Non-isothermal Dilatometry Research Articles

Modeling Solid-State Phase Transformations of 13Cr-4Ni Steels in Welding Heat-Affected Zone

Fatigue damage is commonly encountered by operators of Francis type hydraulic turbine runners made of 13Cr-4Ni soft martensitic stainless steel. These large and complex welded casting assemblies are subjected to fatigue crack initiation and growth in the vicinity of their welded regions. It is well known that fatigue behavior is influenced by residual stresses and the microstructure. By including solid-state phase transformation models in welding simulations, phase distribution can be evaluated along with their respective volumetric change and their effect on residual stresses. Thus, it enables the assessment of welding process on fatigue crack behavior by numerical methods. This paper focuses on modeling solid-state phase transformations of 13Cr-4Ni soft martensitic stainless steel, used for manufacturing hydraulic turbine runners, occurring upon welding. It proposes to determine the material parameters of the models for both the austenitic and the martensitic transformation by nonisothermal dilatometry tests. The experiments are conducted in a quenching dilatometer with applied thermal conditions as experienced in the heat-affected zone of homogeneous welds. The activation energy and the kinetic parameters of the austenitic transformation from fully martensitic state are measured from the experimental results. The martensitic transformation modeling from a fully austenitic domain is also presented.

Investigation of sintering behavior of ZrO2 (Y) ceramic green body by means of non-isothermal dilatometry and thermokinetic analysis

Using the method of dilatometry, kinetic characteristics of compacts shrinkage manufactured from ultrafine plasmochemical ZrO2 (Y) powders and commercial Tosoh’s powders are investigated. The shrinkage curves are constructed with a DIL 402 C high-sensitivity dilatometer in a non-isothermal heating mode at different heating rates (1, 2, 5 and 10 °C min−1). It is shown that the plasmochemical powders are characterized by a lower sintering efficiency than the Tosoh’s powders. The kinetic results are processed using a Netzsch Thermokinetics license software program developed for those who make use of devices manufactured by the Netzsch-Geratebau GmbH. The kinetic characteristics of compact shrinkage are determined as a function of partial length variation using Friedman’s analysis. Considerable differences are found between the values of apparent shrinkage activation energy for plasmochemical and Tosoh’s powders in the initial and final shrinkage stages. We attribute the mentioned differences in phase composition of the powders and their degree of agglomeration. In the intermediate shrinkage stage, the values of the apparent activation energy obtained for both types of powders have only marginal differences.

Solid-state phase transformation kinetics in the near-equilibrium regime

Solid-state phase transformation kinetics in the near-equilibrium regime behaves differently from that in extremely non-equilibrium regime since their thermodynamic states are different. Incorporating temperature- and transformed fraction-dependent thermodynamic terms, a thermo-kinetic model is derived to describe the transformation kinetics in the near-equilibrium regime. The model predicts a sluggish stage in isothermally conducted transformation and a temperature-dependent stage in non-isothermally conducted transformation. Then, the kinetics of γ/α transformations in two binary substitutional Fe-based alloys (i.e. Fe-3.28 at.% Mn and Fe-1.73 at.% Co), measured by isothermal and non-isothermal dilatometry, is investigated by the newly proposed model. The model quantitatively describes the retarded isothermal kinetics in Fe-3.28 at.% Mn alloy and the abnormal non-isothermal kinetics in Fe-1.73 at.% Co alloy.

The determination of the activation energy varying with the precipitated fraction of β″ metastable phase in an Al–Si–Mg alloy using non-isothermal dilatometry

The kinetic study of the β″ phase precipitation in Al–Si–Mg alloy was carried out through nonisothermal dilatometry. Using Kissinger method, the apparent activation energy was first estimated to be 67kJ/mol. In order to investigate the variation of the apparent activation energy (Qα) as a function of the formed fraction (α) of β″ phase, differential isoconversional method of Friedman (FR) and integral isoconversional methods of Kissinger–Akahira–Sunose (KAS) were used. The methods showed an increase in Qα with α. However the dependence of Qα is more pronounced for FR method whose Qα values are higher. Qα values were in the range 77–110kJ/mol and 67–79kJ/mol for FR and KAS methods, respectively. Some disagreement between these results and those obtained in a previous work using DSC experiments are observed, particularly for FR analysis.

On the non-isothermal precipitation of copper-rich phase in 17-4 PH stainless steel using dilatometric techniques

The kinetics of the precipitation of copper-rich phase in 17-4 PH stainless steel was studied in this paper by non-isothermal dilatometric experiments. The dilatometric curve was analyzed and the conversion degree of the precipitates was associated with the area under the derivative curve of the thermal expansion as a function of temperature. The apparent activation energy associated with the formation of the precipitates was calculated and the obtained results were compared with the data calculated from the isothermal dilatometric tests presented in a previous paper. The data well agree, so confirming that non-isothermal dilatometry can be considered a powerful method to study the precipitation kinetics of PH stainless steels. Finally, the conversion degree for any isothermal treatment was calculated starting from the parameters obtained by non-isothermal dilatometric tests and compared with the conversion degree calculated from the hardness values during heat treatments.

