GATING OF STEEL CASTINGSGATING OF STEEL CASTINGS

IMPORTANCE OF GATING SYSTEM

• The design of gating system is as important as risering of a steelcasting.

• It is well known that improper gating practice can result in defects like• It is well known that improper gating practice can result in defects likeCEROXIDE, INCLUSIONS, COLD SHUTS, MISRUNS, HOT TEARS,LOCAL SHRINKAGES, and GAS CAVITIES in a steel casting.

• A gating system should be pre-designed and incorporated in methoddrawing as is the case of risering and not left to the discretion of themolder.

CONSTITUENTS OF GATING SYSTEM

• A gating system for steel castings can be broadly divided into:-

• 1) The entry section – consisting of the pouring basin,sprue and sprue base.sprue and sprue base.

• 2) The distribution section – consisting of the runners and ingates.

FUNCTIONS OF A GATING SYSTEM

• The entry section of a gating has two functions:-1) To supply liquid metal free of entrapped gases, slag and eroded sand.2) To establish a hydraulic pressure head, which will force the metal through the rest ofthe gating system and into the casting.

• The distribution section has five functions:-

1) To decrease the velocity of the metal stream.2) To minimize turbulence, both in the gating system as well as in the mold cavity.3) To avoid mold and core erosion.4) To establish the best possible thermal gradient in the casting.5)To regulate the rate of flow of metal into the mold cavity.

• In addition to these, the gating system should be of such simple design as to facilitatemolding, particularly with mechanical methods, at the same time involving minimumfettling cost and affording maximum casting yield.

• Many of these requirements and functions are conflicting with each other. Effortshould be to harmonize these so as to create conditions conducive to the productionof a defect free casting.

GENERAL PRINCIPLES OF FLOW

• When considering the running systems, it is instructive to bear in mind a few idealizedconditions of flow. It is true that the conditions prevalent in a mold are more complex.However, certain basic patterns of flow are fundamental and the gating system can befully “engineered” from these.

• The requirements of a gating system are the opposite of a hydraulic system. In thelater case, every effort is made to reduce all frictional and kinetic losses to minimumso as to conserve power. In designing a running system of a casting, the reverse is theso as to conserve power. In designing a running system of a casting, the reverse is thecase. The metal entering the mold should have the lowest possible velocity, and yet,should fill up the same at a rate first enough before the loss of temperature rendersthis impossible.

• To obtain a understanding of the fundamentals of metal flow in gating systems, twobasic fluid flow equations are of interest. The first of them is the “Law of continuity”and the second one is “Bernoulli’s Theorem”.

LAW OF CNTINUITY

• The law of continuity states that the flow rate must be the same at agiven time in all portions of a fluid system. It may be written as:-

• Q = A1V1 =A2V2

where Q = metal flow rate in cu.ft/sec

A1 & A2 = cross-sectional area of flow channel at two differentpoints 1 & 2 in sq.ft.points 1 & 2 in sq.ft.

V1 & V2 = metal velocity at points 1 & 2 in ft/sec.

• This would mean that if the flow channel narrows down to half itsoriginal cross-section, the metal velocity would be double, and viceversa. The law of continuity, therefore, can be used to predictquantitatively the effect of variation in channel size on the metalvelocities and flow rates in a gating system.

BERNOULLI’S THEOREM

• Bernoulli’s theorem states that the energy of a liquid at a given point can be separatedinto three parts: energy of velocity, ( v2/2g) energy of pressure, (P1/ρ) and energy ofposition, (h). In the ideal case ( with no energy loss ), when liquid moves from point 1to point 2, it neither gains nor looses energy. Thus, setting the energies equal for twopositions, yields the equation as below:-

• (V12÷2g) + (P1÷ρ) + h1 = (V2

2÷2g) + (P2÷ρ)+ h2

where V1 & V2 ~ metal velocity at two different points 1 & 2, in ft/sec.g ~ acceleration due to gravity.

P & P ~ static pressure in the liquid at points 1 & 2 in lb/sq,in.P1 & P2 ~ static pressure in the liquid at points 1 & 2 in lb/sq,in.h1 & h2 ~ height of liquid at points 1 & 2 in ft.

