Aço para soldagem 101: Por que o aço de baixo carbono é soldável, mas o aço de alto carbono não é?

Welding Steel 101: Why Is Low Carbon Steel Weldable But High Carbon Steel Is Not?

According to the chemical composition of steel, it can be divided into two main categories: carbon steel and alloy steel.

Carbon 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 steels with a carbon content greater than 0.6%.

Mild steel

Carbon steel is a type of carbon steel with a carbon content of less than 0.25%. It is also known as carbon steel due to its low strength, low hardness and softness.

Mild steel covers most of the common carbon structural steel and some of the high-quality carbon structural steel. It is typically used for engineering structural parts that do not require heat treatment and for mechanical parts that require wear resistance, which can be achieved through carburizing or other heat treatments.

Medium carbon steel, which has a carbon content of 0.25% to 0.60%, is known for its good cutting and hot working performance, but poor welding performance. It has greater strength and hardness than low carbon steel, but lower plasticity and toughness. It can be used directly on cold rolled or drawn materials, or after heat treatment without additional treatment.

With adequate quenching and tempering, medium carbon steel has good general mechanical properties, with a maximum hardness of around HRC55 (HB538) and tensile strength of 600 to 1100 MPa. This makes medium carbon steel the most widely used among medium strength materials.

In addition to being used in construction, medium carbon steel is also commonly used in the production of various mechanical parts.

High-carbon steel, with carbon content ranging from 0.60% to 1.70%, is also known as tool steel. It can be quenched and tempered, but it has poor welding performance. Tools like hammers and crowbars are made from 0.75% carbon steel, while cutting tools like drills, screws, and reamers are made from 0.90% to 1.00% carbon steel.

Comparison of welding properties of low carbon steel and high carbon steel

Comparison of welding properties of low carbon steel and high carbon steel

The weldability of steel is mainly determined by its chemical composition, with carbon being the most important factor. Although other alloying elements in steel can also affect its weldability, their impact is generally much smaller compared to carbon.

Low carbon steel typically has good weldability and does not require special measures. However, at low temperatures or when welding thick sheets or those with high demands, it may be necessary to use basic electrodes and preheating. If the carbon and sulfur content of low-carbon steel exceeds the upper limit, measures such as using high-quality low-hydrogen welding rods, preheating and postheating, and appropriate groove selection should be taken to prevent thermal cracking.

Medium carbon steel is prone to cracking during welding, especially when the carbon content is high. This occurs because the higher the carbon content, the greater the tendency for the zone affected by heat to harden, leading to a greater risk of cold cracking and reduced weldability. The carbon content of the weld metal will also increase as the carbon content of the base material increases, further increasing the risk of cracking.

The presence of sulfur in medium carbon steel can increase the risk of thermal cracks during welding. To mitigate this risk, a basic electrode with good crack resistance should be used, and measures such as preheating and postheating can be taken to reduce the tendency to crack.

High-carbon steel has the worst weldability due to its high carbon content, which leads to high welding stresses and a greater risk of hot and cold cracking in the heat-affected zone. Welds in high carbon steel are also more prone to hot cracking compared to medium carbon steel. As a result, high carbon steel is not commonly used in general welding structures and is only used for repair welding or coating castings.

After welding, it is important to temper the welded parts to relieve stress, stabilize the microstructure and prevent cracking, thus improving welding performance.

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