Precipitate Behavior in Fe-20Cr-30Ni-2Nb Austenitic Heat-Resistant Steel

doi:10.1007/s40195-015-0212-1

Abstract

Abstract:

In this study, the precipitation behavior of a new austenitic heat-resistant steel (Fe-20Cr-30Ni-2Nb, in at%) was investigated. The effects of alloying addition of boron (B) and lanthanum (La) on the microstructure of the austenitic steel were scrutinized using SEM, EPMA, TEM, and XRD. The results showed that the addition of B enhanced the precipitation of bar-type Laves phase. A small precipitate with high La concentration was observed at the grain boundary in the alloy without aging; similar precipitates without La also presented in region adjacent to the La single phase. This result indicates that La can exist independently and does not contribute to the formation of new compounds. However, in both B- and La-modified alloy, B appeared in the precipitate free zone. In the alloy containing both B and La, only Fe₂Nb Laves-phase precipitates, as indicated by the XRD result.

Key words: Precipitation free zone, Grain boundary, Phase transformation, Microstructure, Fe-20Cr-30Ni-2Nb austenitic heat-resistant steel

Hiroki Ishikawa, Chi Zhang, Sheng-Wei Chen, Zhi-Gang Yang. Precipitate Behavior in Fe-20Cr-30Ni-2Nb Austenitic Heat-Resistant Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(4): 424-429.

Figures/Tables 11

Table 1 Compositions of the experimental alloys (wt%)

Alloy	Cr	Ni	Nb	B	La	C	Fe
Base alloy	18.35	31.07	3.28			<0.01	Bal.
B-modified	18.35	31.08	3.28	0.01		<0.01	Bal.
La-modified	18.30	30.98	3.27		0.49	<0.01	Bal.
Both-modified	18.30	30.99	3.27	0.01	0.49	<0.01	Bal.

Fig. 1 SEM images of different alloys after heat treated at 1,150 °C for 20 min and then at 800 °C for 12 h followed water quenching: a base alloy; b B-modified alloy; c La-modified alloy; d B and La co-modified alloy

Table 2 Calculated results of the phase amount for different alloys (mol%)

Alloy	fcc matrix	NbNi₃	LaNi₃	Cr₅B₃	FeB
Base alloy	92.5	7.41	-	-	-
B-modified	92.6	7.00	-	0.14	-
La-modified	99.1	-	0.80	-	-
Both-modified	99.1	-	0.80	-	0.05

Fig. 2 XRD patterns in the 2θ range from 34° to 49° of different alloys: a base alloy; b B-modified alloy; c La-modified alloy; d B and La co-modified alloy

Fig. 3 EPMA results of C, Fe, Ni, Cr, and Nb for a precipitate along the grain boundary in Fe-20Cr-30Ni-2Nb alloy

Fig. 4 TEM images of the base alloy showing the precipitate on grain boundary a; Fe2Nb phase in grain b; diffraction pattern of Fe2Nb c

Fig. 5 TEM images of the B-modified alloy showing the precipitate on grain boundary a; Fe2Nb phase in grain b; diffraction pattern of Nb4Ni2C c

Fig. 6 SEM image a; EPMA result of La b of a precipitate in heat-treated La-modified alloy

Fig. 7 TEM image of a precipitate on grain boundary in La-modified alloy a; the diffraction pattern of Nb4Ni2C b

Fig. 8 SEM image a; EPMA results of elements B b; Fe c; Nb d for heat-treated B and La co-modified alloy

Fig. 9 TEM image of the Fe2Nb precipitates on grain boundary in B and La co-modified alloy a; diffraction pattern of Fe2Nb b

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