Comparison of weldability 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, we will mainly discuss the various situations regarding whether carbon steel can be welded. 1. Based on their chemical composition, steels can be divided into two main categories: carbon steels and alloy steels. 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 greater than 0.6%. Mild steel is a type of carbon steel with a carbon content of less than 0.25%. Due to its low strength and hardness, 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 workability and machinability, but poor weldability. Its strength and hardness are higher than those of low-carbon steel, while its plasticity and toughness are lower than those of low-carbon steel. 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 achievable hardness is approximately HRC55 (HB538), with σb ranging from 600 to 1100 MPa. Therefore, among various applications at moderate intensity levels, medium-carbon steel is the most widely used; in addition to being used as building materials, it is also extensively employed in the manufacture of various mechanical parts. High carbon steel, also 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%. 2. 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 usually does not require any special processing measures. Only in cases involving low temperatures, thick plates, or stringent requirements is it necessary to use basic electrodes for welding, along with appropriate preheating. When both the carbon and sulfur contents in low-carbon steel are at the upper limits, in addition to using high-quality low-hydrogen welding electrodes and adopting 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 consequently 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 prone 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 during the welding process. 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 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.