1.4713

Definition of heat-resistant steel and its importance in industry

Heat-resistant steel is a group of iron alloys with alloying additions which, at high temperatures (commonly above 400–500°C), exhibit increased resistance to oxidation and degradation caused by chemical and mechanical factors. A key feature is the ability to form durable, adherent protective oxide layers – most often chromia (Cr2O3) or alumina (Al2O3) – which inhibit further corrosion. In practice, heat-resistant steel is used where a material operates for long periods at high temperatures, subjected to cyclic heating and cooling, aggressive flue-gas atmospheres or contact with oxidising gases.

Its industrial significance is huge: from power generation (burners, boilers, heat exchangers), through metallurgy and the chemical industry (reducers, process piping), to automotive and aerospace sectors (turbochargers, exhaust manifolds). Selecting the correct grade determines the durability, safety and operating costs of equipment.

Basic characteristic features of the grades H6S2, X10CrAlSi7 and X10CrAl7

• H6S2: a GOST/Russian nomenclature grade, designed for moderately high temperatures. It is characterised by the presence of chromium and silicon and a low carbon content. Intended for furnace elements and flue ducts where good oxidation resistance and reasonable ductility are required.
• X10CrAlSi7: a German/EN nomenclature grade in which the key components are chromium (~10%), aluminium and silicon – additions that improve the formation of a protective oxide layer. It features good resistance to high-temperature corrosion and structural stability at elevated temperatures.
• X10CrAl7 / 1.4713: a close relative of the above – the designation 1.4713 is the DIN number for alloys of similar composition. This is an FeCrAl type heat-resistant steel in which aluminium plays a particular role in forming a durable Al2O3 layer that provides resistance in oxidising atmospheres. This steel is often chosen where long-term operation at temperatures of several hundred degrees Celsius is required, together with contact with aggressive gases.

In practice, differences between these grades translate into: the allowable operating temperature range, resistance to cyclic thermal loads, cohesion of the oxidised layer and susceptibility to cracking under thermal stress.

Understanding which elements and in what amounts form these steels allows prediction of their behaviour under extreme conditions. Below are the characteristic constituents and their typical influence; quoted values are indicative and should be verified in normative documentation or manufacturers’ datasheets.

Analysis of the percentage content of chromium, aluminium, silicon and other elements

• Chromium (Cr): a key element for forming a chromia oxide layer. For X10Cr… grades typical chromium content is around 9–12%. In H6S2 the chromium proportion may be lower, depending on production standards, but it always serves as a basic anti-corrosive component.
• Aluminium (Al): in FeCrAl-type alloys (e.g. X10CrAl7/1.4713) aluminium occurs in significant concentrations, which can range from a few to a dozen percent depending on the alloy type. In practice values on the order of 3–7% Al are encountered in grades designed for strong oxidation resistance. Aluminium favours formation of a stable, dense Al2O3 layer.
• Silicon (Si): in grades marked “Si” (X10CrAlSi7) silicon is usually added in amounts of 0.5–2.0%. It improves oxidation resistance, affects stability of the oxide layer and facilitates heat dissipation. Silicon can also improve melt fluidity during production.
• Carbon (C): kept at a low level (typically ≤0.12%) to limit carbide formation, which can weaken the continuity of the protective oxide layer and reduce resistance to high-temperature corrosion.
• Manganese (Mn), phosphorus (P), sulphur (S): present in small amounts in line with permitted contents of raw materials. Phosphorus and sulphur are undesirable in excess – they promote embrittlement and reduce oxidation resistance.
• Alloying additions (e.g. Nb, Ti, Zr, rare earth elements): in some variants micro-additions of niobium, titanium, zirconium or rare earth elements are used to improve microstructure stability, oxide layer adherence and resistance to cracking.

In practice specific compositions vary depending on the standard, manufacturer and intended use of the material. Therefore for orders and material certificates one should always refer to precise tables and certificates.

Influence of individual constituents on heat-resistance properties

• Chromium: forms a protective Cr2O3 layer at moderate temperatures – this layer is effective, but at very high temperatures (or in the presence of sulphur) it can be less stable than an aluminium-based layer.
• Aluminium: the presence of Al promotes the formation of Al2O3 – a layer with excellent adhesion and resistance to aggressive atmospheres. The aluminium-based layer is preferred where long-term protection in high-temperature conditions is required.
• Silicon: improves the ability to form dense oxides, supports protection in atmospheres containing oxygen and sulphur dioxide.
• Low C and controlled impurities: limiting carbon and unfavourable impurities prevents formation of brittle phases and carbides, which affects long-term mechanical and thermal resistance.

