1.4845

Definition and characteristics of the steel grade

The steel designated H23N18 / X8CrNi25-21 / 1.4845 is a grade of austenitic heat‑resistant steel designed for operation at elevated temperatures and in environments promoting oxidation and corrosion. The names used in parallel reflect different classification systems: the common chemical designation (H23N18), the German designation according to standards (X8CrNi25-21) and the EN/DIN system number (1.4845). In practice, this grade sits alongside alloys such as AISI 310/310S and other high‑chromium, high‑nickel austenitic steels, combining oxidation resistance with retained ductility and strength at elevated temperatures.

To describe the material figuratively: it is like an “armour” for components exposed to hot gases and aggressive combustion products – it forms a durable protective layer that slows metal degradation and allows structures to operate longer and more safely.

Unique heat‑resistant properties

Steel 1.4845 is characterised by a combination of several key features:

• high resistance to oxidation at elevated temperatures due to its rich chromium content,
• increased resistance to creep and loss of strength at operating temperature resulting from nickel content and possible alloying additions,
• good ductility and weldability when appropriate procedures are used, which facilitates production of complex components,
• stability of the microstructure over a wide temperature range, provided prolonged exposure to temperature ranges promoting formation of undesirable brittle phases (e.g. sigma) is avoided.

These properties make steel 1.4845 suitable for applications where coexisting resistance to high temperature and corrosion is required, e.g. in furnaces, boilers, exhaust ducts and industrial installations operating in oxidising conditions.

Exact chemical composition depends on the manufacturer’s specification and the standard, but the general profile of steel 1.4845 includes high chromium and significant nickel content, with limited carbon. A typical elemental distribution is approximately:

• Chromium (Cr): around 23–25%,
• Nickel (Ni): around 18–21%,
• Carbon (C): low concentration, usually below 0.08% (sometimes lower to limit carbide formation),
• Manganese (Mn), silicon (Si), phosphorus (P), sulphur (S): elements in trace amounts in accordance with quality standards,
• Nitrogen (N): a controlled addition, can influence strength and austenite stability,
• Alloying additions (e.g. Al, Ti, Nb, Cu, Mo): depending on the alloy variant may be present in small quantities to improve specific properties (stabilisation, yield limits, creep resistance).

In practice, selecting precise contents is a compromise between oxidation resistance, corrosion resistance and mechanical properties.

Chromium content and its role in heat resistance

Chromium is the fundamental element responsible for oxidation and corrosion resistance in heat‑resistant steels. At contents of approximately 23–25% it forms a thin, adherent layer of chromium oxide (Cr2O3) on the metal surface, which acts as a barrier to further oxidation. This layer is self‑healing in oxidising conditions provided it is not mechanically damaged or chemically attacked in a way that prevents its reformation. The higher the chromium content within the alloy’s permissible range, the better the stability of this protective layer; however, increasing Cr can affect brittleness in some conditions and promote formation of brittle phases if the composition is not properly balanced.

Effect of nickel on strength and corrosion resistance

Nickel acts as an austenite stabiliser – it maintains the austenitic structure of the material at room temperature and at elevated temperatures, which results in good ductility, toughness and resistance to cracking. In heat‑resistant steels with high Ni content, general corrosion resistance and resistance in reducing environments improve, and creep resistance increases. In practice, the presence of 18–21% nickel in 1.4845 provides a compromise between structural stability and high temperature resistance.

Importance of alloying additions and other elements

Additions such as titanium (Ti) or niobium (Nb) are used to stabilise carbon, preventing formation of chromium carbides at grain boundaries (which could lead to intergranular corrosion). Nitrogen, in controlled amounts, strengthens the material and stabilises austenite. Molybdenum (if present) improves resistance to corrosion in chloride‑containing environments and to pitting. Silicon and manganese influence mechanical properties and processing behaviour. Each addition modifies the phase balance and the mechanics of the oxide layer, so alloy design is key to achieving the desired properties.

