1.4520

Stainless steels are most often divided by crystal structure and chemical composition into groups: austenitic, ferritic, martensitic and duplex (mixed). Ferritic and superferritic steels are distinguished within this class by a number of features that make them attractive for specific industrial applications.

• Ferritic alloys are single-phase alpha (α) steels — a body-centred cubic (BCC) lattice. They are characterised by chromium contents typically in the range 10–18% and very low or no nickel content.
• Superferritic variants are ferritic alloys with increased chromium content and often additions of molybdenum and other alloying elements, which significantly improve resistance to crevice corrosion and pitting while preserving the single-phase character.

Differences between ferritic, superferritic and austenitic steels

• Structure: Austenitic alloys (e.g. 304, 316) have an FCC structure, are stable at low temperatures and exhibit high ductility; ferritic alloys are single-phase BCC, which affects their stiffness and lower ductility.
• Chemical resistance: Austenitics are more resistant to many forms of corrosion (particularly thanks to Ni and Mo additions), whereas ferritics better tolerate stress corrosion cracking (SCC) and oxidising environments at certain temperatures.
• Mechanical properties: Ferritics generally have lower tensile strength and less ductility than austenitics, but better thermal conductivity and a lower coefficient of thermal expansion.
• Cost and availability: The absence of nickel in ferritics reduces cost compared with austenitics, which, combined with appropriate corrosion properties, makes them an economical choice for specific applications.

Importance of crystal structure in stainless steels

Crystal structure determines not only mechanical and thermal behaviour, but also how the steel responds to heat treatment, welding and chemical environments. The single-phase ferritic structure means there is no austenite–ferrite transformation at useful temperatures, which simplifies prediction of material behaviour during thermal cycles and reduces the risk of precipitation of brittle phases typical for certain transformation temperatures. On the other hand, single-phaseness is associated with greater brittleness at low temperatures and a lower ability to be hardened by phase transformations. In practice, the choice between structures depends on a compromise between corrosion resistance, ductility, thermal stability and cost.

Chemical composition determines the properties of stainless steel. Grades designated X2CrTi17 and X1CrTi15 are titanium-stabilised ferritic alloys. Titanium stabilisation involves binding carbon as titanium carbides, which prevents formation of chromium carbides during thermal processes and in service.

Analysis of chromium, titanium and other element contents

• Chromium (Cr): The most important element for corrosion resistance. In 1.4520 (X2CrTi17) Cr content is approximately 16–18% — a level providing good general resistance and surface passivation. In X1CrTi15 the chromium content is slightly lower, typically around 14–15%, which translates to somewhat reduced resistance in aggressive environments, but still adequate for many industrial applications.
• Titanium (Ti): The presence of Ti in both grades is crucial. Titanium binds carbon as TiC, stabilising the structure and preventing precipitation of chromium carbides. As a result, the steel is less susceptible to intergranular corrosion after welding or heat treatment. Ti content usually ranges from 0.2–0.8% depending on grade and manufacturer.
• Carbon (C): Low carbon content (typically ≤0.08%) minimises the possibility of chromium carbide formation, but the presence of Ti provides additional stability.
• Manganese (Mn), silicon (Si), phosphorus (P), sulphur (S): These elements occur in trace amounts, affecting formability, casting and machinability. Their contents are controlled by standards, typically Mn ≤1%, Si ≤1%, P and S at ppm levels.
• Additional alloying elements: In some superferritic variants molybdenum (Mo) and other elements are added to improve resistance to crevice corrosion and pitting. For the discussed grades X2CrTi17 and X1CrTi15, molybdenum is usually absent or present only in trace amounts.

Effect of alloying additions on material properties

• Titanium: The most significant in the described grades. It stabilises the steel, binds carbon and inhibits the formation of chromium carbides at grain boundaries, protecting against intergranular corrosion. It also reduces sensitisation typical after heat treatment.
• Chromium: Ensures formation of a passive chromium oxide layer, key to corrosion resistance. Higher chromium content increases resistance to oxidation and pitting.
• Molybdenum: (when present) Increases resistance to corrosion in chloride-containing environments; however, it is more expensive and less commonly used in simpler ferritic steels.
• Carbon-control and stabilisers: Low C content together with stabilisers means these materials retain good resistance after heat treatment and welding.

