By C. H. KAIN
This P aper is intended to be a general descrip
tion of the production of carbon steel castings, and is divided into seven headings : — (1) G eneral considerations; (2) preparation o f liquid steel;
(3) pouring practice; (4) m oulding and core prac effect upon structure. It increases the solubility of iron fo r the CO and C 0 2, which are always
region of 1,500 deg. C. Some alloy steels (e.g., m anganese steel) have a carbon content over 1.0 per cent., b u t even these have a m elting point of over 1,450 deg. C. G ood quality cast iron is in the region of 1,250 to 1,350 deg. C.
(phosphoric irons are m uch lower). In each case it is necessary to superheat to enable the
necessary to m ake such extensive provision for feeding the steel.
Preparation o f Liquid Steel
T he m ethods of preparing the liquid steel may be divided into (a) m elting processes, such as the crucible and high-frequency furnaces, in
Fi g. 3 .— On e- To n Co n v e r t e r i n Bl o w i n g Po s i t i o n.
w hich no m ajor change in com position takes place, and (b) conversion or refining processes where im pure metal, such as pig and scrap, is refined to the quality desired. O f these, the m ost im portant conversion processes are the converter (Bessemer), the acid open-hearth (Siemens M artin) and the acid electric furnace.
O f these the only tru e refining furnace is the
TIM E
/
'MWS/Fi g. 2 . — Ge n e r a l Co n s t r u c t i o n o f Hi g h- Fr e q u e n c y Fu r n a c e.
liquid m etal to be handled and poured. This is usually 80 to 120 deg. C., which m eans th at steel is poured at a tem perature between 1,550 and 1,700 deg. C. T h e diagram shows clearly the short freezing range o f steel, and why it is
Fi g. 4 . — Re d u c t i o n o f Ox i d i s a b l e El e m e n t s.
basic electric, as it is the only furnace which can rem ove both sulphur and phosphorus.
Crucible Process
T he crucible process as used in the steel foundry differs little from th a t in use in the 172
Fi g. 1 .— Ir o n/ Ir o n Ca r b i d e Eq u i l i b r i u m Di a g r a m.
non-ferrous foundry. Forced draft is usually used when coke firing, but gas-fired furnaces are often employed. The process, w hether with clay or plum bago crucibles, is acid in character and very small changes in com position take place. C arbon and m anganese are usually som ew hat reduced by oxidation, silicon often
Fi g. 5 .— Ol d Ty p e o f Tw o- El e c t r o d e Th r e e- Ph a s e Fu r n a c e.
increases by the reduction of silica from the crucible, and there is a slight pick-up of sulphur and phosphorus. If the raw m aterials are care
fully selected, crucible steel is of excellent quality.
High-Frequency Process
Fig. 2 shows the general construction of the high-frequency furnace. The m etal is contained in a crucible and heating is obtained by induced current in the metal itself. This is generated by a water-cooled copper coil surrounding the crucible, which was originally of clay, but is now usually ramm ed in place and fritted by the melt. The process may be acid or basic, the basic method being essential for alloy steels containing a high proportion of manganese.
Fi g. 6 .— Op e r a t i n g Ge a r o n La d l e, w i t h La d l e In v e r t e d f o r Pr e h e a t i n g.
Converter Process
The charge of scrap and pig is usually melted in a cupola, but in the “ Stock ” process, m elt
ing takes place in the converter itself, the molten m etal containing: C, approx. 3.0; Si, 1.0 to 2.5; M n, 0.3 to 0.8, and S and P, under 0.06 per cent.
Fi g. 7 .— Co n s t r u c t i o n o f La d l e Pl u g.
Fig. 3 shows a one-ton vessel in the blowing position, the air being introduced on the surface of the m etal at a pressure of 3 to 4 lbs. per sq. in. In the original Bessemer process the air was blown in through the bottom of the vessel at much higher pressure. At first Bessemer heated the vessel, but it was quickly
Fi g. 8 .— Id e a l Co o l i n g a n d Fr e e z i n g Co n d i t i o n s i n Pa r a l l e l a n d Ta p e r e d Si d e d In g o t s.
173
found that the oxidation of the C, Si and M n provided all the heat necessary. T he vessel m ay be lined w ith brick or a ram m ed m aterial, and great care is necessary in drying ou t and preheating. T he m ain reactions are as follow : —
0 2 + 2Fe -> 2FeO (heat)
2FeO + 2C -> 2CO f + 2F e (heat) 2CO + 0 2 -> 2C O z -f (heat and flame) 2FeO + M n -> 2M nO , (slag) + 2Fe (heat) 2FeO + Si -> SiO, (slag) + 2Fe (heat) M ost o f the heating com es from the oxida
tion of the silicon and m anganese, the tim e of the blow varying according to the am ounts present, and m ay be from 10 to 30 mins.
T he end of the blow is shown by the d rop of the flame at the m outh of the vessel, but there may be several “ false drops ” before the final end is reached. Fig. 4 shows diagram - m atically how the reduction of the oxidisable elements takes place.
Fi g. 9 .— Se c t i o no f “ Ke e l ’’- Ty p e Te s t- Bl o c k w i t h Pi p e f o r m e d t h r o u g h Ce n t r e.
