Why can low-carbon steel be welded while high-carbon steel cannot? What is the difference between low-carbon steel and high-carbon steel?

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We know that, based on their chemical composition, steels can be divided into two main categories: carbon steel and alloy steel. Today, the editor will discuss the various situations regarding whether carbon steel can be welded. Based on the chemical composition of steel, it can be divided into two main categories: carbon steel and alloy steel. Carbon steel is further divided into: ① Low-carbon steel, with a carbon content of less than 0.25% ； ②Medium carbon steel, with a carbon content of 0.25%~0.6% ； ③High-carbon steel, with a carbon content of over 0.6%. Mild steel is a carbon steel with a carbon content of less than 0.25%; it is soft due to its low strength and low hardness, which is why it is also known as soft steel. It includes most ordinary carbon structural steels and some high-quality carbon structural steels; most of them are used in engineering structural components without heat treatment, while some are used in 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, whereas its plasticity and toughness are lower. Cold-rolled or cold-drawn materials can be used directly without heat treatment, or they can be used after heat treatment. Medium-carbon steel after quenching and tempering possesses excellent comprehensive mechanical properties. The maximum attainable hardness is approximately HRC55 (HB538), with σb ranging from 600 to 1100 MPa. Therefore, in various applications at moderate intensity levels, medium-carbon steel is the most widely used; aside from being used as building materials, it is also extensively employed in the manufacture of various mechanical parts. High carbon steel, commonly known as tool steel, has a carbon content ranging from 0.60% to 1.70%; it can be quenched and tempered, but its weldability is very 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 of steel with a carbon content of 0.90% to 1.00%. Comparison of welding properties between low-carbon steel and high-carbon steel: The welding quality of steel depends mainly on its chemical composition. Among them, carbon has the greatest impact; in other words, the amount of carbon contained in a metal determines its weldability. Most other alloying elements in steel are also unfavorable for welding, but their impact is generally much less than that of carbon. Generally, low-carbon steel has good weldability and does not require special welding procedures; however, in cases of low temperatures, thick plates, or when higher standards are required, alkaline electrodes must be used along with appropriate preheating. When the carbon and sulfur contents in low-carbon steel are both at the upper limits, in addition to using high-quality low-hydrogen welding electrodes and implementing measures such as preheating and post-heating, it is also necessary to choose the groove shape appropriately and reduce the fusion ratio in order to prevent the formation of hot cracks. Medium carbon steel tends to suffer from cold cracking during welding; the higher the carbon content, the greater the tendency for hardening in the heat-affected zone, and thus the greater the risk of cold cracking as well as the poorer the weldability. As the carbon content in the base metal increases, the carbon content in the weld metal also rises accordingly; coupled with the adverse effects of sulfur, this makes hot cracks likely to form in the weld. Therefore, for welding medium-carbon steel, alkaline electrodes with good crack resistance should be used, and measures such as preheating and post-heating should be adopted to reduce the tendency to cracking. When welding high-carbon steel, the high carbon content of this type of steel results in significant welding stresses. The welded area is prone to hardening and cold cracking, and heat cracks are also more likely to occur in the weld. High-carbon steel is more susceptible to heat cracks during welding than medium-carbon steel; therefore, it has the worst weldability of all such steels. As a result, it is not used in ordinary welded structures, but only for repairing castings or performing surfacing welding. The welded parts should be tempered after welding to relieve stress, stabilize the microstructure, prevent cracks, and improve the properties of the weld.