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Cast Irons 1 Weld Tech News VOL 1. NO. 1 WELD TECH NEWS is a newsletter for welders working primarily in maintenance and repair. Each issue contains useful information on materials (cast irons, steels, aluminum, copper alloys, etc.), welding products, welding techniques and safety. By collecting each issue, the reader will soon have a handy reference manual covering all aspects of welding, brazing and soldering for maintenance and repair. CAST IRONS Types, Properties and Problems “Cast iron” is a general name applied to hundreds of different alloys of iron, carbon (generally between 2.0 and 4.5 percent) and silicon. Small amounts of sulfur and phosphorus are also present as undesirable elements. Other alloying elements (manganese, molybdenum, copper and others) can be added to increase strength and corrosion resistance. Cast irons vary in quality from cheap, low strength “pig iron,” used for counterweights, to high quality castings for railroad wheels, crank shafts, or pump housings. This article discusses the three most common types: grey cast irons, white cast irons, and malleable cast irons. Other types will be discussed in a future article. To understand any cast iron, we need to understand a little about iron and carbon. Carbon dissolves easily in molten iron, like sugar in hot coffee. When iron cools, however, the carbon cannot stay dissolved. It comes out of solution, either as hard iron carbides, or as soft bits of free carbon in the form of graphite (like the lead in a pencil). The hardness, machinability, brittleness, and strength of cast iron mainly depends on three variables: the ratio of graphite to carbide; the type of iron and iron carbide grain structure; and the size and shape of the graphite particles. These three variables depend in turn on the total amount of carbon; the amounts of alloying elements and impurities; the cooling rate when the piece is originally cast; and heat treatment after casting. The last two items above, cooling rates and heat treatment, are very important in determining the type of cast iron and how to weld it.

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Page 1: Cast Irons - Rockmountweldit.com/wp-content/uploads/2013/05/Vol.-1-No.-1-Cast-Irons.pdf · Cast Irons WTN #1 ! 3! White cast irons are made by rapidly chilling the mold, or a part

Cast Irons  

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Weld Tech News

VOL 1. NO. 1

WELD TECH NEWS is a newsletter for welders working primarily in maintenance and repair. Each issue contains useful information on materials (cast irons, steels, aluminum, copper alloys, etc.), welding products, welding techniques and safety. By collecting each issue, the reader will soon have a handy reference manual covering all aspects of welding, brazing and soldering for maintenance and repair.

CAST IRONS

Types, Properties and Problems “Cast iron” is a general name applied to hundreds of different alloys of iron, carbon (generally between 2.0 and 4.5 percent) and silicon. Small amounts of sulfur and phosphorus are also present as undesirable elements. Other alloying elements (manganese, molybdenum, copper and others) can be added to increase strength and corrosion resistance.

Cast irons vary in quality from cheap, low strength “pig iron,” used for counterweights, to high quality castings for railroad wheels, crank shafts, or pump housings.

This article discusses the three most common types: grey cast irons, white cast irons, and malleable cast irons. Other types will be discussed in a future article.

To understand any cast iron, we need to understand a little about iron and carbon.

Carbon dissolves easily in molten iron, like sugar in hot coffee. When iron cools, however, the carbon cannot stay dissolved. It comes out of solution, either as hard iron carbides, or as soft bits of free carbon in the form of graphite (like the lead in a pencil).

The hardness, machinability, brittleness, and strength of cast iron mainly depends on three variables: the ratio of graphite to carbide; the type of iron and iron carbide grain structure; and the size and shape of the graphite particles.

These three variables depend in turn on the total amount of carbon; the amounts of alloying elements and impurities; the cooling rate when the piece is originally cast; and heat treatment after casting.

The last two items above, cooling rates and heat treatment, are very important in determining the type of cast iron and how to weld it.

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If cast iron cools rapidly, more of the carbon comes out of solution in some kind of carbide grain structure. It can be either pure iron carbide (cementite) or various mixed grain structures of iron and iron carbide, such as pearlite. Slow cooling lets more carbon come out of solution as graphite. If the iron is reheated to over 1450° F, the grain structure can be changed, depending on the temperature, the length of heating time, and the rate of cooling.

This is very important for the welder. Any welding will produce some amount of localized heat treatment, or heat affected zone. This can be a problem or a benefit, depending on the materials and techniques involved.

Grey cast irons, the most common forms of cast iron, have all or most of their carbon in the form of graphite flakes. There are many types of grey cast irons, each with slightly different compositions and different properties.

