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274 FTJ November 2010

the University of Birmingham explains why he believes

systems for a good counter-gravity system to improve quality

and save on costs.

The bubbles are air bubbles entrapped during the turbulence of the pour.

to dispersed micro-porosity, cracking, and poor elongation and fatigue

The concept of quality in castings

Figs 1a & 1b

The fact that the properties of metals have

been subject to scatter has been with metallurgists

for so long that the problem is hardly noticed. In

fact a number of quality assurance systems have

developed to allow for the taking of a second

The occurrence of low mechanical properties

has traditionally been assigned to the presence

of porosity in the cast metal. However, the

occurrence of porosity in cast metals has presented

an intractable problem, simply because porosity

cannot, theoretically, occur. There seems no

way to promote the nucleation of porosity, even

when considering the most favourable types of

heterogeneous nuclei. This has been demonstrated

by theoretical treatise many times(1).

The whole problem of the initiation of porosity

is solved at a stroke if entrainment defects are

considered. It is proposed that the content of


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FTJ November 2010 275

Feeding and Gating

entrained defects controls the quality of cast metals.

This defect content can be controlled by good metal

treatment and good casting techniques.

The entrainment processIn their molten state, many metals and alloys exhibit a

(1)

many stainless steels, aluminium bronzes, ductile iron,

and nickel-based superalloys cast in vacuum (since

the so-called vacuum contains plenty of residual

oxygen of course).

that its underside is in perfect atomic contact with

the liquid, since it has grown atom by atom. The top

surface is, however, dry and crystalline. Thus on being

side to dry side, and so guarantees that practically

interface. The defect entrained by this simple folding-

in action has all the features of a crack in the liquid.

with an immense density of cracks.

It is easily shown theoretically that if the liquid has

energy to jump or splash high enough to entrain its

surface during its subsequent fall. The liquid can gain

this energy simply by a fall of only about 12mm (2,3).

Thus all pouring actions of melts are seen to impair

the quality of the metal, some very seriously.

observed(4) by the simple technique of reducing

the pressure on the liquid to 0.1 atmosphere, thus

into compact clumps by the action of the internal

turbulence that always accompanies the pouring of

castings. In this form they are tolerably harmless, the diameter of the defect

open, unravelling, to take on the form of planar cracks. Thus by this action

they effectively increase in size by about a factor of 10, becoming defects

in the range of 1.0 to 10 mm in diameter (although sometimes as large 100

mm diameter in some large stainless steel and Ni-base alloy castings). This

The metamorphosis of the defects from compact to open cracks is driven

by a number of mechanisms:

1/100 atm experiment described above)

during the past year. Interestingly, all these factors effectively embrittle (or

perhaps more accurately, reduce the ductility of) the important structural

aluminium-silicon alloys. Thus the loss of ductility observed as a result of the

actions of hydrogen porosity, shrinkage porosity, high iron levels, and large

grain size, all occur as a result of their action in opening the compact and

the properties and reliability (i.e. the quality) of the castings.

The production of quality castings

such as copper and ferrous alloy, where the previously entrained oxides

in suspension for long periods, many hours, because they are close to being

neutrally buoyant.

Once the melt is clean it is important to transfer it into the mould without,

so far as possible, folding over the surface at any time, thereby folding in an

to live up to their name is not generally realised.

10 mm

10 mm

Figs 2a & 2b


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(a) (b)(5)

cost effectiveness of this process.

accompanied by some damage to the liquid metal. Thus the design of a good

To start with, a major problem is the pouring basin. The universal conical

funnel basin could hardly be worse, dragging in at least 50% air into the melt

such as the offset step design(5), but by far the best solution is to eliminate

basins completely, by converting to contact pouring techniques using a

bottom-stoppered ladle.

systems used for cast irons, and the non-pressurised systems often used

for light alloys, both of which systems are now known to be highly damagingto metal quality. The naturally pressurised system gently pressurises the

inclusions. (If sand inclusions are found in the casting it is a certain indicator

Other features of the naturally pressurised system is that it forbids the use

of wells at the sprue/runner junction which create turbulence and bubbles,

and instead replaces this with a simple radiussed turn which behaves

beautifully, not entraining a single bubble.

The new naturally pressurised system is particularly economical in use of

metal as a result of its extremely slim channels, designed to be only large

the air.

to below 1.0 m/s and preferably to below 0.5 m/s if possible. If this is not

the velocity earlier in the system, usually at the runner entrance, or a surge

control system is needed(5).

