Shan Wu Yang

Dislocation-Precipitate Interaction and Its Effect on Thermostability of Bainite in a Nb-Bearing Steel

After bainitic transformation, the dislocations formed in deformed austenite remained to be pinned by the precipitates so that thermostability of the bainitic ferrite was improved. Coarsening of the precipitates accompanied by their distribution density change occurred during reheating. After long reheating, further precipitates nucleated in bainite. Dislocations inside laths getting rid of pinning of precipitates and their polygonization play the precursor to the evolution of microstructures, in which lath boundaries disappeared gradually.

Formation and Control of the Acicular Ferrite in Low Carbon Microalloying Steel

The influence of processing parameters on the acicular ferrite formation for the low carbon microalloying steel was studied. The results showed that the fraction of acicular ferrite could be controlled by the cooling process. The acicular ferrite/ bainitic ferrite dual phase structure can be formed. The multi-phase microstructure is ultra fine. The hardness is sensitively affected by the acicular ferrite fraction.

In Situ Observation of Microstructure Evolution in Low Carbon Bainite Steels Isothermally Held Below A1 Temperature

The dominant microstructures in low carbon bainite steels such as bainitic ferrite are non-equilibrium phases, which will tend to evolve into equilibrium phases when the steels are subjected to thermal disturbance. In-situ observation by optical and scanning electron microscopy was carried out in this investigation to track the evolution when the steels were isothermally held below A1 temperature. It is found that the primary polygonal ferrite grows slowly during isothermal holding, while bainitic ferrite changes rapidly into polygonal ferrite. Self-tempered bainitic ferrite would recover further and recrystallize. The lower the concentration product of carbon and niobium, the faster is the evolution.

Nucleation of Bainite on Allotriomorphic Ferrite/Austenite Interface in a Low Carbon Steel

. A low carbon steel is austenitized and isothermally held at 680°C to form allotriomorphic ferrite and followed by a holding at lower temperature to form bainite. The morphology of allotriomorphic ferrite/bainite interfaces is studied using optical microscope. Three kinds of combination are observed: Type I: interface on one side is clear while on the other side, unclear; Type II: unclear on both sides; Type III: clear on both sides. Clear interface indicates a large difference in the orientation between the bainite and the ferrite, and unclear interface, a very small difference. The statistical counting shows that the ratio of Type I is about 80-82%, and that of Type II, 7-8%, and Type III, 9-11%. It is observed that this ratio does not change with the austenite grain size and bainite forming temperature. And the clear and unclear side of allotriomorphic ferrite may have different influence on the nucleation rate of bainite at allotriomorhic ferrite/prior austenite interface.

Evolution of Microstructures in a Low Carbon Bainitic Steel during Reheating

Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibited synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650°C or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones, accompanied by obvious change of hardness. The rate of microstructures evolution was closely related to relaxation time of deformed austenite. The sample relaxed for 60s displayed the highest thermal stability, while microstructure evolution was quickest in the sample relaxed for 1000s even though it was softest before reheating. By hardness measurement, it was found that softening was not only process occurring during reheating, in which hardness fluctuated with time. There were two peaks in hardness-time curve of each sample having undergone relaxation, while single peak occurred in the curve of the sample not being relaxed. These results indicate that thermal stability of microstructures is determined by their history of formation.

A Study on Cu and Nb Precipitation during High Temperature Tempering in Low Carbon Steels

The microstructure evolution and precipitation behavior of two low carbon steels are studied, with 0.05C-0.77%Nb added in one steel and (0.03C-)1.63Cu-0.74%Nb added in the other as a comparison. In the Cu-Nb steel tempered at 600°C for 18 hrs, there are two peaks in the particle size distribution figure, one between 2-3nm formed by NbCN precipitates, and the other, 10-12nm for Cu precipitates. The TEM observation on carbon replica shows that the average particle diameter of NbCN precipitate is 2.81±0.78nm in C-Nb steel, while 4.23±0.95 nm in Cu-Nb steel with lower carbon. The analysis shows that this size increase of NbCN not only decreases the precipitation strengthening, but also weakens significantly the pinning effect on the dislocations, which results in a more serious microstructure softening in Cu-Nb steel.

Influence of Austenite Grain Size on the Crystallography of Allotriomorphic Ferrite in a Low Carbon Steel

The influence of prior austenite grain size on the crystallography of allotriomorphic ferrite is investigated in a low carbon steel. The results show that as the prior austenite grain size decreasing, the fraction of allotriomorphic ferrites that do not keep K-S orientation relationship with any surrounding prior austenite grains is increased. It is observed that such ferrites usually form at the grain edges or grain corners. It is known that with the grain size decreasing, the fraction of grain edges and corners increases. It is suggested that the free energy of the defects at such nucleation sites is higher than that at grain faces, and the nucleation barrier of ferrite is lower. As a result, the possibility for the ferrite to form that does not have orientation relationship with all surrounding austenite grains is increased at such sites.

