The structure, mechanical and corrosion properties of duplex stainless weldments

Elsherief, Ahmed Fathy Abd Elshafi (1990) The structure, mechanical and corrosion properties of duplex stainless weldments. University of Southampton, Doctoral Thesis.

Abstract

There have been few systematic studies on the properties of duplex stainless weldments, and so in this study a large number of experimental tests have been carried out on different wrought and cast duplex stainless weldments (SAF-2205 UNS-S31808) and (Ferralium-225) which are characterized by low carbon content and good balance of ferrite and austenite phases (im 50/50). These experiments have sought to assess the following parameters: - The influence of heat input on microstructure, mechanical and corrosion properties. - The effect of ageing on the microstructure and transformation behaviour. - The effect of consumable composition and type on the microstructure, mechanical and corrosion properties. - The effect of gas back purging on phase balance and corrosion resistance. - The influence of multi-run heating on subsequent properties. The current studies have revealed a number of significant conclusions, such as:- - Heat input has a very small influence on mechanical properties. - Toughness values are increased as the heat input is increased because there is an increased austenite level. - The toughness decreased with decreasing testing temperature. - Ageing for long term at 500^o-700^o and 800^oC has significant effects on hardness, mechanical properties and microstructure. Moreover there is evidence of intermetallic formation. - The microstructure was found to be influenced by the welding process, heat input, composition of filler metal and number of weld passes. - The microstructure examination has indicated that high input rates generally promote austenite formation, due to slower post weld cooling. - The welds have an increased ferrite content compared with the parent metal. - There is a significant increase of ferrite in the HAZ, and as a result the HAZ has a low resistance corrosion. - The austenite/ferrite phase balance in the microstructure has a significant influence on the corrosion resistance. - The change in heat input does not appear to have a direct effect on corrosion, but its effect is secondary through its influence on austenite formation. - The minimum austenite level for good pitting corrosion resistance appears to be greater than 32-35%. - The influence of grain growth is found to be important in controlling corrosion, with high heat inputs favouring grain growth. - The cast materials generally behave in an inferior manner compared to the wrought materials. A generalised model for pitting corrosion in these materials is also presented.

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