Dynamic recovery actions in multi-objective liner shipping service with buffer times

This paper investigates the dynamic recovery policies for liner shipping service with the consideration of buffer time allocation and uncertainties. We aim to allocate the buffer time at the tactical level and then determine the optimal policy, including speed optimization strategy, port skipping and acceleration rate choice, for recovering from disruptions due to various uncertainties or random adverse events, which cause vessel delays. To achieve this, we attempt to obtain the optimal balance among economic, environmental and service-reliable objectives. A novel mathematical formulation is introduced to solve the robust vessel scheduling problem with short- and long-term decisions. Furthermore, we propose and test two heuristics to solve the proposed model. Experiments on the container liner shipping service show the validity of the model and some managerial insights are gained from them.

Basic Creep-Fatigue Models Considering Cavitation

Cavitation plays a central role during creep-fatigue. During recent years, fundamental models for initiation and growth of creep cavities that do not involve any adjustable parameters have been developed. These models have successfully been used to predict creep rupture data for austenitic stainless steels again without using adjustable parameters. However, it appears that basic models have not yet been applied to creep-fatigue assessments. A summary of the fundamental cavitation models is given. A model for monotonous deformation is transferred to cyclic loading. The parameter values are kept except that the dynamic recovery constant is raised due to increased interactions between dislocations during cycling. This model is successfully compared with observed LCF and TMF hysteresis loops. A new model for cavity growth due to plastic deformation is presented. The model is formulated in such a way that the condition for constrained growth is automatically satisfied. In this way, it is avoided to overestimate the growth rate.

Current Understanding of Microstructure and Properties of Micro-Alloyed Low Carbon Steels Strengthened by Interphase Precipitation of Nano-Sized Alloy Carbides: A Review

The current understanding of the microstructural features and mechanical properties of micro-alloyed low carbon steels strengthened by interphase precipitation of nano-sized alloy carbides are critically reviewed in this paper. The experimental results obtained via advanced quantitative characterization have revealed that interphase precipitation is promoted at the ferrite/austenite interface with a relatively lower degree of coherency caused by the deviation from the exact Kurdjumov–Sachs orientation relationship. Its dispersion becomes refined by enlarging the driving force for its precipitation, as adjusted by changing the transformation condition and chemical composition. The occurrence of interphase precipitation can significantly increase the strength of steels due to its large precipitation strengthening, and maintain good ductility as a result of enhanced work-hardening and dynamic recovery in different stages of tensile deformation. Finally, the application of interphase precipitation to ferrite/martensite dual-phase steels, together with our outlook on the challenging points in future research, are briefly explained.

Effect of Supra-Transus Deformation Conditions on Recrystallization of Beta Ti Alloy

There is increasing usage of high strength Beta Ti alloy in aerospace components. However, one of the major challenges is to obtain homogeneous refined microstructures via the thermo-mechanical processing. To overcome this issue, an understanding of the hot deformation conditions effect on the microstructure, prior to and after annealing, is needed. In this work, the effect of strain levels, which is more precise than percentage of reduction, and strain rate under supra-transus deformation temperature on beta annealing are studied using a double cone sample. The Electron Backscattered Diffraction (EBSD) is used to determine the deformed microstructure and texture evolution, as well as the static recrystallized grains evolution using the ex situ annealing approach. This work provides evidence that the mechanisms of dynamic recovery and recrystallization, along with texture evolution, are affected by the deformation conditions, which in turn affected the subsequent static recrystallization during annealing. It will also be shown that high levels of strain do not necessarily lead to an increase in the rate of recrystallization. Finally, the results obtained provided several examples of guidance in designing the TMP processes for obtaining not only a refine microstructure, but a more homogeneous beta microstructure during the beta processing of Beta Ti alloy.

Dynamic Recrystallization and Recovery Behaviors in Austenite of a Novel Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo Ultrahigh Strength Steel

Due to the complex composition and high proportion of alloys in traditional ultrahigh strength steel, the dilemma caused by ultrahigh strength and low toughness in casting and forging processes requiring subsequent heat treatment can be mitigated with an efficient and economical rolling process. In this work, the effect of deformation parameters on dynamic recrystallization (DRX) and dynamic recovery (DRV) is discussed through stress-strain analysis, the DRV mathematical model is obtained, and then the dynamic recrystallization activation energy, Zener–Hollomon equation, and hot working equation are obtained. The critical strain of DRX detected by the P-J method is ε c / ε p = 0.631 , which indicates that dynamic recrystallization of this novel steel is relatively easy to achieve by the rolling process. These models and conclusions have potential to be generalized for the formulation of process specification and process configuration without requiring extensive material testing.

Hot working behaviour and processing maps of duplex cast steel

In this paper the hot working behaviour of medium carbon duplex cast steel is studied using uniaxial hot compression tests over a temperature range varying from 700 ˚C to 1 000 °C and at different strain rates ranging from 10–4 to 10–1 s–1. A model based on a variant of a dynamic materials model was employed to construct processing maps. These maps delineate the safe and unsafe domains. The safe domains, associated with dynamic recrystallization and dynamic recovery, can be chosen to optimize the hot workability of the studied material. Whereas, the unsafe domain is to be avoided because it is associated with plastic deformation instabilities. The domain associated with dynamic recrystallization is centred at 1000 °C and 10–4 s–1 with a peak energy dissipation efficiency of about 40%, while the domain associated with dynamic recovery is centred at 700 °C and 10–4 s–1 with a peak energy dissipation efficiency of about 27%. The unsafe hot working domain, spread over the entire temperature range and moderate to high strain rates, predicts the appearance of flow instabilities, in the form of shear bands and intergranular cracks. To validate the obtained results, microstructural observations corresponding to different processing conditions are presented.

Influence of Boundary Migration Induced Softening on the Steady State of Discontinuous Dynamic Recrystallization

During discontinuous dynamic recrystallization (DDRX), new dislocation-free grains progressively replace the initially strain-hardened grains. Furthermore, the grain boundary migration associated with dislocation elimination partially opposes strain hardening, thus adding up to dynamic recovery. This effect, referred to as boundary migration induced softening (BMIS) is generally not accounted for by DDRX models, in particular by “mean-field” approaches. In this paper, BMIS is first defined and then analyzed in detail. The basic equations of a grain scale DDRX model, involving the classical Yoshie–Laasraoui–Jonas equation for strain hardening and dynamic recovery and including BMIS are described. A steady state condition equation is then used to derive the average dislocation density and the average grain size. It is then possible to assess the respective influences of BMIS and dynamic recovery on the strain rate sensitivity, the apparent activation energy, and the relationship between flow stress and average grain size (“Derby exponent”) of the material during steady state DDRX. Finally, the possible influence of BMIS on the estimation of grain boundary mobility and nucleation rate from experimental data is addressed.