ρ ~ density of liquid in lb/ cu.ft.

• As mentioned earlier, Bernoulli’s theorem can only be employed to calculate velocity inideal fluid system i.e.. in systems in which the fluid suffers no energy losses. In realgating systems, besides losses due to friction, energy losses occur at all entrancesand exits, bends, enlargements and contractions. The exit velocity and flow ratesobtained by the above equation would, therefore, be somewhat higher than thosefound in actual practice.

POURING BASIN• A pouring basin can be compared to a tankfull of water with a hole in its base. If theedges of the hole is sharp, then the cross-sectional area of the issuing streamdecreases to a minimum value a little belowthe orifice. The reason for this is that a fluidcannot turn at a sharp angle. Thus, fluid, withthe exception of those at the centre line ofthe orifice, will be traveling in a directioninclined to the centre line. They have toinclined to the centre line. They have totravel a little further before this directionbecomes parallel with the general directionof the stream , resulting in a contraction inthe stream as shown in Fig-1.

A standard design of the pouring basin,generally used in steel foundries, is shown inFig-2.

SPRUE• As the liquid metal enters the sprue from thepouring basin and travels down, it acceleratesunder the influence of gravity. This accelerationhas two effects:-

• 1) The metal stream acquires a high velocity,which, theoretically, is given by the simpleequation, v2= 2gH.

• 2) Due to the acceleration of the freely fallingstream, the cross-sectional area reduces as thevelocity increases; this is because, according tothe law of continuity, volume flowing past onesection must be the same as at any other section.As a result of the above, the metal pulls awayfrom the walls of the sprue with consequentturbulence and aspiration as shown in Fig-3.

• If the walls of the sprue are tapered sufficiently so that metallies firmly against them, aspiration is eliminated. The followingequation may be used with advantage to arrive at the tapernecessary to prevent aspiration.

• A1/A2=√Z2/Z1

• Where A1~ area of the sprue entrance.A2~ area of any other location in the

sprue.Z1~ level of the pouring basin above the

sprue entrance.Z ~ distance from the top of the pouringZ2~ distance from the top of the pouring

basin to the location of A2.

• Although the above equation indicates that the ideal sprueshould have a parabolic taper, straight sided taper has beenfound to suffice in practice as shown in Fig-4.

• In addition to its shape, the height of the sprue also effects itsfilling. It has been shown that short sprue tend to fill upcompletely, when the sprue: runner is1:1. The precise sprueheight at which incomplete filling begins, is determined by thechoke area.

SPRUE BASE

• As it leaves the sprue, the molten metal travels at itshighest velocity and develops its maximum energy. At thesprue base, the direction of flow abruptly change, whichcauses severe turbulence. Therefore, by increasing the areaof sprue base, both the velocity and the turbulence of metalcan be effectively reduced. In addition, as the sprue base isfilled, the molten metal acts as a cushion to absorb theimpact of the falling stream. In order for the sprue base tofunction properly, its bottom surface must be flat. Thisfunction properly, its bottom surface must be flat. Thisbecause curved bottom surface of a sprue base will notabsorb the kinetic energy of the falling stream and willdeflect the molten metal up the sides of the bowl, thuscausing severe turbulence.

• The cross-sectional area of the sprue base should beapproximately 5 times that of the sprue exit, its depth being2 times that of the runner.

RUNNER

� The function of the runner is to change the direction of the flow of metal from verticalto horizontal. Since liquid cannot turn through a right angle instantaneously, acontraction results as shown in Fig-6.

• Although little is known of the optimum radii required to suppress this type ofcontraction, an enlarged sprue base goes long way in meeting the above problem.Also to reduce appreciably the velocity of the metal leaving the sprue or spue base,the cross-sectional area of the runner must be larger than that of the sprue exit. Asmentioned earlier, short sprues tend to fill completely, the reverse is, however, truefor runners. As the metal stream proceeds along the runners, it expands as itsvelocity falls off, and eventually, completely fills the runners. Therefore it may be saidthat short sprues, and long runners are an ideal combination in a running system.

