You’ll get the most out of this guide if you need to learn weld overlay.

Combine this information and you are well on your way to becoming a savvy  weld overlay expert.

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What does weld overlay mean?

Weld overlay is to increase the size of the weldment by welding, restore the weldment, or deposit metal with select properties on the surface. Welding uses a welding heat source to form a surface metallurgy bond between the surface of the substrate and the material.

Weld overlay purpose is not to connect parts, but to deposit one or more layers of materials with the desired properties by welding. It can obtain the required cladding layer with unique features such as wear resistance, heat resistance, and corrosion resistance.

Weld overlay has four categories:

  1. Wear-resistant surfacing: The welding layer used to reduce abrasive wear, corrosion, impact, or cavitation on the surface of the weldment.
  2. Cladding overlay: For surface corrosion-resistant, a certain thickness of filler metal is deposited on the carbon steel or alloy steel base material.
  3. Isolation layer overlay welding: When welding different materials, there may be special requirements. Ensure the quality and performance of the joint; the base material must be isolated in advance.
  4. Thickening surfacing: The metal added on the weldment surface, the edge of the joint, or to restore the required size of the component.

Why weld overlay?

The most widely used industries for overlay welding are roll surface repair and cutter edge repair. Overlay welding is a welding process in which a material with specific properties is covered on the welding surface of a workpiece; the purpose is to restore the hardness of the outer surface of the workpiece or increase the external size of the workpiece. After the welding layer of the workpiece material is worn, re-welding can improve the service life of the entire part. Save manufacturing costs and maintenance costs, reduce the time for repair and replacement of the whole section, reduce the number of production shutdowns and production loss, and reduce the overall cost of production. Surfacing welding can make better use of materials to obtain excellent comprehensive performance, which is also of great significance for improving product design. The content of surfacing welding mainly includes two aspects. One is the welding alloy, and the other is the welding process.

Wheel cutter weldingWheel cutter welding

The function of overlay welding is mainly to improve the resistance of parts to wear, corrosion resistance, impact resistance, and performance at high temperatures. The wear resistance of welding materials refers to the ability of the elements to resist wear under certain friction conditions. It is also the most common use of welding in the manufacture and repair of workpieces. The wear of the workpiece surface includes four cases: adhesion wear, abrasive wear, invasion corrosion, and gas corrosion, and fretting wear. Adhesive wear refers to the wear caused by the contact between two contact surfaces that are fused due to adhesion, and the material on one workpiece surface is transferred to the other workpiece surface. Adhesive wear is divided into three levels according to the degree of wear, the lighter is mild wear, the heavier is severe wear, and the most serious is the surface of the workpiece tearing in a block. Adhesive wear generally uses a welding wear-resistant layer on the surface of the workpiece to reduce adhesion wear. A commonly used welding alloy is a nickel-based wear-resistant welding alloy.

Double-head submerged arc surfacing

Double-head submerged arc surfacing

Abrasive wear refers to hard particles or micro-bumps on the surface where the two workpieces are in contact. The particles and bumps wear away the material during the friction process. According to the magnitude of the force, the abrasive wear can be divided into low-stress abrasive wear, high-stress abrasive wear, and chisel abrasive wear. Common examples of low-stress abrasive wear are surface wear of plowshares, and high-stress abrasive wear is standard in surface wear of ball mill liners and balls. Chiseled abrasive wear is typical in the surface damage of bucket teeth of excavators. This type of wear surfacing generally uses a large amount of carbide-containing alloys.

Gear repair overlay welding

Gear repair overlay welding

Invasive corrosion refers to the abrasion that occurs when a fluid containing microparticles flows on the surface of a pipe or workpiece. The repair or cover surface of invasion corrosion generally uses a wear-resistant surfacing alloy with high carbide content. Bubble corrosion phenomenon refers to the abrasion of metal caused by the shock wave generated when the bubble bursts on the metal surface, which usually occurs in liquid valves or high-speed pumps. Bubble corrosion wear generally uses cobalt-based wear-resistant surfacing alloys. Fretting wear refers to friction and wears caused by small movements on the surface of parts due to low amplitude vibrations at the connection of two or more workpieces. This type of wear is caused by the repeated scraping of the workpiece surface by hard metal oxide particles. This type of wear is usually repaired and covered with a cobalt-based wear-resistant surfacing alloy.

