What Is Cold Shut in Casting?
You broke your 3/4-inch crescent wrench trying to loose that fitting on some stubborn, rusty old plumbing. Let that sink in for a moment. You broke a 3/4-inch crescent wrench. Sure, you really put your back and shoulder into it to add extra torque, but the wrench shouldn’t break like that. Well, before examining your biceps for super-human strength, examine the broken wrench. It may have succumbed to a manufacturing flaw, perhaps a casting defect like cold shut.
What is cold shut in casting? Cold shut in casting presents itself as a line or crack on the surface of metal – the result of improper fusion of molten metal during the casting process. One or more streams of molten material lacked fluidity at the crucial moment, and left a gap in what should have been solid metal.
You’ll find cold shut on a long list of casting defects – defects that can contribute to serious failures in metals. These defects often go undetected, and make their presence felt at the worst possible moment- like the ringing of the Liberty Bell at the celebration of George Washington’s birthday in 1846. The crack that became famous from that moment on actually started much earlier as hairline fractures in the bell. Attempts to repair them proved futile and possibly contributed to it getting worse.
Preventing Cold Shuts
Unless you work at a die casting plant or have a forge in the garage next to your house, your knowledge of the casting process probably trails behind your other areas of expertise. You may not care to know how to prevent cold shuts in casting, but you should know something about them, because you’ll encounter one sooner or later. You may even attempt to repair with welding a cold shut or one of the many other casting defects.
In the casting process, molten metal flows through gates into molds (dies) in the shape of the piece being cast. Powerful injectors push the metal through at high pressure to ensure that it reaches all portions of the mold. Problems occur when the metal takes too long to fill the die and simply stops short of creating a solid product.
The die operator can prevent cold shuts by:
- Optimizing the gating system so that the molten metal arrives swiftly, doesn’t criss-cross and has a short path to the mold.
- Increasing the pouring temperature to prevent premature solidification.
- Improving the functionality of the mold so that the metal item exits cleanly with no residue left behind.
Misrun – A Close Cousin to the Cold Shut
You could say that a cold shut belongs in a sub-group of related misruns. Misruns occur when the molten metal cools too quickly on its way to the mold. Therefore, it doesn’t flow well enough to completely fill the mold cavity. This differs from a cold shut. With a cold shut, the manufactured piece appears completely intact, while with a misrun, the piece is clearly (usually) missing something.
Misrun pieces generally end up in the slag heap for re-melting. Pieces affected by cold shuts often pass visual inspection and go right on out the door as finished products. That gives trouble a head start, if the defect proves substantial.
Cold Shut – What to Look For
You’ve no doubt heard the phrase that a chain is only as strong as its weakest link. That weakest link could suffer from a fatal case of cold shut. A defective tool, auto part or a piece of construction steel can harbor potentially catastrophic hairline fractures. That part has already sneaked past the gatekeepers in the final inspection process, so you have to know what to look for, if you expect to spot a problem.
Before using a critical piece of metal, inspect it for integrity. Look for:
- Lines. They may look like simple scratches, but they could point to a cold shut. What could have left a scratch on your heavy crescent wrench? Not much. So you might want to take a closer look.
- Pits. Your new manifold should not have pits on the surface right out of the box. Check to see how deep they go, and whether they connect.
- Swirls. When the molten metal cools unevenly, you can see curved lines, arcs or even a wave pattern on the surface.
- Bumps. If your metal piece looks like it has a case of acne, beware.
- Cracks. Cracks point more surely toward cold shut than just about any physical sign. They found cracks in the Liberty Bell and look what happened to it!
Some Defects Can Be Forgiven
Just because you found a suspicious looking spot on your elbow conduit doesn’t mean you can’t use it. Cold shuts in non-critical locations can be harmless. You don’t like to see them, but the piece may serve you well enough in spite of the flaws. Frankly, you could spend all day in the hardware store looking for a perfect part and not find one.
Consider the stress points of the item. Evidence of a cold shut anywhere near a major angle or point of great stress could spell trouble. Elsewhere, and maybe you get by with it.
Other Casting Defects – Die Casting
- Shrinkage. Shrinkage occurs when the metal withdraws from the sides of the mold as it cools. Some cavities may form, both internally and externally.
- Holes. Pinholes occur in large numbers near the top edge of a piece of metal. Blowholes run larger than pinholes, but may escape detection until some of the surface metal is machined off. You can see open holes on the surface.
- Run outs and swells. Run outs and swells occur when the molten metal overruns its mold or expands over the top slightly.
Other Casting Defects – Sand Casting
- Cuts and washes. Cuts and washes occur as excess metal on top of the surface. These can usually be machined off without compromising the quality of the metal piece.
- Fusion. Fusion occurs in sand casting when the molten metal fuses with sand. It appears as a crusty patch on the surface.
