Welding Cast Iron and Other Irons

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Understanding Cast Iron Types and Properties

The term cast iron broadly describes many iron types that are castings with varying properties and purposes. Generally, cast iron is an iron, carbon, and silicon alloy containing more carbon (usually 1.7-4.5%) than can be retained in solid solution in austenite at the Eutectic temperature. Pig iron, the blast furnace product cast into pigs or ingots for later remelting, can be considered a form of cast iron. However, gray iron remains the most widely used type, with production exceeding that of any other cast metal.

Gray iron typically has a pearlite matrix structure with graphite flakes dispersed throughout. The automotive industry extensively uses gray cast iron for engine blocks and heads, automatic transmission housings, differential housing, water pump housing, brake drums, and engine pistons. Exceptions are usually aluminum components, which are readily distinguishable from cast iron.

Alloy cast irons contain small amounts of chromium, nickel, molybdenum, copper, or other elements that provide specific properties, particularly higher strength. These higher-strength irons are commonly used for automotive crankshafts and are sometimes called semisteel or given proprietary names.

Austenitic cast iron, modified with nickel and other elements to reduce the transformation temperature, maintains an austenitic structure at room temperature and offers high corrosion resistance.

White cast iron features carbon primarily in combined form, resulting in higher hardness suitable for abrasion resistance applications.

Malleable iron is produced by subjecting white cast iron to a special annealing heat treatment that changes the carbon structure to pearlitic or ferritic, increasing ductility.

Nodular iron and ductile cast iron are more ductile than gray cast iron. The addition of magnesium or aluminum either binds the carbon in a combined state or gives free carbon a spherical/nodular shape rather than the typical flake form in gray cast iron, providing greater ductility or malleability.

Gray cast iron exhibits very low bendability and ductility, with a maximum of about 2% ductility in the extreme low carbon range. This low ductility results from graphite flakes acting as discontinuities.

Most welding processes create expansion and contraction through heating and cooling cycles, generating tensile stresses during contraction. This makes gray cast iron difficult to weld without special precautions. However, ductile cast irons such as malleable, ductile, and nodular iron can be successfully welded, preferably in their annealed condition.

Preparation Techniques for Cast Iron Welding

Proper preparation is essential when welding cast iron components. All surface materials must be completely removed from the weld area, including paint, grease, oil, and other foreign substances. Brief heating of the weld area is advisable to eliminate entrapped gas from the base metal's weld zone.

When grooves are necessary, a V-groove with a 60-90° included angle should be utilized. Complete penetration welds are essential, as any crack or defect not fully removed may quickly reappear under service conditions.

Preheating is beneficial with all welding processes, though it can be reduced when using extremely ductile filler metal. Preheating diminishes the thermal gradient between the weld and the remainder of the cast iron. Appropriate preheat temperatures depend on the welding process, filler metal type, mass, and casting complexity.

Arc Welding Techniques for Cast Iron

The shielded metal arc welding process is effective for cast iron welding. Four filler metal types are available:

• Cast iron covered electrodes
• Covered copper base alloy electrodes
• Covered nickel base alloy electrodes
• Mild steel covered electrodes

Each electrode type serves specific purposes related to deposit machinability, color match, strength, and final weld ductility.

When arc welding with cast iron electrodes, preheating to 120-425°C is necessary, depending on casting size, complexity, and whether the deposit and adjacent areas require machining. Generally, small-size electrodes with relatively low current settings are optimal. A medium arc length is recommended, and welding should ideally be performed in the flat position.

Two types of copper-base electrodes exist: copper-tin alloy (ECuSn-A and C) and copper-aluminum (ECuAl-A2). Copper-zinc alloys are unsuitable for arc welding electrodes due to zinc's low boiling temperature, which causes volatilization in the arc and weld metal porosity. Copper-tin electrodes produce braze welds with good ductility. ECuSn-A contains less tin and serves as a general-purpose electrode, while ECuSn-C provides stronger deposits with higher hardness.

The copper-aluminum alloy electrode (ECuAl-A2) delivers much stronger welds and is suitable for higher-strength alloy cast irons.

When using copper-base electrodes, a 120-200°C preheat is recommended, along with small electrodes and low current. The welding technique should direct the arc against the deposited metal or puddle to prevent penetration and mixing of base metal with weld metal. Slow cooling is recommended after welding. However, copper-base electrodes do not provide a good color match.