Roller repair surfacing

Roller repair surfacing

Another use of stacking is to improve the corrosion resistance of the workpiece. There are two types of workpiece corrosion, one is chemical corrosion, which refers to the chemical reaction between metal and medium, and the other is electrochemical corrosion, which makes the metal contact with the electrolyte solution to react. Workpiece corrosion is commonly found on the inner walls of petrochemical containers. The metals used for this type of surfacing are mainly copper-based, nickel-based, cobalt-based alloys, and nickel-chrome austenitic stainless steel. The surfacing material also needs to be impact resistant, because when the metal surface particles are worn, they will also be impacted by these particles. This impact is usually divided into three categories: mild, moderate, and severe. There is an absolute contradiction between impact resistance, and abrasion resistance, so careful consideration should be made when selecting materials. When the working environment of the surfacing workpiece is at a high temperature, the surfacing content is required to have high temperature rigidity. And it has higher requirements for the material’s thermal strength, hot rigidity, thermal fatigue resistance and oxidation resistance, and high temperature gas corrosion resistance. This type of surfacing should use high chromium martensitic stainless steel, tool steel, mold steel, nickel-based surfacing alloy, cobalt-based surfacing alloy, and other surfacing materials according to different use environments.

The surfacing of drill bits in the oil drilling industry

The surfacing of drill bits in the oil drilling industry

Metal materials for surfacing are divided into five categories: one iron-based surfacing metal, two nickel-based alloys, three cobalt-based alloys, four copper-based alloys, and five carbide alloys. Pearlite surfacing metal. This type of surfacing alloy has excellent welding performance, strong impact resistance, and low hardness. It is mainly used to repair mechanical parts such as shafts and rollers to achieve the restored size. The goal of austenitic surfacing metals, including austenitic manganese steel and chromium-manganese austenitic steel. Austenitic manganese steel is mainly used to repair metal wear and abrasive wear parts under severe impact loads, such as railway turnouts for mining trucks.  Chromium-manganese austenitic steel is primarily used to improve manganese steel and carbon steel parts, which are worn by severely impacted metal parts. Martensitic surfacing metal is used primarily to repair friction and wear between metals, such as gears, chassis, etc. Alloy cast iron surfacing metals includes martensitic alloy, austenitic alloy, and high chromium alloy. Martensitic alloy cast iron surfacing layer has high resistance to abrasive wear, heat resistance, corrosion resistance, and oxidation resistance, and can withstand mild impacts. It is mainly used for surfacing welding of agricultural machinery, mining equipment, and other workpieces. Austenitic alloy cast iron surfacing metal has excellent corrosion resistance and oxidation resistance, has a certain toughness, can withstand moderate impact, and is less sensitive to cracking and peeling. It is mainly used on the occasion of abrasive wear of medium impact, such as excavation Surfacing of bucket teeth. High chromium alloy cast iron has an excellent heat resistance and wear resistance and is widely used for surfacing welding of wear-resistant materials on the surface of construction machinery, agricultural machinery, mining machinery, and other parts.

Shield excavator cutting tool welding

Shield excavator cutting tool welding

The most widely used nickel-based overlay welding technology is the nickel-chromium-borosilicate series alloy. It has excellent resistance to low-stress abrasive wear and intermetallic wear resistance, corrosion resistance, heat resistance, and high temperature oxidation resistance. It is commonly used in corrosive or high-temperature environments subject to low-stress abrasive wear. The most prominent feature of cobalt-based welding metal is that it can still maintain high hardness and strength at a high temperature of about six hundred degrees. So it is often used for surfacing welding on the surface of gears at high temperatures, such as surfacing welding on the surface of parts such as drills and hot punches. There are four types of copper-based alloys: bronze, pure copper, yellow child, and white copper. These materials are mainly suitable for working below 200 ° C. They are primarily used for surfacing welding of bearings, low-pressure valves, sealed end faces, and other parts. Carbide welding metal is characterized by a high melting point, high hardness, and excellent abrasion resistance, but has relatively large brittleness. It is mainly used for severely worn workpieces in the working environment, such as petroleum exploration drilling bits and shield excavators. Cutting tools, and so on.

How to weld overlay?

The weld overlay techniques usually include electrode arc welding, oxygen-acetylene flame welding, submerged arc welding, melting electrode gas shielded arc welding, tungsten arc welding, plasma arc welding, and electro-slag welding. The choice of surfacing method is generally determined according to the site construction conditions and technical requirements. Specific uses and characteristics of the surfacing way are shown in the table.

Uses and characteristics of common surfacing methods

  • Electrode Welding

    Features:

    1. Cheap and lightweight equipment, suitable for on-site surfacing

    2. High flexibility, especially suitable for overlay welding of irregularly shaped workpieces

    3. High productivity and low workpiece deformation

    4. The significant penetration depth and high dilution rate reduce the hardness and wear resistance of the surfacing layer. Usually, 2 to 3 layers are welded, but multi-layer surfacing is natural to cause cracking.

