Heat treatment of duplex stainless steel is usually restricted to their initial production. After forging or rolling, items will be subjected to a solution annealing heat treatment. By holding the item above 1040degC, the composition and microstructure normalise through the item. The hold time is dependent upon the maximum section thickness, in order to allow sufficient time for the whole item to reach the desired temperature and time. Solution annealing is then followed by a rapid quench in water, with a minimum of delay between the furnace and bath.
Stress-relieving is not recommended for duplex or super duplex stainless steels given their potential to form deleterious intermetallic phases at temperatures below the solution annealing temperature.
There are two mains effects of heat treatment for stainless steel, which we have detailed below –
Solution Annealing – At elevated temperatures, the alloying additions effectively ‘dissolve’ into a solution and become more evenly distributed throughout, whilst rapid quenching then ‘freezes’ the composition.
475°C Embrittlement – Duplex stainless steels have high strength, superb resistance to corrosion and good weldability. Following long-term exposure at temperatures between 280 to 450°C, duplex stainless steels are subjected to thermal ageing embrittlement. The ferritic elements of the microstructure begin to decompose into either iron-rich or chromium-rich areas, which significantly reduces corrosion resistance and mechanical properties.
Langley Alloys experience of super duplex stainless steels is unrivalled, holding the most complete product range as solid bars up to 16” (406.4mm) diameter, plate up to 3” (76.2mm) in thickness, plus pipes and fittings.