In short: a combination of appropriate amounts of Cr, Al and Si allows design of a steel that in specific working conditions will form a suitable, durable anti-oxidation barrier.

The properties of heat-resistant steels result not only from their chemical composition but also from manufacturing and heat treatment processes that shape the microstructure and phase distribution in the alloy.

Manufacturing methods and their influence on material structure

• Melting in induction or arc furnaces: most grades are produced by melting under controlled conditions, which allows precise dosing of alloying additions and limiting impurities. For steels with specific properties vacuum degassing and electrochemical refining are used.
• Directional casting and rolling: after melting the steel is formed by hot rolling, and then finished in cold or hot rolling processes. Controlled rolling affects fibre directionality and the material’s ability to work under variable thermal conditions.
• Powder metallurgy and sintering: for components with complex geometry or requiring a very homogeneous structure powder metallurgy methods can be used. They allow obtaining precise constituent distributions and a fine microstructure, which translates to better resistance to thermal cracking.
• Additional technologies: processing in atmospheres with controlled oxidation, microalloying, homogenisation processes – all affect the quality of the oxide layer, compositional uniformity and microstructure stability.

The choice of manufacturing method directly influences final properties – sintered materials may better withstand cyclic loads, whereas rolled ones offer better ductility.

Principles of heat treatment optimising high-temperature resistance

• Homogenising annealing: applied after shaping processes to dissolve excess phases and even out the distribution of elements. This reduces internal stresses and improves resistance to high-temperature corrosion.
• Solution treatment and ageing: in some variants controlled ageing is applied to precipitate fine particles that increase creep resistance and stabilise the microstructure.
• Control of cooling rate: cooling rate determines grain size and carbide distribution. Too rapid cooling can lead to residual stresses, while too slow cooling leads to a coarse-grained structure less resistant to thermal fatigue.
• Heat treatment of welds: welding these alloys requires appropriately chosen procedures – preheating, control of the heat-affected zone temperature and possible post-weld annealing. Aluminium and silicon influence weld zone brittleness, which is why special electrodes and filler wires and techniques minimising contamination are used.

Appropriate heat treatment can significantly extend the service life of components operating at high temperatures, reducing the risk of cracking and fractures.

Heat-resistant steels show specific mechanical behaviour: strength decreases with rising temperature, but oxidation and chemical resistance are increased. Key mechanical aspects important in operation are discussed below.

Resistance to wear, stresses and deformation at high temperatures

• Wear: under normal conditions heat resistance goes hand in hand with moderate wear resistance. In environments containing abrasive particles it is necessary to consider strengthened alloy variants or protective coatings.
• Thermal stresses: cyclic heating and cooling generate thermal stresses that can lead to cracking. Grades with higher aluminium content form oxide layers with better adhesion, which reduces the risk of local spallation and crack propagation.
• Plastic deformation: under constant load at high temperatures (creep) the material undergoes gradual deformation. Creep resistance depends on microstructure and alloying additions. Alloys with fine dispersions of carbides or precipitates provide better creep resistance.

In practice, component design requires consideration of the coefficient of thermal expansion, modulus of elasticity at operating temperature and expected mechanical loads.

Tensile strength and resistance to high-temperature corrosion

• Tensile strength: decreases with increasing temperature, but alloys with appropriately chosen composition retain sufficient load-bearing capacity even at elevated temperatures. Data on yield strength and tensile strength must be read from technical datasheets for a specific grade and heat-treatment condition.
• Resistance to high-temperature corrosion: depends on the ability to form and maintain a protective oxide coating. Layers based on Cr2O3 and Al2O3 differ in protection mechanism; alumina layers are more stable across a broad temperature range and in the presence of certain aggressive gases, while chromia works well in less extreme conditions.

For critical applications engineers use creep-rupture curves, oxidation tests and high-temperature fatigue crack growth studies.

Heat-resistant steels have a wide range of applications. Typical areas where the properties of H6S2, X10CrAlSi7 and X10CrAl7 bring measurable benefits are discussed below.

The role of heat-resistant steel in power plants and the chemical industry

• Burners and combustion chambers: components exposed to direct contact with high-temperature flue gases benefit from these alloys’ resistance to oxidation and to aggressive sulphur compounds and chlorides.
• Heat exchangers and boilers: tubes and casings operating at high temperatures require materials with a stable oxide layer; FeCrAl offers long service life in such applications.
• Reactors and chemical installations: in processes where oxygen and high-temperature oxidising atmospheres occur, materials that form a durable oxide coating are preferred. Applications include pipelines, furnace elements and reaction chambers.
• Petrochemical industry: components that must withstand prolonged exposure to hot gases and by-products benefit from alloys with Al and Si additions.

