Kinetic study of austenite formation during continuous heating of unalloyed ductile iron

The austenite formation kinetics in unalloyed cast ductile iron was studied on the basis of dilatometry measurements, and Avrami＇s equation was used to estimate the material＇s kinetic parameters. A continuous heating transformation diagram was constructed us-ing heating rates in the range of 0.06 to 0.83℃·s^-1. As the heating rate was augmented, the critical temperatures, c1A and Aα, as well as the intercritical range, which was evaluated as the difference between the critical temperatures, α c1Δ T =A_a- A_c1 , increased. At a low heating rate, the kinetics of austenite formation was slow as a consequence of the iron＇s silicon content. The effect of heating rate on k and n, the kinetic parameters of Avrami＇s equation, was also determined. Parameter n, which is associated with nucleation sites and growth geometry, de-creased with an increase in heating rate. In addition, parameter k increased with the increase of heating rate, suggesting that the nucleation and growth rates are carbon-and silicon-diffusion controlled during austenite formation under continuous heating.

Effect of rare earth addition on continuous heating transformation of a high speed steel for rolls

The effect of rare earth（RE） on continuous heating transformation of a high speed steel for rolls was investigated by using differ-ential scanning calorimetry（DSC） with combination of microstructure analysis.Determination of the Ac1 and Ac3,the starting temperature of carbide dissolution and melting upon heating,the enthalpy change for the α→γ transformation and overall carbide dissolution were also es-tablished.It was found that RE could reduce the volume fraction of large eutectic carbides and the chrysanthemum-like eutectic colonies but could not change the phase composition.RE made a little change to Ac1,but the variation in Ac3 and enthalpy for the α→γ followed an in-creasing pattern as the RE addition increased.The start temperatures of carbides dissolution increased with increase of RE addition,which may be associated with the influences of RE on the morphology of carbides,but the overall enthalpy change of carbides dissolution decreases as the RE addition increased.Moreover,the start temperature of melting also increased with increasing RE addition.

Calculation of Austenite Formation Kinetics of Copper-Bearing Steel During Continuous Heating

The kinetics of austenite formation in a new type of copper-bearing steel with initial microstructure composed of ferrite and bainite was investigated by using dilatometric analysis and measurement during continuous heating. The formation of austenite was observed to occur in two stages. The first stage is the dissolution of ferrite and most bainite, followed by the second stage of dissolution of bainite and formation of austenite. The first stage takes place through diffusion and the second stage through shear. The critical temperature of austenite formation during continuous heating increases with increasing heating rates, which therefore exerts a greater influence on the Ast temperature of the austenite formation. Kinetics calculation shows that the process is mainly controlled by diffusion when the heating rate is over 1℃/s.

Research on Energy-Saving Performance of Intermittent Heating for Rooms in Hot Summer&ColdWinter Zone

In the hot summer&cold winter zone in China,intermittent heating space for rooms is widely used.However,in comparison with continuous space heating,the energy-saving performance of intermittent space heating has not been sufficiently investigated.This paper studied the factors influencing the energy performance of intermittent heating for the representativeoffice inhot summer&coldwinter zone.Basedon theheatbalancemethod,adynamic thermalmodel of the intermittent heating roomwas built and tested by experiments.And then,it analyzed the total space heating load,the amount of energy saving and energy saving ratio of the intermittent heating under different preheating hours,occupation hours,required roomtemperatures,air change rates,overall heat transfer coefficients(U-value)of windows and wall materials.If the adjacent rooms were not heated,for a typical room occupied about 10 h a day,the energy-saving ratio of intermittent heating was about 30%compared with continuous heating.But the preheating power was higher than two times of continuous heating.The results also indicated that the occupation hours had a significant effect on energy saving amount and ratio,it should be noted that the energy saving ratio by intermittent heating was much lower than the unoccupied period ratio.Relative to other factors,the heating temperatures,room air change rates and U-value of windows,and room envelope materials had little effect on energy efficiency.If the adjacent rooms were heated in the same manner as the roomin question,the energy-saving ratio of the total load of intermittent heating was heavily reduced to 8.46%.

Electrochemical Corrosion Behavior of the Laser Continuous Heat Treatment Welded Joints of 2205 Duplex Stainless Steel

The electrochemical corrosion behaviors of the welded joints of 2205 duplex stainless steel with the laser continuous heat treatment were investigated. The secondary austenite formation is the outcome of thermodynamic equilibrium breach of the alloy during heat treatment and the result of the continuous heat treatment which has the most important effect on the weld material. The partitioning behaviors of chromium and molybdenum as well as the volume fraction of ferrite and austenite have a remarkable influence on the composition of the individual phase. Mechanical examination of the laser trated weld demonstrates that the tensile strength and yield strength increase with increasing the amount of the secondary austenite. It is shown that the ultimate tensile strength of the 6 kW laser-treated weld is higher about 20 MPa than no heat treatment weld and the ductility can be further improved without compromising strength. The results indicate that the welding alters the corrosion behavior because of different post heat treatment power and the broad active peak is not identified which is attributed to the dissolution of the secondary austenitic in the ferrite phase. It is indicated that pitting resistance equivalent （PRE） values of base metal and 6 kW weld are higher than that of other welds; base metal is 33.7, 6 kW weld 33.3, no treatment 32.4, 4 kW weld 32.8, 8 kW weld 32.5. The extent of corrosion resistance improvement after reheating treatment is mainly caused by the removal of nitrogen from ferritic regions, which occurred as a consequence of secondary austenite growth.

