The Critical Raw Materials (CRMs) list has been defined based on economic importance and supply risk by the European Commission. This review paper describes two issues regarding critical raw materials: the possibilities of their substitution in iron-based alloys and the use of iron-based alloys instead of other materials in order to save CRMs. This review covers strategies for saving chromium in stainless steel, substitution or lowering the amounts of carbide-forming elements (especially tungsten and vanadium) in tool steel and alternative iron-based CRM-free and low-CRM materials: austempered ductile cast iron, high-temperature alloys based on intermetallics of iron and sintered diamond tools with an iron-containing low-cobalt binder.
The burgeoning population and increasing industrialization are leading to an increase in pressure on resources. Other factors such as digitalization, the transition to climate neutrality with metals, minerals and biotic materials used in low emission technologies and products and increasing demand from developing countries are also contributing significantly in addition to this pressure. In order to overcome the issue of demand for virgin materials, robust and fundamental change is required in the manufacturing process. Organisation for Economic Co-operation and Development (OECD) forecasts that global materials demand will more than double from 79 billion tons today to 167 billion tons in 2060 [1]. Assuming no change in the current processes would lead to fierce competition among the nations. One such resource is a group of critical raw materials (e.g., arsenic, cadmium, strontium, zirconium), and the continued dependence on them will soon replace today’s dependence on oil. The new industrial strategy of the European Union focuses on this need of integrating secondary raw materials and has embarked upon the journey of the green deal. This ambitious plan to integrate both primary and secondary raw materials, in particular critical raw materials, for key technologies and strategic sectors as renewable energy, e-mobility, digital, space and defense are one of the pre-requisites for achieving climate neutrality [1].
Critical Raw Materials (CRM) are those, which display a particularly high risk of a supply shortage in the next 10 years and which are particularly important for the value chain. Due to the uncertain geopolitical environment, and during the pandemic scenario, supply chains are disrupted considerably. This leads to adverse conditions for industrial productivity of the member nations of the European Union. The supply risk is linked to the concentration of production in a handful of countries, and the risk is in many cases compounded by the low substitutability and low recycling rates of CRMs [2]. The perpetual supply of these critical raw materials is also important for climate policy objectives and for technological innovation. For example, rare earth metals are essential for high-performance permanent magnets in wind turbines or electric vehicles, catalytic converters for cars, printed circuit boards, optical fibers and high-temperature superconductors. Thus, it becomes imperative to either find alternatives to these raw materials or to bring a fundamental shift in the current manufacturing processes.
Concerned about the uninterrupted supply of these raw materials, the European Commission launched the Raw Materials Initiative (RMI) in 2008 [3]. The policy stresses the need to diversify and secure non-energy raw materials for EU industrial value chains. Diversification of supply concerns reducing dependencies in all dimensions by the sourcing of primary raw materials from the EU and third countries, increasing secondary raw materials supply through resource efficiency and circularity and finding alternatives to scarce raw materials.
The methodology presented here is adopted from the European Union report on critical raw materials [4]. The first assessment conducted in 2011 identified 14 critical raw materials (CRMs) out of the 41 non-energy, non-agricultural candidate raw materials. The second exercise in 2014 leads to an increase in these numbers, 20 raw materials out of 54 candidates. The third attempt in 2017 identified more of these critical raw materials, as 27 CRMs were identified among 78 candidates [5]. The latest assessment conducted in 2020 covers a larger number of materials: 83 individual materials or 66 candidate raw materials comprising 63 individual and 3 grouped materials (ten individual heavy rare earth elements (REEs), five-light REEs and five platinum-group metals (PGMs)). Five new materials (arsenic, cadmium, strontium, zirconium and hydrogen) have been assessed [5].
Of the 83 individual (66 candidates) raw materials assessed, the following 30 were identified as critical in this assessment (Table 1).
