Development of Ferritic High Strength SMAW Electrodes For Steam Generator Application

1.0 INTRODUCTION

It is well understood that the involvement of welding technology in commercial as well as in industrial fields is inevitable. In contrast to commercial needs, the industry demands more stringent requirements on welding technologies. In view of this, new welding technologies are developed world wide to improve the manufacturing and fabrication of industrial components. Besides this, the main focus is also on

• The development of welding consumables for specific applications based on its physical and the mechanical characteristics 
• Tailoring of composition and structure
• Evaluating the weld component performance under severe service conditions
• Accomplishing the design criteria
• Reducing the cost and enhancing the applicability etc. 

It seems that the novel way of developing welding consumable for specific applications in power generation and petrochemical industry is rather a challenging one. As well, the developed welding consumable must possess an international material standard for its applicability in various related applications. For example, the main industrial products such as boilers, heaters, pressure vessels etc. have to be developed accordingly to enhance the process by various means of increasing steam parameters. In the direction of satisfying the enhanced base metal properties, a high performance welding consumables have to be developed and tested for its reliability. In view of this, a low hydrogen SMAW welding consumable with improved mechanical properties have been developed in-house for fabricating pressure vessel component in nuclear applications. This type of electrodes is regarded as advantageous one, because it eliminates the pre-heat process before carrying out to welding stage. Based on the specification prescribed by client on covered ferritic welding

electrodes for shielded metal arc welding process, extensive research and development has been taken place in-house to achieve the required properties. The details of the specification of weld metal required are presented in Table1.

2.0 DEVELOPMENTAL WORK

2.1.1. All Weld-Joint Preparation

During development, several batches of weld joint have been prepared with

slightly modified electrode compositions to optimize the desired composition of the weld metal. For standardization purpose, various aspects that are taken care as follows,

(a) Reduction of impurity elements in the weld metal

(b) Reduction of hydrogen content by choosing a suitable binder

(c) Welding procedural aspects like influence of Inter Pass Temperature, heat input etc.

(d) Adjustment of chemical composition to get desired properties

The above said methods are optimized and the all weld joint assembly is prepared successfully for metallurgical and mechanical evaluation for its suitable applications. The schematic of the all weld preparation procedure is shown in Figure-1. 20MnMoNi55 forge plate of dimensions 450×125×20 mm is prepared with the bevel angle of 10 degrees and a root gap distance of 16 mm supported with backing strip. This specimen is welded with our newly developed electrode by using SMAW process. The optimized welding procedure utilized during the welding process is systematically presented in Table-2 (average of seven layers). The test specimens are machined from the weld joint and are subjected to various analyses such as chemical, metallography, mechanical and radiographic examination.

2.1.2. Chemical Composition

The chemical composition (wt. %) of the weld metal determined using wet chemical analysis is given in Table-3. In addition to this, the resulting composition obtained from the root of the weld after the welding process is also given in Table-3. This has been performed to know the extent of dilution. In Table-4, the composition of the forge plate used as a base material is also tabulated.

2.1.3. Metallography Studies

The optical and hardness studies have been carried out using AXIOVERT 100A Optical microscope and Rockwell Hardness tester (0 – 100 RC). Metallographic specimens have been prepared by adopting standard method of polishing procedures using various grades of emery sheets and cloth impregnated with fine alumina particles. This is followed by cleaning with distilled water and methanol. The etchant used for observing the microstructure is made of aqueous solution containing 4% Picric acid and 1% Nitric acid. The etched specimens have been used further for hardness analysis.

2.1.4. Tensile Studies

The tensile property of the pure weld deposit 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), 200o C and at 350o C respectively.  Figure-2 shows the round specimens of diameter 12.5 mmand guage length 60 mm used for tensile testing prepared as per the ASTMstandard E-21. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (et) and reduction in area.

2.1.5. 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 V-notched, with 2 mm of depth. The Charpy transition curves are obtained from room temperature to sub-zero temperatures. The Charpy impact test is accomplished in compliance to ASTM

E23 standard to determine the ductile to brittle transition temperature.

2.1.6. Bend Tester

The welded specimens have been Bend Tested using AMSLER Bend Tester for the evaluation of the ductility and soundness of the weld.

2.1.7. Die Penetration and Radiography

The weld deposits are analyzed with Die penetration and X-ray radiography for

the evaluation of any presence of crack and inclusions.

2.1.8. Drop weight test results

Drop weight test of eight weld samples were tested and the results are satisfactory. Welding procedure for preparation of weld coupon is given in Table-5. From the above weld coupon final size of the test specimen prepared as per ASTM E208. The details are given in Figure-3.

3.0 RESULTS AND DISCUSSIONS

The optical micrographs of the base metal away from the weld region and the as deposited last bead of the weld are shown in Figure-4. In both the cases, the microstructure consists of ferrite and bainite. It is clear that the strength and toughness of this material emanates from the presence of bainite and ferrite fractions. The average Rockwell (RC) hardness values of the forge plate and the as deposited weld metal are 14 and 16 RC respectively. Figure-4a also shows the microstructure of the HAZ of the specimen. It is clear from the figure that the microstructural features are finer than the base metal and the weld metal. This may be due to the effect of cooling rate and dilution.