MICROSTRUCTURE ANALYSIS AND MECHANICAL PROPERTIES OF Zn-Al ALLOY ROD PRODUCED BY HEATED MOLD CONTINUOUS CASTING

The new technology of continuous casting by heated mold was used to produce directional solidification ZA alloy lines to eliminate the inter defects of these lines and increase their mechanical properties. The results are as follows: (1) The microstruc-ture of the ZA alloy lines is the parallel directional dendritic columnar crystal. Every dendritic crystal of eutectic alloy ZA5 was composed of many layer eutectic β and η phases. The micro structure of hypereutectic ZA alloys is primary dendritic crystal and interdendritic eutectic structure. The primary phase of ZA8 and ZA12 is β, among them, but the primary phase of ZA22 and ZA27 is a. (2) Through the test to the as-cast ZA alloy lines made in continuous casting by heated mold, it is found that the tensile strength and hardness increase greatly, but the elongation decreases. With the increase of aluminum amount from ZA 5 to ZA 12, ZA22 and ZA27, the tensile strength increases gradually. ZA27 has the best comprehensive mechanical properties in these four kinds of ZA alloys. (3) Heat treatment can decrease the dendritic segregation and improve the elongation of ZA alloy, but make their strength decrease slightly.

Relationships between the surface quality of a single crystal copper ingot and the process parameters of a heated mould continuous casting method

The relationships between the surface quality of a single crystal copper ingot and the process parameters of heated mould continuous casting method were studied experimentally using our own design of horizontal heated mould continuous casting apparatus, and the mechanism by which process parameters affect the surface quality of a single crystal copper ingot is analyzed in the present paper. The results show that the process parameters affect the surface quality of a pure copper ingot by affecting the position of the liquid-solid interface in the mould. The position of the liquid-solid interface in the mould must be controlled carefully within an appropriate range, which is determined through a series of experiments, in order to gain a single crystal copper ingot with good surface quality.

Process Parameters of Manufacturing Single Crystal Copper by Heated Mold Continuous Casting

The effect of process parameters on the surface quality of single crystal copper ingot was studied through experiment with a self-designed horizontal heated mould continuous casting apparatus,and the mechanism was analyzed.The results show that the process parameters affect the surface quality of pure copper ingot by affecting the position of the liquid-solid interface in the mould.The position of the liquid-solid interface in the mould must be controlled carefully in an appropriate range determined through experiments in order to gain a single crystal copper ingot with a high surface quality.

Computer Simulation of the Continuous Annealing Recrystallization for Cold-Rolled Strip in Galvanizing Line

The continuous heating transformation program is developed to investigate the influence of continuous annealing technological parameters on recrystalization for cold-rolled strip in galvanizing line.This program can be used for the calculation of the continuous annealing temperature field of strip steel,determination of the start and end time of recrystallization in continuous heating process,and procurement of the continuous heating transformation diagram.The temperature field for continuous annealing is simulated by means of finite difference method.Based on the Scheil superposition principle,the continuous heating transformation curve is obtained by using time temperature transformation(TTT)curve which is measured experimentally.The developed program has been used in practice,and the strip speed on production line increases bv more than 10%.

Microstructural Characterization of Welded Joint in Duplex Stainless Steel by Laser Continuous Heat Treatment

Microstructural characterization in fusion zone of the laser continuous heat treatment welded joint was in vestigated. The results showed that the martensite-like microstructure is the face centered cubic （FCC） crystal structure so that it can be identified as the secondary austenite. The dislocation is observed inside and outside the seconda ry austenite, whereas inclusion is not found in the vicinity of the secondary austenite. In the fusion zone, there is a kind of carbide precipitate which is identified as M23 C6 by the means of transmission electron microscope （TEM）. The carbide precipitate is a representative mode of transformation, which can be generated by the eutectoid reaction. Furthermore, the formation mechanisms of the secondary austenite and chromium carbide are analyzed.

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product.The multiphase flow phenomena of molten steel,steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation,and the effect of electromagnetic brake（EMBR） and argon gas blowing on the process were investigated.The relationship between wavy fluctuation height near meniscus and the level fluctuation index F,which reflects the situation of mold flux entrapment,was clarified.Moreover,based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification,the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux,temperature and stress of shell in slab continuous casting mold were described.

ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy

In this work, the phase transformation sequence during the continuous heating process(3℃/min) was investigated in a near β titanium alloy. The results show that the staring formation of ω phase is about267℃, and the ending precipitation temperature about 386℃ during the heating process. When the heating temperature is greater than 485℃, there are no ω phase detected within the β matrix. Combined with the microstructural characterization, it is found that ω phase facilitates the nucleation of αphase nearby the ω/β interface and has a great effect on the refinement for α phase. As compared with the specimens directly aged, the specimens with ω-assisted refinement of α phase possess high tensile strength, but there is no yield stage detected on their stress-strain curve. Combined with the analyses of the fracture morphology, the specimens with ω-assisted refinement of α phase present a brittle fracture.This is mainly ascribed to its relatively lager width of grain boundaries and the absence of widmanst?ttenα precipitates.