高クロム-フェライト鋼の歪時効について

抄録

High-Cr ferritic steels do not show a clear yielding phenomenon on room temperature tensile test after proper heat treatment, although dislocation locking by C and N atoms is expected in such a body-centred cubic metal as indicated by Cottrell and others. Because this is attributed to the strong chemical affinity of C and N to Cr, it might be well anticipated that the yielding phenomenon will be observed when these steels increase their internal stress by plastic deformation or by quenching from sufficiently high temperature and then being subjected to aging treatment at relatively lower temperatures. Thus at first, the existence of the above mentioned strain-aging phenomenon was confirmed by tensile test of AISI Type 446 steels. A specimen strained 5% in length showed a clear yielding after heating only for 20 min at 200°C while no singularity appeared in the stress-strain diagram of that treated at 100°C up to 100 hr. Then the effect of interstitial atoms such as C and N and that of Cr were investigated. The results revealed that nitrogen is more effective on strain-aging than carbon and that the yield point is more perceptible when the Cr content is increased up to 35%. Although no quench-aging was observed on the specimen quenched in water from 1200°C and aged for 100 hr at 200°C but the specimens air-cooled from 750∼1000°C showed a flattening of stress-strain curve or even a yielding on room temperature tensile test. The hardness increased also by strain aging. In this case, the increase was observed even after an aging at 100°C on specimens prestrained more than 5% in length. Highly strained specimens showed larger increase which completed within an hour and a softening due to overaging is observed by longtime aging at 300°C. The impact test of cold rolled and aged specimens disclosed the fact that the brittle-ductile transition temperature shifts to the higher side and consequently a remarkable embrittlement was observed on the impact test below 50°C. No structural change has been observed on microscopic examination of strain-aged specimens.