Influence of welding process on Type IV cracking behavior of P91 steel M. Divya a , C.R. Das a,n , S.K. Albert a , Sunil Goyal a , P. Ganesh b , R. Kaul b , J. Swaminathan c , B.S. Murty d , L.M. Kukreja b , A.K. Bhaduri a a Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India b Laser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India c National Metallurgical Laboratory, Jamshedpur 831007, India d Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India article info Article history: Received 15 October 2013 Received in revised form 24 June 2014 Accepted 24 June 2014 Available online 1 July 2014 Keywords: Laser welding Shielded metal arc welding P91 steel Type IV cracking Lath martensite Finite element analysis abstract Influence of laser welding (LW) and shielded metal arc welding (SMAW) processes on Type IV cracking behavior of modified 9Cr–1Mo (P91) steel has been investigated in this paper. The study involved comparison of stress rupture lives of modified 9Cr–1Mo steel weldments prepared by SMAW and continuous wave CO 2 laser welding processes. Width of the heat affected zone (HAZ) in laser weldment was found to be 1.0 mm, whereas it was 2.5 mm in SMAW weldment. The rupture lives of laser weldment were found to be higher than SMAW weldment at higher stress level and comparable at lower stress level. Under similar stress levels, the creep rupture lives of 875 1C heat treated specimens were found to be lower than that of the base metal and cross weld specimen. These results clearly suggest that the instability of microstructure in the intercritical heat affected zone (ICHAZ) is responsible for lower creep rupture lives of P91 steel weldment than the base metal. The experimentally observed variations in creep cavitation have been corroborated with the results of finite element (FE) analysis. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Creep strength of the high chromium ferritic steels weldments is inferior to that of the base metal. These weldments fail in the weakest regions of the heat affected zone (HAZ), which is sandwiched between the base metal on one side and the weld metal on the other side. Creep cavitation in these regions becomes the life limiting factor for the welded components during high temperature service. Failure of the weldment in these regions under creep conditions is known as “Type IV” cracking. Type IV cracking is associated with the microstructural degeneration in the HAZ of the weldment during welding and long-term service exposure [1–8]. The microstructure in the HAZ is influenced by the peak temperatures experienced during weld thermal cycles. Based on these peak temperatures, the phase transformation occur locally which causes variations in microstructure in the HAZ. Therefore, the HAZ can be divided into three zones, viz., coarse grain (CGHAZ), fine grain (FGHAZ) and intercritical (ICHAZ) heat affected zone. Among these zones, FGHAZ and ICHAZ are the weakest regions. These regions experience peak temperatures for extremely short duration during welding which are expected to be just above A c3 (FGHAZ) and between A c1 and A c3 (ICHAZ) tem- peratures. This causes solid state phase transformation and the transformation products are distinctly different than those of the base metal and weld metal. Lath martensite forms in the FGHAZ have less carbon than those forms in the CGHAZ. In addition to that, the precipitates do not dissolve completely in this region. As a result, the strength of this region is less compared to the CGHAZ. But in the ICHAZ, the partial transformation of tempered lath martensite to austenite as well as partial dissolution of precipitates takes place during heating cycle which results in formation of low carbon martensite during cooling. During post weld heat treat- ment (PWHT) less precipitates form in these regions compared to the CGHAZ. However, the coarsening of partially dissolved precipitates occurs rapidly resulting in the drop in hardness and strength in the FGHAZ and ICHAZ. Higher strength of CGHAZ/weld metal and the base metal than the weakest regions imposes a microstructural constraint to the deformation behavior in these regions which leads to the development of multi-axial loading conditions under creep condition [1–5]. This constraint effect leads to the creep cavitation induced intergranular failure during long- term service. To mitigate the “Type IV” cracking in 9Cr steels, advanced 9–12Cr ferritic steels have been developed in recent years [6–8]. Stable microstructure of these steels at high tempera- ture improves the creep strength. On the other hand, use of advanced welding processes, like laser and electron beam welding Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.06.089 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 44 27480118. E-mail address: chitta@igcar.gov.in (C.R. Das). Materials Science & Engineering A 613 (2014) 148–158