Environmental cracking of type 316 austenitic stainless steel weldments in high temperature co 2 gas
- PDF / 4,092,751 Bytes
- 16 Pages / 630 x 792 pts Page_size
- 82 Downloads / 270 Views
I.
INTRODUCTION
TYPE316 austenitic stainless steels are used widely within the electricity supply industry for plant which operates under onerous conditions of stress, environment, or at high temperatures, l~l Since these plant components are generally of a complex geometry, many contain weldments that are made using a range of austenitic steel weld metals. All the major welding processes 12'3] are used for the production of the weldments and as a consequence two potential problems must be considered to ensure their integrity. First, cracking can develop within the weld metal due to inadequate control of weld rod composition and welding procedure; second, localized corrosion can occur preferentially in the associated heat affected zone due to the formation of specific microstructures. The latter is of particular significance in relation to the general aqueous corrosion or stress corrosion cracking behavior and is dependent upon the microstructures and residual stresses developed by the welding thermal cycle. [3-9] The phenomenon of environmentally assisted cracking within a material depends upon the complex interaction between the microstructural condition, the environment, and the total stress-time history.rl~ Within a weldment in an austenitic stainless steel a wide range of microstructures and residual stresses are developed as a consequence of the thermal cycle produced during the welding process. Subsequently these weldments can be exposed to aggressive environments developed either by the welding process or arising from the service conditions. In order to assess the integrity of such a weldment it is essential, therefore, to establish the combination of microstructures, stresses, and environments that could render a component containing the
D.J. CHASTELL and P.J. NORMAN, Research Officers, P. DOIG, Group Head, and P. E.J. FLEWITT, Section Manager, are with Central Electricity Generating Board, Scientific and Technical Branch, Gravesend, Kent, United Kingdom. Manuscript submitted April 29, 1987.
METALLURGICALTRANSACTIONS A
weldment susceptible to environmentally assisted cracking. tl~ Cracking may be prevented by reducing or eliminating any one of these contributions since their interaction is synergistic rather than additive. It has been recognized for about three decades that the microstructural feature that renders austenitic stainless steels susceptible to intergranular corrosion is the intergranular precipitation of carbides, usually Mz3C 6 type, with kinetics controlled by the diffusion of substitutional chromium to the grain boundary. 111-14]Slow cooling through, or heat treatment within, a temperature range = 1073 to =773 K affects the precipitation of these chromium rich precipitates at grain boundaries which results in a local depletion in chromium about these boundaries, thereby rendering the material susceptible to stress corrosion cracking or intergranular attack in certain environments, tT'n'15-181 The width of the chromium depleted zone has been estimated using diffusion calculations I14'19
Data Loading...
