Direct Observations of Sigma Phase Formation in Duplex Stainless Steels Using In-Situ Synchrotron X-Ray Diffraction

Abstract

The formation and growth of sigma (σ) phase in 2205 duplex stainless steel was observed and measured in real time using synchrotron radiation during 10 hr isothermal heat treatments at temperatures between 700ºC and 850ºC. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. Differences between the calculated and measured amounts of sigma, ferrite and austenite suggest that the thermodynamic calculations underpredict the sigma dissolution temperature by approximately 50ºC. The data were further analyzed using a modified Johnson-Mehl-Avrami (JMA) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMA exponent, n, at low fractions of sigma was found to be approximately 7.0, however, towards the end of the transformation, n decreased to values of approximately 0.75. The change in the JMA exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMA equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high temperature value to room temperature.

Use of Magnetic Barkhausen Noise (MBN) to Follow Up the Formation of Sigma Phase in Saf2205 (UNS S31803) Duplex Stainless Steel

“…The kinetics of sigma formation measured in this study is consistent with that reported by Magnabosco 4 and with the results of Elmer et al 2 obtained using In-situ Synchrotron X-Ray Diffraction. More important than the kinetic measurements, however, is the indication that the Barkhausen noise signal can be adequately used to assess the decrease in ferromagnetic phase over time, thus making it a valuable non-destructive technique.…”

“…Since σ is the intermetallic phase more prominently formed during field applications of duplex stainless steel, its formation has been the object of several studies throughout the years, mostly focused on morphology and formation kinetics [1][2][3][4][5][6][7][8] , as well as its effects on mechanical properties [9][10][11] and corrosion resistance [12][13][14][15][16][17] . Processes that include high temperature thermal cycling, such as welding, are particularly critical regarding the appearance of σ, due to the temperature range to which the heat affected zone is exposed in a poorly controlled manner.…”

In situ observations of sigma phase dissolution in 2205 duplex stainless steel using synchrotron X-ray diffraction

“…The in-situ x-ray diffraction was performed while heating test coupons in vacuum (10 -4 Torr) using a direct resistance method [15][16][17]. The heating cycle consisted of a 20ºC/sec ramp to the initial temperature of 850ºC where it was held for 30 min to form a measurable amount of sigma.…”

“…Additional details about the data acquisition technique on this beam line are presented elsewhere [15][16][17]. Figure 4 shows a room temperature diffraction pattern (upper line) taken after an 850ºC heat treatment where a significant amount of sigma phase had formed [7].…”

Effect of niobium in the phase transformation and corrosion resistance of one austenitic-ferritic stainless steel

“…Sigma phase reduces the elongation and toughness too, although it increases hardness, yield point and tensile strength in the matrix. Precipitation of 1% of this phase reduces by 50% the initial value of the absorbed energy in the Charpy test 11,12 .…”