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The challenges and techniques of welding high carbon steel

The challenges and techniques of welding high carbon steel

High carbon steel, characterized by a carbon content greater than 0.6%, has a greater tendency to harden compared to medium carbon steel. This increased hardenability results in the formation of high-carbon martensite, making the material more prone to cold cracking. The martensitic structure formed in the heat-affected zone (HAZ) during welding is hard and brittle, significantly reducing the joint’s plasticity and toughness. Consequently, high carbon steel exhibits poor weldability, necessitating special welding techniques to ensure joint performance. As a result, high carbon steel is seldom used in welded structures and is primarily utilized in machine parts requiring high hardness and wear resistance, such as shafts, large gears, and couplings.

To conserve steel and simplify processing techniques, these machine parts are often combined into welded structures. In heavy machinery manufacturing, welding of high carbon steel components is occasionally required. When formulating welding procedures for high carbon steel weldments, it is crucial to thoroughly analyze potential welding defects and implement appropriate welding techniques.

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1.Weldability of High Carbon Steel

1.1 Welding Methods

Given the high hardness and wear resistance required for high carbon steel structures, the primary welding methods include shielded metal arc welding (SMAW), brazing, and submerged arc welding (SAW).

1.2 Welding Materials

Welding high carbon steel typically does not demand joints with strength equivalent to the base metal. In SMAW, electrodes with strong desulfurization capabilities, low hydrogen content in the deposited metal, and good toughness are usually selected. When joint strength equivalent to the base metal is required, corresponding low-hydrogen electrodes should be chosen. If equal strength is not necessary, electrodes with a lower strength grade than the base metal should be used. It is critical not to choose electrodes stronger than the base metal. If preheating the base metal is not feasible, austenitic stainless steel electrodes can be used to achieve a ductile and crack-resistant austenitic structure.

1.3 Groove Preparation

To limit the carbon content in the weld metal, the fusion ratio should be minimized. Therefore, U-shaped or V-shaped grooves are generally used, and the area within 20mm of the groove and its sides must be cleaned of oil, rust, and other contaminants.

1.4 Preheating

When using structural steel electrodes, preheating before welding is essential, with temperatures controlled between 250℃ and 350℃.

1.5 Interpass Treatment

For multi-layer and multi-pass welding, the first pass should use a small-diameter electrode with low current. The workpiece is typically positioned semi-upright, or the electrode is oscillated laterally to ensure the entire HAZ is heated in a short time, achieving preheating and heat retention.

1.6 Post-Weld Heat Treatment

Immediately after welding, the workpiece should be placed in a heating furnace and stress-relief annealed at 650℃.

2.Welding Defects in High Carbon Steel and Preventive Measures

Due to the high hardenability of high carbon steel, it is prone to both hot and cold cracking during welding.

2.1 Preventive Measures for Hot Cracking

Control the chemical composition of the weld, strictly managing sulfur and phosphorus content, and appropriately increasing manganese content to improve weld structure and reduce segregation.

Control the shape of the weld cross-section, maintaining a larger width-to-depth ratio to avoid segregation in the weld center.

For rigid weldments, select appropriate welding parameters, sequence, and direction.

When necessary, apply preheating and slow cooling measures to prevent hot cracking.

Increase the basicity of the electrode or flux to reduce impurities in the weld and improve segregation.

2.2 Preventive Measures for Cold Cracking

Preheating before welding and slow cooling after welding not only reduce hardness and brittleness in the HAZ but also accelerate hydrogen diffusion out of the weld.

Choose appropriate welding methods.

Employ suitable assembly and welding sequences to reduce restraint stress in the joint and improve stress distribution in the weldment.

Select appropriate welding materials, ensure electrodes and flux are dried before use, and use them immediately after removal from storage.

Thoroughly clean the base metal surface around the groove of water, rust, and other contaminants before welding to reduce hydrogen content in the weld.

Perform dehydrogenation treatment immediately before welding to allow hydrogen to escape from the welding joint.

Conduct stress-relief annealing immediately after welding to promote hydrogen diffusion out of the weld.

Welding high carbon steel requires meticulous planning and execution to prevent defects such as hot and cold cracking. By adopting the appropriate welding methods, materials, groove preparation, preheating, interpass treatment, and post-weld heat treatment, the challenges associated with welding high carbon steel can be effectively managed, ensuring the integrity and performance of the welded joints.