Phase Transformations and Microstructural Evolution of Mo-Bearing Stainless Steels

Abstract

The good corrosion resistance of superaustenitic stainless steel (SASS) alloys has been shown to be a direct consequence of high concentrations of Mo, which can have a significant effect on the microstructural development of welds in these alloys. In this research, the microstructural development of welds in the Fe-Ni-Cr-Mo system was analyzed over a wide variety of Cr/Ni ratios and Mo contents. The system was first simulated by construction of multicomponent phase diagrams using the CALPHAD technique. Data from vertical sections of these diagrams are presented over a wide compositional range to produce diagrams that can be used as a guide to understand the influence of composition on microstructural development. A large number of experimental alloys were then prepared via arc-button melting for comparison with the diagrams. Each alloy was characterized using various microscopy techniques. The expected d-ferrite and c-austenite phases were accompanied by martensite at low Cr/Ni ratios and by r phase at high Mo contents. A total of 20 possible phase transformation sequences are proposed, resulting in various amounts and morphologies of the c, d, r, and martensite phases. The results were used to construct a map of expected phase transformation sequence and resultant microstructure as a function of composition. The results of this work provide a working guideline for future base metal and filler metal development of this class of materials.

Influence of Ti and C on the Solidification Microstructure of Fe-10Al-5Cr Alloys

“…Each button was then flipped over and melted once more, to ensure the complete melting of all component materials. Previous work [13] identified the cooling rate for the 50-g buttons to be approximately 30°C/s for these conditions and for alloys with a similar thermal mass. The alloys were chemically analyzed by inductively coupled plasma and combustion-infrared absorbance techniques.…”

“…A Netzsch Instruments STA 409 thermal analyzer (Netzsch Instruments Inc., Burlington, MA) was used to remelt and solidify each alloy at a controlled cooling rate of 0.1°C/s. A laserengineered net-shaping system (Optomec, Albuquerque, NM) was used to apply the laser weld using parameters (370 W power, 4.2 mm/s travel speed) previously identified [13] to generate a cooling rate of approximately 30,000°C/s on arc button melts of similar thermal mass.…”

Prediction of σ Phase Formation in Fe–Cr–Ni–Mo–N Alloys

“…In other words, as the concentration levels of Cr and Mo increased in the primary g solidification, an d phase forms during the terminal stage of solidification at the interdendritic region, resulting in a dual-phase structure of g and d. It is known that the d phase transforms into an intermetallic phase such as s phase during cooling after solidification, especially in the case of high-Mo content alloys. 8,19) Magnetic ferrite scope examination did not detect d-Fe; thus the d phase can be assumed to have transformed into s phase. The amount of the s phase in the 2-mm surface layer of the cast sample is smaller than those at deeper levels, which suggests a dependency on the cooling rate.…”

Effect of Composition on the Formation of Sigma during Single-Pass Welding of Mo-Bearing Stainless Steels

“…Liquidus projection calculations (Thermo-Calc [41,42] using the Fe-Data thermodynamic database [43] ) in the Fe-rich corner ( ‡55 wt pct Fe) of the Fe-Ni-Cr-Mo system show no evidence of a ternary eutectic, and instead exhibit regions of primary c-austenite and primary d-ferrite solidification exclusively. [44] This is in contrast with previous studies of c-austenite solidification in the Ni-rich corner of the Fe-Ni-Cr-Mo system [7] that attributed this coupled c/r morphology to a eutectic reaction, L fi c + r. These calculations [39] suggest that compositions within the range of interest that result in primary c-austenite solidification will either produce a fully austenitic structure or terminate with the formation of eutectic d-ferrite, regardless of the nominal Mo content of the alloy or the shape of the solidification path. Thus, the thermodynamics of this compositional regime in the Fe-Ni-Cr-Mo system dictate that r-sigma will not form during solidification.…”