A t the end o f the blow there is a b ath of highly oxidised alm ost pure iron. Excess oxidation is removed by the addition o f ferro m anganese, ferro-silicon, alum inium or ferro- titanium , and the carbon is adjusted by the addition of cupola m etal. T here is a slight gain of sulphur and phosphorus, due to loss of metal and oxidation o f the constituents.
Sulphur is widely controlled by the use o f the soda ash process in the cupola m etal, bu t it is essential to em ploy low phosphorus materials.
The m elting and conversion losses are heavy, and vary between 15 and 20 per cent, of the total m etal charged to the cupola. Only the acid process is used in the foundry.
Open-Hearth Process
This consists essentially o f a b road shallow bath of m etal heated by producer gas. Both the gas and the air for com bustion are
pre-heated by recuperation. T he charge consists of scrap steel and pig-iron.
In the acid process, the h earth is siliceous, and C, Si and M n are rem oved by sim ilar re
actions to those in the converter, except th a t the oxygen is obtained by the addition of iron ore instead o f from the air. In the basic process the hearth is of dolom ite, and a heavy lime slag is em ployed to carry the phosphorus. Very large tonnages are m ade by this process, furnaces varying from 5 to 200 tons capacity.
The tim e o f the m elt m ay be from 6 to 12 hrs.
The Electrode Furnace
This consists essentially of a steel fram e supporting a hearth in w hich the steel is heated by one o r m ore electric arcs, usually on the surface o f the m etal. Indirect arc furnaces are
Fig. 10.— T y p i c a l S t e e l C a s t i n g s h o w i n g S y s t e m o f R i s e r s .
widely em ployed for cast-iron and non-ferrous metals, but only on a lim ited scale for steel.
Fig. 5 shows an old type of two-electrode three-phase furnace. In this case the hearth form s the third phase connection. This con
struction can only be used in basic furnaces as siliceous m aterials are never conducting. On the o ther hand, basic h earth m aterials become conducting at high tem peratures.
M ost m odern furnaces are of the three-electrode type with rem ovable roofs for rapid charging. T he electrodes are suspended and balanced for easy control, and the m otion is usually hydraulic. M echanical screw and pinion elevating gear is very rarely used nowa
days.
In the acid process the hearth is siliceous, and the oxidisable elements are rem oved by re
actions sim ilar to the converter and open hearth. In the basic process the h earth is
usually o f dolom ite, and a heavy lime slag is carried. P hosphorus is rem oved in the oxidising period, after which follows the unique feature o f the basic electric process, the reducing slag.
It is usual to rem ove the oxidising slag before starting the reducing period, but occasionally a one-slag process is w orked. T he reducing slag is m ade up from lime, carbon (as crushed coke, coal, graphite, etc.), fluorspar and pow dered ferro-silicon. D uring this period the sulphur is removed into the slag as calcium sulphide, although some m ay pass into the atmosphere.
The reactions are complex, but the necessary conditions are readily recognised by the characteristic slag, which is white on cooling and evolves acetylene when wetted.
Other m elting processes such as rotary fu r
naces, pulverised fuel furnaces and oil furnaces are com ing into use, but the greatest output at
Fireclay : 1,650 to 1,750 deg. C.
Pouring Practice
T he difficulties o f pouring are caused by the very high tem perature and the very short freez
ing range of the metal. In addition, the slag is no t easy to control. A cid slags are usually viscous and h ard and basic slags very fluid and run easily with the metal.
Plain ladles can be used w ith acid steel if the lining is thick and adequately preheated.
It is usual to form a bridge of slag at the lip through which the m etal is poured. This bridge may be form ed by the addition of sand o r by using a fireclay brick. Plain ladles are not suc
cessful with basic steel as the slag cannot be controlled.
Fi g. 11.— Ty p ic a l St e e l Ca s t in g w i t h Ru n n e r a n d Ri s e r s.
present is by one o f the processes referred to above.
Refractories
The selection of suitable refractories is a difficult problem for the steelfounder. Table II shows the softening point of the com m oner materials from which it will be seen that these are only just above the tem perature of the steel.
T a b l e I I .— “ Softening Point ” of Refractories.
Max. safe working tem perature
1,545 deg. C.
Silica : 1,600 to 1,790 deg. C.
Qanister : 1,545 to 1,650 deg. C.
Dolomite : 1,600 to 1,700 deg. C.
In addition it m ust be borne in m ind that the figures given are ideal values, and in all probability com m ercial m aterials fall below these figures.
Fi g. 12.— Ty p ic a l St e e l Ca s t in g w i t h Ru n n e r a n d Ri s e r s.
Teapot ladles are used successfully if the spout is large enough and if pouring from the ladle is frequent or continuous. Otherwise the teapot is no t very successful as the spout tends to freeze up.
The bottom pour ladle is in m ost general use, the metal being poured through the nozzle at the bottom of the ladle w hich is opened and closed by a refractory plug. Fig. 6 shows the m echanism of the operating gear and in the background can be seen a ladle inverted for preheating. Fig. 7 shows the construction of the plug or stopper. It is usual that the actual stopper end should be m ade of graphite whilst the sleeves are of fireclay supported on a steel rod. It is im portant that these should be free to expand during heating.
