Hot Working Temperature Range Research Articles

Fracture Characteristics of Super Duplex Stainless Steel after Hot Tensile Testing

This study’s temperature range of hot tensile testing is 1000–1200 °C, which is consistent with ordinary industrial hot working temperatures. The fractography of super duplex stainless steel S32750 after hot tensile testing is investigated with Optical Microscopy and Scanning Electron Microscopy, and Electron Back-scatter Diffraction and Transmission Electron Microscopy study hot deformation behaviors of the necking area. The results reveal that ~1050–1150 °C is the optimal hot working temperature range regarding hot ductility and cracking sensitivity. The micro-voids nucleate at the ferrite-austenite interphase boundaries following the Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationship at 1000 °C. Therefore, a quasi-cleavage fracture forms along the interphase boundaries. At the temperature range 1050–1150 °C, a uniform distribution of fracture surface is formed, and the highest Reduction in Area appears at 1150 °C. Dynamic phase transformation occurs with the formation of secondary austenite at ferrite grain boundaries at 1200 °C, which results in an intergranular fracture surface morphology.

A new constitutive model and hot processing map of 5A06 aluminum alloy based on high-temperature rheological behavior and higher-order gradients

To develop a high-precision constitutive model and hot processing map for 5A06 aluminum alloy, the hot deformation behavior of the alloy was investigated through isothermal compression experiments and microstructure analysis experiments, which were conducted at temperatures ranging from 573 K to 773 K and at strain rates ranging from 0.01 to 10 s−1. Firstly, a new constitutive model, which is based on the partial derivatives of the experimental flow data and does not significantly increase the material parameters, was proposed. Secondly, a comparison and analysis of the predictive accuracies of both the new model and the classic model were conducted. Finally, the new constitutive model was used to construct the hot processing maps of the 5A06 aluminum alloy, and the optimal hot processing parameters were validated through metallographic experiments. The results show that a high-precision constitutive model can be constructed with a small number of material parameters by considering the second-order approximation between logarithmic stress and temperature, as well as the third-order approximation between logarithmic stress and logarithmic strain rate. The mean square errors of the new model were significantly lower than those of the Arrhenius model and Hense-Spittle model for all strain rates and temperatures. There were significant differences in the prediction accuracy of the AH and HS models at different temperatures and strain rates, indicating their accuracy was dependent on temperature and strain rate. The coefficients of the new model were significantly higher than those of the AH and HS models. Moreover, the proximity of predicted values to experimental values was ranked in descending order as follows: new model, AH model, and HS model. The energy dissipation rate is relatively high when the strain rate is lower than 10 s-1 and the temperature is higher than 623 K. The instability factor is larger than zero for all conditions except when the strain rate is equal to 10 s−1. The optimal hot working temperature range for 5A06 aluminum alloy is 623–700 K, and the strain rate should be in the range of 0.01 s−1 to 1 s−1. The microstructure analysis matches the predicted results from hot processing maps, which can guide the hot working process of 5A06 aluminum alloy.

Effect of Thermal History on Hot Workability of Fe–Cr Multiphase Steel during Hot Working

In the hot working of multiphase steels, the thermal history before hot working affects the phase balance. The aim of this study is to investigate the effect of the thermal history immediately before hot working on hot workability by performing hot uniaxial tensile tests. Duplex stainless steel was chosen as the test material, as this material has a typical dual-phase structure consisting of δ ferrite and austenite, and its phase balance changes with the working temperature. The phase ratio of δ ferrite is higher at high temperatures, and the ratio of the austenite phase increases as the temperature decreases. To clarify the effect of the cooling rate immediately before hot working on hot workability, a hot tensile test was carried out with two extremely different cooling rates (0.3°C/s, 10.0°C/s) from the heating temperature of 1250°C. The hot working test temperature was in the range of 650°C to 1150°C. Hot workability was higher under the rapid cooling condition than under the slow cooling condition in the hot working temperature range of 850°C to 1150°C, but this tendency was eliminated or reversed at temperatures below 750°C. The reason for this change was clarified by investigating both the phase balance immediately before hot working, which depends on the cooling rate, and the strength of each single phase. Therefore, in order to accurately measure the hot workability of a multiphase material, it is important to consider the thermal history immediately before hot working.