• To ensure that only clean metal enters the gates, and thereby, the mold cavity, therunners should be filled before the gates. It is, therefore, best to place runners in thedrag and gates in the cope.

• The molten metal that first enters the running system is usually contaminated due toturbulence, aspiration and eroded sand. Runner bar extensions are, therefore, usedwith advantage to prevent this metal from entering the mold cavity. The runnerextension must, however, be extended far enough beyond the last gate to prevent thebackwash of unclean metal from entering the gate.

GATES

• Similar conditions of flow exist at the junction of eachgate and runner bar as the junction of sprue andrunner. The resulting contraction that takes place inthe former is shown in Fig:-8.

• It can be seen that the contraction at the leadingedge is rather slight, but at the trailing edge it isedge is rather slight, but at the trailing edge it isconsiderably pronounced.

• Suffice it to say that unless the degree ofcontractions at various junctions, as enumeratedabove, are known, or suppressed altogether, it is notpossible to talk with any precision about the cross-sectional areas of a running system.

• Research has shown that since, in the case of multiple gating, thetendency of the stream of molten metal is to flow the path of leastresistance, a large portion of metal will flow through the last gateattached to the runner.

• Following Newton’s First Law of Motion, a moving object, in this case the• Following Newton’s First Law of Motion, a moving object, in this case thestream of metal, tends to continue moving in the same direction untilsome outside force is exerted to change it. The reduction of the cross-sectional area of the runner just beyond the first gate, acts as that force.It restricts the flow of metal to a certain extent and builds up a slight backpressure, thereby making the stream of metal turn and flow through thefirst gate. The amount by which the cross-sectional area must bereduced at each gate is dictated by the gating ration being used.

1. When gating ratio is 1:1:1, decrease area of runner by the area of gate.

2. When gating ratio is 1:2:1.5, decrease area of the runner in proportion tothe number of gates passed.

GATING SYSTEM

• Theoretically, the best way to fill a mould with liquid metal is to pour themetal straight through the riser. This will create the ideal conditions fordirectional solidification of castings. However, the method is not applicablein its entirety, particularly to steel castings made in sand mould for obviousin its entirety, particularly to steel castings made in sand mould for obviousreasons. Hence, the need for a gating system. Some of the gates commonlyused in steel foundry are described below:-

TOP GATE

• Top gates are usually limited to relatively small castings of simple design.The turbulence of metal as it enters the mould cavity causes erosion, whichis a major problem in the manufacture of steel castings. As such, top gatesis a major problem in the manufacture of steel castings. As such, top gatesare used in steel foundries only for broad shapes of low heights.

BOTTOM GATE

• Bottom gating reduces the turbulence and erosion of the mould to aminimum, but creates unfavorable thermal gradients. Whereas local hotminimum, but creates unfavorable thermal gradients. Whereas local hotspots results at the gate entrance, cold metal appears in the riser.

Foundry men have devised various means of to find a compromise betweenthese basic forms of gating. It my be stated that bottom gating is mostdesirable where risers or atmospheric risers are used to feed sections deepdesirable where risers or atmospheric risers are used to feed sections deepin the mould.

HORN GATE

• This gate, so called because of its shape, is a variety of bottom gating. Themain objection to its use is that the metal enters mould in a fountain like jet,causing turbulence, aspiration of air etc. Horn gate is probably the greatestsingle cause of gas cavities resulting from trapped air, and is notsingle cause of gas cavities resulting from trapped air, and is notrecommended for gating steel castings. Experiments have shown that theabove fountain effect can be considerably reduced by enlarging the cross-sectional area of the exit end of the horn gate into the mould to twice thearea of its entrance from the runner.

PARTING LINE GATE

• This particular form of gating is a compromise between top and bottomgating. They are often chosen more as a molding expedient than for theintrinsic value. In this case, metal enters the mould cavity at the same levelas the mould joint or parting line. Molten metal enters through the sprue andreaches the parting surface where the sprue is connected to the runner orgates in a direction horizontal to the casting. The arrangement of providing areaches the parting surface where the sprue is connected to the runner orgates in a direction horizontal to the casting. The arrangement of providing agate at the parting line allows the use of devices that can effectively trapany slag, dirt, or sand, which passes with the metal down the sprue.

