The optimization direction of the weld structure of low-alloy high-strength steel is to generate more acicular ferrite. Studies have shown that acicular ferrite has fine grain size and high density of dislocations. When its content is greater than 65% and the average lath size is about 1 μm, the weld metal can have excellent strength and toughness.
How to obtain the above organization?
Appropriate welding material alloy composition design is the key.
- C. The C content is generally controlled at 0.05% to 0.10%, and the carbon equivalent is less than 0.39. In this range, side slat ferrite and acicular ferrite can be generated, and the reheat zone of the weld is transformed into equiaxed massive ferrite.
- Mn. Mn has two opposite effects on weld metal refinement and hardening. A proper Mn content can obtain more acicular ferrite. However, excessive addition of Mn will cause the grain boundary nucleation rate of bainite to be higher than the intragranular nucleation rate of acicular ferrite, which will increase the hardness of the weld. The Mn content is generally controlled at 0.6 to 1.8%.
- Cr. As the Cr content in the weld metal increases, the number of acicular ferrite increases, the microstructure of the weld is refined, and the proeutectoid ferrite in the columnar zone and the coarse-grained zone decreases. The impact toughness decreases with the increase of Cu content and increases with the increase of Cr content. The Cr content is generally 0.9 to 1.0%.
- Ti. The Ti content range for obtaining the optimal combination of structure and impact performance is 0.02% to 0.05%. Studies have shown that when the Ti content in the weld metal is 0.014% to 0.048%, the weld structure of the Q235 plate is mainly composed of equiaxed ferrite and acicular ferrite. With the increase of Ti content, acicular ferrite The increase in content and the decrease in length will increase the toughness of the weld metal. This is due to the formation of TiO2 inclusions when the Ti content in the weld metal is higher than the Al content, which is conducive to the nucleation of acicular ferrite.
- B. Experiments show that when the B content is between 0.0032% and 0.0103%, the acicular ferrite decreases with the increase of the B content, and the impact energy is significantly reduced. This is due to the decrease of the eutectoid temperature caused by B. However, if higher Ti is added to the welding wire and lower alkalinity is ensured, and when the ratio of boron to nitrogen (B/N) in the weld metal is in the range of 0.6 to 0.8, the formation of needles can be beneficial. The ferrite nucleated Ti-containing oxide inclusions can obtain a higher impact toughness value for the weld metal; if the B/N is higher than 0.8, the impact toughness will decrease. The B content is generally controlled within 0.003～0.006%.
- O. O is a restrictive impurity element in the weld metal. Al, Mg and other strong reducing agents are often added to the weld metal to deoxidize and fix nitrogen, but it is easy to generate polygonal AlN brittle inclusions, which seriously damage the low temperature toughness of the weld metal. It is proposed in the literature that adding an appropriate amount of LiF to the flux core will generate Li3N with N in the arc zone, thereby significantly reducing the N content in the weld metal and reducing the number of harmful AlN inclusions. There is also a view that the formation of V(C, N) phase by precipitation of N and V can promote the nucleation of acicular ferrite. It is also pointed out in the literature that the addition of appropriate amounts of Fe2O3, MnO2, etc. to the core can increase the O content in the weld metal, but form circular inclusions dominated by Al2O3, which is still conducive to obtaining acicular ferrite. Weld organization. It is generally believed that within the limit of Al/O = 0.45, an appropriate increase in the Al/O ratio in the low alloy steel weld metal can promote the generation of more inclusions with a size of 0.2 to 0.8 μm, promote the nucleation of acicular ferrite, and thus Toughness is favorable.
- S. S is also a restrictive impurity element in weld metal, but FeS particles containing a small amount of Mn and Cu are effective for nucleation of acicular ferrite.
In addition, the control of the size and shape of inclusions is also very important. It is generally believed that spherical particles mainly composed of MnS and other amorphous phases with a size of 0.5-0.8 μm and a thin layer of TiO on the surface are most advantageous for acicular ferrite nucleation.
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