1.4749

X18CrN28 with material number 1.4749 is an austenitic heat-resistant steel designed to operate at elevated temperatures and in aggressive atmospheres. In industrial terminology it serves as a bridge between stainless steels and nickel alloys: it combines good oxidation and corrosion resistance at high temperatures with a relatively favourable property-to-cost ratio. This is a material whose “temperature resistance” is like armour plating — it protects structures from the destructive effects of high temperature while retaining the ductility and malleability needed for assembly and operation.

Origin and history of heat-resistant steels

The history of heat-resistant steels is a story of continual efforts to maintain structural integrity under extreme thermal conditions. The first classes of alloy steels appeared in the 19th century with the development of metallurgy and the power industry. In the 20th century, alongside rapid development in power generation, the chemical and aerospace industries, the need arose for materials that combined resistance to high-temperature oxidation with good ductility and corrosion resistance.

Austenitic steels enriched with chromium and nickel, with additions of nitrogen or other stabilisers, began to play a key role in furnace constructions, boilers and chemical installations. X18CrN28 emerged in this context as a response to the demand for a material with very high chromium content (to protect surfaces from oxidation) and significant nickel and nitrogen contents (to stabilise austenite and increase strength at elevated temperatures).

Main industrial applications of the heat-resistant steel

X18CrN28 is used where materials must withstand prolonged exposure to high temperature and aggressive gaseous environments. The most important applications include:

• elements of industrial furnaces and combustion chambers;
• tubes and heat exchangers in power and chemical installations;
• components of gas turbines and exhaust systems;
• shields and liners in heat-treatment equipment;
• parts of boilers and burners operating in oxidising atmospheres.

In practice X18CrN28 is used where a combination of high-temperature resistance and resistance to corrosive action of combustion products and flue gases is expected. As a result it finds applications in the power, cement, petrochemical and waste‑to‑energy industries.

Exact alloy composition may vary depending on the manufacturer and order specification. Below are typical, indicative ranges of element contents for the grade designated X18CrN28 (1.4749).

Percentage elemental content

• Chromium (Cr): 27–29%
• Nickel (Ni): 16–19% (often indicated in the name as “18”)
• Carbon (C): ≤ 0.08%
• Manganese (Mn): ≤ 2.0%
• Silicon (Si): ≤ 1.0%
• Nitrogen (N): 0.06–0.20% (depending on the degree of nitrogenation of the steel)
• Phosphorus (P): ≤ 0.035%
• Sulphur (S): ≤ 0.015%
• Iron (Fe): balance (to 100%)

Additional alloying elements, if present, appear in trace amounts or as deliberate modifications: molybdenum (Mo) in small amounts, copper (Cu), niobium (Nb) or titanium (Ti) — depending on specialised variants of the material.

Role of each element in the steel structure

• Chromium (Cr): The key element for oxidation resistance and surface passivation. At contents above ~25% it forms stable chromium oxide layers that protect the steel from further oxidation at high temperature.
• Nickel (Ni): Stabilises the austenitic structure, improves ductility and malleability, and increases resistance to brittle fracture at low temperatures. At high temperatures it also raises mechanical strength and reduces the tendency to embrittlement.
• Nitrogen (N): Strengthens the steel by solid solution strengthening, increases yield strength and tensile strength, improves resistance to intergranular corrosion and stabilises austenite.
• Carbon (C): In small amounts affects strength and hardness, but excessive carbon can promote carbide formation and weaken resistance to intergranular corrosion. Therefore its content is controlled.
• Manganese (Mn) and silicon (Si): Act as deoxidisers during melting and influence forming processes and mechanical properties. Excess manganese, however, can reduce high-temperature resistance.
• Phosphorus (P) and sulphur (S): Impurities that should be minimised, as they can promote brittle fracture and reduce ductility.

Understanding the role of individual constituents is crucial when designing heat treatments and selecting the material for a specific application. A high share of chromium and nickel together with controlled nitrogen content is the combination that determines the specific properties of X18CrN28.

The microstructure of the steel determines its behaviour at elevated temperatures. In the case of X18CrN28 we are dealing with an austenitic microstructure with possible inclusions and secondary precipitates.

