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Metal Casting Technologies : June 2007
50 www.metals.rala.com.au BacktoBASICS T Hot-Tears in Steel Castings he occurrence of hot-tears in steel castings is an all too common event for the steel foundryman. Whilst steel is poured at high temperature, this temperature is low relative to the high freezing temperature range for steel. The result of this is that steel starts to freeze very soon after being poured, often before the mould and cores have been heated to the point where they have lost their bond strengths. These facts, coupled with the relatively large linear contraction of steel alloys make steel castings particularly susceptible to hot-tears. Figure 1. Detection and repair of hot-tears can often involve many hours of work and ultimately mean the difference between profit and loss to the foundry. MECHANISM OF HOT-TEAR FORMATION Hot-tearing is initiated at a temperature within the solidification range of the alloy, that is, at sometime before the solidus temperature is reached. Further opening and extension of the tear can, however, occur at lower temperatures due to continuing linear contraction. Generally, two conditions are necessary for hot-tearing to occur: 1. There must be resistance to contraction, and 2. There must be variable temperature gradients within the casting section PREVENTION OF HOT-TEARS There is usually no easy or single cure for consistent elimination of hot-tears, as many factors can contribute to their occurrence. To overcome, or at least control hot-tearing, the steel foundryman must often have to choose between conflicting remedies. Some of the most common precautionary measures are: Moulding Materials Strong, hard moulds and cores that collapse slowly under heat are more likely to cause tears than weak, easily collapsible moulds and cores. Increased mould and core density increases the intensity (length) of tears. Clay and sodium silicate bonded sand mixes generally offer lower resistance to contraction and hence greater resistance to tearing than resin bonded sand mixes. Wherever possible, mould and core sections should be hollowed out or a collapsible insert such as polystyrene incorporated into the section as close as possible to the casting interface. Binder levels and rammed densities should be kept to a minimum level conducive with maintaining acceptable surface finish. Steel Quality Sulphur and phosphorous should be maintained at a minimum. Both should be less than 0.02% and the Mn:S ratio at least 25:1. Deoxidise at a level which will avoid deleterious Type II grain boundary inclusions. Pouring Temperature Pour at a minimum temperature consistent with avoiding cold metal defects and maintain high fill rates. Feeding Practice Feeding practice can have a major influence on the occurrence of hot-tears. Adequate feeding of isolated hot-spots can prevent the formation and often also "repair" tears which may initiate. Excessively large feeders which may hinder contraction should be avoided. As should the placement of feeders, sprues, vents etc in close proximity to moulding box bars which may also hinder natural contraction. Gating Practice Employ multiple gates to even out temperature gradients and avoid locally overheated mould parts. Care should be taken to ensure the design of runners and gates is not "tying" the casting and restricting normal contraction. Overfilling of the pouring bush which results in flash across the top of the moulding box can also contribute to contraction restraint. By J. F. Meredith - Casting Solutions Pty Ltd Figure 1. Hot-tear, highlighted by dye-penetrant on the surface of a steel casting.