Why can low carbon steel be welded, high carbon steel can not be welded
According to the chemical composition, steel can be divided into carbon steel and alloy steel two categories. Today, we will talk about whether carbon steel can be welded. According to the chemical composition of steel can be divided into carbon steel and alloy steel two categories.
Carbon steel is divided into:
① Low carbon steel, carbon content 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 a carbon steel with a carbon content of less than 0.25%, which is soft because of its low strength and low hardness, so it is also called mild steel. It includes most of the ordinary carbon structural steel and a portion of high-quality carbon structural steel, most of which are not heat treated for engineering structural parts, and some are carburized and other heat treated for mechanical parts requiring wear resistance.
The hot working and cutting properties of medium carbon steel are good, but the welding properties are poor. The strength and hardness are higher than low carbon steel, while the plasticity and toughness are lower than low carbon steel. Without heat treatment, directly use cold rolled materials, cold drawn materials, can also be used after heat treatment. The medium carbon steel after quenching and tempering has good comprehensive mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), σb is 600 ~ 1100MPa. Therefore, in the various uses of medium strength level, medium carbon steel is the most widely used, in addition to as a building material, but also a large number of used in the manufacture of various mechanical parts.
High Carbon Steel (High Carbon Steel) is often called tool steel, carbon content from 0.60% to 1.70%, can be quenched and tempered, welding performance is poor. Hammers, crowbars, etc. are made of steel with 0.75% carbon content; Cutting tools such as drills, taps and reamers are manufactured from steel with a carbon content of 0.90% to 1.00%.
Comparison of weldability of mild steel and high carbon steel
The weldability of steel mainly depends 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 impact is generally much smaller than that of carbon.
General low carbon steel weldability is good, generally do not need to use special process measures, only at low temperature, thick plate or have high requirements, only need to use alkaline electrode welding, and appropriate preheating. When the carbon and sulfur content of low carbon steel are inclined to the upper limit, in addition to requiring the use of high quality low hydrogen electrode, preheating and post-heating measures, it should also be reasonable to choose the groove form and reduce the fusion ratio to prevent hot cracks.
The welding tendency of medium carbon steel is cold cracking, the higher the carbon content, the greater the hardening tendency of heat affected zone, the greater the cold cracking tendency, and the worse the weldability. With the increase of carbon content of the base material, the carbon content of the weld metal will also increase correspondingly, coupled with the adverse effect of sulfur, which is easy to form hot cracks in the weld. Therefore, the welding of medium carbon steel should use alkaline electrode with good crack resistance, and take measures such as preheating and post-heating to reduce the crack tendency.
High carbon steel welding, due to the high carbon content of this steel, welding will produce a large welding stress, welding heat affected zone hardening and cold crack tendency is larger, while the weld is also more prone to hot cracks, high carbon steel welding than carbon steel is more prone to hot cracks, so this kind of steel weldability is the worst, so in the general welding structure is not used, only for casting repair welding or surfacing welding. After welding, the weldment should be tempered to eliminate stress, fix the structure, prevent cracks and improve the performance of the weld.