Characteristics of metallic phases in the alloy

• Austenite (γ): The dominant phase in X18CrN28, stabilised by nickel and nitrogen. Austenite provides excellent ductility, impact toughness and resistance to brittle fracture, which is especially valuable during operation under variable loading and temperature conditions.
• Carbides and nitrides: In the presence of carbon and nitrogen chromium carbides (Cr23C6) and nitrides and carbonitrides may form. These precipitates affect strength and creep resistance, but uncontrolled precipitation at grain boundaries can reduce resistance to intergranular corrosion.
• Brittle phases (sigma, chi): In steels with very high chromium content precipitation of brittle phases such as sigma (σ) is possible, particularly after prolonged exposure in the temperature range of about 600–900°C. The presence of the sigma phase reduces ductility and fracture resistance.
• Surface oxides: As a result of service in oxidising conditions, surface oxide layers rich in chromium oxides form, which act protectively. The thickness and adherence of these layers determine the effectiveness of protection.

Influence of heat treatment on the microstructure

Heat treatment of X18CrN28 is critical for controlling microstructure and achieving desired mechanical properties:

• Solution annealing: Typical temperature range is 1050–1150°C, followed by rapid cooling (for example in water or by intensive air cooling) to inhibit precipitation of brittle phases and carbides. This procedure restores a homogeneous austenitic structure and maximises corrosion resistance.
• Stabilisation by additions (e.g. Nb, Ti): In certain variants niobium or titanium additions are used to bind carbon into carbides of these elements, reducing the amount of chromium carbides precipitating at grain boundaries and improving intergranular corrosion resistance.
• Ageing processes: Prolonged exposure at intermediate temperatures can lead to precipitation of hardening phases, increasing hardness and reducing ductility. Control of temperature cycles and time is therefore necessary to avoid property degradation.

The effect of microstructure can be compared to the arrangement of bricks in a wall: a uniform, stable austenitic structure is a solid wall, whereas local precipitates of brittle phases are “cracks” in the mortar that weaken the construction.

X18CrN28 has been designed to withstand high temperatures while protecting against aggressive combustion products and corrosive gases. This resistance stems mainly from the high chromium content and the presence of nickel and nitrogen.

Mechanisms of heat resistance

• Formation of a protective oxide layer: Chromium reacts with oxygen to form a thin, passive chromium oxide layer (Cr2O3). This layer is continuous, adherent and poorly permeable to oxygen, which limits further oxidation. With a favourable atmosphere (low contamination) this layer is self‑regenerating.
• Stabilisation of austenite by nickel and nitrogen: This prevents the steel from transforming to ferromagnetic structures or becoming brittle due to phase changes at high temperatures. Austenite retains ductility and allows deformation without cracking.
• Control of precipitates: Through appropriate heat treatment and alloying additions harmful phase precipitation can be minimised, which slows down degradation during service.

Resistance to oxidation and corrosion in industrial conditions

• Oxidation resistance: X18CrN28 shows good oxidation resistance at temperatures around 900–1100°C. Under short exposures and oxidising conditions the steel forms a stable protective scale. Prolonged service at very high temperatures and the presence of aggressive compounds (sulphur, hydrogen chloride) can, however, accelerate degradation.
• Corrosion resistance in flue gases: In flue gas environments, especially those containing sulphur or chlorides, complex corrosion mechanisms occur. High chromium limits oxidation, while nitrogen and nickel improve resistance to stress corrosion cracking. Nevertheless, in the presence of chlorinated species and sulphur oxides caution is required — local corrosion forms such as pitting or crevice corrosion are possible, particularly when stresses are present.
• Corrosion resistance in process chemistry: X18CrN28 performs well in oxidising environments and where combustion products are present. In strongly reducing environments or those containing aggressive ions (e.g. chlorides) nickel alloys or steels with additional alloying elements may provide better results.

In practice the durability of X18CrN28 components depends not only on composition but also on operating conditions: temperature, gas atmosphere, exposure time and mechanical loads.

Producing X18CrN28 requires precise control of the process from melting raw materials to final processing. Each stage affects the final properties of the material.

Melting and forming processes

• Electric melting (EAF): An electric arc furnace is used to produce the steel, enabling accurate dosing of ingredients and control of contamination. The high chromium content requires careful deoxidation and control of dissolved gases.
• Refining trials (VAR, ESR): For high-purity requirements vacuum arc remelting (VAR) or electroslag remelting (ESR) processes are used to reduce impurities and chemical inhomogeneities.
• Rolling and forming: After casting, hot rolling and possibly subsequent cooling and cold rolling are performed to shape the final product (sheets, tubes, strips). Control of rolling parameters allows attainment of desired grain size and structural homogeneity.