Melting and shaping methods

Production of steel 1.4845 begins with melting the alloy in electric furnaces (EAF – Electric Arc Furnace), often supplemented by refining processes such as VOD (Vacuum Oxygen Decarburisation) or VAR (Vacuum Arc Remelting) for demanding applications where metal cleanliness and control of dissolved gases is critical. Further homogenisation can be carried out in vacuum or induction furnaces.

After melting and casting, shaping is performed by rolling or forging depending on the final product (plates, sheets, pipes, forgings). Control of cooling and plastic working is important to obtain the appropriate microstructure and grain size.

Removal of non‑metallic inclusions and control of dissolved gases (H, N, O) are also important in production, as their excess can degrade high‑temperature properties.

Heat treatment and its effect on material structure

Key heat treatments are solution (homogenising) annealing and, where appropriate, careful stabilising procedures. Standard solution annealing involves heating the material to temperatures typical for austenites (e.g. 1000–1150°C depending on specific specification) and rapid cooling, which dissolves possible carbides and restores a homogeneous austenitic structure. Stabilising annealing with Ti or Nb may be used to prevent precipitation of chromium carbides.

Heat treatment directly affects grain size, phase distribution and carbide dispersion, and thus creep resistance, strength and ductility. Improper procedures (e.g. prolonged exposure in the 600–900°C range) promote precipitation of brittle phases, significantly reducing the suitability of the steel for industrial applications.

Austenitic structure and its characteristics

The basic structure of steel 1.4845 is austenite – an ordered crystal lattice providing ductility and resistance to fracture. Austenite favours good impact toughness at low temperatures and retention of ductility after heat treatment. Thanks to the presence of nickel the austenitic structure remains stable over a wide temperature range, which is crucial in thermal applications.

Austenite characteristics in this alloy include:

• high ductility and toughness,
• good weldability (when appropriate procedures are applied),
• ability to undergo plastic deformation without brittleness, facilitating forming.

Phase analysis and their stability at high temperatures

When operating at elevated temperatures phase stability is important. Main concerns relate to precipitation of carbides (e.g. Cr23C6) and intermetallic phases such as sigma (σ), which are chromium‑rich and can greatly reduce ductility and corrosion resistance. Sigma typically forms in the temperature range of about 600–900°C, particularly with prolonged exposure in this thermal range.

Control of composition (e.g. low carbon content or Ti/Nb stabilisation) and appropriate heat treatments minimise the risk of adverse phase formation. High temperatures can also increase diffusion of elements and change the thickness of the protective oxide layer – in this context time and the working atmosphere determine the actual durability of the material.

Mechanisms of protection against oxidation

The fundamental protective mechanism is the formation and maintenance of a thin, compact chromium oxide layer. This layer is self‑regenerating in the presence of oxygen and provides surface passivation. In environments rich in sulphur oxides or phosphorus the layer can be modified, so alloying additions and metal structure influence its stability. At high temperatures metal oxides can grow, forming layers with varying porosity – a less porous and more consolidated layer means better protection.

These mechanisms work effectively in oxidising environments; in reducing atmospheres or those containing chlorides, sulphur or halogen compounds, protection may be weakened, which requires greater caution when selecting material.

Use in high‑temperature and corrosive conditions

Steel 1.4845 is used where there is a combination of high temperature and moderate chemical aggressiveness: exhaust ducts, furnace components, heat exchangers, pipes and thermal shields. In chloride‑containing environments or under stress corrosion cracking (SCC) conditions, high‑nickel austenitic steels are more resistant than ferritic steels, but they are not fully immune – particularly in the presence of high temperatures and stresses.

In practice designers select this material where long‑term operation at temperatures up to several hundred degrees Celsius is required; in extreme cases special coatings or other alloys with enhanced corrosion resistance (typically containing molybdenum, aluminium or different nickel proportions) are used.

Power industry – boilers and turbines

In the power sector steel 1.4845 is used in components exposed to high temperature and oxidative corrosion: boiler parts, exhaust collectors, exhaust pipes and furnace components. Its oxidation resistance and retention of mechanical properties at elevated temperatures make it a choice for components where long‑term durability is critical.

