Cast Iron Welding Rods: Selection & Use

(1) Cold and Hot Welding Methods for Cast Iron

The welding process for the deposition metal of cast iron welding rods can be divided into arc welding, gas welding, and arc cold welding for cast iron-type deposition metal, while it is arc cold welding for non-cast iron-type deposition metal.

Arc Hot Welding for Cast Iron-type Cast Iron Welding Rods

Preheat the entire workpiece or locally preheat the defects and surrounding areas to 500~700°C. The preheated area should appear dark red, then commence welding. The welding process should involve post-weld slow cooling for cast iron welding repair.

Gas Welding

Use an oxygen-acetylene flame, suitable for welding thin-walled cast iron. Preheat the area to be welded to 500~700°C using an oxygen-acetylene flame, then perform gas welding repair using a suitable cast iron welding core for the defects. The welding flame should be neutral or slightly carburizing. During the repair process, alkaline oxides should be added to ensure timely removal of oxidized silicon by the flame.

After the repair, the welded part should be slowly cooled, which can be achieved by using furnace insulation or post-weld heat treatment with an oxygen-acetylene flame.

Arc Cold Welding for Cast Iron-type Cast Iron Welding Rods

The characteristic of this method is that the workpiece is not preheated before welding repair. This method provides good working conditions, but the welded joint is prone to white mouth and hardened structure.

There are two solutions: by using welding rods with a large amount of graphitizing elements or by using large-diameter welding rods and high welding currents to increase the welding heat input, which helps in slow cooling of the workpiece and promotes graphitization. The above solutions have a certain effect on the deposition metal, but it is difficult to completely eliminate the tendency of white mouth in the heat-affected zone.

Therefore, this method is suitable for repairing large and medium-sized gray cast iron defects with not too much rigidity.

Arc Cold Welding for Non-cast Iron-type Cast Iron Welding Rods

This method, also known as arc cold welding for heterogeneous weld seams, uses welding rods made of deposition metal that do not produce white mouth and hardened structures, such as nickel-based welding rods, nickel-iron welding rods, etc. When combined with the welding repair process with no preheating and low heat input, it can effectively solve the occurrence of white mouth and hardening in the deposition metal and heat-affected zone.

(2) Key Points for Selecting Cast Iron Welding Rods

Welding Rods for Gray Cast Iron

For gray cast iron, the following welding rods can be used: Z208 and Z248 for homogeneous welding (used in hot welding); Z308 for heterogeneous welding (used for workpieces with high post-weld machining requirements) and Z408 (used for high-strength gray cast iron), used in cold welding; Z508 welding rods have lower strength and poor crack resistance, and are only used for welding low-strength processed workpieces.

Other steel-based and copper-based welding rods have a significantly different color on the weld seam surface compared to the base material, so they are generally rarely used.

Welding Rods for Nodular Cast Iron

For nodular cast iron, the following welding rods can be used: Z238 and Z258 for homogeneous welding; Z408 for heterogeneous welding (used for welding machined surfaces) and Z116, Z117 (used for non-machined surfaces).

Welding Rods for Malleable Cast Iron

For non-machined surface welding, Z116 and Z117 welding rods can be used; for machined surface welding, Z408 welding rods are recommended.

Welding Rods for Vermicular Cast Iron

When performing arc cold welding, it is advisable to use Z408 welding rods.

The various types of welding rods compatible with cast iron and their applicable ranges are summarized in Table 2-72 for reference when selecting welding rods.

Table 2-72 Various Types of Welding Rods Compatible with Cast Iron and Their Applicable Range

Base MetalApplication RangeTypes of Welding Rods
Z308Z408Z508Z208 Z248Z116 Z117
Gray Cast IronShrinkage Cavity Welding RepairA
Joining
Crack Welding Repair
Ductile IronShrinkage Cavity Welding Repair
JoiningC
Crack Welding RepairC
Malleable IronShrinkage Cavity Welding RepairC
Joining
Crack Welding RepairBAEED
Note: A is excellent; B is good; C is fair; D is poor; E is very poor.

(3) Precautions for Using Cast Iron Welding Rods

Preparation of the base material: If the cast iron has been contaminated with oil, the base material should be heated to 400°C before welding to burn off the oil. Other impurities should also be removed before welding. When repairing defects, the defects should be completely removed by mechanical processing or grinding wheel before welding.

If there is a risk of crack extension during the removal of crack defects, stop-crack holes should be drilled at both ends of the crack before welding.

For hot welding workpieces, preheat them to 500-700°C, then weld continuously, maintaining a temperature close to this range throughout the welding process. After welding, cover the workpiece with insulating material while it is still red-hot to allow for gradual cooling, promoting graphite formation.

For cold welding workpieces, to prevent excessive melting of the base metal and reduce the white layer, use small current, short arc, and narrow weld bead as much as possible. The length of each weld bead should generally not exceed 50mm. Immediately after each weld bead, perform hammering until the weld bead disappears. The temperature between weld beads should not exceed 60°C.

To prevent cracks, it is recommended to use back-stepping, symmetrical welding, and intermittent welding methods when welding long weld beads. For deep groove workpieces, pre-deposition edge welding is recommended to create a transition layer on the groove surface.

When terminating the arc, ensure that the crater is filled to prevent crater cracking.

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