    Usage:

    Mainly used to produce small batches of surfacings and repair worn parts

  • Oxygen-acetylene Flame Welding

    Features:

    1. Due to manual operation, high labor intensity and low welding speed

    2. When high-quality overlays are required, the operation skills of the welder are top.

    3. If a particular oxygen-acetylene flame spray gun is used to spray alloy powder, small penetration depth and a thin overlay layer can also be obtained

    Usage:

    It is mainly used for surfacing welding of parts with smooth surface, high quality, and precision parts, as well as small-area surfacing welding on small and medium-sized workpieces.

  • Submerged Arc Welding

    Features:

    1. The welding process is highly mechanized, usually using large currents (300 ~ 500A), sometimes up to 900A, so the welding speed is significant, and the productivity is high.

    2. The performance of the surfacing layer is stable, the formation is beautiful, and defects such as pores and slag inclusions rarely occur

    3. The flux completely covers the arc, there is no arc radiation, no spatter, the operator does not need special protection, and the workload is reduced

    4.The equipment is not convenient to move; the drying and storage of the flux are complicated, which is not conducive to the on-site surfacing.

    5. The dilution rate is high, and it is often necessary to stack 2 to 3 layers to ensure the required performance. Due to the large thermal gradient of the workpiece during crack welding, it is easy to crack, so measures such as preheating and slow cooling are often required. But when the preheating temperature is too high, it will cause difficulty in de-slagging

    6. Large molten pool, only suitable for surfacing welding in a horizontal position

    Usage:

    It is the most widely used one of various overlay welding methods. It is widely used in the machinery manufacturing industry, especially in the manufacture of petroleum and chemical equipment. It is suitable for welding on larger surfaces, such as flat, cylindrical and large-diameter vessels. It is not suitable for surfacing small workpieces. Mainly for surfacing iron-based materials, such as surfacing corrosion-resistant layers on the inner walls of large-diameter vessels Overlay welding of wear-resistant layers on rolling mill rolls, etc.

  • Tungsten Arc Welding

    Features:

    1. DC positive connection can reduce the contamination of the surfacing layer by the tungsten electrode

    2. Stable arc, less spatter and excellent visibility during welding

    3. The feed of the surfacing welding material and the arc are adjusted separately, so the shape of the surfacing layer is easy to control, and the quality is good, but the deposition speed is not high

    Usage:

    It is suitable for surfacing welding of small size, high quality requirements and complex shapes. Such as overlaying very thin cobalt-based surfacing alloys on steam turbine blades, etc.

  • Plasma Welding

    Features:

    1. Due to the high plasma arc temperature, the surfacing speed is fast. The workpiece does not need to be preheated and insulated before and after surfacing, and no defects such as cracks and porosity

    2. Plasma arc surfacing also has the characteristics of shallow and wide penetration, thereby reducing the dilution rate of the surfacing metal by the parent metal. which not only stabilizes the hardness of the surfacing layer metal and has a uniform structure, but also allows the selection of thinner surfacing layers. To save precious metals

    3. The bead formation of plasma arc surfacing is very smooth and tidy, which can reduce defects and the amount of processing after surfacing

    Usage:

    Plasma flames can be used to overlay alloy materials that cannot be surmounted by other processes. In addition to brass, it can be used to overlay many alloys and non-ferrous metals. Such as cobalt-based cemented carbide, stainless steel, copper, aluminum iron and manganese bronze, etc.

  • Electroslag Welding

    Features:

    1. The workpieces deposited by electro-slag surfacing have even penetration depth, low dilution rate, high deposition speed, and less flux consumption. The thickness of the surfacing layer ranges from 15 to 90 mm.

    2. In addition to the electrode, alloy powder can be added to the slag bath or used as the coating of the wire to infiltrate the alloy.

    3. The composition change near the fusion line is too steep, and the overlay layer is easy to peel off at high temperatures.

    Usage:

    Commonly used for surfacing stainless steel and nichrome

How to measure weld overlay thickness?

It is best to use UT for testing. UT thickness measurement is possible. The echo from the interface between the two materials is visible due to the slight velocity difference between carbon steel and stainless steel cladding. However, repetition is minimal. Do not use a thickness gauge with a digital display. UT instrument with A-Scan presentation is necessary.

If UT is not available, can measure after grinding, the welding layer shall be visible, flat and no unevenness, and the average thickness is measured at multiple points.