In gas turbines and certain parts of industrial engines the material can be used in shields, ducts and auxiliary parts where temperatures do not reach the extremes requiring nickel superalloys.

Chemical and petrochemical industry

In chemical installations steel 1.4845 is used where high process temperatures occur and the environment is not strongly acidic or containing aggressive anions (such as chloride). Typical applications are heat exchangers, pipelines in hot zones of straightforward processes, equipment shields and fast‑change heat transfer elements.

In petrochemistry, in environmentally moderate installations, this steel allows safe operation of heating and supply components at a reasonable cost–benefit ratio.

Other specialised application areas

Additional uses include the metallurgical industry, equipment for melting and remelting metals, components of heat‑treatment furnaces and specialist shields for sensors and instruments operating in hot atmospheres. Good weldability enables fabrication of complex structures requiring both thermal resistance and a degree of ductility.

Alternative steel grades and their parameters

Closest comparisons are steels such as:

• AISI 310/310S (X10CrNi25-21): similar chromium and nickel content; 310S has lower carbon and better resistance to intergranular attack,
• 253 MA (1.4981): an aluminium‑ and cerium‑containing steel used for high oxidation resistance in specific atmospheres,
• Other austenitic alloys with molybdenum additions (e.g. 316/316L) – these offer better resistance to chloride corrosion but lower oxidation resistance at the highest temperatures than 1.4845.

Each of these alloys has strengths and weaknesses: some perform better in alkaline or chloride environments, others provide superior oxidation resistance at extreme temperatures. The choice therefore depends on the specific application, working atmosphere and durability requirements.

Advantages and limitations of steel 1.4845

Advantages:

• high oxidation resistance at elevated temperatures,
• austenite stability and good mechanical properties at elevated temperatures,
• favourable weldability and capability to manufacture complex components.

Limitations:

• sensitivity to precipitation of brittle phases (e.g. sigma) under inappropriate temperature conditions,
• limited resistance in aggressive environments containing chlorides and sulphur compared with some specialised alloys,
• higher cost compared with conventional stainless steels with lower alloying levels.

Decisions to use 1.4845 are usually based on a balance: where oxidation at high temperatures is the dominant factor, this steel grade is often the optimal choice.

International quality standards

Steel 1.4845 appears in European (EN/DIN) standard catalogues and is associated with standards such as EN 10088 (stainless and corrosion‑resistant steels). Production and deliveries are typically certified according to relevant quality standards, which specify chemical composition, dimensional tolerances, mechanical tests and batch documentation.

Quality documentation often includes a 3.1 certificate according to EN10204, confirming chemical and mechanical test results carried out on representative samples.

Test procedures and quality control

Quality control includes:

• chemical analyses (spectrometry, OES) to verify element contents,
• mechanical tests: tensile tests, hardness, impact toughness, creep testing at operating temperature,
• metallographic examinations: assessment of microstructure, presence of non‑metallic phases, grain size,
• corrosion resistance tests: salt spray tests, pitting tests and accelerated high‑temperature oxidation tests,
• non‑destructive testing where required: radiography, ultrasonic testing, leak testing and dye penetrant testing.

Such comprehensive control ensures the material meets the expectations for a given application and reduces the risk of failures due to material defects.

Challenges related to processing and welding

Although austenitic steels are generally well weldable, there are specific challenges:

• risk of forming brittle phases during high temperature welding and cooling if proper procedures are not followed,
• potential contamination and introduction of oxides which may disrupt formation of the protective oxide layer,
• control of heat input to avoid excessive grain growth and precipitations affecting mechanical properties.

Recommended methods and technologies

Recommended welding techniques include TIG (GTAW), MIG/MAG (in appropriate atmospheres), as well as laser and PAW (plasma) welding for precise, low heat‑input applications. Filler materials are often chosen to have compositions similar to the base material or with slight modification to prevent precipitations (e.g. Ti/Nb additions in the wire to stabilise carbides).

