According to the chemical composition of steel, it can be divided into two main categories: carbon steel and alloy steel.
Carbon steels are subdivided into:
(i) Low carbon steel with a carbon content of less than 0.25%;
(ii) Medium carbon steel with a carbon content of 0.25 – 0.6%;
(iii) High-carbon steel with carbon content larger than 0.6%.
Mild steel (mild steel) is carbon steel with less than 0.25% carbon content.
Because of its low strength, low hardness and softness, it is also called mild steel.
It includes most of the ordinary carbon structural steel and part of the high-quality carbon structural steel, which is mostly used for engineering structural parts without heat treatment and mechanical parts that require wear resistance by carburizing or other heating treatments.
Medium carbon steel has good hot working and cutting performance, but poor welding performance.
Its strength and hardness are higher than low carbon steel, while plasticity and toughness are lower than low carbon steel.
It can directly use cold-rolled material, cold-drawn material after heat treatment without heat treatment,.
After quenching and tempering, the medium-carbon steel has good comprehensive mechanical properties.
The highest hardness is about HRC55 (HB538), σb is 600 ~ 1100MPa.
Therefore, among the various uses of medium strength level, medium carbon steel is the most widely used.
In addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.
High Carbon Steel is often called tool steel and its carbon content is from 0.60% to 1.70%.
It can be quenched and tempered and welding performance is very poor.
Hammers and crowbars etc. are made of 0.75% carbon steel; cutting tools such as drills, screws and reamers etc. are made of 0.90% to 1.00% carbon steel.
Comparison of welding properties of low carbon steel and high carbon steel
The weldability of steel depends mainly on its chemical composition.
The most influential factor is the carbon, which means that the carbon content of the metal determines its weldability.
Most of the other alloying elements in steel are not conducive to welding, but their content of influence is generally much smaller than that of carbon.
Generally, low-carbon steel has good weldability and does not require special process measures.
Only at low temperatures, thick plates or high requirements, it needs to weld with the basic electrode and be preheat appropriately.
When the carbon and sulfur content of low-carbon steel is above the upper limit, in addition to the use of high-quality low-hydrogen welding rods, preheating and post-heating and other measures, the groove form should be selected reasonably and the fusion ratio should be reduced to prevent thermal cracks.
Medium-carbon steel has a tendency to cold crack during welding.
The higher the carbon content, the greater the hardening tendency of the heat-affected zone, the greater the tendency of cold cracking and the worse the weldability.
With the increase in the carbon content of the base material, the carbon content of the weld metal will also be increased accordingly.
Being coupled with the adverse effects of sulfur, it is easy to form thermal cracks in the weld.
Therefore, the welding of medium-carbon steel should use a basic electrode with good crack resistance.
After taking measures such as preheating and post-heating to reduce the tendency of cracking.
When welding high-carbon steel, due to the high carbon content of this steel, the welding will produce a large welding stress, the hardening and cold cracking tendency of the welded heat-affected zone is greater.
At the same time, the welds are also more prone to hot cracks and high-carbon steels are more prone to hot cracks than medium-carbon steels.
Therefore, this type of steel has the worst weldability, so it is not used in general welding structures and is only used for repair welding or surfacing of castings.
After welding, the welded parts should be tempered to eliminate stress, then to fix the organization to prevent cracks and improve the performance of the weld.