The constitution of the welded joint in the GX4CrNil3-4 stainless steel Boštjan Markoli in Savo Spaic Oddelek za materiale in metalurgijo, Naravoslovnotehniška fakulteta, Ljubljana Received: December I, 2003 Accepted: December 10, 2003 Abstract: The constitution of the base material (GX4CrNil3-4) and the filler material (CN 13-4IG) was investigated in initial and heat-treated state using modern investigative techniques. The base material and the weld have the same constitution with martensite aw as matrix, and a^- and a^-ferrite along with precipitates of (Cr, Fe)pQC6 carbide. The hardness of the weld is lowered and more equal through the weldment after the stage III heat treatment. The weldment is free of typical welding defects such as porosity, macro-and microcracks, inclusions etc. Key words: constitution, microstructure, welding, stainless steel Introduction Welding is a widely used joining technique also suitable for, e.g., to correct typical casting defects occurring in manufacturing turbines for water power plants. In order to produce durable defect free joints in GX4CrNil3-4 stainless steel turbines, which are also corrosion resistant the appropriate welding process as well as convenient filler material has to be chosen. Welding under C02 with DC and polarity (+), and CN 13-4 IG as filler material seem to meet these requirements [1_3]. The usage of C02 prevents the gas intake thus porosity and consequently crack formation in and around the weld [4,51. The chemical composition of the filler material insures the constitution and microstructure, which is very similar to the constitution and microstructure of the base material [6], the addition of ruthenium and rhodium [7_18] in filler material on the other side improves the corrosion resistance of the weldment. Since the welding process is accompanied with fast crystallisation of the molten metal in the bead there is unavoidable presence of microsegregation of base and alloying elements as well as strain. This means that heat treatment of the weldment is needed to eliminate the initial morphology, microsegrega-tion in the weld and anneal the matrix thus lowering the strain in the weld and surrounding base material. The heat treatment procedure has to be followed precisely to achieve the intended constitution and microstructure of the weldment. In this paper the constitution and development of microstructure of the base material and the weld in as-welded and heat-treated state was investigated using metallograpic methods, such as: optical microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray phase analysis and hardness measurements (HV 0.5). Experimental The GX4CrNil3-4 stainless steel was analysed in as cast and heat-treated state. The filler material (consumable) CN 13-4 IG was also analysed in as delivered commercial state and both as welded and heat-treated state. The weldment was cut into suitable sized specimens as shown in the Figure 1 [19], which were analysed using optical microscopy, REM with energy-dispersive spectroscopy, X-ray phase analysis and hardness measurements (HV 0.5). The chemical composition (Table 1) of base material GX4CrNi13-4 and filler material CN 13-4 IG was also checked with energy-dispersive spectroscopy. 103 mm 14 mm 27 mm 31 mm 17 mm 20 mm 26 mm 107 mm base material Figure I. Schematic presentation of specimen assembling for investigation: specimens denoted with numbers t-S, specimen No. 2 was also used for X-ray analysis. Table I. Chemical composition of the base material (GXCrNil3-4) and filler material (CN 13-4 IG) in mass % Element Base material Filler material C 0.02 0.035 Si 0.43 0.30 Mn 0.68 - P 0.033 - S 0.010 - Cr 12.9 12.20 Mo 0.56 0.50 Ni 3.72 4.50 Ru - 0.754 Rh - 0.265 The base material GXCrNil3-4 was heat treated prior to welding as follows: Stage I: heating (6S °C/h) to 10S0 °C and holding for 6h, then cooling in the air. Stage II: annealing (40 °C/h) to 610 °C and holding for 6\ then cooling in the furnace (40 °C/h) to 2S0 °C, then cooling in the air. Stage III: annealing (40 °C/h) to S90 °C and holding for 6\ then cooling in the furnace (40 °C /h) to 2S0 °C, then cooling in the air. The weldment itself was also heat treated in the same way as the base material in the stage III. Results and discussion The optical micrograph (Figure 2) of the base material in the heat-treated state show the presence of martensite matrix aw with typical lath-like morphology which is in accordance with reference [2]. Within the microstructure of the base material there is also evidence of light phase in form of small islets, which are clearly seen in the Figure 3. The EDS analyser mounted on the REM revealed the composition of the light phase (Table 2), which excludes the presence of the c-CrFe phase. This has further been confirmed by the X-ray phase analysis of the base material in the heat-treated state (Figure 4). The diffraction pattern in the Figure 4 shows presence of the multicomponent martensite matrix aM, mul-ticomponent ôFe-ferrite, (Cr, Fe)23CT type carbide, and multicomponent a^-ferrite. Closer inspection of the diffraction pattern namely