Use in the manufacture of engine components and turbo assemblies

• Turbocharger components and exhaust manifolds: heat-resistant alloys are used in parts exposed to rapid heating and cooling cycles, where resistance to thermal cracking and exhaust corrosion is crucial.
• Ancillaries for diesel engines and exhaust system parts: short- and long-term exposure to exhaust gases requires materials that maintain their protective oxide layer.
• Components intended for direct flame contact: nozzles, fuel inserts and thermal shields.

In practice the choice of a specific grade depends on the combination of requirements: maximum operating temperature, presence of aggressive species in exhausts and cyclicity of thermal loads.

Material selection is a compromise between cost, availability and properties. Below is a simple overview of the strengths and weaknesses of the discussed grades.

Effectiveness in different operating conditions

• H6S2:

– Strengths: favourable price-to-performance ratio at moderate temperatures; good machinability and weldability in versions with controlled composition.
– Weaknesses: lower resistance compared with FeCrAl in very high temperatures or in the presence of strongly aggressive gases.

• X10CrAlSi7:

– Strengths: the Cr–Al–Si composition yields a stable oxide layer, good oxidation resistance and better protection in the presence of sulphur compounds.
– Weaknesses: may require precise welding procedures and quality control during production of thin-walled components.

• X10CrAl7 / 1.4713:

– Strengths: strong protective effect due to Al2O3 layer; excellent durability in long-term operation at high temperatures; wide use in the energy industry.
– Weaknesses: material costs can be higher, weldability and machining require specialised procedures; in some mechanical conditions aluminium can contribute to embrittlement.

In practice an engineer selects material according to the specific duty cycle: static long-term operation at elevated temperature – X10CrAl7; dynamic service with cyclic heating – consider X10CrAlSi7 or specialised variants with stabilising additions.

Production costs and material availability

• H6S2 is often more economical and more readily available in regions where GOST standards or local specifications apply.
• Grades like X10CrAlSi7 and X10CrAl7 /1.4713 can be more expensive due to higher aluminium contents and more stringent manufacturing processes. Their availability depends on manufacturers specialising in high-temperature steels.
• Price and availability directly influence design decisions – where it is possible to use a cheaper material without loss of durability, economical solutions are preferred.

Total life-cycle cost often favours more expensive materials that provide longer service life, less frequent overhauls and lower failure risk.

Conformity with appropriate standards is key for safety and acceptance of components into industrial use. Standards define chemical composition, mechanical properties and test procedures.

Certificates and technical requirements according to ISO, DIN and EN standards

• European (EN) and German (DIN) standards provide standardised designations and specifications for heat-resistant steels. Designations such as X10CrAlSi7 and the DIN numbering (e.g. 1.4713) facilitate unambiguous identification. These documents define composition tolerances, required mechanical tests and delivery and inspection rules.
• International ISO standards define test methods and classifications of properties, enabling comparison of materials on the global market.
• For pressure and critical applications certificates compliant with ASME, PED and industry-specific specifications (e.g. power industry standards) are often required. Compliance checks include material certificates, mechanical and microstructural test reports and welding procedure documentation.

Compliance with standards guarantees repeatability of properties, safety and acceptance in tenders and industry.

Importance of material compliance with standards in industry

Conformity with standards affects:

• operational safety (elimination of unexpected failures),
• the possibility of using materials in critical installations,
• ease in warranty settlements and technical acceptance,
• confidence that material parameters are repeatable and comparable.

Therefore every material order for high-temperature applications should include references to applicable standards and required tests and certificates.

The effectiveness of heat-resistant steel in practice is assessed in laboratories and simulated conditions. Below are methods most commonly used in quality control.

Microstructural examination methods and mechanical property tests

• Optical microscopy and SEM (scanning electron microscopy): allow assessment of grain distribution, precipitates and microstructural defects.
• EDS (energy-dispersive X-ray spectroscopy): enables local determination of chemical composition and analysis of oxide layers.
• XRD (X-ray diffraction): identification of phases and oxides on the material surface.
• Hardness tests and tensile tests: determine mechanical properties at room and elevated temperatures.
• Creep tests: long-term tests under load at a set temperature providing information on material durability under static high-temperature conditions.

