Role of Boron in Steels

Boron is a versatile and most significant alloying element in steel production, primarily known for its ability to enhance the mechanical properties of various steel grades. By introducing even minimal concentrations of boron—typically between 0.0005% and 0.003% by weight—steel manufacturers can markedly improve martensite formation (vs. ferrite formation) and hence hardenability, wear resistance, interface cohesion, hydrogen embrittlement resistance, and overall mechanical performance, making boron containing steels particularly suitable for demanding applications in industries such as automotive, manufacturing, and construction.

Boron's unique properties and its ability to enhance steel's mechanical performance make it therefore a critical element in steel production, with applications spanning various industries. However, the need for careful management of its use highlights the complexities involved in optimizing its benefits in steels.

The efficiency of boron in promoting hardening during heat treatment processes allows it to serve as a cost-effective alternative to more expensive alloying elements like chromium and vanadium, reducing material costs while maintaining essential properties.

The efficiency of boron in promoting hardening during heat treatment processes allows it to serve as a cost-effective alternative to more expensive alloying elements like chromium and vanadium, reducing material costs while maintaining essential properties.

Boron’s role in steel production is not without its challenges; while it significantly increases hardness and strength, excessive amounts can lead to brittleness and reduced ductility, which pose risks during welding and structural applications.[5]

 This delicate balance necessitates precise control over boron content during the alloying process to maximize its benefits while minimizing potential drawbacks.

 Among the widely used boron steel grades are 10B21 and 15B24, known for their excellent weldability and responsiveness to heat treatment, making them ideal for automotive components such as seating mechanisms and structural elements.

 The integration of boron in steel also raises considerations regarding environmental sustainability and economic feasibility. The energy-intensive processes required to achieve high hardness can contribute to a larger carbon footprint, prompting ongoing research into cleaner production techniques and recycling practices within the industry.

 As demand for high-performance materials continues to rise, innovations in boron alloy production are likely to play a crucial role in meeting the evolving requirements of modern engineering while addressing the challenges of sustainability.

Boron (B) has the atomic number 5, situated in Group 13 of the periodic table. It exhibits a combination of metallic and non-metallic properties, behaving as a semiconductor. At elevated temperatures, boron acts as a conductor of electricity, whereas it functions as an insulator at lower temperatures. In its crystalline form, boron appears as a dark, brittle, and lustrous metalloid, with hardness measured at 9.3 on the Mohs scale, which allows it to scratch some abrasives, but its brittleness limits its utility in tools. Boron exists in multiple allotropic forms, with at least four crystalline modifications identified. These forms feature closed cages containing 12 boron atoms arranged in an icosahedral structure. Chemically, boron is relatively inert at room temperature, showing minimal reactivity with substances like boiling hydrochloric acid and hot concentrated nitric acid. However, its amorphous form is more reactive, allowing boron to readily bond with other elements to form stable covalent compounds. In nature, boron is predominantly found in the form of borate minerals such as borax (Na2B4O5(OH)4·8H2O) and boric acid, and it is distributed across the Earth's crust in very small quantities—approximately 0.001 percent by weight. It typically occurs as a mixture of isotopes, primarily boron-10 and boron-11, in a natural ratio of 19.9% to 80.1%, respectively. The inclusion of boron in steel enhances several mechanical properties, such as hardness, wear resistance, and overall performance. Boron significantly improves the hardenability of steel, allowing it to maintain high strength even at elevated temperatures. The addition of boron can also promote fine austenite grain structures, which are crucial for achieving desirable toughness and strength in various steels. However, while boron increases the hardness of steel, it also tends to reduce its ductility, making boron-alloyed steels more prone to brittleness under certain conditions.

The primary function of boron in steel is to improve hardenability, which is the ability of the steel to harden in response to heat treatment. Boron achieves this by promoting the formation of a martensitic structure (against ferrite formation) during quenching, leading to increased hardness and strength. For instance, boron can be a substitute for other alloying elements that require higher concentrations to achieve similar hardening effects, such as chromium and vanadium. This efficiency makes boron a quite cost-effective option in the formulation of medium carbon steels and also recently of medium Mn steels.

Its further unique properties as an alloying element make it also efficient in  not only enhancing hardenability, but also by altering grain boundary behavior, such through improved GB cohesion, further influencing mechanical performance.

More specific we can identify 3 aeras of interst in current reserach along these lines:

1. Hardenability Enhancer: Boron’s unique ability to enhance hardenability stems from its occupancy of interstitial positions in the iron lattice. It modifies the kinetics of phase transformations by reducing the critical cooling rate required for martensitic transformation. This is particularly beneficial in low-alloy steels, where other alloying elements like manganese or chromium may not fully achieve the desired hardenability.
2. Grain Boundary Behavior: Boron’s tendency to segregate at grain boundaries impacts both strength and embrittlement. In low-carbon steels, boron segregation strengthens grain boundaries by reducing the tendency for intergranular cracking. Conversely, in complex steels such as austenitic lightweight alloys, excessive boron segregation and subsequent precipitation at grain boundaries can lead to brittle failure under certain mechanical or thermal conditions.
3. Interaction with Nitrogen and Titanium: Boron strongly interacts with nitrogen to form stable nitrides (e.g., BN), which prevents nitrogen from forming deleterious compounds that impair toughness. However, to maximize boron’s hardenability effects, its free (uncombined) state must be preserved, often achieved by combining nitrogen with titanium, which preferentially forms TiN precipitates.

