Formation of heat affected zone (HAZ) when welding is considered as weakest link across transition joint. Characterization of this zone is a key issue to reduce the effect of embrittlement; hence appropriate welding parameters were adopted. In the present study, HAZ simulation was done for boron modified P91 steel using Gleeble. It has been observed that matrix was martensitic in nature for coarse-grain HAZ (CGHAZ)/fine-grain HAZ (FGHAZ) whereas for inter-critical HAZ (ICHAZ), matrix was ferrite + martensite containing primary carbides. The size and fraction of carbides varied with peak temperature. Presence of boron influenced the microstructural characteristics of different zones by influencing precipitation nature and fractional characteristic of phases. Substantial variation in hardness was obtained before and after post-weld heat treatment (PWHT). In this respect, simulated samples exhibited maximum hardness at CGHAZ and after PWHT the same region showed minimum hardness. Short-term impression creep testing of simulated sub-HAZs was done, which revealed embrittling effect in ICHAZ, whereas embrittlement shifted to CGHAZ after PWHT. Based on conventional nomenclature for weldment failures, it is confirmed that PWHT shifted type-IV failure (ICHAZ) to type-III failure (CGHAZ) in boron modified P91 steel.

Evolution of Microstructure during Welding Simulation of Boron Modified P91 Steel

Boron in Heat-Treatable Steels: A Review

The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications, however, it is desirable to combine direct quenching with tempering, and, bearing in mind the form of the semi-finished product, it is of interest to study the effect of tempering whole coils in a bell furnace. Here, the effects of boron, carbon, titanium, vanadium and tempering temperature on the microstructure, crystallography and mechanical properties of direct-quenched steels has been studied with the aid of simulated bell furnace heating and cooling cycles. All steels contained (in wt.%) 0.2Si–1Mn–1Cr-0.65Mo-0.03Al, while there were two levels of C (0.095 /0.140), V (0 /0.08), Ti (0 /0.025) and B (0 /0.0015). Tempering was performed with peak temperatures at 180 and 570 °C. The paper reveals several possible alloying and processing routes to strong and tough low-C steel. Carbon controls the strength and toughness, while titanium and boron affects the grain size of coarsest grains (d90%), Vanadium has a strong effect on strength retention during tempering at 570 °C: an addition of 0.08 wt% vanadium increases yield strength by 70 MPa and ultimate tensile strength by 100 MPa. The removal of boron from the steel is shown to have a huge impact not only on the microstructure but also on the impact toughness.

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Direct-quenched and tempered low-C high-strength structural steel: The role of chemical composition on microstructure and mechanical properties

The boron (Z = 5) element is unique. Boron-based (nano-)materials are equally unique. Accordingly, the present special issue is dedicated to crystalline boron-based (nano-)materials and gathers a series of nine review and research articles dealing with different boron-based compounds. Boranes, borohydrides, polyhedral boranes and carboranes, boronate anions/ligands, boron nitride (hexagonal structure), and elemental boron are considered. Importantly, large sections are dedicated to fundamentals, with a special focus on crystal structures. The application potentials are widely discussed on the basis of the materials’ physical and chemical properties. It stands out that crystalline boron-based (nano-)materials have many technological opportunities in fields such as energy storage, gas sorption (depollution), medicine, and optical and electronic devices. The present special issue is further evidence of the wealth of boron science, especially in terms of crystalline (nano-)materials.

https://www.mdpi.com/2073-4352/6/9/118/pdf

Boron-Based (Nano-)Materials: Fundamentals and Applications

Single-pass compression tests were performed to investigate the hot deformation behavior of low-carbon boron microalloyed steel containing three various vanadium contents at 900–1100 °C and strain rate of 0.01–10 s−1 using the MMS-300 thermal mechanical simulator. The flow stress curves of investigated steels were obtained under the different deformation conditions, and the effects of the deformation temperature and strain rate on the flow stress were discussed. The characteristic points of flow stress were obtained from the stress dependence of strain hardening rate; the activation energy of investigated steels was determined by the regression analysis; the flow stress constitutive equations were developed; the effect of vanadium content on the flow stress and dynamic recrystallization (DRX) was investigated. The result showed that the flow stress and activation energy (316.5 kJ mol−1) of the steel containing 0.18 wt% V were significantly higher than those of the steels with 0.042 wt% and 0.086 wt% V, which was related to the increase in solute drag and precipitation effects for higher vanadium content. DRX analysis showed that the addition of vanadium can delay the initiation and the rate of DRX.

Influence of Vanadium Content on Hot Deformation Behavior of Low-Carbon Boron Microalloyed Steel

The initial austenite grain size of boron microalloyed steel with three different amounts of boron (20, 40, and 60 ppm) was investigated under different heating temperatures (1150, 1100, and 1050 °C), and hot compression tests of samples in a wide range of temperature (900–1100 °C) and strain rate (0.1–10 s−1) were conducted. It was found that the initial austenite grain size increases with increasing temperature and boron content. The flow stress decreased with increasing boron content at lower strain rates. The flow stress constitutive equation of hot deformation was developed for the experimental steels; results showed that boron addition has the trend to reduce the hot deformation activation energy. The characteristic points of the flow curves were analyzed. Results revealed that the peak and critical stress decreased in response to an increase of boron content. The work-hardening behavior of both steels was investigated, and it was found that boron addition can decrease the work-hardening rate when strained at lower strain rates. On the contrary, peak and critical strains increased as boron content increased, indicating that boron has the ability to delay the onset of dynamic recrystallization.

/pdf/influence-of-boron-on-initial-austenite-grain-size-and-hot-3u0e6gds8d.pdf

Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels

The dynamic recrystallization behavior of low-carbon steel containing three different amounts of boron (0.002, 0.004, and 0.006 wt%) was systematically investigated under various experimental conditions. Low-carbon steel was exposed to temperatures ranging from 900 to 1100 °C, strain rates from 0.1 to 10 s−1, and inspection of the initial austenite grain size at 1150 °C. The resulting stress–strain curves are observed to possess two classifications of behavior, dynamic recovery and dynamic recrystallization, while the initial austenite grain size increases directly proportional to boron concentration. Additionally, the characteristic points of the flow curves were analyzed by regression method in which the peak and critical stresses decreased in response to an increase in boron composition, indicating that a softening effect appears with the addition of boron. On the contrary, peak and critical strains increased as boron content increased, indicating that boron has the ability to delay the onset of dynamic recrystallization. Lastly, the kinetics model of dynamic recrystallization for three boron-treated steels was established.

Yong-liang Gao

Papers

Influence of Boron on Initial Austenite Grain Size and Hot Deformation Behavior of Boron Microalloyed Steels

Effect of Boron Concentration on Dynamic Recrystallization Behavior of Low-Carbon Steel