Non-isothermal phase transformation kinetics of ω phase in TB-13 titanium alloys

The ω phase transformation kinetics of TB-13 titanium alloy has been analyzed by non-isothermal dilatometry. The average value of local activation energy determined is about 91.7 kJ/mol. The nucleation and growth mechanism of the ω phase has been investigated by the non-isothermal local Avrami exponent. Both the local Avrami exponent and the local activation energy during the ω phase transformation process change significantly with the transformed volume fraction, indicating that the ω phase transformation mechanism in this alloy varies at different stages. The local Avrami exponent lies between 1 and 2 in a wide transformed volume fraction range of 0.17–0.95, indicating that the dominating mechanism of ω phase transformation in TB-13 titanium alloy is the three-dimensional growth with a near-zero nucleation rate.

Kinetic parameters during the tempering of low-alloy steel through the non-isothermal dilatometry

The kinetic study of the tempering reactions of a low-alloy steel (AISI 1050) was carried out through non-isothermal dilatometry. The kinetic parameters of the first and third stages on tempering (here referred to as processes I and II) are calculated by procedures which assume that the nucleation and growth reactions obey a Kolmogorov–Johnson–Mehl–Avrami (KJMA) kinetic model. The recipes to obtain the kinetic parameters (E, K0, n) of the reactions on tempering are presented. The first stage of tempering is characterized by the growth of the transition carbide nuclei formed during the quenching, n = 1 (site saturation situation). This stage is controlled by the pipe diffusion of the iron atoms. The third stage of tempering is characterized by the cementite nucleation on dislocations due to the gradual dissolution of the transition carbide, n = 0.66. The cementite growth is controlled by diffusion of the iron atoms through dislocations and in the matrix.

Densification and micro-structure evolution of BaTiO 3/ZrO 2 composites

The sintering of ZrO 2/BaTiO 3 composites was studied by stepwise, individual and non-isothermal dilatometry methods. The composites were prepared by slurry mixing followed by drying, sieving and powder compaction by uniaxial pressing. High density sintered compacts were obtained by a variable time stepwise densification method using a starting mixture with 10 mol% zirconia. Addition of ZrO 2 to BaTiO 3 reduced the temperature at which sintering began and a broad range of temperature of high sintering rate was observed for the composite with 10 mol% ZrO 2 in the initial mixture. The addition of ZrO 2 suppressed abnormal grain growth and allowed more than 97% dense ceramics to be produced by stepwise isothermal densification in short dwell time. Kinetic parameters of sintering (exponent n, specific rate constant K t and apparent activation energy E) were evaluated from which it can be found that the mechanism of sintering is constant in the temperature range from 1260 to 1310 °C.

The influence of nitrogen on the structure and properties of Fe–10Cr–N and Fe–10Cr–1Mo–N steels after tempering in the temperature range of 650–750 °C

The influence of nitrogen on the structure and properties of the Fe–10Cr–N and Fe–10Cr–1Mo–N ferritic steels after tempering in the temperature range of 650–750°C was studied. Before selecting tempering temperatures, the investigation of precipitation processes during continuous heating of the quenched steels (studied here) was carried out using non-isothermal dilatometry and TEM. For better understanding of the observed phenomena, thermodynamic calculations were performed using the Thermo-Calc™ programme. Change of nitrogen content shifts the temperature of the maximum length change rate, which correspond to Cr2N or (Cr, Mo)2N and M23C6 precipitation processes at higher temperatures. The decrease of tensile strength, yield point, and hardness with tempering temperature rise in steels with higher nitrogen contents is less intensive. It was observed that the deterioration of impact toughness, elongation, and the reduction of area becomes more evident after increase of the nitrogen content.

Grain growth and densification study of β-Ta 2O 5

The sintering and grain growth behaviour of the low temperature structure of Ta 2O 5 has been studied using non-isothermal dilatometry up to 1350°C. Grain growth followed a classical law, G m - G m 0 = kt, with m = 2. The sintering mechanism is well described by Coble's model. The activation energies characterizing the process are evaluated to be 260 kJ mol −1 for low temperature and grain boundary diffusion and 590 kJ mol −1 for high temperature and volume diffusion.

Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

The effect of microstructure on the slow bending stress and fracture energy in 0.5C-steel processed by high temperature thermomechanical treatment (processed by forging) (HTMT) was studied to understand which microstructural factors contribute to the strength and toughness of a HTMT steel. Significant improvement was achieved in the slow bending fracture energy, with moderate increase in the slow bending stress when the steel was deformed by 50% at 1473K followed by direct water quenching and subsequent tempering at 453 K. When the steel was deformed by 50% reduction at 1173 K followed by direct water quenching and subsequent tempering at 423 K, the slow bending stress significantly increased though the increase in the fracture energy was not as great as that of the 1473 K forged steel. However, an abrupt reduction occurred in the fracture energy above suitable tempering temperatures, so above these temperatures, there was little difference between the properties of the HTMT and conventional heat-treated steels. Microstructural factors contributing to the mechanical properties are discussed in terms of thin-foil transmission electron microscopy, non-isothermal dilatometry, and X-ray measurements.

Analysis of transformation kinetics by nonisothermal dilatometry

Nonisothermal dilatometry was shown to provide a powerful method for the analysis of solid-state transformation kinetics by measuring the shifts of the temperatures of inflection points on dilatometric curves as a function of heating rate. The method was applied to iron-nitrogen martensite specimens. To that end iron was nitrided in a NH3/H2 gas mixture and subsequently quenched. The experiments allowed the assessment of residual terms in the theoretical development. To demonstrate the high sensitivity of the method, the activation energy was determined for the initial stage of tempering iron-nitrogen martensite, involving a minute decrease of specific volume. The method developed is equally applicable to methods where other physical properties of the material under investigation(e.g., electrical resistivity, magnetization) are measured on isochronal annealing.