STEP GATE

This takes the advantage of bottom gating, at the sametime allowing hot metal to enter directly into the riser. Insome instances, it is possible to arrange a series ofgates at several levels.

Metal flows through the bottom ingates, until the mold isfilled to the level of the next higher ingate. At this point,metal is expected to start flowing through this ingate andthrough successively higher ones, as the mould gatesfilled. However, in practice, step gates do not function inthis ideal manner. The inertia of the metal falling throughfilled. However, in practice, step gates do not function inthis ideal manner. The inertia of the metal falling throughthe sprue and the resulting low pressure areas createdat the entrance of the top gates, as shown in Fig-10,carries the metal past the higher ingates and nearly allof it flows through the bottom ingates only.

Through experimentation, it has been observed that byslanting the ingates upward at an angle to the casting,and designing the gates for relatively increasingresistance to flow at lower levels, step gates can bemade to function properly.

WHIRL GATE

• A whirl gate is the most positive device for preventing dirt from entering the mould cavity.Although steel foundry men have used them sporadically for many years, great interest intheir use has been taken only very recently as the demand for more cleaner steel castingsincreased. Cast irons can be effectively filtered by using variety of Filters, But for steel,development successful filters is still awaited. The following parameters have beenrecommended for whirl gates used for steel castings:

1) Ratio of ingate to outgate cross-sectional area should be 1.5:1.

2) Optimum whirl gate basin diameter appears to be between three to five times the ingate width,2) Optimum whirl gate basin diameter appears to be between three to five times the ingate width,and the height about 1.5 the ingate height.

3) Whirl gate performance is improved increasing the angular displacement (recommendedorientation: 1800 apart.)

DESIGN OF GATING SYSTEM

• There are two major steps in designing a gating system:

i) Calculation of the ingate area.

ii) Derivation of the size of other components, such as runner,sprue etc.

GATING RATIO

Gating ratios recommended by various theoreticians in the literature vary over a wide range.For steel castings, a mildly pressurized gating system is generally used. This has thefollowing advantages:

i) The gating system is kept full of metal. The back pressure due to the restriction of at thegates tends to minimize the danger of the metal pulling away from the mold walls, causingthe consequent aspiration, turbulence and sand erosion.

ii) In case of multiple gating system, the flow from the gates of equal area is uniform. Sincethe kinetic effect of the metal stream is dampened by the back pressure created.

A non-pressurized gating system, wherein the area of runners and gates is larger than thatof the sprue i.e.. 1:2:2 or 1:4:4, offers a rapid filling, the low velocity metal stream resultingin materially reduced mold erosion. Such systems, however, favor oxidation of metal andmay be partially responsible for the formation of ceroxide defect. Also metal flow is non-uniform, when the gate area equals the runner area. A slight change in in the non-pressurized system of 1:2:2 to the gating ration of 1:2:1.5 will produce steel castings nearlyfree from sand erosion, will minimize oxidation in the gating system and will produceuniform flow. It is reported that general application of this ratio reduced the percentage ofsteel castings requiring welding from about 10 to 2%.

GATING CALCULATION

• A number of methods for calculating gating systems are available intechnical literatures today. The method consist of calculating the optimumpouring time of the casting, which is cross checked with minimum rate ofpouring time of the casting, which is cross checked with minimum rate ofrise of metal in the mould. The next step is to determine the total ingatearea, from which the size of the individual gate, runner and sprue arederived, depending upon the gating ratio being used.

TABLE-1

• For determination of pouring time, the following empirical formula can beused:-

t = S 3√VGwhere t is pouring time in seconds,

S is time-coefficient for steel castings(Table-1)V is mean section thickness of casting in millimeter.G is weight of casting and risers in Kg.

Pouring temperature & fluidityPouring temperature & fluidity Bottom gatingBottom gating Side gatingSide gating Top gatingTop gating

NormalNormal 1.31.3 1.41.4 1.5 to 1.61.5 to 1.6

It has been reported that the following values for coefficient ‘S’ have foundto be suitable in actual production condition of steel castings over aconsiderable long period.

