Microstructural and Property Changes during Short-Term Ageing of Cast Austenitic Stainless Steels
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MICROSTRUCTURAL AND PROPERTY CHANGES DURING SHORT-TERM AGEING OF CAST AUSTENITIC STAINLESS STEELS M. Hodgkinson and T.A. Towers Department of Mechanical Engineering & Metallurgy, Teesside Polytechnic,
UK.
ABSTRACT 0
The Charpy v-notch impact properties at -196 C of casts of Type 316 austenitic stainless steel have been compared, after ageing at 650%C for time periods up to 90 minutes. The impact energy of all steels depends on ageing time; the degree of response to ageing depends on original delta ferrite content, which varied from -0.5% to 5.0% in the steels studied. For steels with delta ferrite content in the lower half of this range the impact energy, during ageing, initially decreases to a minimum, subsequently rises to a maximum and finally decreases. Steels with delta ferrite contents near 5% suffer a continuous decrease in impact energy as ageing time increases. Microstructural investigation reveals, during early stages of ageing, the rapid formation of sigma phase, at the ferrite-austenite interphase boundaries. Some delta concurrently transforms to new austenite. Microfractographs indicate a change in the nature of the fracture as ageing progresses. Initially, fractures have large dimples, but as ageing progresses, a finer dimpling appears and extends in area. These dimples contain fine particles, identified mainly as MnS. The variations in impact behaviour are explained on the basis of the structural changes observed. INTRODUCTION
316 austenitic stainless steel has been widely used in high-temperature applications, such as power plant, including nuclear reactor design. It was originally chosen for its oxidation resistance and increased in use owing to its creep resistance. However, low creep rupture ductility may occur owing to the instability of the structure leading to the formation of sigma and other phases. The tendency for sigma formation is increased by the presence of delta-ferrite which contains a higher proportion of the "sigma forming" elements Cr, Mo and Si than the matrix austenite and acts as a focus for the transformations occurring at elevated temperatures. Type 316 weld metals contain 4-10% delta-ferrite, deliberately introduced to overcome hot-cracking problems which can be particularly severe in fully austenitic deposits. For these reasons a great deal of research has taken place into the stability of austenitic stainless steels on long-term exposure at elevated temperatures [1,2,3,4]. Austenitic stainless steels are also widely used in cryogenic applications, being regarded as sufficiently tough at temperatures as low as 0 -196 C. However, there have been indications [5] that even short-term heating of type 316, as may be experienced in multi-pass weld deposits, can adversely affect mechanical properties by the promotion of undesirable phase transformations. In particular, impact tests indicate that a lower-shelf energy may arise which, although above minimum values required by welding codes, may not guarantee immunity from low-energy fracture [6]. Szumachowski and Reid [7] studied
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