Welding and Characterization of HSLA Steel

High strength low alloy steels HSLA are thermo-mechanically controlled rolling (TMCP) steels that have high strength, toughness, and good weldability, formability [1,2]. HSLA steels gain relevance due to their low carbon content that improves weldability and formability. These HSLA steels contain small additions of principally vanadium, niobium, and/or titanium but occasionally Zr and Ta [3,4]. These elements are referred to as microalloying elements because only small amounts of them are required, and the steels are called micro alloyed steels [4]. Typically, HSLA steels have microstructures consisting mainly of ferrite, pearlite, small number of, carbonitrides, carbides and nitrides depending on the heat treatment and processing [5]. High Strength Low Alloy (HSLA) steels have been widely applied for transporting oil and gas [6,7]. Submerged arc welding is one of the welding processes used to weld and construct line pipes. In the petroleum business, high strength low alloy steels are commonly utilized to carry crude oil pipelines. The steel pipe industry has made significant efforts to manufacture line pipe steel grades with outstanding mechanical and metallurgical qualities [2]. The microstructure as well as mechanical properties of HSLA steels can be influenced by the chemical composition, welding parameters [8]. The Heat Affected Zone (HAZ) and microstructure of weld metal can directly affected by the welding thermal cycle, thus affecting the welding joint mechanical properties [1,9]. Bainitic microstructures are now being increasingly used to produce the strength and toughness levels required for high and ultra-high strength line pipe steels [10]. Additionally, the microstructural refinement and precipitation of titanium carbide can contribute to achieving high levels of mechanical properties in HSLA steels [11].

In the present research work HSLA steel was procured from source. Welded using submerged Arc welding process. Characterization for Hardness, DWTT, Impact strength, Tensile strength, Microscopy using optical and scanning electron microscope was done for samples.

HSLA steel was procured from the industry which was Themo mechanically rolled plates. Chemical composition was checked by spectroscopy. Chemistry for HSLA steel is given in Table 1. HSLA steel was Welded using SAW process with filler wire EM12K & EA2. Tendom SAW process was carried out in two passes and welding parameters as given in Table 2. The first Top pass was on the top side of plate and second pass on bottom side of plate. Welded plates of HSLA steel are shown in Figure 1.

Table 1:Chemical composition of HSLA steel.

Table 2:Welding parameters for HSLA steel.

Figure 1:SAW welded HSLA steel.

Further characterization of welded samples is carried out. Tensile testing, impact testing, DWTT, hardness testing, microstructure observation and SEM were performed on the welded high strength low alloy steel samples.

Tensile testing

Tensile testing results show that yield strength, tensile strength, and percent elongation of welded HSLA steel 520Mpa, 678Mpa and 36.94% respectively. Even after welding it offers a very good combination of strength and ductility property (Table 3 & Figure 2).

Figure 2:Tensile testing property.

Table 3:Tensile testing result of welded HSLA steel sample.

Charpy impact test

Figure 3:Impact property of welded HSLA steel.

Impact strength of welded HSLA steel sample in base, weld and HAZ is 322J, 108J & 295J respectively. Impact testing is carried out at 0 C temperature. The result shows excellent value of toughness property at lower temperature. The addition of titanium, ranging from 0.005-0.02%, can enhance the toughness of the heat-affected weld zone by inhibiting the growth of austenite grains [12]; (Table 4 & Figure 3).

Table 4:Charpy impact testing result of welded HSLA steel.

Hardness testing

Hardness testing result shows that the hardness value of welded HSLA steel sample in base, HAZ and weld is 211HV, 216HV, 214HV respectively. The hardness value shows that base and weld offer relatively high hardness as compared to base metal. It is mainly due to microstructure morphology. Weld contains acicular fine ferrite that leads to increase in hardness (Table 5 & Figure 4).

Figure 4:Hardness property of welded HSLA steel.

Table 5:Hardness value of welded HSLA steel.

Drop Weight Tear Test (DWTT)

Drop Weight Tear Test (DWTT) Drop Weight Tear Tests (DWTT) are widely used in the gas pipeline industry to determine material characteristics such as brittle fracture resistance arrest in the seamless or welded. The drop weight tear test showed that the average percentage shear area is 96.42 at 0 ℃ temperature. Which indicates ductile type failure (Table 6).

Table 6:DWTT test of welded HSLA steel.

Microstructure observation

Figure 5a-5e shows the microstructure of the base, HAZ, and weld at 100X & 400X. Base metal Microstructure is containing predominantly very fine-grain ferrite and a minor mixture of ferrite and carbides due hot rolling and recrystallization process. As carbon content is lower in HSLA steel; thus, the amount of the second phase is lower [12,13]. HAZ contains pro eutectoid or grain boundary ferrite (GF), ferrite side plates (SP) and Granular bainite, growth, and formation of bainite are the dominant features of HAZ [14-17] weld microstructure shows Grain Boundary Allotriomorphic Ferrite (GBF) or polygonal ferrite with very fine Acicular Ferrite (AF) [18].

Figure 5:Hardness property of welded HSLA steel.

Scanning electron microscope

(Figure 6 - Figure 11)

SEM images at 500X reveal the structure of ferrite and carbides present in the microstructure. In the HAZ prior austenite grain boundaries are also visible at 1000X. Weld metal shows the acicular ferrite and grain boundary ferrite [21]. Granular bainite is seen in HAZ which is a mixture of ferrite and carbides [10,18,22,23]. Acicular ferrite which forms in weld metal nucleates from inclusions within the prior austenite grains and is characterized by fine ferrite plates as shown in Figure 9c; [20]. Grain boundary or allotriomorphic ferrite or polygonal ferrite is characterized its presence at grain boundary as it is seen in HAZ and Weld metal. In HAZ as the cooling rate becomes slow polygonal ferrite increases with a decrease in acicular ferrite amount as compared to weld Metal [24]; Figure 10 & 11 shows how different morphologies of ferrite nucleate and grow.

Figure 6:SEM image at 500X a) Parent Metal b) HAZ c) Weld.

Figure 7:SEM image at 1000X a) Parent Metal b) HAZ c) Weld.

Figure 8:SEM image at 2000X a) Parent Metal b) HAZ c) Weld.

Figure 9:SEM images with negative effect.

Figure 10:An illustration of the essential constituents of the primary microstructure of a steel weld deposit [19].

Figure 11:Schematic diagram shows the effect of prior austenite grain size and non-metallic inclusions on the nucleation of bainite, Widmanstättenferrite and acicular ferrite [19,20].

After welding of HSLA steel with SAW process heat input of 2.75 bottom side and 3.44 on the top side the results are
a) Tensile testing of weld metal gave yield strength, UTS & ductility of 520Mpa, 678Mpa & 36.9% which is an excellent combination of strength and ductility.
b) The Charpy impact test gave impact strength of the base, HAZ and weld are 322J, 295J & 108J respectively which confirm very good toughness properties.
c) Hardness values in base, HAZ & weld are 211HV, 216HV 214HV respectively. Almost similar hardness value is found in all three regions.
d) The DWTT test gave an average shear area of 96.42%
e) Microstructural observation revealed the presence of acicular ferrite and grain boundary ferrite or polygonal ferrite or allotriomorphic ferrite in weld metal. SEM at higher magnification revealed the formation of acicular ferrite from inclusion. In HAZ acicular ferrite decreases polygonal ferrite increases along with a mixture of ferrite and carbide is also present.

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