Advanced Thermomechanical Processing Research Articles

Investigation and Simulation of the Effects of nm-Scale γ′ Precipitates on the Recrystallization of Ni-based Superalloys

Superalloys are critical materials for the hottest sections of stationary gas turbines and aircraft engines. Homogeneously fine-grained microstructures are essential to unlock their superior high-temperature strength but are challenging to achieve in γ′-containing Ni-based superalloys. Such microstructures are achieved by recrystallization through hot working and grain boundary pinning viaμm-scale second phase particles. Discontinuous dynamic recrystallization is the predominant restoration mechanism, where grain growth is restricted by Zener pinning. Nanometer-scale γ′ precipitates may exercise similar pinning during the nucleation stage and thus delay recrystallization. While the effects of coarse, μm-scale, precipitates during recrystallization and grain growth are well-known, descriptions for fine coherent precipitates are currently lacking. To address this scarcity of knowledge, both γ′-rich and -lean microstructures of the γ′-containing Ni-base superalloy René 41 underwent identical uniaxial hot compression tests. Flow stress and microstructural analyses reveal the inhibition of recrystallization by nm-scale γ′ precipitates during both nucleation and growth stages. This effect is successfully described using thermo-kinetic modeling through application of a driving-force based model. These unique insights provide a novel pathway to unlock homogeneously fine-grained microstructures in γ′-containing Ni-based superalloys via advanced thermo-mechanical processing routes, required for applications in future generations of gas turbines and aircraft engines.

Chemical heterogeneity enhances hydrogen resistance in high-strength steels

The antagonism between strength and resistance to hydrogen embrittlement in metallic materials is an intrinsic obstacle to the design of lightweight yet reliable structural components operated in hydrogen-containing environments. Economical and scalable microstructural solutions to this challenge must be found. Here, we introduce a counterintuitive strategy to exploit the typically undesired chemical heterogeneity within the material’s microstructure that enables local enhancement of crack resistance and local hydrogen trapping. We use this approach in a manganese-containing high-strength steel and produce a high dispersion of manganese-rich zones within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting hydrogen-induced microcracks and thus interrupting the percolation of hydrogen-assisted damage. This results in a superior hydrogen embrittlement resistance (better by a factor of two) without sacrificing the material’s strength and ductility. The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing.

An Initial Report on the Structure–Property Relationships of a High‐Strength Low‐Alloy Steel Subjected to Advanced Thermomechanical Processing in Ferrite

An Initial Report on the Structure–Property Relationships of a High‐Strength Low‐Alloy Steel Subjected to Advanced Thermomechanical Processing in Ferrite

An Initial Report on the Structure–Property Relationships of a High‐Strength Low‐Alloy Steel Subjected to Advanced Thermomechanical Processing in Ferrite

Advanced thermomechanical processing (aTMP) of high‐strength low‐alloy (HSLA) steels has been applied to achieve ultrafine ferrite grains decorated with nanoscale precipitates, clusters, and solute segregation, potentially leading to an increase in strength, toughness, and ductility. In our own previous research on a modern Mo–Ti–Nb HSLA steel processed in ferrite, this was only confirmed using hardness testing. Reports on the success of similar aTMP routes in achieving superior mechanical properties so far only provided results from subsize specimens or simpler steels under large strain conditions. Therefore, herein, an initial report on the mechanical properties of the previously studied Mo–Ti–Nb HSLA steel subjected to warm rolling and aging is provided. A reduction of 55% at 650 °C leads to an ultimate tensile strength (UTS) of 650 MPa, a yield to an ultimate tensile strength ratio of 0.95, and a total elongation of 14% in the as‐rolled condition, similar to mild steels deformed to larger strains. The low yield to UTS ratio is explained by precipitate coarsening. Delamination occurs in the low‐temperature region of Charpy impact testing in both longitudinal and transversal directions. Direct aging significantly increases the room temperature impact energy due to the onset of grain growth.