For castings weighing from 10 to 50 MT :- 1.8 to 2.8For castings weighing from 1.0 to 10.0 MT :- 1.2 TO 2.0For castings weighing up to 1.0 MT :- 1.0 TO 1.5

NormalNormal 1.31.3 1.41.4 1.5 to 1.61.5 to 1.6

IncreasedIncreased 1.4 to 1.51.4 to 1.5 1.5 to 1.61.5 to 1.6 1.6 to 1.81.6 to 1.8

Table-2

• In addition to the determination of pouring time of the casting, dueconsideration must be given to the rate of rise of metal in the mould. As it iswell known, besides casting miss-run, cold shut etc, too slow a rate of rise ofmetal in the mould tend to give rise to scabbing defects on the cope surface.Table-2 gives the minimum rate recommended for the rise in the mould forsteel castings.

Section thickness in mmSection thickness in mm Min rate of rise of metal in mm/secMin rate of rise of metal in mm/secSection thickness in mmSection thickness in mm Min rate of rise of metal in mm/secMin rate of rise of metal in mm/sec

Below 4Below 4 11

6 to 106 to 10 22

10 to 4010 to 40 11

Above 40Above 40 0.80.8

• Having determined the optimum pouring time of the casting, the cross-sectionalarea of the ingate may be calculated according to the following formula:-

F = G ÷(0.31u√hst.t)where F = Cross-sectional area of ingate, cm2

G = Weight of the casting and risers, Kgu = Flow coefficientt = Optimum pouring timehst = Mean ferro static pressure during pouring, cm

The flow coefficient ‘u’ represent the inverse value of the resistance offeredby the mould and the running system. Values applicable to steel castings are

Table-3

by the mould and the running system. Values applicable to steel castings aregiven in Table-3.

Type of MouldType of Mould Resistance of mouldResistance of mould

HighHigh MediumMedium LowLow

Green SandGreen Sand 0.250.25 0.320.32 0.240.24

Dry SandDry Sand 0.300.30 0.380.38 0.500.50

• Table-3 represents castings made without any open risers or flow-offs in amoulding sand of average permeability, cast at normal pouring temperature.

• The following factors, therefore, influences the value of coefficient ‘u’ :-

FactorsFactors Change in values of ‘u’Change in values of ‘u’

1. Increase in pouring temperature per 501. Increase in pouring temperature per 5000CC Up to +0.05 Up to +0.05

2. Open risers & Flow2. Open risers & Flow--offsoffs From + 0.05 to + 0.30From + 0.05 to + 0.30

3. Increase gating ratio:3. Increase gating ratio: From +0.05 to +0.02From +0.05 to +0.02

• Maximum possible value of coefficient ‘u’ = 0.75

3. Increase gating ratio:3. Increase gating ratio:

if Sprue area/Gate area>2 if Sprue area/Gate area>2

and Runner area/Gate area>1.5and Runner area/Gate area>1.5

From +0.05 to +0.02From +0.05 to +0.02

4. Complex multiple gating4. Complex multiple gating From +0.05 to From +0.05 to --0.100.10

5. Low permeability of mould5. Low permeability of mould Up to Up to --0.050.05

• The mean ferro static pressure hst (Fig-12)during pouring is calculated from the equation:

hst = H0 – (P2÷2C)

where

H0 is Height of sprue ( from top of metal level inpouring basin to the ingate level) in cm.pouring basin to the ingate level) in cm.

P is the height of the casting above theingate level in cm.

C is the total height of the casting in ascast condition in cm.

A Practical Example of Gating Calculation

• For the method drawing shown,the basic data available forgating calculation are asfollows:-

Casting Weight:- 128 Kgs.

Weight of casting includingriser:-145 Kgs.

