Mechanical properties and hydrogen induced cracking behaviour of API X70 SAW weldments
Graphical abstract
Introduction
High strength low alloy steels have been extensively used for the production of pipelines to meet the demand for oil and gas industries. Pipeline grades are being continuously improved due to the increasing demand for the high strength-toughness combination. This demand is directly related to the increase in consumption of petroleum products worldwide. To meet this demand it is necessary that large diameter pipes should work under high pressure without any failure (e.g. good mechanical and corrosion resistance properties). Pipeline steels should possess some basic requirements such as good mechanical strength in combination with high toughness at low temperature and excellent weldability. Catastrophic failure can occur when pipeline steel is subjected to operational cyclic loads [[1], [2], [3], [4], [5], [6], [7], [8]]. Apart from the manufacturing process, suitable welding technique, welding process parameters, flux/wire composition, inclusion content and presence of microalloying elements strictly control the mechanical properties of pipeline weld joint [9,10]. Control of weld metal composition requires a crucial understanding of the interaction of the flux, core wire, and base metal [11]. Mechanical properties (such as tensile strength, impact toughness etc.) and microstructural properties depend upon the type of flux selected (such as basic flux, acidic flux or neutral flux) during submerged arc welding. In submerged arc welding excessive heat input uniformly heats the joint area and molten flux protects the weld pool from oxidation and contamination. Oxygen has a great influence on the structure of steel and then on its notch properties. Higher oxide inclusion content promotes poor notch toughness while the addition of micro-alloying elements such as titanium, vanadium and molybdenum promote acicular ferrite microstructure which is responsible for enhancing mechanical properties of steel [[12], [13], [14], [15],43]. In submerged arc welding, the various physicochemical and thermomechanical interactions exist in the weld pool. The transferring metallic elements are comprised of various chemical blends such as fluorides, oxides, and carbonates and during welding; these elements promote phase transformation [15,16,44]. Different elements and oxygen disintegrate in the weld pool, due to the transfer of oxides from the fluxes. Oxide inclusions are formed when mineral constituents react with oxygen and play as nucleation sites for the development of important phase such as acicular ferrite during submerged arc welding process. The presence of these phases in the fusion zone enhances the impact properties [13,21]. The behaviour of SAW fluxes is similar to that of flux coating used in manual metal arc welding. In both cases, the role of the mineral flux mixture is to protect the welded joint from welding defects and atmospheric gases and there is transferring of elements in the weld pool during slag-metal and gas-metal interactions [15,42]. Depending upon the different applications, different types of SAW fluxes are used such as agglomerated flux, fused flux, bonded flux and mechanical-mixed fluxes. Different flux constituents such as Al2O3, SiO2, MgO, CaO, MnO, FeO, TiO2, CaF2 and Na2SiO3/K2 SiO3 as binder is utilized for the flux preparation either by fused or agglomeration technique. SAW fluxes can be categorized according to the chemical nature of the flux i.e. acidic, basic, semi-basic or highly basic. CaO, BaO, CaF2, MnO, K2O, MgO, Na2O etc. show basic behaviour while SiO2, TiO2 and Al2O3 are acidic in nature. BI index of fluxes is decided by the chemical behaviour of the fluxes and it is the ratio of the basic to acidic oxides. Various physicochemical behaviour (slag detachability, viscosity, density, etc.), as well as mechanical properties (tensile strength, hardness toughness, etc.), are affected by the basicity index (BI) of fluxes [[45], [46], [47], [48]]. High current carrying capacity, good bead appearance, good slag detachability and poor mechanical behaviour were observed with low basicity fluxes while high basic fluxes show good crack resistance behaviour [[15], [16], [17], [18], [19], [20]].
Section snippets
Material and experimental procedure
The following steps were followed:
- 1)
Twenty one agglomerated submerged arc fluxes (for three flux systems i.e. basic, rutile basic and rutile acidic) were developed by combination of different mineral constituents (e.g. silica, rutile, fluorspar, calcite, talc and calcinated bauxite powders) in varying proportions by applying extreme vertices design methodology as suggested by Mclean and Anderson [[22], [23], [24]] and the multi-pass bead on plate experiments were performed (Fig. 1).
- 2)
On the basis
Chemical analysis
API X70 steel shown in (Table 4) is a low carbon (0.06 wt %) steel, with manganese, molybdenum, titanium, chromium, niobium, nickel etc. being the main micro-alloying and precipitate hardening elements. These elements provide a good combination of toughness and strength. Carbide former alloying elements (such as niobium, chromium, titanium and boron) cause low-temperature phase transformation. Table 3 demonstrates the chemical composition of the parent metal, filler wire and seven weld joints.
Conclusion
The mechanical properties (impact toughness, tensile properties and microhardness), chemical composition, microstructure and hydrogen induced cracking (HIC) features of API X70 line pipe steel welds were for SAW line-pipe weldments (using commercial as well as laboratory prepared agglomerated fluxes). The tested weld specimens have boron, niobium, manganese, molybdenum, chromium, copper and titanium as the primary major alloying elements.
- 1.
It is observed that Ni and Nb content in the weld metal
Acknowledgement
Material (API X70) and testing support by Jindal Steel and Power Limited Angul, plant and Jindal SAW limited Mundra, plant is greatly acknowledged.
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