Noboru Kinoshita

Effects of Metallurgical and Mechanical Factors on Charpy Impact Toughness of Extra-Low Interstitial Ferritic Stainless Steels

The effects of precipitates, grain size, plate thickness, and notch sharpness on the Charpy impact toughness of extra-low interstitial 18Cr-2Mo, 26Cr-IMo, and 29Cr-2Mo steels were investigated. The steels exhibit good toughness in the solution-treated condition at 1000 to 1200°C and subsequently in the 2-mm V-notched condition. If the steels are subjected to fatigue cracks or brittle weld cracks, their transition temperatures increase and their shelf energies decrease. When the steels are slowly cooled from the solution temperatures or reheated at intermediate temperatures, 700 to 900°C, after solution treatments, precipitation of second phases occurs, such as Laves phase in 18Cr-2Mo steel and sigma phase in 26Cr-lMo and 29Cr-2Mo steels. The steels are embrittled by these precipitates and behave very similarly to the solution-treated specimens, having sharp notches such as fatigue cracks or brittle weld cracks. A decrease in grain size or thickness is beneficial in lowering the transition temperatures of solution-treated and 2-mm V-notched specimens. This effect, however, decreases, when the specimens have brittle weld cracks. These test results suggest that the solution-treated steels have good resistance to brittle fracture initiation and poor resistance to brittle crack propagation. Precipitation of second phases assists mainly crack initiation but also crack propagation to a lesser extent.