Review of type IV cracking of weldments in 9–12%Cr creep strength enhanced ferritic steels

Abstract

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.

The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel

“…A shorter creep rupture life and higher creep strain were identified in material that was exposed to the thermal cycles with a peak temperature close to the Ac 3 temperature. [19] Microstructural characterization was also conducted on HAZ simulation materials in more recent work (e.g., References 8 and 20). Evidence showing an inhomogeneous distribution of precipitate particles due to microsegregation was indicated after weld thermal cycle simulations with a peak temperature between the Ac 1 and Ac 3 temperatures.…”

“…[7,19] These materials are typically produced by exposing the raw parent metal materials to controlled heat cycles that simulate weld thermal cycles using a Gleeble simulator or a dilatometer. [8,20] In these works, comparisons of creep behavior were made between the HAZ simulation materials that were exposed to heat cycles with various peak temperatures under a range of creep testing conditions.…”

Study of cracks in the weld metal joint of p91 steel of a superheater steam pipe

“…Creep cracks can manifest in preferential places of the joint. Several authors [5][6][7] have classified these cracks in type I to IV, according to the location in the joint. Types I and II are formed in the weld metal (WM), whereas type II propagates to the heat affected zone (HAZ).…”

Potential drop monitoring of creep damage at a weld

“…Overwhelmingly, concern around girth weld failure is associated with Type IV failure [1][2][3][4][5]. High chrome steels are known to be particularly susceptible and the widespread adoption of P91 material means that there is considerable concern in the power industry at present [1,2,6,7]. Type IV damage accumulates in the intercritical heat-affected zone (ICHAZ) of a weld, a narrow band of material located at the edge of the HAZ adjacent to the parent material as shown in Fig.…”

“…It is usually assumed that the axial stresses are much smaller than the circumferential stresses [1] and therefore girth welds tend to fail in a 'leak before break' manner. Overwhelmingly, concern around girth weld failure is associated with Type IV failure [1][2][3][4][5]. High chrome steels are known to be particularly susceptible and the widespread adoption of P91 material means that there is considerable concern in the power industry at present [1,2,6,7].…”