7/29/2019 Hot tears

1/5

Hot tears

Hot tears or pulls are characterised by irregular form, partial or complete fracturefollowing an intergranular path. Tears are often located at changes in section, wherestress concentration is associated with locally delayed cooling (see Figure 5.24). Insome cases, however, a tear may be wholly internal; the distinction between such a tear

and some forms of shrinkageis indefinite, since restraint of solid contraction and lack ofliquid feed may both be involved during the later stages of freezing. It is well establishedthat the critical temperature range for tear formation lies in the region of the solidus,where the contraction behaviour of a solid is combined with extreme weakness and littlecapacity for plastic flow (stage 2 in the previously outlined cooling cycle)86,87. Thus,even the comparatively low strength of the moulding material may produce fracture. Theoccurrence of hot tears is influenced by three factors, namely alloy composition, thedesign of the individual casting and foundry technique.

Vacuum meltingThe most radical technique for production of gas-free metal is vacuum melting. Fourspecific effects of vacuum conditions are relevant:

1. atmospheric contamination is excluded,2. dissolved gases are extracted through the influence of reduced pressure on gasmetal equilibria,3. pressure dependent reactions assist the elimination of elements which precipitatecompound gases during freezing,4. elements with high vapour pressures are preferentially evaporated.

These phenomena are invaluable for refining but can render compositional control moredifficult through their effects upon alloying elements. The technique most generallyapplicable to the production of castings is vacuum induction melting, in which the moltenmetal may be removed from the furnace and cast in air, or in which the entire casting

cycle can be conducted under vacuum. In the latter case the moulds must be free ofvolatile constituents and the process is largely confined to the production of precisioninvestment castings. Vacuum arc melting also has limited application in this field: inconjunction with skull melting, shaped castings can be produced in highly reactivemetals such as titanium, q.v. Chapter 10. Although vacuum melting offers cleartechnical advantages, its application in founding is limited by considerations of cost andscale. Capital and operating costs being extremely high, the process can only beadopted where exceptional quality requirements justify a substantial premium onthe selling price. This has so far been principally for high duty precision castings inspecial alloys. It seems probable that vacuum techniques for general casting productionwill be confined to the more economical degassing treatments.

The degassing of molten metal

Although gas absorption can be reduced by precautions in material selection andmelting, it is difficult to avoid the presence of some gas at melt-out under routinefoundry conditions. This can be eliminated by various types of degassing treatment.Most degassing treatments make use of the equilibrium existing between a melt and agas atmosphere. Contact with an atmosphere having a low partial pressure of the

7/29/2019 Hot tears

2/5

gaseous impurity brings about transfer of the gas in accordance with the equilibrium.Slow degassing thus occurs from the surface of any bath in contact with a cleanatmosphere.

Gas scavenging.

One of the most effective methods for gas extraction uses a flushing or scavenging gaswhich, bubbled through the melt, provides a large gasmetal interface and generalagitation. Flushing treatment is most extensively applied in aluminium alloy founding,where there is particular susceptibility to hydrogen absorption and where the highoxygen affinity of the alloy precludes protection by oxidizing conditions. Argon, nitrogenor chlorine can be employed. Chlorine is especially effective and can be generatedwithin the melt through the decomposition of unstable compounds such ashexachloroethane. Tablets of compound are plunged and held within the melt andcontrolled amounts of chlorine are liberated: this process has, however, declined infavour of inert gas treatment on environmental and health grounds. A further example ofgas scavenging from a solid compound entails the use of calcium carbonate to generate

carbon dioxide in copper base alloy melts, a much less toxic operation. Flushingtreatments with inert gas can be applied through a simple immersed tube or lance, butmeasures to generate smaller bubbles greatly increase the efficiency of the process.Booth and Clegg demonstrated marked improvements obtained by attachment ofporous ceramic plug diffusers for this purpose67. An important further development inthe treatment of aluminium alloys came with the introduction of rotary degassing, inwhich inert gas is injected into the bath through a lance provided with a rapidly rotatingimpeller. This shears the emerging bubbles to generate a fine dispersion, greatlyreducing the diffusion distance and increasing the surface area through which thedissolved impurity gas can be transferred. The available contact time is also extended,since the smaller bubbles rise more slowly through the melt. Inclusion separation is alsoassisted as in a froth flotation system. Rotary degassing of aluminium alloys has beensubject to detailed comparsion with simple lance degassing68 and shown to be muchmore effective, subject to selection of the optimum process parameters. Figure 5.13illustrates the importance of adequate rotational speed, whilst the avoidance ofexcessive treatment temperatures is another major requirement, as in other degassingsystems.

