Development of Basic Coated SMAW Electrode.

The nominal composition (wt %) of this weld metal is 15.5 Cr, 8.5 Ni, 1.5 Mo. These electrodes are used for welding stainless steels, such as Types 16-8-2, 316, and 347, for high-temperature, high –pressure piping systems and for dissimilar welding. The weld deposit generally has a Ferrite Number in between 0 to 5 FN. The weld deposit produced with these electrodes has good hot ductility properties that reduces crater cracking problem even in high-restraint condition. The weld metal is usable in as welded condition as per codes but it can also be used in solution-treated condition. The chemical composition of these electrodes should be balanced very carefully to develop their fullest properties. In 16Cr-8Ni-1.5Mo properties include excellent high temperature microstructural stability, high resistance to hot cracking at very low ferrite (FN) levels, and good cryogenic toughness. Table 1 and Table 2 gives the AWS specification limits for SMAW (MMA) weld metal.

Table 1 - Chemical composition of E 16-8-2-XX as per AWS SFA5.4 

Table 2 - Mechanical properties of E 16-8-2-XX as per AWS SFA 5.4

During the primary phase of consumable development we faced problem in percentage of elongation as per AWS standard. This SMAW covered electrode fulfills the requirement of ASME section II part C, which can be verified with the weld metal chemical composition and mechanical property as shown in Table 1 and in Table 2. This advanced SMAW covered are characterized by a sophisticated chemical composition that provides the weld metal with sufficient impact toughness, Tensile strength with elongation. The Babcock and Wilcox was the first company that published with reference to 16.8.2 depositing weld metal with about 0.07C-15.6Cr-8.2Ni-1.5Mo. [2] This composition of weld metal is in the region of austenite „nose‟ of the Austenite + Martensite (A+M) and Austenite + Ferrite (A+F) boundaries in the Schaeffler diagram shown in figure 1, assuming 0.04- 0.1%C and constant values of 0.5%Si-1.5%Mn-0.05%N.

However, As per AWS A5.4 and A5.9 state that 16.8.2 weld metal has usually below 5FN. The box shows that there is a mild chance of martensite formation in the as-deposited weld metal yet there is no confirmation that commercial weld metals contain martensite. In 1992 Kotecki corrects the misleading by proposed a new diagram in which he examined the formation of martensite in composition. He developed boundaries for 1wt% Mn, 4wt%Mn, and 10 wt% Mn. Since Mn does not show up in the nickel equivalent of WRC-1992 diagram, a different limit is required for every Mn level.

As per this new diagram of kotecki martensite-free microstructure of 16.8.2 is observed shown in Figure 2. The lower boundary of 16.8.2 composition box shows there is a probability of as-deposited martensite. Diagram also includes coefficient of 0.25 for copper in the nickel equivalent to observe the effect of copper content on ferrite number.

Several trials have been taken and established the weld metal chemistry to improve the low temperature toughness tensile strength and percentage elongation. The weld metal meeting the chemical composition requirement is confirmed by optical emission spectroscopy. The all weld test coupons were prepared by these electrodes. These test coupons are taken up for characterization and mechanical property evaluations.

The weld deposit after welding is evaluated with Dye penetration and X-ray radiography as per ASME Sec V code. Defect free welded joints were taken for mechanical testing. Once the mechanical properties evaluation was completed, the weld cross section was cut in order to characterize the weldment with macro structural analysis for metallurgical study. For micro structural analysis the specimen was milled, ground, polished and then etched using the Aqua regia reagent (15 ml HCL, 5 ml nitric acid) and examined under the microscope.

2.1. Tensile Test

The tensile property of the weld specimen is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 200 kN. The tensile measurements have been conducted at room temperature (RT). The round specimens of diameter 12.45 mm and guage length 50 mm used for tensile testing prepared as per the ASTM standard E-21. The tensile data is analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (TE) and RA (%).

2.2. Metallographic Studies

The optical studies have been carried out using optical microscope. Metallographic specimens have been prepared and cross section area which is parallel and perpendicular to welding direction were polished using various grades of emery papers and cloth with fine alumina particles. This is followed by cleaning with distilled water and methanol. The aqua regia etchant is used for observing the microstructure is 65 % HCL and 35% HNO3 solution.

3. Results and Discussion:

In severe applications such as in Petrochemical, Chemical process plants, Power generation industries a high performance weld metal is required. The present study contains mechanical properties evaluation of the high performance SMAW electrode is discussed and challenges faced during consumable development. The tensile test is performed on the pure weld deposit at room temperature, resulting values of tensile test suggests that the tensile strength possessed by the weld specimen is satisfactory for Petrochemical, Chemical process plants, Power generation industries applications. From Table 1 it is clear that weld metal strength is higher than the requirement.

In Figure 3(i) and 3(ii) the composition of weld metal on modified WRC-1992 diagram is near to martensite boundary zone with around 1%Mn that corresponds closely to as-deposited martensite resulting lower ductility and brittle fracture even with higher ferrite number where as if weld metal composition is shifted towards right as shown in figure 3(iii) and 3(iv) resulting higher ductility and ductile fracture is observed in tensile test even at very low ferrite number.

Table 4 - Weld deposit composition with various batches

3.1. Tensile Test The figure shown in 4(a) and 4(b) is representative of brittle fracture with a very little elongation whereas in figure 4(c) and 4(d) shows ductile fracture with higher elongation.

The tensile test data as shown in Table 5 are analyzed to evaluate the ultimate tensile strength (UTS), yield strength (YS), %elongation (EI) and reduction in area.

 

It is clear from the tensile test results data that the batches with higher chromium and nickel content as in batch 3 and batch 4 shows ductile fracture as compare to batch 1 and batch 2.

3.1. Microstructure

The weld metal microstructure consists with austenite and ferrite in small proportion and there is no evidence of as-deposited martensite formation in the weld metal but there should be a possibility of staininduced martensite formation if the composition of weld metal lies close to the martensite boundary and if carbide precipates during post weld heat treatment which leads to raise the martensite start temperature. In figure 5(a) and 5(b) irregularities can be seen, these irregularities in microstructure resulting brittle fracture or broke off during tensile testing where as in fig 5(c) and 5(d) uniform microstructure without irregularities gives ductile fracture in tensile test. Microstructural stability is obtained by various combination of composition with low ferrite number as show in figure 5.

3.5 Charpy Impact Testing

For charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10×10×55 mm and type Vnotched, with 2mm of depth. The charpy impact test is performed as per ASTM E23 standard to determine the toughness of the material. Weld metal toughness values at -196°C, -120°C and at -90°C is show in Table-6.

3.6 Inter-granular corrosion test

Susceptibility of weld metal produced with E-16/8/2 to intergranular attack is determined by Practice E test as per ASTM-A262 standard. The test has conducted on as-welded specimen as well as on specimen after sensitizing heat treatment at 700oC for 30 minutes. The test specimens are immersed in boiling solution of Cu-Copper Sulfate- 16% Sulfuric acid for 15 hours. The test procedure is specified in Table 7.

To evaluate micro fissure and cracks 1T bend test has conducted on specimens after exposed to acidified Cu-CuSO4 H2SO4 test solution. The bent specimen has examined under 50x magnification is shown in figure 6.

Metallographic examinations at 50x of the bent specimen showed that there is no evident of micro fissure and cracks in as-welded specimen and in specimen after sensitizing heat treatment at 700 deg C. 

4. Conclusion:

Basic coated SMAW process consumable was developed in our lab meeting AWS: SFA: E 16/8/2 -15 class requirements. Optimum chemistry required for meeting percentage elongation, corrosion practice E and toughness even at -196°C were formulated.