Impact toughness is a metal's ability to withstand a suddenly applied shock load, measured by the energy absorbed during impact. A tough metal combines high strength and ductility, absorbing more energy through deformation before it fractures.
Crystal structure and toughness
Metals with a face-centred cubic crystal structure - such as austenitic stainless steels and nickel alloys - keep a reasonably consistent impact toughness regardless of temperature, staying ductile down to low temperatures.
Metals with a body-centred cubic structure - most carbon steels and ferritic stainless steels - lose impact toughness below a certain temperature, becoming brittle as their ductility falls.
Plotting impact toughness against temperature produces an S-shaped curve. The temperature at which the ductile-to-brittle transition occurs is of particular interest to engineers and designers.
Because duplex stainless steels contain both austenitic and ferritic microstructures, their toughness is limited by the ferrite. Their ductile-to-brittle transition temperature is approximately -50°C.
Testing impact toughness
The most widely used test is the Charpy test: a notched specimen is struck by a weighted pendulum and the absorbed energy recorded. Results are usually taken in triplicate to limit variability, at a temperature related to the alloy. The most referenced standards for duplex stainless steels are ASTM A370 and BS EN ISO 148-1:2009.
The Norsok specification for duplex and super duplex stainless steels requires impact toughness measured at -46 degrees centigrade - at or close to the ductile-to-brittle transition, so a modest result is expected compared with room temperature. The current specification calls for impact toughness at -46degC greater than 45J on average, with no single result below 35J; at room temperature the figure may be several hundred joules.
A quick check on microstructure
Beyond confirming toughness, this test is a fast, low-cost way to confirm a well-controlled microstructure. If processing, heat treatment or quenching are not done at the correct temperatures, deleterious phases such as sigma can form. Because these phases sharply reduce both toughness and corrosion resistance, impact testing gives quick, clear confirmation of product quality alongside corrosion testing.
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