Evaluation of AH 36 microalloyed steel welded joint by submerged arc welding process with one and two wires

Shipbuilding is going through a period of revitalization, growth and technological advancement. One component of these innovations is to improve welding techniques and materials for optimizing processing time, reducing costs and to improve properties and ship performance. The aim of the present work is to evaluate the welding of microalloyed shipbuilding steel, AH36, using the submerged arc welding (SAW) process with one and two wires. The mechanical properties of the welded joint will be presented using microhardness and Charpy V-notch testing. For metallographic characterization, the base metal and welded joint were etched with 2% Nital and Klemm's I reagent. Scanning electron microscopy, X-ray diffraction and colored etching were used to quantify and verify the presence of martensite and retained austenite, the MA microconstituent. The results obtained from Charpy impact tests and another mechanical test can be correlated with the associated microstructure.


Introduction
The shipbuilding industry requires technological advances, such as research in welding techniques and appropriate materials to reduce processing time and costs without impairing the desired mechanical properties.Recently, researchers around the world have focused on research and development of high strength low alloy steels (HSLA), as well as the investigation of various factors influencing their weldability.
Steel plates produced by thermo-mechanical control processing (TMCP) combined with various microalloyed elements have provided an effective solution to those requirements, offering better combinations of strength and toughness without impairing the weldability 1,2 .Welding accounts for a one third of all services performed in the shipbuilding industry, with submerged arc welding (SAW) being one of the most widely used processes 3 .Tandem submerged arc welding has been studied to increase the welding productivity.
The tandem technique process uses two or more wires aligned in the welding direction to increase the deposition rate and welding speed 4 .The tandem SAW has been used as an alternative to the conventional single wire SAW process, mainly in plates of medium and large thickness (greater or equal to 20 mm) 5 .Tandem SAW creates stronger and tougher weld joints due to a suitable thermal cycle in the heat affected zone, when compared with thermal cycles for a one wire welding process.The purpose of this work is to evaluate the welding of microalloyed shipbuilding steel, AH36, produced via TMCP, using flux cored arc welding (FCAW) at the root and filling with SAW with one wire and with two wires.

Experimental Procedure
The base metal used in this work was microalloyed shipbuilding steel, AH36, produced via TMCP, with a 20 mm thickness.Table 1 shows the chemical composition and the mechanical properties of the base metal based on the ASTM A131 standard 6 and the equivalent carbon in accordance with IIW.The geometry of the joints is shown in Figure 1.
Both single wire (S-SAW) and two wires tandem SAW (T-SAW) were used in this work.The root pass, for both welds used the flux-cored arc welding E71T-1 filler metal with a diameter of 1.20 mm 7 and 100% CO 2 as the shielding gas.The chemical compositions and mechanical properties of the FCAW root pass consumable and the SAW electrode are presented in Table 2 and Table 3.The welding parameters are typical of industrial practice and are shown in Table 4 and Table 5.The inter-pass temperature was controlled to be less than 250°C.The heat input was calculated with the conventional heat input equation using a coefficient of heat transfer into the plate of 0.95 8 .
Optical microscopy characterization was done by polishing and etching the samples using 2% Nital and Klemm 1.This reagent colors the microstructure and reveals martensite and austenite microconstituents (MA) as white and ferrite as blue.An image analysis program was used to measure the MA volume fraction.A scanning electron microscope (SEM) was used to examine the fracture surface of various specimens.Microhardness testing was performed in accordance with ASTM E384 9 , and conducted using a LECO's LM Series microhardness testing machine with a 300 gf load and a hold time of 10 s.A 0.2 mm grid spacing was used, revealing the distribution and individual hardness values in selected regions of the weld joints, as depicted in Figure 2. Charpy V impact fracture toughness tests were performed according to ASTM E23 standard 10 for the base metal and the weld metal.Tests were done for three samples each at -60, -40, -20, 0 and 25°C.Samples from the base metal were taken from the longitudinal rolling direction (LRD) and transverse rolling direction (TRD).

