The toughness of the different types of stainless steels shows considerable variation, ranging from excellent toughness at all temperatures for austenitic steels to the relatively brittle behaviour of martensitic steels. Toughness is dependent on temperature and generally improves with increasing temperatures.
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One measure of toughness is the impact toughness, i.e. the toughness measured on rapid loading. The figure shows categories of stainless steel at temperatures from -200°C to + 100°C.
It is apparent from the diagram that there is a fundamental difference at low temperatures between austenitic steels on the one hand and martensitic, ferritic and ferritic-austenitic steels on the other.
Martensitic, ferritic and ferritic-austenitic steels are characterised by a transition in toughness, from tough to brittle behaviour, at a certain temperature, the transition temperature.
For ferritic steel the transition temperature increases with increasing carbon and nitrogen content, i.e. the steel becomes brittle at successively higher temperatures. For ferritic-austenitic steels, an increased ferrite content gives a higher transition temperature, i.e. more brittle behaviour. |
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The figure shows categories of stainless steel at
temperatures from -200°C to + 100°C
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Martensitic stainless steels have transition temperatures around or slightly below room temperature, while those for ferritic and ferritic-austenitic steels are in the range 0 to - 50°C, with ferritic steels in the upper part of this range.
Austenitic steels do not exhibit a toughness transition as do the other steel types, but have excellent toughness at all temperatures, although the toughness decreases slightly with decreasing temperature.