H25T/1.4746

H25T, also designated X8CrTi25 or material number 1.4746, is a type of heat‑resistant steel characterised by a very high chromium content and the addition of titanium as a stabilising element. It is intended for service at elevated temperatures where resistance to oxidation, high‑temperature corrosion and structural stability under prolonged heat exposure are important. In practice H25T combines features of high‑chromium ferritic steels with the ability to inhibit harmful intergranular processes by stabilising carbides with titanium.

Definition and classification of heat‑resistant steels

Heat‑resistant steels are a group of iron alloys with added chromium and other alloying elements, designed to retain useful mechanical properties and resistance to oxidation at high temperatures. Depending on composition and microstructure, we distinguish ferritic, austenitic and other phase alloys. H25T belongs to the group of ferritic or ferritic‑martensitic steels with a high chromium content, with a special addition of titanium that stabilises the structure and improves resistance to carbon migration and strength at elevated temperatures.

Origin of the designations: H25T, X8CrTi25 and number 1.4746

The designations of this grade arise from different classification systems:

• H25T is the trade/technical name, in which “H” suggests high‑temperature application (heat‑resistant), “25” refers to the approximate chromium content, and “T” indicates the presence of titanium as a stabiliser.
• X8CrTi25 is the European convention designation, where “X” denotes an alloy steel, “8” indicates the approximate carbon limit (about 0.08% C), “CrTi” informs about the main alloying elements – chromium and titanium, and “25” denotes the approximate chromium percentage.
• The number 1.4746 is the material number according to the German DIN/EN system, used in technical documentation and orders, facilitating identification of the grade in material catalogues and standards.

This multiple nomenclature allows unambiguous recognition of the steel across different regulatory and commercial systems, which is practically important for purchasing, design and service documentation.

Accurate knowledge of chemical composition is the foundation for understanding the steel’s properties. H25T is designed for maximum high‑temperature resistance, hence the high chromium and stabilising additions.

Main alloying elements and their percentage amounts

The chemical composition of H25T may vary slightly depending on the manufacturer and standard, but a typical distribution of elements is as follows (indicative values and typical ranges used in specifications):

• Chromium (Cr): approximately 24–26% – key for oxidation resistance and formation of a protective oxide layer.
• Carbon (C): typically ≤ 0.08% – low carbon content prevents excessive formation of carbides.
• Titanium (Ti): about 0.4–1.0% – a carbide stabiliser that prevents precipitation of chromium carbides at grain boundaries.
• Silicon (Si): usually ≤ 1.0% – improves oxidation resistance and affects high‑temperature strength.
• Manganese (Mn): usually ≤ 1.0% – controlled in small amounts.
• Nickel (Ni): trace amounts or very low concentration – H25T is low in nickel.
• Phosphorus (P) and sulphur (S): very low trace amounts, in line with quality requirements.

The remainder is iron (Fe) as the base of the alloy. This combination of elements creates a material that exhibits stable mechanical properties at operating temperatures and reduced tendency to embrittlement associated with chromium carbide precipitation.

The role of titanium (Ti) in the alloy

Titanium in this steel acts as a carbide stabiliser. At high temperatures chromium carbides (Cr23C6) can precipitate at grain boundaries, leading to chromium‑depleted zones adjacent to grain boundaries, which in turn increases susceptibility to intergranular corrosion and reduces high‑temperature resistance. The addition of Ti binds carbon and nitrogen as stable titanium carbides/nitrides (TiC, TiN), thereby protecting chromium from carbide formation, maintaining phase homogeneity and anti‑corrosion properties. This effect is particularly important during welding and heating, where local changes in temperature and composition favour carbide precipitation.

Comparison of composition with other heat‑resistant steel grades

• Compared with austenitic steels (e.g. 310 – X10CrNi25‑21), H25T has lower or no nickel content while maintaining a similarly high chromium level. The consequence is lower cost and different corrosion resistance in specific environments.
• Compared with other ferritics with lower chromium (e.g. 1.4016 or 1.4017), H25T offers significantly better oxidation protection due to its 25% Cr.
• Compared with heat‑resistant steels stabilised with zirconium (Zr), Ti has different precipitation behaviour, which affects the microstructure and conditions material selection depending on service conditions.

