THE CONTRADICTORY ASPECTS OF CERTAIN FACTORSAFFECTING THE QUALITY OF BRASS INGOT

FOR WORKING

M. Al. RAY

Indian Copper Corporation Ltd., Ghatsila

Abstract

Practical difficulties for attaining a sound brass

ingot have been outlined. The influences of casting

temperature on entrapping of gases and dross,

surface conditions of the ingot, crystallization,

primary pipe and presence of dissolved gases have

been outlined. The effects of rate of pouring and

different mould dressings on the quality of the

ingot are given. The importance of factors like the

rate of cooling and the prevention of wide pasty

zone has been stressed.

Introduction

N the course of the general discussion on

I papers read at a " Symposium on Metal-

lurgical Aspects of Non-Ferrous Metal

Melting and Casting of Ingots for Working "

held in London in 1949, it was remarked:"InIn order to get tranquil pouring you

dropped a stream of molten metal about

three feet through the air on to a rising liquid

surface in a mould. I also read of the neces-

sity for achieving a fine macrostructure, and

that in order to achieve that, one forced gas

from a flaming dressing through metal inorder to break up the structure and get a

fine macrostructure. It does seem to methat the methods do not smack of the scienti-

fic or of the precise." It is indeed true that

conventional methods and techniques ofcasting brass ingots appear to have been

guided by the dictates of practical conve-

nience rather than by metallurgical require-

ments for production of sound ingots. Thusfeatures have been introduced which are

contradictory to those necessary to obtain

ingots and sheets free from defects. Theposition has, however, been made worse by

the inevitable influence of factors whose

effects are contradictory to one another and

it is proposed to consider in this paper afew of these factors on which the quality of

a brass ingot depends. The consideration

will be entirely qualitative and will be res-

tricted to binary brass slabs for rolling intosheet or strip.

All brass foundry men experience difficulty

in minimizing the conflicting nature of thesefactors in spite of a sound metallurgical under-

standing of them and in spite of the vast

amount of research work done on the subject.

On a commercial scale it is impossible to

produce a really sound ingot, each foundryattempting to keep the unsoundness downto a minimum. In fact to quote again from

a paper read at the above-mentioned sympo-

sium 1: " a satisfactory casting maybe definedas one which is compositionally correct andwhich on processing by the appropriate

technique yields an acceptably high propor-tion of marketable material ". This is avery appropriate definition and clearly shows

the acceptance by metallurgists of the lack

of attainable perfection in the technique ofmanufacture of sound ingots.

Casting Temperature

Entrapping of Gases and Dross - The im-portance of the temperature at which brass

is cast cannot be over-estimated. When

molten brass is poured into a mould, thefalling stream produces an injector effect anddraws gas as well as the rapidly forming zinc

oxide into the mass of liquid metal. If

solidification is delayed until after the metalattains a tranquil state, the injected gas and

184

RAY-FACTORS AFFECTING QUALITY OF BRASS INGOT

zinc oxide, both of which are inclined to rise

rapidly to the surface, are allowed to escape.A melt solidifying quickly will increase thechance of entrapping of gases and dross in the

ingot. The temperature of the metal willaffect in the same way the gases liberated bythe combustion of ' flaming ' mould dress-ings which are very frequently used in brasscasting practice. An ingot cast at a lowtemperature is found on examination to con-tain numerous subsurface spherical cavities.An unsoundness of this nature reveals itselfwhen the ingot is rolled into sheet as that

characteristic defect known as a ` spill'which is only too familiar to a brass sheet

producer.Surface Conditions of Ingot - High tem-

perature pouring normally produces a cleansurface as it prevents folds and remelts

solidified globules of metal splashed on tothe mould- face.

Effects of Crystallization - Columnar andEquiaxial - On the other hand, the hot metalmaintains a steep temperature gradient be-tween the mould faces and the centre of

the casting, a condition favourable to colum-nar crystallization. This causes the crystalswhich are rapidly advancing from the twofaces to form bridges which in turn preventthe feeding of liquid metal and thus createinterdendritic or shrinkage cavities of seriousdimensions. Incidentally, while consideringtemperature gradients within an ingot mould

the importance of the vertical gradientshould not be underestimated. The effectof the steep lateral temperature gradient

mentioned here is dependent on the verticaltemperature gradient being less and in allsubsequent discussions on lateral temperaturegradients this relative aspect of the twogradients must be borne in mind. Lowtemperature pouring promotes undercoolingof the metal in the centre of the mould andequiaxed crystals begin to form between theadvancing planes of the columnar crystals

around numerous independent nuclei pro-vided by the suspended solid particles eitherinjected with the pouring stream or loosened

185

by erosion of the solidifying metal. The

formation of these equiaxed crystals, though

not undesirable, gives rise to small contrac-tion cavities at the grain boundaries. Suchcavities cannot be fed and produce a regionof unsoundness at the centre of the ingot.The columnar crystals are substantially freefrom shrinkage cavities except, as has alreadybeen pointed out, when they meet from op-posite directions. The unsoundness caused

by equiaxial crystallization is, however,less serious than that caused by the bridg-ing of columnar crystals, and from this

point of view, therefore, pouring at tempera-tures within reasonable limits lower than thecorrect temperature is to be preferred.

