Austenite Formation and Decomposition in a High-Strength Alloy Subjected to Heating and Cooling Cycles

Modern heat resistant 9–12% Cr steels require optimised heat treatments and welding strategies to receive the best mechanical and long-term creep properties during the fabrication of power plant components. Therefore, the phase transformation temperatures—especially the austenite and martensite transformation temperatures—have to be well known to define optimised heat treatment and interpass temperatures as well as heating and cooling rates. Since phase transformations are influenced by the chemical composition of the materials and other numerous factors, it is important to pay attention to the circumstances surrounding the determination of these temperatures. To determine specific phase transformation temperatures various methods and procedures like dilatometry, single-sensor differential thermal analysis (SS DTA) or differential scanning calorimetry (DSC) were established. The presented comparative study was initiated to investigate and quantify possible scattering within the measured values of participating institutions. Therefore, the commercially available martensitic heat resistant steel P91 was used to compare specific phase transformation temperatures determined by several participating institutes and laboratories. Two different simplified temperature cycles were defined to identify possible scattering and differences within the determined phase transformation temperatures. Furthermore, possible differing results regarding the evaluation of a given dilatometry data set by various institutes and laboratories were discussed. The presented round robin test shows that institutes and laboratories although using standard methods of analysis—which are said to be state of the art—are reporting variable values for the critical phase transformation temperatures in the steel of interest. It is also shown that the amount of scattering can vary widely depending strongly on the specific phase transformation temperature which has to be determined.

A good selection of the thermomechanical processing parameters will optimize the function of alloying elements to get the most of mechanical properties in Advanced High-Strength Steels for automotive components, where high resistance is required for passenger safety. As such, critical processing temperatures must be defined taking into account alloy composition, in order for effective thermomechanical processing schedules to be designed. These critical temperatures mainly include the recrystallization stop temperature (T5%) and the transformation temperatures (Ar1, Ar3, Bs, etc.). These critical processing temperatures were characterized using different thermomechanical conditions. T5% was determined through the softening evaluation on double hit tests and the observation of prior austenite grain boundaries on the microstructure. Phase transformation temperatures were measured by dilatometry experiments at different cooling rates. The results indicate that the strain per pass and the interpass time will influence the most on the determination of T5%. The range of temperatures between the recrystallized and non-recrystallized regions can be as narrow as 30 °C at a higher amount of strain. The proposed controlled thermomechanical processing schedule involves getting a severely deformed austenite with a high dislocation density and deformation bands to increase the nucleation sites to start the transformation products. This microstructure along with a proper cooling strategy will lead to an enhancement in the final mechanical properties of a particular steel composition.

The paper deals with theoretical and experimental study of phase transformation temperatures of steels in high temperature region (above 1000 °C), with focus on the solidus temperature, peritectic transformation temperature and liquidus temperature of multicomponent steels. Experimental data were obtained using Differential Thermal Analysis and “direct” thermal analysis. The experimental data were assessed by basic statistics. The calculations were performed using InterDendritic Solidification software and Thermo-Calc software. Also, selected empirically based models were used for calculations. The study presents the basic principles of theoretical and experimental methods, characteristics, advantages and disadvantages. Both used thermo-analytical methods are set correctly; the results are reproducible, comparable and close to equilibrium temperatures. Furthermore, comprehensive comparisons between the calculated and measured phase transformation temperatures show that the experimental data is satisfactorily accounted for by the present thermodynamic description.

Experiments using a hot-stage confocal scanning laser microscope (CSLM) have been carried out to observe phase transformations in two steels: Si-killed resulfurized Fe-0.38 wt pct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite colonies and subsequently in the ferrite phase. The measured advance rates of the γ/pearlite front were roughly twice those of the γ/α front and both interfaces were found to be curved. The γ/pearlite migration rate was found to be in qualitative agreement with published rate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstatten ferrite laths precipitate as distinct colonies from the existing allotriomorphic ferrite phase but then also at MnS precipitates. The electron backscatter diffraction (EBSD) analysis showed that all of the laths in a particular colony exhibit similar orientation to one another but a slightly different orientation than the parent allotriomorph, supporting a sympathetic nucleation mechanism. The growth rate of the laths was found to vary widely within a range of 1.5 to 11 μm/s. The ferrite formation is finally halted by impingement with other advancing fronts. The results are presented in a phenomological discussion, with some quantitative analysis of the transformation kinetics.

