Papers by Author: Ping Guang Xu

Abstract: The warm-compression microstructures of two nickel-containing tempered martensite steels with and without carbon were investigated. The EBSD measurement and TEM observation/Kikuchi pattern analysis confirm that the ultrafine microstructures contain ferriteaustenite dual phases with more than 18% austenite. The difference in the austenite amount before and after warm compression proves that the dynamic austenite precipitation occurs during warm deformation and the carbon addition is beneficial to the austenite precipitation. It is also found that the ultrafine austenite grains as a hard phase promote the dynamic recrystallization of ferrite during warm deformation.

Abstract: The low carbon steel, SM490 was austenized at different temperatures followed by quenching into water to obtain martensite microstructures with different grain sizes. Then specimens were heated up to 600°C followed by warm-compression at έ=1.7x10-3 s-1 (strain rate) to investigate the dynamic recrystallization behavior. The influence of pre-tempering before compression was also investigated. The microstructure observations were performed with FE-SEM and orientation imaging analysis with EBSD. It is confirmed that the dynamic recrystallization occurred in the tempered martensite as well as the as-quenched marteniste, resulting in fine grained ferrite microstructure with about 2μm. The dynamic recrystallization grain size is hardly dependent on the block size of initial martensite.

Abstract: A challenge was made to examine the micro-structural evolution during thermomechanically controlled processing (TMCP) by in situ neutron diffraction. Since the neutron beam is too weak to achieve a time-division measurement to follow a rapid transformation in a
low carbon steel, 2%Mn was added to make the austenite to ferrite transformation slower. Round bar specimens were heated up to 900°C with an electrical resistance method, then cooled down to 700°C, and compressed by 25% followed by step-by-step cooling. During the step-by-step cooling, neutron diffraction profiles were obtained and the volume fraction of ferrite, phase stresses and FWHM were analyzed. Using a similar TMCP simulator, specimens were quenched into water at several stages of the heat schedule to freeze the corresponding microstructures, which were observed with OM and SEM. As results, the ferrite volume fraction determined by neutron diffraction on cooling agrees well with that by microscopy. It is found that the austenite deformation and/or Nb addition accelerate the ferrite transformation to result in finer grain size.

Abstract: We investigated the dendrite structures and the as-cast textures of low-carbon steel strips cast at different cooling rates in order to relate the δ-ferrite dendrite structure with the ferrite phase texture. Observations revealed that the orientation intensity of the texture component {111} was stronger than the texture component {001} in the as-cast steel strips obtained at different solidification cooling rates. These two texture components show a roof-shaped variation with the gradual decrease of the solidification cooling rate, reaching a maximum intensity at a cooling rate of about 2.0 K/s. This suggests that the solidification rate strongly influences the as-cast texture of low-carbon steel strips through changing the δ-ferrite dendrite structure.