Mean Casting thickness:- 75mm

Height of metal level in pouringbasin from ingate level:- 250mm

• Calculate Pouring time where time coefficient ‘S’ for side gating and normal pouringtemperature is 1.4 (from Table-1), Casting weight with risers ‘G’ is 145Kgs andMean section thickness ‘V’ is 75mm.

t = S 3√VG = 1.4 3√145X75 = 31 seconds

• Calculate Mean ferro static pressure ‘hst’ where ‘H0’ is 25cm, ‘P’ is 0 as total heightof the casting is below the ingate level and ‘C’ is 7.5cm:-

hst = H0 – (P2÷2C) = 25 – (0÷2X7.5) = 25 – 0 = 25cm

� Calculate ingate area ‘F’ where ‘G’ is 145Kgs, Flow coefficient ‘u’ is 0.6, Mean ferrostatic pressure ‘hst’ is 25cm and Pouring time ‘t’ is 31sconds :-

F = G ÷(0.31u√hst.t) = 145÷(0.31x0.6x√25x31) = 28cm2

� There are two ingates for the casting and as such cross-sectional area of eachingate will be 14cm2 i.e.. 54mm wide and 26mm thick.

• Calculate area of runner and sprue :-

A gating ratio i.e.. Sprue area: Runner area:Ingate area =1:2:1.5 to be usedfor steel castings.

As such the runner area will be (ingate area÷1.5)x2 = (14÷1.5)x2 = 18.66cm2.

So the runner dimension is 54mmx36mm.

In case of sprue area, since it is feeding both the ingates, total area ofingates i.e. 28cm2 to be taken into account,

So the sprue area is 28÷1.5 = 18.66cm2

As such the sprue diameter at the end of the taper is 48.75mm≈49mm.

POINTS TO REMEMBER1. Contraction in the metal stream occur at the various junctions of a running system even after

calculating a gating system accurately, defects in casting may appear unless steps are taken tosuppress these contractions.

2. Short, tapered sprues and long runners with a large well at sprue base, ensure the complete filling ofthe system with minimum turbulence, aspiration etc, thereby causing less mould erosion.

3. Runner bar extensions, whirl gate and runners in drag & gates in cope, areeffective dirt trap.

4. Faster flow rates with low metal stream velocities ensure castings with least moulderosion.

5. Horn gate cause more air entrapment in steel castings and, therefore, are notrecommended.

6. Step gates do not function as expected. In practice, most of the metal tends to flowfrom the bottom gate unless means are employed to obviate the above condition.

7. Multiple gating produces less mould erosion than a single ingate system.

8. A mildly pressurizes system with a gating ratio of 1:2:1.5 has been found to givevery satisfactory results in steel castings.

GATING & CASTING QUALITY

Before any of the studies on gating can be applied in production, the followingfundamental precautions must be observed. It has been found that moresand inclusions in castings result following improper moulding practices, thanfrom the failure to apply scientific gating system.

1. New facing sand must be used for forming the gates, since the latter has towithstand more erosive forces than any other portion of the mould.

2. The gates must be rammed at least as hard as the mould cavity, harder ifpossible. This is particularly applicable to sprue.

3. Rather than the gates cut by moulder, the gating system should form a part ofthe pattern equipment, wherever possible, as the former practice give rise toeasily eroded sand surface.

4. Various portions of the gating system must be fully matched, for if they arenot, the projections coming in the path of the stream are continually washedaway into the mold cavity.

5. Most of all, the gating system must be free from loose sand prior to the entryof the molten metal. The practice of aspirating dirt with compressed air aftermold assembly and placing of coverings over risers and sprue openings aremold assembly and placing of coverings over risers and sprue openings areexcellent quality control operation.

These precautions may seem too elementary to discuss and should betaken for granted. Perhaps they are taken too much for granted.

• Having discussed the common practical safeguards to be takenduring preparation of mould, some of the defects commonly foundin steel castings, which can be minimized by the application ofproper gating practice, may be now discussed.

MOULD EROSION

Research work done on the flow of liquid steel, by taking actual motion picture, has shownthat the steel flows discontinuously over a flat surface. The stream emerging from a gategenerally moves with a sidewise, whip like motion. Consequently the sand on which thestream edges run is alternatively covered and uncovered by metal, thereby burning out theroom temperature bonds. The temperature at this stage on these edges is not yet highenough to fuse the sand grains or bentonite with a high temperature bond. The next wave ofmetal, therefore encounters sand that is not bonded, which is then easily eroded by thestream and may be lodged in the cope or other surfaces of the mold. This means that gatingvertical member, pouring the mould up-hill, to allow the metal to move as a body over flatsurfaces, should be resorted to for obviating mold erosion on account of the abovephenomenon.phenomenon.