Primary Pipe - The steep lateral tempera-ture gradient caused by casting at a high

temperature increases the depth of the pri-mary pipe. The formation of the pipe canbe restricted by increasing the vertical

temperature gradient relative to the lateralgradient thus causing solidification to pro-ceed from the bottom upwards. This willbe discussed in detail later.

Dissolved Gases - The solubility of gases

in a molten metal increases with the rise intemperature. These dissolved gases are libe-rated at the time of solidification and consti-tute one of the important sources of porosity.It can, however, be ignored in the case ofbrasses with high zinc content as the solutionof gases is effectively inhibited by the highvapour pressure of zinc which is of the order

of 500-600 mm. of mercury at the castingtemperature. Therefore, from the point ofview of solution of gases in brass high casting

temperature can be used without danger.

Rate of Pouring

Before considering the way in which acompromise has been reached between theconflicting effects of casting temperatures,it is proposed to discuss how the quality of a

brass ingot is influenced by the pouringspeed or, in other words, the rate of rise ofthe liquid metal in the mould. The tasting

186 SYMPOSIUM ON NON- FERROUS METAL INDUSTRY IN INDIA

temperature and the pouring speed are so

intimately related to each other that it isalmost impossible to treat them separately.

Similarity to Casting Tem perature - In some

of its effects a high rate of rise is very similar

to casting at a high temperature. It ensuresa clean ingot surface as the rapidly risingliquid metal maintains a high temperature

enough to prevent folds and cold shuts. The

fact that the metal is not likely to solidify

while the turbulent state exists is conduciveto soundness of ingots. Fast pouring as a

means of deferring solidification is of parti-cular importance where a thin ingot is con-cerned, since in a thick ingot solidification is

normally retarded on account of its mass.Greater pouring speed, however, means in-

creased momentum and a greater injectoreffect drawing the dross and gases deeper intothe metal and increasing the likelihood of

their being trapped.Depth of Penetration - The deep penetra-

tion caused by fast pouring increases thelateral temperature gradient and reduces thevertical. Unless the temperature of themetal is very near the liquidus such a combi-nation of temperature gradients will promote

bridging with concomitant porosity.Ideal Conditions of Directional Solidifica-

tion - The ideal way to prevent contractioncavities is to direct the solidification of theingot from the bottom to the top and the

nearest approach to this ideal can be securedby casting the metal at as low a temperatureas possible and by decreasing the rate ofrise of the metal in the mould. Directionof solidification is governed by the tempera-ture isothermals inside the mould and slowpouring results in these isothermals beingmore horizontal which is a prerequisite to theprogress of solidification from the bottomupwards. In this ideal state of directionalsolidification, where the rate of solidificationequals that of the rate of rise of the metal,an adequate supply of liquid metal is alwaysavailable to feed shrinkage caused by solidi-fication. Consequently, neither a large pipe

nor any shrinkage cavity will form except

for a certain amount of porosity at the ingot

top where no additional molten metal isavailable to fill the cavities. This applies

also to brasses with long ranges of solidi-fication in which it is more difficult to secure

freedom from contraction cavities than in60/40 brass which has a short range.

This ideal solidification technique can

be readily applied to continuous casting

methods, but is not practicable for individual

ingot casting as a low rate of rise will producea poor surface unless the temperature of

pouring is high.Undecomposed Mould Dressing - The rate

of rise is also very closely related to the

effect of the mould dressing particularly ifthe latter is of the flaming type. If the metal

rises rapidly, it reaches the top before theevolution of the gases has ceased and hencethe likelihood of gas entrapping increases.

In order to obtain a clean surface completecombustion of the flaming dressing isnecessary. Undecomposed mould dressingentrapped by fast rising metal is likely

to produce a poor surface. Undecom-posed substances may also be absorb-ed into the liquid metal and give rise to

defects similar to those caused by drossinclusion.

High Temperature and Slow Pouring - Inpractice a compromise has been effectivelyfound by raising the casting temperature and

reducing the pouring speed. The high tem-perature allows all gases and non-metallic

inclusions to rise to the surface, which pro-cess is aided by the lower pouring rate asalready pointed out.

