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Welding Procedures and Type IV Cracking Tendency - an Experimental Study

Welding Procedures and Type IV Cracking Tendency - an Experimental Study. J.A. Francis School of Materials University of Manchester V. Mazur Manufacturing and Materials Technology CSIRO Australia H.K.D.H. Bhadeshia Materials Science and Metallurgy University of Cambridge. Background.

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Welding Procedures and Type IV Cracking Tendency - an Experimental Study

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  1. Welding Procedures and Type IV Cracking Tendency - an Experimental Study J.A. Francis School of Materials University of Manchester V. Mazur Manufacturing and Materials Technology CSIRO Australia H.K.D.H. Bhadeshia Materials Science and Metallurgy University of Cambridge

  2. Background • Part of a collaborative project involving CSIRO Australia University of Cambridge. • Concerned with failure of welds in 9-12 wt % Cr ferritic power plant steels. • If implemented, these steels enable higher steam temperatures and pressures, greater thermodynamic efficiency, when compared with 2.25Cr-1Mo grades. • Aim: to develop technologies, procedures that ameliorate type IV cracking.

  3. Features of Type IV failures in 9-12 Cr Steels Parent material austenitised at 1040-1080 oC, tempered at 750-780 oC Intended service temperatures of ~ 600 oC Micrographs: V. Karthik et al., Welding Journal, 81 (12), 265s, 2002. • During service, localised void formation in FGHAZ/ICHAZ • Type IV limited creep life significantly lower than for parent plate. • Type IV failures predominate when applied stress is below a threshold. ICHAZ has lowest hardness after PWHT Rupture location tends from ICHAZ to FGHAZ as stress decreases

  4. Analysis of Published Cross-Weld Stress-Rupture Data Why Bayesian Neural Networks? Typical Network Structure • Most type IV studies have focused on the metallurgy. • Neural networks can capture mathematical relationships when physical models do not exist. • Neural networks can also perceive the relative importance of each input variable. • In a Bayesian framework, the dangers in extrapolating non-linear functions are reduced.

  5. The Database • 53 type IV failures included in database. • Overambitious set of variables can limit data available for analysis.

  6. Trends Results Significance of Input Variables

  7. Creep Testing P91 pipe welds, OD 356 mm, wall thickness 51 mm Root pass + hot pass : GTAW Filling passes: FCAW Filler: 1.2 mm Supercore F91 PWHT: 760 oC for 2 hours

  8. Stress (MPa) 0.8 kJ/mm, 30o, 250 oC 1.6 kJ/mm, 30o, 250 oC 2.4 kJ/mm, 30o, 250 oC 1.6 kJ/mm, 90o, 250 oC 1.6 kJ/mm, 0o, 250 oC 1.6 kJ/mm, 10o, 350 oC log (10) of creep life (h) Creep Testing Parameters varied: heat input, preheat temperature, joint preparation

  9. 1 mm Typical rupture surface corresponding to 0.8 kJ/mm, 250 oC, 30o

  10. Tests in Progress 5 further tests being initiated (“follow-up programme”) Aim to separate effects of preheat temperature and joint preparation Follow up programme Initial programme 250 oC 350 oC 250 oC 350 oC 350 oC 250 oC

  11. Conclusions • There is scope to improve “resistance” to type IV cracking through optimisation of welding procedures. • The weld heat input does not have a significant influence on propensity for type IV failure, which is in agreement with neural network predictions. • The joint preparation angle has a significant influence on type IV creep life, but the mechanism remains unclear. • Higher preheat temperatures have been predicted to improve resistance to type IV failures. Preliminary results suggest this is plausible but further tests are required to confirm effect.

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