Grain refinement technology for duplex stainless steel using rapid cooling immediately before hot working

For the purpose of grain refinement and improvement of the hot formability of duplex stainless steel, a new concept of the thermomechanical processing method using the austenite phase transformation from the δ ferrite phase in the hot working temperature range was proposed. The supercooled δ ferrite phase is generated by rapid cooling immediately before hot working, and crystal grains are refined by the austenite transformation induced by hot working immediately after rapid cooling. Furthermore, since the new thermomechanical processing method is characterized by a large amount of soft δ ferrite phase during hot working, reduction of hot flow stress and improvement of hot ductility are possible in addition to grain refinement. The material used here was 25 % Cr duplex stainless steel, and the temperature history was controlled by combining induction heating and nitrogen gas cooling. It was confirmed that the δ ferrite phase was obtained as the parent phase by initial heating at 1320 °C, and a supercooled δ ferrite phase was generated by rapid cooling at 30.0 °C/s from the initial heating temperature to the hot working temperature of 1000 ℃. When uniaxial compression of 60 % at strain rates of 0.1, 1.0 and 10.0 s −1 was applied to the supercooled δ ferrite phase after cooling to 1000 ℃, nucleation of austenite grains was induced by the degree of supercooling and compressive strain, and as a result, the microstructure was successfully refined by strain-induced fine austenite grains. The grain diameter of the austenite grains obtained by strain-induced transformation was several μ m. The flow stress under thermomechanical processing, which succeeded in microstructure refinement, was much lower than that under the condition simulating the conventional process. The hot ductility investigated in the uniaxial tensile test was also greatly improved under thermomechanical processing. Finally, the results of a detailed crystal orientation analysis revealed that the nucleated fine austenite grains obtained by strain-induced transformation have a K–S relationship with the supercooled δ -ferrite parent phase.

Effect of Zn addition on the microstructure and mechanical properties of Mg-0.5Ca-0.5RE magnesium alloy

The microstructure and mechanical properties of Mg–Zn-0.5Ca-0.5RE magnesium alloy, with zinc contents of 0, 2, 4, and 6 wt%, were studied. A significant effect of Ca and rare earth (RE) elements (via mischmetal addition) on the grain refinement of cast ingots was observed with the consequent enhancement of ultimate tensile strength (UTS). Moreover, while the grain size was refined by the addition of Zn, no significant α-Mg grain refinement occurred beyond 2 wt% Zn addition. The Mg12RE intermetallic compound was observed in all alloys while the Mg7Zn3 phase formed at the grain boundaries of Zn-containing alloys. Based on the competition of grain refinement as a favorable factor and formation of unfavorable grain boundary phases, the Mg–4Zn-0.5RE-0.5Ca (ZEX400) alloy showed the highest UTS among the cast alloys. Intense grain refinement introduced by the extrusion process and the fracturing and dispersion of the grain-boundary phases resulted in the significant enhancement of strength and ductility and the appearance of ductile fracture surface for the extruded Mg–4Zn-0.5RE-0.5Ca alloy. It was found that high Zn content (∼6 wt%) limits the usability and hot working temperature range of the alloys due to the problem of hot shortness and intergranular brittle fracture. Therefore, the addition of 4 wt% Zn was recommended for industrial practice. This work revealed the significant effects of alloying (with Ca, RE, and Zn) and hot extrusion process for microstructural refinement and enhancement of mechanical properties of Mg alloys.

Flow behavior and processing maps of high-strength low-alloy steel during hot compression

Deformation behavior of a new kind of high-strength low-alloy steel was investigated by isothermal compression tests at temperatures of 800–1200 °C and strain rates of 0.01–10 s−1. The flow behavior of the steel was characterized through the analysis of true stress-strain curves. A relevant constitutive equation was established, and proved to be suitable by comparing experimental and calculated data under various deformation conditions. Processing maps based on the dynamic material model were constructed with strains of 0.3, 0.5 and 0.7. The maps showed that the optimal hot-working temperature range of the new steel was 1050–1200 °C, while the optimal strain rate range was 0.018–0.562 s−1, which was consistent with the microstructure of the deformed specimens. Dislocations and substructures under different deformation conditions were observed and analyzed by TEM. The results indicated that there were two kinds of DRX nucleation mechanisms during hot deformation.

