One Minute Mentor: Annealing of Austenitic Stainless Steels
June 29, 2016
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The graph shows isothermal precipitation kinetics for detrimental σ/χ phases for the 254 SMO (S31254), 904L, and 317LMN alloys. For comparison, the stability curve for the formation of chromium carbide in type 316 stainless steel is presented.
As the alloying content increases, the time to precipitate these undesirable phases decreases, and the more highly alloyed superaustenitic grades have very limited time (less than a minute) before the onset of precipitation at temperatures in the 1470 to 1700°F range.
Because of this, cooling from the anneal temperature must be controlled to maximize the quench rate and minimize the time in the 1020 to 1920°F temperature range. This is particularly a problem with thick sections, for which rapid cooling rates are difficult to achieve in the center section.
Annealing temperatures for these more highly alloyed austenitic grades tend to be higher than those of the conventional austenitic grades. These high annealing temperatures ensure dissolution of undesirable secondary phases and promote homogenization of as-solidified structures.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Annealing
The graph shows isothermal precipitation kinetics for detrimental ?/? phases for the 254 SMO (S31254), 904L, and 317LMN alloys.
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One Minute Mentor: Highly Alloyed Austenitic Stainless Steels
June 23, 2016
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Highly alloyed austenitic stainless steels typically contain larger amounts of molybdenum to provide very good resistance to chloride corrosion. These alloys are austenitic in the mill-annealed condition but do form σ (sigma) and to a lesser extent χ (chi) phases under certain conditions of heat treatment or service. The σ and χ phases are hard, brittle intermetallic compounds that form in the temperature range of approximately 1020 to 1920°F. The formation of these undesirable secondary phases causes reduced corrosion resistance and a loss of ductility and toughness. Because of these detrimental effects, it is important to avoid the formation of σ and χ during heat treatment and quenching of these grades of stainless steel.
The graph shows isocritical pitting temperature curves that demonstrate the effects of isothermal anneals in the temperature range of sigma formation.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Annealing
Highly alloyed austenitic stainless steels typically contain larger amounts of molybdenum to provide very good resistance to chloride corrosion.
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One Minute Mentor: Time-Temperature Sensitization Curves for Austenitic Stainless Steel
June 17, 2016
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The graph shows a Time-Temperature-Sensitization diagram (TTS) for 304 austenitic stainless steel with various carbon contents. Decreasing carbon content significantly delays the formation of chromium carbides and the onset of intergranular corrosion. In low carbon grades, defined as those with carbon contents below 0.030%, an exposure of about eight hours is necessary before sensitization occurs.
Low-carbon austenitics are intermediate in tendency to precipitate chromium carbides to the stabilized and unstabilized grades. The lower carbon content in an austenitic stainless steel will slow the kinetics of carbide precipitation, which allows longer time at temperature before the onset of sensitization, as shown in the graph.
Carbon contents of 0.03% and lower allow short excursions of several hours into the sensitizing temperature range without loss of corrosion resistance. Carbon in austenite can be retained in supersaturation for extended periods of time. This is why austenitic L grades do not sensitize even though they are slightly supersaturated. Sensitization occurs at higher levels of carbon by prolonged heating at 600 to 850°C.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Heat Treating of Austenitic and Duplex Stainless Steels ⇒ Annealing
The graph shows a Time-Temperature-Sensitization diagram (TTS) for 304 austenitic stainless steel with various carbon contents.
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One Minute Mentor: Heat Treatment Applied to Tool Steels
September 30, 2015
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After hot forming, tool steel usually is exposed to a heat treatment process done to produce a low-hardness condition. Low hardness is important for machining, but the treatment also should promote a homogeneous microstructure, usually referred to as the annealed condition. The graph shows the process condition as a function of time, temperature, and phase transformation.
Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels -> Introduction to Heat Treating of Tool Steels -> Heat Treatment Applied to Tool Steels
After hot forming, tool steel usually is exposed to a heat treatment process done to produce a low-hardness condition.
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