reveals the split of peaks for the major {110}, {200}, {211} and {220} planes of martensite aM and that intensity for the ôFe-ferrite are very low. According to the chemical composition of base material in the Schâffler diagram p] the microstructure should consist Table 2. EDS analysis of the light phase (islets) Element Si Cr Mn Fe Ni Mo Mass % 1.166 19.220 0.247 77.506 1.106 0.756 Figure 2. Microstructure of the base material (GX4CrNil3-4) in the heat-treated state Figure 3. Microstructure of the base material (GX4CrNil3-4) in the heat-treated state: presence of the white islets of multicomponent d^-ferrite phase of martensite aM and approximately of 15 % ferrite. There is no clear distinction or explanation in the reference [1] whatsoever, if this ferrite is either 5Fe- or a^-ferrite. Thus it can be concluded based on the assessment of the volume fraction of the microstructural constituents by optical microscopy (Figure 3), X-ray phase analysis and comparison of reference data [1] and the constitution of the Fe-rich corner of the binary Fe-Cr system [6] that the multicomponent a^-ferrite is present in the microstructure of the base material. Since the chemical composition of the base material and filler material are very similar (Table I) the same constitution can be expected in the weld itself. There is on the other hand compositional difference with regard o O in o O 8 S3 280 S 260 "E « K 240 220 200 base HAZ weld HAZ base material C mater — o O o 3 o - o ° °o O °o° o — o o° o ° o D — o O O o o — o - 3 o °o o 00 - 1 1 1 1 1 1 1 10 20 30 40 50 60 70 73 Figure S. Macrostructure and hardness (HV O.S) across the weldment in as-welded state Macroscopic investigations of the weldment in the heat-treated state The Figure 6. reveals the development of the macrostructure in the weldment after the stage III heat treatment. The HAZ is more distinct from the base material with pronounced darker zone and thinner lighter zone next to the fusion line. Macrostructure of each bead (totally 26 beads made) with typical columnar morphology can also be recognized. Figure 6. Macrostructure and hardness (HV O.S) across the weldment in heat-treated state Hardness measurements across the weldment show that the overall hardness is lower and more equalized than in the as-welded state. Still the hardness of the weld itself is higher with -280 HV from the hardness of the base material with -260 HV. The hardness is also highest in the light zone next to the fusion line. This is on the other hand the area, which is most likely to initiate or to form microcracks [4] when material is exposed to cyclic mechanical strain. Comparison of macrostructure in Figures 5 and 6 shows that the weld and the HAZ as well as the base material in as-welded and heat-treated state are actually completely free of typical welding defects apart from one localized inclusion in the weld after heat treatment (Figure 6). Microscopic investigations of the weldment In the micrograph of the weld prior to the stage III heat treatment (Figure 7) made by optical microscopy the columnar-dendritic morphology is evident which is typical for higher crystallisation rates that can be expected during the process of welding. This is also associated with occurrence of micro-segregation of base and alloying elements and enrichment of the base material next to the fusion line mostly with the nickel as confirmed by the EDS analysis of the area adjacent to the fusion line (Figure 8 and Table Figure 7. Microstructure of the weld (filler material) prior to heat treatment Figure 8. Optical microscopy of the fusion line in the weldment in as-welded state: A-weld; B-base material in conjunction with Table 3 Table 3. EDS analysis of the area adjacent to the fusion line in conjunction with Figure 8 Filler material (weld) A Base material B Element Mass % Element Mass % A1 0.039 A1 0.004 Si 0.558 Si 0.254 Cr 11.015 Cr 11.772 Mn 0.353 Mn 0.368 Fe 83.225 Fe 83.750 Ni 4.626 Ni 3.702 Ru 0.152 Ru 0.039 Rh 0.033 Rh 0.112 3). The EDS analysis in addition revealed the diffusion of ruthenium and rhodium from the filler material in the weld into the base material (Table 3), with both elements added to increase the corrosion resistance of the weldment as described in reference [7_18]. The initial morphology of the weld is eliminated during the heat treatment (Figure 9) where some remains of this morphology are mostly restricted to the root and surface of the weld. The microstructure of the weld has finer martensitic laths than in the base material and the fusion line seen in the Figure 8 becomes wider and actually disappears as such during the heat treatment. The constitution of the weld and base material are actually the same as established with the X-ray analysis of the weld in heat-treated state (Figure 10) and comparison of both diffraction patterns (Figure 4 and Figure 10). The matrix of the weld is lath-like martensite aM (Figure 9) next to ôFe-ferrite which appears in the microstructure as white spots or islets, Figure 9. Microstructure of the weld in heat-treated state o « S ÇN. 5 S 0 1 5 o o tN, O LL o 3 n ._. ^ o> o o o 1