Microstructure and corrosion behavior of the heat affected zone of a stainless steel 308L-316L weld joint

doi:10.1016/j.jmst.2017.12.016

Abstract

Abstract:

Microstructure of the heat affected zone (HAZ) of a 308L-316L stainless steel (SS) weld joint and its corrosion behavior in high temperature water were studied. Peak of the residual strain was observed to approach to the fusion boundary in the HAZ while the strain increased from the top to root areas of the HAZ. The root area of the HAZ shows a lower corrosion resistance in high temperature water than the top and middle areas of the HAZ. This is attributed to a higher level of residual strain in association with a higher density of tangled dislocations in the top area of the HAZ. The results suggest that the residual strain in the HAZ could also promote the SCC through its effect on corrosion, in addition to that on the local microstructure and mechanical property of the steel.

Key words: Stainless steel, HAZ, Weld joint, Corrosion, High temperature water

Cheng Ma, Qunjia Peng, Jinna Mei, En-Hou Han, Wei Ke. Microstructure and corrosion behavior of the heat affected zone of a stainless steel 308L-316L weld joint[J]. J. Mater. Sci. Technol., 2018, 34(10): 1823-1834.

Figures/Tables 19

Table 1 Chemical composition (wt%) of the base and weld metals of the weld joint.

	C	Si	Mn	Mo	Ni	Cr	Fe
316L	0.025	0.52	1.71	2.4	11.7	17.9	Bal.
308L	<0.3	0.35	0.9	-	11	20	Bal.

Fig 1. Schematic drawing showing the geometry of the weld joint. The red squares denoting the locations of the three specimens for corrosion and EBSD analysis. The purple lines denoting two plate-type specimens for the TEM analysis extracted in the HAZ with an orientation parallel to the fusion boundary.

Fig. 2. Metallographic observation of the fusion zone of the weld joint.

Fig. 3. KAM maps of the top (a), middle (b) and root (c) areas of the FB region of the weld joint.

Fig. 4. Average KAM distribution in the top, middle and root areas of the HAZ of 316L SS analyzed on specimens A, B and C.

Fig. 5. Grain boundary character distribution in the top, middle and root areas of the HAZ of 316L SS analyzed on specimens A, B and C.

Fig. 6. TEM observation of top (a) and root (b) areas of the HAZ in 316L SS.

Fig. 7. Microhardness variation in the top and the root areas of the fusion boundary region of the 316L-308L weld joint.

Fig. 8. SEM observation of the morphology of the oxide scales formed on specimens A, B and C during the exposures in high temperature water.

Fig. 9. GIXRD patterns of the oxide scale formed on the HAZ of 316L SS following the 500-h exposure in high temperature water.

Table 2 Indexed spinel oxide diffraction peaks analyzed on the oxide scale formed on the 316L stainless steel specimens.

Reflection plane (hkl)	Fe₃O₄, 2θ (deg.)	NiFe₂O₄, 2θ (deg.)
2 2 0	30.092	30.102
3 1 1	35.451	35.460
2 2 2	37.072	37.101
4 0 0	43.086	43.095

Fig. 10. XPS depth profiles of the oxide scales formed on the HAZ of the three specimens following the exposure in high temperature water. The dash line refers to the interface between the oxide and the substrate. The black left axis is atomic concentration of [Cr, Fe or Ni/(Cr + Fe + Ni)]. The red right axis is atomic concentration of [O/(O+ Cr + Fe + Ni)].

Fig. 11. Thickness of the oxide film on specimens A, B and C as a function of exposure time.

Fig. 12. XPS spectra of Cr 2p3/2 for the oxide scales formed on specimens A and C after the 25 h exposure in high temperature water.

Fig. 13. XPS spectra of Fe 2p3/2 for the oxide scales formed on specimens A and C after the 25 h exposure in high temperature water.

Fig. 14. XPS spectra of Ni 2p3/2 for the oxide scales formed on specimens A and C after the 25 h exposure in high temperature water.

Fig. 15. TEM observation of the cross-section of the oxide scale formed on the HAZ region of 316L SS following the 500 h exposure in high temperature water. (a) Pictures showing the area analyzed; (b) EDX mappings for Fe, O, Cr and Ni, respectively; (c, d) EDX compositional profiles along the brown lines presented in (a), The left axis is atomic concentration of [O/(O+ Cr + Fe + Ni)], the right axis is atomic concentration of [Cr, Fe or Ni/(Cr + Fe + Ni)].

Table 3 Results of the EDX point analysis of the oxide scale shown in Fig. 15 and the calculated stoichiometry of the spinel oxide.

Point	O (at.%)	Cr (at.%)	Fe (at.%)	Ni (at.%)	Spinel stoichiometry
point I (outer)	57.14	0.61	40.32	1.92	(Ni_0.14Fe_0.86)Fe₂O₄
point II (outer)	60.39	0.82	30.13	8.66	(Ni_0.67Fe_0.33)Fe₂O₄
point III (inner)	63.69	19.85	12.69	3.77	(Ni_0.48Fe_0.52)(Fe_0.46Cr0.₅₃)₂O₄

Fig. 16. Schematic showing the dislocation-accelerated diffusion behavior (a) and the influence of diffusion on the formation of the oxide scale in the top and root areas at the early stage (b) and latter stage (c) of the oxidation.

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