Development of SMAW Electrode for Welding SS 316LN Material for Use in Fast Reactor Components
1. Introduction
Austenitic stainless steel of 316 LN SS is the leading potential candidate structural material for most of the high temperature applications (>427°C) in Prototype Fast Breeder Reactor (PFBR) at IGCAR. Fabrication of these components requires extensive use of welding. Worldwide and IGCAR’s experience on austenitic weldments is mainly concerned about the development of weld metal and heat affected zone (HAZ) against hot and liquation cracking.
It is understood that the resulting solidification transformation mode experienced during welding, segregation of the impure elements to grain boundary forming low melting eutectics play a foremost role in cracking tendency. In order to improve these aspects, the critical factors that have to be considered are:
(i) Optimization of welding consumable composition besides weldability considerations
(ii) Controlling of impurities which causes property degradation
(iii) Knowledge about the effect of heat input
(iv) Kinetic factors associated with solidification of weld
(v) Control of delta ferrite content in weld metal
(vi) sensitization effects.
IGCAR’s extensive long-term research on austenitic welds related to solidification cracking, effect of nitrogen on fusion zone and HAZ cracking, effect of ferrite content on embrittlement of weldments due to sigma-
phase formation, improvement of creep properties by adding N content, weldability studies by controlling Ti, Ta and Nb contents provided enormous inputs to improve the properties of austenitic weldments. Based on
this, they have planned to develop special purpose welding electrodes namely, modified E316-15 SS to achieve the desired properties of the weldments.
To meet these challenging requirements of PFBR components, E316-15 (Modified) welding consumable of various diameters have been developed successfully in-house and the properties of weld metal are evaluated jointly in collaboration with IGCAR for its applications. The details of the specification [PFBR/30000/SP/1032/R-1] of weld metal required by IGCAR are presented in Table 1.
2. Developmental Work
In order to develop the electrode material E316– 15 (Modified), the pre-planned activities aimed at developing austenitic stainless weld metal are given as follows;
(i) Optimization of chemical composition of the core wire as per AWS Section II, SFA5.9 to maintain the ferrite level to its 3-7 range for improving hot cracking resistance and avoiding the sigma embrittlement due to service exposures.
(ii) Formulation of ingredients present in the flux to improve the slag detachability and make the electrode user friendly. The formulation is
based on adjusting the ratio of the contents of all basic oxides to all acidic oxides known as basicity index (BI) to achieve the above mentioned properties. The adopted formula for BI in this present work is as follows,
BI = CaO+MgO+CaF2+Na2O+K2O+0.5(FeO+MaO) / SiO2 + 0.5 (TiO2+Al2O3+ZrO2) (1)The basicity index has been fixed as 1.35, which has demonstrated good slag detachability.
(iii) Effect of basicity index on bead appearance, mechanical properties, slagdetachability.
(iv) Optimization of welding parameters in accordance to electrode wire size and effect of voltage, current and speed of deposition on stability of arc, porosity, shape of bead, bead size, depth of penetration and weld distortion.
(v) Careful optimization of each variable to achieve the specified properties such as solidification mode, tensile properties, toughness requirements on as-welded and heat treated specimens, resistance to cracking test.
2.1.1 Chemical Analysis and All Weld-Joint Preparation
In order to develop the electrode of required composition, various weld pads have been made with different flux combinations. The electrode composition has been fixed on the basis of
control of carbon in the core wire
selecting a core wire having very minimal phosphorus and sulfur content
strict control of elemental content in both flux and core wire.
The composition test assemblies made as per AWS section II, SFA-5.4/SFA-5.4M is subjected to accurate evaluation of chemical composition by optical emission spectroscopy. The weld composition having the required/specified composition mentioned by IGCAR has been identified from the various weld pads. A careful observation of the composition reveals that the weld specimen prepared from different batches satisfied the required criteria of composition.
Although the composition of all the weld pads prepared seems to have identical values, the presence of Ti+Nb+Ta content in one batch is found to be 0.030 % and it is within the specified limit. The oxygen content determined is also found to be 525 ppm. The typical compositional (wt. %) details of the weld metal prepared with optimized electrode batch is listed in Table 2.
In addition to this, the composition details of core wire with the IGCAR specification for weld metal composition are also listed in Table 2. Followed by this identification, the all-weld single V joint is prepared with the electrode of the interest. The weld assembly consists of a 316 L plate of dimensions 300×150×16 mm with the groove angle of 45 degrees and a root gap distance of 6.5 mm. This joint is supported with a backing strip made of SS 316 having dimensions of 325×32×6 mm. The welding conditions used during making of the weld assemblies with the E316-15 (modified) electrode using MMAW
process are tabulated in Table 3.