Interface segregation affects the microstructure evolution and mechanical properties of alloys, including strength, ductility and damage tolerance. This is particularly true for multiphase high-strength steels containing multiple types of interfaces whose characteristics are key factors influencing the steels’  mechanical performance. The different tendencies of solute segregation to different types of interfaces can lead to complex segregation behavior, which needs to be understood. In our studies, we focus on the segregation behavior of B in a high-Mn, high-Al lightweight steel with a two-phase austenite-ferrite microstructure. We find distinct B segregation at both austenite and ferrite grain boundaries as well as at austenite-ferrite phase boundaries after high temperature annealing (1100◦C) and fast quenching. The segregation process is governed by local equilibrium between bulk and interfaces as discussed in terms of thermodynamic and ab initio calculations. Our findings reveal a dependence
of B segregation on the interface structure regardless of the adjacent phases, which can be explained in terms of respective interfacial energy in accord with the Gibbs adsorption isotherm. In addition, co-segregation of B and C is observed at both high-angle and low-angle ferrite grain boundaries due to the attractive interaction between the two solutes in the bulk ferrite phase. In contrast, for austenite grain boundaries, C depletion is observed owing to its site competition effect and repulsive interaction with B in austenite. These  observations help to guide interface segregation engineering in complex multiphase lightweight steels to improve their mechanical performance.

In response to the automotive industry's growing demand for lightweight materials that do not compromise safety, advanced high-strength steels (AHSSs) with complex microstructures are emerging as key solutions. Among these, high-Mn and high-Al lightweight steels, featuring austenite and ferrite phases, are particularly attractive due to their low density and exceptional mechanical properties. These materials offer ultimate tensile strengths approaching 1 GPa, combined with elongation rates up to 60%. A critical aspect of their performance is the behavior of solutes, such as boron (B), at grain and phase boundaries.

Interface segregation, wherein solutes preferentially localize at planar defects like grain and phase boundaries, plays a pivotal role in defining the mechanical behavior of multiphase steels. This process minimizes the system's free energy and is influenced by factors such as bulk solubility and the specific characteristics of the interface. Boron, known for its potent effect in enhancing steel's hardenability and damage tolerance, exemplifies this phenomenon. Through segregation at grain boundaries, boron delays ferrite nucleation during the austenite-to-ferrite transformation, thereby improving hardenability. Moreover, boron strengthens grain boundary cohesion, mitigating hydrogen-induced intergranular cracking.

Boron segregation can occur via two distinct mechanisms:

• Non-Equilibrium Segregation: This form arises due to rapid cooling, which creates oversaturated vacancy populations. Interfaces act as vacancy sinks, attracting boron-vacancy complexes, especially in scenarios involving high cooling rates.
• Equilibrium Segregation: Governed by the minimization of Gibbs free energy, this mechanism involves the diffusion of boron to interfaces to establish a local equilibrium. Factors such as temperature, bulk composition, and interfacial energy strongly influence this process.

In high-Mn and high-Al steels, segregation is observed across various types of interfaces, including low-angle and high-angle grain boundaries, twin boundaries, and phase boundaries between austenite and ferrite. These boundaries differ in crystallographic and structural features, leading to complex segregation behavior.

Research reveals that boron segregation is interface-specific, with higher concentrations observed at high-angle grain boundaries compared to low-angle ones. In ferrite, boron tends to co-segregate with carbon due to their mutual attraction, whereas in austenite, boron and carbon exhibit repulsive interactions, leading to carbon depletion at grain boundaries.

Austenite-ferrite phase boundaries further complicate segregation behavior. Semi-coherent phase boundaries with Kurdjumov-Sachs orientation relationships show lower boron segregation levels compared to incoherent boundaries. This discrepancy is attributed to differences in interfacial energy and atomic structure.

Understanding boron's segregation behavior offers opportunities to engineer steels with tailored microstructures. By manipulating phase fractions and interface types through precise annealing treatments, it is possible to control solute segregation. For example, optimizing boron distribution at critical boundaries can enhance resistance to hydrogen embrittlement, intergranular cracking, and other failure mechanisms, improving the overall damage tolerance of lightweight steels.

This knowledge contributes to advancing steel design strategies, ensuring high performance and safety in lightweight applications while addressing the automotive industry's pressing demands for weight reduction and sustainability.

In our work the interaction between boron and hydrogen at grain boundaries is investigated experimentally and numerically in boron-doped and boron-free martensitic steels using thermal desorption spectrometry (TDS) and ab initio calculations. The calculations show that boron and hydrogen are attracted to grain boundaries but boron can repel hydrogen. This behavior has also been observed using TDS measurements, with the disappearance of one peak when boron is incorporated into the microstructure. Additionally, the microstructure
of both steels has been studied through electron backscattered diffraction, synchrotron X-ray measurements, and correlative transmission Kikuchi diffraction-atom probe tomography measurements. While they have a similar grain size, grain boundary distribution, and dislocation densities, pronounced boron segregation into PAGBs is observed for boron-doped steels. It indicates that boron in PAGBs is responsible for the disappearance of the TDS peaks for the boron-doped steel. Then, the equilibrium hydrogen concentration in different trapping sites has been evaluated using the Langmuir–McLean approximation. This thermodynamic model shows that the distribution of hydrogen is identical for all traps when the total hydrogen concentration is low for boron-free steel. However, when it increases, traps of the lowest segregation energies (mostly PAGBs) are firstly saturated, which promotes failure initiation at this defect type. This finding partially explains why PAGBs are the weakest microstructure feature when martensitic steels are exposed to hydrogen-containing environments.