A further variation of flushing treatment is the jet degassing process described byHoyle69: hydrogen contents in steel were successfully reducedto below 2 cm3/100 gusing argon jets on the surface of the metal bath. Inert gas purging can also be used inconjunction with filtration in installations designed for the simultaneous reduction of gasand inclusion contents. The function of a scavenging gas is also performed by theoxygen injected for removal of carbon and other elements in steelmaking: hydrogen andnitrogen contents are reduced during the process. A carbon monoxide boil promoted byslag oxidation is similarly valued for its physical side effect in eliminating dissolvedgases.

7/29/2019 Hot tears

3/5

Vacuum degassing.Extraction of gases under vacuum occurs comparatively rapidly, so that this particularadvantage of vacuum melting can be obtained by brief vacuum treatment of moltenmetal. Several processes of vacuum degassing have been developed, all depending onoverthrow of the gasmetal equilibrium by reduction of the external pressure. The rate

of degassing depends partly upon the geometry of the system the surface area andmass of metal and partly upon the extent of the agitation and stirring which acceleratethe transport of dissolved gas to the nearest surface. Processes are designed toimprove these conditions and provide effective treatment in the limited time availableduring the cooling of superheated metal to the pouring temperature. Since mostmeasures which accelerate gas removal also accelerate cooling, possibilities arerestricted by the need for heat conservation.

A number of industrially established processes are schematically illustratedin Figure 5.14:(a) Static bath treatment. The simplest form of vacuum treatment involves the enclosure

of a ladle of molten metal within a chamber which can be sealed and evacuated. Givensuitable modifications, the ladle itself may be sealed with a cover, the space above themolten metal forming the vacuum chamber. The latter system gives the advantage of asmall pump down volume, so reducing the pump capacity requirement.

(b) Induction degassing. Molten metal is transferred from the melting unit into apreheated bath surrounded by an induction stirring coil, the whole being located within achamber which can be sealed and rapidly evacuated. This type of unit is essentially asimplified vacuum induction melting furnace. A still simpler arrangement is to utilize aninduction stirring coil in conjunction with an evacuated ladle.

(c) Fractional degassing. The contents of a ladle are progressively degassed bytreatment of fractions in a separate vacuum vessel. Two notable examples are themethod developed by Dortmund Horder Huttenunion AG, in which successivefractions of molten metal are raised through a suction nozzle into a vacuum chamber byalternate raising and lowering of the chamber71, and the continuous process developedby Ruhrstahl, AG, in which metal flows up a tube into the vacuum chamber under theinfluence of inert gas bubbles bled into the base of the tube; the degassed metal returnsto the ladle through a second tube so that continuous circulation is achieved72. Thecirculation can alternatively be effected by electromagnetic pumping. These processesalso enable reactive alloy conditions to be made under vacuum.

(d) Stream droplet degassing. A ladle is tapped through a sealed annulus into apreviously evacuated chamber containing a second ladle, degassing beingaccomplished during exposure of the falling stream to the vacuum. Gas evolutiondisintegrates the stream into small droplets so that the geometrical conditions are suitedto rapid gas extraction even though the period of exposure is extremely short. Of theestablished processes, fractional and droplet degassing have hitherto been confined tothe bulk tonnage treatment of liquid steel73, where the fall in temperature is minimal.Heat loss does however present an obstacle in treating smaller batches of foundry

7/29/2019 Hot tears

4/5

metal, where the problem of maintaining temperature is critical. Induction degassing canoffer the combined advantages of induction heating and circulation, the sharp distinctionbetween brief degassing and full vacuum melting disappearing in favour of a widechoice of treatment times. The high cost of generating equipment, however, makes theadoption of this process a major undertaking unless existing facilities can be used,

as in foundries already operating induction melting plant. The ideal process for generalfoundry application would accomplish the treatment in either the melting or the castingvessel, eliminating heat losses involved in transfer to a further vessel. Simple static bathtreatment can be effective with small volumes and shallow baths: low cost installationscould make feasible the routine treatment of much foundry metal used in small batches.Installations of this kind, involving treatment in the ladle or the crucible, have beendescribed in the literature74,75.

7/29/2019 Hot tears

5/5

(2)Hot tears :- They are also known as pulls or hot

cracks. The main reasons of their occurrence is the low

strength of metal after solidification, causing the

metal to fail in coping up with the excessively high

stresses set up by the solid shrinkage of the metal.

These cracks may be external or internal. They are

supposed to be more harmful when they are presentinternally, because in that case their occurrence is not

revealed without machining or radiographic testing.

Their presence is identified by an oxidised surface

showing an irregular and ragged appearance on fracture.

The main reasons of their occurrence are lack of

collapsibility in the core and mould, faulty design

leading to exceptionally high residual stresses at

certain portions in the casting and very hard ramming ofsand resulting in restricted contraction of casting. An

improvement over these shortcomings will help

elimination of hot tears.