Microstructure and properties of inter-critically reheated coarse grained heat-affected zones in X70 steels with different Ti/N ratios

This work investigates the effect of different levels of Ti, N and Ti/N ratios on microstructure and properties in the inter-critically reheated coarse grained heat affected zone (IC-CGHAZ) of double submerged arc welds (DSAW) in API 5L grade X70 pipe. Gleeble simulation was employed to reproduce the IC-CGHAZ of the real DSAW welds. Hardness and Charpy V-notch (CVN) tests were performed on the simulated samples. The microstructure of the simulated IC-CGHAZ was characterised by optical microscopy and scanning electron microscopy. LePera colour etching technique was employed to identify and quantify the martensitic-austenitic (M-A) constituent. Results show that the simulated IC-CGHAZ exhibited extremely low toughness, but in the studied range of the Ti and N, there was no correlation with Ti/N ratio. The beneficial effect of near-stoichiometric Ti/N ratio observed in CGHAZ did not translate to IC-CGHAZ. This was due to the negative effect of the blocky M-A constituent formed on prior austenite grain boundaries overriding the slight improvement arising from austenite grain size control.

Zhu, Z., Kuzmikova, L., Li, H., Barbaro, F., Fletcher, L., Zheng, L. & Bai, M. (2015). Microstructure and properties of inter-critically reheated coarse grained heat-affected zones in X70 steels with different Ti/N ratios. 9th Pipeline Technology Conference 2014 (pp. 1-13).

This work investigates the effect of different levels of Ti, N and Ti/N ratios on microstructure and properties in the inter-critically reheated coarse grained heat affected zone (IC-CGHAZ) of double submerged arc welds (DSAW) in API 5L grade X70 pipe. Gleeble simulation was employed to reproduce the IC-CGHAZ of the real DSAW welds. Hardness and Charpy V-notch (CVN) tests were performed on the simulated samples. The microstructure of the simulated IC-CGHAZ was characterised by optical microscopy and scanning electron microscopy. LePera colour etching technique was employed to identify and quantify the martensitic-austenitic (M-A) constituent. Results show that the simulated IC-CGHAZ exhibited extremely low toughness, but in the studied range of the Ti and N, there was no correlation with Ti/N ratio. The beneficial effect of near-stoichiometric Ti/N ratio observed in CGHAZ did not translate to IC-CGHAZ. This was due to the negative effect of the blocky M-A constituent formed on prior austenite grain boundaries overriding the slight improvement arising from austenite grain size control.

Zhu, Z., Kuzmikova, L., Li, H., Barbaro, F., Fletcher, L., Zheng, L. & Bai, M. (2015). Microstructure and properties of inter-critically reheated coarse grained heat-affected zones in X70 steels with different Ti/N ratios. 9th Pipeline Technology Conference 2014 (pp. 1-13).

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