Grey cast irons are cooled relatively slowly after the casting is poured. If broken, they have a characteristic grey, dull appearance, caused by the flat graphite flakes, often shaped like small potato chips. (See Figure 1.) Since the graphite is soft and weak, grey irons generally have relatively low tensile strength and are subject to brittle fracture.

If a grey iron casting has thick and thin sections, the thinner sections sometimes cool more quickly, giving mottled or white iron "chill spots”, where there is more hard carbide and less graphite.

Grey irons are easier to weld than most other cast irons. They can be machined at low speeds, but bits of sand in the surface (from sand casting molds) and hard chill spots are often hard on cutting tools.

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White cast irons are made by rapidly chilling the mold, or a part of the mold, just after the casting is poured. When white cast irons are broken, the high percentage of iron carbide grains and absence of any free graphite give the surface a characteristic silvery or whitish color. White cast irons are extremely hard, brittle, and non-machinable. They are usually impossible to weld.

Pure white cast irons are limited to applications where hardness is important and brittleness is not important such as extrusion nozzles.

Often castings are made of “chilled iron”, which combines a surface layer of hard, white cast iron with a grey iron interior having greater impact resistance. Chilled iron is used for railway car wheels, crushing rolls and heavy duty machine parts.

Malleable cast irons are made by annealing white cast iron of the proper composition. The castings are heated slowly in a furnace to over 1600°F, held there for several days, and then cooled slowly and carefully. The iron carbide converts to pure, soft iron, plus graphite. The graphite particles are more rounded than in grey irons. (See Figure 2.) As a result, malleable cast irons do not have the brittleness caused by the flat graphite flakes in grey cast irons. Malleable cast irons are used for things like cast iron pipe, where impact resistance and ductility are important.

The Welding Shop

WELDING CAST IRONS – MATERIALS AND TECHNIQUES

Most cast irons can be welded successfully – if you know how, and use the right materials. The following techniques have been developed by engineers of Rockmount Research & Alloys lnc. for use with the Rockmount maintenance and repair alloys mentioned below.

The keys to welding cast irons are choice of materials, joint preparation, temperature control, and welding procedures.

Materials

There are several types of materials that can repair cast iron, depending on the individual problem and the equipment available. These Rockmount products provide a complete range of solutions to all cast iron repair problems.

Jupiter-A, Jupiter-AAA, Jupiter-B, Jupiter-BBB and Jupiter-NM nickel bearing electrodes are the most widely used and most flexible materials for cast iron repairs. They combine high strength and high ductility. Jupiter-A is the strongest (79,000 p.s.i.) of the Jupiter arc products. Jupiter-B and Jupiter-BBB are slightly more machinable. Jupiter-BBB has a special non-conductive coating, which permits welding in tight spaces without side arcing. Jupiter-NM is used for welding large areas in castings where machinability of weld deposit is not required. All Jupiter electrodes are designed to run at low amperage and have a special flux coating that assists in removing contaminants in the slag, so they can be used even

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with dirty, oil-soaked castings. Jupiter TIG-B produces a similar deposit to Jupiter-B with maximum machinability.

Venus-A and Venus-B electrodes require even less heat per pound of filler metal and produce a deposit with a bronze tone. Venus-A is used with DC welders, while Venus-B can be run on either AC or DC.

Jupiter-G is a flux coated, special bronze alloy brazing rod with outstanding ability to wet out on cast iron. Since it can be applied at relatively low temperature, the heating effects on the cast iron are minimized. Jupiter-GB is the uncoated version of Jupiter-G.

Jupiter-GC is a brazing rod that provides a good color match with grey cast iron. Deposits match the base metal in expansion and contraction rate and can be porcelainized. lt deposits true cast iron without requiring high fusion temperatures.

In addition, metallic powders can be used to build up thin coatings on cast irons. Jupiter Powder is especially good for producing a machinable buildup to compensate for overmachining an area, or to repair casting defects. This and other powders, such as Apollo Powder for hardfacing, will be discussed in detail in a future issue of WELD TECH NEWS.

The choice of materials will depend on the need for strength and ductility, equipment and welder skills available for arc or gas, whether or not color match is needed with the base cast iron, and the size and location of the casting. In general, it is better to arc weld larger pieces, especially if they are outside or in a location where preheating is not practical.

