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, carbon content is less than 0.25%; ② medium carbon steel, carbon content is 0.25%-0.6%; ③ high carbon steel, carbon content is greater than 0.6%;

Mild steel is carbon steel with a carbon content of less than 0.25%. Because of its low strength, low hardness and softness, it is also called mild steel. It includes most ordinary 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 performance, but poor welding performance. Strength and hardness are higher than low carbon steel, while plasticity and toughness are lower than low carbon steel. It can be used directly without heat treatment, cold-rolled material, cold-drawn material, or after heat treatment. The quenched and tempered medium carbon steel has good comprehensive mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600～1100MPa. Therefore, among the various uses of medium strength level, medium carbon steel is the most widely used. In addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.

High Carbon Steel (High Carbon Steel) is often called tool steel. The carbon content ranges from 0.60% to 1.70%. It can be quenched and tempered and has poor welding performance. 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 mainly depends on its chemical composition. The most influential element is carbon, which means that the amount of carbon in a metal determines its weldability. Most of the other alloying elements in steel are also not conducive to welding, but their degree of influence is generally much smaller than that of carbon.

Generally, low-carbon steel has good weldability and generally does not need to adopt special process measures. It is only necessary to weld with alkaline electrodes when low temperature, thick plates or high requirements are required, and proper preheating. When the carbon and sulfur content of low-carbon steel is above the upper limit, in addition to the use of high-quality low-hydrogen welding rods, preheating and post-heating, etc., the groove form should be selected reasonably and the fusion ratio should be reduced to prevent thermal cracks. .

Medium carbon steel has a tendency of cold cracking during welding. The higher the carbon content, the greater the hardening tendency of the heat-affected zone, the greater the tendency of cold cracking, and the worse the weldability. As the carbon content of the base metal increases, the carbon content of the weld metal will also increase accordingly. Coupled with the adverse effect of sulfur, it is easy to form hot cracks in the weld. Therefore, the welding of medium carbon steel should use alkaline electrodes with good crack resistance, and take preheating and postheating measures to reduce the tendency of cracks.

When welding high-carbon steel, due to the high carbon content of this steel, a large welding stress will be generated during welding. The hardening and cold cracking tendency of the welding heat-affected zone is relatively large, and the weld is also more prone to hot cracks. High-carbon steel is more prone to hot cracks than medium-carbon steel when welding, so this type of steel has the worst weldability, so it is not used in general welding structures and is only used for casting repair welding or surfacing. After welding, the weldment should be tempered to eliminate stress, fix the structure, prevent cracks and improve the performance of the weld