Heat treatment and its parameters

• Solution annealing: As mentioned, typical temperatures are 1050–1150°C with rapid cooling. The aim is to dissolve carbides and restore a homogeneous austenitic microstructure.
• Normalising and stress relieving: These actions reduce internal stresses arising during forming and rolling and improve mechanical uniformity.
• Avoiding prolonged heating in the 600–900°C range: This temperature band favours sigma phase precipitation and other degradations; processing and service conditions must be selected to minimise the time the material spends in this range.

Mechanical processing methods

• Cutting, turning, milling: X18CrN28 is machined similarly to other austenitic steels, although the high alloy content requires appropriate tooling (carbide inserts, diamond-like coatings) and cutting parameters.
• Welding: Welding austenitic steels with high Cr and Ni content requires suitable welding wires and procedures (e.g. TIG, MIG) and control of the weld chemistry to avoid cracking and unwanted precipitates. Techniques that reduce stress and ensure microstructural homogeneity are used.
• Surface treatment: Protective coatings, thermal chromising, controlled oxidation or spraying can be applied to increase the durability of parts exposed to extreme conditions.

Correct execution of each manufacturing and processing stage is a prerequisite for achieving the properties declared for this steel grade.

X18CrN28 finds wide application in sectors where components are exposed to high temperatures and corrosive media.

Components of high‑temperature equipment

• Tubes and burner shields: In boilers and burners this steel is used for tubes, shields and flue gas guiding elements.
• Heat exchangers: Where a combination of oxidation resistance and good thermal conductivity with mechanical durability is required.
• Industrial furnace components: Grates, doors, tube liners and other parts exposed to long-term hot gas exposure.
• Gas turbines — secondary components: In positions not reaching the extreme core temperatures of the turbine, but exposed to high temperature and corrosion, X18CrN28 can be a favourable choice.

Use in corrosion‑resistant installations

• Flue gas systems and exhaust handling: Pipes and ducts in district heating and flue systems exposed to nitrogen oxides, sulphur oxides and other aggressive combustion products.
• Chemical processes and petrochemicals: Process equipment components such as exchangers or conduits where thermal resistance must be combined with chemical resistance in oxidising conditions.
• Cement and lime industries: Furnace parts, dryers and sliding shields where hot, abrasive and chemically active gases are present.

Use of X18CrN28 should always consider the specifics of the working environment: in atmospheres containing chlorinated species or sulphur protective coatings or an alternative alloy may be required.

To select the correct material it is necessary to compare the features of X18CrN28 with other steels and alloys used in similar applications.

Basic compositional differences and their significance

• Compared with typical austenitic steels 304/316, X18CrN28 has a significantly higher chromium and nickel content and nitrogen addition, which translates to better high-temperature and oxidation resistance.
• Compared with nickel alloys (e.g. Inconel), X18CrN28 is cheaper and sufficiently resistant in many applications, but has inferior properties in extremely aggressive conditions (e.g. high-temperature chlorides) and for very long-term creep. Nickel alloys outperform the steel in strength at extreme temperatures, but their cost is significantly higher.
• Compared with chromium‑aluminium steels used in some boiler components, X18CrN28 offers better ductility and resistance to cracking, but may require additional corrosion protection in specific atmospheres.

Advantages and limitations compared with competing grades

Advantages of X18CrN28:

• High resistance to oxidation and corrosion at temperatures up to about 1000–1100°C.
• Good ductility and crack resistance due to the austenitic structure.
• Better cost-to-performance ratio than nickel alloys in many applications.
• Ability to be produced as tubes, sheets and profiles using standard metallurgical technologies.

Limitations:

• Tendency for precipitation of brittle phases (sigma) during prolonged exposure in a specific temperature range.
• Lower resistance than nickel alloys in very aggressive corrosive environments.
• Requirement for careful control of heat treatment and welding to avoid property degradation.

Choosing between X18CrN28 and alternative materials requires analysis of operating conditions, expected component life and life‑cycle costs.

Materials intended for industrial use must meet specific standards and have documentation confirming their properties.

International production and quality standards

• X18CrN28 is designated by material number 1.4749 in European metal material catalogues. For stainless and heat‑resistant steels primary references are EN 10088 (regulating grades and requirements for stainless steels) and specifications related to high‑temperature applications.
• In technical documentation manufacturers often refer to standards concerning quality control of the melting process, such as PN‑EN/ISO standards related to metallography, non‑destructive testing and chemical composition control.
• For pressure and boiler applications standards and guidelines from organisations such as ASME (construction codes for pressure equipment) also apply — materials for such uses must meet ASME I/VIII conditions, depending on purpose.

