According to the chemical composition, steel can be divided into two categories: carbon steel and alloy steel.
Carbon steel is subdivided into:
Low carbon steel with less than 0.25% carbon.
Medium carbon steel with 0.25%-0.6% carbon content.
High carbon steel with a carbon content above 0.6%.
Low carbon steel is carbon steel with a carbon content of less than 0.25%, including most conventional carbon structural steels and some high quality carbon structural steels, most of which are used for engineering structural parts that do not require heat treatment. Some of them are also carburized or heat treated.
Medium carbon steel has good hot working and cutting properties, but poor welding properties. Its strength and hardness are higher than mild steel, and its ductility and toughness are lower than mild steel. It can be cold rolled without heat treatment, and machined or forged after heat treatment. Hardened medium carbon steel has excellent overall mechanical properties. The maximum achievable hardness is about HRC55 (HB538) and σb is 600-1100 MPa. Therefore, medium carbon steels are widely used in various applications with a moderate level of strength. It is not only widely used as a building material, but also used to make various machine parts.
Tool steel is often referred to as high carbon steel, with a carbon content between 0.60% and 1.70%. It is quenchable and temperable, and its welding performance is poor. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%. Cutting tools such as drills, taps and reamers are made from steel with a carbon content between 0.90% and 1.00%.
The weldability of steel depends primarily on its chemical composition. The most influential element is carbon, and the higher the carbon content of the steel, the more difficult it is to weld. Most other alloying elements in steel also do not contribute to welding, but the degree of influence is usually much less than that of carbon.
Generally speaking, mild steel has good weldability and does not require special processing methods. Welding with basic electrodes is only necessary when low temperatures, thick plates or high requirements are required and proper preheating is required. When the content of carbon and sulfur in mild steel exceeds the upper limit, in addition to the reasonable selection of the shape of the groove, high-quality electrodes with low hydrogen content, pre-heating and post-heating are used to prevent thermal cracks from reducing the degree of melting. .
Medium carbon steel is prone to cold cracking when welded. The higher the carbon content, the greater the tendency to hardenheat-affected zone, more prone to cold cracking and worse weldability. As the carbon content of the steel increases, so does the carbon content of the weld metal. Combined with the negative effects of sulfur, welds are prone to hot cracking. Therefore, when welding medium carbon steel, it is necessary to use base electrodes with excellent crack resistance, and pre-heating and post-heating measures must be taken to suppress the occurrence of cracks.
When welding high carbon steel, due to the high carbon content of this steel, large welding stresses will be created during welding. The tendency for hardening and cold cracking in the heat-affected zone of the weld is large, and the weld zone is also prone to hot cracking. High-carbon steel during welding more often than medium-carbon steel gives hot cracks, therefore it has worse weldability and is not used for welding general-purpose structures, but only for repair welding and surface treatment of castings. After welding, the weld must be released to relieve stress, fix the structure, prevent cracking, and improve weld performance.