Aging Precipitation Behavior And Strengthening Mechanism In Super304H Austenitic Heat Resistant Steel

With the rapid development of thermal power generating units, the steam pressure and temperature parameters of units operating increase continuously. Thus, the performance requirements of heat resistant steels for units also become higher. Super304 H austenitic heat resistant steel is based on 18/8 Cr-Ni stainless steel alloyed mainly with about 3 wt% Cu and a small amount of Nb, which is widely used in the superheater and reheater of ultra-super critical(USC) unit due to its high strength and good oxidation resistance at elevated temperatures. The outstanding high-temperature strength of Super304 H steel mainly arises from the precipitation of nanoscale Cu-rich phase and MX(Nb(C, N)) phase in the austenitic matrix during service at high temperatures. In addition, M23C6 and Z phase(Nb Cr N) play a minor role in precipitation strengthening in the Super304 H austenitic steel. However, the aging precipitation behavior and strengthening effect of these precipitates in the Super304 H steel remain unclear until now. The study on these two problems are of great significance to further understand the aging precipitation behavior and strengthening effect of these precipitates in the Super304 H austenitic steel, which can provide a experimental evidence using for the performance evaluation of this steel during service at elevated temperatures. In this thesis, the aging precipitation behavior of the Super304 H austenitic steel have been investigated through observing and analyzing the microstructures of this steel deeply by scanning electron microscope(SEM) and transmission electron microscope(TEM). Combining the aging precipitation behavior with mechanical properties of the Super304 H austenitic steel, precipitation strengthening mechanism is discussed in detail. The main conclusions are summaried as followings:1. There is a small amount of bulk Nb-rich MX particles and circular inclusions with large size in the initial microstructure of the solution-treated Super304 H steel. The microstructure of Super304 H austenitic steel after aging at 650 °C for 5000 h have been investigated. The results show that there are some precipitates including Nb-rich MX phases, M23C6 carbides and Cu-rich phases in the austenitic matrix. Among these precipitates, the nanosized and incoherent Nb-rich MX precipitates were found occasionally within grains and keep a cubic-on-cubic crystallographic relationship with the austenitic matrix. M23C6 carbides were found to nucleate at Nb-rich MX interface forming a core-shell structure and occasionally precipitated in the austenitic matrix. Additionally, discontinuous chains of M23C6 were often found at grain boundaries. M23C6 usually keep a cubic-on-cubic crystallographic relationship with the austenitic matrix. A large number of nanosized Cu-rich precipitates were found within grains and have a cubic-on-cubic crystallographic relationship and coherent interface with the austenitic matrix.2. The coarsening behavior of Cu-rich precipitates in Super304 H austenitic steel aged at 650, 700 and 750 °C have been investigated. The results show that the Cu-rich precipitates always keep a cubic-on-cubic crystallographic relationship and coherent interface with the austenitic matrix during coarsening process and the coarsening behavior of the Cu-rich particles can be predicted by the Lifshitz-Slyozov-Wagner(LSW) theory. The activation energy for coarsening of the Cu-rich precipitates was evaluated to be 212±3 k J/mol. The coarsening of Cu-rich precipitates is controlled mainly by the volume diffusion of copper atoms in the austenitic matrix.3. Tensile yield behaviors of the solution- and aging-treated Super304 H austenitic steel have been investigated at temperatures from 25 to 650 °C. The results show that the contribution to yield stress from precipitation strengthening of Cu-rich phases is 17~23%. The higher thermal activation volume and energy imply an interaction between dislocation and Cu-rich precipitates. The precipitation strengthening mainly arises from coherency strain and partially from stacking fault strengthening during tensile deformation at room temperature. The calculated shear stress from precipitation strengthening agrees reasonably with the experimental result. The hardening behavior of Cu-rich precipitates in Super304 H austenitic steel aged at 650, 700 and 750 °C have been investigated. The results show that the contribution to maximum microhardness occuring at different aging temperatures from precipitation strengthening are about 17~25%. The strengthening of the Cu-rich precipitates also arises mainly from the coherency strain and partially from stacking fault strengthening.4. Creep behavior of Super304 H austenitic steel has been investigated at elevated temperatures of 650~700 °C and at applied stress of 190~210 MPa. The results showed that the average apparent stress exponent and activation energy for creep deformation are 21.5 and 687.3 k J/mol, respectively. These high values imply the presence of threshold stress due to an interaction between the dislocation and Cu-rich precipitate during creep deformation. The creep mechanism is associated with the dislocation climbing governed by the matrix lattice diffusion. The origin of threshold stress is mainly attributed to the coherency strain induced in the matrix by the Cu-rich precipitates due to a positive lattice parameter mismatch. The theoretically-estimated threshold stresses from Cu-rich precipitates agree reasonably with the experimental results. The microstructure of Super304 H austenitic steel after creep test under 250 MPa at 650 °C for 447 hours has been investigated. The results show that the nanosized and cubical-shaped incoherent Nb-rich MX phases were found within grains in the austenitic matrix. In addition, the nanosized Nb-rich MX phases easily nucleate on the interface between the Cu-rich phase and austenitic matrix and precipitate on dislocations during creep.