Holed Tundish - Reduction of turbulenceby the low pouring speed decreases the extent

of oxidation of the zinc. The absence of deeppenetration of the molten metal stream, an-other result of a lower pouring speed, reducesthe injection of zinc oxide and gases. Thehigh temperature steepens the temperature

gradient between the mould faces and thecentre of the casting, but this is counter-

balanced by the lower penetration, whichdecreases the slope of the gradient, the

RAY - FACTORS AFFECTING QUALITY OF BRASS INGOT 187

resultant effect being a substantial reductionin shrinkage porosity in the central region.This position can be further improved by

using a tundish preferably with holes, asboth the tundish as well as small individualstreams reduce the kinetic energy of eachof these pouring streams on which depends

penetration.

Rate of Cooling

The next subject to be considered is therate of heat extraction from the mould,which has a profound bearing on the qualityof an ingot . More rapid heat extractioninduces a steeper lateral temperature gradientand automatically increases the rate of solidi-fication. The effect of these features hasalready been mentioned while discussingcasting temperatures and rates of pouring-

Zones of Solidification -Adverse Effect ofWide Pasty Zone-The zones of solidificationhave an important influence on the soundnessof the resulting brass ingot . The pasty zoneintermediate between the solid and liquidin a solidifying alloy consists of numerousnarrow deep fissures lying between advancingdendrites and become likely source of con-traction cavities. The thicker the pastyzone, the greater will be the probability ofunsoundness . The thickness of this zoneis mainly affected by the solidification rangeof the alloy and by its rate of cooling. Thefaster the cooling , the greater is the depar-ture from equilibrium conditions and itfollows, therefore , that . a -rapid cooling willincrease the range of solidification. As thethickness of the pasty zone is dependent onthe length of the solidification range , it wouldbe thought that an increase in the rate ofcooling would adversely affect the soundnessof the ingot. In actual fact , however, anincreased rate of cooling steepens the tem-perature gradient and thus narrows thepasty zone.

Stresses Caused by Body Contraction - Onthe other hand, when body contractionoccurs after solidification is complete, the

contraction differential caused by a steeptemperature gradient may be very large.This sets up tensile and compressive stressesin cooling layers in alternate relation to eachother - a condition not only promoting hotcracking but also giving rise to residualstresses of a considerable magnitude.

In considering the whole question of therate of cooling it must be remembered thatthe thermal conductivity of the mould mate-rial is by no means the sole factor governing

the rate of heat extraction. The flow of heatthrough a good conductor is mitigated to agreat extent by the mould dressing and bythe air gap between the mould faces andthe solidified shell of the cast metal. The

thermal characteristics of the liquid, solidand the solidifying metal also play an im-portant part.

Junker Moulds - A compromise betweenthese contradictory features is generally ob-tained by using water-cooled Junker typemoulds in which the severe chilling effect isreduced by a suitable mould dressing. Therate of cooling in these moulds can also beeffectively controlled by regulating the tem-perature and the rate of flow of the cooling

water.

Mould Dressings

Extraction of heat from the mould isinseparably linked with mould dressings, theeffect of which will now be considered.

Flaming and Inert Dressings- Mould dress-ings are nearly as many in number as foun-dries that use them, but they can all bedivided into two broad classes, viz. flamingand inert. Each foundry favours a parti-

cular variation of one or other of these two.In brass foundries today flaming dressingsare most widely used and the advantagesand disadvantages of these dressings will be

discussed in some detail.Gases from Flaming Dressings - Reference

has already been made to the entrapping of

gases evolved by the combustion of the

mould dressing. In fact, the source of the

188 SYMPOSIUM ON NON-FERROUS METAL INDUSTRY IN INDIA

majority of the trapped gas cavities is the

mould dressing, and this contributes subs-tantially to subsurface unsoundness. Theturbulence maintained by the liberated gasesreduces the crystal size as well as the tem-

perature gradient in the mould - a condi-tion not conducive to the formation of colum-nar crystals.

Grain Refinement Caused by Flaming Dress-

ing - With a large volume of metal solidi-fying simultaneously into small equiaxed

crystals, a widely distributed region is likelyto be full of fine shrinkage cavities. In thecase of 60/40 brass this effect is less severeby virtue of its short range of solidificationwhich gives it a strong preference for colum-nar crystallization. It is doubtful whether

the grain refining effect of a flaming dressinghas any distinct advantage in brasses thoughit is essential for successful plastic deforma-tion in cases of metals such as zinc whichcrystallize hexagonally. Although hot roll-

ing properties of brass, especially in regard.

to edge-cracking, seem to be influenced bythe grain size, orientation of crystals is pro-bably the over-riding factor.