Three nickel electrode types are used for cast iron welding: ENiFe-CI (approximately 50% nickel with iron), ENiCI (about 85% nickel), and ENiCu (containing nickel and copper).

The less expensive ENiFeCI electrode delivers results approximately equal to the high-nickel electrode. These electrodes can be used without preheat, though heating to 40°C is recommended. Nickel and nickel-iron deposits are extremely ductile and resist brittleness from carbon pickup. Minimizing penetration into the cast iron base metal can reduce heat-affected zone hardness.

The copper-nickel type comes in two grades: ENiCu-A (55% nickel, 40% copper) and ENiCu-B (65% nickel, 30% copper). Either electrode can be used similarly to nickel or nickel-iron electrodes with comparable technique and results. However, these electrodes do not provide a color match.

Mild steel electrodes (E St) are not recommended for cast iron welding if the deposit requires machining. The mild steel deposit will absorb sufficient carbon to create a high-carbon deposit that resists machining. Additionally, the mild steel deposit will have reduced ductility due to increased carbon content. These electrodes should only be used for small repairs where machining is unnecessary.

For small repair jobs, minimal preheat is possible. Small electrodes at low current are recommended to minimize dilution and avoid shrinkage stress concentration. Short welds using a wandering sequence should be employed, and the weld should be peened as quickly as possible after welding. Mild steel electrode deposits provide a fair color match.

Oxy-Fuel Gas Welding for Cast Iron

The oxy-fuel gas process is commonly used for cast iron welding. Most fuel gases are suitable, and the flame should be neutral to slightly reducing. Flux should be used. Two filler metal types are available: cast iron rods (RCI and A and B) and copper-zinc rods (RCuZn-B and C).

Welds made with the proper cast iron electrode will match the base metal's strength. The RCI classification is used for ordinary gray cast iron. RCI-A contains small alloy amounts for high-strength alloy cast irons, while RCI-B is used for malleable and nodular cast iron welding. All these welding rods provide good color matches. Optimal welding procedures regarding joint preparation, preheat, and post-heat should be followed.

Copper-zinc rods produce braze welds and come in two classifications: RCuZn-B (manganese bronze) and RCuZn-C (low-fuming bronze). The deposited bronze offers relatively high ductility but does not provide a color match. The deposited bronze offers relatively high ductility but does not provide a color match.

Modern Welding Processes for Cast Iron

Gas Metal Arc Welding

The gas metal arc welding process is effective for joining malleable iron and carbon steels. Several electrode wire types can be used:

• Mild Steel (E70S-3) using 75% Argon + 25% CO₂ for shielding
• Nickel Copper (ENiCu-B) using 100% Argon for shielding
• Silicon Bronze (ECuZn-C) using 50% Argon + 50% Helium for shielding

Small-diameter electrode wire should be used at low current in all cases. With mild steel electrode wire, the Argon-CO₂ shielding gas mixture minimizes penetration. Nickel-base and copper-base filler metals produce extremely ductile deposits. Mild steel provides a fair color match. Higher preheat is usually necessary to reduce residual stresses and cracking tendencies.

Flux-Cored Arc Welding

This relatively recent process has been successfully applied to cast iron welding. The most successful application uses a nickel-base flux-cored wire that produces a weld metal deposit similar to the 50% nickel deposit provided by the ENiFe-CI covered electrode. This electrode wire typically operates with CO₂ shielding gas but can function without external shielding gas when lower mechanical properties are acceptable. Minimal preheat temperatures can be used, and the technique should minimize penetration into the cast iron base metal. Postheating is normally unnecessary. However, a color match is not achieved.

Flux-cored self-shielding electrode wires (E60T-7), operating with electrode negative (straight polarity), have also been used for certain cast iron to mild steel applications. This produces a minimum penetration weld, and proper technique should keep penetration minimal. It is not recommended for deposits requiring machining.

Alternative Welding Processes for Cast Iron

Other welding processes can also be applied to cast iron. Thermit welding has been used to repair certain cast iron machine tool parts. The procedure matches that used for welding steel, except that a special thermit mixture is required.

Soldering can join cast iron and is occasionally used to repair small defects in small castings. Soldering is performed with an iron or torch. Flash welding is another viable method for welding cast iron.