The research team set out to answer a deceptively simple question: what happens to the microscopic oxides that inevitably form during laser melting when the processing efficiency is doubled? In LPBF, a focused laser beam scans across a thin bed of metallic powder, melting it into a small melt pool that solidifies within microseconds. Oxygen dissolved in the powder or picked up from the atmosphere reacts with alloying elements during this brief molten lifetime, generating oxide particles that become trapped in the solidifying metal. These inclusions range from nanometer-scale dispersoids to micron-sized particles, and they act as potential crack nucleation sites that can degrade impact toughness. Because manufacturers increasingly push LPBF machines toward higher productivity, understanding how efficiency changes oxide behavior is essential for qualifying printed parts.

The experimental comparison produced strikingly detailed data. Samples built with doubled processing efficiency contained coarser and more abundant micron-scale oxides, with an average size of 2.24 micrometers and a number density of 41.7 per square millimeter, compared with 2.17 micrometers and 27 per square millimeter in low-efficiency samples. At the nanometer scale, the trend reversed in an unexpected way: high-efficiency samples had fewer but larger nano-oxides, with a number density of 6.17 times ten to the nineteenth per cubic meter and an average diameter of 31 nanometers, whereas low-efficiency samples contained 1.35 times ten to the twentieth per cubic meter of nano-oxides averaging 27 nanometers. This dual-scale characterization demonstrates that build efficiency does not simply scale oxide content uniformly; it selectively alters different size populations through distinct physical mechanisms.

To explain these observations, the researchers turned to classical nucleation and growth theory, modeling how oxide particles form and evolve under the thermal conditions of each processing regime. Their analysis suggests that slower cooling rates in high-efficiency processing suppress the nucleation of new oxide particles while enhancing the growth of those that do form. Fewer nuclei, given more time in the molten state, grow into larger particles. Meanwhile, stronger convection within the melt pool drives oxides toward the pool surface, where they can be expelled during track overlaps. The consequence is that more micron-sized oxides remain trapped internally in the solidified metal after successive laser passes, while the nano-scale population becomes sparser but coarser. This mechanistic picture links macroscopic processing parameters directly to the microscopic inclusion architecture through well-established physical principles of deoxidation chemistry and melt pool hydrodynamics.

Heat treatment, a standard post-processing step for maraging steels, added another layer of nuance to the findings. The researchers found that heat treatment did not change the oxide inclusions themselves, confirming that these particles are stable remnants of the melting process rather than products of subsequent thermal cycles. What heat treatment did accomplish was the elimination of the cellular substructures characteristic of as-built LPBF material, the fine dislocation networks and solute-decorated cell walls that form during rapid solidification. After heat treatment, both sample groups produced similar amounts of retained austenite, the metastable face-centered cubic phase that can influence cryogenic toughness. This means that the microstructural differences responsible for property variations between the two build efficiencies were almost entirely attributable to the oxides rather than to matrix features.

The mechanical property data revealed the practical importance of these oxide differences. Despite comparable tensile strengths between the two sample groups, low-efficiency samples exhibited substantially higher impact toughness. At room temperature, low-efficiency samples absorbed 195 joules of impact energy compared with 160 joules for high-efficiency samples. The gap persisted at cryogenic temperature, with low-efficiency samples absorbing 106 joules at minus 196 degrees Celsius versus 88 joules for high-efficiency samples. Because heat treatment had equalized matrix microstructure and retained austenite content, the researchers could attribute the toughness differences primarily to the oxide inclusion populations. Larger and more abundant micron-scale oxides in high-efficiency samples provide more crack nucleation sites and easier crack propagation paths, degrading impact performance at both ambient and cryogenic temperatures.

These findings carry significant implications for the industrial deployment of additively manufactured maraging stainless steels. Manufacturers seeking to maximize productivity by increasing laser power, scan speed, or layer thickness may inadvertently create a tougher-to-qualify material with degraded impact resistance. The study demonstrates that build efficiency must be carefully balanced rather than simply maximized. A processing window that optimizes oxide behavior, suppressing the formation of large micron-scale inclusions while managing the nano-scale dispersoid population, is essential for achieving the combination of strength and toughness that these steels are designed to deliver. This is particularly relevant for cryogenic applications, where the toughness gap between processing regimes remained substantial even at minus 196 degrees Celsius.

