How To Braze Cast Iron?


Cast iron is particularly made of an artificial alloy of iron, silicon, and carbon. In cast iron, a part of the carbon is graphite element. The total number of contents range between 1.7 to 4.5 percent. The fundamental usage of cast iron is to make water pipes, transmission housing, machine tool casting, stove castings, pistons, engine blocks, etc.

You will find the metal in a bronze welded state. Besides, you will see it in a gas and arc welded, machined or hardened form. One specific type of limitation of cash iron is – you must heat it before using in a welding project. If you think about repairing a cast iron, the best way is brazing, especially if the parts require rebuilding and machined to a specific tolerance level.

In case you want to braze cast iron, you do not have to melt the base metal because the rod possesses an integrated flux. The clearance level is at.003 (estimated) for full tensile and shear strength. The rod will take a maximum level of heat at more than 1400 degrees for penetrating the openings of the metal. Additionally, the rod used to braze cast iron can weld all types of cast iron with brass, copper, nickel, and many other types of steel.

 At first, you need to open the pours of the areas that need repairing. Then you will drill and grind out a shape like ‘U’ in the metal until it does not look like a hairline crack. Now, test with the cast iron while crunching a slot 1/16th to 1/8th of an inch into the metal. You should use a cutting or brazing tip for bringing the metal up to 1400 degrees. At 600 degrees, it will turn red; however, you must continue heating the metal.

After that, you will need to test the rod. Remember that the rod is in an outer position on the metal 1/4 inch the bulk of the flame. There is a flux core within the rod, which will melt down quicker than the rod. When the heat level is around 1400 degrees, it may seem the metal would have a hole due to extensive heat, but don’t worry as it will not happen.

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If you observe that the rod is melting, you should graze the rod in the area where it needs repairing. Apparently, it will soften and fuse down into the metal because of the flux core. After pulling out the torch, for some 20 seconds, it will seem just like a cherry red element. Anyone following the aforementioned instructions will be able to make the sturdiest weld made with metal. Afterwards, you will just need to grind, shape, paint, and polish the weld according to your needs.

The Properties of Cast Iron

Cast iron properties include a ‘Brinell hardness number’ of 150 to 220 without any alloys and 300 to 600 (alloyed). It also has a workable strength of 25,000 to 50,000 psi and 50,000 to 100,000 psi. The first strength consists of no allows and the second one does have alloys. The definite gravity of 7.6 and the high compressive strength is four times the workable strength, making it a highly rigid element. The cast iron also comes with wear resistance and fair corrosion resistance features.

What is Gray Cast Iron?

If you allow liquefied pig iron to cool down slowly, you will see the chemical composite of iron and carbon will break into a particular level. The carbon will separate as small bits of graphite, lying randomly throughout the metal. The graphite-like carbon characterizes the gray cast iron. This type of carbon is different from the combined carbon element with a gray appearance.

As you know that graphite is an exceptional lubricant type, the metal within the graphite is shot all over with small flaky cleavages. You can machine the gray cast iron smoothly, but it will not hold onto a substantial shock. Additionally, the gray cast iron includes 90 to 94 percent metallic iron, having a blend of carbon manganese with phosphorus, silicon, and sulfur. The high-strength grade of metal of gray cast iron consists of 0.75 to 1.50 percent nickel and 0.25 – 0.50 percent chromium.

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Overall, the more there is free carbon in the cast iron, the lower the combined level of carbon material and the softer the iron will be.

What Are The Tests for Gray Cast Iron?

The Appearance Test

The gray cast iron casts a gray and dull colored surface. It might be roughened due to the use of the sand mold in casting the part. Typically, the cast iron casts are not machined and un-machined moldings are used to eliminate the bumpy edges.

The Fracture Test

To execute this test, you have to scratch a corner from every side with a hacksaw or chisel. You need to hit the corner severely with a hammer. The fine black dots of carbon (will seem like graphite) will turn dark gray color (on the broken surface). When you fracture the cast iron, it will come short. Once they are molded, the small and stiff chips made of from chisel will break off.

The Spark Test

Small portions of dull-red sparks, which will follow the straight-line (near to the wheel) are given off. These sparks would break up into several recurring spurts that will change into a straw color.

The Torch Test

This test will provide a pool of liquefied metal (it is noiseless and possesses consistency similar to that of a jelly). Once you raise the torch flame, the depression in the surface of the pool of liquefied metal will disappear quickly. The pool of liquefied metal will take time for hardening and will give off no sparks.

What Are The Different Brazing Cast Irons?

White Cast Iron

Once the gray cast iron transforms into a molten state (for the heating procedure) the carbon will entirely dissolve within the iron. If you cool down the molten metal quickly, there will two elements remaining in a combined state. In the meantime, the cast iron will form. You will see the carbon is more than 2.5 to 4.5 percent by weight and it will be a part of combined carbon type. Usually, the white cast iron is hard and stiff with silvery white fracture, which is difficult to machine.

Malleable Cast Iron

You can get this type of cast iron through white cast iron. The required temperature level for heating would be 1400 to 1700°F (760 and 927°C) and you need around 150 hours to keep it in the boxes with hematite ore or iron scale. This procedure will tear apart the joint carbon and change it into a free state. The free carbon isolates in an altered way than gray cast iron and known as temper carbon.

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In addition, it exists as small rounded particles of carbon. It usually provides malleable iron castings the capacity to bend prior to breaking. It can survive a substantial amount of shock than the gray cast iron. The castings include pure iron properties such as – ductility, capacity for resisting shock, toughness, and high strength. You can weld and braze malleable cast iron and all welded part will be hardened after welding.

The Appearance Test

A malleable cast iron’s surface is similar to that of gray cast iron. However, it is free from any sand and comes in a dull, light color than gray cast iron.

The Fracture Test

Once you fracture the malleable cast iron, the main part of the broken surface will change to a dark gray color with lively steel-like band at the ends. A picture frame best describes this appearance. The is stronger than other types of cast irons. When nicked, they will not break short.

The Spark Test

If you ground malleable cast iron, the outer layers will be sunny and will give off sparkling sparks. When the sparks reach to the interior section, they will transform to a light red color, close to the wheel. These sparks typically within the inner section are similar to that of cast iron. Nevertheless, they are much longer in shape and they are present with a big volume.

The Torch Test

To initiate the torch test, you would begin with boiling melted malleable cast iron beneath the torch flame. When the flame is taken off, the surface will include blowholes. If you fracture the liquefied parts, they will become solid and stiff with an appearance of cast iron. The outer, steel-like band will give off sparks, but the middle part will not give off any sparks.

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