Development of MMAW Inconel Consumable for Nuclear Steam Generator Applications

1.0 Introduction

As per the published literature, India aims to produce 20 000 MW nuclear power by 2020.

Several structural candidate materials are used for fabricating nuclear materials. Some of the materials which are specified are, austenitic base Inconel alloys of grade 600, 690, 625, 625, 718, 800 etc and stainless steels of 316LN, 304LN etc. Besides to this, ferritic base 9-12 wt. % Cr alloys having optimized additions of Mo, W, V, Nb, N etc have also been used. For most of the materials, welding consumables are available indigenously. 

But in case of Inconel category, we are still importing consumables from overseas to meet certain stringent requirements on specific forged materials for overlay applications. Therefore development of suitable MMAW consumables for overlay applications has become very much important for self reliance. In view of this, MMAW electrodes of ≈ ENiCrFe-3 have been developed indigenously for weld overlaying and joining applications. In this paper, the welding of 20MnMoNi55 steel with ≈ ENiCrFe-3 electrode by MMAW 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 characteristics and the essential properties of this weld metal for its suitability in overlay applications related to heat exchangers. The design requirement of this weld metal is tabulated in Table 1. 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.

2.0 Experimental Details

2.1 Cladding Assemblies Preparation

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 Ni – based 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 bottom of the weld pad. 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 assemblies. 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 clad assemblies 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 40 h. This is followed by cooling the weld assemblies to 450°C at 30° C h-1.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 Metallography

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.

3.0 Discussions

In order to design a Nickel based welding consumable, it is important to gather systematic

information on the metallurgical evolution of weld with respect to composition. As per the requirements, optimized welding consumable variables such as composition, flux and

electrode size have been established as a result of enormous laboratory tests. The process parameters such as voltage, current, speed are optimized to achieve good quality weld and all-weld joint. The micro-structural changes occurred across the weld assemblies as a result of welding process is not found to differ much and this subsequently improves the all-weld properties. In addition to this, the optimization of heat input is an important factor in determining the solidification microstructure of nickel base welds. To avoid coarser and columnar dendritic grains besides segregation in the solidification microstructure, the welding parameters must be carefully controlled, since the morphology of dendrites has a major influence on tensile and hardness properties. The Charpy toughness of the weld metal is found to be well above the required value and this confirms that the steel-diluted Ni-Cr-Fe weld metal is not significantly affected. The weld metal microstructure stability is also found to be good after prolonged exposure at elevated temperature and thermal cycling treatment. From the above study, it is concluded that the weld and all weld metal properties meet the requirement for heat exchangers applications.

4.0 Conclusions

Major conclusions that are drawn from the present study is as follows,

1. Indigenous development of Inconel base welding consumable is developed successfully to meet steam generator applications.

2. Optimization of composition is based on choosing core wire and flux formulations.

3. Control over heat input avoids modification of microstructure in the diluted region.

4. The stability of weld microstructure at elevated temperatures is important for achieving adequate mechanical properties.

5. The weld metal microstructure is free from hot cracking.