Toshiyuki Manabe

Change in the microstructure and mechanical properties of drawn pearlitic steel with low-temperature aging

Hydrogen embrittlement is a serious problem in high-strength steels. Drawn pearlitic steel shows excellent resistance to hydrogen embrittlement despite its high strength, and aging treatment at a low temperature can simultaneously improve its strength and hydrogen-embrittlement resistance. To clarify the mechanism for this we have used thermal desorption analysis (TDA) and the newly developed precession electron diffraction analysis method in the transmission electron microscope. After aging at 100 °C for 10 min, the amount of hydrogen seen amount on the TDA curve reduced at around 100 °C. In contrast, when aging was performed at 300 °C, the hydrogen amount further reduced at around 100 °C and the unevenly deformed lamellar ferrite zone was locally recovered. For the samples that were aged at the low temperature, we confirmed that their yield strength and relaxation stress ratios increased simultaneously with improvement in the hydrogen-embrittlement property. We infer that segregation of carbon or formation of very fine carbide in dislocations during aging is the cause of these behaviors.

Effect of Carbon Content on Bainite Transformation Start Temperature on Fe–9Ni–C Alloys

Upper bainite in steels has many common features with lath martensite in steels. But there are some studies that indicate that bainite transformation start temperature (Bs) is greater than T0 on steels containing high carbon content. We measured Bs on Fe–9 mass% Ni alloys containing 0.003–0.89 mass% C. In low carbon alloys, Bs is below T0, and the increasing of carbon content decreases Bs standing in a line that is parallel to T0. On the other hand, in high carbon alloys, carbon content does not affect Bs which stands around 753 K. The border between these two tendencies and Bs in high carbon alloys seem to correspond to the intersection point between the line of Bs on low carbon alloys and the calculated γ/(γ + θ) phase boundary.