Microstructure control to improve creep strength of alumina-forming austenitic heat-resistant steel

Abstract

In this dissertation, the the relationship between microstructural evolution and creep strength of alumina-forming austenitic heat-resistant (AFA) steel was investigated. AFA steels have higher oxidation resistance than commercial austenitic heat resistant steels due to the formation of Al2O3 on the surface. AFA steels contain high Nb content (approximately 0.9wt%) to enhance precipitation hardening through the formation of fine and uniformly distributed secondary NbC and Laves phases (Fe2M, M= Nb, Mo, W). to improve the creep strength of the alumina-forming austenitic heat-resistant steel, microstructures were controlled using alloying elements and fabrication processes. The changes of microstructure at high temperature, especially the relation between precipitation behavior and creep strength, were studied and the results were summarized. Before studying the effects of alloying elements and fabrication processes, microstructural evolution and basic mechanical properties of AFA steel were measured. The high temperature tensile was measured at 700 ° C and the creep test conditions were set based on the yield strength obtained. The creep test was carried out at 700 ℃, 160 MPa, which is less than the yield strength at that temperature, and the results are shown in two graphs of creep life Vs. creep strain, creep life Vs. creep strain rate. The creep-fracture behavior was analyzed by observation of the fracture and fracture surface after the creep test, and the basis of the microstructure design for improving the creep strength in the future was provided. In order to simulate microstructural evolution and precipitation behavior during the creep test, aging tests were carried out at the same temperature with time using a heat treatment box furnace. The degree of precipitation strengthening was qualitatively measured using Vickers hardness. In the case of AFA steel, NbC, M23C6 → NiAl Laves phase was precipitated with the aging time, and hardness increased explosively after 50 hours which the precipitation of Laves phase starts. In the case of NbC, it was formed at the early stage and its size was very fine, which was analyzed as the main cause of improvement of creep strength in addition to Laves phase. Therefore, in this study, the alloy elements were controlled to maximize the precipitation strengthening effect of Laves phase and NbC, and to improve the creep strength by the room temperature plastic deformation to introduce additional strengthening mechanism. Phosphours (P) which is known to be closely related to M23C6 precipitation which does not contribute to precipitation strengthening. Low P AFA (AFALP) was prepared to inhibit the precipitation of grain boundary M23C6 and to promote NbC precipitation. The creep rupture time and hardness of the investigated steels were improved with the decrease of the P content. The TEM and EELS analyses confirmed an enhanced P concentration in the M23C6 precipitates along the grain boundaries. The P promoted the precipitation of M23C6 and also suppressed the precipitation of secondary NbC in the earlier stage of aging. The creep rupture time of AFA became shorter compared to AFALP due to the reduction in the fraction of secondary NbC which acts as a dominant strengthening phase during creep. The suppression of secondary NbC precipitation by P increased the Nb content in the austenite matrix and accelerated the precipitation of the Laves phase in the intermediate stage. However, the difference in Vickers hardness caused by Laves phase was not large in both alloys because the volume fraction of Laves phase strongly depended on the nominal content of Mo rather than Nb. The Vickers hardness at the earlier stage is determined by the precipitation of secondary NbC and the difference of the hardness maintained at the final stage. Thus, it is found that P degrades the creep rupture time and P is a detrimental element in terms of creep and precipitation hardening in AFA steels. The major factors of decrease in the precipitation strengthening effect of AFA steel at 700 ℃ with time are coarsening of Laves and NiAl phase and formation of Sigma phase. In this study, The coarsening of Laves phase can be suppressed and controlled the formation of Sigma phase by W-addition. The effect of W addition on the creep and precipitation behavior of AFA steels at 700 °C and 160 MPa was investigated. The results showed that the creep rupture time of the W containing AFA (WAFA) increased more than that of the AFALP due to enhanced precipitation hardening of Laves phase and suppression of sigma phase formation. W increased the volume fraction of Laves phase and enhanced precipitation hardening by higher driving force for the precipitation and affinity of the Laves phase. Therefore most W were consumed for the precipitation of the Laves phase, however Mo content contributed to the precipitation of M23C6 as well as precipitation of Laves phase due to the lower affinity of Laves phase. The M23C6 in AFALP contained enough Mo to form the sigma phase, which served as an initiation site for cavitation and crack propagation. The sigma phase grew and coarsened at the expense of M23C6 and Mo-rich Laves phase to match the stoichiometric composition of the sigma phase, and finally creep strength of AFALP was greatly reduced In order to elucidate the optimal processing conditions of the developed AFA, the dynamic recrystallization phenomenon of WAFA and AFA was studied. It found that the recrystallization was greatly suppressed in the recrystallization temperature range of general austenitic stainless steels by Nb, W, and Mo added to improve the creep strength of the AFA. Based on this, we tried to use work hardening as an additional strengthening mechanism to improve creep strength of AFA. At 700 °C and 200 MPa, the creep rupture time of WAFA steel increases from 1044 to 3119 h by pre-strain. Pre-strain introduces a lot of dislocation into the matrix and most of the dislocations remain stable to the end of creep without recovery and recrystallization. In addition, the dislocations have enabled secondary NbC and Laves phase to be uniformly distributed and refined. However, the dislocations lower grain boundary strength due to the coarsening of precipitates along grain boundaries. Therefore, the increase in the creep rupture time in spite of deterioration of grain boundary strength is attributed to the dislocation strengthening and enhanced precipitation hardening within grains.