Effect of the microstructure evolution on the high-temperature strength of P92 heat-resistant steel for different service times

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Highlights

  • The microstructure evolution of P92 and the effect on tensile strength was quantitative study during long term service, the result shows Lath hardening and dislocation hardening change to lath and precipitation hardening with the service time increased.

Abstract

Grade 92 heat-resistant steel is an important material for ultra-supercritical units. The degradation of the P92 properties affects the safety of structural components during long-term service. In this paper, the microstructural evolution of P92 depending on different service times was quantitatively studied using scanning electron microscopy and transmission electron microscopy. The effect of each strengthening mechanism on the strength of P92 is discussed. The results show that the dimensions of M23C6 and Laves phases increase with increasing service time, but the growth rate of the Laves phase is higher than that of M23C6 and the size of MX carbonitrides remains almost unchanged. The width of the martensite lath increases from the initial 350 nm–750 nm after 70,000 h of service. The dislocation density significantly decreases and then stabilizes with increasing service time. The high- and room temperature tensile properties worsen and then stabilize during long-term service. The thermal yield stress of P92 during service was calculated. The results show that the high-temperature strength of P92 steel is mainly affected by lath and dislocation hardening during the initial service time; as the service time increases, the strengthening mechanism transforms into lath and precipitate hardening.

Introduction

Grade 92 heat-resistant steel is widely used in ultra-supercritical units due to its excellent creep properties, oxidation-resistance at high-temperature, and good processing performance [[1], [2], [3]]. Based on previous studies, these excellent properties depend on the substructure of the P92 heat-resistant steel, which leads to various strengthening mechanisms in P92 such as precipitation and dislocation hardening [4,5]. It has been reported that the microstructure of P92 significantly changes under long-term high-temperature conditions[6]. These changes include the precipitation and growth of typical M23C6, Laves, and MX particles [7]; widening of martensite laths [8]; and dislocation reversion and annihilation [9]. These microstructural changes lead to a decreased performance of the material at high temperature [10], which seriously affects the safety of the structural T/P92 components.

Many studies focused on the correlation between the microstructure evolution and high-temperature strength of T/P92 materials. However, in most of the studies, the microstructure evolution and property changes were accelerated by long-term aging or creep at high temperature [[11], [12], [13], [14]]. There is a lack of research on the microstructure evolution and strength change of P92 during long-term service. High-temperature-accelerated tests can simulate the corresponding microstructure evolution of T/P92 steel at high temperature. However, the current experimental time for this research is relatively short, that is, ≤10,000 h, because of economic reasons [14,15], but the microstructure and properties of T/P92 after long-term service differ and there is a lack of references for practical engineering applications.

This research work is based on one of the earliest ultra-supercritical units in China since 2007. The same batch of P92 heat-resistant steel was used at different service times accumulated through multiple different maintenances. The microstructure of P92 after different service times was quantitatively characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tensile strength of P92 under different conditions was also tested. Finally, the athermal yield stress model was used to analyze the microstructure evolution based on the strength degradation of P92 after long-term service.

Section snippets

Material and experiment procedure

The P92 experimental materials were obtained from the main steam pipe of one of the earliest Chinese ultra-supercritical power plants after different service times: 0 h, ~30,000 h (30 kh), ~49,000 h (49 kh), and ~70,000 h (70 kh). The designed service temperature and pressure were 610 °C and 27.15 MPa, respectively. The actual operating temperature and pressure were 605 °C and 26.15 MPa, respectively. The manufacturer is the Mannesmann AG from Germany. The chemical composition of the materials

Results and discussion

Previous studies indicated the following four strengthening mechanisms for P92 heat-resistant steel: solute hardening, precipitation or dispersion hardening, dislocation hardening, and subboundary hardening [2,6]. The change of the mechanical properties is related to the microstructure evolution associated with these four strengthening mechanisms during service. However, based on previous research results, the effect of solute hardening is greatly reduced compared with other strengthening

Conclusions

In this paper, the microstructure evolution and its effect on the high-temperature strength of P92 steel after service for 0, 30, 49, and 70 kh were quantitatively studied using SEM and TEM. The conclusions are as follows:

  • (1)

    The dimensions of the M23C6 and Laves phases increase with increasing service time, but the growth rate of the Laves phase is much higher than that of M23C6, while it was hardly observed that the size of MX carbonitrides changes. The width of the martensite lath increases from

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2017YFB0702200).

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