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Cylinder Head Crack Repairs
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Furnace Welding Cast Iron
Furnace welding cast iron is often called the "black art" of crack repair because it requires lots of heat and operator skill. Learning how to furnace weld cast iron is not something an inexperienced welder can pick up quickly. Those who have mastered the process say it takes at least six months to a year of constant practice to master it. If it were that easy, everybody would be doing it. It isn't, and that's why some shops have created a successful niche for themselves by specializing in furnace welding cast iron heads.
To furnace weld a cast iron head, the head is first preheated to 1,300° F (cherry red) in an oven. This step is absolutely essential to minimize thermal shock and to relax the metal so it won't distort when the torch is applied to the casting.
When the head preheat temperature has stabilized (it takes about an hour), a "neutral flame" oxyacetylene torch with slightly more acetylene than oxygen is used to melt the cast iron (which melts at 2,400° to 2,600° F). The crack can then be filled using a 1/4" cast iron filler rod and Borax flux. The trick here is to keep the weld clean by adding a little flux so the impurities will rise to the top. The impurities can then be floated out of the repair area with the torch.
When building up a worn or damaged valve seat in a cast iron head, a carbon graphite plug can be used to fill the hole. The weld is then built up around the plug. The puddle will be about half an inch deep and maybe two inches in diameter. It takes a lot of heat to do this, about 5,000° F.
After the crack has been filled comes a long, slow cool down. This step is also essential to prevent the head from recracking. If cast iron cools too quickly, the surrounding metal can shrink away from the weld causing new cracks to open. Carbon in the iron can also turn to carbide making the metal too hard and brittle to machine. The casting must therefore be cooled very slowly to prevent these undesirable metallurgical changes. Slow cooling can be accomplished by wrapping the head in an insulating blanket and keeping the head in a hot box so it will cool at a rate of no more than 200° F per hour. At this rate, it may take 8 to 12 hours to slow cool back down to room temperature. Once the head has cooled, it can be cleaned to remove scale, machined and pressure tested a second time to recheck for leaks. If no leaks are found, the final machine work and assembly can be done. Some shops will even pressure test the head a third time if it has pressed-in valve seats to make sure no cracks opened up after the seats were installed.
Because of the high temperatures involved in furnace welding, the induction hardening of integral valve seats is usually destroyed. This may require replacing the exhaust seats so the seats don't pound out when the head is returned to service. For this reason, some shops prefer to braze weld cracks rather than furnace weld. With braze welding, the job can be done at 800° to 900° F so the head doesn't get as hot.
Flame Spray Welding
Another technique for repairing cracks in cast iron heads and blocks is flame spray (powder) welding. The process is similar to brazing but uses nickel powder and a special torch to produce high strength repairs.
The basic technique for flame spray welding cast iron has been around for 40 years but has only recently been adapted for automotive applications. The main advantages of this technique is that it is easier and faster than furnace welding.
Flame spray welding can create a strong and long-lasting repair the same as furnace welding but without as much heat. The head still has to be preheated and postheated, but not as much nor as long. Spray welding needs about 1,000° to 1,400° F of preheat. Only the weld area gets hot so there is little risk of distortion.
Flame spray welding uses nickel-based powders and requires a special torch. The oxyacetylene spray welding torch has a trigger-operated hopper that feeds nickel powder into the flame. When the powder is exposed to the flame, it melts and fuses to the sides of the crack to fill the repair area. The process is similar to brazing but results in a much stronger repair. The powder can be used to fill holes and cracks, and to build up worn or damaged valve seats.
The process goes very quickly and can fill a three inch hole in less than a minute. With furnace welding, it might take three times as long to fill the same size hole - and then the part must be slow cooled for hours to keep it from cracking.
Flame spray welding is easier to learn than furnace welding, but still takes training and time to learn. Cracks must be properly prepared by grinding out damaged areas with a V-shaped chamfer. Both sides of the repair area must be clean. The key to success is applying the right amount of heat to the repair area. A dull cherry red, or about 1,300° to 1,400° F is needed; otherwise the nickel powder may not adhere properly.
TIG Welding Aluminum
Cracks in aluminum heads are most often repaired by TIG welding (though pinning also works with small accessible cracks). The first step in welding aluminum is preparing the crack. The head must be clean, grease-free and dry before grinding out the crack. The crack must be ground all the way out. Just grinding the surface and welding over a crack will likely be a short-lived temporary fix because the underlying crack is still there and will continue to grow. After grinding, the surface of the metal should be cleaned with a stainless steel wire brush.
When exposed to air, aluminum forms an oxide coating that contaminates the weld and interferes with fusing. A TIG welder prevents the formation of the oxide layer by bathing the weld with a steady supply of inert gas (usually argon). An alternating current is used to alternately heat the metal and burn off any oxide that forms.
