Effects of heat treatment and testing temperature on fracture mechanics behavior of low-Si CA-15 stainless steel
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20/1/04
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Effects of Heat Treatment and Testing Temperature on Fracture Mechanics Behavior of Low-Si CA-15 Stainless Steel CHENG-HSUN HSU, HWEI-YUAN TENG, and SHEN-CHIH LEE This research studied the effects of heat treatment and testing temperature on fracture mechanics behavior of Si-modified CA-15 martensitic stainless steel (MSS), which is similar to AISI 403 grade stainless steel, which has been widely used in wall and blanket structures and in the pipe of nuclear power plant reactors, turbine blades, and nozzles. The results indicated that fracture toughness of low-Si CA-15 MSS is better than that of AISI 403. The specimens of the low-Si CA-15 MSS after austenitization at 1010 °C and then tempering at 300 °C have higher plane-strain fracture toughness (KIC) values for both 25 °C and 150 °C testing temperatures. However, the specimens tested at 150 °C cannot satisfy the plane-strain fracture toughness criteria. The fatigue crack growth rate is the slowest after austenitization at 1010 °C for 2 hours and tempering at 400 °C. Observing the crack propagation paths using a metallographic test, it was found that the cracking paths preferred orientation and branched along ferrite phase, owing to martensite-phase strengthening and grain-boundary-carbide retarding after 300 °C to 400 °C tempering. Also, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.
I. INTRODUCTION
AISI 403 martensitic stainless steel (MSS) possesses excellent high-temperature strength, corrosion resistance, and fracture toughness, and it has been widely used in wall and blanket structures, pipes in nuclear power plant reactors, turbine blades, and nozzles.[1,2] AISI 403 MSS is derived from AISI 410 MSS, with a lower Si content being made to specific quality requirements for turbines and highly stressed parts. A casting with grade similar to AISI 410 MSS is CA-15 MSS. The cast MSS normally contains higher Si than its wrought alloy counterpart. The increased Si level is for the better fluidity of the molten steel, which promotes castability of the casting. However, Si is also a ferrite former that increases the ferrite content in the martensitic-ferritic duplex structure of the MSS. Although this low-Si CA-15 alloy may appear to have the same chemical composition range as the wrought 403 SS alloy, due to the slower cooling rate of its casting process and the possible segregation, shrinkage, and other potential defects inherent to cast products, it is important to evaluate the heat treatment effect for fracture mechanics behavior. However, there are limited research reports[3] on the properties of CA-15 MSS at room temperature and virtually no available data as to its fracture mechanics behavior. Therefore, it is the purpose of this research to study the fatigue crack growth rate, and middle (150 °C), room (25 °C), and subzero (150 °C) temperature fracture toughnesses of low-Si-modified
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