The improvement of thermal efficiency of power plants has provided the incentive for the development of the martensitic-ferritic 9-12%Cr creep-resistant steels. Good progress has been made in developing such steels, which are being used particularly in the wrought form as tubes and pipes for fossil fuelled power stations. They are also finding use in high temperature process plant within the oil and gas sector, and are being considered for use in generation IV nuclear designs. The high temperature conditions that these steels operate under in fossil fuelled power stations induce type IV cracking. This type of cracking occurs in the intercritical or fine grain region of the heated affected zone via a creep mechanism, and results in fractures with relatively little total cross-weld strain. Despite the occurrence of type IV cracking experienced in lower alloy predecessors, successor alloys have been introduced and widely used with insufficient consideration given to the consequences of welding them. Unfortunately, the newer steels suffer from reduced cross-weld creep strength due to type IV cracking to a greater degree in the temperature range of operation expected of them, and thus many failures by this mechanism have occurred. The subject of type IV cracking has been an area of active research interest. This review aims to serve as an update, drawing selectively on some of the vast amount of literature that has been published over the last 30 years.

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The improvement of tensile and creep rupture strength, achieved by the development of martensiticferritic 9 -12%Cr creep-resistant steels, known as creep strength enhanced ferritic (CSEF) steels, is attractive to designers who want to take advantage of them to improve power plant efficiency and reduce component wall thickness. However, although parent alloy developments have been making good progress, weldment performance has been of increasing concern. In particular, the cross-weld creep performance for CSEF steels is an issue, with Type IV cracking characterised by rupture in the outer region of the heat affected zone (HAZ) and low strain to failure being the typical features of failure in cross-weld tests. In this investigation, narrow gap TIG welding, flux-cored arc welding (FCAW) and reduced pressure electron beam (RPEB) welding were employed to produce a series of butt welds in a new European steel -FB2 -a wrought, boron-containing 9%Cr steel. These welds were subjected to metallurgical examination and mechanical testing, including toughness and cross-weld creep rupture tests. Results are compared with the well known steel grades 91 and 92. Welding process was found to have a marked effect on weld metal toughness, but not on the long term crossweld creep rupture performance. FB2 weldments were found to out-perform those of grades 91 and 92 in long term creep tests, but they did suffer from type IV cracking and reduced rupture strength, compared with the parent steel. The paper draws conclusions on the performance of the welded joints, and makes recommendations for further work and possible application within the power industry.

Boiler and steam piping components in power plants are fabricated using creep strength enhanced ferritic (CSEF) steels, which often operate at temperatures above 550°C. Modification of alloy content within these steels has produced better creep performance and higher operating temperatures, which increases the process efficiency of power plants. The improved materials, however, are susceptible to type IV cracking at the welded regions. A better understanding of type IV cracking in these materials is required and is the basis of the Technology Strategy Board (TSB) UK funded VALID (Verified Approaches to Life Management & Improved Design of High Temperature Steels for Advanced Steam Plants) project. In order to study the relationship between creep performance and heat input during welding, several welds with varying amounts of heat input and resultant HAZ widths were produced using the electron beam welding process. The welding parameters were developed with the aid of weld process modeling using the finite element (FE) method, in which the welding parameters were optimized to produce low, medium and high heat input welds. In this paper, the modeling approach and the development of electron beam welds in ASTM A387 grade P92 pipe material are presented. Creep specimens were extracted from the welded pipes and testing is ongoing. The authors acknowledge the VALID project partners, contributors and funding body: Air Liquide, Metrode, Polysoude, E.ON New Build & Technology Ltd, UKE.ON, Doosan, Centrica Energy, SSE, Tenaris, TU Chemnitz, The University of Nottingham, The Open University and UK TSB. Paper published with permission.

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