A Study of Microstructure and Mechanical Properties for the Autogenous Single-Pass Butt Weldment of a Ferritic/Martensitic Steel Using Gas Tungsten Arc Welding

doi:10.1007/s40195-020-01146-5

Abstract

Abstract:

A 12% Cr ferritic/martensitic steel, HT-9, has been used as a primary core material for nuclear reactors. The microstructure and mechanical properties of gas tungsten arc butt welded joints of HT-9 in as-welded, and as-tempered conditions have been explored. In as-welded condition, the fusion zone (FZ) contained a fresh martensite matrix with delta (δ)-ferrite. The δ-ferrite was rich in Cr and depleted in C compared with the matrix. The heat-affected zone (HAZ) could be divided into three areas as the distance from the fusion line increased: δ-ferrite/martensite duplex zone, fully recrystallized zone, and partly recrystallized zone. Prior austenitic grains did not coarsen in the δ-ferrite/martensite duplex zone due to the newly nucleated δ-ferrite grains and incompletely ferritizing (δ-ferrite) during the welding thermal cycle. The weldment microhardness distributed heterogeneously with values above 600 HV_1.0 in the HAZ and FZ and 250 HV_1.0 in the base metal (BM). Solute C in the matrix, induced by the dissolution of carbide during the welding process, dominated the microhardness variation. Low toughness was observed in the FZ with a quasi-cleavage fracture tested from - 80 to 20 °C. The tensile fracture occurred in the relatively soft BM tested from 20 to 600 °C. In as-tempered condition (760 °C for 1 h), M₂₃C₆-type carbides precipitated within the martensitic laths, the lath boundaries, and the δ-ferrite/martensite interfaces. Moreover, V, Cr, Mo-rich nitrides with very small size also precipitated in the δ-ferrite/martensite interface. The tempering treatment improved the homogenous distribution of weldment hardness significantly. Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600 °C. The impact toughness improved significantly, but the ductile-brittle transaction temperature was - 12 °C which was higher than that of the normalized and tempered (N&T) BM. δ-ferrite was considered to be one of the major factors aggravating the impact toughness in the FZ.

Key words: Martensitic stainless steel, Gas tungsten arc welding (GTAW), Post-weld heat treatment, Microstructure, Mechanical properties

Dong Wu, Shitong Wei, Shanping Lu. A Study of Microstructure and Mechanical Properties for the Autogenous Single-Pass Butt Weldment of a Ferritic/Martensitic Steel Using Gas Tungsten Arc Welding[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(5): 628-638.

Figures/Tables 12

Table 1 Chemical composition of HT-9 steel (wt%)

C	Si	Mn	Cr	Ni	Mo	W	V	N	Fe
0.20	0.24	0.51	11.96	0.45	0.99	0.50	0.30	0.053	Bal.

Table 2 Main welding parameters

Welding current (A)	Arc voltage (V)	Welding speed (mm/s)	Electrode diameter (mm)	Shielding gas	Flowing rate of shielding gas (L/min)
132	13-13.5	1.7	2.4	99.999% pure Argon	15

Fig. 1 Schematics of the weldment and location of mechanical-test specimens. The RT and HT denoting room temperature and high temperature, respectively

Fig. 2 a Calculated phase fractions versus temperature diagrams and b the magnification of a

Fig. 3 a Cross section of HT-9 weldment and the corresponding OM image of b FZ, c δ-ferrite/martensite duplex zone, d fully recrystallized zone, e partly recrystallized zone and (f) BM

Fig. 4 a SEM image of the FZ in the HT-9 weldment and the corresponding EPMA mapping of b C and c Cr elements

Fig. 5 SEM images of the a FZ, c δ-ferrite/martensite duplex zone, e fully recrystallized zone, g partly recrystallized zone and i BM in as-welded condition, comparing with the b FZ, d δ-ferrite/martensite duplex zone, f fully recrystallized zone, h partly recrystallized zone and j BM in as-tempered condition

Fig. 6 TEM bright-field images of a martensite laths and b δ-ferrite in the FZ of the HT-9 weldment and c the ChemiSTEM maps showing the element distribution within the martensite/δ-ferrite interface in as-tempered condition. The α’ and δ denoting martensite and δ-ferrite, respectively

Fig. 7 Microhardness profiles of the HT-9 weldments in a as-welded and b as-tempered condition. The areas labeled as No. 1-No. 5 indicating the FZ, δ-ferrite/martensite duplex zone, fully recrystallized zone, partly recrystallized zone, and BM, respectively

Fig. 8 Evolution of tensile strength with temperature for the HT-9 weldments in as-welded (AW) and as-tempered (AT) conditions. The inset photographs showing the typical locations of the fracture in the tensile specimens

Fig. 9 Comparison of Charpy curves for one-fourth size specimens of FZ in the HT9 weldment in as-welded and as-tempered conditions

Fig. 10 SEM fractographs of the impact specimens tested at a 20 °C and b - 80 °C in as-welded condition, c 20 °C and d - 80 °C in as-tempered condition

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