A novel solid-state joining method called Cold Spot Joining (CSJ) has been successfully developed. In this joining concept, the material near the joining interface is plastically deformed under high pressure to form a joining interface, resulting in the fragmentation of oxide films at the joining interface and the formation of strong interface. Medium carbon steel sheets were CS-joined under various process conditions. The joining temperature can be varied by the applied pressure during CSJ. Microstructural observations and hardness distribution indicated that the appropriate pressurization resulted in joining temperatures below the A₁ point and suppressed the formation of the brittle martensitic structure. By providing appropriate applied pressure, sound S45C spot-welded joints were successfully produced, showing plug failure of the base metal in both tensile shear and cross-tension tests. Further investigation into the mechanism of interface formation reveals that the oxide film at the interface is fragmented and expelled. At the same time, dynamic recrystallization occurs at the interface and extremely fine new grains with dispersed fine cementite are formed at the interface to achieve the sound joining with sufficient strength.

In this paper, fatigue crack initiation and propagation properties of three structural steels with different static strength and cyclic softening behavior were evaluated and compared with the estimation results based on the local strain response. Steel C had the highest cyclic softening rate, which was 10 times that of steel A and twice that of steel B. The relationship between strain range and fatigue life in the fatigue life region shorter than 10⁵ cycles was almost the same regardless of the test steels. The fatigue crack initiation life from the notch bottom of the SENT specimen was almost the same independent of static strength and cyclic softening rate. The crack initiation life estimated from the Strain range versus fatigue life equation using the local strain response measured by DIC was roughly in agreement with the experimental results. Steel C had the highest crack opening load and the slowest fatigue crack propagation rate compared to the other two steels. The local strain range at the fatigue crack tip showed a good correlation with the fatigue crack propagation rate irrespective of the steel grade. In addition, the estimates of fatigue crack propagation rate based on the local strain response were in almost agreement with the experimental results.

The effect of boron addition on low cycle bending fatigue strength of carburized steel was investigated. The low-cycle bending fatigue strength of B-added steel was improved compared to SCM420. When the fatigue failure was divided into the crack initiation process and the crack propagation process, the addition of boron did not affect the crack initiation life, but improved the crack propagation life. The presence of boron in B-added steels was evaluated using TOF-SIMS and AES measurements. Grain boundary segregation of boron was not observed in the gas carburized surface layer, but precipitation of BN was observed. On the other hand, grain boundary segregation of boron was observed from the carburized surface layer toward the 400 μm core side. Therefore, it is considered that the grain boundary strengthening effect of boron was not obtained in the carburized surface layer, and the crack initiation life did not change.

In order to clarify the reason why the crack propagation life is improved by boron addition, the initial crack observation after crack initiation and the crack propagation test was carried out. From the observation of the pop-in crack, it was found that the pop-in crack length was shorter in the B-added steel than in the SCM420 steel. On the other hand, the crack propagation test showed no difference in crack propagation rate between SCM420 and B-added steel. Therefore, the decrease in pop-in cracks due to the addition of boron is considered to be the main factor for the improvement in crack propagation life.