According to the chemical composition of the steel can be divided into two categories: carbon steel and alloy steel.
Carbon steels are further divided into.
① low carbon steel, with a carbon content of less than 0.25%.
② Medium carbon steel, with a carbon content of 0.25% to 0.6%.
③ high carbon steel, carbon content greater than 0.6%.
Low carbon steel (mild steel) for the carbon content of less than 0.25% carbon steel, because of its low strength, low hardness and soft, so also known as soft steel. It includes most of the ordinary carbon structural steel and part of the quality carbon structural steel, mostly without heat treatment for engineering structural parts, some by carburizing and other heat treatment for the requirements of wear-resistant mechanical parts.
Medium carbon steel hot processing and cutting performance is good, welding performance is poor. Strength, hardness than low carbon steel, while plasticity and toughness than low carbon steel. Can not be heat-treated, the direct use of cold-rolled material, cold-drawn material, but also after heat treatment. The quenched and tempered medium carbon steel has good overall mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), σb for 600 ~ 1100MPa. so in the medium strength level of a variety of uses, medium carbon steel is the most widely used, in addition to as a building material, but also a large number of mechanical parts for the manufacture of a variety of.
High Carbon Steel (High Carbon Steel) often called tool steel , carbon content from 0.60% to 1.70%, can be quenched and tempered, welding performance is very poor. Hammers, crowbars, etc. are made from steel with a carbon content of 0.75%; cutting tools such as drills, taps and reamers are made from steel with a carbon content of 0.90% to 1.00%.
Comparison of the welding properties of low-carbon and high-carbon steels
The weldability of a steel depends mainly on its chemical composition. The most influential element is carbon, which means that the amount of carbon in the metal determines its weldability. Most of the other alloying elements in steel are also detrimental to welding, but their degree of influence is generally much smaller than that of carbon.
General low carbon steel weldability, generally do not need to use special process measures, only in low temperature, thick plate or have higher requirements, only need to use alkaline electrode welding, and appropriate preheating. When low carbon steel in carbon, sulfur content are on the upper limit, in addition to the requirement to use high-quality low-hydrogen welding rod, preheating and post-heating and other measures, but also a reasonable choice of bevel form, reduce the fusion ratio, to prevent thermal cracking.
The higher the carbon content, the greater the tendency to harden the heat affected zone, the greater the tendency to cold cracking, the worse the weldability. With the increase in the carbon content of the base material, will also make the weld metal carbon content correspondingly high, coupled with the adverse effects of sulfur, easy to form thermal cracks in the weld. Therefore, the welding of medium carbon steel should be used to resist cracking good alkaline welding rod, and take preheating and post-heating and other measures to reduce the cracking tendency.
High carbon steel welding, due to the high carbon content of this steel, welding will produce a lot of welding stress, welding heat affected zone of hardening and cold cracking tendency is greater, while the weld is also more likely to produce thermal cracking, high carbon steel welding than carbon steel is more likely to produce thermal cracking, so this type of steel welding is the worst, so in general welding structure is not used, only for casting welding or overlay welding. Welded parts should be tempered after welding to eliminate stress, fixed organization, prevent cracking and improve the performance of the weld.