Microstructural characterization
Microstructural characterization of the joints is shown in Figure 3 to 8. Figure 3.a presents the microstructure of the base metal.It can be seen to be ferrite and (F) and pearlite (P).Table 6 shows a comparison between volumetric fractions of the F and P in the HAZ and BM.
Figure 3.b shows the microstructure of the base metal etched with Klemm I reagent.This reagent is able to show ferrite as blue or brown, depending upon the crystallographic orientation.In addition, it identifies the presence of MA       microconstituents as white regions.In this image, there is only ferrite in blue and pearlite as dark gray.The grain size in the base metal was measured.The results show a range from 8 to 17 µm, which agrees with past work 11 .Specifically, the ferrite grain size and the banded structure are due to microstructural changes that occur during TMCP processing.Figure 4.a presents the microstructure in the FGHAZ region (fine grain heat affected zone) in the welded joint for S-SAW.The observed constituents are polygonal ferrite (PF), ferrite with non-lamellar carbides (FC) and perlite (P).The mean grain size for this region was between 10 and 12 µm, which is slightly lower than that for the base metal.The temperature during the welding process is high enough to allow partial austenite transformation in the intercritical region.However, the region from the FGHAZ to the fusion line does not undergo significant grain growth of austenite, since the cooling rate drives the austenitic transformation in polygonal ferrite grains 12 .Likewise, the FGHAZ was etched with Klemm I reagent (Figure 4.b) and no MA microconstituents are present.The large amount of PF in the FGHAZ occurs because the cooling rate in this region is lower than the cooling rate for the CGHAZ region (coarse grain heat affected zone).A cooling rate between 1 and 5°C/s in the AH36 steel promotes the formation of polygonal ferrite and perlite 13 .
The microstructure in the CGHAZ for S-SAW (Figure 5.a) was composed of grain boundary ferrite (GF), ferrite with a second phase FS (aligned FS(A) and nonaligned FS(NA)), polygonal ferrite (PF) and ferrite with non-lamellar carbides (FC).These microstructural changes occur when the temperature in the CGHAZ region is above the A c3 temperature for AH36 steel, thus promoting the growth of austenite grains.If the peak temperature during the last pass exceeds the A c3 temperature, the resulting austenite after cooling may decompose in Widmanstätten ferrite, bainite and MA microconstituents, which can be seen in Figure 5.b. as white dots.
Figure 6 depicts the weld metal microstructure for S-SAW.Figure 6.a shows acicular ferrite (AF), ferrite with second phase FS (aligned FS(A) and nonaligned FS(NA)) and grain boundary ferrite (GF).In Figure 6.b one can see the MA microconstituents with a volumetric fraction comparable to the CGHAZ of S-SAW.
The microstructure presented in Figure 7 is similar to Figure 5.The difference is a coarser microstructure and the amount of MA microconstituent, which is larger for T-SAW.These results are likely due to the higher heat input used for T-SAW, which produced higher peak temperatures for comparable thermal cycles.These characteristics in the thermal cycles produced the changes in the microstructure observed.
Figure 8 shows the weld metal microstructure for T-SAW.The phases are identical to those in Figure 6.In Figure 8.b one can see the MA microconstituents with a volumetric fraction higher than that for the WM (weld metal) of S-SAW.
The MA microconstituent is associated with the dissolution of carbides and a decrease in the M s and M f temperatures.The amount of retained austenite depends on the cooling rate of the joint; the faster the cooling rate, the larger the volumetric fraction of austenite.This was observed for the case of T-SAW.
The volumetric fraction of MA microconstituent was measured by the colored etching technique.The results are presented in Figure 9.However, this metallographic technique is not sufficient to prove, in a precise way, the presence of austenite.Thus, X-ray diffraction was used to identify austenite, martensite and ferrite phases in the HAZ of the welded joint made by S-SAW and T-SAW.This technique identifies phases with at least 5% in volume fraction.X-ray diffraction was carried out in the CGHAZ regions, which according to image analysis had higher volume fractions of MA.The regions showed the presence of ferrite, austenite and martensite.Table 7 and 8 present the identification of the planes of each phase with higher intensity.The results of the diffraction patterns are depicted in Figure 10 and Figure 11.
The calculations of the crystal parameters of the phases found in the diffraction patterns are presented in Table 9, to verify the planes and phases identified in Table 7 and  Table 8.The analysis by X-ray diffraction confirmed the presence of retained austenite and martensite phases in the experiments under study; in other words, the presence of MA microconstituent.
The volume fractions of the MA microconstituents for S-SAW and T-SAW, for the separate weld and HAZ regions, are shown in Figure 9.There is a larger quantity of MA in the CGHAZ for the two welding conditions, with the T-SAW sample having a fraction about 60% higher than the S-SAW sample.It is possible that the higher heat input in T-SAW is responsible for this result.Figure 12 shows the heat input for each pass.The higher the heat input, the higher the peak temperature at a location in the HAZ.In addition, the cooling rate is slower, allowing more time at a given temperature in the HAZ.This increases the grain size of the regions which, together with carbide dissolution, increases the hardenability of the region in the HAZ.As a consequence, the M s and M f temperatures decrease, and more retained austenite is formed together with untempered martensite.