Strength tests and high-temperature resistance tests

• Oxidation tests: exposure in controlled atmospheres (oxygen, flue gas, presence of sulphur) to assess mass gain (scale) and the stability of the oxide layer.
• Thermal cycling tests: repeated heating and cooling to simulate real operating conditions and check resistance to thermal cracking.
• High-temperature fatigue tests: assessment of durability under cyclic loads at elevated temperatures.
• Corrosion tests under chemical conditions: exposure to aggressive media (acids, chloride salts, sulphur compounds) to evaluate resistance.

Only a comprehensive set of tests provides a complete picture of material suitability for a given application.

Operating conditions determine the real durability of the material; the composition table alone is insufficient to predict steel behaviour in actual installations.

Effects of atmospheric, chemical and thermal factors

• Oxidising atmosphere: promotes formation of an oxide layer; the stability of this layer depends on alloy composition – Al2O3 has the advantage in very high temperature conditions.
• Atmosphere containing sulphur or chlorine: can destabilise a Cr2O3 layer, leading to sulphidation corrosion and surface degradation. Alloys with Al show better resistance in these conditions.
• Thermal cycling: promotes spallation of oxide layers and generates stresses that lead to cracking; materials used under cyclic service should have good oxide adhesion and adequate thermal fatigue strength.
• Contact with solid particles: can cause abrasion and local damage to the protective coating; in such cases additional surface protection or thicker material is required.

Protection strategies and extending the service life of steel components

• Oxide and protective coatings: sprayed ceramic or metallic coatings can extend the life of parts exposed to abrasion or aggressive environments.
• Control of operating parameters: limiting peak temperatures, managing thermal shocks and preventing rapid load changes.
• Monitoring technical condition: regular inspections, wall-thickness measurements (ultrasonic), non-destructive testing (NDT) and analysis of spalled oxide fragments.
• Appropriate welds and fastenings: use of appropriate welding procedures and consumables to avoid local defects and embrittlement.

A well-designed maintenance strategy significantly extends the service life of components made from these alloys.

Choosing the right grade and its correct operation determines the efficiency and durability of installations.

Criteria for selecting material relative to application and working conditions

• Maximum operating temperature: if long-term operation at very high temperatures in an oxidising atmosphere is required – FeCrAl alloys (e.g. X10CrAl7) are preferred.
• Cyclic loading: in applications with frequent heating/cooling cycles consider materials with better oxide adhesion and lower tendency to crack – X10CrAlSi7 may be a good compromise.
• Aggressiveness of the atmosphere: presence of sulphur, chlorine or combustion products from fuels with high impurity content affects steel selection and the need for protective coatings.
• Machining and welding capability: for components requiring intensive machining, grades with more predictable weldability (with proven procedures and consumables) are preferable.
• Cost and availability: balancing material and operating costs; a more expensive grade may pay off by extending intervals between overhauls.

Maintenance and monitoring rules for steels in equipment

• Regular visual inspections: checking surfaces for spalling of the oxide layer, cracks and local corrosion.
• Non-destructive testing: ultrasonic wall-thickness measurements, penetrant and magnetic tests where applicable.
• Maintenance and overhaul schedules: planning replacements and repairs based on predicted oxide mass gain and creep test results.
• Recording operating parameters: documentation of temperatures, operating times and cycle counts, which helps in failure analysis and optimisation.

Applying these principles translates into fewer shutdowns and lower operating costs.

Materials technology does not stand still; the development of heat-resistant alloys and their manufacturing techniques opens up new application possibilities and property improvements.

Modern technologies and composition improvements

• Micro-alloying and microstructure modification: addition of rare earth elements, niobium or zirconium improves oxide layer adhesion and resistance to cracking; fine precipitates strengthen creep resistance.
• Design of multi-component alloys: optimisation of Cr–Al–Si ratios and introduction of new additions to achieve simultaneous resistance to oxidation, sulphidation and mechanical durability.
• Functional coatings and nanostructures: ceramic nanocomposites, graded-composition coatings and repair coatings improve surface resistance and component life.

Potential applications in future industry sectors

• High-efficiency power generation: heat-resistant alloys enable operation at higher temperatures, which translates to better efficiency of combustion and energy conversion processes.
• Clean combustion technologies and alternative fuels: materials resistant to aggressive combustion products and high temperatures will be key in adapting installations to new fuels.
• Additive manufacturing (3D printing): using metal additive manufacturing to produce complex components from high-temperature alloys will allow optimisation of geometry and mass reduction while retaining heat-resistant properties.
• Aerospace and space industries: lightweight, heat-resistant structures for propulsion components and thermal protection will benefit from material improvements.

Advances in alloy synthesis, processing and coatings open new horizons of application while reducing operating costs and environmental impact.

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