By always ensuring to gate at the lowest point on the casting, and at

of the mould also takes place in a progressive way, pushing air ahead,

and rolling out the surface oxide against the walls of the mould as the melt

a mechanical barrier between the melt and the mould, helping to bridge

perturbations such as sand grains. Grey iron castings made in this way in

broken out of the mould (the lustrous carbon only becomes a problem when

of a crack).

Quality featuresThe features of a casting made with good quality

metal, and which has entered the mould without

undue surface turbulence are impressive. There

absent), no bubbles entrained during pouring, no

hot tearing (it is likely that all hot tears are formed on

crack initiating sites, which are mostly, if not always,

In addition, the leak tightness and corrosion

and corrodants cannot penetrate the casting along

the easy pathways provided by the inner unbonded

little-understood problem of stress corrosion cracking

might also be eliminated in quality castings.

Castings are also resistant to blister defects that

arise during high temperature heat treatment. This

diffusion of hydrogen at high temperatures.

ConclusionFoundrymen are strongly urged to upgrade their

pressurised system, or, even better, abandon gravity

Castings are not only improved technically, and

References

alloys

Invisible macrodefects in castings.

au Journal de Physique III 1993, 3, 8610872.

. ScriptaMat. 2000, 43, (10) 881-886.

Casting, Elsevier 2004.

Feeding and Gating


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Here Bob Puhakka of Alloy Casting Industries

(Ontario, Canada) describes the background

that led him to pursue a new approach to

the methoding of steel castings which has

revolutionised the company.

a commercial testing laboratory performing full,

comprehensive analysis on thousands of metal

castings of every conceivable alloy type and

machined, ground, prepared, polished, macroetched,

microetched, pulled, bent, impacted, indented all

performs each and every step of a comprehensive

failure analysis with their own hands from beginning-

to-end a few repeatable features gradually and

obvious: the casting defects causing failure in almost

every single casting were of the same morphology

- independent of alloy or moulding method. Forinstance, a high-pressure aluminium diecasting, a

bronze gravity sand casting and a titanium investment

casting all failed for exactly the same reason.

anomalous feature was present on the fracture

surface, discernible quite clearly under the scanning

electron microscope (SEM). Furthermore, its presence

in associated mechanical properties.

nature of this defect. Bob referred to an industry bible

'Castings'by Prof John Campbell(1).

He then set about dedicating the following four years

perfecting a metal delivery system design ideally

90% of the work being accomplished in the past 18

months.

MethodologyTo avoid oxides it seemed rational to adopt a system that would assist with

(2). This is a system in which

V, at

each fall distance, h, from the melt level in the pouring basin, assuming no

friction. The approach, which is well-known to casting method engineers,

is therefore a simple balance between potential energy, mgh, and kinetic

energy, mV2/2. The difference in this situation is to accept these areas and

onlythese calculated areas at every point

throughout the downsprue and runners. Only the gates would be increased

in size to reduce the velocity of entry to the mould to the critical 0.5 m/s if

possible (on occasions this would be raised to 1.0 m/s if necessary, but not

gate ratio for such a system might vary from 1:1:4 to 1:1:20. It must be

stated however that the pre-selection of such a ratio has no part to play in

result of the design process.

It is also worth noting that, to the authors knowledge, the standard

technique of increasing the area of the runner to reduce the velocity of

(2).

The system requires a specially designed pouring basin of an offset

stepped type(2), to reduce so far as possible the ingress of air into the

(3).

(Campbell(2) describes a contact pour technique using bottom-teemed ladles

to exclude air at the sprue entrance but our lip pour techniques suited to our

size of casting did not require this particular solution).

recommends a curved hyperbolic taper(2) it was judged that the castings at

sophistication. Thus so far they have calculated only the sprue entrance andthe sprue exit and connected the two with a straight taper which works well

with the limited size of casting poured there (1-2,500kg).

The other major feature of this approach is that only bottom gating into

Steel foundry quality revolution

Fig. 1 A 250kgcarbon steel pump

casing immediatelyafter removal from

the mould andinitial shot-blasting.

Traditional fillsystem designs

cannot produce acasting with this

level of cleanliness


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Feeding and Gating

cavity, exceeding the critical fall distance of a few millimeters in which gravity

accelerates it to above its critical velocity, so that it starts to jump and splash,

creating surface turbulence and entraining defects such as bubbles and

mould joint (for traditional horizontally parted moulds) has become a feature

serves as a corner-cutting, cost-saving practice that produces damaged

castings.