A non-isothermal transformation model was proposed to determine the austenite formation kinetics in a steel alloyed with 2.6% wt. Si by dilatometric analysis, considering that the nucleation mechanism does not change with the heating rate. From the dilatometric analysis, it was observed that the austenite formation occurs in two stages; critical temperatures, degree and austenite formation rate were determined. The activation energies associated with each of the stages were obtained employing the Kissinger method (226.67 and 198.37 kJ·mol−1 for the first and second stage) which was used in concert with the austenite formation rate in the non-isothermal model as a first approximation, with acceptable results in the second stage, but not in the first due to the activation energies magnitude. Then, the activation energies were adjusted by minimizing the minimal squares error between estimated and experimental austenite formation degree, obtaining values of 158.50 kJ·mol−1 for the first and 165.50 kJ·mol−1 for the second stage. These values are consistent with those reported for the diffusion of carbon in austenite-FCC in silicon steels. With these activation energies it was possible to predict the austenite formation degree with a better level of convergence when implementing the non-isothermal model.

In binary NiTi shape memory alloys (SMA), the highest martensite (M) and austenite (A) transformation temperatures (TT) occur in the annealed condition. The highest TT also occur in near equiatomic NiTi alloys that have an excess of Ti. However, the NiTi alloy composition and condition that have the highest TT produce actuating elements that are generally short lived, have poor mechanical properties, and plastically deform (creep) under low stress levels. Cold working the SMA followed by a memory imparting stress relieving heat treatment (HT) produces actuating elements that are long lived, have good mechanical properties, and are resistant to creep under moderate stress levels. However, in obtaining these desirable properties through thermal-mechanical processing, the M and A TT are significantly decreased, which limits the upper ambient temperature in which the actuating element can operate.A dimensionally stable actuating member with high TT can be achieved by thermal cycling (under stress) a NiTi SMA wire that has received prior thermal-mechanical processing. Cycling under an applied axial stress can increase the M TT of a SMA wire. Data showing the influence of thermal cycling on the TT of axially stressed SMA wires, that were cold drawn followed by HT at different memory imparting temperatures, are presented and discussed. For a NiTi SMA wire (A finish TT = 111 °C in the annealed condition) having approximately 40% cold reduction in area, 400°C for 1 hour memory imparting HT, and 10 Ksi axial stress, the M start TT (Ms) and A finish TT (Af) increase from 26°C and 79°C respectively after 10 thermal transformation cycles to 62 °C and 83.5 °C respectively after 10,000 thermal transformation cycles.

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

We use the statistical moment method to study the dependence of the critical temperature Tc for Cu3Au on pressure in the interval from 0 to 30 kbar. The calculated mean speed of changing critical temperature to pressure is 1.8 K/kbar. This result is in a good agreement with the experimental data.

This work deals with determining temperatures of phase transformations in steel S34MnV in a low-temperature region (below 900 °C). Although S34MnV is a significant tool steel, in the literature, there are only a few works dealing with the study of the thermo-physical properties of this steel. For the study of phase transformation temperatures of steel S34MnV, a differential thermal analysis and dilatometry were used in this study. Both methods are used to determine the phase transformation temperatures of steel. Dilatometry, however, unlike differential thermal analysis, is commonly used to determine the temperature of nonequilibrium phase transformations during cooling. Temperatures of the eutectoid phase transformation (A c1) and temperatures of the end of the ferrite to austenite transformation (A c3) were obtained at heating, and temperatures of the start of the ferrite formation (A r3), the temperature of the start of the pearlite formation (A r1) and the temperature of the start of the bainite formation (B S) were obtained at cooling using these methods. The temperatures obtained using the both methods were compared and discussed. The original thermo-physical data on steel S34MnV were obtained under precisely defined conditions. For the complexity of the study of the steel, a metallographic analysis of samples was also conducted after thermal analysis, which enables determining the phases occurring in the final structure and their quantity. The experimentally obtained data were compared with data calculated by the software QTSteel.

