Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures

doi:10.1016/j.jmst.2016.12.001

摘要/Abstract

Abstract:

The high cycle fatigue (HCF) tests of modified 9Cr-1Mo dissimilarly welded joint were carried out at different elevated temperatures and the fracture mechanism was systematically revealed. The fatigue strength at 10⁸ cycles based on S-N curve can be estimated as a half of weld joint’s yield strength for all conducted temperatures, which can be a reliable criterion in predicting the fatigue life. The results show that the inter-critical heat affected zones (IC-HAZs) of both sides are the weak zones due to their low hardness and inferior fatigue resistance property. HAZ of COST-FB2 (BM2) is the weakest zone at room temperature due to the existence of numerously distributed defects and the initiation of cracks, either in the surface or interior zone, impacting a crucial effect on the fatigue life of the joint. While at elevated temperatures, fatigue life was controlled mostly by the intrusion-extrusion mechanism at the specimen surface under high stress level and subsurface non-defect fatigue crack origin (SNDFCO) from the interior material under low stress amplitude. With increasing temperature, more and more fatigue failures began to occur at the HAZ of COST-E (BM1) due to its higher susceptibility of temperature. Besides, it is found that the δ-ferrite in the BM1 has no harm to the HCF behavior of the joint at the conducted temperatures.

Key words: High cycle fatigue, Dissimilarly welded joint, Life time, Fatigue failure, Fracture mode

. [J]. 材料科学与技术, 2017, 33(12): 1610-1620.

Shao Chendong, Lu Fenggui, Wang Xiongfei, Ding Yuming, Li Zhuguo. Microstructure characterization and HCF fracture mode transition for modified 9Cr-1Mo dissimilarly welded joint at different elevated temperatures[J]. J. Mater. Sci. Technol., 2017, 33(12): 1610-1620.

图/表 17

Table 1 Chemical composition of BM1, BM2 and filler wire (wt%).

Materials	C	Si	Mn	Cr	Mo	W	Ni	V	Nb	B	N	Co	Fe
BM1	0.12	0.1	0.45	10.4	1.06	0.81	0.74	0.2	0.08	-	0.06	-	Bal.
BM2	0.13	0.05	0.4	9.3	1.5	-	0.15	0.2	0.05	0.01	0.02	1.0	Bal.
US-9 (filler)	0.1	<0.50	<1.25	9.5	1.0	-	<1.0	0.2	0.05	-	-	-	Bal.

Table 2 Mechanical properties of the modified 9Cr-1Mo dissimilarly welded joint.

Temperature (°C)	0.2% proof strength, R_p0.2 (MPa)	Tensile strength, R_m (MPa)	Elongation, A (%)	Reduction of area, Z (%)
RT	640	772	16.7	66.8
500	502	566	16.1	73.6
538	461	508.5	14.8	76.5
566	433	480	17.4	83.3

Fig. 1. Schematic of the welded joint and fatigue specimen.

Fig. 2. S-N curves of the modified 9Cr-1Mo dissimilarly welded joint.

Table 3 Fatigue parameters for the modified 9Cr-1Mo dissimilarly welded joint.

Test temperature	σf' (MPa)	b	Fatigue strength (MPa) at 10⁸ cycles
RT	429.4	-0.01420	327
500	303.6	-0.01352	234
538	375.3	-0.02647	226
566	294.0	-0.01772	210

Fig. 3. Relationship between two kinds of fatigue strength ratios and the temperature.

Fig. 4. Different HCF failure locations regardless of temperature: (a) Failure location at the middle of WM (RT, σmax = 335 MPa, Nf = 4.13 × 107), (b) Failure location at fusion line of BM1 side (500 °C, σmax = 235 MPa, Nf = 1.24 × 107), (c) Failure location at the BM1-HAZ (538 °C, σmax = 250 MPa, Nf = 4.52 × 106), (d) Failure location at the BM2-HAZ (566 °C, σmax = 220 MPa, Nf = 3.39 × 106).

Fig. 5. Hardness distribution of the dissimilarly welded joint before and after test at different temperatures: (a) BM1 HAZ side, (b) BM2 HAZ side.

Fig. 6. Fracture location distribution of BM1-ICHAZ and BM2-ICHAZ at different temperatures.

Fig. 7. Microstructure of the whole dissimilarly welded joint, HAZs and WM: (a) the overall microstructure of the modified 9Cr-1Mo dissimilarly welded joint, (b) the overall microstructure of BM1-HAZ, (c) the overall microstructure of BM2-HAZ, (d) the overall microstructure at the middle of WM revealing the multi-layer and double pass feature, (e) tempered martensite microstructure in the WM. Zones B, C and D marked with yellow rectangles in (a) reveal locations of (b), (c) and (d) in the joint respectively.

Fig. 8. Fractography of the fatigue specimens tested at RT: (a) surface shrinkage porosity induced failure at BM1-HAZ (σmax = 340 MPa, Nf = 2.74 × 105), (b) interior shrinkage porosity induced failure at BM2-HAZ (σmax = 340 MPa, Nf = 8.85 × 107), (c) interior enriched elements induced failure at BM2-HAZ (σmax = 320 MPa, Nf = 1.17 × 107), (d) interior inclusion induced failure at middle of WM (σmax = 370 MPa, Nf = 1.63 × 107).

Fig. 9. BN inclusion observed in the BM2-HAZ: (a, b) BN inclusions observed in the BM2-HAZ and (c) (d) EDX mapping results of B and N of (b).

Fig. 10. Fractography of the fatigue specimens tested at high temperatures with surface crack initiation: (a) surface inclusion induced failure at BM1-FL (500 °C, σmax = 235 MPa, Nf = 1.24 × 107), (b) magnified image of crack ignition of (a), (c) surface crack initiation induced failure at BM1-HAZ (566 °C, σmax = 240 MPa, Nf = 7.19 × 104), (d) surface crack initiation induced failure at BM2-HAZ (566 °C, σmax = 210 MPa, Nf = 1.68 × 106).

Fig. 11. Fractography of the fatigue specimens tested at high temperature with interior crack initiation: (a) interior inhomogeneous microstructure induced failure at BM2-HAZ (538 °C, σmax = 250 MPa, Nf = 8.66 × 105), (b) interior inhomogeneous microstructure induced failure at BM2-HAZ (538 °C, σmax = 260 MPa, Nf = 1.77 × 106), (c) interior inhomogeneous microstructure induced failure at BM2-HAZ (538 °C, σmax = 235 MPa, Nf = 2.85 × 107), (d) interior inhomogeneous microstructure induced failure at BM2-HAZ (500 °C, σmax = 235 MPa, Nf = 7.06 × 107).

Fig. 12. Proposed mechanism of fatigue crack initiation: (a) fatigue crack initiation from surface defect without fish-eye area, (b) fatigue crack initiation from interior defect with fish-eye area, (c) fatigue crack initiation due to intrusion and extrusion in surface grains, (d) subsurface non-defect fatigue crack origin with fish-eye area.

Fig. 13. Fatigue crack initiation sites at different temperatures.

Fig. 14. δ-ferrite observed at different zones of BM1 side in a tested specimen without failure (566 °C, σmax = 200 MPa, N = 1.06 × 108): (a, b) δ-ferrite in the BM1 without cavity, (c, d) δ-ferrite in the BM1-FGZ without cavity, (e, f) δ-ferrite in the IC-HAZ with massive cavities at the phase boundary and inside the δ-ferrite phase.

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