Such pockets allow turbulence and the entrainment of air and oxides. The

(the natural pressurisation concept, arising from the friction generated by the

system is then checked to ensure that there is no danger of shrinkage. For

some castings this exercise is not always straightforward. The provision of

a bottom gate and high level (top if possible) feeders sometimes requires

turning the casting through 180 before a workable solution can be found.

Occasionally, much use of heavy chills is required to generate favourable

temperature gradients.

It is at this step that one notices the vast difference in approach compared

to the traditional methoding practices. The traditional approach selected the

system being only an after-thought. The new approach begins with assessing

action of the feeders is seen to be adequate to all locations of the casting,

interesting to note that the pouring of the mould is no longer viewed as being

methoding technique.

That is not to say that failures have not been experienced. These have

occasionally occurred as a result, for instance, of

forgetfulness or oversight of some key aspect of the

design prior to pouring. Such mistakes have been

embarrassing. They illustrate the fallibility of a regime

in which a single person is totally responsible, but at

the same time working under pressure in a production

environment. Clearly, like all professional engineering

designs, at least one other competent person checks

and signs off the design prior to manufacture.

produced from the entire family of ferrous engineering

alloys in sizes ranging from 1 to 2,500kg. The design

parameters remain fully intact, and are scalable to any

casting one wishes to pour.

Cast steel components are notorious for their need

for excessive dressing, rework and cosmetic repair.

The majority of the imperfections visible on the

surface are reoxidation inclusions(4) resulting from the

is no better way to establish the initial quality of a steel

casting than to view the casting immediately following

Its a situation that does not allow for the disguising of

quality defects.

The carbon steel (ASTM A216 WCB) pump casing

put through the coarse shot blaster to remove the

remaining adhering mould material. The casting is

incredibly clean and devoid of all manner of inclusions

and evidence of shrinkage and tearing. This is a pump

subjected to full magnetic particle inspection (MPI),

liquid penetrant inspection (LPI) and radiographictesting (RT) to insure this condition.

Fig. 3 is a photograph of the essentially vacant weld

repair booth illustrating the real-world impact the new

concepts and methods have had.

Looking to the future, it seems reasonable to

suppose that the solution proposed in this article for

alloyed and stainless steels will similarly apply to

Ni-base superalloys. This system has already been

proved to be effective in the casting of a wide variety

of Ni-base superalloys, brasses, bronzes and high-

conductivity (essentially pure) copper.

ConclusionsThe majority of casting defects (including gas porosity,

shrinkage porosity, hot tears, leakers and reoxidation

inclusion) are the result of defects (mainly double

pouring basins, non-tapered sprue, choked runners,

parting line gated, etc. cannot solve these problems.

can practically eliminate entrained defects,

giving essentially perfect castings. Castings are

leaks, surface inclusions, porosity, hot tears and

cracks.

absence of rework avoids the unintentional masking

Fig. 2 A further image of the carbon steel pump casing shown in fig.1. Note the

impressive cleanliness and the complete lack of shrinkage or tearing at the changes

in geometry. Reoxidation inclusions are completely eliminated by using a naturally

pressurised fill system


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Feeding and Gating

of initially unseen defects that may contribute to

subsequent service failures.

It seems reasonable to suppose that the casting

manufacturing systems presented here would apply

to metals and alloy of all types. The author has

validated the success of these techniques on a very

broad range of alloys in the steel, white/grey/ductile

iron, brass, bronze, Ni-base superalloys and high-

conductivity (pure copper) alloy families.

Acknowledgements

I am grateful to Prof John Campbell, author

ofCastingsfor his continued advice and

encouragement.

The volume of work required to complete a full

conversion to the new concepts and methods is

weeks and hundreds of heated exchanges could

have brought the process to a halt if not for the

perseverance of the leaders within our skilled

workforce. My sincere thanks to my excellent co-

References1. Campbell J. Castings, 2003 Elsevier.

2. Campbell J. Casting Practice, 2004 Elsevier

Chennai, India 2008 (February) pp 483-48.

About the Author

Casting Industries in New Hamburg, Ontario, Canada. Bob is a foundry

engineering professional specialising in metal casting technology and

process metallurgy.

blogspot.com) Bob continues to be directly involved in the technology

developments shaping the future of the metal casting industry.

email: bobpuhakka.blogspot.com

Fig. 3 An example of what the companys weld repair booth looks like these days;

essentially vacant. The cast product no longer requires upgrading. The company

now dresses contact points and parting line features only

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