The test specimens that are machined from the weld joint assembly made of optimized batch have been subjected to metallography, mechanical, corrosion and cracking test, fillet weld test, radiographic examinations. The procedure given in AWS SFA-5.4 section is followed for the analyses wherever it is necessary. In addition to this, some of properties of the weld are also tested in parallel at IGCAR and a comparison is made for its quality and reproducibility.
2.1.2 Determination of Ferrite Content
The amount of ferrite present in the weld samples have been determined using Ferrite scope and as per AWS SFA 5.4.
2.1.3 Metallographic Studies
The optical studies have been carried out using AXIOVERT 100A Optical microscope. 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 electrolytic etchant used for observing the microstructure is 10 % Oxalic acid aqueous solution.
2.1.4 Tensile Studies
The tensile property of the weld specimen is analyzed using AMSLER Universal Tensile Testing Machine with a load capacity of 100kN. The tensile measurements have been conducted at room temperature (RT) and at
550°C respectively as per the ASTM standard A370 and E21. Fig 1 show the round specimens of diameter 4.0 mm and guage length 20.0 mm used for hot tensile testing. The tensile data are analyzed to estimate the yield strength (YS), ultimate tensile strength (UTS), total elongation (et) and RA (%).
2.1.5 Charpy U-notch Impact Test
For charpy impact testing, the specimens used are cut across the welded joints having dimensions of 10×10×55 mm and type U- notched, with 2mm of depth. The charpy energy of the as-welded specimen as well as the thermally aged sample (750°C / 100 h) is obtained at room temperature. The charpy impact test is accomplished in compliance to ASTM E23 standard to determine the
toughness values.
2.1.6 Inter-granular Corrosion Test in As- welded state
Micro fissure and crack evaluation has been performed on the weld specimen having dimensions of 70×10×4 mm using Amsler bend tester. Before carrying out the bend test (90°angle around a mandrel 6 mm), the weld
specimens with copper turnings are immersed in boiling solution of 10 wt. % CuSO4.5H2O and 10 wt. % H2So4 for 24 h.
2.1.7 Cracking Susceptibility Test
The procedure recommended by IGCAR for evaluating the non-cracking tendency of weld specimen deposited in 1G position is performed to evaluate the cracking susceptibility. The schematic of such assembly is shown in Fig 2. ASTM SA516 Grade 70 equivalent steel plate of thickness 50 mm having a V groove of 80° bevel angle and root gap distance of 2 mm buttered with the electrode to protect from dilution is prepared. The exact procedure mentioned by IGCAR is adopted to achieve the length of bead, distance between the adjacent weld beads, better fusion by using 1/3rd end of an electrode etc. This is followed by the visual, liquid penetrant, microscopic examination of the weld assembly for crack evaluation.
2.1.8 Fillet Weld Test
As per the AWS: SFA-5.4 Clause-13, fillet weld test has been performed on the fillet joints prepared in vertical, horizontal and overhead positions. The visual and the convexity of the fillet weld are measured as per the AWS specifications.
2.1.9 Slag Detachability
Slag detachability test has been performed to know the ease of slag removal.
3. Results
3.1 Metallography examination of weld
Fig 3 displays the optical micrograph of the solidification microstructure of the weld metal and the aged weld specimen (750°C for 100 h). Fig 3a reveals the dendritic solidified grains believed to be enriched with austenite stabilized elements. The presence of very minimal ferritic inter-dendritic liquids seen as
black in color may have ferrite stabilizers. Fig 3b reveals the microstructure of 7500 C /100 h Aged weld specimen.
It is established that the metastable ferrite phase forms as a result of rapid solidification during welding process along dendritic boundaries having vermicular shape (black colour). In addition to this, the chromium carbides of type M23C6 also form during ageing above 450°C at grain boundaries and grain interior. The ferrite phase and the M23C6 carbides are clearly evident in Fig 3b. The ferrite content in weld metal determined by Ferritoscope was found about 6.1 and by WRC diagram it is about 4.0 respectively.
3.1.1 Tensile Studies on All Weld Metal
In Fig 4, the extension of the weld specimen recorded at 550°C with increasing load is shown. The yield strength, ultimate tensile strength, percentage elongation and reduction in area are determined from the tensile graph. The room temperature and elevated temperature tensile properties of the all-weld are presented in Table 4. It is from the Table 4 that the values of the tensile strengths, elongation and reduction in area are well above the limits specified by IGCAR for PFBR applications.
