each
element must be removed in its conversion to steel, certainly from basic pig
iron. Table 2 displays these differences.
Since the unwanted presence of these elements in the pig iron is due to the
powerful reducing conditions in the blast furnace, the opposite conditions are
used for their removal, i.e. oxidizing conditions. The oxidizing agent employed
is oxygen, as gas, or combined as oxygen in iron ore, but mainly as ferrous oxide
in the slag. Scrap of about the composition desired in the finished steel may be
APPLIED SCIENCE IN THE CASTING OF METALS
TABLE 2
Pig irons Non-alloy steels
Hematite Basic Mild Medium
Carbon 3-5 3-5 015-0-25 0-45
Silicon 2-5 0-9 003 0-15-0-25
Manganese + 0-5 10 0-6 0-7
Sulphur 005 006 005 005
Phosphorus 005 1-5 005 005
melted directly in an electric furnace and oxidized or reduced as necessary.
Processes employing gas/metal reactions are very much faster than those in
which the oxygen is combined as iron oxide.
The heat of oxidation of the elements it is desired to remove, in the case of
oxidation with oxygen gas, are not only sufficient to raise the temperature of the
molten pig iron above that of steel ready for tapping, but also to melt a quantity
of scrap, which usually constitutes up to one-third the total charge weight.
Silicon can contribute a 300°C rise for 1 % of the element present, phosphorus
200°C and manganese 100°C. Some of the oxide metal reactions are as follows:
FeO + C ► Fe + CO
Fe 0 + 4C ► 3Fe + 4CO
3 4
Fe 0 + 3C ► 2Fe + 3CO
2 3
2FeO + Si - 2Fe + Si0
2
5FeO + 3CaO + 2P ► 5Fe + 3CaO P 0
2 5
Oxygen/Oxygen-Flux High Pressure Lance
LD,LD-OLP&C
Rotate Furnace
to fill Blow& Tap
Part Roof Absent in Surface
and for Kaldo
Blown Processes
FOS -&c
Oxygen & Fuel
Carbon Electrode
Arc Furnace. Injection 02 &<=
Flame Out to
Regenerator
Heated Air, (02)+Fuel
from Regenerator.O.H.
Charging Doors
Slag /Metal Interface Slag
Tap Hole •Metal
Electro-Magnetic Bottom (or Side) Blast
Stirring Pipes. Bessemer &
Thomas
FIG, 2, Converter or furnace for steel-making reactions.