Table 1. 2020 EU Critical Raw Materials list (* N 2020—CRMs in 2020, non-CRMs in 2017, ** HREEs—heavy rare earth elements, *** LREEs—light rare earth elements, **** PGMs—platinum group metals).
The overall results of the 2020 criticality assessment are presented in Figure 1. Critical raw materials (CRMs) are highlighted by red dots and are located within the criticality zone (Supply Risk (SR) ≥ 1 and Economic importance (EI) ≥ 2.8) of the graph. Blue dots represent non-critical raw materials.
Figure 1. Assessment of the criticality dynamics in 2017–2020 (individual materials and groups), adopted from [5].
The key changes in the 2020 CRMs list compared to the 2014 CRMs list are the 2020 assessment-confirmed 19 CRMs from the 2014 list, whereas 8 (Baryte, Bauxite, Hf, Natural Rubber, Sc, Ta, Ti and V) of the non-critical materials in 2014 shifted to being critical in 2020 [5]. The 2020 CRMs list includes 26 of the CRMs identified in 2017. Only helium, which was listed in 2017, shifted out of the list. Compared to the 2017 CRM list, four additional raw materials (bauxite, Li, Ti and Sr) are identified as critical and enter the 2020 CRMs list [5].
In the CRM list, there are several elements, which are important for the production of the iron-based alloys. Tungsten and vanadium are used as carbide-forming elements in tool steel. Vanadium is usually bound in MC carbides, having a cubic structure. These carbides, due to high thermal stability, arise already in the first stage of crystallization, forming as primary carbides. The presence of these carbides as precipitates was also proved during heat treatment [6]. Tungsten is usually contained in M 6 C primary carbides having a cubic structure, while chromium which was considered as a CRM recently, could be present in “blocky” M 7 C 3 carbides (hexagonal structure) and M 23 C 6 (cubic), which could arise during heat treatment. Cobalt is also added to high-speed steel in order to prevent thermal degradation during the service [7].
Among the elements contained in the various grades of stainless steels (SSs), chromium plays the fundamental role in maintaining their corrosion resistance properties in several application fields and in corrosive exposures. At the same time, this element can be considered a critical raw material, although the supply risks (SRs) have decreased in the last six years within the EU from a commercial and industrial standpoint.
In general, various alloys contain Cr for increasing their corrosion resistance, both in dry and in wet exposure conditions, but SSs are the most important ones within this materials family. They used 74% of the Cr among the different market goods [8], therefore it is considered a pillar of the EU economy. The EU produces 21.1% of worldwide steel output [9], second only to China (45.5%), but it does not have easy access to the raw materials (i.e., Ni, Cr, Mo, etc.) or sufficient resources of Cr required for producing SSs.
Because of this reason, Cr has gained attention during the last decade, becoming a topic of supply and economic analyses, as shown in the EU reports on CRMs from 2014 to 2020 [5,9,10]. It has been found inside the criticality area only in the 2014 report [9] on the SR vs Economic Importance (EI) plot. However, it must be specified that Cr is in the border of this area in the 2014 report and a little bit out of the border in 2017 and 2020 reports, especially in terms of SRs. Furthermore, the result of these analyses is considered “a false impression created by the change in methodology” [11] passing from the old report to the new ones. Therefore, Cr remains nowadays of high strategic value for the EU processing industry and it can still be considered a CRM.
Cr is obtained from chromite ore, refined to become ferrochromium, in particular in the manufacturing of SSs. Concerning the ores, the highest amounts are present in South Africa (46%), Zimbabwe, Kazakhstan and India. Ferrochromium is obtained mainly in China (37%), which dominates the market for this material in recent years for domestic use and exports [8]. Looking at the EU domestic sources, chromite is mainly present in Finland (Kemi mine), while the ferrochromium production is again mainly concentrated in this country, but also in Sweden and Germany.
