Research on Welding Hot Cracks of Stainless Steel Materials
Due to its strong corrosion resistance, oxidation resistance and high temperature resistance, stainless steel materials are widely used in petrochemical, aerospace, nuclear energy, metallurgy, electric power, transportation and electronics sectors. Because there are many alloying elements and complex structural systems in the chemical composition of stainless steel materials, these have caused greater difficulties in welding processing, especially the problem of welding hot cracks has attracted more attention.
There are generally three types of thermal cracks. The thermal cracks generated in the weld metal are also called solidification cracks. The thermal cracks formed in the heat-affected zone or the reheating zone of the multi-layer weld bead are called liquefaction cracks. Both of these cracks are in the liquid phase. A high-temperature region that coexists with the solid phase and is formed under the action of thermal strain. The third type belongs to low plasticity cracks. The temperature of this type of cracks is relatively low, and their appearance has nothing to do with the liquid phase. They are generally located in the coarse-grained area of the heat-affected zone.
In the process of stainless steel welding, solidification cracks of the weld metal are a more critical issue. The sensitivity of solidification cracks is dependent on the content of delta ferrite. When the ferrite content is in the range of 5%-20%, The sensitivity of cracks is the lowest. When the single γ phase or ferrite content is above 40%, the sensitivity of cracks will also increase significantly. By optimizing the chemical composition of the weld metal and adjusting the fusion ratio, the amount of ferrite in the weld metal can be controlled within a few percent range, which can prevent the formation of solidification cracks.
Among various nickel-based alloys, since the brittleness temperature range of nickel 625 or 718 is larger than that of Hastelloy X or C-276, the solidification crack sensitivity of the former is slightly higher. This is due to their comparative The most abundant alloying element is niobium, which is the element that generates NbC, γ phase (Ni3Nb) and Laves phase. When welding alloys with this chemical composition, it is easy to form γ/NbC or γ/Laves phase eutectics in the final solidification zone. This type of eutectic has a low melting point, so solidification cracks are easy to form. In addition to Nb, carbon and silicon are also elements that can increase the susceptibility to solidification cracks. In addition, when there is unsolidified NbC in the base material, liquefaction will occur at the γ/NbC interface, and liquefaction cracks will also occur along with the liquefaction of the grain boundaries.
In general, the formation of various types of welding hot cracks in stainless steel materials is not only related to the alloy composition and structure of the material itself, but also to the inclusions in the material and solute elements such as phosphorus, sulfur, Pb, Sn, Zn, etc. Their more important role is that they produce low melting point substances, causing the residual liquid phase to cover around the grain boundaries, which will cause cracking under the action of a small thermal strain.
For austenitic-ferritic stainless steel welds, the ferrite structure content has an important influence on the susceptibility to solidification cracks. For pure austenitic stainless steel welds, the phosphorus and sulfur content also have an impact on the BTR. It has a very significant effect, the lower its content, the lower the susceptibility to hot cracking. In the welding of nickel-based heat-resistant and corrosion-resistant alloys, elements such as niobium, carbon and silicon are prone to form low-melting-point eutectics, which will also increase the susceptibility to solidification cracks. The segregation of impurities such as phosphorus and sulfur at the grain boundaries can also cause hot cracks. the final formation.