3.1.2 Charpy ‘U’ Notch Impact Toughness
Charpy U-notch impact test has been conducted both in the as-welded and aged conditions at 7500 C / 100 h. The charpy energy found both in the as-welded and aged conditions are 8.9 daJ cm-2 and 4.5 daj cm-2 respectively.
3.1.3 Inter-granular Corrosion Test in As-Welded state
The fissure and cracking analysis test conducted on the weld metal as per IGCAR specifications for evaluating the susceptibility against intergranular attack ensures that no crack has been developed in the 90° bentregion of the weld.
3.1.4 Cracking Susceptibility Test
The cracking susceptibility test carried out as per IGCAR specifications on the crack test assembly made in 1G position confirmed that no crack is observed and hence it meets the IGCAR requirement.
3.1.5 Fillet Test
The result of the fillet test conducted on the fillet weld joints is listed in Table 5. It is clear from the table that the fillet weld test has been passed and it meets the requirement of IGCAR.
4. Discussion
During welding of austenitic stainless steels, one may encounter four possible solidification modes in the weld fusion zone depending on the composition. With regard to 316 LN austenitic welds, the welding metallurgy essentially concerns about how to control the solidification structure to avoid cracking in the weld metal. The knowledge on the phase diagram specifies that, if the Creq/Nieq ratio of electrode is ~1.5, then the solidification mode is situated along the boundary between the ferrite and austenite region. As a result, the solidification mode may be either austenitic or ferrite-austenite type.
Under non-equilibrium conditions, rapid cooling causes the boundary to shift which in turn affects the solidification mode. Hence the prevailing weld zone composition as well as the welding method strongly decides the solidification structure. Based on the cracking susceptibility studies, it is observed that the weld solidified primarily by austenitic mode is susceptible to cracking due to segregation of impure elements (S+P), formation of low melting eutectics by reaction of Ti and Nb with C, N and S, sensitization by chromium depletion at high temperatures.
In other sense, the weld having austenitic-ferritic structure is better than fully austenitic structure because of higher solubility of impurities in ferrite phase, protection to chromium depletion, effect on ferrite on sensitization kinetics. However long term research on austenitic weldments signifies that, this type of weld is also failed in service due to
sigma phase formation during prolonged high temperature exposure, transformation of ferrite to carbides at high temperatures
Formation of nitrides leading to pitting corrosion. It is found that the sensitization of the heat affected zone (HAZ) is not a problem, if the carbon content of the base metal is ≤ 0.03 wt.-%
Based on the extensive long term research carried out by IGCAR on developing the performance of the austenitic weld and weldments against hot cracking and their prevention, enhancement of creep strength, fatigue and creep-fatigue life have drawn major conclusions. As a result, E316-15 (Modified) electrode for welding PFBR components has been specified by IGCAR. This includes the control over the impurity levels of the weld, control of carbon and nitrogen level in the weld for ensuring improved creep strength and freedom from sensitization.
The permissible amount of ferrite in the weld is specified between 3 to 7 FN. This is followed by specification of hot tensile, toughness properties, bend test etc. The E316-15 (Modified) electrode specified by IGCAR has been developed successfully in- house as a result of sheer hard work with proper planning and execution.
The optimization of flux combination for the welding consumables, welding parameters to achieve high quality weld and the slag detachability characteristics to make welder friendly yielded fruitful results.
The weld assembly made with the optimized electrode composition has been subjected to radiographic examination. It ensures that the weld assembly is free from crack and porosity. The metallurgical testing suggests that it is highly recommended for PFBR applications in IGCAR. The evaluation test performed by IGCAR on the determination of chemical composition of the weld, delta ferrite content in weld metal, mechanical properties, IGC test, fillet test and cracking test provides satisfactory results and as a result, our developed product has been approved by IGCAR for welding PFBR components. However, the creep rupture test of the weld specimen at 650°C has been planned in near future to evaluate the creep properties. As per IGCAR requirement, the weld pad made in 1G position has to be evaluated for creep rupture analysis under a stress of 130 MPa.
5. Conclusion
(i) Balancing of the composition of the weld to achieve ~5% of delta ferrite makes our product suitable for welding 316 LN austenitic steels.
(ii) By using this welding consumable, it is possible to obtain sound weld joints with high temperature strengths comparable to base metal.
(iii) It is essential to control the heat input to obtain a suitable austenite-ferrite balance in the weld which avoids detrimental phase formation.
(iv) From corrosion point of view, it is beneficial to do post weld cleaning of the weld mechanically.
