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Type IV Cracking in 9-12%Cr Creep Strength Enhanced Steels

 

Review of Type IV Cracking in 9-12%Cr Creep Strength Enhanced Ferritic Steels

TWI Industrial Member Report Summary 958/2010

D J Abson and J S Rothwell

Background

Improved thermal efficiency of power plants has been the main driver for the development of the martensitic-ferritic 9-12%Cr creep-resistant steels, which are commonly known as creep strength enhanced ferritic (CSEF) 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, but are also finding use in high temperature process plant within the oil and gas sector. Furthermore, the nuclear industry is considering these steels for use in future, generation IV nuclear designs. The high temperature conditions that these steels operate under in fossil fuelled power stations encourage type IV cracking. This type of cracking occurs in the intercritical or fine grain region of the HAZ via a creep mechanism and results in fractures with relatively little total strain across the weld. Despite the occurrence of type IV cracking experienced in lower alloy predecessors, successor alloys have been introduced and widely used with apparently little consideration being given to the consequences of welding them. Unfortunately the newer steels have been found to 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 failures by this mechanism continue to occur. The subject of type IV cracking was reviewed previously at TWI (Bell, 1997) but this has been an area of very active research interest and an update of the review was required. This review aims to update the earlier review, drawing selectively on some of the vast amount of literature that has been published in the intervening years.

Objective

Review the phenomenon of type IV cracking, particularly with respect to 9-12%Cr steels, and to identify any associated technology gaps that exist.

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