Improving Fatigue Properties of 316L Stainless Steel Welded Joints by Surface Spinning Strengthening

doi:10.1007/s40195-024-01668-2

Abstract

Abstract:

The surface spinning strengthening (3S) mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests. Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening, as well as in the welding zone composed of deformed δ-phases and nanotwins. The fatigue strength of welded joint after 3S significantly rises by 32% (from 190 to 250 MPa), which is attributed to the effective elimination of surface geometric defects, discrete refinement of δ-Fe phases and the appropriate improvement in the surface strength, collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer. The redistributed fine δ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking, causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip, further suppressing the initiation and early propagation of fatigue cracks.

Key words: 316L stainless steel, Welded joint, Surface spinning strengthening, Nanotwin, Fatigue life

Dongqiqiong Wang, Qiang Wang, Xiaowu Li, Zhefeng Zhang. Improving Fatigue Properties of 316L Stainless Steel Welded Joints by Surface Spinning Strengthening[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(5): 840-854.

Figures/Tables 13

Table 1 Main welding parameters of the tungsten inert gas (TIG) welding

Welding mode	Single-sided multi-layer
Welding voltage	22-25 V
Welding current	100-110 A
Welding speed	100 mm/min
Shielding gas	100% Ar
Gas flux	10 L/min
Welding heat input	760 J/mm

Table 2 Chemical compositions of 316L stainless steel and ER316L stainless steel solder (wt.%)

Material	Cr	Ni	Mo	C	Si	Mn	S	P	Fe
Base metal	17.5	14	2.5	0.03	1.00	2.00	0.03	0.035	Bal
Filler metal	18.73	12.5	2.36	0.02	0.38	1.85	≤ 0.03	≤ 0.04	Bal

Fig. 1 a Schematic diagram of welded joint treated by 3S, b schematic diagrams of the shapes and dimensions of fatigue specimens

Fig. 2 Microstructures of 316L stainless steel welded joint. a-a'' Evolution of the surface gradient microstructure in the BM: a' outermost layer in the BM, a' original microstructure in the BM; d-d'' the evolution of the surface gradient microstructure in the WZ: d' outermost layer in the WZ, d'' original microstructure in the WZ; b, e IPF and c, f KAM mappings from typical cross-sectional EBSD corresponding a, d

Fig. 3 TEM micromorphologies of welded joint. a1-a4 3S BM, b1-b4 3S WZ, a1, b1 top-surface layer 50-100 µm, a2, b2 subsurface layer 100-200 µm, a3, b3 subsurface layer 200-300 µm, a4, b4 matrix layer

Fig. 4 Microhardness distribution of the 316L stainless steel welded joint. a Surface microhardness distribution with distance from treated surface of the WZ and BM, b the microhardness distribution across the welded joint at 250 µm below the surface before and after 3S

Table 3 Main parameters of the surface microhardness curves

Region	$H_{{\text{m}}}$ (HV)	$H_{{\text{M}}}$ (HV)	$\lambda$ (µm)	$R$ (10⁻³)
Weld zone	182	369	1080	2.364
Base metal	175	398	871	3.175

Fig. 5 Diagrammatic sketch of surface strengthening mechanism for 3S 316L stainless steel welded joint. a 3S BM, b 3S WZ. (AT: annealing twin, DT: deformation twin, CG: coarse grain)

Fig. 6 Tension-compression fatigue life distributions for the as-welded, smooth-welded and 3S-welded specimens of the 316L stainless steel welded joint. a S-N curves, b histogram of the fatigue strength comparison

Fig. 7 Fatigue fracture surface features of the as-welded specimens. a, a', a'' Specimen failed at the weld toe (WT), Δσ/2 = 220 MPa, Nf = 407,807; b, b', b'' specimen failed at the weld root (WR), Δσ/2 = 230 MPa, Nf = 230,399. a, b Full views of fracture surfaces; a', b' fatigue crack sources; a'', b'' later period of crack stable growth

Fig. 8 Fatigue fracture surface features of the smooth-welded specimens. a, a', a'' For #1, Δσ/2 = 250 MPa, Nf = 488,882; b, b', b'' for #2, Δσ/2 = 270 MPa, Nf = 69,700. a, b Full views of fracture surfaces; a', b' fatigue crack sources; a'' initial crack propagation stage of intergranular fracture for #1; b'' stable crack propagation stage of tearing columnar-grain for #2

Fig. 9 Fatigue fracture surface features of the 3S-welded specimens. a, a', a'' For #1, Δσ/2 = 260 MPa, Nf = 649,952; b, b', b'' for #2, Δσ/2 = 290 MPa, Nf = 60,486. a, b Full views of fracture surfaces; a', b' fatigue crack sources; a'', b'' surface morphologies near the fatigue sources corresponding a', b'

Fig. 10 Surface microstructure characteristics near the fatigue fractures. a Microcracks and long cracks of the smooth-welded specimen; c surface microstructure near the fatigue fracture of the 3S-welded specimen; typical fore-scatter detector (FSD) diagrams along with the phase mixing diagrams of b the smooth-welded specimen and d the 3S-welded specimen

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