4. Welding hot cracking (weld crystal crack, heat affected zone liquefaction cracking)
Thermal cracking sensitivity depends primarily on the chemical composition, organization and properties of the material. Ni and S, P and other impurities to form low melting compounds or eutectic, boron, silicon and other segregation, will lead to thermal cracking. Welds are easy to form strong directional columnar grain structure, is conducive to harmful impurities and elements of segregation. Thereby promoting the formation of a continuous intergranular liquid film, increasing the sensitivity of thermal cracking. If the welding is not uniform heating, it is easy to form a larger tensile stress, to promote the production of hot welding cracks. Top Quality no.4 Stainless Steel Sheet 316l
A. Strict control of harmful impurities S, P content.
B. Adjust the organization of the weld metal. The two-phase microstructures have good crack resistance, and the δ phase in the weld can refine the grains, eliminate the directivity of the single-phase austenite, reduce the segregation of harmful impurities in the grain boundary, and dissolve more Of the S, P, and can reduce the interface energy, the formation of intergranular liquid film.
C. Adjust the weld metal alloy composition. In the single-phase austenitic steel to increase the content of Mn, C, N, add a small amount of cerium, ho, tantalum and other trace elements (can refine the weld organization, clean grain boundary), can reduce the thermal cracking sensitivity.
D. Process measures. Minimize the bath overheating to prevent the formation of coarse columnar crystals, using small wire energy and small cross-section bead.
For example, 25-20 type austenitic steel prone to liquefied cracking. Can be strictly limited by the base material impurity content and grain size, the use of high energy density welding methods, small line energy and improve the cooling rate of joints and other measures.