Influence of Ti/N ratio on simulated CGHAZ microstructure and toughness in X70 steels

Abstract

Three API 5L X70 steels with different Ti and N contents and otherwise identical chemistry were selected to investigate the effect of Ti/N ratio on the toughness in coarse grained heat affected zone (CGHAZ). A Gleeble 3500 thermomechanical simulator was used to simulate the thermal profile of CGHAZ of double submerged arc welding process. The microstructure was examined by optical microscopy. Statistics of CGHAZ grain coarsening were compiled by measuring the prior austenite grain size. Toughness of the simulated CGHAZ regions was evaluated by Charpy V-notch testing at −20 and −40°C. Morphology of the impact fracture surface was investigated using SEM. Steel B with Ti/N ratio of 3·22 (slightly below stoichiometric) showed slightly higher toughness in the simulated CGHAZ due to higher volume fraction of austenite grains less than 80 μm in diameter.

Keywords

Austenite grain size Charpy impact toughness CGAHZ X70 pipeline steels Ti/N ratio Gleeble simulation

Get full access to this article

View all access options for this article.

References

, Lee

, Jang

: ‘Fracture toughness anisotropy in a API steel line-pipe’, Mater. Lett., 2007, 61, 5178–5180.

Pan

, Lee

: ‘Development of TiO_x-bearing steels with superior heat-affected zone toughness’, Mater. Des., 1994, 15, 331–338.

Tomita

, Saito

, Tsuzuki

, Tokunaga

, Okamoto

: ‘Improvement in HAZ toughness of steel by TiN-MnS addition’, ISIJ Int., 1994, 34, 829–835.

Mitchell

, Hart

, Morrison

: ‘The effect of microalloying on HAZ toughness’, Proc. Int. Conf. on ‘Microalloying ’95’, Pittsburgh, PA, USA, June 1995, the Iron and Steel Society, 149–162.

Suzuki

, Ichimiya

, Akita

: ‘High tensile strength steel plates with excellent HAZ toughness for shipbuilding: JFE EWEL technology for excellent quality in HAZ of high heat input welded joints’, JFE Technical Report No. 5, 2005, 24–29.

Laitinen

: ‘Improvement of weld HAZ toughness at low heat input by controlling the distribution of M-A constituents’; 53; 2006, Oulu, Oulu University Press.

Ei-Kashif

TKE

:‘Alloy steel – properties and use’( Morales

E V

, ), Chapter 4, 87–108; 2011, Croatia, InTech.

Batte

, Boothby

, Rothwell

: ‘Understanding the weldability of niobium-bearing HSLA steels’, Proc. Int. Symp. on ‘Niobium’, Orlando, FL, USA, December 2001, Minerals, Metals and Materials Society, 931–958.

Gladman

, Dulieu

: ‘Grain-size control in steels’, Met. Sci., 1974, 8, 167–176.

10.

Pickering

: ‘Titanium nitride technology’, Proc. 35th Mechanical Working and Steel Processing Conf., Pittsburgh, PA, USA, October 1993, Iron & Steel Society, 477–491.

11.

Kejian

, Baker

: ‘The effects of small titanium additions on the mechanical properties and the microstructures of controlled rolled niobium-bearing HSLA plate steels’, Mater. Sci. Eng. A, 1993, A169, 53–65.

12.

Penniston

, Collins

, Hamad

: ‘Effects of Ti, C and N on weld HAZ toughness of high strength line pipe’, Proc. 7th Int. Pipeline Conf., Calgary, Canada, September–October 2008, International Petroleum Technology Institute and the Pipeline Division, 75–83.

13.

Smaill

, Keown

, Erasmus

: ‘Effect of titanium additions on the strain aging characteristics and mechanical properties of carbon-manganese reinforcing steels’, Met. Technol., April 1976, 194–201.

14.

Mukae

, Nishio

, Katoh

: ‘Solution of TiN during synthetic weld thermal cycling and heat affected zone toughness in low carbon steels’, Trans. Jpn Weld. Soc., 1987, 18, 148–158.

15.

Bhadeshia

: ‘Thermodynamic analysis of isothermal transformation diagrams’, Met. Sci., 1982, 16, 159–166.

16.

Babu

: ‘Acicular ferrite and bainite in steels’, PhD thesis, University of Cambridge, Cambridge, UK, 1992.

17.

McGurk

, Speer

, Matlock

: ‘Effect of alloy modifications on haz microstructure and properties of HSLA X80 pipeline steel’, Mater. Sci. Technol., 2006, 6, 347.

18.

Kim

, Lee

: ‘Effects of grain size on fracture toughness in transition temperature region of Mn-Mo-Ni low-alloy steels’, Mater. Sci. Eng. A, 2003, A359, 198–209.