Functions of Mould Dressings - The mostimportant functions of mould dressings are-

a) to protect the mould faces

b) to liberate a large volume of reducinggases

c) to prevent a too severe chilling effectd) to prevent ` blowing ' of ingot surface

when cast iron moulds are usede) to prevent solidification of metal

splashes.Reducing Gases from Flaming Dressing and

Its Drawbacks - Of these functions, protec-tion of the mould face is afforded and, thoughless efficiently, the chilling effect and theblowing of ingot surface are prevented by

inert mould dressings. Evolution of reduc-ing gases, however, is only possible by thecomplete or partial burning of an inflammabledressing. These reducing gases protect fromoxidation the down-coming stream as well

as the rising surface of liquid metal, oxide

inclusions being one of the major causes of

defective ingots. The flaming dressings also

assist in remelting any splashes which have

adhered to the mould faces. It has beenfurther suggested that the agitation producedby combustion of the dressing helps by a

` scouring ' effect the removal of dross anddirt and to prevent their inclusion in theingot. These functions are so importantthat flaming dressings are generally preferredto inert dressings in spite of the fact that theformer give rise to subsurface cavities and

inhibit columnar crystallization. These de=fects in the ingot, however, are very real andare responsible for the rejection of a consi-

derable proportion of rolled sheet, so that

efforts have been made to overcome them.The following two methods have been

suggested as possible remedies:(a) Inert Dressing with Reducing Atmo-

sphere - To obtain a reducing atmo-sphere without the harmful effects ofa flaming dressing, the mould is coated

with an inert dressing and the reducingatmosphere is created in and over themould by partially burning a carbona-ceous gas such as coal gas.

(b) Preference for Thicker Ingots - Increas-ed thickness of the ingot reduces theunsoundness caused by metal rising in

the mould faster than the rate of com-bustion of the mould dressing, andhence the slab is cast as thick as rollingand other conditions permit. In the

case of a thin chill cast ingot solidi-fication is so rapid that no compositiongradient can be set up. In a thickerslab solidification from the mould faceinwards proceeds slowly enough to af-ford time for coring and grain growth.As a consequence, segregation is likely

to take place, the residual liquid be-coming richer and richer in zinc. The

disadvantages, however, of segregationon a microscopic as well as macro-scopic scale are minor as compared tothe advantages already discussed andthicker ingots are, therefore, generallyto be preferred.

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RAY -FACTORS AFFECTING QUALITY OF BRASS INGOT 189

Tilted Moulds

Efforts have been made to produce soundingots by inclining the moulds at an angle

to the pouring stream. The points in favourof this practice are the absence of the centralregion of unsoundness due to shrinkage poro-

sity and all the good effects associated withmore quiescent pouring,

Increased Entrapping of Gases - The gasbubbles, however, while rising to the surfaceare apt to be trapped against the upper faceof the mould. This happens not only to theproducts of combustion of the mould dressingbut also to the gases drawn in by the injectoreffect of the pouring stream which is notentirely absent. The compromise in thiscase has been made by reducing the angle oftilt to about 5 to the vertical. The first

impingement is taken by the lower mouldplate and pouring is quiet for about one-thirdof the height of the mould. Above thispoint the short distance through which themetal has to fall produces a less turbulencethan would normally occur.

Reduced Angle of Tilt --- The slight tilt doesnot significantly increase the entrapping ofgas bubbles against the mould face. Anygas cavities are found to be concentrated onthe upper surface of the ingot and heavyscalping of this surface substantially reduces` spills ' in the finished sheet. The impactof liquid metal against the mould platecauses local overheating and it is necessaryto guard against the consequent danger ofblowing by applying an adequate thicknessof mould dressing. In the case of water-cooled copper moulds the belief that the

impingement of the molten metal on the

mould plates increases their wear does notappear to be correct.

Conclusion

In conclusion I should like to emphasizethat these various factors are really too

closely related to one another to be discussedseparately. However, to make the presen-tation clearer, I have endeavoured to dealwith them under separate heads and this hasled to a certain amount of unavoidable re-

petition.

Acknowledgement

I am also grateful to Mr. E. R. Dempster,General Manager, Indian Copper CorporationLtd., for permission to read this paper andto those of my colleagues who have helpedme with their criticism and suggestions.

References

1. SMITH , CHRISTOPHER , Chief Metallurgist, James

Booth & Co. Ltd., Birmingham.

2. COOK , MAURICE & FLETCHER, N. F.

Discussion

MR. P. N. GANDHI ( Indian Ordnance Fac-tories )

Could you enlighten me as to what are the factorsthat produce blisters in rolled brass sheet ?

MR. M. M. RAY ( Indian Copper Corporation,Ghatsila )In my opinion a blister is the same as a deep-

seated ' spill '. A gas cavity that is too deep-seated to tear the skin under stress may produceenough pressure to cause a bulge in the sheet givingrise to a blister, All factors producing gas cavitiesinside a brass ingot are, therefore, likely to originate

blisters.