Stainless, Duplex, Super Duplex Steel Castings
Stainless Steel Castings
Stainless steels castings are a class of chromium-containing steels commonly used for corrosion resistance in aqueous environments and for service at elevated temperatures.
Stainless steels castings are distinguished from other steels by the improved corrosion and oxidation resistance created by chromium additions. Chromium imparts passivity to ferrous alloys when present in amounts greater than about 11%, especially if conditions are strongly oxidizing.
As a result, steels containing more than 10 or 12% Cr are sometimes described as stainless steels. These highly alloyed cast steels usually contain more than 10% Cr and consist mainly of stainless steel compositions.
The real difference between heat resistant and corrosion resistant cast steels is based on carbon content. Austenitic stainless steel castings usually contain small amounts of ferrite. The presence of ferrite in castings is desirable to facilitate weld repair and ferrite also increases resistance to the formation of stress-corrosion cracks.
Stainless steels are broadly classified into five groups:
- Martensitic stainless steels
- Ferritic stainless steels
- Austenitic stainless steels
- Duplex (ferritic-austenitic) stainless steels
- Precipitation hardening stainless steels
Duplex Steel Castings
Duplex Steel Castings usually contain between 5% and 40% ferrite, depending on the particular grade and the balance between ferrite-promoting and austenite-promoting elements in the chemical composition. Duplex Steel Castings offer superior strength, corrosion resistance and weldability service properties.
Duplex Steel Castings are primarily focused on CF grades, especially by the power generation industry. Strengthening in cast CF grade alloys is essentially limited to the strengthening that can be achieved by incorporating ferrite into the austenite matrix phase.
Duplex Steel Castings cannot be strengthened by heat treatment as in cast martensitic alloys, nor by hot or cold working as in wrought austenitic alloys. Therefore, alloys are effectively strengthened by stabilizing the alloy composition to produce a duplex microstructure consisting of ferrite (up to 40% by volume) dispersed within an austenite matrix.
Super Duplex Steel Castings
The term “Super Duplex” was first utilized in the 1980’s to denote highly alloyed, high performance Duplex steel with a pitting resistance (Pren) equivalent of >40 (based on Cr% + 3.3Mo% + 16N%).
With its high level of chromium, Super-Duplex steels provides outstanding resistance to acids, acid chlorides, caustic solutions and other environments within the chemical/petrochemical, pulp and paper industries, often replacing 300 series chrome steel, high nickel superaustenitic steels and nickel-based alloys.
The chemical composition supported high contents of chromium, nickel and molybdenum improves intergranular and pitting corrosion resistance. Additions of nitrogen promote structural hardening by interstitial primary solid solution mechanism, which raises the yield strength and supreme strength values without impairing toughness.
Recently, highstrength, corrosion resistant super duplex steel has been used in coil manufacturing, marine and chemical industries, architectural and mast riggings, cable wires, lifting tools and pulley manufacturing.