Microstructural studies of advanced austenitic steels

Abstract

This report presents •the first. complete microstructural and analytical electron microscopy study of Alloy AXS, one of a series 'of advanced austenitic steels developed by Maziasz and co-workers [1-7] at Oak Ridge National Laboratory, for their potential application as reheater and superheater materials in power plants that.will reach the end of their design lives .in the 1990'.s.. The advanced steels are modified with carbide forming elements such as titanium, niobium and vanadium. When combined with optimized thermo-mechanical treatments, the advanced steels exhibit significantly improved creep rupture properties compared to commercially available 316 stainless steels, 17-14 Cu-Mo and 800 H steels. The importance of microstructure in•controlling these improvements has been demonstrated for selected alloys, using stress relaxation testing as an accelerated test method. The microstructural features responsible for the imp~oved creep strengths have been identified by studying the thermal aging kinetics• of one of the 16Ni-14Cr advanced steels, Alloy AXS, in both the solution annealed and the solution annealed plus cold worked conditions. Timetemperature-precipitation. diagrams have been developed for the temperature range 600 C to 900 C and for times from 1 h to 3000 h. Particle size measurements have shown that the MC precipitates formed on matrix dislocations resist coarsening for up to 3000 h at 900 C.

Influence of Hold Time on Creep-Fatigue Behavior of an Advanced Austenitic Alloy

“…This observation is consistent with previous work on AX-5, an HT-UPS alloy of similar composition [6]. After a solutionannealing treatment at 1250°C, Todd and Ren reported the exclusive presence of larger precipitates, which were classified as likely remnants of carbonitrides, and no dispersion of fine MC precipitates [6]. Aging of the AX-5 for 10 hours at 700°C resulted in an extensive amount of fine MC precipitates, whereas aging at the reduced temperature of 600°C revealed the presence of relatively few MC precipitates even after 300 hours [6].…”

“…Fine MC precipitates have not yet formed in the as-received material despite the presence of a discernable dislocation density remaining from the hot rolling operation. This observation is consistent with previous work on AX-5, an HT-UPS alloy of similar composition [6]. After a solutionannealing treatment at 1250°C, Todd and Ren reported the exclusive presence of larger precipitates, which were classified as likely remnants of carbonitrides, and no dispersion of fine MC precipitates [6].…”

“…This finely-precipitated MC phase, which readily pins dislocations and prevents recovery of the deformed dislocation substructure [3], is reported to be rich in Ti and Nb [2,3,5] in HT-UPS alloys that are very similar in composition to the HT-UPS alloy investigated in this work. The fine MC precipitates are approximately 2 to 10 nm in diameter depending on the previous processing conditions and have a complex FCC crystal structure oriented in a cube-cube relationship with the austenitic matrix [6], characteristics that are confirmed (Figure 1) for the alloy being studied in this work. Larger MC and M 23 C 6 precipitates, commonly seen in standard 316 SS, are present along the grain boundaries.…”

“…In this manner, the precipitation structure is already widespread throughout the matrix prior to employment of the material in the application. As a follow-on to work that was performed which demonstrates that precipitation of the ultra-fine particles will not occur until some level of deformation dislocation density has been established [6], the creep-fatigue testing and analysis reported here was conducted on material in the hot rolled condition with no addition of cold work. This condition allows for both full characterization in the un-precipitated state and for ex-situ observations of the precipitate morphology as deformation is introduced through fatigue and creep-fatigue exposure.…”

“…HT-UPS alloys offer improved strength and creep resistance over conventional stainless steels, particularly at temperatures in excess of 600 °C [2][3][4]. Developed in the 1980s through a study to develop alloys for elevated temperature fossil fuel applications, HT-UPS has already been subjected to extensive studies covering the basic mechanical properties [2][3][4][5] and related microstructural evolution [6] following exposure to standard creep testing, but relatively little research has been carried out to specifically elucidate the combined creep-fatigue behavior of any of the HT-UPS variants [7]. In service, components will be subjected to pressure-and temperature-related stresses that are prone to fluctuations along with being held constant for extended periods of time.…”