Objectives
The main objective of the MOWSES project is to define the conditions for safe utilization of green structural steels, for European infrastructure projects made of medium strength (S355) to ultra-high strength (S960) steels. These green steels, produced with an increased amount of recycled scrap metal, are likely to contain higher concentrations of residual elements that unintentionally alter the steel’s chemical composition. This recycling process poses challenges, particularly for the welded joints of these steels, as the residual elements influence how the material responds to thermal cycles such as those imposed by welding. A critical area of focus is the heat-affected zone (HAZ), where the base material’s chemical composition remains unchanged because no melting or filler metal addition occurs, but the mechanical properties are significantly modified by the largely uncontrolled thermal cycles of welding and, in some cases, post-weld heat treatment (PWHT). Since these changes could affect the safety and integrity of welded constructions, the project seeks to understand and address these effects through a combination of laboratory experiments, welding simulations, fracture testing, and numerical modelling. This integrated approach will help define the conditions for the safe application of green steels in critical infrastructure, potentially without relying on scarce strategic microalloying elements (e.g. Nb, Ti, V).
To reach the main objective, six different steps were defined:
Objective 1: Screen the influence of residual and critical elements on the strength, toughness and microstructure of structural steel in the base material and in the heat-affected zone.
The recycling of scrap metals inevitably leads to the ‘unintentional’ alloying of the steel produced with “residual” elements. The amount of the residual elements will depend on various factors, including the quality of scrap, the prior application from which the scrap steel originates and the efficiency of scrap sorting. Depending on their interaction with the steel matrix, they can strengthen or embrittle the material, change how it reacts to heat and affect toughness. For instance, elements like Sn and Sb can segregate at grain boundaries, reducing cohesion and increasing crack susceptibility. Similarly, when copper gathers on the surface, it can cause problems during manufacturing, such as making the material brittle and more likely to crack when heated. MOWSES stands out by expanding research beyond common residual elements to include less-studied ones like As, Pb and others. The project explores their effects not only individually but also in combination, addressing gaps in understanding synergetic interactions.
Objective 2: Develop automated method to analyse the microstructure of the heat-affected zones.
Traditional microstructure quantification relies on manual, labour-intensive evaluation by experts. However, advances in artificial intelligence (AI) and machine learning (ML) now enable automated, objective, and reproducible analysis, offering unprecedented precision and the ability to quantify complex microstructures. These advancements accelerate the establishment of process-microstructure-property relationships, supporting faster, microstructure-based materials development. Despite these benefits, most ML applications to date focus on well-curated datasets, limiting model robustness and generalizability. This consortium addresses these challenges by fostering collaboration across multiple groups and laboratories, creating an environment to develop reliable ML models. These models aim to enhance microstructure analysis, particularly for simulated heat-affected zone (HAZ) and real weld microstructures, including those with gradients. Going beyond state-of-the-art methods, MOWSES will establish ML as a foundational tool for future microstructure analysis and materials innovation.
Objective 3: Identification of the microstructural feature(s) controlling fracture in welded steels with increased amounts of residual elements.
In a structure, welds are often critical because they can be at a location that is more heavily loaded and/or suffering from stress concentrations on the one hand, and because there is an intrinsic risk of deteriorated properties and the presence of defects on the other. In addition, welded connections can have residual stresses or complex shapes, leading to high levels of restraint. Investigating the toughness and the weldability of green steel is therefore critical for safe application of these steels in large infrastructure components. In fusion welds, the Heat-Affected Zone (HAZ), and in particular the Coarse Grain Heat Affected Zone (CGHAZ), is often the area with the poorest toughness. In welding, the properties of the HAZ can only be influenced by the thermal cycles during welding and, if needed, by post-weld heat treatment (PWHT). During welding, the temperatures in the CGHAZ can reach between 1300°C and 1500°C, causing rapid grain growth and often resulting in large austenite grains (over 200 micrometers in size). In modern steels, the cooling process after welding can form upper or lower bainite, a phase that affects the material’s properties. Research has shown that the toughness of the material is linked to the size of laths (thin plates of bainite), and these laths are directly related to the prior austenite grain size. An innovation of this project lies in performing Gleeble simulations to assist in establishing the missing link between the residual elements, the microstructure of HAZs, and their mechanical performance for different chemical compositions, enabling fracture micro-mechanisms to be unravelled. This knowledge is crucial for the weld integrity of green steel structures and thereby the advancement of sustainable practices in the steel industry.
Objective 4: Investigate the weldability of green steel through experimental welding tests and develop a procedure for post-weld heat treatment considering the mechanical properties of the HAZ.
Weldability tests using real welding experiments are crucial for evaluating the usability of steel grades under specific process conditions, forming a key part of material development. Current knowledge on how residual elements influence weldability largely derives from Basic Oxygen Furnace (BOF) steels, leaving gaps in understanding the impact of specific elemental concentrations on HAZ properties. Predicting weld behaviour under restraint conditions remains challenging due to the complex interaction of factors, making experimental investigations indispensable. In addition, if there is a hardening tendency in the HAZ, the hydrogen diffusion behaviour and the risk of the formation of brittle microstructural components in the HAZ are particularly decisive with regard to the weldability of a steel through the risk of Hydrogen Induced Cold Cracking (HICC). The transformation-delaying effect of alloying and residual elements are responsible for hardening, and their effect on the formation and properties of the HAZ will therefore be investigated as part of the project. MOWSES aims to advance beyond conventional practices that focus solely on hydrogen content and hardness by incorporating the effects of chemical composition, microstructure, and predictive models for weldability. Additionally, the project will explore how post-weld heat treatment influences hydrogen distribution, residual stress, and microstructure to mitigate cold cracking risks.
Objective 5: Develop a modelling technique that can predict the fracture of steel in the ductile to brittle transition, with an application to the heat-affected zone.
Current modelling approaches capture ductile and brittle fracture in steels but often require a posteriori modifications to account for behaviour in the upper ductile-to-brittle transition. This is probably due to interactions between the ductile and brittle failure modes that happen on a very local scale, which are too fine for typical Finite Element Analysis (FEA) models to capture. Capturing fracture behaviour in the ductile-to-brittle transition is crucial for Heat-Affected Zones (HAZ), as many structures operate in this temperature range. Current models often lack microstructural data, but incorporating these parameters is essential to assess the impact of residual elements on the safety of structures made from new, green steels. A key innovation of this project is to develop a model that predicts the fracture of steel in the upper ductile-to-brittle transition while also accounting for the specific hard particles contained in that steel. Additionally, the project explores the use of machine learning (ML)-based surrogate models to approximate physical simulations, reducing computational demand and accelerating materials development. These ML models will complement physical models, especially in complex scenarios where conventional methods struggle.
Objective 6: Engage with stakeholders, including the steel industry, the Clean Steel Partnership, the fabrication sector, and the academic and research community.
The MOWSES project aims to drive the adoption of its results by partners and stakeholders, fostering a competitive and innovative green steel industry in Europe that produces high-quality products. The project will benefit various groups, including the steel and manufacturing industries, infrastructure owners, and educational and research institutions. To achieve this, MOWSES will employ clear communication and dissemination strategies, such as Open Access publications, presentations, workshops, and participation in EU events and trade fairs, to share its findings and inspire further research. The project will also contribute to education and training, with academic and industrial partners integrating the results into university courses and hiring five PhD and postdoctoral researchers to support the project. Additionally, the consortium has strong ties to the Clean Steel Partnership, where results will be shared through workshops, leveraging the involvement of partners like OCAS, Dillinger, Comtes FHT a.s., RWTH Aachen, TU Delft and UGent.