Because aluminum can crack when subjected to too much heat in a concentrated area, the head must be preheated in an oven to 450° to 550° F to eliminate thermal stress. The added heat also makes the head easier to weld because aluminum conducts heat away from the weld area rapidly. Keeping the head hot is one of the tricks to successfully welding aluminum, which requires working in an area free from drafts and occasionally reapplying heat to the head.
Welding techniques vary, but the basic idea is to melt the surrounding metal and fill the crack with molten metal and filler rod. An experienced welder can even "recast" a badly damaged area, saving a head that would otherwise be junk. The strongest welds are achieved by using a filler rod that's the same alloy as the head, or very close to it. Two filler rods that work well are #4043 and #5356 with 5 percent magnesium. The type of electrode that's used on the welder can also make a difference. Tungsten thorium electrodes work well with aluminum but zirconium tungsten electrodes (which are much more expensive) are considered the best.
After welding, the head must be allowed to cool slowly. This is done by placing the head back in the oven or wrapping it in an insulating blanket. A long slow cool down relieves stress in the metal, which if not relieved could lead to cracking.
Pinning is the most commonly used technique for repairing cracks in cast iron heads because it's fast, reliable and cheap. It can also be used to repair aluminum castings, too. Pinning is a relatively easy technique to learn and use, doesn't require any special tools other than a drill, guide fixture and tap, and uses no heat.
The technique involves drilling holes in both ends of the crack to keep it from spreading, then drilling holes at various intervals along the length of the crack, installing overlapping pins to fill the crack, then peening over the pins with an air hammer to seal and blend the surface. Either tapered pins or straight pins may be used.
Tapered pins pull themselves into a crack as they are tightened to provide a tight seal along the entire length of the pin. This occurs because the threads on both the tapered pin and hole have an interference fit. Sealer really isn't necessary, but may be used for added insurance. The holes for tapered pins must be carefully hand tapped with a tapered tap, and the pins hand tightened.
Straight pins, by comparison, can be installed with an ordinary straight tap and a power drill. Straight pins, however, must be sealed by a tapered shoulder on one end of the pin and/or with sealer.
If a crack is along an outside edge or corner that require support to hold the sides of the crack together, or if the crack is in an area that would open up or pull apart when the casting is under load or gets hot, ordinary pins won't work. One solution is to use "locks" to hold the two sides of the crack together, and/or to use special pins that have a "spiral hook" or "reverse pitch" thread pattern. These pins can actually hold a crack together rather than just fill it.
Cracks in thin areas of a casting (thinner than 1/8") can be difficult to repair because the metal isn't thick enough to support the threads on a standard pin. For these applications, very small pins must be used to fill the crack.
On some applications, such as the Dodge 318 Magnum heads, the crack between the valve seats can often be repaired with a single soft steel pin that has a countersunk shoulder. A steel pin works best in this application because it can withstand heat better than a cast iron pin. After the crack has been fixed, the seats can be remachined. There should be no need to cut the head to accept valve seat inserts.
If a valve seat is badly damaged or recessed and must have a new seat installed, use the smallest diameter and shallowest seat that will fit the application. This will reduce the risk of creating future cracks from overstressing the head.
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TROUBLESHOOTING FLAME-SPRAY PROBLEMS
If you're having trouble achieving good results with flame-spray welding, here are some hints that may improve your success:
If the nickel powder just lays there or stacks up instead of wetting out when starting a weld, the work is too cold. The chamfer must be a dull cherry red, or about 1,300° to 1,400° F. Trying to force the base metal up to temperature by heating it though a thick .020" or greater layer of unfused powder will likely burn the flux out of the powder.
If the nickel powder boils up when starting a weld and you get a volcanic like reaction when the initial burst of powder is sprayed into the chamfer, the work is too hot. Back off the torch, allow the area to cool to a dull cherry red color, then hold the tip of the blue cone in the flame about 1/8" to 1/4" from the deposit and add fresh powder in short bursts. This should add enough new flux to wet out the powder properly. If this doesn't work, stop, scrape the chamfer clean and start over.
If too much powder is applied during the initial spray, it won't wet out. When unfused powder builds up to .020" or thicker, it acts like an insulator between the flame and base metal. Stop, scrape away the unfused powder, and start over.
Improper flame angle can also cause the powder to ball up and not wet out. If the flame angle is less than 75° F, powder can blow ahead of the puddle and stick to the chamfers. When unfused powder stacks up to about .020" or thicker, it won't wet out properly. Trying to force it out with the torch will create porosity and hardness problems. Stop, scrape or brush away the unfused powder, hold the torch at the proper angle and start welding again.
Porosity and hardness problems in the weld can be caused by surface contamination adjacent to the "V" chamfer. On some castings, manifolds in particular, the surface may contain a lot of "burned" iron. This should be ground away leaving sound, clean metal at least 3/8" to 1/2" on either side of the "V" chamfer. Rusty, dirty or burned metal will generally pop and crack when a flame is applied to it.
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