By V. N. W O O D T he production of sound cupola-melted
blackheart castings is a series of processes which necessitates strict m etallurgical control throughout, otherwise m any difficulties will appear. M ost of these have root at the m elt
ing operation, and prevail right through to any m achining operations which m ay be necessary on the finished product. A lthough any Paper on this subject m ust necessarily introduce some m etallurgical considerations, the author will en
deavour to avoid any involved technicalities and present the P aper from as practical a viewpoint as possible.
Choice o f Raw Materials and Metal Composition
D uring the present times, the choice of the correct raw m aterials constitutes a problem in itself, and the careful utilisation of substitutes is a m atter which dem ands a great deal of atten
tion from the m etallurgist if he is to m aintain the quality of the product a t the standard pos
sible in norm al times and using standard materials.
The successful m anufacture o f blackheart malleable castings initially depends on produc
ing a white iron within narrow limits o f com position w hich are such th at a rather small variation in any one of its vital constituents is likely to introduce difficulties by either giving an iron too grey in character to be of any use as malleable iron or conversely a completely white iron having a deficiency in such elements as silicon and carbon. These are necessary in certain am ounts to prom ote satisfactory annealing treatm ent later in the process.
A typical com position for cupola-melted blackheart m alleable iron i s : —T.C, 3.1; Si, 0.7;
Mn, 0.50; S, 0.15; and P, 0.10 per cent.
The cupola was considered some years ago as being unsuitable for melting iron for m al
leable castings. It is now recognised, however, th a t a regular supply o f ho t m etal of the desired uniform ity can be obtained from the cupola for this purpose, provided that attention be given to every detail of cupola operation.
In the first place, owing to the lack o f lati
tude permissible in the chemical com position of
the iron, it is im portant th a t the charges of raw m aterials which generally consist o f hard white iron scrap, hem atite pig, steel scrap, small percentages of annealed scrap and ferro-alloys such as speigel and ferro-silicon, should be care
fully weighed in their correct proportion. One consideration at the present tim e m ay be the variation in the size o f steel scrap. One con
signm ent m ay consist of clean pieces o f sub
stantial dimensions, whilst others m ay be of small rusty variety. On no account should the type of steel vary from charge to charge, it being a fa r wiser plan to mix small and large together. This should assist in preventing ex
cessive oxidation, as would be the case if all small steel was used. As small steel would tend to be selectively m elted m uch quicker than the larger accom panying pieces of pig-iron and iron scrap, the foregoing suggestion should pre
vent different melting rates of the charges and consequent variations in the m etal from ta p to tap, especially if the well of the cupola is of small capacity. If a large mixing ladle or re
ceiver is used, variation from this source may not be so prevalent.
M anganese steel should be strictly avoided and care should be taken in the handling of machinery steel, w hich is often of the alloy type and m ay contain considerable am ounts o f chro
mium. The presence of small am ounts of chrom ium in standard malleable castings ren
ders them useless for the subsequent annealing process. T he chrom ium content of the metal when tapped should no t exceed 0.05 per cent.
Special care is necessary to ensure th a t the coke bed is m ade up to the same height for each successive melt. T oo high a bed will give a m etal o f too high a carbon content, and the fracture will tend to be grey, giving danger of prim ary graphite in the castings, which is a great source of m echanical weakness in the annealed product. On the other hand, a low bed will give rise to m etal of low carbon con
tent, which will generally be cold and sluggish in character. Even with the best of conditions, molten white iron for malleable castings has a short “ life,” due to relatively low silicon, carbon and phosphorus contents, and it is essential that 157
it should be transported and poured into the
M etal Consistency Controlled by Analyses and Fracture m etal thicknesses will give totally different fractures w hen poured w ith m etal o f the same
Annealing Furnace and Loading of Product A n annealing furnace o f the batch type con thick, well-fitting firebrick lined door.
T he castings are placed in round, square or
annealing furnace. Beneath the bottom po t a robust stool w ith good strong legs should be used. This serves two purposes, first, to facili
tate the handling of a full tier o f castings by allowing the prongs of the lifting gear on a petrol or electric truck to pass underneath, and secondly to enable the h o t furnace atm osphere to circulate beneath th e bottom pot o f the tier.
If the base o f the bottom po t were resting on the floor o f the furnace the tem perature o f this p a rt o f the p o t w ould lag behind the rest.
