Chapter 17. Stainless Steels. Identification of Stainless Steels • Stainless Steel Manufacturing • Corrosion Resistance.
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Chapter 17 Stainless Steels Identification of Stainless Steels • Stainless Steel Manufacturing • Corrosion Resistance
There are five families of wrought stainless steels, which include martensitic, ferritic, austenitic, precipitation-hardening, and duplex stainless steels. Cast stainless steels exhibit various types of metallurgical structures and are classified as a sixth family.
With >12% Cr addition, the austenite phase field becomes an island, forming a gamma loop on the phase diagram.
The base composition contains 18% Cr because greater or lesser amounts of chromium require more nickel to ensure a completely austenitic structure at room temperature.
Ferritizers, austenitizers, and carbide formers are added to stainless steels for their effects on properties.
To predict the metallurgical structure from the composition, the CrE is plotted against the NiE on the Schaefflerdiagram.
The basic martensitic stainless steel is type 410 (S41000), and it is closely related to other martensitic stainless steel family members.
High-carbon martensitic stainless steels contain >0.15% C, which expands the gamma loop to allow chromium additions up to 18%.
The base composition of the ferritic stainless steels is type 430 (S43000) and is closely related to other members of the ferritic stainless steel family.
Sigma phase embrittlement occurs during prolonged heating in the range 540°C to 760°C (1000°F to 1400°F).
The basic austenitic stainless steel alloy is type 302 (S30200), and the majority of the grades belong to the 300 series.
Type 301 is the most work hardenable of the low-nickel austenitic stainless steels and is used extensively in sheet form for structural applications.
The stress relieving procedure must be carefully selected to achieve the required result without compromising other properties.
When heated into the sensitization temperature range, carbon and chromium combine to form discrete precipitates of chromium carbide.
Austenitic stainless steels are the strongest of all stainless steels used in services above 540°C (1000°F). Depending on the operating stress, they are used from 760°C to 870°C (1400°F to 1600°F).
Most precipitation-hardening stainless steels are designated by the AISI 600 series, but they are better known by trade designations.
Most duplex stainless steels contain approximately 70% Fe, 20% Cr to 25% Cr, 4% Ni to 7% Ni, and 2% Mo to 4% Mo.
The room temperature microstructure of the duplex stainless steels contains approximately equal amounts of ferrite and austenite.
Corrosion-resistant castings are designated by the uppercase letter C followed by a letter that indicates the approximate alloy content.
The Schaeffler diagram indicates the amount of ferrite in austenitic castings.
Heat-resistant castings are designated with the uppercase letter H followed by a letter that indicates the approximate alloy content.
Many welding filler metals are produced for welding the various stainless steels.
The preheat temperatures for martensitic stainless steels are dictated by the carbon content.
The correct geometry must be used on tools for machining stainless steels.
Surface finishes for stainless steels are designated with a number that indicates the degree of surface roughness.
Chloride stress-corrosion cracking can occur at tube-tubesheet joints in heat exchangers or under thermal insulation, or in any location where the chloride ion concentrates under the right temperature conditions.