Influence of shielding gas flow rate on properties of steel S960QL welded joint

Abstract

The experiment aimed on influence of shielding gas flow rate on properties of single V groove butt weld of high strength ultra-fine grained steel S960QL (XABO 960) and filler material G 89 6 M21 Mn4Ni2CrMo (X90) is described in this article. The steel S960QL is sensitive to presence of hydrogen in material structure, in exceed of maximal recommended content (<5ml/100g) the cold cracking may take place. During the experiment specimen 1 with reduced flow rate 5l.min-1 and specimen 2 with recommended flow rate 14l.min-1 was welded. The active shielding gas Corgon with the 82% Ar and 18% CO2 ratio was used. Flow rate of shielding gas has a noticeable influence on properties of welded joint. Because of specimen 1 reduced flow rate the full penetration was not achieved, chemical composition evaluation of weld metal shows fewer deoxidizing elements (Si, Mn, Ti) and increased hydrogen contents (≈18 ml/100g) as a result of chemical reaction between weld metal and air during welding. The increased hydrogen content, type of base metal and lack of penetration were the reasons of fracture of the specimen 1. Cold cracking was confirmed by hydrogen content measurement and by fracture surface evaluation by optical and electron microscopy. The porosity, fish eyes and cleavage facets typical for brittle fracture was found by microscopy. The high-quality weld with low content of hydrogen can be obtained with the recommended shielding gas flow rate, the flow rate adjustment is one of method how to avoid cold cracking of welds.