Production and processing influence microstructure and final properties of the steel. For titanium-stabilised ferritic steels it is important to maintain appropriate parameters during manufacture, annealing and welding.

Melting and refining methods

• Electric furnace melting: The standard method is electric steelmaking (EAF), often with controlled addition of scrap and pig iron. Refining in an electric arc furnace allows precise dosing of chromium and titanium.
• Vacuum degassing and vacuum refining: Used to achieve low gas contents and excellent composition control. This minimises defects and inclusions that could weaken mechanical properties and corrosion resistance.
• Secondary processing (e.g. forming and hot rolling): Control of rolling temperature influences grain structure. For ferritics it is important to avoid excessive grain growth, which degrades mechanical properties.

Heat treatment and its influence on microstructure

• Homogenising anneal: Allows dissolution of irregularities and homogenisation of composition, and stabilisation of titanium carbides. For Ti-stabilised steels typical annealing temperatures ensure stress relief and homogenisation without forming chromium carbides.
• No hardenability by quenching: Ferritic stainless steels are not hardened by phase transformations like martensitics. Heat treatment is mainly annealing and normalising in character.
• Risk of thermal embrittlement: Prolonged exposure to temperatures of about 300–800°C can lead to precipitation of the alpha’ (α’) phase, causing so-called embrittlement — hence control of service temperatures is important.

Forming and welding of ferritic steels

• Forming: Ferritic steels are relatively ductile at room temperature but have lower elongation than austenitics. Bending and stamping require consideration of bend radii and the risk of cracking in profiles with sharp corners.
• Welding: Possible by TIG, MIG/MAG and arc methods. Key principles:

– Use low and controlled heat input to limit grain growth and excessive hardening of the heat-affected zone.
– Choose filler materials with similar chemistry; for Ti-stabilised steels there are suitable welding wires that contain stabilisers or have comparable composition.
– Avoid prolonged dwell at temperatures that could lead to decarburisation or reduction of corrosion resistance.
– Titanium stabilisation facilitates welding because it reduces the risk of intergranular corrosion without the need for stress-relief annealing after welding.
Practical experience shows that using appropriate welding procedures and consumables can achieve joints with good corrosion and mechanical resistance.

These properties determine where and how the discussed grades can be used in practice.

Tensile strength, hardness and ductility

• Tensile strength: Titanium-stabilised ferritics typically exhibit moderate tensile strength. Typical values for grades such as 1.4520 and X1CrTi15 in the as-delivered condition fall within a broad range depending on mechanical processing and thickness, but generally:

– Tensile strength: approximately 380 MPa to 600 MPa.
– Yield strength (Rp0.2): typically 200–350 MPa.
– Elongation at fracture: usually 15–25% for typical sheet thicknesses.
– Hardness: on the Brinell or Vickers scale corresponds to typical low-hardness values similar to low-hardness austenitics — not intended to be work-hardened materials.

• Ductility: Lower than in austenitic steel, which should be considered when designing parts subject to large deformations.

Corrosion resistance in various environments

• General resistance: Alloys with 15–17% chromium form a durable chromium oxide passive film that protects against general corrosion in neutral and mildly oxidising environments.
• Intergranular corrosion: Thanks to titanium stabilisation the risk of intergranular corrosion is significantly reduced even after heat treatment and welding.
• Crevice corrosion and pitting: Compared with molybdenum-bearing austenitics, resistance to pitting in chloride environments is lower; in environments with high chloride ion concentration materials with higher Cr and Mo content are preferable.
• Resistance to atmospheres and exhaust gases: They perform well in atmospheres of moderate aggressiveness, in the food and automotive industries, and in installations where operation is conducted without excessive exposure to chlorides.

