1.4724

What is heat‑resistant steel? Basic properties

Heat‑resistant steel is a group of steel alloys designed to operate at elevated temperatures where oxidation, aggressive corrosive agents and long‑term deformation under load (creep) dominate. The most important task of these materials is to retain mechanical integrity and surface resistance at temperatures at which ordinary steels rapidly lose strength and become covered with a thick layer of flaking oxides. In practice, heat‑resistant steel combines several features:

• high resistance to oxidation in a high‑temperature environment;
• mechanical stability (strength and ductility) at elevated temperatures;
• adequate corrosion resistance against aggressive combustion products and chemicals;
• good thermal conductivity and a relatively low coefficient of thermal expansion (for some grades).

Metaphor: heat‑resistant steel works like a blacksmith’s armour — it keeps its shape and function despite the heat, and the alloying additions (Cr, Al, Si, Ni, etc.) are the plates that reinforce that armour.

Differences between heat‑resistant steel and stainless steel

Both groups — stainless steel and heat‑resistant steel — use chromium and other elements to protect the surface against corrosion. However, the differences are significant:

• Design purpose: stainless steel is mainly designed for corrosion resistance in atmospheric, chemical and humid conditions at ambient temperatures; heat‑resistant steel is formulated for long‑term contact with high temperatures, where the protective tool becomes an oxide layer (mainly Cr2O3 and/or Al2O3).
• Alloy composition: heat‑resistant alloys often contain aluminium and silicon, which stabilise and strengthen the oxide film at high temperatures; stainless alloys, especially austenitic ones, have more nickel to maintain an austenitic structure and ductility at low temperatures.
• Mechanical properties: heat‑resistant steel can be designed for creep resistance and thermal ageing, while stainless steel places more emphasis on general corrosion resistance and ductility.
• Behaviour in extremes: under conditions >500–600°C the passive layer of stainless steels may not be sufficient, whereas heat‑resistant alloys thanks to Al and Si form more stable, less spalling oxide layers.

Origin and classification of the individual grades

• X10CrAl13 (EN 1.4724) is a classic heat‑resistant steel grade developed for components operating at elevated temperatures, mainly in boiler and power plant industry. The designation X10CrAl13 indicates an approximate Cr content (~10%) and Al (~1–1.5%), which enables the formation of a protective alumina oxide layer, increasing oxidation resistance. In European countries and in Russia this grade has been and remains widely used for tubes, burners and heating elements.
• X10CrAlSi13 is a variant of X10CrAl13 enriched with a higher silicon (Si) content, which enhances oxidation resistance and improves the formation of a uniform oxide layer. The Si addition positively affects the adhesion and continuity of the oxide layer during cyclic temperature changes.
• H13JS is a designation more commonly found in technical documentation and in the product offers of manufacturers from Central and Eastern Europe. It may refer to heat‑resistant steel with a composition adapted to specific operating conditions, combining features of heat‑resistant steels with resistance to thermal shock. H13JS functions in practice as a trade name or a local designation for a group of heat‑resistant alloys. For an engineer, the declared composition, standards and properties provided by the manufacturer are more important than the designation itself.

Basic standards and designations of these grades

• X10CrAl13 = EN/DIN 1.4724 — this is the official assignment in European material catalogues. Products made from this steel are also described in standards for tubes and boiler components (e.g. EN 10216/10217 for tubes, EN 10095 for heat‑resistant tubes).
• X10CrAlSi13 is described similarly, but with an additional note regarding the Si content — manufacturer standards and quality test parameters are important.
• H13JS — a market designation; when purchasing components it is important to refer to material certificates, results of chemical analysis (3.1/3.2 certificates according to EN), mechanical test reports and oxidation resistance testing.

International and European standards relevant to these types of materials include EN 10095 (heat‑resistant tubes), EN 10216/10217 (seamless and welded steel tubes), EN 10028/10029 (plates), as well as material testing standards: ISO, ASTM with respect to test methods. In technical documentation one should always verify specific links to the applicable standards.