Good practices include:

• cleaning the welding area of contaminants,
• avoiding excessive heating and using low heat input,
• controlling interpass temperature,
• applying appropriate post‑weld treatments such as solution annealing where necessary,
• using high‑purity shielding gases (argon).

Applying these procedures minimises the risk of degradation and ensures welds have mechanical and corrosion properties close to those of the base material.

Optimal storage conditions

Components made of 1.4845 should be stored in dry, well‑ventilated warehouses, away from sources of contamination and acidic or alkaline environments. It is important to avoid direct contact with superior materials (e.g. carbon, cast iron) which could cause surface contamination and subsequent intergranular corrosion.

Packaging should allow ventilation and protect against moisture. For longer‑term storage it is advisable to inspect the surface condition and, if necessary, clean deposits and dust.

Rules for maintaining properties during service

In operation it is important to:

• avoid prolonged exposure of the material in temperature ranges that favour formation of brittle phases (600–900°C),
• monitor the condition of the surface and protective oxide layer; in case of damage apply coating repairs or local replacement panels,
• control stresses and thermal overloads, which combined with aggressive environments can lead to cracking,
• perform regular non‑destructive inspections at critical points of the structure.

Good operational practice extends service life and minimises the risk of failures from corrosion or thermal fatigue.

New manufacturing technologies and alloy modification

Development perspectives include:

• use of vacuum melting methods and micro‑alloying to increase steel cleanliness and control dissolved gases,
• development of alloys with optimised compositions of stabilising elements to retain austenitic structure while reducing nickel cost,
• powder technologies and 3D printing (additive manufacturing) enabling production of complex parts with minimal waste and control of microstructure,
• surface modifications and protective coatings (e.g. aluminising, ceramic coatings) to extend oxidation resistance in extreme conditions.

At the same time, simulation tools are being developed to predict formation of undesirable phases and material behaviour in complex thermo‑chemical conditions.

Applications in modern structures and equipment

With the evolution of the power and chemical industries, expanded use of advanced heat‑resistant steels in installations with higher thermal efficiency, lower emissions and higher operating temperatures is expected. Technologies that reduce material volume while increasing durability will favour the use of alloys such as 1.4845 in precise heat‑exchanger components, thermal shields in higher‑efficiency turbines and in waste‑to‑energy and energy recovery applications.

Moreover, developments in the aerospace and space industries pose material challenges that could lead to adaptation of such alloys in hypersonic elements and engine/landing shields with appropriate modification of composition and structure.

• Steel 1.4845 (H23N18 / X8CrNi25-21) is a high‑chromium, high‑nickel austenitic grade designed for operation at elevated temperatures and in oxidising conditions.
• The main constituents determining its properties are about 23–25% chromium and 18–21% nickel; the composition is completed by controlled amounts of carbon, nitrogen and stabilising elements.
• Chromium forms a protective, self‑regenerating oxide layer, and nickel stabilises the austenitic structure, which together provide oxidation resistance and good strength at temperature.
• Production includes EAF melting, vacuum refining and precise forming; correct heat treatments are crucial to avoid precipitation of brittle phases (e.g. sigma).
• Applications: boiler and furnace components, thermal shields, heat exchangers, pipelines in high‑temperature industrial areas.
• Compared with grades such as AISI 310/310S, this steel has strengths in oxidation resistance but requires care in chloride‑containing environments and with prolonged exposure to temperatures that promote precipitations.
• Quality control based on standards (e.g. EN) and mechanical and metallographic tests is essential for reliable service.
• Welding requires appropriate technologies (TIG, MIG, laser) and filler materials similar in composition to the base, observing procedures that limit heat input.
• The future for these alloys includes composition optimisation, powder technologies and protective coatings, as well as applications in structures demanding higher thermal efficiency.

Each application of steel 1.4845 requires individual analysis of working conditions, composition control and processing to ensure long‑term and safe operation.