Namin Xiao

Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 ‐ C and 850 ‐ C leads to flne ferrite grains. The microstructure characterization and mechanism of the flne ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered difiraction (EBSD) technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries (HAGBs) are dominant in both of the deformation induced microstructures occurring below and above Ae3, with only a few low angle grain boundaries (LAGBs) existing in the grain interiors. It implies that the dynamic strain-induced transformation (DSIT) happens above and below Ae3 temperature and has the same phase transition mechanisms. The reflnement of ferrite is the cooperative efiect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon difiusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with reflned ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the reflned structure is partly responsible for the ductility loss.

Mechanical Properties and Temper Resistance of Deformation Induced Ferrite in a Low Carbon Steel

The microstructures and mechanical properties of deformation induced ferrite (DIF) in the low carbon steel Q235 under different deformation temperatures have been investigated systematically. Through deformation induced ferrite transformation (DIFT), ferrite grain can be refined to 3 mu m and accounts for above 85% of the overall fraction. Yield strength of DIF (>500 MPa) is increased by up to 100% compared with the conventional low carbon steel. Comparison of microstructure and mechanical properties in the Q235 steel with DIF and tempered DIF microstructure illustrates that the strengthening mechanism of DIF microstructure is the combination of grain boundary strengthening and carbon supersaturated strengthening. Electron back-scattered diffraction (EBSD) analysis and high magnification scanning electron microscopy (SEM) observation denote that high-angle grain boundary among ultrafine ferrite grain and the transformation product of retain austenite membrane along ferrite boundaries are responsible for the stability of ferrite grain size during tempering process. Transmission electron microscopy (TEM) analysis demonstrates that the transformation product of retained austenite membrane between ferrite grain boundaries is cementite.

Transformation Behavior of Precipitates in a W-alloyed 10 wt pct Cr Steel for Ultra-supercritical Power Plants

Thermodynamic calculation, thermal analysis, and identification and observation of precipitates have been carried out on a W-alloyed 10 wt pct Cr steel by means of ThermoCalc program, differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) with an energy dispersive X-ray spectrometer, respectively. Several critical phase transformation points were determined by combining experimental results with calculations. Two individually stable phases of Nb(C, N) and VN, instead of one single phase MX (X: C, N), M(23)C(6) or Laves phases, were predicted in the calculated equilibrium phase diagram. An unstable elongated M(7)C(3) with relatively higher Cr was detected unexpectedly in the normalized and tempered steel. Two kinds of spherical Nb(C, N) with different size were recognized as the primary and the secondary precipitates of Nb(C, N) which contain different V contents. It was observed that one kind of complex precipitate in a special V-wing shape was resulted from two plate-like VN adhering to coarse primary spherical Nb(C, N).