Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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47 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 environment and observes the changes of SCC behavior between the base metal and its welded joints. Those results were stud- ied and compared with base metal and welded joints exposed to a 3.5% NaCl solu- tion , w hich simulat es a marin e atmo- spheric environment. T h e m a t e r i a l s u s e d i n t h i s stu dy included a 3-mm thick rolled sheet of 09CuPCrNi commercial weathering steel as base metal, and J502NiCrCu welding wire with 2.5-mm diameter as filler metal. Their chemical compositions are given in Table 1. The welded joints were made by shielded metal arc welding. The samples were polished using SiC p a p e r s t e p - b y - s t e p u n t i l 2 0 0 0 # w a s reached. They were then buffed and etched with 4% natal. The… microstructure… of the b a s e m e t a l a n d w e l d e d j o i n t s w e re observed by scanning electron microscopy. T h e h a rd n e s s d i s t r i b u t i o n f r o m t h e weld…seam and base metal were measured by a MH-5 † HV Vickers hardness tester. The distance between test points was 0.5 mm. SCC behaviors were evaluated by slow strain rate testing (SSRT) using the Chinese national standard of sample processing (GB/T15970.7-2000 12 ). The rolling direction of the material was the length direction of the sample and the welded joints were in the middle of the sample. The gauge…lengths of tensile… specimens were polished using SiC paper from 260# to 1000# step-by-step, cleaned with deionized water and alcohol, and dried by air blower. The tensile strain rate was 1 × 10 –6 s –1 , and the corrosion mediums were respectively 3.5% NaCl solu- tion and 0.01 mol/L (0.052%) NaHSO 3 solu- tion at room temperature. The loss of elongation could be…used to characterize the SCC sensitivity to quantify † Trade name. TABLE 1. CHEMICAL COMPOSITION OF BASE METAL AND FILLER METAL (WT%) Materials C P S Si Mn Ni Cr Cu Mo Base metal 0.0897 0.0343 0.0037 0.358 0.314 0.299 0.383 0.321 <0.0001 Filler metal 0.029 0.0154 0.0189 0.177 0.409 0.389 1.04 0.345 0.0147 FIGURE 1 Morphologies of base metal (a) and weld zone (b). the correlation between the SCC sensitivity of 09CuPCrNi welded joints and the envi- ronment medium (Equation [1]): I – a c a (1) where I b is the SCC sensitive index, and b a and b c are th e elongation in th e in ert medium and corrosive… medium, respec- tively. The higher the I b , the more…SCC…sen- sitivity the material has. Results Morpholog y and Hardness Distribution Before and After Welding Because the material is so thin, only the… morphologies of the weld… zone of the welded joints were observed, which are shown in Figure 1. The bright phase is ferrite and the dark is pearlite. It is clearly seen that the base metal consists of evenly distributed polygonal ferrite and pearlite. After welding, the welded joint mainly consists of acicular ferrite, columnar ferrite, and an amount of coarse grain pearlite, which shows relatively poor microstructural homogeneity. It can be seen from Table 1 that there is obviously a higher content of Cr in the filler metal than that in the base metal. It should be noted that Cr has the same…crystal struc- ture and similar atomic radius as _-Fe, so that it can form complete solid solution with _-Fe. When heated during the welding, a Cr atom can replace the Fe atom in the…crystal lattice, which disrupts the lattice arrange- ment and creates… distortion. Furthermore, the plastic deformation with the change to acicular ferrite will create…dislocation, which will change the mechanical properties of the material. The tests of hardness show that the result of the weld zone is 172 to 179 HV, which is higher than the base metal's 153 to 158 HV (not including the heat-af fected zone [HAZ]). It shows that hardening of the structure caused by welding will have nega- tive effects on SCC behaviors of the welded joints. SCC Sensitivity Figure 2 shows the stress-strain curves of 09CuPNiCr base metal and welded joints in 3.5% NaCl and 0.052% NaHSO 3 solutions. There are obvious differences in the curves b e tw e e n t h e tw o s o l u t i o n s a n d t h o s e exposed to air, whether the base metal or the welded joint, and especially the welded joint. Table 2 shows the mechanical…parameters and SCC…susceptive…index I b of SSRT tests.

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