Joint Preparation

It is important to prepare the cast iron surfaces to bond to the weld filler metal and to shape the joint to allow filler metal penetration, minimize expansion cracking, and “key-in” the base metal and the filler metal.

Any welding is better on clean surfaces. However, there are two special problems in cleaning cast iron. First, the surface is often slightly porous and usually has soaked up oil and dirt. The main problem, however, is that the free graphite in the cast iron sometimes makes it hard to bond the filler metal. Grinding the joint may just smear the graphite, making it worse. The easiest (and best) method is to gouge out the joint, using a gouging electrode like Electra. This will remove un-wanted material, shape the joint properly, burn out contaminants, and leave the joint in perfect condition to receive the filler metal.

For any arc welding, the joint must be open enough to let the first joining pass penetrate to the bottom. This usually means cutting a vee-joint, as shown in Figure 3. The vee should be open at the bottom. If the piece is very thick, and the back is accessible, a double vee can be cut.

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To “key in” the weld, grooves can be cut in the side of the joint as shown. In addition, it is often helpful to drill and tap large castings and screw in threaded inserts to key in the weld metal (and sometimes to hold the broken pieces in place while welding).

If there is a crack in the middle of a casting, it is very important to drill a hole at each end of the crack. This will prevent the crack from extending when the casting is heated during welding. Then the crack should be gouged out with Electra to remove any damaged metal and allow room for proper welding.

Temperature Control

Improper temperature control in welding cast iron can cause cracking from, expansion and contraction stresses and from conversion of graphite into brittle iron carbide.

The first point is shown in Figs. 4a, b, and c. In each case, the heated area is shaded and the expansion is shown by arrows.

In Figure 4a, the heating produces little stress, since the part is free to expand. In Figure 4b, the heat expansion tries to produce a crack at the weak section at point x. Figure 4c shows an existing crack in the middle of a casting. If the crack is not stopped, by drilling a hole at each end, the heat of repair welding will try to extend the crack at points y and z.

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Remember that expansion stresses are caused by differences in temperature. If the entire piece is heated or cooled evenly, there is no problem. The problem starts when one area, such as a welding zone, is heated much more than the rest of the casting.

If cast iron is heated over 1400°F, the grain structure will start to change. This can occur next to the weld zone. If this heat affected zone is then chilled too rapidly (by exposure to air or by conduction into adjacent cold parts of the casting, for example), a very hard, brittle zone of iron carbide can be produced. This is why improperly welded iron castings sometimes crack in the base metal next to the weld.

There are two different approaches to controlling the heat input and its effect on expansion stresses and carbide embrittlement.

The first approach, which is generally suitable for smaller pieces and shops where heating furnaces are available, is to preheat the entire piece evenly.

Preheating temperatures can be 500°-1200°F for arc welding, and 900°-1200°F for gas welding, although much lower preheat temperatures may be adequate. Remember, some preheat is better than none.

Preheating reduces the wide temperature differences caused by welding and slows down the rate of cooling from the weld zone into the rest of the casting.

In many cases, however, the casting is too big or is located so that preheating is not practical. In these cases, we need another approach.

The other approach is to add as little heat us possible, thereby keeping the heat affected zone as small as possible. The casting is not preheated at all. The filler metal is added a few inches at a time and allowed to cool before adding more metal. This method is especially suitable for use with the Jupiter and Venus electrodes that are especially engineered to require minimum heat input. Each pass creates a minimum amount of carbide in the base casting and tends to anneal any minor brittleness caused by previous passes.

As metal cools, it shrinks. Depending on the shape of the casting and the weld, the weld may try to pull away from the base metal. The solution is to choose an especially ductile filler metal, such as Jupiter-A, AAA, B, or BBB; keep the piece as cool as possible, and peen each pass of the weld metal with light blows of a round-nosed hammer while it is hot. This peening will cause the filler metal to stretch slightly and fill the required space without cracking.

After welding, the piece should always be cooled as slowly and evenly as possible to minimize carbide formation.

Welding Procedures

Whenever possible, the edges to be joined should be “buttered” first with weld metal and then joined, as shown in Figure 5. The buttering seals in contaminants, adds some preheat to the area, and allows the metal

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in the joint area to expand without restraint. In joints that are not preheated by buttering, the root pass should be fairly heavy. If it is too small, it will cool too quickly, starting a crack that will tend to continue up through later passes. When the final passes are made to close the joint, they add just enough heat to help anneal the heat affected zone caused by the buttering and the root pass.