D ifferent types of castings require different means o f packing before annealing.
It is im portant th a t thin and intricate cast
ings should be carefully packed in some inert
rendered absolutely airtight by m eans o f an application o f luting mixture.
D uring the firing o f a furnace in the initial stages, the location of the zone o f com bustion is generally som ew hat better than the rem ainder o f the furnace, so judgm ent should be exercised in the distribution o f the weight o f product to be placed in the furnace. Tiers o f pots con
taining heavy castings o r small jobs with a large ratio of m etal weight to air space should be placed in the parts o f the furnace which are know n to receive the m ost heat. This precau
tion o f placing the m ost weight in the area of the furnace which receives the m ost heat tends to even out any tem perature differences likely
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F i g . 1.— C o m p l e t e l y A n n e a l e d M e t a l . U n e t c h e d , x 5 0 .
F i g . 2 .— C o m p l e t e l y A n n e a l e d M e t a l . E t c h e d , x 100.
m aterial such as burnt sand in order to pre
vent warping. M ore robust products may be loaded into pots w ithout packing m aterial p ro viding that sound pots and lids are used. It is also very im portant that the furnace atm o
sphere which at times m ay be of an oxidising nature, should not come into direct contact with the castings. To minimise this danger a liberal application of a strong w et luting m ix
ture, w hich m ay consist o f fine sand and intim ately mixed clay, is well rubbed into the joints form ed by the bottom o f each po t on the projecting flange o f the one directly underneath.
A sound and well fitting lid should cover the top po t o f the tier which in turn should be
to occur, particularly during the initial stages o f the firing. T he tiers should be loaded into the furnace so that the air space between each is even and sufficiently large to allow ample circulation of the furnace atmosphere.
F o r the best results, a soft rolling flame m ust be m aintained during the whole firing operation.
This is produced by using the m inim um am ount of air required in excess of th a t necessary for com plete com bustion o f the fuel. In addition it is essential to have as p a rt of the air used, an adequate am ount o f d raft through the fur
nace to allow flexibility o f flame control. The control should be carefully exercised by suitable flue dam pers, as too m uch d raft is likely to
159
cause a rather fierce flame which m ay result in ho t spots or zones. D u rin g the w hole anneal
ing process the tem perature m ust be strictly controlled by m eans o f reliable tem perature recorders.
Stages in the Annealing Cycle A nnealing of blackheart m alleable iron co n sists o f three essential stages.
(1) H eating-up Period - -A period in w hich to raise the fu rn ace to a j.redeterm ined tem pera
ture, know n as the “ at h ea t ” tem perature, generally betw een 850 and 950 deg. C., the choice depending up o n the desired class and quality of the product, an d to a great extent upon the average com position of the m etal.
T he tem perature gradient m ust n o t be forced, b u t m aintained a t such a rate th a t the heating is uniform th ro u g h o u t the furnace. F o r in
stance, w ith too rapid heating, the outside p o r
tion of a tier o f castings m ay be really hot, w hilst the castings in the centre m ay be com paratively cold.
T h e rate of beating is, of course, controlled by the size and capacity o f the furnace, and w ith large furnaces holding 40 tons o r m ore of m etal the tim e required m ay be in the region o f 36 to 40 Ins. or m ore. T h e rate of heating then, to the at heat ” tem perature, should be so gradual as to ensure thorough and uniform h eat penetration of the m ass o f pro d u ct in the furnace.
(2) Soaking Period.— The next stage is _Ip,.
m aintain th e fu rn ace at the required tem pera
ture fo r a sufficiently long period to prom ote and com plete first-stage graphitisation. This operation is the breaking up o f the massive cem entite or carbide o f iron, w hich is the hardest structural chief constituent o f white iron, into a softer m aterial, know n as pearlite, along with nodules o f graphite com m only know n as tem per carbon. T he length o f time required for this purpose depends up o n the control tem perature. T he higher the tem pera
ture, the quicker the reaction. A t 850 deg. C.
the period required fo r the conversion would be 60 to 70 hr., whilst at 950 deg. C. the re
action m ay be com plete in ab o u t 36 hr.
A finer-grained m etal is obtained by the use o f lower tem peratures, but the tim e factor in- io lv e d is a great disadvantage in quantity p ro duction w ork. W ith m etal o f suitable analysis, reasonably fine-grained fra c tu re s . can be obtained with higher annealing tem peratures, though excessively high tem peratures give poor, coarse-grained m etal.
