The effects of tensile and compressive dwells on creep-fatigue behavior and fracture mechanism in welded joint of P92 steel

Highlights

• Longer CF life under tensile dwell in comparison to compressive dwell.

• Different local creep behaviors were observed for the two dwell modes.

• The hardness, elastic modulus and creep resistances near fracture edges were clearly reduced.

• Two independent fracture mechanisms were suggested for tensile and compressive dwells.

Abstract

The creep-fatigue interactions on the long-term service damage of P92 welded joint were investigated on the tensile and compressive holdings with a series of applied strains, respectively. The local mechanical properties including hardness, elastic modulus and creep resistance of welded joint were uncovered using nanoindentation. The features of fracture morphology and internal defects after CF tests were studied by scanning electron microscope. Based on the characteristic of microstructural evolution and the variation in local mechanical properties, holding type effects of creep-fatigue interaction on the local creep behavior and fracture mechanism of the welds were systematically investigated.

Introduction

Welding technologies are widely used for construction of modern fossil fuel and nuclear power plants [[1], [2], [3]]. Welded joints are a critical part of the welded structure and typically have strongly graded microstructure, transitioning from PP through the heat affected zone (HAZ, which can be divided into fine-grained and coarse-grained regions) to the fusion zone, or WM. The heterogeneity in microstructure and local mechanical properties of the welded joint leads to high-temperature instability of the welded structure [[4], [5], [6]]. In power plants, welded joints experience CF loading due to frequent start-ups and shut-downs coupled with high temperature operation [[7], [8], [9]]. Under high-temperature CF loading, several damage processes occur simultaneously: softening under cyclic loading, accumulation of creep damage during holding periods (dwell) and a contribution from creep-fatigue [[8], [9], [10]]. This is a complex interaction that can result in premature failure of the welded joints, particularly at operating temperatures beyond 600 °C for steels [[11], [12], [13]]. To assist in an improved evaluation of the remaining life of existing power plant, it is therefore essential to understand local creep behavior and the fracture mechanisms in welded joints after they have experienced prior CF loading.

In recent decades, investigations on the remnant creep properties of homogeneous materials after prior fatigue or CF loading have attracted significant attention [[14], [15], [16], [17], [18]]. For example, Sarker et al. [14] performed conventional creep tests after prior fatigue loading, and found that the reduction in remaining creep life was related to the number of prior fatigue cycles. Zhang et al. [18] studied the remnant tensile strength and creep behavior of P92 steel after prior CF and concluded that the reduction in remnant creep resistance was commonly caused by the growth of martensitic laths. Other work has found that mode of holding period, i.e. tensile or compressive, strongly affects the creep-fatigue resistance of steels [19,20]. Fournier et al. [19], in work on P91 steel, found that periods of compressive dwell were more deleterious than tensile dwell and that this phenomenon was more pronounced at low strain ranges. Gao et al. [20] found that intergranular long cracks were formed under tensile stresses, while transgranular short cracks were occurred under compressive stresses for 316 stainless steel. However, there are still only a limited number of investigations that have focused on the remnant creep properties of homogeneous materials after prior CF loading with tensile and compressive dwell periods of differing length. Since the CG and FG HAZ microstructures are narrow (1–2 mm) their mechanical heterogeneity cannot be precisely measured using conventional uniaxial tension, compression or bending tests. In this scenario, nanoindentation dwell tests have become a popular and effective method to study creep deformation in numerous materials, particularly high entropy alloys, e.g. metallic glasses, where the work uses small samples, often with surface coatings and a strongly heterogeneous structure [[21], [22], [23], [24], [25]]. Nanoindentation offers the advantages of ultra-high sensitivity to displacement and load and provides a high shear-compressive stress region beneath the indenter, allowing creep deformation to be clearly observed at ambient temperatures even for materials with a high-melting point [[21], [22], [23]]. The local creep behavior of the HAZ regions in welded joints of SA508 Gr3 steel has also been studied using nanoindentation [26]. Under long-term high-temperature creep flow, voids have been observed to form at grain boundaries in the HAZ, with their density being significantly higher in the FG HAZ and also increasing with creep time [18,27]. Creep voids act as local stress concentration sites during periods of dwell loading and promote further creep deformation and voiding, eventually leading to the initiation of small cracks. Creep during nanoindentation is induced from the high-stress region beneath the indenter and can therefore be used to investigate the local creep resistance of alloys that have experienced previous creep or CF damage.

Furthermore, under CF loading, crack propagation in fatigue is closely related to the local creep resistance near the crack tip [[28], [29], [30]], and accumulated creep damage near the fatigue crack tip would further increase the fatigue crack propagation rate [[28], [29], [30]]. To date, creep damage has been more widely investigated via FE stress analysis rather than by using experimental methods such as mechanical testing. Since creep resistance near the crack is difficult to measure the fracture mechanisms of welded joints experiencing CF rupture have not been extensively studied. However, nanoindentation techniques now offer the opportunity to measure the local creep resistance and other mechanical properties near the fractured edge of specimens that have undergone CF rupture. Investigation of the mechanical properties and microstructural evolution near the fracture surface can elucidate the mechanisms involved in fracture of a welded joint under CF loading. Our previous work [[28], [29], [30], [31]]on P92 steel investigated the local creep behavior of a welded joint under CF loading. However, the effect of the type of dwell period, i.e. whether it is tensile or compressive, on the creep behavior of welded joints is still an open question.

ASTM A355 P92 steel is 9Cr–2Mo steel that is widely used as a new generation creep strength-enhanced ferritic steel for steam pipes and super-header components in nuclear and fossil-fuel power plants [31]. The work reported in the current paper focuses on the local creep behavior and fracture mechanism of P92 steel welds after prior loading with tensile and compressive dwell periods. Relying on nanoindentation, hardness, elastic modulus and creep deformation were individually measured near the fractured edge (Crack region), as well as in the PP, FGHAZ, CGHAZ and WM regions of specimens after CF loading with tensile or compressive dwell at low strain amplitudes. The fracture surface features at the welded joint under CF loading with long-term tensile or compressive dwell were characterized using SEM. Thus the present work systematically investigates local creep behavior and fracture mechanisms in P92 steel welded joints after different loading sequences and their correlation with the local microstructural evolution.