Percentage contents of key elements

The table below provides typical ranges of key element contents for the discussed grades. Real values may differ depending on the manufacturer and the standard; the data below should be treated as indicative.

• X10CrAl13 (1.4724) — typical composition:

– C: ≤ 0.12%
– Si: 0.3–1.0%
– Mn: ≤ 1.0%
– Cr: 10.0–11.5%
– Al: 0.9–1.6%
– Ni: ≤ 0.6%
– P: ≤ 0.04%
– S: ≤ 0.03%
– Fe: balance

• X10CrAlSi13 — a variant of X10CrAl13 with higher Si:

– C: ≤ 0.12%
– Si: 0.5–1.5% (usually closer to 0.7–1.0%)
– Mn: ≤ 1.0%
– Cr: 10.0–11.5%
– Al: 0.9–1.6%
– Ni: ≤ 0.6%
– P, S: in low trace amounts
– Fe: balance

• H13JS — indicative composition (should be verified with the supplier):

– C: approximately 0.06–0.20% (depending on subtype)
– Si: 0.3–1.5%
– Mn: 0.2–1.0%
– Cr: 1.0–13.0% (depending on specification; some variants are enriched in chromium to improve resistance)
– Al: often present in auxiliary amounts (0.2–1.5%)
– Ni, Mo, V: may be present in trace amounts or in added amounts to improve strength
– Fe: remainder

Note: H13JS as a market designation may cover different formulations. Therefore the designation should always be confirmed by the manufacturer’s certificate.

Effect of individual elements on material properties

• Chromium (Cr): key for forming a protective chromia (Cr2O3) oxide layer. In heat‑resistant grades, in combination with aluminium, it increases the stability of the protective layer at higher temperatures. It also increases strength at elevated temperatures.
• Aluminium (Al): forms a very stable and adherent alumina (Al2O3) oxide layer, which is more resistant to spalling at very high temperatures than chromia. Al provides exceptional oxidation resistance under very high‑temperature conditions.
• Silicon (Si): supports oxide formation and improves adhesion of the protective layer, which is particularly useful during cyclic temperature changes. Si also increases resistance to corrosion in flue gases.
• Carbon (C): affects hardness and strength; its content is kept relatively low to avoid excessive brittleness and carbide formation, which reduce oxidation resistance.
• Manganese (Mn): improves machinability and influences hardness; however, too much Mn can be detrimental at high temperatures.
• Nickel (Ni): usually limited in ferritic heat‑resistant steels; small Ni additions stabilise the austenitic structure and improve ductility, but in this type of steel it is not often used in large amounts.
• Alloying additions (Mo, V, Ti): used locally to increase strength, creep resistance and thermal stability, but they raise costs.

Each element brings specific benefits, but the essence of alloy design is a compromise between oxidation resistance, mechanical strength and cost.

Manufacturing methods for H13JS and X10CrAl13

Production of heat‑resistant steels is based on several standard stages, using technologies aimed at maintaining the desired microstructures and minimising contamination:

• Melting: an induction furnace (EAF) or an electric arc furnace is used to melt the steel with appropriate alloying additions. Refining (AOD/VOD) is then carried out to reduce gas content and impurities.
• Casting: steel may be cast continuously (ingots/blooms) or directly in continuous casting, which influences the uniformity of composition and structure.
• Plastic working: hot forging, rolling and drawing form semi‑finished products (bars, plates, tubes). Temperatures are controlled during working to avoid introducing excessive oxide or carbide contamination.
• Surface treatment: passivation, grit blasting, pickling and annealing in a controlled atmosphere to reduce stresses and achieve the required surface cleanliness.
• Quality control: ICP/OES spectrometry for chemical analysis, mechanical and microstructural tests and oxidation resistance tests.

Grades such as H13JS are produced in forms such as boiler tubes, bars and forged parts, while X10CrAl13 is often available as tube and plate for high‑temperature installations.