The research also connects to a broader scientific conversation about oxygen inclusions in LPBF-processed stainless steels. Previous studies have documented the presence of oxide inclusions in laser additively manufactured austenitic stainless steels and their effects on impact toughness and stress corrosion cracking behavior. Other work has explored the origin and formation of oxygen inclusions during powder bed fusion, tracing them to powder surface chemistry and atmospheric exposure. The present study extends this understanding to maraging stainless steels, a class of materials where the interplay between oxide inclusions, retained austenite, and precipitation strengthening is especially intricate. By isolating the effect of build efficiency through careful experimental design, the researchers have provided a clean dataset that other investigators can use to validate and refine predictive models of inclusion evolution during additive manufacturing.

Looking ahead, the mechanistic framework developed in this study opens pathways for rational process optimization. If slower cooling suppresses nucleation while enhancing growth, and if stronger convection redistributes oxides toward the melt pool surface, then targeted modifications to scanning strategies, such as adjusting laser parameters, hatch spacing, or interlayer time, could be used to steer the inclusion population toward a more favorable architecture. Numerical modeling of oxide particle evolution during additive manufacturing, building on classical deoxidation theory from steelmaking, offers a route to predicting inclusion outcomes before parts are ever printed. As additive manufacturing matures from rapid prototyping into certified production of safety-critical components, the ability to control not just bulk chemistry but also the microscopic inclusion landscape will separate qualified, reliable parts from those that fail unexpectedly. This study demonstrates that the invisible oxides inside printed steel are not incidental artifacts but controllable features that demand the same engineering attention as any other aspect of the manufacturing process.

Subject of Research: Effect of LPBF build efficiency on oxide inclusions and toughness of maraging stainless steel

Article Title: Effect of LPBF build efficiency on oxide inclusions and toughness of maraging stainless steel

Article References: Wang, C., Wang, L., Lin, X., Sun, Y., Liu, G., & Su, J. (2026). Effect of LPBF build efficiency on oxide inclusions and toughness of maraging stainless steel. Journal of Materials Science, 61(43), 33956-33973. https://doi.org/10.1007/s10853-026-13698-5

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13698-5

Keywords: LPBF, maraging stainless steel, oxide inclusions, toughness, additive manufacturing, build efficiency, nucleation and growth, cryogenic properties, heat treatment, retained austenite, melt pool convection, Journal of Materials Science

The stakes could hardly be higher. Advanced high-strength steels underpin the lightweighting of the automotive body-in-white, cutting vehicle mass by 30 to 50 percent compared with conventional low-strength carbon steel. That mass reduction translates directly into lower fuel consumption for combustion vehicles and longer driving range for electric ones, which is why steel remains the dominant structural material for A-pillars, B-pillars, bumpers, and roof rails. Yet the review highlights a troubling trend: as strength climbs, so does vulnerability. Studies cited in the review show that 2 GPa-grade press-hardened steel faces a significantly greater embrittlement risk than its 1.5 GPa counterpart, so severe that hydrogen contents above roughly 0.5 weight parts per million can entirely negate the weight-saving benefit of moving to the stronger grade. In laboratory bending tests, as little as 0.37 ppm of dissolved hydrogen collapsed the bendability of AlSi-coated 2 GPa press-hardened steel from 39.1 degrees to just 10.1 degrees.

Understanding why hydrogen is so corrosive to toughness requires descending to the atomic scale, and the review devotes detailed attention to the competing mechanisms proposed over nearly 150 years of research, dating back to the first documented observation of hydrogen’s effect on iron by William Johnson in 1875. Three frameworks dominate modern thinking. The hydrogen-enhanced decohesion model holds that hydrogen’s 1s electron donates into iron’s unfilled 3d shell, weakening interatomic bonds so that grain boundaries part more easily, producing the intergranular fracture surfaces characteristic of embrittled martensite. The hydrogen-enhanced localized plasticity mechanism describes a subtler process: dissolved hydrogen lowers the shear stress needed for dislocations to move, so deformation concentrates into narrow bands at crack tips, and moving dislocations ferry hydrogen ever deeper into the material, a feedback loop that nucleates microcracks. A third mechanism, adsorption-induced dislocation emission, proposes that hydrogen adsorbed at crack surfaces injects dislocations into the crystal on favorable slip planes, such as the {110} planes of body-centered cubic iron, allowing the crack to advance with far less plastic work than in hydrogen-free metal.