(3) C ooling-D ow n Period.—T he third and final stage of the annealing process is the cool
ing-dow n period, during which second-stage graphitisation takes place. W hen the furnace
has been m aintained at the desired tem perature for the required tim e, the fuel is shut off, any dam pers closed, and all openings to the atm o sphere should be filled u p with refractory m aterial, thereby com pletely insulating or seal
ing off the furnace. T he rate o f cooling is ex
trem ely im portant, especially in the range 730 to 700 deg. C., and care is necessary to ensure th a t it does n o t exceed 7 deg. per hr. through this range, an d preferably slower th a n this rate.
F urnaces m ay be so sm all th a t the cooling rate is in excess of th a t desired, b u t it m ay be re
tarded by relighting the furnace when the tem peratu re has fallen to 730 deg. C., and
gradu-F i g . 3.— W h i t e I r o n . E t c h e d , x 5 0 .
ally reducing the fuel supply so th a t it takes up to 10 hrs. fo r the tem perature to fall from 730 to 700 deg. C. W ith the larger types o f fu r
naces this procedure is usually unnecessary, as the cooling rate is naturally slow, due to the larger bulk contained therein, bu t nevertheless the cooling rate should be carefully observed at the critical period as a precaution against leaking flues or faulty insulation. This slow rate o f cooling is necessary in o rd er to decom pose the pearlite, w hich consists of alternate bands of cem entite and ferrite. T he cem entite o f the pearlite contains all the residual com bined carbon, the latter being set free as tem per carbon in fine, evenly dispersed nodules and ferrite, id est carbonless iro n in w hich is dis
solved the m anganese and silicon. T h e
struc-tu ral com position of fully annealed blackheart m alleable iron, then, is a m atrix of ferrite to which the iron owes its ductility, and fine, evenly distributed particles of free carbon, know n as tem per carbon (Figs. 1 and 2). The structure of the m etal in the “ as cast ” or unannealed condition is th at of a norm al white iron (see Fig. 3), i.e., a cem entite-pearlite structure. To obtain the condition of m axim um strength and ductility, the annealing procedure m ust be given the correct attention.
Some Possible Failures after Annealing A fter annealing, a good blackheart malleable iron has a fine black fracture, as shown in Fig. 4 (1). This fracture exhibits a silky sheen when turned at certain angles to the light. H ow ever, a small num ber of waster castings may be found after annealing treatm ent, due either to undesirable m etal com position or to other
causes. D istorted castings are those which have usually been subject to an annealing tem perature high enough to render the material sufficiently plastic under heat to deform under their own weight, or the weight of other m aterial directly above. Such castings usually have a coarse fracture, as shown in Fig. 4 (2).
A white and hard fracture, after annealing, can be attributed to several causes, such as in
sufficient time at the annealing tem perature, or too low an annealing tem perature, due to either faulty pyrom etric equipm ent o r cold spots in a badly designed or controlled furnace. It may also be caused by very low silicon, alone or together w ith low carbon content, the lack of sufficient quantities of which, and especially in the case o f silicon, greatly retards the rate of graphitisation. M anganese also affects the rate o f annealing to some degree. li should be present a little in excess of that required to com bine with the sulphur to form manganese
sulphide; at the same time the excess should not be too great, otherwise under-annealing diffi
culties m ay be introduced due to the stabilising effect of manganese on carbide, especially d u r
ing second-stage graphitisation and possibly due to the introduction of manganese carbide itself.
The presence o f chrom ium even in very small am ounts, is capable of retarding graphitisation to such an extent that successful and complete annealing is virtually impossible.
A casting will sometimes be found which is weak and brittle, and exam ination of the frac
ture reveals a rather dull black interior, with a considerably lighter coloured lustreless edge, as is illustrated in Fig. 4 (3). A defective casting of this type is caused by metal which in the hard state was m ottled with prim ary graphite. A m icroscopical exam ination will show large segregations of graphite in the centre of the casting, many of which follow
Fi g. 4 .— (1) Go o d Fr a c t u r e; (2) Co a r s e Fr a c t u r e s u c h AS OBTAINED BY TREATM ENT a t Hi g h Te m p e r a t u r e s; (3) Me t a l i n c l i n e d t o b e Gr e y; (4 ) De c a r b u r is e d; (5) Pe a r l it ic; (6 ) Co a r s e Pe a r l- i t i c-l o w Si a n d C.
