DEVELOPMENT OF SMAW CONSUMABLES FOR OVERLAY & JOINING OF LOW ALLOY FORGE MATERIAL USED IN NUCLEAR SG APPLICATIONS

1 INTRODUCTION

Several structural candidate materials are used for fabricating nuclear power plant materials. For most of the materials, welding consumables are available indigenously. But in case of welding consumables for nuclear applications, we are still importing consumables from overseas to meet certain stringent requirements on specific forged materials for overlay and joining applications. In view of this, SMAW electrodes of ≈ ENiCrFe-3 and low hydrogen high strength low alloy steel welding consumable with improved mechanical properties have been developed indigenously for fabricating pressure vessel component in nuclear applications.

In this paper, overlaying on 20MnMoNi55 steel with ≈ ENiCrFe-3 and for joining low hydrogen high strength low alloy steel electrode by SMAW process is reported. The overlaid structural components find application in petrochemical, nuclear, oil & gas industries etc. For such applications, the components should possess good mechanical properties and corrosion resistance.

Hence the objective of this present study is to evaluate the weldability and the essential properties of this weld metal for its suitability in overlay and joining applications related to heat exchangers. The design requirement of this weld metal is tabulated in Table 1 & 6. It is clear from the table that strict control over the weld metal composition besides established welding parameters is necessary to achieve the specified properties.

2 EXPERMENTAL DETAILS FOR INCONEL CONSUMABLE

2.1 Overlay on forge plates

20MnMoNi55 plate of dimension 700×400×20 mm has been used as a base material for cladding purpose. Cladding has been done with ≈ ENiCrFe-3 electrode of three different sizes. The typical weld assemblies made using MMAW process is shown in Figure 1. The optimized welding procedure adopted during welding of these cladding assemblies is listed in Table 2.

2.2 Chemical Composition of welding consumable

The chemical composition of the Inconel welding consumable has been optimized on the basis of experience without compromising on weldability characteristics. Following points are kept in mind while designing the product. (i) Effect of impurity elements such as P, B and S contents on solidification cracking of the weld 

(ii) Effect of alloying elements on wetting characteristics to avoid micro-cracking 

(iii) Effect of Si and Fe on formation of low melting laves phase 

(iv) Optimization of Mn and Si to counteract the detrimental effects of S and P (v) Addition of strengthening constituents such as Al and Ti contents.

Few trials have been taken after studying the core wire chemistry with different formulations.

Then it was tested after depositing on a forge plate. Specimen taken from indicated location (CH1) in Figure 1. 

The chemical composition of the weld metal is analyzed by optical emission spectroscopy at five different locations of each weld pad. The locations are 5 mm and 6 mm height from the fusion line. The required weld metal composition is identified from each set of weld pads that are prepared using different sizes of electrodes and the results of the optimized chemical compositions of the weld metal are listed in Table 3.

2.3 Non Destructive Evaluation of Weld

The surface of the prepared weld assemblies has been subjected to liquid penetrant test and ultrasonic test for surface and internal weld defect inspection. Ultrasonic examination has been performed to investigate the presence of any weld bead crack or bonding defects in the weld overlay. As per the requirement, focused 70o angle beam is used for inspection of weld coupons.

2.4 Heat Treatment of Weld Assemblies

As per the requirement, the claded plates have been subjected to a simulated heat treatment cycle, before carrying out any mechanical tests. The overview of the heat treatment procedure is mentioned in Table 1. It consists of heating the weld assemblies that is isothermally held at 300°C to 550°C at a rate of 30°C h-1. At this temperature the weld assemblies is being soaked for over 40h. This is followed by cooling the weld assemblies to 450°C at 30°C h-1. And then, the assemblies are taken to 600°C at a rate of 30°C h-1 and held at this temperature for 8h and cooled to 450°C. This particular heating, holding and cooling cycle (600°C/30°C h-1, 8h, and 450°C/ 30°C h-1) is repeated for three times before cooling to room temperature. This simulated heat treatment process is followed by mechanical evaluation of the weld metal. In general, the heat treatments that are subjected to weld assemblies are aimed at optimizing the properties as well as tempering and promoting stress relief.

2.5 Mechanical Test

2.5.1 Tensile Test

The tensile tests have been carried out at room temperature using an Amsler Universal Testing Machine having a load capacity of 20kN. The two numbers tensile specimens taken from indicated locations (T1 & T2) in Figure 1 of the prepared clad assemblies. Tensile properties of the test specimens are presented in Table 4.

2.5.2 Charpy Impact Test

Charpy impact test is carried out to evaluate the toughness of the welding joints at 20°C. Charpy tests are conducted on the machined specimens having a 2 mm notch positioned at the centre of the weld. Impact specimens are machined from indicated locations (IP 1-3) in Figure 1 of the prepared clad assemblies. Values for the test specimens are presented in Table 5. An average value of 98-110 J is obtained in this present investigation which is found to be well above the requirement specifications.

