Abstract
The effects of restoration and certain elements, nitrogen, sulphur, calcium and
Misch metal, on the hot ductility of austenitic, high-alloyed austenitic and
duplex stainless steels have been investigated by means of hot rolling, hot
tensile, hot bending and stress relaxation tests. The results of these different
testing methods indicated that hot rolling experiments using stepped specimens is
the most effective way to investigate the relationship between the softening and
cracking phenomena under hot rolling conditions. For as-cast, high-alloyed and
duplex stainless steels with a low impurity level, the cracking tendency was
observed to increase with increasing pass strain and temperature, being minimal
for the small strain of 0.1. No cracking occurred in these steels when rolled in
the wrought condition. It could be concluded that the cracking problems are only
exhibited by the cast structure with the hot ductility of even partially
recrystallised steel being perfectly adequate. However, the recrystallisation
kinetics of the high-alloyed austenitic stainless steels, determined by stress
relaxation and double-pass rolling tests, were found to be so slow that only
partial softening can be expected to occur between roughing passes under normal
rolling conditions. In the duplex steel, the restoration is fairly fast so that
complete softening can occur within typical interpass times in hot rolling, while
certain changes in the phase structure take place as well.
Sulphur was found to be an extremely harmful element in duplex stainless steel
with regard to their hot ductility so that severe cracking can take place with
sulphur content above 30 ppm. However, the effect of sulphur can be eliminated by
reducing its content and by calcium or Misch metal treatments that significantly
increase the number and decrease the average size of the inclusions. It seems
that the desulphurisation capacity of an element is the most important property
for assessing its usefulness in reducing the detrimental influence of sulphur.
The hot ductility of type 316L stainless steel determined by tensile tests was
found to be better for nitrogen content of 0.05 wt-% than 0.02%, while in
double-hit tensile tests the hot ductility values were identical. The mechanism
whereby nitrogen affects hot ductility remains unclear but a retarding effect on
static recrystallisation was observed.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-6560-2 |
| Date | 09 November 2001 |
| Creators | Kömi, J. (Jukka) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2001 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3213, info:eu-repo/semantics/altIdentifier/eissn/1796-2226 |
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