Specifics of heat treatment and influence on steel structure

Heat treatment of heat‑resistant steels has a dual purpose: removal of stresses after plastic working and obtaining a microstructure that provides mechanical durability and creep resistance.

Typical processes:

• Homogenising annealing: carried out at temperatures that dissolve unwanted phases and ensure even distribution of alloying elements. This stabilises the microstructure and reduces segregation.
• Normalising: in some grades normalising is used to obtain a uniform, fine grain structure, which improves mechanical properties.
• Stress‑relief annealing: used after forming and welding to reduce internal stresses and prevent thermal cracking.
• Ageing/precise heat treatments: in grades with strength‑increasing alloying additions, controlled thermal treatments are used to precipitate strengthening phases.

Effect on structure:

• In ferritic steels (e.g. X10CrAl13) the aim of treatment is not to form austenite, but to maintain a fine‑grained ferritic structure with a uniform distribution of oxide phases.
• In hybrid grades or those containing additives, heating can lead to precipitation of fine particles that strengthen the material at high temperatures.

Heat treatment and microstructure control have a key impact on creep resistance, hardness, fracture resistance and adhesion of the oxide layer.

High‑temperature strength and corrosion resistance

Heat‑resistant steels of the X10CrAl13 class and its variants show good oxidation resistance up to temperatures of several hundred degrees Celsius, and under certain conditions even up to 900–1000°C for short periods. The actual permissible operating temperature depends on:

• duration of exposure (continuous vs intermittent operation),
• atmosphere (dry vs humid, presence of sulfur or chlorides),
• mechanical and thermal stresses.

In the presence of aggressive gases (sulfur, chlorides) resistance decreases and specialised alloys or protective coatings are required. Compared with standard stainless steels, alloys with Al and Si cope better with extreme oxidation because a stable Al2O3 layer forms.

Oxidation resistance and thermal deformation

• Oxidation: formation of a thin, continuous oxide layer (Al2O3/Cr2O3) is crucial. X10CrAlSi13, thanks to increased Si, better controls oxide growth and limits spalling during thermal cycles.
• Thermal deformation: ferritic alloys usually have a lower coefficient of thermal expansion than austenitic ones, which reduces the risk of cracking under diverse thermal gradients. However, cyclic heating and cooling induce stresses that over time lead to degradation. Here uniform composition and appropriate heat treatment are critical.

Analysis of hardness, ductility and fatigue resistance

• Hardness: maintained at a level that enables resistance to plastic deformation at high temperatures. Depending on composition and heat treatment, hardness can be tailored to the application.
• Ductility: in ferritic steels it is generally sufficient for most boiler and tubing applications, but in applications exposed to impact and dynamic stresses variants with improved composition (e.g. Ni additions) are preferred.
• Fatigue resistance: critical in cyclic applications such as turbine components or tubes subjected to frequent temperature and pressure changes. Proper control of contaminants, a uniform structure and the presence of small, stable strengthening phases improve resistance to crack initiation and propagation.

Practical example: a boiler tube made of X10CrAl13 in a steam boiler may operate for years if well chosen with respect to operating temperature, weld quality and fuel control; however sudden deterioration in flue gas quality (e.g. presence of sulfur and chlorides) can significantly shorten service life.

Power industry and power plants – roles of H13JS and X10CrAl13

Heat‑resistant steels have wide application in the power industry:

• boiler tubes and components,
• flue gas ducts and exhaust channels,
• heat exchangers and boiler furnaces,
• gas turbine components in low and medium thermal load areas.

In coal and biomass power plants, where aggressive flue gas environments and high temperatures occur simultaneously, alloys such as X10CrAl13 are used where oxidation resistance and material stability over long service are required.

Use in manufacture of boiler components and heating tubes

• Heating and boiler tubes: X10CrAl13 is a typical choice for tubes exposed to direct flame and hot flue gases.
• Burner components and shields: due to resistance to oxide layer spalling, they are used where intense thermal action occurs.
• Seals and fittings: in locations where both temperature resistance and resistance to aggressive flue gas chemistry are required.

