IN SITU SYNCHROTRON X-RAY DIFFRACTION INVESTIGATION ON TENSILE PROPERTIES OF 9Ni STEEL

9Ni steel has been widely used in liquid natural gas tanks and pipelines because of its excellent toughness at low temperature after quenching, larmellarizing and tempering heat treatment. Recently, in the cryogenic field it is used in some forgings, which have a strict demanding on the strength of this material. In order to clarify the relationship between the strength and the reversed austenite in the 9Ni steel after different temperature tempering, a systematic investigation on the amount of reversed austenite, deformation induced phase transformation (DIPT) of reversed austenite and its influence on the mechanical properties of 9Ni steel has been carried out by dilatometer, in situ synchrotron high-energy X-ray diffraction, XRD and TEM. The experimental results indicated that the amount of reversed austenite showed a parabolic trend with increase of tempering temperature and obtained the highest value after 600 degrees C tempering. And the DIPT of reversed austenite occurred after yielding during uniaxial tension test. This phenomenon induced that the yield strength of the experimental steel decreased to a minimum value after 600 degrees C tempering, and then, the value increased with further the increase of tempering temperature. However, the tensile strength of experimental steel increased with the increase of tempering temperature and reached the maximum after 640 degrees C tempering, because almost all of the reversed austenite transforms to martensite before necking.