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|Fig. 1. Section of a cast HH (25% Cr, 12% Ni) stainless steel furnace load-lifting hook that failed due to sigma-phase embrittlement. (Photograph courtesy of George F. Vander Voort, Vander Voort Consulting LLC)|
|Fig. 2. Iron-chromium phase diagram|
Many stainless steels and other iron-chromium alloys are susceptible to a grain boundary phenomenon known as sigma-phase embrittlement. This type of embrittlement has been shown to cause severe loss of ductility, toughness, and corrosion resistance resulting in cracking (Fig. 1) and failure of components, especially those subjected to impact loads or excessive stress. As heat treaters we need to know more about what sigma-phase embrittlement is and how to avoid its occurrence. Let’s learn more.
Prolonged exposure in the temperature range of 565-925°C (1050-1700°F) results in chromium depletion from the grain boundaries, making them susceptible to intergranular corrosion. The most rapid sigma-phase formation occurs in the range of 700-900°C (1290-1650°F). Alloy elements such as molybdenum, titanium and silicon promote the formation of sigma phase, while nitrogen and carbon reduce its tendency to form.
Sigma phase is an intermetallic compound consisting of chromium and iron, which is hard, brittle and non-magnetic. Pure sigma forms between 42% and 50% chromium and is one of the equilibrium phases in the iron-chromium phase diagram (Fig. 2). A duplex structure (sigma and alpha phases) has been found to form in alloys with as little as 20% chromium and as much as 70% chromium when exposed to the critical temperature range noted above. At chromium contents of less than 20%, sigma phase is difficult to form, but the presence of molybdenum, silicon, manganese or nickel have a tendency to shift the lower limit down.
Molybdenum reportedly promotes sigma-phase formation much more effectively than chromium, particularly at temperatures around 900°C (1650°C). This is why, in the HH cast stainless example shown, the molybdenum content of the alloy is deliberately kept around 0.5%. Austenite-forming elements such as nickel or nitrogen can also accelerate the nucleation and growth of the sigma phase, although these elements may reduce the total amount formed because of the smaller volume fraction of ferrite. Sigma typically nucleates in the austenite-ferrite grain boundaries and grows into the adjacent ferrite. Additional austenite often forms in the areas of chromium depletion adjacent to the sigma phase.
Although the formation of sigma phase is sluggish, cold working enhances the precipitation rate considerably, and sigma phase has even been found in the air-cooled, as-cast structures in very high chromium content alloys. Sigma phase usually appears as a continuous network in the microstructure. Since sigma has a significantly lower corrosion resistance compared to the ferrite matrix, its presence can be detected by etching in a metallographic examination (Fig. 3).
The temperature range of rapid sigma formation coincides with the normal temperatures used for annealing ferritic stainless steels. Consequently, highly alloyed ferritic stainless steels must be annealed in the 1050°C (1925°F) range and rapidly cooled through the critical range to avoid sigma-phase embrittlement. Any sigma phase already formed can be dissolved again by a solution-annealing process performed above 800-850°C (1470-1560°F) for relatively short times – approximately an hour (once the entire part has reached temperature) – followed by air cooling.
Other Forms of Embrittlement
475°C (885°F) Embrittlement
Iron-chromium alloys containing 15-70% chromium may exhibit a pronounced increase in hardness accompanied by severe loss of ductility and corrosion resistance if exposed to the temperature range of 400-540°C (750-1005°F) for significantly shorter time periods than is required for sigma-phase formation. The name of this phenomenon comes from the fact that the peak hardness usually occurs at 475°C (885°F). In fact, it can occur during slow cooling from an elevated temperature as well as during elevated-temperature service. For alloys containing 18% Cr, the onset of embrittlement is fast enough to require rapid cooling from the annealing temperature to extend below 400°C (750°F) in order to ensure optimal ductility.
In service, alloys containing greater than 16% Cr should not be used at 375-540°C (707-1004°F) for extended periods of time or cycled from room temperature through this critical range. This embrittlement phenomenon is believed to be due to the formation of a submicroscopic, coherent precipitate that is induced by the presence of a solubility gap below approximately 550°C (1020°F) in a chromium range where sigma phase forms at higher temperatures. Cold work intensifies the rate of 475°C (885°F) embrittlement, especially for the higher-chromium alloys. Reheating the alloy to above 550°C (1022°F) for a few minutes completely removes this form of embrittlement.
Medium- and high-chromium ferritic alloys containing moderate amounts of carbon and/or nitrogen develop high-temperature brittleness if cooled slowly from above 950°C (1740°F). The mechanism is similar to that of sensitization and leads to severe intergranular corrosion. Work on two wrought ferritic stainless steels containing 18 and 25% Cr, respectively, has shown that the maximum amount of carbon plus nitrogen tolerable for good room-temperature toughness is 0.055% for the 18% Cr alloy and 0.035% for the 25% Cr alloy.
Duplex Steels Not Immune
In general, the presence of a high percentage of sigma phase is undesirable in duplex stainless steels due to its detrimental influence on corrosion (e.g., pitting) and mechanical properties. Duplex and super-duplex stainless steels are ferrous alloys with up to 26% chromium, 8% nickel, 5% molybdenum and 0.3% nitrogen and are intended for service in corrosive applications. The metallurgy of duplex and super-duplex stainless steels (especially castings) is complex due to high sensitiveness to sigma-phase precipitation on cooling from solidification temperature as well as from heat treatment.
The hardness of these materials is a strong indication of the presence of sigma phase in the microstructure. It has been found that the material hardness is inversely proportional to the heat-treatment temperature. When the heat-treatment temperature during solution treatment increases, the sigma-phase content in the microstructure decreases. Consequently, the material hardness diminishes. When the sigma phase is completely dissolved by the heat treatment, the material hardness is influenced only due to ferrite and austenite contents in the microstructure.
The soak temperature also influences the percentage of sigma phase present in solution (as well as in the volumetric concentrations of the ferrite and austenite phases). The ferrite percentage increases with the increasing heat-treating temperature. From 1060°C (1940°F) and up, the sigma-phase quantity is eliminated and the volume fractions of ferrite and austenite each approach 50%.
The presence of sigma phase in stainless steels and iron-chromium alloys should be cause for concern among heat treaters, but awareness of what can trigger this form of embrittlement and what can be done to negate its effects are worth our time and effort. IH