Differential barothermal analysis (DBA) is used to analyze the effect of hydrostatic pressure of 100 MPa on characteristic temperatures of a number of eutectic alloys (wt %), such as Al–10Si, Al–12Si, Al‒22Cu, Al–33Cu, and Al–7Cu–7Si. According to DBA data, the increase in pressure resulted in an increase in the temperatures of phase transformation; in practice, the most important of them is the nonvariant eutectic transformation temperature that determines the solidus of alloys. It was found that, for the binary systems, the temperatures of the nonvariant eutectic transformation L → (Al) + Si and L → (Al) + Al2Cu increase by 6 and 11°С (from 577 to 583°С and from 548 to 559°С, respectively); for the ternary system, the temperature of transformation L → (Al) +Al2Cu + Si increases by 6°С (from 520 to 526°С). Theoretical analysis, performed using thermodynamic models and Thermo-Calc software, shows that the increase in the eutectic transformation temperature with increasing pressure is directly dependent on the relative decrease in the molar volume of system upon associated eutectic transformation. In this case, the excess dissolution of silicon in (Al) under high pressure can lead to an additional decrease in the molar volume of the system, whereas the increase in the copper solubility is thermodynamically unfavorable.

The thermo-mechanical behavior of NiTi-X shape memory alloys

In this study, a Ti-rich Ti–Ni alloy (Ti54Ni46) was prepared by rapid solidification technique through vacuum suction casting into a water-cooled copper mold. The microstructure, thermal expansion, and phase transformation behavior of the alloy were studied systematically. The results show that the rapidly solidified Ti54Ni46 alloy exhibits negative thermal expansion (NTE) response in both vertical and horizontal measuring directions upon heating and cooling. The discrepancy in the NTE response between the two mutually perpendicular directions of the alloy is small, indicating an implicit anisotropic NTE behavior. A one-to-one correspondence exists between the characteristic temperatures of phase transformation and NTE, as well as between their changes during thermal cycling. It is conclusive that the NTE strains generated upon heating and cooling originate from the volume changes accompanying the forward and reverse martensitic transformations in Ti54Ni46 alloy. Characteristic temperatures of both phase transformation and NTE of the alloy rapidly shift to lower temperatures due to the multiplication of dislocations during the initial approximately 20 thermal cycles, and then tend to be relatively unchanged in subsequent thermal cycling as the transformation-induced defects reach saturation. The absolute values of the coefficient of thermal expansion of the NTE stage upon heating and cooling decrease rapidly during the initial approximately 20 thermal cycles, and thereafter become relatively stable with the increase of thermal cycle number, which is mainly attributed to the decrease of the effective fraction of the B19′ martensite participating in the forward and reverse martensitic transformations.

Research in copper selenide thermoelectric (TE) alloys has raised the possibility of a significant enhancement of the TE figure-of-merit ZT when the Seebeck coefficient is affected by a concurrent phase transformation. This ZT increase has also been related to a radical reduction of the thermal conductivity evaluated by transient laser flash thermal diffusivity measurements. In contrast, steady-state Harman-based measurements do not support a significant ZT increase only a modest one, because the thermal conductivity instead of decreasing goes through a sharp maximum as it approaches the critical phase transformation temperature of 407 K. The nature of this sharp increase of heat transfer has not been related to the well-known electronic or phononic contributions. Below the critical temperature, when the alloy is exposed to a steady-state temperature gradient, an additional heat transfer phenomenon takes place, induced by the ongoing gradual phase transition. We show that the enthalpy associated to the α to β and β to α phase transformations can lead to heat flow in the direction of the temperature gradient above and beyond conventional heat conduction. This unconventional heat transfer mechanism disappears when the temperature rises above the critical temperature where only a stable β phase remains. We propose a model of such a heat transport which leads to the sharp maximum of the related thermal conductivity. Numerical results obtained from the model compare favorably to the experimentally measured thermal conductivity.

A systematic experimental investigation was conducted using lab processed low carbon 0.08C-2.0Mn-Cr-Mo steel microalloyed with Ti/Nb to evaluate the influence of initial hot-rolled microstructures on the kinetics of austenite formation and decomposition after cold-rolling and subsequent annealing. Coiling temperature as a major hot rolling parameter was used to obtain different types of hot-rolled microstructures. Dilatometer and continuous annealing simulator were employed for austenite formation studies and annealing simulations, respectively. It was found that the coiling temperature affects the processes occurring during heat treatment in continuous annealing lines of full hard material: ferrite recrystallization, austenite formation during continuous heating and austenite decomposition during cooling. A decrease in coiling temperature accelerates the recrystallization of ferrite and nucleation of austenite, which results in formation of refined ferrite-martensite structure. The effect of initial hot rolled structure on final mechanical properties after continuous annealing was also investigated.

Multi-scale evaluation of an R290 variable geometry ejector