Certification requirements for specialised applications

• Material certificates 3.1/3.2 in accordance with EN 10204 confirm batch conformity with declared properties and test results.
• For critical components in the power and petrochemical industries additional tests are often required, such as creep tests, pitting and crevice corrosion resistance tests, and reports from non‑destructive testing (UT, RT, PT, MT).
• For international orders it is advisable to ensure compliance with local regulatory requirements and acceptance by the equipment manufacturer.

Meeting standards and holding the appropriate certification is a guarantee of safe use and longer service life of installations.

Proper handling of the material from delivery to installation significantly affects its durability and properties.

Storage conditions ensuring material longevity

• Store in a dry, well‑ventilated place, away from direct contact with the ground and moisture; use pallets and spacers to prevent condensation and contact corrosion.
• Protect from metallic contaminants (dust, splinters from carbon steel), which can cause localised corrosion.
• Batch scanning and identification: keep material documentation with batches to ensure traceability of chemical composition and processing parameters.

Safe transport procedures

• Transport elements to prevent dents and deformation. In some cases protective packaging against scratches and moisture is required.
• For pipeline and long elements use supports to prevent sagging and stresses.
• Avoid carrying together with corrosive materials and chemical reagents without appropriate protection.

Recommendations for operation at high temperatures

• Design components to avoid prolonged residence of material zones in the temperature range that favours sigma phase precipitation (around 600–900°C).
• Provide the possibility of cyclic cooling and controlled heating to limit accumulation of thermal stresses.
• Monitor the working environment for aggressive components in flue gases (chlorinated species, sulphur compounds, alkalis) and apply protective coatings if there is a risk of local corrosion.
• In welding and repairs use procedures that minimise heat‑affect zone (HAZ) impact and, if necessary, perform solution annealing after welding operations.

Good practice in storage, transport and operation extends component life and reduces the risk of failure.

Materials technology does not stand still. New methods to improve properties and durability are also being developed for grades such as X18CrN28.

Modern production techniques and property modification

• Control of microstructure through advanced heat treatments: precise annealing and cooling cycles and techniques of partial heating allow minimisation of brittle phase precipitates.
• Alloy composition modifications: adding microalloying elements (Nb, Ti) and controlling nitrogen and carbon contents enable optimisation of mechanical properties without significant cost increases.
• Coatings and surface modifications: techniques such as plasma spraying, laser cladding or thermal chromising increase surface resistance to erosion and corrosion, allowing the material to be used in more demanding conditions.
• Additive manufacturing (metal 3D printing): selective laser melting (SLM, DMLS) and powder‑binding techniques allow production of complex geometries and internal structures from X18CrN28. Development of high‑alloy powders and printing parameters expands possibilities for aerospace and power industry applications.

Prospects for use in new industrial sectors

• Renewable energy: elements in geothermal and solar installations that carry high temperature and are exposed to aggressive conditions may benefit from X18CrN28 properties.
• Energy recovery and waste‑to‑energy industries: high‑temperature boilers and combustion chambers require materials resistant to oxidation and corrosive combustion products.
• New propulsion systems and aerospace: where a combination of thermal resistance and ductility is required, advanced variants of the steel may find application in secondary turbine components or high‑temperature resistant systems.

Technological innovations and development of powder production and coatings open new perspectives for steels such as X18CrN28, enabling their use in more demanding environments.

Main advantages and limitations of the material

• Advantages:

– High resistance to oxidation and corrosion at elevated temperatures thanks to high chromium content.
– Stable austenitic structure providing good ductility and resistance to cracking.
– Additional strengthening by nitrogen, increasing strength without significantly reducing ductility.
– More cost‑effective than nickel alloys while retaining many desirable thermal properties.

• Limitations:

– Possibility of precipitation of brittle phases (e.g. sigma) during prolonged service in the 600–900°C range.
– Lower resistance than some nickel alloys in extremely aggressive corrosive conditions, especially in the presence of chlorides.
– Requirement for strict control of welding and heat treatment processes.

When and why to use X18CrN28 1.4749

X18CrN28 is recommended wherever a combination of good oxidation resistance at high temperatures, ductility and reasonable cost is required. It is an optimal material for:

• components of industrial furnaces and combustion chambers,
• heat exchangers and flue gas shields,
• power and industrial installations where operating conditions do not reach extremes that require nickel alloys,
• applications where ease of machining and weldability are important while retaining high‑temperature properties.

The decision to use it should be based on a comprehensive analysis of operating conditions (temperature, exposure time, atmosphere composition, mechanical loads) and comparison with alternative materials. In appropriately chosen applications X18CrN28 delivers a good compromise between durability and cost, making it an effective choice for many industrial sectors.