Why is high carbon steel more difficult to weld?

High-carbon steel refers to carbon steel with w(C) higher than 0.6%, which has a greater tendency to harden than medium-carbon steel, and forms high-carbon martensite, which is more sensitive to the formation of cold cracks. At the same time, the martensitic structure formed in the welding heat-affected zone is hard and brittle, which leads to a great decrease in the plasticity and toughness of the joint. Therefore, the weldability of high-carbon steel is quite poor, and a special welding process must be adopted to ensure the performance of the joint. 

Therefore, it is generally rarely used in welded structures. High carbon steel is mainly used for machine parts that require high hardness and wear resistance, such as shafts, large gears and couplings.

In order to save steel and simplify the processing technology, these machine parts are often combined with welded structures. Welding of high carbon steel components is also encountered in heavy machine building. When formulating the welding process of high carbon steel weldment, all kinds of welding defects that may occur should be comprehensively analyzed, and corresponding welding process measures should be taken.

  • Weldability of High Carbon Steels

1.1 Welding method

High carbon steel is mainly used in structures with high hardness and high wear resistance, so the main welding methods are electrode arc welding, brazing and submerged arc welding.

1.2 Welding materials

Welding of high carbon steel generally does not require the same strength between the joint and the base metal. Low-hydrogen electrodes with strong desulfurization ability, low diffusible hydrogen content of deposited metal, and good toughness are generally selected for electrode arc welding.

When the strength of the weld metal and the base metal is required, a low-hydrogen electrode of the corresponding level should be selected; when the strength of the weld metal and the base metal is not required, a low-hydrogen electrode with a strength level lower than that of the base metal should be selected. An electrode with a higher strength level than the base metal cannot be selected.

If the base metal is not allowed to be preheated during welding, in order to prevent cold cracks in the heat-affected zone, austenitic stainless steel electrodes can be used to obtain an austenite structure with good plasticity and strong crack resistance.

1.3 Groove preparation

In order to limit the mass fraction of carbon in the weld metal, the fusion ratio should be reduced, so U-shaped or V-shaped grooves are generally used during welding, and care should be taken to clean the groove and the oil stains and rust within 20mm on both sides of the groove.

1.4 Preheating

When welding with structural steel electrodes, it must be preheated before welding, and the preheating temperature should be controlled at 250°C to 350°C.

1.5 Interlayer Processing

For multi-layer multi-pass welding, the first pass uses small-diameter electrodes and low-current welding. Generally, the workpiece is placed in semi-vertical welding or the welding rod is used to swing laterally, so that the entire heat-affected zone of the base metal is heated in a short time to obtain preheating and heat preservation effects.

1.6 Post-weld heat treatment

Immediately after welding, put the workpiece into the heating furnace, and carry out heat preservation at 650°C for stress relief annealing.

  • Welding Defects and Preventive Measures of High Carbon Steel

Due to the high tendency of high carbon steel to be hardened, hot cracks and cold cracks are prone to occur during welding.

2.1 Preventive measures for thermal cracks

1) Control the chemical composition of the weld, strictly control the content of sulfur and phosphorus, and appropriately increase the manganese content to improve the structure of the weld and reduce segregation.

2) Control the cross-sectional shape of the weld, and the width-to-depth ratio should be slightly larger to avoid segregation in the center of the weld.

3) For weldments with high rigidity, appropriate welding parameters, appropriate welding sequence and direction should be selected.

4) Take preheating and slow cooling measures when necessary to prevent thermal cracks.

5) Increase the alkalinity of the electrode or flux to reduce the impurity content in the weld and improve the degree of segregation.

2.2 Preventive measures for cold cracks

1) Preheating before welding and slow cooling after welding can not only reduce the hardness and brittleness of the heat-affected zone, but also accelerate the outward diffusion of hydrogen in the weld.

2) Select the appropriate welding measures.

3) Adopt appropriate assembly and welding sequence to reduce the restraint stress of welded joints and improve the stress state of weldments.

4) Choose suitable welding materials, dry the welding rod and flux before welding, and make sure that they are ready for use.

5) Before welding, the water, rust and other dirt on the basic metal surface around the groove should be carefully removed to reduce the content of diffusible hydrogen in the weld.

6) Dehydrogenation treatment should be carried out immediately before welding, so that hydrogen can fully escape from the welded joint.

7) Annealing treatment for stress relief should be carried out immediately after welding to promote the outward diffusion of hydrogen in the weld.

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