- Rat tails, veins and buckles. Irregular lines or cracks develop when the sand buckles up. Buckles- more severe occurrences – pose the greatest risk to the integrity of the finished metal.
- Metal penetration. Examples of this defect are visible on the surface of the casting as snake-like lines of excess metal. This occurs when molten metal flows into gaps in the casting sand.
Repairing Casting Defects
A few repair techniques exist for repairing surface defects on base metal. Some involve welding, but some do not. The greatest chance for success resides with repairing surface defects – not suspicious-looking lines, cracks and sub-surface holes. Even if you consider yourself a particularly skilled welder, you should start with smaller, simpler projects and work your way up to more challenging projects.
Most casting defects occur with aluminum and iron, two metals that welders find more difficult to work with.
Chemical hardeners stand out as a viable solution to surface defects. Currently, the resin-and-epoxy combos that comprise most of the chemical hardeners for this purpose have to match the metal. In recent years, new products have come onto the marketplace that are multi-purpose. With this process – also called cold welding – the user fills small pits, holes and gaps in the base metal with a dual-component epoxy, smooths it down and polishes it until it matches the surface of the base metal.
This technique generally serves as a safe and appropriate repair for surface defects. In some cases the cold weld material out-performs the base metal in terms of hardness and endurance. Among this process’ many uses are metal pipes, conduits, car parts, bed frames, fence gates and more.
MIG welding techniques, with its wire feed system, can affect some repairs to surfaces marked with pits and holes. Where you would normally apply a lateral bead, you instead let fill material fall into the hole until the fill material mounds up over the top. After the project cools, any fill metal can be filed down with a mini-grinder.
Die casters mold cast iron at around 1450 degrees, a critical point for the molecular structure of iron.
Tips for welding repair of cast iron include:
- Use heavy coated nickel alloy electrodes. For deep defects, use a root pass electrode, like Nickel 99 in the beginning, and finish with Nickel 55.
- Preheat the entire casting, if possible. Otherwise, preheat as large of a general area as you can. Do not heat it warmer than 1400 degrees, however.
- Allow the casting to cool slowly. Wrapping it in a thermal blanket or burying it in dry sand can help.
- Watch for cracks as you weld. Cracks commonly appear alongside welding beads. These constitute a problem only if the casting was intended to be waterproof.
- The studding method. For larger defects, drill holes into the worst areas and thread screws into the holes, leaving 3/16 inch to be filled. Machine the area flush after it cools.
Cold Shuts in the Welding Process
Cold shuts can occur in the welding process as well as with the casting process. When it occurs in welding, it may go by the name of lack of fusion. The principle remains the same as with cold shuts within the casting process. Due to temperature differences, the two metals fail to completely fuse.
Often, the blame goes to a poor welding technique. If welder moves too slowly, the weld puddle becomes too large and weld metal runs ahead of the arc. If the welder moves too rapidly, his arc runs ahead of the weld puddle. Ideally, the arc should intersect the leading edge of the weld puddle as it moves across the base metal.
Forming too large of a weld joint can also cause cold shutting. If the arc moves down the center of the joint, the molten weld material will dash against the side walls of the base plate without melting them. Narrow the bed, or direct the arc more toward the side walls. A common, but incorrect technique is to cover multiple root passes with one giant finishing pass. Use a split-bread technique instead in this situation.
Avoiding Cold Shuts When Welding Aluminum
Cold shuts can occur when aluminum oxide is present. Aluminum oxide is insoluble in molten aluminum, so fusion with the weld metal fails. Therefore, remove all oxide just prior to welding.
Step one of this process involves the removal of any oil, grease or water vapor from the surface of the base metal. Use a lint-free cloth dipped in an environmentally-friendly (legal) solvent like acetone, methyl ethyl ketone, lacquer thinner or toluene. Do not use a wire brush. Using a wire brush only spreads the grease and oil around and drives them into crevices within the metal.
Mild alkaline solutions can also prove effective. But whatever solvent you use, make certain you wipe the base metal down thoroughly. After the work metal has been degreased, you can then use a fine-bristled steel brush, one used only for aluminum to rid the surface of aluminum oxide. Don’t bear down too hard, though, or you’ll force it into the metal rather than remove it. Lastly, wipe the metal down and make sure it dries completely.
Welders disagree on how soon you should work on aluminum after ridding it of aluminum oxide. Some say you should work on it within two hours at the very least. Others say you can wait a week or so. Oxide begins rebuilding itself immediately, but if you keep the piece dry at warm temperatures, the build-up is slow, and won’t reach the point where it could affect welding for months.
Cold shut can be a minor nuisance or a major headache, depending on its location and severity. A few surface dings on a metal work table won’t sink the Titanic, but a crack on a support beam should send out an a alarm of trouble ahead. Carefully inspecting metal for signs of cold shut or other casting defects isn’t OCD. It’s just good technique.