Wet charge materials, sealed scrap, and bridging are all safety hazards that can lead to catastrophic explosions in foundries. By diligently examining and treating scrap, following induction furnace safety procedures, properly training and retraining personnel, and use of automated melt shop equipment, founders can be confident their operations are as safe as possible.

Wet charge materials are a serious safety hazard in all foundries. Water, moisture, or any liquid-bearing material instantaneously turns to steam when coming in contact with molten metal — expanding to 1,600 times its original volume and producing a violent explosion. This occurs without warning and throws molten metal and possibly high-temperature solids out of the furnace, putting workers, the furnace itself, and nearby plant and equipment at risk.

A water/molten metal explosion can occur in any type of furnace. For an induction furnace, however, the aftereffects may be more serious, including the possibility of additional explosions caused by liquid in a ruptured cooling system coming in contact with molten metal in the bath. Molten metal need not be present in the furnace for a water/molten metal explosion to occur. Explosions also can occur if sealed drums or containers containing water are charged into an empty but hot furnace. In this case, the force of the explosion will eject the newly charged material and quite likely damage the refractory lining as well.

The violent and unpredictable nature of a water/molten metal explosion makes the wearing of appropriate Personal Protective Equipment (PPE) by melt shop workers imperative. PPE can help to prevent disfiguring, incapacitating or fatal burns.

Some foundries reduce the possibility of water/molten metal explosions by storing scrap under cover for a least one day and then carefully inspecting bales and containers for any residual moisture. But a more reliable solution employed by an increasing number of foundries is to use remote charging systems with charge dryers or preheaters. Remote charging systems permit the operator to stand safely back from the furnace or behind protective screens during charging. Dryers and preheaters maximize the removal of water and moisture before the scrap enters the bath.

Sealed Containers

An easily overlooked danger is posed by sealed containers and sections of tubing or piping that are sheared-closed on both ends. Obviously, containers holding combustible liquids or their fumes will explode long before the scrap itself melts. Preheating sealed material will not prevent this hazard. In fact, there is a risk that a sealed container will explode inside the preheating systems. Operator vigilance is the only preventive measure. Sealed material must never be permitted into the furnace or preheater. Sheared sections of scrap tube and pipe and apparently empty sealed containers may seem less dangerous but can be equally hazardous.

Even though they do not contain combustible liquids, the air inside them can rapidly expand in the heat. In extreme cases, the pressure buildup will be sufficient to breach the container wall or escape through a sheared-closed end. If this occurs, the forceful expulsion of gas can propel the hot scrap out of the furnace or smash it into the furnace lining, causing damage.

Cold or Easily Fragmented Charge

Cold charge or tools and easily fragmented materials pose a special hazard for induction furnaces and their operators because they may contain a thin layer of surface or absorbed moisture. On contact with the bath, the moisture turns to steam, causing spitting or splashing. Appropriate protective equipment and face and eye protection normally will protect the operator. Preheating the charge and tools prevents many splashing injuries.

In ferrous metal foundries the greatest splashing risk occurs toward the end of the melt, when a foundry worker adds ferroalloys or introduces tools into the melt. Ferro-alloy materials can absorb moisture from their surroundings. Sampling spoons and slag rakes collect moisture as a thin film of condensation. Manufacturers’ instructions must be followed for storing alloying materials and preheating tools to minimize moisture accumulation and reduce the risk of splashing.

In a nonferrous foundry, spitting or splashing can accompany the introduction of ingots into the melt, as surface condensation comes in contact with molten metal. Ideally, ingots should be placed only in an empty furnace or on top of solid foundry returns. Ingots added to a molten pool should be preheated or introduced using a remote charging system.

Centrifugally Cast Scrap Rolls

Special steps need to be taken when charging a furnace with centrifugally cast scrap rolls. Ideally, this type of scrap should not be melted in an induction furnace. The hazard stems from the possibliity that a roll may contain a ductile inner core surrounded by a brittle outer layer. The different rates of expansion can cause the surface material to explosively separate from the roll injuring personnel and damaging equipment. Breaking the scrap rolls can minimize the fragmenting hazard.