Lattice Misfit between Inclusion and Acicular Ferrite in Weld Metal of Low Carbon Low Alloy Steel
To clarify the mechanism of acicular ferrite formation in low carbon submerged arc weld metals of a Ti-B system with different aluminum contents, the lattice misfit between inclusion and acicular ferrite was investigated. The inclusions were directly sliced into thin foils by a focused ion beam device and crystallographic analyses were performed using a transmission electron microscope. These inclusions were surrounded by a narrow Ti-enriched layer which was identified as TiO. Our results showed that the Baker-Nutting orientation relationship was satisfied between the narrow TiO on the inclusion surface and acicular ferrite.
Ferrite Nucleation at Ceramic/Austenite Interfaces
Pure polycrystal ceramics (TiC, TiN, TiO, VN, AIN and Al2O3) were heat-pressure-bonded to an Fe-0.1 mass%C-2 mass%Mn alloy and were isothermally reacted below the Ae3 temperature for the proeutectoid ferrite reaction to occur. The number of ferrite particles per unit area of bonded interfaces was measured using Schwartz-Saltykov method and was compared with the number of particles formed at grain boundaries. VN was most effective in stimulating ferrite nucleation whereas TiO was least effective. All other ceramics showed more or less similar potencies for ferrite formation. VN is capable of forming lower energy interfaces with ferrite than other ceramics, through the possibility of V diffusion into the alloy during and after bonding may not be ruled out in this experiment. On the other hand, the least effectiveness of TiO in stimulating ferrite nucleation is possibly due to the proximity of thermal expansion coefficient to iron.
A Review on Inclusion-Assisted Microstructure Control in C-Mn and Low-Alloy Steel Welds
Inclusion-assisted microstructure control has been a key technology to improve the toughness of C-Mn and low-alloy steel welds over the last two to three decades. Microstructure of weld metals and heat-affected zone (HAZ) is refined by different inclusions, some of which work as a nucleation site of acicular ferrite or intragranular ferrite and some of which as pinning particles against the grain growth of austenite. In the present paper, mechanisms proposed for intragranular ferrite formation in weld metals and HAZ are reviewed and the feasibility of each mechanism is examined. Factors affecting the intragranular ferrite formation in weld metals and HAZ are compared to find consistent explanation applicable to the both. As the result, by considering the difference of microstructural characteristics between weld metals and HAZ, rational, self-consistent explanations are given to why lattice matching is preferred when acicular ferrite is formed on inclusions in weld metals and why manganese depletion works only in HAZ for the formation of polygonal intragranular ferrite.