Thermal resistance and thermal contraction

• Service temperature range: Titanium-stabilised ferritics are suitable for use up to about 600°C in continuous service, although specific limits depend on exposure time and environment. Prolonged exposure in the 400–800°C range may cause ageing and alpha’ precipitation leading to increased brittleness.
• Coefficient of thermal expansion: Lower than in austenitics, which can be an advantage when joining components made of different materials. Better thermal conductivity facilitates heat dissipation.
• Dimensional stability: Thanks to titanium stabilisation, migration of carbon and sensitisation are limited, which translates into dimensional stability under cyclic heating and cooling.

Such steels are used where corrosion resistance is required together with reasonable cost and resistance to thermal stresses.

Automotive and heavy industry

• Exhaust systems and thermal shrouds: Due to oxidation resistance and suitability to temperatures up to 600°C and a favourable cost-to-performance ratio, titanium-stabilised ferritics are a popular choice for exhaust components, shields and silencers.
• Body panels and trim: The aesthetically passive surface and ease of forming allow use of these steels in automotive trim and details.
• Heavy industry: In heat exchangers, boiler cladding and valves where temperature resistance and moderate chemical aggressiveness are required.

Food and chemical industry

• Food processing equipment: Sheets and forming tools for food processing, tanks, pipes and components of installations where hygiene and surface corrosion resistance are key.
• Chemical apparatus: In installations where strongly chlorinated environments do not occur, ferritic alloys offer good resistance and ease of cleaning.
• Applications where titanium stabilisation is important: Welded and irregularly processed components that must retain corrosion resistance after heat treatment.

Construction and architecture – possibilities and limitations

• Facades and architectural details: An aesthetic surface, good atmospheric resistance and lower cost compared with some austenitics make these alloys useful in architectural practice.
• Limitations: In locations with high exposure to chlorides (e.g. coastal structures) alloys with higher Cr and Mo content or austenitics are a better choice.

Material selection is always based on compromises. Below are the main advantages and disadvantages of ferritic stainless steels compared with other groups.

Advantages of ferritic stainless steels over austenitics

• Cost: Absence of nickel reduces material costs, which matters for mass-production.
• Resistance to stress corrosion cracking: Ferritics are less susceptible to SCC than austenitics in chloride environments, which is an advantage in applications exposed to such phenomena.
• Thermal stability and thermal conductivity: Better thermal conductivity and lower coefficient of thermal expansion facilitate thermal applications and joining with other materials.
• Ease of welding due to Ti stabilisation (in stabilised grades): Ability to retain corrosion resistance after welding without special stress-relief treatments.

Drawbacks and application limitations

• Resistance to pitting and crevice corrosion: Less effective than Mo-bearing austenitics; in chloride-rich environments other grades should be selected.
• Ductility and impact toughness: Lower ductility and poorer toughness at low temperatures than austenitic steel.
• Temperature limitations: Long-term exposure above ~600°C can lead to precipitation of undesired phases and loss of mechanical properties.

Titanium is a key element in the design of these steel grades. Its role is both chemical and microstructural.

Role of titanium in counteracting chromium carbides

• Carbon binding: Titanium has a strong affinity for carbon and forms stable TiC carbides, which are less harmful to the continuity of the passive oxide layer than chromium carbides.
• Prevention of sensitisation: In standard chromium steels, exposure in the range of about 450–850°C leads to precipitation of Cr23C6 carbides at grain boundaries. This reduces chromium content in the grain boundary zone and causes susceptibility to intergranular corrosion. Titanium addition captures carbon before it combines with chromium, preventing this process.
• Stability after heat treatment: Thanks to Ti, the steel maintains its resistance after welding and heating in service, which greatly facilitates its use in welded structures.

Improvement of resistance to intergranular corrosion

• Practical effect: Components made from Ti-stabilised steel can be subject to heat treatment and welding without the need for costly stress-relief anneals to restore corrosion resistance.
• Durability: Reduced susceptibility to intergranular corrosion extends service life of components in chemical, food and automotive installations where temperature cycles and varied loads occur.

Understanding material designations is essential for correct ordering and use of components.