Applications in automotive and aerospace industries

• Automotive: parts of exhaust systems, turbochargers, exhaust manifold components and engine elements close to high‑temperature sources. In commercial automotive applications alloys that withstand variable loads and rapid heating/cooling cycles are important.
• Aerospace: applications are more limited due to creep strength and strength‑to‑weight ratio requirements, but exhaust system components, jet engine shields and engine hardware can use special variants of heat‑resistant alloys. Aerospace more commonly uses nickel‑ and titanium‑based superalloys, but Al‑Cr alloys have niche applications.

Analysis of operating costs and material lifetime

Comparison of H13JS with X10CrAl13/X10CrAlSi13 should consider:

• Material cost: grades with additional alloying elements (Mo, V, Ni) are more expensive. X10CrAl13 is often an economical choice for boiler applications.
• Fabrication and welding cost: some alloys are more difficult to weld and require special procedures, increasing installation cost.
• Service life: alloys well matched to operating conditions (temperature, flue gas composition) generally deliver lower total operating costs due to less frequent repairs and replacements.
• Downtime costs: using a cheaper material that degrades sooner can generate costs many times greater than using a more expensive but more durable alloy.

In practice X10CrAl13 offers a good compromise between cost and oxidation resistance for typical boiler applications. H13JS, depending on exact composition, may cope better with thermal shock, extending service life in applications with rapid temperature changes.

Impact of alloy innovations on competitiveness of grades

• Micro‑alloying and contamination control: small additions (e.g. Nb, Ti) can prevent grain growth and improve creep resistance.
• Protective coatings and ceramic coverings: use of coatings extends the service range of steels with lower inherent resistance, which affects cost‑effectiveness.
• Chemical optimisation: development of alloys with an optimal Cr/Al/Si ratio allows cost reduction while meeting operational requirements.
• Manufacturing methods: processes such as powder metallurgy or advanced casting techniques enable the production of more homogeneous materials with better high‑temperature properties.

These innovations mean that grades such as X10CrAlSi13 can in some applications replace more expensive superalloys, offering a balanced cost‑to‑performance ratio.

European and international material standards

• EN/DIN: X10CrAl13 is assigned the designation 1.4724 in the EN/DIN system. These standards regulate requirements for chemical composition, mechanical properties and testing.
• EN 10095 and EN 10216/10217: concern production of heat‑resistant tubes and test methods.
• ISO/ASTM: international methods for mechanical, chemical and corrosion testing used as complements to EN requirements.

In practice it is important that purchased material has quality documentation: chemical certificate, mechanical test report, NDT documentation and a declaration of conformity with relevant standards.

Testing procedures and quality control

• Chemical analysis: spectrometry (OES/ICP) confirms alloying element contents.
• Mechanical tests: tensile, elongation, hardness, impact (Charpy), creep tests for long‑term applications.
• Oxidation resistance tests: exposure in controlled high‑temperature atmospheres and assessment of oxide growth and spallation.
• Microstructural studies: metallography, analysis of carbide distribution and phases.
• Non‑destructive testing: ultrasonic, magnetic‑particle, penetrant tests for detecting internal and surface defects.
• Weld inspection: welding procedure tests, macro‑examination of welds, leak tests, e.g. for tubes.

Thorough quality control from raw material to finished component is key to installation safety and minimising operating costs.

Technical and environmental factors affecting material selection

When selecting a heat‑resistant steel consider:

• Operating temperature (continuous vs short‑term): affects alloy selection and determination of maximum permissible operating temperature.
• Operating atmosphere: presence of oxygen, sulfur, hydrogen chloride, steam, salts and other compounds modifies alloy choice.
• Mechanical loads: static, dynamic, impact and creep risk.
• Thermal cycling: number and amplitude of heating‑cooling cycles — the more frequent, the higher the requirements for oxide layer adhesion.
• Welding and fabrication conditions: production requirements, availability of welding consumables and need for PWHT (post‑weld heat treatment).
• Costs: project budget versus maintenance costs and potential downtime.

