Double-hit Compression Tests Research Articles

Metadynamic Recrystallization of the Nickel-Based Superalloy Allvac 718Plus<sup>TM</sup>

During hot working of the recently developed nickel based alloy Allvac 718PlusTM softening kinetics like dynamic, metadynamic as well as static recrystallization govern the microstructure evolution during and after hot forming and hence the final mechanical properties. In this work the metadynamic recrystallization was investigated using double hit compression tests. The classical methodology of offset stress comparison was not usable because of discontinuous yielding of this material. Thus a new method to describe the softening during metadynamic recrystallization was developed, which compares the deformation energy at equal strains, i.e. the area beneath the true strain vs. true stress curve, of the first and second hit as well as at steady state.

Study of fractional softening of AZ31 magnesium alloy under multistage hot deformation

In the present study flow softening behaviour of AZ31 magnesium alloy was investigated by double hit compression tests in the temperature range 250–400°C and strain rate range 10–3–10–1 s–1. The tests were conducted with delay times varying 4–250 s after achieving the critical strain ϵc in each deformation condition. It has been found that static restoration processes (recovery and recrystallisation) were intensely depended on strain rate and deformation temperature. Fractional softening values increased with increasing strain rate and deformation temperature. Accordingly the softening curves were divided into three regions. The softening in the short interpass times was attributed to the occurrence of static recovery and followed by static recrystallisation and grain growth as dominant softening mechanisms for the second and third regions respectively. In addition static recrystallisation kinetics was interpreted by Avrami equation. Analysis of the results indicated that Avrami constant was changed by varying temperature.

Austenite Recrystallization–Precipitation Interaction in Niobium Microalloyed Steels

A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at lower temperatures by solute drag or by precipitation pinning. In this study, the recrystallization behavior of four Nb-microalloyed model alloys which were designed to show either extensive or almost no precipitation, was compared by multi-hit torsion tests and double hit compression tests. A good consistency between the different types of tests was found and the results were verified by optical micrographs. Further, by construction of softening‐ time‐temperature diagrams the recrystallization behavior was linked to the precipitation state of the material which was investigated by thermodynamical equilibrium calculations and by experimental observations from TEM-EDX, Inductively Coupled Plasma Mass Spectroscopy and X-ray Diffraction. Quantitative agreement between the experimental measurements and the calculations for precipitated mass fraction and precipitate composition as a function of temperature and steel composition is demonstrated.

Investigation of Retained Strains and Microstructures for a Hot Deformed Stainless Steel

For the investigation of retained strains during hot forming, which are related to the dislocation structure, single and double hit compression tests were carried out at different temperatures and strain rates for a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations in both the deformed and recrystallized grains, respectively. At a given total amount of deformation in a double hit compression, the retained strains for the as deformed unrecrystallized grains are reduced for a lower deformation in the first hit. For the recrystallized grains the retained strain increases, when comparing double hit with single hit compression.

AVALIAÇÃO DO AMACIAMENTO ESTÁTICO A QUENTE E A MORNO DE UM AÇO SILICIOSO

Double hit compression tests were performed on an ultra low carbon Si-Al alloyed steel. The true strain in both deformation steps was kept constant (0.2), while varying the strain rate (1 s -1 , 10 s -1 and 30 s -1 ), the temperature (1,050°C-austenite phase, 950°C-intercritical region and 850°C-ferritic phase) and the interpass time (0.5; 1; 5 and 50 s). The temperatures used are within the industrial range applied in the finishing rolling for this steel. The analysis of the flow curves obtained allowed the evaluation of the static softening characteristics of the steel. It was shown that the steel has a similar softening condition between passes at the three temperatures studied, with the softening fraction increasing with the increasing time and strain rate. The characteristics of the softening curves and microstructural features indicate that at the two higher temperatures (austenite phase and upper part of the intercritical region) the softening process is governed by recrystallization, with the nucleation and growth of subgrains. At the lower temperature, however, the softening is mainly due to strain induced boundary movement (SIBM) and recovery.

Model Alloys to Study the Solute Drag and Precipitation Effect on the Recrystallization Kinetics of Nb-Microalloyed Steels

A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures by either a solute drag or by a precipitation pinning (when bonded to C or N) effect. Both mechanisms result in improved mechanical properties due to grain size refinement of the transformed ferrite. In this study, 3 Nb-microalloyed model alloys were designed to investigate the solute drag and the precipitation effect separately. The first alloy, containing a stoechiometric ratio of Nb and C, was designed to study the retarding effect of NbC on the recrystallization behavior. A second alloy, containing Nb and only few ppm C, was casted in order to study the effect of Nb in solid solution. The two alloys were compared with a C-Mn reference alloy. The recrystallization behavior of the three alloys were compared by multi-hit torsion tests and double hit compression tests. The Nb-C and the Nb-very low C showed small differences in recrystallization behavior. These results show that Nb delays the recrystallization by a solute drag effect or by the formation of a very small amount of precipitates.

Investigation of Retained Strains during Multipass Deformation

Double-hit compression tests were carried out at different temperatures and strain rates for a nickel based alloy and a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations. It is shown that at a given total amount of deformation, the retained strain is reduced for the as deformed grains that have not been recrystallized yet, but increased for the recrystallized grains, when comparing double hit with single hit compression tests.

