ABSTRACT

The microstructure and hydrogen induced cracking of SAW welds in API 5L X70 grades with different amount of titanium were studied. The investigation was carried out on the base metal, heat affected zone and weld metal with three different titanium amounts in weld metal containing 1.4%Mn and 1.9%Mn respectively. The results showed that the centerline segregation region of the base metal was sensitive to hydrogen induced cracking while the heat affected zone did not show any failure. Furthermore, were observed that with addition of titanium, the formation of acicular ferrite in the microstructure and hardness values were increased and stress oriented hydrogen induced cracks appeared in the weld metal. The optimum combination of the microstructure and hydrogen induced cracking resistance was observed in 1.4%Mn-0.11%Ti and 1.9%Mn-0.05 Ti% respectively.

INTRODUCTION

In pipeline industries, the primary interest is to obtain the best possible combination of strength and toughness. The high strength low alloy pipeline steels (HSLA) have a good combination of strength, toughness and weld ability. They have been widely used in the construction of long-distance oil and gas transportation systems.1The microstructure of these steels depends on the alloy elements, thermo-mechanical processing and cooling rate. Generally, the microstructure of conventional C-Mn weld metals consists of varying amounts of acicular ferrite (AF), widmannstätten ferrite (WF) and micro phases, with a yield strength ranging from 350- 450 MPa.2 Some high strength low alloy weld metals, such as C-Mn with titanium (Ti) and / or vanadium (V) and niobium (Nb) additions, exhibit similar microstructures to the C-Mn welds; however, they have higher yield strength, usually in the range of 500-700 MPa. It was reported that a predominant acicular ferrite microstructure with martensite / Austenite (M/A) islands as a second phase, exhibited optimum mechanical properties. Keehan et al.4 found that once Ni exceeds a critical point, which depends on manganese (Mn) concentration, the charpy toughness of these steels at -40°C decreased. The addition of Mn and Ni together has been reported to harden weld metal and therefore decrease the impact toughness.5 Bhole et al.6 found that Mo addition of 0.881 wt% in the weld metal gave the optimal impact toughness at -45 °C with a microstructure of 77% acicular ferrite (AF) and 20% granular bainite (GB). Also, manganese content is an important alloying element for solid solution strengthening. Mo and Nb containing steels are commonly used in pipeline applications because it is observed that the HSLA steels containing Mo and Nb exhibit superior strength and toughness combination as compared to the HSLA steels containing Nb and V.7 Inclusions are known to be an important factor in controlling the microstructure and toughness of weld metals, acting as nuclei for acicular ferrite formation and initiation sites for the cleavage fracture process. Bose .Filho et al.2 found that in the weld metals with low Ti content, manganese and silicon were the main chemical elements present in inclusions

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