Taking into account that these EU Cr sources are not sufficient, the import of the above-mentioned materials is of crucial importance, given the EU SSs demand. The main non-EU suppliers, as expected based on the above data, are South Africa and China, but it must be considered that the former is a “critical country” in terms of governance, rule of law, etc. and the latter increased the domestic use over time and applies a 20% export tax. In general, the import of elements necessary for SSs manufacturing coming from non-EU markets is an issue related to uncertain political stability and an exporting policy of these countries. Consequently, the strong dependence of the EU on these market issues can be easily understood, and the SRs of Cr cannot be neglected. This has prompted the EU to find strategies and solutions to save the demand/use of Cr in the production of SSs or other stainless Fe-based alloys but preserve the corrosion resistance, which represents a huge challenge [12].
This review aims to show the ways for:
A possible solution to reduce the consumption of Cr in the obtainment of SSs is material recycling by the use of scraps, which can reach 60% of the industrial production [8], even if it is complicated to sort them from those coming from more common steels, with the consequence of producing only low-performance steels. Considering also the availability of domestic SS scraps and the low recycling rate of Cr [9], it seems to be a limiting strategy to save Cr. On the other hand, considering the increasing demand for SSs, some authors [13] report that the use of scraps is unavoidable, due to the large consumption of iron ore resources and a general Cr and Ni shortage. The use of scraps determines in the manufactured alloys the presence of more noble elements such as Cu, As and Sn because they cannot easily be removed. The advantage is that elements such as Sn give a significant improvement in corrosion resistance of low-Cr (14 wt.%) ferritic SSs in less severe environments, compared to the common 18 wt.%-SSs. Alloys such as those without Ni, Mo, Cu, but with a 16 wt.% of Cr and 0.3 wt.% of Sn gave the same corrosion resistance as 304 SSs, saving 40% of the total Ni and Cr consumption on the production of SSs and so reducing the external market dependence in industrial processing.
In terms of corrosion resistance, the presence of Cr in SSs, in a sufficient amount, guarantees their strong corrosion resistance, oxidation resistance and/or heat resistance. Typically, it ranges from 10.5 wt.% to 30 wt.% [14,15]. More specifically, reacting with oxygen from the atmosphere, Cr determines the passivation of SSs, producing an adherent, insoluble, ultrathin (1–2 nm) film that protects the alloy against corrosion, in particular under wet conditions, where acidic species and/or chlorides-contaminated environments are present. For stable surface passivation, the content of Cr should be at least 10.5 wt.% [8,14,16,17,18]. Furthermore, the Cr present in the bulk of SSs guarantees self-healing properties, which enable restoring their corrosion resistance by forming a new passive Cr-based oxide layer in the presence of oxygen in the event of surface mechanical scratches or damage [14,17]. The formation of a Cr 2 O 3 thin layer on SSs by Cr gives also the necessary protection in high-temperature applications [14].
A partial substitution with limited options [9] and/or a reduction of Cr in SSs steel alloys could be possible, maintaining the corrosion properties of these alloys [19]. It seems that there is not a chance to completely substitute this element within the corrosion performance of SSs [8]; it is a utopia (or “a dream” [16]) and probably remains an old question [20] without a response, in particular in those applications, such as severe aqueous environments where the high corrosion resistance properties of SSs are needed. Several U.S. researchers have faced up to this question during the 1980s and 1990s because it has been considered a fundamental issue due to the possible import vulnerability of their country in supplying strategic materials [21], such as the situation previously discussed, coming from EU reports on analyzing CRMs.
Dealing with the corrosion performances of metallic materials, the difference between the resistance to oxidation in high-temperature applications (“dry corrosion”) and the resistance to corrosive electrolytic aqueous environments (“wet corrosion”) must be considered. In the first case, the substitution of Cr is possible, while in the second case, it is more difficult. In fact, the corrosion resistance in aqueous environments is a more complex phenomenon, which is not only related to the presence of Cr in the alloy composition, but also to the presence of other elements, such as Ni and Mo, which can improve the corrosion resistance in specific environments. Therefore, the substitution of Cr in SSs for wet corrosion applications is a more challenging task.