The Strain-Independence of Interpass Softening during the Hot Compression of 304 H Stainless Steel

The kinetics of interpass softening and the strain (ε*) at which they become strain-independent were determined by means of double-hit hot compression tests. For this purpose, interrupted compression tests were conducted at strain rates of 0.01 and 0.1 s-1 to initial strains ranging from that corresponding to the initiation of DRX (εc) to the onset of steady state flow (εs.). Test temperatures between 1000 and 1100 °C (inclusive) were employed. Interpass times were varied from 0.3 to 1000 seconds. The fractional softening was determined using the 0.2% offset method. It is clear from the results that there is a transition strain (ε*) that separates the straindependent range of post-dynamic softening from the strain-independent range. The value of ε* obtained in this work was ε* = 4/3 εp. It was also found that the strain hardening rate was identical at all the critical strains (ε*) and took the value -22 MPa.

MODELING OF MICROSTRUCTURAL EVOLUTION IN MICROALLOYED STEEL DURING HOT FORGING PROCESS

The microstructural evolution of microalloyed steel during hot forging process was investigated using physical simulation experiments. The dynamic recrystallized fraction was described by modifying Avrami's equation, the parameters of which were determined by single hit compression tests. Double hit compression tests were performed to model the equation describing the static recrystallized fraction, and the obtained predicted values were in good agreement with the measured values. Austenitic grain growth was modeled as: D i n c 5 = D 0 5 + 1.6 × 10 32 t ⋅ exp ( - 716870 RT ) using isothermal tests. Furthermore, an equation describing the dynamic recrystallized grain size was given as D dyn = 3771·Z −0.2 . The models of microstructural evolution could be applied to the numerical simulation of hot forging.

MODELING OF STATIC RECRYSTALLIZATION KINETICS OF Nb-BEARING STEELS

By using isothermal double hit compression tests and applying the 2% offset method, a new model was developed to predict the microstructural evolution of Nb-bearing steels at temperatures above and below the start temperature of strain-induced precipitation (T P ). The T P was developed as a function of true strain, initial austenite grain size and the Nb content. The activation energy of static recrystallization (Q rex ) was expressed as a function of the content of different alloy elements. It was found that Nb played the most important role in increasing the value of Q rex The microstructural observations and measurements confirmed the validity of the model developed in the present investigation.

Kinetics of Metadynamic Recrystallization in Microalloyed Hypereutectoid Steels

The isothermal kinetics of the recrystallization processes of vanadium microalloyed high carbon steels has been measured and modelled. The overall softening data were obtained by double hit compression tests performed at temperatures between 900 to 1 050°C, strain rates of 0.01 to 1 s -1 , and inter-pass times of 0.1 to 30 s. The recrystallization behavior above and below and the critical strain for dynamic recrystallization was investigated. The results show that there is a transition strain region between where both static and metadynamic recrystallization take place during the inter-pass time. The results also revealed that V and Si have a strong solute drag effect, on the kinetics of metadynamic recrystallization. A kinetic model is proposed which takes the V and Si concentrations into account.

Effect of Silicon on the Kinetics of Nb(C, N) Precipitation during the Hot Working of Nb-bearing Steels.

The effect of Si on the rate of Nb(C, N) precipitation was investigated by using fractional softening measurements. Compression specimens, with diameters of 7.6 mm and lengths of 11.4 mm, were prepared from four Nb microalloyed steels containing a range of Si concentrations from 0.01 mass% to 0.41 mass%. Double-hit compression tests, with a strain of 0.3 for each pass and a strain rate of 0.1/sec., were employed. A solution heat treatment was applied immediately prior to each test. It was found that the rate of Nb(C,N) precipitation in hot deformed austenite increases with Si concentration. This phenomenon is attributed to the increases in the activities of C and N that result from Si addition.

Prediction of the Onset of Static Recrystallization after Hot Deformation.

In order to determine the kinetics of recrystallization in plain carbon steels, double-hit compression tests have been performed using a computerized Gleeble machine. Based on the nucleation mechanism of subgrain coalescence and the measured softening data, a kinetic model was developed to predict the start times for the static recrystallization occuring after hot deformation. Both the model calculations and experimental observations indicate that the onset of recrystallization is decelerated with increasing reheating temperatue but speeded up as the deformation temperature is increased. The former case can be attributed to the larger initial austenite grain size produced at higher reheating temperatures, which in turn results in larger subgrains and reduces the rate of coalescence. The latter case is due to the higher boundary diffusivity and mobility attained at higher deformation temperatures, which accelerates the process of subgrain coalescence. The accelerating effects of strain and strain rate can also be similarly explained by means of the higher dislocation density and finer subgrains generated during the higher strain and strain rate deformation. The good agreement obtained between model predictions and experimental results finally supports the view point that the coalescence of subgrains could be the rate controlling mechanism for the nucleation process of static recrystallization in austenite.

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Siderurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.

Recrystallization of austenite after deformation at high temperatures and strain rates—Analysis and modeling

Double-hit compression tests were performed on low-carbon steels containing various combinations of niobium, boron, and copper over a wide range of temperatures and strain rates pertinent to hot rolling. The kinetics of static recrystallization are characterized in terms of the mean flow stresses, which lead to more accurate results than alternative methods. The fractional softening defined by the mean flow stress method was first corrected for adiabatic heating using a simple procedure. Appropriate expressions are given for the recrystallization kinetics as a function of predeformation, temperature, and strain rate. Particular attention is paid to the effect of preloading strain rate on recrystallization kinetics. It is shown that there is a one order of magnitude increase in softening rate when the strain rate is increased by two orders of magnitude. Thus, Simple extrapolations of laboratory data determined at conventional strain rates to high-speed mill conditions are likely to be inaccurate.