Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions

The Avrami kinetics of dynamic recrystallization

The dynamic recrystallization behavior in 42CrMo steel was investigated by hot compression tests. The effects of deformation temperature, strain rate, and initial austenite grain size on the dynamic recrystallization behavior were discussed. Based on the experimental results, the kinetic equations for the dynamic recrystallization behavior of 42CrMo steel were proposed. Results indicate that the effects of the deformation temperature, strain rate and initial austenitic grain size on the dynamic recrystallization behavior in 42CrMo are significant. The dynamic recrystallization in 42CrMo steel easily occurs at high deformation temperature, low strain rate and fine initial austenitic grain. A good agreement between the experimental and predicted results shows that the proposed kinetic equations can give an accurate estimate of the dynamic recrystallization behavior in hot deformed 42CrMo steel.

The kinetics of dynamic recrystallization of 42CrMo steel

This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds. The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses. The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness. In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness. These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands.

Microstructure and Local Brittle Zone Phenomena in High-Strength Low-Alloy Steel Welds

A new ultrahigh strength niobium-microalloyed pipeline steel of yield strength ∼700 MPa has been processed. The Charpy impact toughness at 0 °C was 27 J and tensile elongation was 16%. The ultrahigh strength is derived from the cumulative combination of fine grain size, solid solution strengthening with additional interstitial hardening, precipitation hardening from carbides, dislocation hardening, and mixed microstructure. The microstructure was characterized by polygonal ferrite, upper bainite, degenerated pearlite, and martensite–austenite (MA) constituents. The microstructure of weld and heat-affected zone (HAZ) was similar to the base metal such that the hardness is retained in the weld region implying insignificant softening in the weld zone. Niobium and titanium precipitates of different morphology and size range evolved during thermomechanical processing and include rectangular (∼500 nm), irregular (∼240–500 nm), cuboidal/spherical (∼125–300 nm), and very fine (

Microstructure and high strength–toughness combination of a new 700 MPa Nb-microalloyed pipeline steel

The relationship between mechanical properties and microstructural characteristics of pearlite was investigated using various heat treatments on a hypereutectoid steel. The materials were reheated between 900 and 1200 °C and these microstructures were then subjected to isothermal transformation at temperatures of 550, 580 and 620 °C. For the hypereutectoid steel, the mean value of the interlamellar spacing was observed to increase with increasing reheat and transformation temperatures. Examination of the mechanical properties of the resulting pearlitic microstructures indicated that the strength was related primarily to the interlamellar spacing by a Hall–Petch type relationship, while the ductility was dependent also on the prior-austenite grain size and pearlite colony size.

The effect of microstructural characteristics of pearlite on the mechanical properties of hypereutectoid steel

Two important parameters for dynamic recrystallisation can be derived from changes in the strain hardening rate: the critical strain for initiation of dynamic recrystallisation and the point of maximum softening. In the present work, these values are determined from stress–strain data obtained by compression testing over the range of 900–1100°C. The resulting strains are used to derive a kinetic model of dynamic recrystallisation for two materials: 304 austenitic stainless steel and a hypereutectoid plain carbon steel. The values of the mechanical parameters used to define the proposed model are confirmed with the aid of metallographic analysis of the recrystallised microstructures.

Modelling of dynamic recrystallisation kinetics in austenitic stainless and hypereutectoid steels

Hypereutectoid steels of 1% carbon, alloyed with high silicon and microalloying levels of vanadium were subjected to dynamic recrystallization. It was found that an increase in carbon and vanadium content leads to smaller grain size. To characterize the dynamic recrystallization behavior of these steels, compression tests were performed over the temperature range 900–1050 °C using strain rates of 0.01, 0.1 and 1 s −1 . Equations were generated that can be used to predict the critical strain for dynamic recrystallization. Of interest is the finding that there is an activation energy for deformation specifically associated with dynamic recrystallization (i.e. peak strain). This activation energy associated with the peak strain is lower than that associated with the steady state stress. This is contrary to Sellar's original observation that the peak strain is a function of the activation energy for deformation according to the Zener–Hollomon relationship.

Dynamic recrystallization of austenite in microalloyed high carbon steels

In order to design thermomechanical schedules for processing low carbon microalloyed steels, the various critical transformation temperatures, i.e. the start and finish of the austenite transformation (Ar3, Ar1) and the non-recrystallization temperature (Tnr), must be determined. Continuous cooling torsion and compression testing are useful ways to measure these values. In this study six low carbon microalloyed steels with different additions (Nb, Cu, Si and Mo) were examined using these techniques. Moreover, the equilibrium phase diagrams for each alloy were calculated using FactSage. The comparison of the thermomechanical testing results with the thermodynamic calculations leads to a better understanding of the effect of the different elements on the transformation behaviour of pipeline steels. Regarding transformation temperatures, Cu in residual contents showed a strong effect on decreasing both Ar3 and Ar1, which indicates a hardenability effect of this element. On the other hand, increasing Nb contents increased Tnr by accelerating Nb(C,N) precipitation. However, when Si was added to a Nb-microalloyed steel, the Tnr decreased.

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

In the present study, it was established that a pressure level of ∼2 GPa applied to nanoparticles can generate metallurgical bonds within an agglomerate. The shockwave consolidation technique was therefore used to produce bulk nanostructured silver samples from nano-sized powders. The grain size evolution was characterized during compaction and the mechanical properties of the bulk components were analyzed using X-ray diffraction, transmission electron microscopy and microhardness and compression testing at room temperature. Upon consolidation, an average grain size of 49 ± 22 nm was measured and constant hardness across the diameter with an average of 83 HV was obtained. Compression results showed strength of approximately 390 ± 10 MPa and ductility of 22 ± 2%. This study therefore demonstrates the possibility of fabricating reliable bulk nanostructured materials by imposing high pressure on nano-sized powders.

A.M. Elwazri

Papers

The effect of microstructural characteristics of pearlite on the mechanical properties of hypereutectoid steel

Modelling of dynamic recrystallisation kinetics in austenitic stainless and hypereutectoid steels

Dynamic recrystallization of austenite in microalloyed high carbon steels

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

Fabrication of bulk nanostructured silver material from nanopowders using shockwave consolidation technique