International classification systems and symbols

• EN (European Standard): The system used in Europe assigns numbers to grades, e.g. 1.4520, and descriptive symbols such as X2CrTi17. These designations contain information about the content of main elements and stabilisers.
• AISI/ASTM: American and international systems sometimes use different designations, and equivalents should be compared using conversion tables. For titanium-stabilised steels there are equivalents in the AISI system (e.g. 430Ti is a commercial name corresponding to the stabilised steel category in the American region).
• ISO: The International Organization for Standardization publishes standards that facilitate identification and use of materials in global markets.

Application of EN, AISI and ISO standards

• When ordering materials: always require the standard specification (e.g. EN 10088 for stainless steels) and give the material number (e.g. 1.4520) and commercial designation (X2CrTi17).
• In technical documentation and orders: clearly specify requirements for weldability, chemical and mechanical tolerances and the form of delivery (bars, pipes, sheets).
• In quality control: perform metallographic, chemical and mechanical tests based on standards to confirm batch compliance with requirements.

Despite many advantages, users must be aware of typical limitations and know how to mitigate them.

Corrosion phenomena and methods to minimise them

• Pitting and crevice corrosion in chloride environments: Avoid use in direct contact with strong brines; if necessary choose alloys with Mo addition or austenitic grades with better pitting resistance.
• Stress corrosion cracking: Although ferritics are more resistant to SCC than austenitics, residual stresses should be minimised through appropriate design and, where necessary, stress-relief annealing or stress relaxation processes should be applied.
• Contaminants and deposits: Regular cleaning and prevention of salt and aggressive chemical deposits significantly prolong component life.

Problems related to processing and welding

• Grain growth in the heat-affected zone: Control heat input, use rapid cooling techniques where possible, and select filler materials with appropriate composition.
• Brittleness at low temperatures: For cryogenic or low-temperature applications consider alternatives with better impact toughness.
• Limitations in bending and forming: When designing parts consider bend radii and processing procedures to avoid cracking.

Materials do not stand still; steel technology development is directed at increasing performance and sustainable resource use.

Modern techniques for composition modification

• Microalloy optimisation: Adding small amounts of elements such as Nb, Ta or N to improve mechanical properties and corrosion resistance.
• Precise control of Ti content: Reducing batch variability through better control of the melting process improves stability of final properties.
• Superferritic compositions: Increasing Cr content and selective addition of Mo and N to create ferritics with pitting resistance approaching that of austenitics while retaining the economic and thermal advantages of ferritics.

Applications in advanced technologies and ecology

• Energy industry: New alloy variants are used for construction of heat exchangers and installations in power plants where temperature and media resistance are critical.
• Ecology and recycling: Ferritic steels, lacking nickel, are easier to integrate into recycling streams, reducing the carbon footprint of production.
• 3D printing and additive manufacturing: Possibilities of processing ferritic stainless alloys in additive technologies are being investigated, enabling complex, lightweight structures with favourable properties.

• Material selection for environment: In situations with chlorides, high salt concentrations or aggressive halogen acids, prefer materials with Mo addition or austenitics with greater pitting resistance. For general, atmospheric, food and automotive applications 1.4520 and X1CrTi15 are a good choice.
• Detail design: Take into account lower elongation and greater tendency to brittleness than in austenitics. Good practice is to use larger bend radii and avoid sharp edges.
• Welding and assembly: Use procedures that limit heat input, select appropriate filler metals and control interpass temperature. For welded structures a weld qualification and corrosion test is recommended.
• Maintenance: Regular removal of deposits and surface inspection prevent localised corrosion attacks. In marine atmospheres use components with higher Cr content or additional surface protection.
• Documentation and standards: When ordering always request material specification with the EN number (e.g. 1.4520) and information on delivery condition and quality certificates.

Ferritic and superferritic stainless steels, particularly titanium-stabilised grades such as 1.4520 (X2CrTi17) and X1CrTi15, represent a pragmatic choice in many industries. They combine corrosion resistance with favourable cost, good thermal properties and ease of processing and welding while maintaining stability after heat treatment. Their use, however, requires a conscious approach to design and operation to make the most of the material’s strengths and minimise its limitations.