Beyond these classic mechanisms, the review discusses the hydrogen pressure theory, in which recombining hydrogen atoms form molecular gas that pressurizes internal voids and explains hydrogen blistering, and the newer hydrogen-enhanced strain-induced vacancies mechanism, in which plastic strain multiplied by hydrogen generates dense populations of vacancies that coalesce ahead of crack tips, degrading fracture toughness. Notably, brittle hydride formation, a major embrittlement route in metals like titanium and zirconium, is rarely observed in steels because the thermodynamic conditions for hydride stability are too harsh. The practical consequence is that no single mechanism yet allows engineers to predict hydrogen-induced fracture from first principles; instead, quantitative assessment depends on measuring where hydrogen resides and how fast it moves.

That measurement challenge is formidable. Hydrogen concentrations in martensitic automotive steel typically sit between 0.1 and a few weight parts per million, while diffusivity exceeds 1 x 10^-5 mm^2 per second at room temperature, meaning the tracer escapes before conventional instruments can map it. The review catalogs an expanding arsenal of characterization techniques. Thermal desorption spectrometry, though lacking spatial resolution, reveals hydrogen trapping energies by recording desorption peaks as specimens are heated; running the measurement at multiple heating rates yields activation energies for each trap population, from weakly bound solute hydrogen desorbing at cryogenic temperatures to deeply trapped hydrogen released only above several hundred degrees Celsius. Secondary ion mass spectrometry provides micron-scale three-dimensional mapping by sputtering the surface with an ion beam and counting emitted deuterium ions. Scanning Kelvin probe force microscopy detects hydrogen indirectly through hydrogen-induced drops in surface potential, and can resolve hydrogen diffusion across individual ferrite and austenite grains over hours, especially when a thin palladium coating traps escaping hydrogen and extends the observation window. Atom probe tomography now reaches near-lattice resolution, and cryogenic operation has captured deuterium atoms clustered around vanadium carbide precipitates, direct visual proof that designed nanoprecipitates intercept diffusing hydrogen. The hydrogen microprint technique, in which dissolved hydrogen reduces silver ions in solution onto the specimen surface, recently visualized hydrogen segregation at crack tips in bent 2 GPa press-hardened steel.

Tracking hydrogen is only half the assessment problem; engineers also need tests that predict whether a component will fail in service. The workhorse is the slow strain rate tensile test, typically run at strain rates around 1 x 10^-5 per second so that hydrogen has time to diffuse and concentrate. Electrochemical charging in solutions such as 3 percent sodium chloride with ammonium thiocyanate, or high-pressure gas charging, loads specimens with hydrogen before testing, and losses in strength and ductility quantify susceptibility. The review is candid about the method’s blind spots: pre-charged hydrogen distributes unevenly, concentrating near surfaces where fracture is most dangerous, and effuses from the specimen during deformation, potentially underestimating risk. Constant load tests, four-point bending, slow-strain-rate bending, biaxial tensile tests, U-bend experiments on sheared edges, and salt-spray exposure of deep-drawn cups all add realism, because real automotive parts experience multiaxial stresses, residual stresses, and crash-rate loading rather than simple uniaxial tension. Emerging approaches even charge hydrogen during austenitization under controlled furnace dew point, mimicking the exact pathway by which industrial press-hardened parts absorb hydrogen in production.