2.5.3 Bend Test (Side and Face)

Face and side bend test of the weld specimen has been carried out with Amsler Bend Tester. Side bend test specimens are machined from indicated locations (SB 1-4) in Figure 1 and face bend test specimens are machined from indicated locations (FB 1-2) in Figure 1 of the prepared clad assemblies. The thickness of the weld specimen is machined to about 1/4th of mandrel diameter. The test specimens are bent through an angle of 180° slowly to check for it soundness and nature of the defects introduced at the bent side.

2.6 Hot Crack Test

Hot crack test specimens are machined from indicated locations (HC 1-3) in Figure 1 of the prepared clad assemblies. To check resistance to hot cracking, depositing a sequence of cross welded stringer beads on the HC 1-3, after simulated heat treatment. No preheating applied. The beads sequences are displayed in Figure 2. The welding parameters used same as used in the cladding. Liquid penetrant test conducted after grinding. After further grind in steps each of 0.5 mm such that the underlying layer is reached. Conducted liquid penetrant test for every steps and found satisfactory.

2.7 Hot Cracking Sensitivity Test (Thomas Schaeffler Test)

Four numbers of test specimens with a dimension of 45 mm × 45 mm × 25 mm have been machined from the SS 347 base material. The schematic of the hot cracking test specimen is shown in Figure 3. This figure demonstrates that how the four pieces of test specimens (A, B, C and D) are arranged for the preparation of hot cracking test. The squarely arranged test specimens having 90 mm length and breadth are welded up to a length of only 50 mm in both directions. After joining, a single V groove is made on this test specimen, whose side view is shown in Figure 3. The side view of the grooved joint has a depth of 12.5 mm and angle 60°. After making this groove, the weld metal is deposited onto the groove in a clockwise direction by a continuous single pass. The specified discontinuous deposition procedure consists of depositing the weld from a particular point (X) marked on the test specimen to a certain distance (Y) and followed by cleaning and subsequent deposition of remaining portion of the groove from Y to X. The test assemblies are prepared as per the procedure is subjected to liquid penetrant test for crack inspection. The photograph of the grooved and weld deposited test specimen are displayed in Figure 3.

2.8 Metallographic Study

The different microstructures that form during welding govern the toughness and other mechanical properties of a material under investigation. Therefore, the knowledge of compositional effects and welding parameters on micro-structural evolution is important for achieving good weld properties. In Figure 4, the optical micrographs of the weld metal and the HAZ portion of the base metal are shown. The etchants used for revealing the respective microstructure are 10% Oxalic acid for Inconel and 4% Picric acid with 1% Nitric acid for base metal respectively. The dendrite morphology of the weld is found to be composed of fine features of columnar and equiaxed grains. In general, the bright and dark dendrite regions are recognized in the solidification microstructure of the Inconel alloys is due to the segregation of low melting phases such as Nb-rich Laves phases and topologically close packed phases such as sigma, P and μ phase. The investigations of the secondary phases in the present material are currently underway and hence a correct description of secondary phases is not dealt with this present paper. The HAZ regions of the base metal show finer as well as coarser features of ferrite + bainite. This may be due to the effect of maximum temperature reached and the cooling rate influenced by the HAZ region during multi pass welding. This observation suggests that the micro-structural features are not much influenced by the heat input utilized during welding. The typical optical micrograph of the base metal is also shown in Figure 4. The ferrite + bainite structure is clearly evident from this figure.

3 EXPERMENTAL DETAILS FOR LOW ALLOY STEEL CONSUMABLE

3.1 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-5. 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-7. The test specimens are machined from the weld joint and are subjected to various analyses such as chemical, metallography, mechanical and radiographic examination.

3.2 Chemical Composition

The chemical composition (wt. %) of the weld metal determined using wet chemical analysis is given in Table-9. In addition to this, the resulting composition obtained from the root of the weld after the welding process is also given in Table-9. This has been performed to know the extent of dilution. In Table-8, the composition of the forge plate used as a base material is also tabulated.

3.3 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.

3.4 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), 200oC and at 350o C respectively. Figure-6 shows the round specimens of diameter 12.5 mm and guage length 60 mm used for tensile testing prepared as per the ASTM standard E-21. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (El) and reduction in area. Results are given in Table-11.

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 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. Results are given in Table-12.

3.6 Bend Tester

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

3.7 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.

3.8 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-10. From the above weld coupon final size of the test specimen prepared as per ASTM E208. The details are given in Figure-7.

3.9 Metallographic Study

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-8. In both the cases, the microstructure consists of ferrite and bainite (Table-13). It is clear that the strength and toughness of this material emanates from the presence of bainite and ferrite fractions.

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. 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:

• A high performance low hydrogen E9018-G SMAW electrode meeting specifications is successfully developed.
• 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.
• The control of hydrogen content in the weld is advantageous to obtain good ductility of the weld. Major conclusions that are drawn from the present study for Inconel consumable is as follows
• Indigenously developed successfully to meet steam generator applications.
• Optimization of composition is based on choosing core wire and flux formulations.
• Control over heat input avoids modification of microstructure in the diluted region.
• The stability of weld microstructure at elevated temperatures is important for achieving adequate mechanical properties.
• The weld metal microstructure is free from hot cracking.