Long weld beads should be avoided. Deposit narrow beads in 2-3" lengths, allowing each bead to cool until it can be touched by the hand before laying another bead next to it. While one bead is cooling, lay another bead in another area, as shown in Figure 6. This “skip welding" technique will reduce the stress from heating and cooling, keep the heat affected zone as small as possible, and provide some annealing of the heat affected zone in subsequent passes.

The above techniques can all be used with grey cast iron, except that preheat must be used with Jupiter GC. Malleable cast iron, however, will turn back into white cast iron if it is melted and then cooled at a normal rate. This will create a very brittle area, which can be re-annealed by a heat soaking and cooling cycle of several days, like the original malleabilizing treatment. Another method is to preheat the entire casting to 900°F and braze with a Jupiter-G rod. The cast iron surface should be heated just enough to “tin” it with the Jupiter rod and then the joint filled with the least possible heating of the casting.

More information about welding cast iron will be contained in a later issue of WELD TECH NEWS. Meanwhile, for help with specific problems or for more information on the products mentioned in this issue, contact your Rockmount sales representative.

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Safety Tips for Welders

WELDING FUMES AND HEALTH

All welders know they should keep from breathing welding fumes. Most do not know why. This article explains some of the problems, and some of the precautions welders can take.

There are four problems: Particulates, metal fumes, non-metallic fumes, and ozone.

Particulates are small bits of dust-like material that are carried in the hot air and gases generated by the welding heat. Some of these produce lung and nasal irritation, which can be serious if breathed in large amounts over a long period. The work area should be adequately ventilated, as discussed below. lf this requires mechanical ventilation, the particulates can be removed in a filtration system.

Metal fumes are gases from base metal or filler metal that is vaporized by the heat of the arc or torch flame. Fumes from several metals produce metal fume fever, with symptoms of nasal congestion, nausea, vomiting, stomach pains, diarrhea and muscle cramps, especially in the legs.

Metal fume fever is caused by short term, high exposures to zinc, magnesium, copper, and a few other metallic fumes. Zinc fumes can be caused by welding galvanized pieces or from work on some brasses. Copper fumes can come from copper coated MIG wire, for example.

Metal fume fever is a temporary condition and can be easily prevented by proper ventilation. lt can be serious, however, if normal precautions are not taken.

Non-metallic fumes come from vaporization of electrode coatings or brazing fluxes, or from grease, paint, cleaning compounds or other materials. The greatest problems in welding come from fluoride fumes generated from some coatings and fluxes. The solution is to clean the base metal, and to use proper ventilation.

Ozone is created by an electric arc reacting on oxygen from the air. Ozone is produced by all stick, TIG, or MIG welding. Ozone irritates the lungs and can be a problem to welders working in closed spaces. lt can be serious, but is easily controlled by proper ventilation.

What is proper ventilation? ln a large room with high ceilings, natural air circulation may be enough. Even so, it is common sense to keep your head to one side, not directly over the fumes rising from the weld zone.

If the ceiling is low or if the space per welder is relatively small, mechanical ventilation may be needed. There are three methods: area ventilation, local exhaust, or air supplied respirators.

Area ventilation can include room exhaust fans, welding booth hoods, and/or fans blowing the welding fumes away from the welder. Remember that you need to get the fumes away from the welder, not just stir them around.

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Local exhaust systems have small, high-velocity, exhaust ducts with a flexible hose leading to a suction nozzle that can be positioned next to the weld zone. The suction should produce an effective draft at the weld zone of at least 100 feet per minute. This means that a small 3” nozzle must be very close to the work (4-6 inches). As the distance increases, the effectiveness drops off sharply, unless the duct size and the exhaust fan capacity are greatly increased.

Sometimes welders have to work in tight spaces where neither area ventilation nor local exhaust is practical. The solution then is to use a fresh air respirator.

Remember: high ceilings and large spaces… or ventilation, the more ventilation, the better... and keep your head out of the fumes. Follow these simple rules and enjoy good health.

(For more details, read ANSl Standard Z49.1, “Safety in Welding and Cutting”, available by sending $10.00 to the American Welding Society, 2501 N.W. 7th Street, Miami, Florida 33125)

WELD TECH NEWS is published by Rockmount Research & Alloys lnc. Vancouver, Washington - Denver, Colorado ©Copyright 2013 by Rockmount Research & Alloys lnc.