What is the Impact Toughness of Duplex and Super Duplex Stainless Steels? Stainless Steels
Impact toughness is the ability of a metal to withstand a suddenly applied shock load and is quantified by the amount of energy absorbed during impact. A tough metal possesses a combination of high strength and ductility, so that more energy is absorbed during deformation before fracture.
Metals with a face-centred cubic crystal structure, such as austenitic stainless steels and nickel alloys, have a reasonably consistent impact toughness irrespective of the temperature. By association, they maintain their ductility down to low temperatures. However, metals with a body-centred cubic crystal structure, such as most carbon steels and ferritic stainless steels, suffer a drop in their impact toughness below a certain temperature. Their ductility reduces at lower temperature and they become brittle in nature. When plotting impact toughness measurements at different temperatures, the results appear as an S-shaped curve. Of interest to many engineers and designers is the temperature at which this ductile-to-brittle transition takes place.
As duplex stainless steels contain both austenitic and ferritic microstructures, their impact toughness is limited by the presence of the ferritic microstructure. The ductile-to-brittle transition temperature is approximately –50°C.
The most widely used test of impact toughness is the Charpy test. A notched specimen is struck by a weighted pendulum, and the energy absorbed during impact recorded. Results are typically taken in triplicate, to limit the potential variability of this test method and the testing is undertaken at a specific temperature often related to the alloy. The most referenced standards for the impact testing of duplex stainless steels are ASTM A370 and BS EN ISO 148-1:2009.
The Norsok specification for duplex and super duplex stainless steels specify that impact toughness measurements are taken at -46degC. This temperature is at or close to the ductile-to-brittle transition temperature, so a modest impact toughness is expected compared with at room temperature. The current specification calls for impact toughness at -46degC to be greater than 45J on average, with no single test result less than 35J, whereas at room temperature the impact toughness may be several hundred joules.
However, besides confirming the impact toughness of the alloy, this test is a quick and low-cost method to confirm a well-controlled microstructure. If processing, heat treatment or quenching operations are not undertaken at the correct temperatures then deleterious phases such as sigma may form. As the presence of these phases greatly reduce the impact toughness as well as corrosion resistance, impact testing provides quick and clear confirmation of product quality in addition to corrosion testing.
Langley Alloys offers the most comprehensive range of duplex and super duplex stainless steels, operating from locations in the UK and USA (Portland, Houston).
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