Shanks are also used for pouring small cast
ings in conjunction with a large ladle, but for this very hot steel is essential.
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M oulding an d C ore Practice
This is governed by the very high tem pera
ture and considerable shrinkage and contraction o f the metal. T able III shows the shrinkage frequently em ployed. These usually operate, either by low ering the m elting p o int o f the metal
very successfully, especially on large headers.
Figs. 10 to 14 show a num ber o f castings with the gates and headers in position. These show clearly the large am o u n t o f feeder m etal which is necessary to ensure soundness.
F ettling
T he trim m ing o f steel castings is a very expensive series o f operations. Sandblasting is essential ow ing to the close ap p ro ach o f the tinuously thickening envelope from the outside tow ards the centre an d th a t the head o f m etal used. Sometimes sandblasting is carried out before fettling begins b u t in som e cases it is
O ther operations such as chipping, grinding, 176
peening, etc., are sim ilar to iron practice and F o r stress-relieving the most im portant thing is are governed chiefly by the size o f the casting. to ensure slow cooling. F or annealing, normalis-Pneum atic ham m ers are in universal use, as are ing and quenching, it is im portant to be sure
Fi g. 13.— Ty p ic a l St e e l Ca s t in g w i t h Ru n n e r a n d Ri s e r s.
Fi g. 14.— Ty p ic a l St e e l Ca s t in g s h o w i n g Me t h o d o f Ru n n i n g.
high-speed bakelite-bonded wheels. Swing wheels are very widely employed but are ex
pensive in upkeep.
Welding
The repair of surface defects is often neces
sary where sand has fused or where some foreign m atter in the m oulding m aterials has
T a b l e V .—Heat-Treatment of Steel Castings.
Stress relieving.—Castings heated to 740 to 760 deg. C. and cooled slowly. ■ Full Anneal.—Castings heated to 880 to 950 deg. C., soaked for a suitable period, and cooled slowly.
Normalising.—Castings heated to 880 to 950 deg. C., soaked for a suitable period, and cooled in air.
A ir Quench and Temper.—Castings heated to 850 to 920 deg. C., soaked for a suitable period and cooled quickh in air. R eheated to 500 to 780 deg. C. and cooled (usually slowly).
that the desired tem perature is obtained throughout the furnace and that the castings are soaked for the correct period.
The eutectoid region of the iron-iron carbide equilibrium diagram in Fig. 16 shows clearly why these tem peratures are used. Table VI shows typical results from steels made to meet the first two grades of B.S.S. 592-1940. These
melted or blown. Also, in spite o f the most extensive provision for feeding, it is sometimes necessary to repair cavities in bosses and heavy sections. H ot tears can sometimes be made good but, more generally, torn or cracked cast
ings are scrapped because it is difficult to know the extent o f the defect. All the usual welding processes are in use, oxy-acetylene, direct arc and carbon arc, and the choice is often a m atter of personal preference, but the m odern tendency is towards the use o f covered electrodes only.
Heat-Treatment
H eat-treatm ent is carried out in all types of furnaces— coal, pulverised coal, gas, oil and electrical heating. The objects are: (1) To re
move stresses in the casting caused during cool
ing in the m ould and during subsequent opera
tions, and (2) to obtain maxim um properties in the metal itself. Table V shows the tem pera
tures usually em ployed for carbon steel castings.
T a b l e V I.— Laboratory Test Report of Properties and Co mposition.
Test No. A. 3074 C. 10552
Original dimensions—
Diam., in. 0.564 0.505
Area, sq. in. 0.25 0 . 2 0
Distance between gauge
points, in. 2 2
Yield point, tons per sq. in. 19.68 25.25 Max. stress, tons per sq. in. 31.40 44.60
Elong. 2 in., per cent. 36 26
Red. of area, per cent.
(
52.30 32.00
1 in. X J in. 1 in. x J in., Bend ... 130 deg. 90 deg.
unbroken. unbroken.
Izod im pact, ft.-lbs. 45 28
Analysis— Per cent.
C 0 . 2 1 0.44
Si 0.26 0.30
Mn 0.89 1 . 1 2
S 0.015 0.027
P 0.019 0 . 0 1 2
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figures are from electric furnace steel which has been norm alised. A som ew hat m odified heat- treatm ent w ould be necessary in the case o f converter o r open-hearth steels which co n tain slightly higher sulphur and phosphorus contents. T he properties show n here can be
considerably m odified in all types o f steel by suitable heat-treatm ent.
A cknow ledgm ents
T he author wishes to acknow ledge the assist
ance given to him by M r. T. W. Ruffle and Mr. C. E. R ayner in preparing the illustrations.
W ithout their help it w ould no t have been pos
sible to give this Paper.
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Fi g. 15.— Cu t t i n g He a d e r o f f Pu m p- En d Ca s t i n g.
Fi g. 16.— Ir o n/ Ir o n Ca r b id e Eu t e c t o e d Dia g r a m.
178