Stress corrosion cracking (SCC) occurs in shrouds and piping of low carbon austenitic stainless steels at nuclear power plants. A work-hardened layer, where the transgranular SCC initiates, is considered to be one of the probable cause for this occurrence. In order to clarify the microstructural characteristics of work-hardened layer at the surface of shrouds or piping, the strengthen analysis of low carbon austenitic stainless steel, 316SS, rolled at the reduction in area,
RA, of 10, 20, 30, 40 and 50% at room temperature were conducted on a nanoscopic scale, using an ultra-microhardness tester, TEM and SEM. TEM and SEM observation showed that the microstructural parameters are the dislocation cell size,
dcel, coarse slip spacing,
lcsl, and austenitic grain size,
dγ. Referring 10
dcel and 10
lcsl, Vickers hardness,
HV, corresponding to macro strength was expressed as
Hv=
Hvbas*+
Hvsol*+
Hvdis*+
Hvcel*+
Hvcsl*.
Hvbas* (= 100) is the base hardness,
Hvsol* is the solid solution strengthening hardness,
Hvdis* is the dislocation strengthening hardness in the dislocation cell, and
Hvcel* and
Hvcsl* are the fine grain strengthening hardness due to the dislocation cell and coarse slip.
Hvsol* was about 50, independently of
RA.
Hvdis* was zero at
RA<30% and increased at
RA>30%.
Hvcel* and
Hvcsl* increased with increasing in
RA and were kept constant at about 50 and 120 at
RA=20 and 30%, respectively. It was suggested from these results that all dislocations introduced by rolling might be dissipated for the creation of dislocation cells and coarse slips at
RA<30% and that the microstructure contributing to the fine grain strengthening due to the dislocation cell and coarse slip might be accomplished at
RA=30%. The dislocation strengthening in the dislocation cell might begin to operate at
RA>30%.
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