Acicular ferrite (AF) formation potency of Ti-Rare earth metal (REM)-Zr (TRZ) complex oxide has been investigated in the simulated heat affected zone of low carbon steel. The TRZ complex oxide shows higher AF formation potency than Ti and Al oxides. A TRZ oxide particle is composed of REM-rich and Zrrich phases. AF crystals nucleate on the interface between austenite and the Zr-rich oxide phase, having a crystal structure that is face-centered cubic with lattice parameter of 0.44 nm. The Zr-rich phase and AF have an orientation relationship described by (011)AF//(011)Oxide, [100]AF//[ ]Oxide (the AF-TRZ orientation relationship), which allows good lattice coherency. It is suggested that the formation of this orientation relationship promotes AF nucleation on TRZ complex oxides. The AF also satisfies the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. It is considered that the coexistence of the AF-TRZ and K-S "three phase" orientation relationships is caused by variant selection of AF in addition to the formation of a rational orientation relationship between the Zr-rich oxide phase and the austenite matrix during the HAZ thermal cycle.KEY WORDS: heat affected zone; Ti-Rare earth metal-Zr complex oxide; acicular ferrite.

Hydrogen induced cracking (HIC) remains a prominent issue for oil and gas exploration in challenging environments. This assessment discusses HIC in light of hydrogen transport through pipeline steel microstructures and crack initiation and propagation processes. While there has been significant research in hydrogen permeation through steel alloys, additional understanding is necessary in microstructures specific to pipeline steels. Furthermore, a standard model for crack initiation and propagation processes needs to be established; a fracture mechanics based model, which has been used by some researchers, is presented in the present paper to predict crack propagation. Advanced characterisation techniques can help elucidate mechanisms of hydrogen induced crack growth. Ultimately, linking hydrogen transport and cracking processes during HIC will enable optimised alloy and microstructure design.

Crystal orientation relationships between acicular ferrite (AF), oxide and the austenite matrix have been investigated in low carbon steel weld by the submerged arc welding process. In particular, this study focused on the formation mechanism of the crystal orientation relationships. The AF microstructure was observed in weld metal containing titanium. Oxide particles were composed of MnTi2O4, amorphous phase and TiO2. The AF nucleated on MnTi2O4 having the Baker-Nutting (B-N) orientation relationship with the MnTi2O4 and Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. This result implies that the MnTi2O4 had a rational orientation relationship with the austenite matrix. The orientation relationship is considered to be (001)MnTi2O4//( 11)γ, [100]MnTi2O4//[211]γ from the viewpoint of the lattice coherency. It is supposed that the MnTi2O4 can be formed within oxide particles having this orientation relationship with the austenite matrix at high temperature during welding process. This mechanism allows the coexistence of both B-N and K-S orientation relationships, which lowers the AF/MnTi2O4 and AF/austenite interfacial energies. This results in the decrease of the activation energy for AF nucleation.KEY WORDS: acicular ferrite; weld metal; Mn-Ti oxide.

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