About the prediction and assessment of thermal embrittlement of Cr–Ni austenitic–ferritic weld metal and castings at the ageing temperatures 260–425°C

Cast duplex stainless steel (CDSS) components suffer embrittlement after long-term thermal aging. The deformation and fracture behaviors of un-aged and thermally aged (at 400 °C for 20,000 h) CDSS were investigated using in situ scanning electron microscopy (SEM). The tensile strength of CDSS had a small increase, and the tensile fracture changed from ductile to brittle after thermal aging. Observations using in situ SEM indicated that the initial cracks appeared in the ferrite perpendicular to the loading direction after the macroscopic stress exceeded a critical value. The premature fracture of ferrite grains caused stress on the phase boundaries, leading the cracks to grow into austenite. The cleavage fracture of ferrite accelerated the shearing of austenite and reduced the plasticity of the thermally aged CDSS.

Section: Tem Observation Of Deformed Microstructures In the Aged Cdssmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

In Situ Observation of the Deformation and Fracture Behaviors of Long-Term Thermally Aged Cast Duplex Stainless Steels

Wang

2019

“…It has been reported that the thermal aging embrittlement in CDSS was caused by the precipitation that formed in the ferrite phase after long-term thermal aging, e.g., the G-phase precipitates and the spinodal decomposition during which the ferrite phase decomposes into a Fe-rich α-phase and a Cr-rich α'-phase [10,11,[13][14][15]. Many researchers have found that the mechanical properties of CDSS are strongly dependent on the strength of each individual phase [16,17] and their microstructures, e.g., the volume fraction and the morphology of the ferrite phase, have meaningful influence on the fracture toughness, thermal aging embrittlement, high-temperature ductility, impact properties and corrosion resistance of duplex stainless steel [18][19][20][21][22][23][24].…”

Section: Introductionmentioning

confidence: 99%

Mechanical Properties of a Thermally-aged Cast Duplex Stainless Steel by in Situ Tensile Test at the Service Temperature

Zhang

Jing

2019

Z3CN20.09M cast duplex stainless steel (CDSS) has been used for primary coolant water pipes in pressurized water reactors due to its excellent mechanical properties. Such pipes operate at an elevated service temperature (~320 °C) and experience issues of thermal aging embrittlement. In situ tensile tests were conducted to investigate the deformation mechanisms of Z3CN20.09M CDSS after long-term thermal aging at 475 °C for up to 2000 h in both optical microscope and scanning electron microscope at 320 °C. For the 320 °C tests, the tensile stress and other mechanical properties, e.g. the yield stress and the ultimate tensile strength, increase during the thermal aging process and recover to almost the same level as the unaged condition after annealing heat-treatment, which is caused by the formation and dissolution of precipitation during aging and anneal heat-treatment, respectively. For the slip mechanism, straight slip lines form first in the austenite phase. When these slip lines reach the austenite/ferrite interface, three kinds of slip systems are found in the ferrite phase. During the fracture process, the austenite phase is torn apart and the ferrite phase shows a significant elongation. The role of the ferrite phase is to hold the austenite matrix, thus increasing the tensile strength of this steel.

“…Cast duplex stainless steels (CDSSs) have excellent performances on mechanical properties, corrosion resistance, and weldability because of their dual phase microstructure of austenite and ferrite [1][2][3]. CDSS components are known to be sensitive to embrittlement after long-term thermal aging at moderate temperatures [4][5][6][7]. Spinodal decomposition in ferrite of CDSS during thermal aging is widely regarded as the primary mechanism of embrittlement [8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Effects of Thermal Aging on the Low Cycle Fatigue Behaviors of Cast Duplex Stainless Steels

Wang

2019

The low cycle fatigue (LCF) behaviors of cast duplex stainless steel (CDSS) thermally aged at different times were investigated under different strain amplitudes. The effects of thermal aging on the LCF lives of CDSS are closely related to the strain amplitude. At a low strain amplitude, the fatigue life of the material increases significantly after thermal aging, while the LCF life decreases with an increasing aging time at a high strain amplitude. After thermal aging at 400 °C for 10,000 h, the fatigue fracture morphologies of CDSS change from fatigue fringes to mixture features including fatigue fringes in austenite and cleavage cracks in ferrite. Severe plastic deformation in ferrite of the aged CDSS under a high strain amplitude causes the cleavage cracking of ferrite. The premature failure of ferrite accelerates the propagation of fatigue crack and shortens the fatigue life at a high strain amplitude.