The heart of the review lies in steel-specific analysis. Quenching and partitioning steels, third-generation advanced high-strength steels retaining 10 to 25 percent metastable austenite, illustrate how one microstructural constituent can be both savior and saboteur. Retained austenite dissolves far more hydrogen than martensite, in one QP980 study trapping more than three times as much, and its transformation-induced plasticity effect boosts ductility. But unstable austenite transforms to fresh, hydrogen-vulnerable martensite during deformation, and its interfaces with martensite and ferrite can serve as crack initiation sites. In QP980 charged with low hydrogen, cracks initiate at martensite and martensite-ferrite boundaries but are blunted by ductile ferrite; at intermediate hydrogen, crack propagation follows the localized plasticity mechanism; above 3 ppm, ferrite itself cleaves along {110} planes via decohesion. Comparisons between commercial QP980 and QP1180 grades reveal a cautionary lesson: opposite embrittlement rankings have been reported for the same grades because alloy compositions differ between suppliers, meaning microstructural design, not grade name, must guide hydrogen-resistant engineering.

Medium manganese steels, containing 3 to 12 percent manganese and reaching 900 to 1600 MPa, showcase perhaps the most creative austenite engineering in the field. Intercritical annealing reverts martensite to austenite, but annealing temperature, duration, and deformation route all tune austenite stability, morphology, and topology in ways that determine whether hydrogen is harmlessly trapped or funneled into strain-localized failure sites. Flash annealing, which freezes nonequilibrium manganese distributions, creates chemically heterogeneous austenite in which manganese-poor regions nucleate hydrogen cracks but manganese-rich regions arrest them, a strategy demonstrated in Nature Materials in 2021 and extended with ordered core-shell austenite designs in which manganese-rich shells block crack propagation entirely. Warm rolling adds topological defense, producing elongated banded microstructures that deflect transverse hydrogen cracks, promote delamination, and reduce stress triaxiality, while microalloying with niobium, vanadium, molybdenum, and boron adds carbide traps and grain-boundary strengthening. Even pulsed electric current treatment has been shown to fully restore ductility lost to hydrogen charging in medium manganese steel, a striking demonstration that hydrogen damage can be at least partly reversible.

Press-hardened steels and their welded derivatives present distinct problems tied to processing. The fully martensitic microstructure of hot-stamped 22MnB5 boron steel delivers 1500 MPa strength but concentrates hydrogen at prior austenite grain boundaries, where dislocation transport delivers it during deformation; elegant cryogenic preloading experiments have proven this segregation drives the intergranular fracture. The mandatory AlSi coating is itself a hydrogen source, because molten aluminum reacts with furnace moisture during austenitization and pumps hydrogen into the steel. Mitigation strategies are multiplying: nanostructured niobium, vanadium, and titanium carbide precipitates act as deep irreversible traps with activation energies as high as 81.8 kJ per mole; low-temperature tempering at just 90 degrees Celsius relaxes the extreme residual stress of quenching and dramatically improves hydrogen resistance without alloy changes; and redesigned surface structures that replace brittle aluminum-rich intermetallics with ductile low-aluminum ferrite delocalize hydrogen and suppress crack initiation. In tailor-welded blanks, filler-wire laser welding that stabilizes austenite, combined with post-weld hot stamping that refines grains and precipitates nano-scaled carbides, shifts fracture away from the vulnerable fusion zone entirely. The unifying principle emerging across all four steel families is that hydrogen embrittlement is governed not by strength alone but by the hierarchy of hydrogen traps, the topology of strain partitioning, and the microstructural crack path, an integrated design philosophy that the review’s authors argue will be essential for the next generation of 2 GPa-class automotive steels to reach the road safely.

Subject of Research: Hydrogen embrittlement mechanisms, characterization, and mitigation in advanced high-strength automotive steels

Article Title: Hydrogen embrittlement risk of automotive steel

Article References: Guan, M., Liu, Y. X., & Huang, M. X. (2026). Hydrogen embrittlement risk of automotive steel. Advanced Materials Joining, 1(1), Article 5. https://doi.org/10.1007/s44500-026-00009-w

Image Credits: AI Generated

DOI: 10.1007/s44500-026-00009-w

Keywords: hydrogen embrittlement, advanced high-strength steel, automotive steel, press-hardened steel, quenching and partitioning, medium manganese steel, tailor-welded blanks, hydrogen trapping, retained austenite, thermal desorption spectrometry, atom probe tomography, delayed fracture