Rheological Characteristics of Fe-C-Cr(Ni) Alloys

The principal objective of this project was to investigate the rheological properties of Fe-C-Cr and Fe-C-Ni-based low-alloy steels using an Anton Paar high-temperature rotational viscometer up to 1550 °C. The emphasis was placed on determining the liquidus temperatures and evaluating the flow and viscosity curves and the temperature dependence of dynamic viscosity. All were studied depending on the change in the content of chromium (0.010-4.863 wt%), nickel (0.001-4.495 wt%), and carbon (0.043-1.563 wt%). It was shown that the dynamic viscosity decreases with increasing nickel content and increases with increasing carbon and chromium content. The experimental data of the flow curves were fitted using the Herschel-Bulkley model with a good agreement between the measured and calculated values. Characterization of the internal structure was performed by SEM and EDX analyses, confirming non-significant changes in the microstructure of the original and remelted samples. The phase composition of the selected samples was also determined using JMatPro 12.0 simulation software (Sente Software Ltd., Guildford, UK).

On the Recycling of Water Atomized Powder and the Effects on Properties of L-PBF Processed 4130 Low-Alloy Steel

The microstructure and mechanical properties of a 4130-grade steel processed by L-PBF using a feedstock of low-cost water atomized powder have been investigated considering the effects of powder recycling. Chemical analysis of the recycled powder showed a constant amount of alloying elements with a slight reduction in oxygen content. The as-built microstructure was mainly composed of a martensitic structure separated by a high fraction of low-angle grain boundaries, suggesting the application of a direct tempering treatment starting from the as-built condition as a cost-effective post-process thermal treatment rather than the conventional quench and tempering treatment. Moreover, the degree of anisotropy generated by L-PBF in as-built specimens could be reduced after performing either the direct tempering or the quench and tempering treatments. The possible degradation of powder properties on the steel performance was also investigated. After various powder recycling events, no significant deterioration in tensile properties was measured, indicating that the water atomized powder could be a sustainable feedstock candidate for L-PBF.

Correlation between Microstructure and Hydrogen Degradation of 690 MPa Grade Marine Engineering Steel

Electrochemical H charging, hydrogen permeation, and hydrogen-induced cracking (HIC) behavior of 690 MPa grade steel substrate and different heat-treatment states (annealed, quenched, normalized, tempered) are investigated by cyclic voltammetry (CV), hydrogen permeation, electrochemical H charging, and slow strain rate tensile test (SSRT). The results show that hydrogen diffuses through the steel with the highest rate in base metal and the lowest rate in annealed steel. The hydrogen-induced cracks in base metal show obvious step shape with tiny cracks near the main crack. The cracks of annealed steel are mainly distributed along pearlite. The crack propagation of quenched steel is mainly transgranular, while the hydrogen-induced crack propagation of tempered steel is along the prior austenite grain boundary. HIC sensitivity of base metal is the lowest due to its fine homogeneous grain structure, small hydrogen diffusion coefficient, and small hydrogen diffusion rate. There are many hydrogen traps in annealed steel, such as the two-phase interface which provides accommodation sites for H atoms and increases the HIC susceptibility.

Effects of Ti and Cu Addition on Inclusion Modification and Corrosion Behavior in Simulated Coarse-Grained Heat-Affected Zone of Low-Alloy Steels

In this paper, the effects of Ti and Cu addition on inclusion modification and corrosion behavior in the simulated coarse-grained heat-affected zone (CGHAZ) of low-alloy steels were investigated by using in-situ scanning vibration electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), and electrochemical workstation. The results demonstrated that the complex inclusions formed in Cu-bearing steel were (Ti, Al, Mn)-O_x-MnS, which was similar to that in base steel. Hence, localized corrosion was initiated by the dissolution of MnS. However, the main inclusions in Ti-bearing steels were modified into TiN-Al₂O₃/TiN, and the localized corrosion was initiated by the dissolution of high deformation region at inclusion/matrix interface. With increased interface density of inclusions in steels, the corrosion rate increased in the following order: Base steel ≈ Cu-bearing steel < Ti-bearing steel. Owing to the existence of Cu-enriched rust layer, the Cu-bearing steel shows a similar corrosion resistance with base steel.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel

The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 °C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 °C led to the significant acceleration of the austenite → ferrite and austenite → pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 °C·s^-1. The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 µm. As the analysis of the stress-strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.