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the grain boundaries, whilst at the edge there will be a band of finer and m ore plentiful prim ary graphite. This structure is shown in Figs. 5 and 6. The prim ary graphite flakes greatly break up the continuity of the ferrite ground mass, causing a serious loss o f strength and malleability.
A nother defect, which at first sight appears to be similar to that of grey metal just referred to, is that o f décarburisation, which is the burn
ing or oxidising away of the surface carbon.
A closer exam ination of the fracture will show that the appearance o f the inside o f the casting is quite norm al but the outside layer is rather lighter and som ew hat steely in aspect as is shown in Fig. 4 (4). D écarburisation is always caused by the castings being subjected to oxi
dising conditions and the depth o f décarburisa
tion varies according to the severity of, and the time exposed to these conditions (Fig. 7).
W hen they are particularly severe the carbon
1 M
m ay be com pletely rem oved th ro u g h o u t cast
ings o f thin section, a n d in m any such cases there is oxide penetration into the iron as show n in Fig. 8, especially a t the grain boundaries at the edge o f the casting. T h ere are several causes o f décarburisation—em ploym ent o f air m uch in excess o f th a t required for com plete com bustion of the fuel, an d especially is this the case w hen pots are n o t airtight due to in
efficient luting, o r p rem ature failure o f the pot itself by the em ploym ent o f one w hich has insufficient m etal thickness to w ithstand the oxidising conditions. D écarburisation is gener
ally associated w ith a considerable am o u n t of scaling o f the castings an d after cleaning they m ay be appreciably “ off size.”
F i g . 5 .— G r e y M e t a l E d g e o f C a s t i n g . U n e t c h e d , x 5 0 .
Incidentally the life o f the annealing pots, w hich should allow for use several tim es, is considerably curtailed if constantly used in furnaces w here oxidising conditions prevail. To m inim ise the occurrence of this furnace con
dition, and to correct, if possible, w hen present, it is a wise precaution to take gas analyses or check with a p ortable furnace atm osphere analyser. Castings, w hich by reason o f their shape o r small thickness, are packed in sand o r other substances, m ay also suffer som e dé
carburisation if the packing m aterial contains any red oxide o f iron, and is no t com pletely b u rn t before use.
Cases som etim es occur w here the m etal has a tensile strength well above the average but
only a small am o u n t o f ductility. N odules o f graphite or tem per carbon m ay be plentiful and can be s e e D w ith the naked eye, giving a fine m ottled appearance to the fractu re an d th e m aterial is no t unduly hard. This type of m aterial, show n in Fig. 4 (5), is know n as- pearlitic— the hardest constituent o f the original white iron, nam ely, cem entite— has no t been wholly decom posed, an d som e o f the pearlite which contains a quantity o f the carbon in the com bined form still persists. This is show n in Fig. 9. A com m on cause o f this type o f frac
ture after annealing is too rapid a cooling rate through the critical tem perature range 730 to 700 deg. C. Also too high a m anganese con
tent along w ith low silicon an d carbon m ay
F i g . 6 .— G r e y M e t a l ; C e n t r e o f C a s t i n g . x 1 0 0.
be responsible as is illustrated in Fig. 4 (6).
Less frequently, exceptionally low m anganese and silicon will also p io d u c e pearlitic malleable.
Cases m ay occur w here the fracture is not com pletely pearlitic, bu t the pearlite m ay be present as a w hite rim running a t various depths near the edges o f the castings an d m aterial o f this description is often referred to as “ picture fram e ” m alleable, and the cause can often be traced to either very' high o r very low m an
ganese.
Behaviour of Various Structures in Machining Several o f these divergences from the norm al desired structure frequently give trouble when the castings are subjected to any m achining
operations. Theoretically, owing to the soft nature of blackheart m alleable iron and the presence of fine, evenly-distributed temper carbon, m achining should not introduce any difficulties; in fact, m achinability is invariably excellent. If the m etal is sound and correctly annealed, tool life should be long, and it should be possible to m aintain high cutting-speeds.
The presence o f tem per carbon in blackheart m alleable iron assists m achining both by break
ing up the chip and by acting as a lubricant
ing up the chip and by acting as a lubricant