Amelioration of surface cracking during hot rolling of AISI D2 tool steel

ABSTRACTReported is a relationship between a profile edge cracking during hot rolling of AISI D2 tool steel and material and processing parameters. Several months of observation of industrial hot rolling was done for neural network analysis and complemented with equilibrium thermodynamics calculations and laboratory hot deformation tests. Industrial results, in general, show that for the same chemical composition, hot rolling yield decreases with an increased profile aspect ratio. Cr content is significant for the soaking and strongly correlated with a hot workability at upper and lower limits of the hot working temperature range. Laboratory hot compression tests were employed to determine the optimal soaking temperature and to study hot workability to expand safe hot working temperature window.

On the increase of intrinsic workability and hot working temperature range of M42 ledeburitic super high steel in as-cast and wrought states

In order to remain competitive to powder metallurgy and spray forming, further improvements of a classical production route are needed and desired. This paper describes the means by which the intrinsic hot deformability may be improved and the temperature range of the safe hot working extended for AISI M42 super high speed steel, one of the most challenging materials with respect to hot deformability. A special hot compression procedure is described to determine appropriate soaking temperatures of as-cast and semi-wrought states. Moreover, conventional hot compression tests in the extended temperature range (850–1150°C) and strain rates range 0.001–5s−1 were carried out. The complex behaviour of carbides is discussed, which depends on the soaking temperature and time, deformation temperature and strain rate. Deformation conditions for improved crushing of initial eutectic carbides networks during the first stages of hot working are identified. Apparent activation energies for hot working in two temperature ranges and constants of the hyperbolic sine function are determined. The microstructure and respective carbides are characterised by field emission scanning electron microscope, optical microscopy, X-ray diffraction and the results compared to equilibrium thermodynamic calculations using ThermoCalc.

Hot deformation and static softening behavior of vanadium microalloyed high manganese austenitic steels

The hot deformation and static softening behavior of various high Mn (20–30wt%) austenitic steels microalloyed with different V (0.1, 0.2wt%), C (0.2, 0.6, 1wt%) and N (0.005–0.025wt%) contents were investigated. Double-hit torsion tests at temperatures in the range 700–1100°C were carried out and specimens quenched at selected conditions were examined using advanced microscopy techniques (EBSD-TEM) to characterize the recrystallization and strain-induced precipitation behavior. The results show that precipitation of vanadium at the hot working temperature range is sluggish. It mainly occurs for the combinations of 20%Mn–0.6%C–0.2%V and 30%Mn–1%C–0.1%V. When the carbon content is reduced to 0.2%C, strain-induced precipitation is suppressed at typical hot working temperatures, independently of the N level. The flow stress behavior was affected by the amount of C and by modifying the base composition from 30%Mn to 20%Mn–1.5%Al. However, the effect is complex and depends on deformation conditions. In the absence of strain-induced precipitation, the static softening kinetics was accelerated by increasing C content. However, no effect of Mn or V in solid solution was observed. In those cases where strain-induced precipitation took place, static recrystallization was severely delayed, leading to a major contribution of recovery to softening kinetics.

DEVELOPMENT AND RESEARCH OF A NEW METHOD OF STAMPING

The article describes the results of theoretical and experimental studies of a new method of stamping with regulated heating of workpiece to a predetermined temperature and its subsequent deformation by bilateral exposure of combustion products of gas mixtures. The authors demonstrated the possibility of heating the formed preform directly into the die cavity until the hot working temperature range. At this time, heating of the billet is less than 1 s, and the pressure of the combustible gas mixture depending on the material and thickness of the preform is 0.5 ... 2.0 MPa. The dependences of the parameters are determined for the calculation of the stamping process. Experimental studies have shown that this method by heating the workpiece creates favorable conditions for its deformation. This provides stamping parts with complex shapes in a single technological transition, using relatively simple die tooling, signifi cantly reduces the cost of the stamped parts. Due to this and the low cost of the used equipment, this method of stamping can fi nd effective application in industrial production.

