Why Is High Carbon Steel Difficult to Weld?

High carbon steel refers to carbon steel with w (c) higher than 0.6%.

It has a greater hardening tendency than medium carbon steel and forms high-carbon martensite, which is more sensitive to the formation of cold cracks.

At the same time, the martensite structure formed in the welding heat-affected zone has hard and brittle properties, resulting in a great decline 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 rarely used in welded structures.

High carbon steel is mainly used for machine parts requiring high hardness and wear resistance, such as rotating shafts, large gears and couplings.

In order to save steel and simplify processing technology, these machine parts are often combined by welding structure.

In the manufacture of heavy machinery, the welding of high carbon steel parts will also be encountered.

When formulating the welding process of high carbon steel weldments, various possible welding defects shall be comprehensively analyzed and corresponding welding process measures shall be taken.

Why Is High Carbon Steel Difficult To Weld

1. Weldability of high carbon steel

1.1 Welding method

High carbon steel is mainly used for 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

High carbon steel welding generally does not require equal 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 weld metal and base metal is required, low hydrogen electrode of the corresponding grade shall be selected;

When the strength of weld metal and base metal is not required, the low hydrogen electrode with strength level lower than the base metal shall be selected.

Remember not to select the electrode with strength level higher than the base metal.

If preheating of base metal is not allowed during welding, austenitic stainless steel electrode can be selected to prevent cold crack in heat affected zone, so as to obtain austenitic 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. Therefore, U-shaped or V-shaped groove is generally used during welding, and pay attention to clean the oil stain and rust within 20mm on both sides of the groove.

1.4 Preheating

When welding with structural steel electrodes, preheating must be carried out before welding, and the preheating temperature shall be controlled at 250 ℃ ~ 350 ℃.

1.5 Interlayer treatment

In case of multi-layer and multi pass welding, the first pass welding adopts small diameter electrode 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 whole heat affected zone of the base metal is heated in a short time, so as to obtain the effect of preheating and heat preservation.

1.6 Post weld heat treatment

Put the workpiece into the heating furnace immediately after welding, and conduct thermal insulation at 650 ℃ for stress relief annealing.

2. Welding defects of high carbon steel and preventive measures

Due to the great hardening tendency of high carbon steel, hot cracks and cold cracks are easy to appear during welding.

Why Is High Carbon Steel Difficult To Weld

2.1 Prevention measures for thermal cracks

1) Control the chemical composition of the weld

Strictly control the content of sulfur and phosphorus and appropriately increase the content of manganese to improve the weld structure and reduce segregation.

2) Control weld section shape

The aspect ratio should be slightly larger to avoid segregation in the center of the weld.

3) Weldment with high rigidity

For weldments with high rigidity, appropriate welding parameters, appropriate welding sequence and direction shall be selected.

4) Preheating and slow cooling measures

If necessary, preheating and slow cooling measures shall be taken to prevent hot cracks.

5) Increase the alkalinity of electrode or flux

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

2.2 Cold crack prevention measures [4]

1) Preheating before welding and slow cooling after welding

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

2) Select appropriate welding measures.

3) Use proper assembly and welding sequence

Proper assembly and welding sequence shall be adopted to reduce the restraint stress of welded joints and improve the stress state of weldments.

Why Is High Carbon Steel Difficult To Weld

4) Select appropriate welding materials

Before welding, the welding rod and flux shall be dried and used at any time.

5) Remove dirt

Before welding, the water, rust and other dirt on the base metal surface around the groove shall be carefully removed to reduce the content of diffused hydrogen in the weld.

6) Dehydrogenation

Dehydrogenation treatment shall be carried out immediately before welding to fully escape hydrogen from the welded joint.

7) Annealing treatment

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

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