Ageing Phenomena of Tensile Properties and Behaviours of Hydrogen in Killed Steel

Abstract

Ageing behaipurs of tensile properties and correlation of the content of hydrogen in killed steels have been carried out and the results obtained are summarized as follows:
1. There was a clear recovery of ductility in terms of total elongation and reduction of area owing to ageing.
2. Change in yield stress or 0.2% proof stress was shown to be considerably large in some cases. This phenomenon seemed to be more complex in comparison with that of ductility because the change in yield stress was much dependent on preparation of tensile specimens; yield stress was lowered as the result of ageing in the test pieces with the full thickness or diameter of hot rolled products, and in the test pieces machined out from the center of the products, yield stress was increased, and the smaller the test pieces was, the more increase the yield stress was.
3. Although the ageing phenomena may not be directly related to hydrogen in the steel, the hydrogen contents of specimens were analysed on various locations in the fractured test pieces.
When ageing was in progress, hydrogen was extremely enriched in the region near the fractured part of the specimen, and when hydrogen level was naturally decreased to some extent after the completion of the ageing, and the enrichment of hydrogen near the fractured edge also became very slight.
4. The loss of ductility is thought to be caused by an extreme enrichment of hydrogen to the stressed region and not primarily dependent on the whole concentration of hydrogen. The recovery of ductility due to ageing could also be explained in terms of hydrogen contents. But, because of different ageing behaviour, the change in yield stress during aging was not attributable to hydrogen behaviour.