Advanced Thermo-mechanical Process for Homogenous Hierarchical Microstructures in HSLA Steels

Engineering microstructures in high-strength low-alloy steels via advanced thermo-mechanical processing is a promising approach to overcome challenges around low work hardenability and toughness in ultrafine-grained mild steels. Recently, multiscale-hierarchical microstructures with ultrafine grains and two populations of precipitates decorating high-angle grain boundaries and dislocation structures were achieved by some of the current authors in a modern Ti-Mo-Nb high-strength low-alloy steel. However, the high-strain rate of 10 s−1 during single-pass plane-strain compression at 600 °C led to the formation of macroscopic shear bands. Here, we propose an optimized advanced multi-hit thermo-mechanical process for achieving homogenous hierarchical microstructures in the same steel without strain localization. This is verified via microscopy and thermo-kinetic modelling, using the software MatCalc. A typical body-centred-cubic rolling texture is achieved in contrast to previous process design. Ultrafine crystallites confined by a mixture of high-angle gain and subgrain boundaries are formed, decorated by two types of precipitates. Large FeMnC-rich cementite particles are found on grain boundaries and smaller TiNbC-rich precipitates on dislocations and subgrain boundaries. It is shown that TiNbC particles transform to a core-shell structure when subjected to direct aging. Thermo-kinetic modelling underpins experimental results concerning the detailed evolution of crystallite size, precipitate morphology and composition, enabling a through-process description of the microstructural evolution.

On the Microstructural Characteristics of UFG Microalloyed Steel Influencing the Mechanical Behavior under Dynamic Loading

This paper presents a summary of a preliminary research aimed at producing ultrafine-grained (UFG) and heterogeneous microstructure in microalloyed steel and testing these materials under dynamic loading conditions (strain rates 800 s-1 and 1800s-1). The UFG and bimodal-structures, due to grain size, structural composition or morphology of structural components, were produced by an advanced thermomechanical processing, namely rolling in: hot, two-phase and cold-hot combined conditions. The advantage of bimodal microstructures is their maximization of mechanical behavior under extreme loading conditions due to promoted accumulation and interactions of geometrically necessary dislocations. The dynamic work-hardening behavior has been studied as a function of solute atoms and fine-scale, second-phase particles in the UFG and bimodal-structures. The substantial complexity of the phenomena, which occur through the evolution of microstructure and texture in response to dynamic loading, presents formidable challenges to theoretical model development of plastic deformation of UFG and bimodal-structures. Such an extraordinary work hardening provides an attractive strategy to develop optimal combination of mechanical properties i.e. strength/ductility ratio. A multi-scale analysis capable of including material behavior in different scales should be applied to discuss mechanical response of mentioned above microstructures and to help to analyze their influence on mechanical behavior under dynamic loading. The investigation was performed for a material of common application: high strength microalloyed steel X70. The experimental results show that strain rate sensitivity of the heterogeneous microstructures obtained by various thermomechanical rolling routes are significant, but not by a similar magnitude with the microstructure compositions and increasing strain rate.

Engineering Hierarchical Microstructures via Advanced Thermo-Mechanical Processing of a Modern HSLA Steel

Advanced thermo-mechanical processing of mild steels in the ferrite phase field has recently achieved breakthrough in grain refinement into the submicron regime. However, these steels often suffer from grain boundary failure and low rates of work hardening. A potential approach to overcome these challenges is to process modern high-strength low-alloy steels with multi-scale hierarchical microstructures. Thus, the applicability of advanced thermo-mechanical processing for achieving such microstructures in a high-strength low-alloy steel was studied. The microstructural evolution during warm deformation of a martensitic/bainitic starting microstructure using a Gleeble 3500 thermo-mechanical simulator at 600 °C followed by a direct aging step was investigated. The strain rate of 10 s−1 led to strain localization and, therefore, the formation of a macroscopic shear band. High-resolution characterization techniques such as electron channeling contrast imaging, electron backscatter diffraction, and transmission electron microscopy were used to reveal the ultrafine grain sizes (~ 0.5 μm) in this shear band. The mechanism behind this refinement is continuous dynamic recrystallization, as the initial grains subdivided into smaller crystallites that are confined by a mix of subgrain and high-angle grain boundaries. Two populations of precipitates were formed. Larger precipitates (mean diameter ~ 150 nm) decorate grain boundaries, whereas smaller precipitates (~ 15 nm) nucleate on dislocations and subgrain boundaries.

