Welding of medium carbon steel
- Welding characteristics of medium carbon steel
The carbon content of medium carbon steel is only 0.2%~0.3% higher than that of low carbon steel, but this change has caused a serious deterioration in weldability. At the same time, in terms of physical properties, medium carbon steel has a slightly higher linear expansion coefficient and a slightly lower thermal conductivity than low carbon steel, which increases the thermal stress and overheating tendency of medium carbon steel welding.
Due to the increase in carbon content, the strength of medium carbon steel increases, but it is more difficult to protect carbon from burning. The reduction reaction of carbon and FeO, the generated CO may promote the formation of pores.
When Wc in steel is greater than 0.15%, the segregation of carbon itself and its promotion of segregation of other elements such as sulfur (S) become obvious, which will increase the tendency of steel to crack. In order to avoid the formation of film-like distribution of low-melting sulfides, the Mn content must be increased, which will cause the Mn content of medium carbon steel to exceed the normal value, thereby causing certain difficulties in metallurgy. Therefore, the S and P contents of medium carbon steel used for welding need to be strictly limited.
The main reason why carbon deteriorates the weldability of steel is that it increases the hardenability of steel. Whether it is quenching in the weld zone or quenching in the heat-affected zone, especially overheating, the martensite of medium carbon steel is more brittle and is prone to cold cracking and brittle fracture under the action of welding stress and diffused hydrogen.
In short, the weldability of medium carbon steel is poor, and the weldability will deteriorate with the increase of the mass fraction of carbon in the steel. The main welding defects are hot cracks, cold cracks, pores and joint brittleness, and sometimes the strength in the heat-affected zone will decrease. When there are more impurities in the steel and the rigidity of the weldment is greater, the welding problem will be more prominent.
- Welding process of medium carbon steel
(1) Welding method When welding medium carbon steel weldments, arc welding is the most appropriate welding method, and alkaline low-hydrogen type electrodes of corresponding strength levels should be used.
(2) Bevel preparation When welding medium carbon steel, in order to limit the carbon content in the weld and reduce the fusion ratio, a U-shaped or V-shaped groove with a blunt edge is generally opened, and the oil, rust and other dirt on both sides of the groove are cleaned.
(3) Preheating In most cases, medium carbon steel welding requires preheating and control of the interlayer temperature to reduce the cooling rate of the weld and heat-affected zone and prevent the formation of martensite. The preheating temperature depends on the carbon equivalent, parent material thickness, structural stiffness, electrode type and process method. Usually, the preheating temperature of 35 steel and 45 steel can reach 150~250℃. If the carbon content is higher, or the thickness is large, or the toughness is high, the preheating temperature can reach 250~400℃.
(4) Welding power supply Generally, a DC arc welding power supply is selected, and the reverse polarity is connected, which can reduce the penetration depth and play a role in reducing the crack tendency and porosity sensitivity.
(5) Post-weld heat treatment After welding, stress relief heat treatment should be performed as soon as possible, especially for thick or rigid workpieces. The stress relief tempering temperature is generally 600~800℃.
If stress relief heat treatment cannot be performed immediately after welding, post-heating is also required, that is, insulation and slow cooling measures are taken to allow diffused hydrogen to escape in order to reduce the occurrence of cracks.
