Mechanical properties and hydrogen induced cracking behaviour of API X70 SAW weldments

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Highlights

  • SAW flux mixtures have been developed using extreme vertices design method.

  • Mechanical and microstructural behaviour of agglomerated and commercial flux welds were studied.

  • HIC susceptibility of agglomerated and commercial flux welds was studied.

  • F3RA and F19RA show maximum CSR % in hydrogen induced cracking measurements.

  • Base metal, F5B and C.F weld specimen show lower CSR percentage.

Abstract

Pipeline welding is an integral part of oil and gas exploration industries. Often the weld failures are observed due to lack of weld quality, improper heat treatment and poor workmanship. Further, the use of new materials in pipeline industry puts focus on a better understanding of requirements for welding and reducing the failures in future. This necessitates the need for development and design of suitable welding fluxes for joining of these materials. In this paper an attempt is made to analyse the effect of commercial submerged arc welding flux and laboratory prepared agglomerated submerged arc welding fluxes (of basic, rutile basic and rutile acidic type) on the weldability, microstructural evolution as well as the structural integrity issues in API X70 line pipe steel welds. The mechanical and microstructural behaviour of weld joints was observed for submerged arc welding fluxes. The maximum tensile strength of 613 N/mm2 was observed for weld joint prepared using commercial flux (C.F). While in case of agglomerated fluxes the maximum value of tensile strength observed was 561 N/mm2 for F15B basic flux. Impact toughness for all the weld joints was evaluated both at room temperature and at −65 °C. For the weld as well as the heat-affected zone, the maximum impact toughness (160 J and 436 J) was observed for basic flux F5B at room temperature while at −65 °C (16 J and 30 J) impact toughness was obtained which was similar to that obtained with commercial flux. Microhardness of commercial weld joint (232 HV) is maximum as compared to the other weld joints. Maximum microhardness of 222 HV was observed for weld joint fabricated with flux F15B. Flux F15B shows a maximum value of microhardness (218 HV) for the heat-affected zone. Weld joints fabricated by using F3RA and F19RA fluxes exhibit high susceptibility to hydrogen induced cracking as compared to the remaining weld joints. The crack sensitivity ratio (CSR %) is high for weld specimen F3RA (54.64%) and F19RA (50.26%) while weld specimen fabricated using (F5B and C.F) fluxes show minimum crack sensitivity ratio (27.54% & 30.97%) respectively. The high susceptibility towards hydrogen induced cracking may be attributed to the very low value of carbon equivalent as compared to the remaining weld joints.

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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