Fabricating ultrafine grain by advanced thermomechanical processing on low-carbon microalloyed steel

The ultrafine grain with the effective grain size of 1.10 μm was fabricated by intercritical deformation on low-carbon microalloyed steel, which was finer than the deformation induced ferrite transformation (DIFT) at 1.55 μm. During intercritical deformation, the DIFT process was enhanced, showing a colony distribution of the refined grains. Meanwhile, the continuous dynamic recrystallization of pro-eutectoid ferrite was developed by both the subgrains progressively trapping dislocations and the serrated boundary pinching off the elongated grains. These processes were directly observed through the grain average misorientation maps of electron back-scattered diffraction and confirmed by transmission electron microscopy micrographs.

The mechanical response of UFG and nanostructured microalloyed steels subjected to dynamic loading conditions

As the number of available, advanced high-strength metallic materials possibilities increases due to advancements in processing (for example advanced thermomechanical processing - ATP or severe plastic deformation - SPD), experimental comparisons alone are not sufficient for determination of the most ideal microstructures for specific applications. Our study deals with the dynamic behaviour of high strength steels and in particular with ultrafine-grained (UFG) microalloyed ferrite and austenite. The forming processes of modern UFG materials require rheological models describing the materials behaviour at large strains and strain rates up to over 1000 s-1. In our case, the mechanical response of UFG steels (produced using MaxStrain system) was investigated with split Hopkinson pressure bar (SHPB) tests, performed at room temperature. The dynamic work-hardening behaviour as a function of solute atoms and fine-scale, secondphase particles in the nano-structures of microalloyed ferrite and austenite has been compared to the mechanical response of these materials under quasi-static loading conditions.

The development of ultrafine-grained hot rolling products using advanced thermomechanical processing

The aim of the work is development of industry guidance concerning production of ultrafine-grained (UFG) High Strength Low Alloy (HSLA) steels using strain-induced dynamic phase transformations during advanced thermomechanical processing. In the first part of the work, the effect of processing parameters on the grain refinement was studied. Based on the obtained results, a multiscale computer model was developed in the second part of the work that was subsequently used to predict the mechanical response of studied structures. As an overall outcome, a process window was established for the production of UFG steels that can be adopted in existing hot rolling mills.

Analysis of change in microstructure and properties during high temperature processing of ultralow C and high Nb microalloyed steel

To further improve the strength and toughness, the advanced thermomechanical controlled processing has been applied in the development of an ultralow C and high Nb bearing steel. In the present investigation, the effects of processing parameters, consisting of the coiling and starting temperatures in non-recrystallisation region, on the final microstructure and mechanical properties of this steel have been studied by tensile, Charpy impact tests, optical microscopy and transmission electron microscopy. Results indicate that the acicular ferrite dominated microstructure can be greatly refined in grain size with decreasing the starting temperature of finishing rolling. However, for high Nb steels, the too low starting temperature would promote the formation of high temperature transformation products and consequently make against the improvement of mechanical properties. In addition, the optimum temperature window of finishing rolling is found to be also related to alloying levels of austenite stabilising elements. At the high starting temperature of finishing rolling, the precipitation strength contribution increases with increasing coiling temperature. However, the increase in strain accumulation associated with low temperature processing greatly reduces the sensitivity of the precipitation strength contribution to coiling temperature.

Microstructure and mechanical properties of bainitic low carbon high strength plate steels

This paper reports the results of an investigation of plates produced by the advanced thermomechanical processing (TMP) schedules, which were designed using the results of a laboratory study. There were two steel compositions that corresponded to X-80 with carbon contents 0.04 and 0.07 wt.%, respectively. The variation in microstructure, hardness, tensile properties and Charpy impact properties with TMP schedule were determined, and compared with the expected requirements for X-100 linepipe steel. The relationships between microstructure and mechanical properties were experimentally obtained and discussed.