In Figure-5, the response of weld tensile specimen with the application of load is portrayed. The collage in Figure-5, tested at different temperatures clearly reveals that the tensile strength (YS, UTS) of all the specimens are almost identical, except during the occurrence of fracture. Moreover the tensile strength at high temperature is also not drastically different for 200o C and 350o C tested specimens. It suggests that the required hot tensile property of the weld is attained in accordance to ASME specifications. For comparison purpose, the ASTM value and literature data on tensile properties of base material are gathered. It is observed that the tensile property of the developed weld metal is comparable to the literature data, signifying the strength and quality of the weld. Table-6 also lists the average of three tensile tested specimen properties that are obtained at room temperature and elevated temperatures respectively besides some literature data. In Figure-6, the impact toughness of the weld joints tested at room temperature and at subzero temperatures are displayed. In this figure, the available toughness data on base material as well as similar weld metal is co-plotted. It is seen fromFigure-6 that the toughness values of weld metal is inferior to base metal. In general, the weld samples may have micro-defects, impurities, non-uniform properties due to multi-pass weld, cooling rate dependent transformations etc. However in the present study these parameters are taken care to obtain a high quality weld and hence the improved toughness properties are obtained. This is also made clear by the highlighted data of about 54 J specified by AWS. In Table- 7, the toughness values of the weld metal tested at various temperatures are listed. In addition to this, data on lateral expansion and the percentage of shear fracture area determined from the fracture surface is also listed. In Figure-7, the fracture dependent lateral expansion and the percentage of shear fracture area calculated from the fracture surface is plotted. Both of the parameters decrease gradually with the temperature signifying the change in the fracture mode i.e. ductile to brittle.

In general, a high performance weld material is chosen for stringent applications in nuclear industries. For example, the reactor pressure vessels are made with nuclear grade high strength steel which requires equivalent high strength weld materials for fabrication purposes. In view of this, the results obtained in the present study with regard to mechanical properties evaluation of the high performance SMAW ferritic electrode are discussed below.

The optical microstructures determined at locations of weld centre line, HAZ and base metal suggested the formation of bainite and ferrite phases. The variation of the phase fractions of bainite and ferrite between the base metal, weld and HAZ specimens are determined using the Image Analysis® software. Because this provides the information about the effect of cooling rate, compositional re- adjustments on the phase transformation of γ-austenite → α-ferrite transformation.

It is observed from the image analysis results that the phase fraction of ferrite and bainite is high in base metal as compared to weld and HAZ specimens. This may be due to the effect of cooling rate / composition on the kinetics of γ → α phase transformation. The phase fraction values have been determined at different locations (more than 10 locations at each region) and the average value is tabulated in Table-8. A typical threshold image of the base metal obtained using Image Analysis is shown in Figure-8 for a comparison. Since the value of the phase fraction depends on the threshold fixing, care is taken in determining the values of respective phase fractions. In Figure-8(b) the white ferrite regions are undecorated with red colour while the dark regions of bainite are decorated with red color using Image anlaysis procedure is shown.

The evaluation of the tensile results of the weld specimen tested at RT, 200 o C and 350 o C suggests that the tensile strength possessed by the weld specimen is adequate for high temperature pressure vessel applications. Besides, the decrease in the yield strength of the weld specimen at 350oC is about 25% and the ultimate tensile strength is about 13% as compared to RT data. 

The decrease in the tensile strengths is attributed to the enhanced thermal activation of dislocations at elevated temperatures. In most cases, the material under fracture investigation must possess homogeneous structure to yield the scatter-less tensile properties. The presence of HAZ and brittle zones in the welded joint has inhomogeneous tensile properties leading to dispersion of the data. But this property is also very important in design considerations [8-11]. In the present study, only the pure weld deposit is tensile tested and the average values of the tensile strengths are presented. It is also clear from the Table-6, the tensile strength of the weld deposit is markedly higher than the base material. The results of the Charpy transition curves of the weld joints having a notch at the weld deposit region clearly suggest that the weld deposit have sufficient toughness to resist the fracture at sub-zero temperatures i.e > 54 J. Even at minus 76o C, the toughness value is found to be 60 J. It is known that the primary design criteria of a component under stringent conditions depend upon the strength and the stability of the microstructure. The fracture toughness is also one of the design criterions, which has to be determined for structural integrity assessments.

4.0 SUMMARY AND CONCLUSION

It is clear from our evaluated results of the weld metal that the SMAW electrode developed in-house has met the specified requirements of BHEL. Careful optimization of the composition of the weld metal yielded good hot tensile as well as toughness properties. The liquid penetrant and radiography test have confirmed that no cracks and inclusions are present on the surface of weld metal.  In addition to this, the diffusible hydrogen mercury test also shows the average level of hydrogen is about 3.9 ml in 100 gms of weld. Hence the optimization of composition of welding consumable, maintenance of weld metal quality, achievement of superior high temperature and low temperature properties makes this product applicable for steam generator applications as well as various demanding structural applications mentioned earlier in this paper.

The major conclusion drawn from this work is as follows:

I.  A high performance low hydrogen E9018-G SMAW electrode meeting specifications is successfully developed.

II. Effect of heat input, Inter Pass Temperature (IPT) on weld metal properties have been analysed and found that lower heat input, interpass temperature of about 100°C is beneficial for desired mechanical properties. Hence a careful selection of welding parameters is always recommended.

III. The control of hydrogen content in the weld is advantageous to obtain good  ductility of the weld.