Solid state crack repair by friction stir processing in 304L stainless steel

doi:10.1016/j.jmst.2017.10.023

Abstract

Abstract:

Friction stir processing (FSP) was investigated as a method of repairing cracks in 12 mm thick 304L stainless steel plate. Healing feasibility was demonstrated by processing a tapered crack using a PCBN/W-Re tool with a 25 mm diameter shoulder and a pin length of 6.4 mm. The experiment showed that it was possible to heal a crack that begins narrow and then progressively grows up to a width of 2 mm. Bead on plate experiments were used to find the best parameters for creating a consolidated stir zone with the least amount of hardness difference compared to the base metal. Grain refinement in some specimens resulted in much higher stir zone hardness, compared to base metal. A plot of grain size versus microhardness showed a very strong inverse correlation between grain size and hardness, as expected from the Hall-Petch relationship. Corrosion testing was carried out in order to evaluate the effect of FSP on potential sensitization of the stir zone. After 1000 h of intermittent immersion in 3.5% saline solution at room temperature it was found that no corrosion products formed on the base material controls or on any of the friction stir processed specimens.

Key words: Stainless steel, Crack healing, Friction stir processing, Corrosion resistance

C. Gunter, M.P. Miles, F.C. Liu, T.W Nelson. Solid state crack repair by friction stir processing in 304L stainless steel[J]. J. Mater. Sci. Technol., 2018, 34(1): 140-147.

Figures/Tables 17

Table 1 Process parameters used for FSP experiments.

Parameter set	Tool speed (RPM)	Feed rate (mm/min)	Power (kW)
1	50	50	3.2
2	60	50	3.5
3	80	50	3.3
4	100	50	3.6
5	100	100	3.8
6	100	150	4.0
7	125	150	4.8
8	150	50	4.8
9	150	100	4.6
10	150	150	5.6
11	175	150	5.9
12	200	100	5.6
13	250	100	6.2

Table 2 Composition of 304L stainless steel (wt%).

C	Mn	P	S	Si	Cr	Ni	N	Fe
0.08	2.00	0.045	0.030	0.75	18-20	8-12	0.10	Bal.

Fig. 1. Friction stir processing tool (70% PCBN/30% W-Re), with a 6.4 mm long conical pin and 25 mm diameter convex shoulder.

Fig. 2. Schematic of tapered crack machined into 12 mm thick 304L stainless steel plate. The crack was 400 mm long with a linear taper that terminated with a 2 mm final width. All dimensions are mm.

Fig. 3. Cross section and U-bend specimens for corrosion testing were cut by waterjet from the friction stir processed 304L plate.

Fig. 4. Alternate immersion corrosion testing apparatus, programmed so that each basket was submerged in 3.5 wt% NaCl solution for 10 min out of each hour.

Fig. 5. Microstructure for as-received 304L stainless steel plate: (a) EBSD map, (b) misorientation distribution.

Fig. 6. Steady-state tool temperature as a function of spindle power during FSP experiments.

Fig. 7. Cross sections of bead-on-plate stir zones for different parameters. (a) 80 rpm-50 mm/min, (b) 125 rpm-150 mm/min, (c) 150 rpm-50 mm/min, (d) 150 rpm-100 mm/min, (e) 175 rpm-150 mm/min, (f) 250 rpm-100 mm/min (AS: advancing side; RS: retreating side).

Fig. 8. U-bend specimens after 1000 h corrosion test (a) 80 rpm - 50 mm/min, (b) 150 rpm - 50 mm/min, (c) 150 rpm-100 mm/min, (d) 250 rpm-100 mm/min.

Fig. 9. Microhardness maps for bead-on-plate cross sections. (a) 80 rpm-50 mm/min, (b) 150 rpm-50 mm/min, (c) 150 rpm-100 mm/min, (d) 250 rpm-100 mm/min.

Fig. 10. Microhardness in the stir zone as a function of grain size. Greater hardness is strongly correlated to smaller grain size, which follows the Hall-Petch relationship.

Fig. 11. Grain structure maps obtained at the AS of the stir zone. (a) 80 rpm - 50 mm/min, (b) 150 rpm-10 mm/min, (c) 150 rpm-50 mm/min, (d) 250 rpm-100 mm/min.

Fig. 12. Grain size distribution obtained at the AS of the stir zone. (a) 80 rpm-50 mm/min, (b) 250 rpm-100 mm/min.

Fig. 13. Misorientaton distribution obtained at the AS of the stir zone. (a) 80 rpm-50 mm/min, (b) 250 rpm-100 mm/min.

Fig. 14. Cross sections from tapered crack healing experiment. (a) base material just before the crack, (b) 45 mm from beginning of tapered crack (0.22 mm crack width), (c) 205 mm from beginning of crack (1.02 mm crack width), (d) 385 mm from beginning of crack (1.92 mm crack width).

Fig. 15. Probe-driven material flow around the tool during FSP. (a) Schematic of FSP performed on crack-free stainless steel, and (b) Schematic of FSP was performed along a crack.

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