Hot working behavior of a super high strength Cu–Ni–Si alloy

Hot working behavior of a novel super high strength Cu–6.0Ni–1.0Si–0.5Al–0.15Mg–0.1Cr alloy has been studied systematically. Experiment of hot compression deformation with temperature ranging from 750°C to 900°C and strain rate from 0.001s−1 to 1s−1 indicated that both strain rate and deforming temperature affect the flow stress. Flow stress increased to a peak value, and then decreased with increase of deformation time, showing continuous dynamic recrystallization behavior. In the deformation temperature zone of 750–800°C, nucleation of the recrystallization grains occurred in the interior grains at special locations such as deformation bands and shear bands. At the high temperature zone (850–900°C), nucleation of the recrystallization grains occurred both at the grain boundaries and inside the grains of the matrix. Three typical textures appeared after hot compressive deformation and the order of the texture intensity from largest to smallest was: Copper texture of {112}〈111〉, S texture of {123}〈634〉, Gauss texture of {011}〈100〉. On the basis of established processing map, appropriate hot working temperature range was 850–875°C. With the optimised hot working parameters and appropriate thermal mechanical treatment, the designed alloy has achieved excellent properties: hardness 341HV, tensile strength 1090MPa, yield strength 940MPa, elongation 3.5%, electrical conductivity 26.5% IACS, and the stress relaxation rate 8.52% as tested at 150°C for 100h.

Mechanical Characteristics of Superaustenitic Stainless Steel Type 30Cr25Ni32Mo3 at Elevated Temperatures

In making tubes of corrosion resistant and hardly deformed steels and alloys, the pilger rolling method is used for hot rolling of final thick-walled tubes or mother tubes of large diameters (above 300 mm) and small quantities of other size tubes when no other, more efficient tube rolling or extrusion equipment is available. To clarify individual parameters of the production process and make choice of the deformation-and-temperature parameters, mechanical properties of the alloy type 30Cr25Ni32Mo3 Superaustenitic Stainless Steel at various temperatures were studied. The tests have been performed using samples taken from the forged 400 mm diameter billet to determine strength and plastic properties of the billet metal at various temperatures and its macro- and microstructure. The test results will be used in the choice of optimum conditions of preheating of the billets and hot rolling of tubes. On the whole, it should be stated that as-forged alloy 30Cr25Ni32Mo3 features a favorable combination of strength and plastic properties in the hot-working temperature range of 1075-1200?C.

Mechanical Characteristics of Superaustenitic Stainless Steel Type 30Cr25Ni32Mo3 at Elevated Temperatures

In making tubes of corrosion resistant and hardly deformed steels and alloys, the pilger rolling method is used for hot rolling of final thick-walled tubes or mother tubes of large diameters (above 300 mm) and small quantities of other size tubes when no other, more efficient tube rolling or extrusion equipment is available. To clarify individual parameters of the production process and make choice of the deformation-and-temperature parameters, mechanical properties of the alloy type 30Cr25Ni32Mo3 Superaustenitic Stainless Steel at various temperatures were studied. The tests have been performed using samples taken from the forged 400 mm diameter billet to determine strength and plastic properties of the billet metal at various temperatures and its macro- and microstructure. The test results will be used in the choice of optimum conditions of preheating of the billets and hot rolling of tubes. On the whole, it should be stated that as-forged alloy 30Cr25Ni32Mo3 features a favorable combination of strength and plastic properties in the hot-working temperature range of 1075-1200?C.

The Hot Workability of Cromanite<sup>TM</sup>, a High Nitrogen Austenitic Stainless Steel

Columbus Joint Venture produces a High Nitrogen Austenitic Stainless Steel, called CROMANITE (EN 10 088 type 1 3820), containing a nominal 19% chromium, 10% manganese, 0.5% nitrogen and less than 1% nickel. At hot working temperatures, High Nitrogen Austenitic Stainless Steels are known to have a lower ductility than conventional austenitic stainless steels The factors which affect the hot workability of CROMANITE under production conditions have been studied and the hot working temperature range for this steel has been optimised From the literature it is known that high nitrogen steels are prone to edge cracking, if the δ-ferrite content is too high The effect of composition and slab reheat temperature on δ-ferrite content was determined and it was found that the δ-ferrite content varies with the slab reheat temperature. It was found that in some alloys, the amount of δ-ferrite increased substantially if the slab was heated above 1200°C In one case, the δ-ferrite increased from 3% at 1100°C to 30% at 1300°C. In addition to δ-ferrite content, it was found that chromium had a detrimental effect on hot ductility, while intermediate amounts of boron were beneficial With the production of CROMANITE, the nitrogen solubility, which has a strong affect on the δ-ferrite content, is determined principally by the Cr and Mn levels, the liquid metal temperature in the ladle and the nitrogen partial pressure at the slag-metal interface. From an understanding of these effects, the composition is selected to ensure that at least the minimum specified nitrogen content is obtained and that the hot workability of the steel is maximised Optimisation of the chemistry also takes into account alloying costs and steelmaking constraints The above has resulted in successfully hot rolling CROMANITE in gauges ranging from 50mm down to 3mm on a production scale.

