It is more difficult to weld hot rolled and normalized steel with the increase of strength grade and alloying element content. The main problems of this kind of steel welding are embrittlement of heat affected zone and various cracks.
1) Over-heat zone embrittlement over-heat zone refers to the area near the fusion line in the heat-affected zone where the base material is heated to more than 1100℃, also known as coarse-crystal zone. Due to the high temperature, the austenite grains increase significantly and some refractory particles melt into the zone, resulting in the property change. This variation is not only related to the type of steel, the alloy system, but also to the welding heat input, because the heat input directly affects the residence time and cooling rate at high temperature.
Hot rolled steel is solid solution reinforced steel, used under hot rolling condition. The reasons for embrittlement of hot rolled steel are as follows: high welding input makes austenite grain increase seriously and stability increase. In addition to precipitation along grain boundaries, the transformation products also extend to the grain, forming weistenite structure and other mixed structures with low plasticity, thus making the superhot zone embrittlement. Therefore, the key to prevent embrittlement of hot rolled steel such as Q345 (16Mn) is to adopt appropriate low input heat and other technological measures to restrain the growth of austenite grain and the appearance of weistenite structure in the overheat zone during welding.
The thermal sensitivity of hot rolled steel under embrittlement in overheat zone of normalizing steel is higher than that of hot rolled steel, because of different alloying modes. For 15MnTi and Q420 (15MnVN) containing Ti and V
The results show that with the increase of welding heat input and the extension of residence time at high temperature, the more fully dissolved Ti and V are, the embrittlement is significant. Therefore, using small heat input is an effective measure to avoid embrittlement of this kind of normalizing steel.
If large heat input welding is used to improve the welding efficiency of normal steel, in this case, normal heat treatment at 800 ~ 1100℃ should be used after welding to improve the joint toughness.
2) Thermal strain embrittlement refers to the phenomenon that the fracture toughness of steel decreases obviously and the brittle transition temperature increases obviously after the steel is subjected to a large plastic deformation (5%-10%) within the temperature range of 200℃ ~ Ac1. The study on Q345 (16Mn) and Q420 (15MnVN) steels shows that these steels have a certain tendency of thermal strain embrittlement. The effective measure to eliminate and embrittlement of thermal strain is annealing after welding. After stress relief annealing at about 600℃, the toughness of the material can be almost restored to the original level.
1) Hot crack of weld metal Hot rolled normalizing steel generally has low carbon content and high manganese content. Their W (Mn)/ W (S) value is relatively large, so it has good thermal cracking resistance. Under normal conditions, there will be no crack in the weld. However, when the material composition is unqualified, or there is serious segregation, so that the local carbon and sulfur content is high, its W (Mn)/ W (S) value is low, it is easy to crack. Controlling the content of carbon and sulfur in base metal and welding material, reducing the fusion ratio and increasing the forming coefficient of weld metal are beneficial to prevent hot cracks.
2) The three main factors leading to cold welding cracks in steel are the hardening tendency of steel, the diffuse hydrogen content of weld and the constraint stress of joint, among which the hardening tendency is decisive.
It is generally considered that steels with carbon equivalent CE < 0.4% have no hardening tendency and good weldability. The hot-rolled steel with σ S =295 ~ 390MPa, such as 09Mn, O9MnNb and 12Mn, basically belongs to this category. In addition to the steel plate is very thick, the environmental temperature is very low, and welding low carbon steel generally do not need preheating before welding and strict control of welding heat input, will not cause cold cracks. The carbon equivalent of Q345 (16Mn), Q390 (15MnV) and other hot-rolled steels are slightly higher than the above steels, and the hardening tendency is correspondingly higher. When the cooling rate is fast, martensite hardening structure may be generated. It is necessary to take proper measures to prevent cold cracks when the confining stress is high and the content of diffused hydrogen is high. The steel with carbon equivalent CE= 0.4% ~ 0.6% basically belongs to the steel with hardening tendency, while the normalizing steel with σ S =440 ~ 490MPa is in this range. When CE is less than 0.5%, the hardening is not serious and the weldability is good. However, with the increase of plate thickness, some preheating measures must be taken to avoid cold cracks. The steel with CE above 0.5% has a significant hardening tendency and is easy to cold crack. Therefore, it is necessary to control the welding input and take some technological measures such as preheating and post-heat treatment to prevent cold crack.