Influence of Allotriomorphic Ferrite under Different Growth Modes on the Variant Selection of Bainite in a Low Carbon Steel

A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Rust Layer Structure and Corrosion Resistance of High Strength Microalloyed Steels

Corrosion behavior and corrosion resistance of two laboratory prepared microalloyed steels (steel 1 and steel 2) and a commercial traditional weathering steel (TWS) was compared. All these steels contain 0.03% C, 0.50% Si, 1.35%Mn, 0.30%Mo, 0.030Nb, 0.35%Cu%, 0.30%Ni and 0.040%Ti. In addition, steel 1 contains 0.0028%B, steel 2 contains 0.0026%B, 0.66%Cr, TWS contains 0.63%Cr. It has been found that a noticeable change occurs in optical microstructures attributed to a very little amount of boron. The mechanical properties of steel 2 are much better than that of TWS. The corrosion behavior of the three steels has been investigated by an alternating wet-dry experiment. Small amount of pearlite appears in TWS, which lead to its higher corrosion rate. The corrosion depth of steel 2 is nearly thrice that of steel 1. The compound components of inner rust in steel 1 and steel 2 are similar. The Nyquist diagrams indicate that the corrosion rate of steel 1 is controlled by the diffusion of oxygen in rust layer while it in steel 2 is controlled by chemical reaction in rust, which result in loosened rust layer and higher corrosion rate to steel 2. Addition of chromium cannot enhance the corrosion resistance of steel, but reduce the corrosion resistance in environments with high levels of chlorides.

Formation and Flanging Burrs Control of Three-Dimensional Substructure on Copper Surface

In this paper, the model of petal-shaped fin is developed, based on the traditional two-dimensional substructure in heat transfer enhancement. The petal-shaped fin is a three-dimensional substructure and is applied on the surface of pin fin heat sink. The processing method of petal-shaped fin on cylindrical surface is studied. The formative mechanism of flanging burrs that are harmful for heat transfer is researched. The effectiveness of triangle broaching cutting tool’s rake angle o γ , tool angle β and broaching deepness p a on the occurrence of flanging burrs are discussed by experimental methods. It is proved that the three-dimensional substructure used in heat sink is useful for heat transfer enhancement, by means of the contrast tests. Introduction It is well known for all of us that high temperature will effect the performance of the chip, endanger the node of the semi-conductor, destroy the joint interface of the circuit, increase the resistance of the conductor and form the thermal stress damage. For the high-integrate chip, it will greatly decline the failure rate when the temperature decreases 1 . It is very important for the reliability of CMOS chip. In the daily life, the heat amount of CPU has increased much more than the past, with the basic frequency increasing. Now, the basing frequent of Pentium IV CPU has passed over 3GHz. It’s thermal density is very large, while the size is minute. It requires a more efficient heat transfer system to solve the thermal problem. Air cooling is the most common heat transfer technology used in CPU for this kind of heat sink is cheaper and safer than other kinds. The characters of air cooling heat sink have changed greatly, from no-fan passive heat transfer to active heat transfer with fan, from traditional small volume to the big one, form continuous plate fin to interval and pin fin, from aluminum material to copper one. The purpose of above changes is to improve the effective area and coefficient of heat transfer. At the same time, it will increase the weight of heat sink and the volume occupation in host computer box. Therefore, in order to enhance heat transfer more effectively, we must launch on the surface substructure of heat sink. Making substructure on surface is one of the main methods to increase thermal coefficient. Turbulent flow boundary layer is composed of three layers: viscous sublayer, buffer layer and core layer. In the viscous sub layer, velocity gradient is high and viscous shearing force is dominant. It is where the main heat resistance in for the flow mode is laminar [1]. By constructing substructure element can destroy the viscous layer, increase the insatiability of the boundary layer and enhance the surface thermal coefficient At the same time, the substructure surface has great effect on the fluid frictional resistance, so it will generate a minus effect if the designing and processing of substructure unit is not optimum. Substructure is mostly used on CPU plate fin heat sink. It is more difficult to apply it on CPU pin fin heat sink for the pin fin diameter is only 2-3mm. Therefore considering the process feasibility, it is very necessary to optimize the design of substructure on cylindrical surface and further study its forming mechanics. Materials Science Forum Online: 2004-12-15 ISSN: 1662-9752, Vols. 471-472, pp 248-254 doi:10.4028/www.scientific.net/MSF.471-472.248