Effects of hot working on the martensitic transformation of NiTi alloy

Among shape memory alloys, Ni-Ti alloys of near-equiatomic composition have been used for various commercial applications, since they have good mechanical properties and well-known shape memory effects. Processing disadvantages, however, include the difficulties and expenses of processing. Moreover, cold working to high reduction is difficult and expensive because limited cold workability and high work hardening, makes hot working indispensable. Therefore, optimization of hot working parameters and control of shape memory behavior is necessary. It is generally known that the influence of work hardening and subsequent isothermal annealing on the stability of the individual phases formed in Ni-Ti shape memory alloys has been investigated. Work hardening promotes the formation of the rhombohedral R phase. The matrix work hardening suppresses the phase transformation B2[leftrightarrow]B19[prime]. In this study, the equiatomic Ni-Ti alloy is plastic deformed by hot compression at various temperatures. The effects of hot compression on the martensitic transformation are investigated with differential scanning calorimeter measurements and hardness tests. The optimal hot working temperature range is also discussed.

Behavior of a nickel-base high-temperature alloy under hot-working conditions

The behavior of a Ni--Cr--Co base alloy with significant additions of Mo, Ti and Al (Ni-monic 105) under hot working conditions was studied using hot compression tests in the temperature range of 1223 to 1523/sup 0/K and strain rates between 0.38 and 64.3 s/sup -1/. The microstructure of the Nimonic 105 is complex and the matrix contains second phases in the form of Ni/sub 3/ (Ti, Al) dispersion (..gamma..'), various Cr and Ti carbides and titanium cyanonitrides inclusions. However, the results show that above the dissolution temperature of the ..gamma..' phase, the alloy behaves like a single phase nickel-base solid solution from the point of view of steady state flow stress-temperature-strain rate relationships, and the activation energies for hot working and static recrystallization. Under deformation conditions where the ..gamma..' phase is present, as in the case of creep; the activation energy is almost doubled. The hot working temperature range giving sound product is 1280 to 1450/sup 0/K at a strain rate of 0.4 s/sup -1/ and decreases to 1400 to 1480/sup 0/K at a strain rate of 65 s/sup -1/. At temperatures above the higher limit the alloy suffers intercrystalline cracks due to hot shortness and at temperatures below themore » lower limit the alloy suffers transcrystalline cracks due to excessive strain hardening.« less

Compositional and structural aspects of processing nickel-base alloys

The effects of the principal alloying additions to nickel on the hot-working properties are considered and illustrated in terms of hot ductility and resistance to deformation. The reduction of hot-ductility and hot-working temperature range brought about by some major alloying additions is considered in relation to the restriction placed on the choice of hot-working processes. The effects on hot workability of minor elements are examined in two respects: a number of undesirable elements produce deleterious effects, whereas certain other elements are deliberately added to improve workability; precise control is required in the latter case. Macro- and microsegregation and grain size are reviewed in relation to the hot working of, primarily, the high-strength, high-temperature alloys. The benefits to be derived here from consumable electrode remelting and homogenizing sequences are considered. Recent developments which significantly influence the processing of the less easily worked alloys are examined, including thermomechanical processing and the pre-alloyed powder techniques for complex alloy manufacture.

Recrystallization behavior in deformed austenite of high strength low alloy (HSLA) steels

High temperature deformation experiments were conducted to monitor the recrystallization process in the austenite phase of a vanadium and columbium-treated HSLA steel alloy. The results show that columbium is much more effective than vanadium in retarding recrystallization. As the austenitizing temperature, above 2000°F, or time is increased the austenite recrystallization in the 1500° to 1700°F hot working temperature range is correspondingly increased. If, following the austenitization, the samples are held in the hot working temperature range for up to 5 min before hot working recrystallization occurs at a much faster rate. Several mechanisms are offered to explain the experimental results.

Effect of Iron Powders on High-Density Sintered Iron-Base Alloys

High-density iron-base parts has been developed to meet the requirement for high strength sintered materials. The process consists of mixing, compacting, sintering and repressing (hot working temperature range), and is characterized by rapid heating and rapid forming under pressure. Effects of iron powders from different sources on the ductility of sintered products were studied. It is shown that the non metallic inclusions have deleterious effect.