In practice an engineer selects material from corrosion/temperature resistance tables for realistic operating parameters, including safety margins.

Economic analysis and operational aspects

Cost analysis should include not only unit material price but also:

• Installation cost (e.g. need for specialised welding)
• Operating cost (frequency of repairs, replacements)
• Downtime costs (failures, maintenance)
• Possibilities for component regeneration (cleaning, recoating)
• Service life and residual value of the material

An economic decision should be based on LCC (life‑cycle cost) analysis, not merely CAPEX. Sometimes a more expensive alloy with longer durability proves cheaper over the full life cycle of the installation.

Alloy development and innovative design solutions

• New compositions: research into micro‑alloying and optimisation of the Cr/Al/Si ratio aims to improve strength and oxidation resistance at lower cost.
• Powder metallurgy and sintering: allow production of homogeneous, fine‑grained microstructures with favourable high‑temperature properties.
• Hybrid solutions: combining suitably chosen alloys with special ceramic or metallic coatings extends component life in the most aggressive conditions.
• Topology‑optimised and lightweight designs: enable mass savings while retaining thermal resistance, which matters in automotive and aerospace applications.

Use in ecological and energy‑efficient industrial systems

• Renewable energy: biomass boilers and incinerators require materials resistant to aggressive combustion products; heat‑resistant alloys are key here.
• Flue gas recirculation and heat recovery systems: materials with high oxidation and corrosion resistance in flue gases enable improved equipment efficiency.
• Hydrogen and future fuels: new combustion media and the presence of hydrogen may require adaptation of alloy compositions to prevent new degradation forms (e.g. hydrogen embrittlement) — development of hydrogen‑resistant alloys is an active research area.
• Energy‑efficient industrial processes: better materials allow operation at higher temperatures without loss of service life, translating to higher thermal efficiency of equipment and lower emissions.

Industry trends move towards multifunctional materials that combine high thermal resistance with good fabricability and lower life‑cycle cost.

Optimal storage conditions and avoiding corrosion

• Dry and ventilated: store heat‑resistant steel components in dry, ventilated areas, away from sources of moisture.
• Isolation from the ground: use spacers (e.g. plastic, wood) to avoid contact with ground contaminants and condensation.
• Chemical protection: avoid contact with corrosive substances (salts, acids, alkalis) and with materials that can cause localised corrosion (e.g. coarse iron with condensate).
• Surface protection: for long‑term storage it is advisable to protect surfaces with protective film, anti‑corrosion oil or corrosion inhibitors, especially for cut or welded materials.
• Documentation and stock rotation: batch control and documentation (certificates) should go hand in hand with FIFO to minimise storage time.

Methods for regeneration and extending component life

• Mechanical cleaning: grit blasting, brushing and abrasion can remove loose oxide deposits, but avoid overly aggressive cleaning that would expose a “fresh” surface prone to rapid oxidation.
• Chemical degreasing and pickling: used to remove deposits and contaminants; however these require neutralisation and subsequent passivation to restore the protective oxide layer.
• Protective coatings: in areas of extreme corrosive‑thermal conditions ceramic or metallic coatings are applied to protect against direct exposure to aggressive media.
• Weld repairs: for repairs use appropriate welding consumables and procedures, control the weld microstructure and perform necessary stress‑relief annealing.
• Regular inspections: visual inspection, wall thickness measurements (ultrasonic), NDT and monitoring of oxide growth help predict the need for replacement and prevent failures.

Practical example: a boiler tube with an oxide deposit that is regularly cleaned and maintained can retain adequate function for longer than an unmaintained tube even with a similar initial material composition.

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This guide presents comprehensive knowledge about heat‑resistant steels using X10CrAl13 (1.4724), X10CrAlSi13 and the H13JS designation as examples. The descriptions combine definitions, historical context and practical technical guidelines to facilitate engineering decisions regarding material selection, processing and maintenance in real industrial conditions.