Microhardness test
Figures 13 to 15 present a microhardness profiles for lines 1, 2 and 3, respectively, in Figure 2. Line 1, in Figure 13, corresponds to the filling pass and line 3, in Figure 15, is the finishing pass region.Figure 13 shows that the values of microhardness along line 1 in S-SAW are similar to line 1 in T-SAW.In both cases, reheating due to subsequent passes can produce a kind of a "localized heat treatment", keeping both hardness profiles almost the same.
By comparing S-SAW and T-SAW, it is possible to observe significant differences in microhardness values, in particular in the CGHAZ.Besides the heat input, the values of microhardness vary due to the different microstructures present in the welded joint and due to reheating thermal cycles during welding, which act as a kind of non-isothermal heat treatment.
Figure 14 shows the results of the Vickers microhardness measurements for line 2, the root pass for S-SAW and T-SAW.The difference in microhardness values are due to the peak temperature, which is a function of the heat input, (Figure 12).

Charpy V notch impact test
The results obtained in the Charpy V notch impact test are depicted in Figure 16.An ANOVA analysis, with 95% confidence, showed no difference in weld metal absorbed energy, comparing samples from the two conditions (S-SAW and T-SAW).However, when comparing different regions of the welded joint, the weld metal had the lowest impact toughness.This deleterious behavior is due to an as-solidified weld metal with a coarse grain size and also the presence of microsegregation. 14ccording to the fracture images obtained through scanning electron microscopy, shown in Figure 17, it is possible to identify transgranular fractures and dimples.This is the typical appearance of specimens that suffered ductile       fracture.The Charpy impact test, for a notch situated 5 mm away from fusion line (FL+5), offers the highest average value of absorbed energy (220 J) in the FG HAZ region for S-SAW.That is a zone of grain refinement, which underwent reheating by subsequent welding passes.The average grain size is 5.5 μm and the predominant microstructure consists of polygonal ferrite with a smaller fraction of pearlite.The same zone observed in T-SAW not only consists of polygonal ferrite and pearlite, but also a microstructure with zones of Widmanstätten ferrite (WF).The presence of those morphologies has contributed to a slight reduction in the amount of absorbed energy, which is about 10% lower than that for S-SAW.
The results of the ANOVA analysis, with 95% confidence, presented no difference between the absorbed energy values in the Charpy impact test for the region situated 1 mm

Figure 1 :
Figure 1: (a) Dimensions of plates for welding and (b) detail of the bevel adopted for the joint.

Figure 9 :
Figure 9: Volume fraction of MA microconstituent present in the WM and regions of the HAZ for the two experiments, S-SAW and T-SAW.

Figure 10 :
Figure 10: Diffraction pattern for sample welded by S-SAW.

Figure 11 :
Figure 11: Diffraction pattern for sample welded by T-SAW.

Figure 12 :
Figure 12: Heat input for each pass in S-SAW and T-SAW experiments.

Figure 13 :
Figure 13: Results of microhardness for line 1, as presented in Figure 2.

Figure 14 :
Figure 14: Results of microhardness for line 2, as presented in Figure 2.

Figure 15 :
Figure 15: Results of microhardness for line 3, as presented in Figure 2.

Figure 16 :
Figure 16: Results of Charpy V tests at 0°C for different regions of the welded joint; weld metal, 1 mm away from the fusion line and 5 mm from the fusion line.

Figure 17 :
Figure 17: (a) and (b) SEM fractographs of Charpy samples at 0°C of the weld metal for S-SAW; (c) and (d) SEM fractographs of Charpy fractographs at 0°C of the weld metal for T-SAW.

Table 1 :
Chemical composition and mechanical properties of the base metal.

Table 2 :
Chemical composition and mechanical properties of the consumable for the root pass.

Table 3 :
Chemical composition and mechanical properties of the flux and electrode for SAW.

Table 4 :
Welding parameters for one wire SAW (S-SAW).

Table 5 :
Welding parameters for two wires tandem SAW (T-SAW) (*) The heat input of each wire was added per pass.

Table 7 :
Phases identified by X-rays diffraction in the CGHAZ region of one wire SAW joint (S-SAW).

Table 8 :
Phases identified by X-ray diffraction in the CGHAZ region of two wires SAW joint (T-SAW).
(*) Identification in the diffraction pattern background

Table 9 :
Crystal parameters for the identified phases by X-ray diffraction.