Microstructures and Mechanical Properties of a Newly Developed Austenitic Heat Resistant Steel

doi:10.1007/s40195-018-0770-0

Abstract

Abstract:

The effect of heat treatment on the microstructures and mechanical properties of a newly developed austenitic heat resistant steel (named as T8 alloy) for ultra-supercritical applications have been studied. Results show that the main phases in the alloy after solution treatment are γ and primary MX. Subsequent aging treatment causes the precipitation of M₂₃C₆ carbides along the grain boundaries and a small number of nanoscale MX inside the grains. In addition, with increasing the aging temperature and time, the morphology of M₂₃C₆ carbides changes from semi-continuous chain to continuous network. Compared with a commercial HR3C alloy, T8 alloy has comparable tensile strength, but higher stress rupture strength. The dominant cracking mechanism of the alloy during tensile test at room temperature is transgranular, while at high temperature, intergranular cracking becomes the main cracking mode, which may be caused by the precipitation of continuous M₂₃C₆ carbides along the grain boundaries. Typical intergranular cracking is the dominant cracking mode of the alloy at all stress rupture tests.

Key words: Austenitic, heat, resistant, steel, -, Microstructure, -, Mechanical, property, -, Fracture, mode

Peng Liu, Zhao-Kuang Chu, Yong Yuan, Dao-Hong Wang, Chuan-Yong Cui, Gui-Chen Hou, Yi-Zhou Zhou, Xiao-Feng Sun. Microstructures and Mechanical Properties of a Newly Developed Austenitic Heat Resistant Steel[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(4): 517-525.

Figures/Tables 13

Table 1 Chemical composition (wt%) of T8 alloy

Ni	Cr	Nb	Si	N	C	B	P	Co	V	Fe
19.5	23.6	0.58	0.46	0.23	0.17	0.002	0.027	1.96	0.1	Bal.

Fig. 1 Predicted equilibrium phase diagram of alloy

Fig. 2 XRD pattern of alloy after solution treatment

Fig. 3 a Image of T8 alloy illustrating twin and MX phase in grains, b TEM morphology of MX phase, and corresponding SAED pattern (inset) c EDS result from position of pentagram in Fig. 3b

Fig. 4 Microstructures of alloy after different aging treatments (the inset image is the higher magnification figure of nanoscale MX phases): a 750 °C, 0.5 h; b 750 °C, 4 h; c 800 °C, 0.5 h; d 800 °C, 4 h

Fig. 5 a TEM morphology of M23C6 carbides along grain boundaries and corresponding SAED pattern (inset), b EDS result from position of pentagram in Fig. 5a

Fig. 6 Distribution and morphology of M23C6 carbides along grain boundaries after different aging treatments: a 750 °C, 0.5 h; b 750 °C, 4 h; c 800 °C, 0.5 h; d 800 °C, 4 h

Table 2 Tensile test results of T8 and HR3C alloys at room temperature (25 °C), 650 and 700 °C

Test temperature (°C)	Yield strength (MPa)		Ultimate tensile strength (MPa)		Elongation (%)
Test temperature (°C)	T8	HR3C	T8	HR3C	T8	HR3C
25	333	350	756	750	61	50
650	179	200	508	500	30	39
700	187	190	443	450	21	30

Fig. 7 Fracture surfaces of tensile tests under room temperature a, d, 650 °C b, e, 700 °C c, f at low a-c, high d-f magnification

Fig. 8 Longitudinal section of fracture surfaces of tensile tests under room temperature a, d, 650 °C b, e, 700 °C c, f at low a-c, high d-f magnification

Table 3 Stress rupture properties of T8 and HR3C alloys tested at various conditions

Test condition	Life (h)		Elongation (%)
Test condition	T8	HR3C	T8	HR3C
650 °C/250 MPa	1409	988	9	9
700 °C/180 MPa	685	500	17	19
750 °C/130 MPa	567	398	24	23

Fig. 9 Larson-Miller parameters of T8 and HR3C alloys (T temperature; t time)

Fig. 10 Fracture surfaces a-c and longitudinal section of fracture surfaces at low d-f, high g-i magnification of T8 alloy crept at 650 °C/250 MPa a, d, g, 700 °C/180 MPa b, e, h, 750 °C/130 MPa c, f, i

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