The Influence of Different Microstructure on Tensile Deformation and Acoustic Emission Behaviors of Low-Alloy Steel

The effect of different microstructures, obtained under different tempering temperatures on acoustic emission (AE) characteristics and source mechanisms during tensile deformation, was investigated in this study. Different heat treatments were carried out on hot-rolled low-alloy steels to obtain different microstructures (ferrite/pearlite, tempered martensite and tempered sorbite) and the AE was used to monitor the deformation and fracture process of samples of different types (BM, 200 °C tempered and 600 °C tempered). The results showed that the microstructure had different influences on the high amplitude burst-type signals and low amplitude continuous-type signals during the deformation and fracture process of low-alloy steels. In the 200 °C tempered sample, the continuous-type signals were enhanced by the high yield stress and dislocation velocity induced by the block of the lath martensite whose substructure was high-density dislocation. On the other hand, the interaction of the precipitates with the local dislocations increased the intensity of AE events, thus generating burst-type signals with higher amplitude in the 600 °C tempered samples.

Analysis of Environmental Factors Affecting the Atmospheric Corrosion Rate of Low-Alloy Steel Using Random Forest-Based Models

As one of the factors (e.g., material properties, surface quality, etc.) influencing the corrosion processes, researchers have always been exploring the role of environmental factors to understand the mechanism of atmospheric corrosion. This study proposes a random forest algorithm-based modeling method that successfully maps both the steel’s chemical composition and environmental factors to the corrosion rate of low-alloy steel under the corresponding environmental conditions. Using the random forest models based on the corrosion data of three different atmospheric environments, the environmental factors were proved to have different importance sequence in determining the environmental corrosivity of open and sheltered exposure test conditions. For each exposure test site, the importance of environmental features to the corrosion rate is also ranked and analyzed. Additionally, the feasibility of the random forest model to predict the corrosion rate of steel samples in the new environment is also demonstrated. The volume and representativeness of the corrosion data in the training data are considered to be the critical factors in determining its prediction performance. The above results prove that machine learning provides a useful tool for the analysis of atmospheric corrosion mechanisms and the evaluation of corrosion resistance.

Effect of Thermal Cycle on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Low-Alloy Steel

Low-alloy steel samples were successfully fabricated by selective laser melting (SLM). The evolution of the microstructure and the mechanical properties were investigated with different values of the energy area density (EAD). The results revealed that the initial solidification microstructures of the single tracks with different EADs were all martensite. However, the microstructures of bulk samples under different EADs were not martensite and differed significantly even from one another. When EAD increased from 47 to 142 J/mm², the mixed lower bainite and martensite austenite microstructure changed to granular bainite; further, the morphology of bainite ferrite gradually changed from lath to multilateral. Moreover, with the increase of EAD, the grain size was remarkably reduced because of the increasing austenitizing periods and temperature during thermal cycling. The average grain size was 1.56 µm, 3.98 µm, and 6.31 µm with EADs of 142 J/mm², 71 J/mm², and 47 J/mm², respectively. Yield strength and tensile strength of the SLM low-alloy steel increased with the increase in EAD; these values were significantly more than those of the alloys prepared by traditional methods. The microstructure of the SLM low-alloy steel samples is not uniform, and the inhomogeneity becomes more significant as EAD decreases. Simultaneously, when EAD decreases, the fracture mechanism changes from ductile to a mixture of ductile and brittle fracture; this is in contrast to the samples prepared by traditional methods. This study also found a stress concentration mechanism around large pores during plastic deformation that resulted in a brittle fracture. This indicates that large-sized pores significantly degrade the mechanical properties of the specimens.

Influence of Magnesium Ions in the Seawater Environment on the Improvement of the Corrosion Resistance of Low-Chromium-Alloy Steel

This study examined the synergic effect of alloying the element Cr and the environmental element Mg²⁺ ions on the corrosion property of a low-alloy steel in seawater at 60 °C, by means of electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR) tests and weight-loss tests. The Mg²⁺ ions in seawater played an important role in lowering the electron transfer of the rust layer in the Cr-containing steel. The corrosion resistance of the Cr-containing steel is superior to that of blank steel in Mg²⁺ ions containing seawater. XPS and XRD results indicated that the formation of MgFe₂O₄ and a mixed layer (Cr oxide + FeCr₂O₄ + MgCr₂O₄) improved the corrosion resistance of the low-alloy steel in the seawater.