A summery of what had been said earlier would show that, in order to minimize the moulderosion, the best gating system would be a double ingate with a central sprue, a rapid filling,low velocity system of properly proportioned runners and gates and short sprues with anenlarged well base.

Besides, dirt trap in the form of whirl gates, runner bar extensions and provision of runners onthe drag and ingate in the cope are effectively used by most steel foundries to ensure that, asfar as possible, only clean metal enters into the mold cavity.

POROSITY

Normally, porosity or gas cavities in steel castings are not associated with gating practice. However, certain factors pertaining to gating system are sometimes responsible for isolated gas cavities.

As mentioned earlier, horn gate is probably the greatest single cause for entrapped air in steel castings. Its use is, therefore, to be strongly discouraged. A changed to taper sprue and deeper pouring basin also goes a long way to minimize aspiration.

CEROXIDE

Ample opportunities for oxidation of molten steel exist in an improperlydesigned gating system, before it enters the mold cavity. The oxidation iscaused by aspiration of air into the molten stream in the gating system andby an oxidizing atmosphere in the mould cavity. The resulting corrosiveconstituents, according to one school of thought, reacts with the mouldingmaterial, particularly eroded sand from the gate, to form a viscous materialcalled ‘Ceroxide’ which is lodged usually at the cope surfaces of steelcastings.

Poor gating system apparently add to the amount of ceroxide in at leasttwo ways:-

- turbulent flow produces excessive aspiration, and increased turbulencecauses more mould erosion – both of which are contributive towards theproduction of more ceroxide of higher viscosity resulting in deeper andmore pronounced defects in the castings.

COLD-SHUT & MISRUN

Too slow a rate of flow, as well as rate of rise of steel in the mould, resultsin misrun castings with wrinkles and cold-shut surfaces. Under the aboveconditions, temporary solidification takes place and further flow of metal isnot sufficient to erase the cold-shuts by re-melting these surfaces.

Increasing the gate area is not a panacea of every misrun problem. Amultiple gating system so designed, that each gate receives supply ofmetal uniformly, reduces the casting area served by each gate, therebyoffering a effective solution.

SHRINKAGE CAVITIES & HOT TEARS

Both these defects can be caused by the existence of local hot spots,resulting from gating system. Gating practice may have marked effect onthe temperature gradients in the casting. Ingates are potential hot spots, inthe mould area adjacent to the ingate absorbs heat and become as hot asthe metal itself, thus delaying solidification of the casting at this area. Thismay be very severe where a single ingate, and a slow pouring rate, isemployed.

Therefore to avoid shrinkage cavities and hot tears, multiple gating shouldbe used so as to provide a flatter temperature gradient in the casting. Tworules of thumb employed by steel foundries to minimize the above defectsare: to keep the cross-sectional area of the ingate smaller than that of thecasting and to cut ‘cracker ribs’ in the mold or core surface in front of theingates.

• A word about the inter-relationship of riser and ingatepositioning in steel casting. Control directional solidificationalong the casting towards the riser should not be disturbedby improperly placed ingates, since the feeding range ofrisers may be reduced materially on account of therisers may be reduced materially on account of thesuppression of end effect by the ingates.

REFERENCES

1. S. Bharadwaja, Indian Foundry Journal’1969

2. Bidulya, “Steel Foundry Practice”

3. Report of Sub-committee TS54 of the Technical council: Investigation of flow-phenomenon invarious running and gating system. The British Foundrymen, May, 1965

4. Basic principles of gating, AFS.1967

5. Taylor, Fleming and Wulf, Foundry Engineering.

6. Caine: AFS Symposium on principle of gating,1951

7. Brigg: Gating steel castings, Foundry, June,1960

8. SFSA Research report No.31: The performance of whirl gate with liquid steel. December,1953