We know that according to the chemical composition, steel can be divided into two categories: carbon steel and alloy steel. Today, I will talk about several situations about whether carbon steel can be welded. According to the chemical composition of steel, it can be divided into two categories: carbon steel and alloy steel.

Carbon steel is divided into:

① Low carbon steel, with a carbon content of less than 0.25%;

② Medium carbon steel, with a carbon content of 0.25%~0.6%;

③ High carbon steel, with a carbon content greater than 0.6%.

Low carbon steel is carbon steel with a carbon content of less than 0.25%. It is also called soft steel because of its low strength, low hardness and softness. It includes most common carbon structural steels and some high-quality carbon structural steels. Most of them are used for engineering structural parts without heat treatment, and some are used for mechanical parts that require wear resistance after carburizing and other heat treatments.

Medium carbon steel has good hot working and cutting properties, but poor welding properties. Its strength and hardness are higher than those of low carbon steel, but its plasticity and toughness are lower than those of low carbon steel. It can be used directly as cold rolled or cold drawn material without heat treatment, or after heat treatment. Medium carbon steel has good comprehensive mechanical properties after quenching and tempering. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600-1100MPa. Therefore, medium carbon steel is most widely used in various uses with medium strength levels. In addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.

High Carbon Steel is often called tool steel, with a carbon content ranging from 0.60% to 1.70%. It can be quenched and tempered, but has poor welding properties. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%; cutting tools such as drills, taps, and reamers are made of steel with a carbon content of 0.90% to 1.00%.

Comparison of welding performance between low carbon steel and high carbon steel

The welding performance of steel depends mainly on its chemical composition. The most influential element is carbon, that is, the amount of carbon in the metal determines its weldability. Most of the other alloying elements in steel are also not conducive to welding, but their influence is generally much smaller than that of carbon.

Generally, low carbon steel has good weldability and does not require special process measures. Only when welding at low temperature, thick plates or with higher requirements, it is necessary to use basic welding rods and preheat appropriately. When the carbon and sulfur content in low carbon steel are both closer to the upper limit, in addition to requiring the use of high-quality low-hydrogen welding rods, preheating and post-heating measures, it is also necessary to reasonably select the groove form and reduce the fusion ratio to prevent thermal cracks.

Medium carbon steel has a tendency to cold crack when welding. The higher the carbon content, the greater the tendency of hardening in the heat affected zone, the greater the tendency of cold cracking, and the worse the weldability. As the carbon content of the parent material increases, the carbon content of the weld metal will also increase accordingly. Coupled with the adverse effect of sulfur, it is easy to form thermal cracks in the weld. Therefore, basic welding rods with good crack resistance should be used for medium carbon steel welding, and measures such as preheating and post-heating should be taken to reduce the tendency of cracking.

When welding high carbon steel, due to the high carbon content of this steel, a large welding stress will be generated during welding, and the hardening and cold cracking tendency of the welding heat affected zone is greater. At the same time, the weld is more likely to produce thermal cracks. High carbon steel is more likely to produce thermal cracks than medium carbon steel when welding. Therefore, this type of steel has the worst weldability, so it is not used in general welding structures and is only used for repair welding or surfacing of castings. After welding, the weldment should be tempered to eliminate stress, fix the structure, prevent cracks and improve the performance of the weld.

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