Such comparison shows that H25T occupies a position between economical ferritic steels and more expensive austenitic heat‑resistant steels, offering a favourable compromise between high‑temperature resistance and cost.

The properties of H25T result directly from its composition and microstructure. They are important when selecting material for a specific application.

Resistance to high temperatures and thermal shocks

H25T shows good oxidation resistance at service temperatures typical for boilers and industrial furnaces. Thanks to the high chromium content a stable, protective oxide layer forms, which limits further oxidation and mass loss. Practical applications indicate that this steel performs well at temperatures on the order of several hundred degrees Celsius, and it can be used at higher temperatures for short periods; however, long‑term thermal loading should be determined according to manufacturer specifications and standards.

Resistance to thermal shock depends on the ferritic microstructure and the titanium addition. H25T has better structural stability under rapid temperature changes than some austenitic steels, but components should still be designed to limit local thermal stresses and temperature differentials.

Strength against corrosion and oxidation

The high chromium content provides significant resistance to oxidation and to corrosion in dry and moderately aggressive atmospheres. Titanium as a stabiliser reduces the tendency to intergranular corrosion after welding and prolonged exposure to temperature. In environments containing sulphur, hydrogen chloride or halogen compounds, the resistance characteristics may be worsened – in such conditions tailored tests are required for the specific application.

In practice H25T cannot compete with the highest‑class alloys designed for the very highest temperatures and extremely aggressive environments (e.g. in the presence of molten salts or strong reducers), but in most industrial applications it offers favourable long‑term resistance.

Hardness and ductility characteristics

H25T has a ferritic structure, which translates into:

• moderate hardness and good ductility within the range of service temperatures,
• relatively low creep strength compared with the highest nickel alloys or some austenitic heat‑resistant steels,
• good plasticity above room temperature, which facilitates machining and hot forming.

Mechanical properties such as yield strength, tensile strength and impact toughness should always be verified according to standards and supplier specifications, as values depend on heat treatment and material condition.

Production and processing of H25T must take into account the role of titanium and the high chromium content to achieve the desired characteristics.

Production methods and quality control

Basic production stages include:

• melting and refining (EAF – electric arc furnace, often with further AOD or VOD treatment to achieve the required low levels of impurities),
• casting and hot rolling to produce plates, sheets or bars,
• drawing, bending and final machining.

Quality control includes chemical analyses (spectrometry), mechanical testing (tensile, hardness), microstructural examinations (metallography) and tests for high‑temperature resistance and oxidation. In practice certificates of conformity with standards and material documentation confirming composition and mechanical parameters are required.

Heat treatment and mechanical processing options

Heat treatment of H25T focuses on processes that ensure compositional homogenisation and dissolution of undesirable precipitates:

• homogenising and stabilising annealing: intended to dissolve carbides and bind carbon with titanium, restoring corrosion resistance and ductility,
• hardening and tempering typical for carbon steels are relatively rarely applied, because ferritic heat‑resistant alloys have a different phase behaviour.

Mechanical processing (turning, milling, bending) is possible with appropriately selected cutting parameters. Thanks to good plasticity H25T is suitable for manufacture of complex components, but cutting tools and coolants should be chosen with regard to the material’s properties.

Recommendations for welding and soldering

Welding H25T is generally feasible but requires adherence to several rules:

• use of welding wires or electrodes with composition close to the base material or with added stabilising titanium to avoid chromium carbide precipitation,
• control of temperature in the heat‑affected zone, as excessive heating may induce unfavourable transformations and worsen corrosion resistance,
• post‑weld full tempering is often not necessary if the material was stabilised before welding and an appropriate filler material was used; however, testing and weld assessment are recommended for critical applications,
• soft soldering and brazing are possible, but selection of solders and processes must consider differences in thermal expansion and temperature durability of the joint.

In practice designers and welders often consult the material supplier to select welding procedures and filler materials with suitable composition and properties.

H25T finds use where requirements for high‑temperature and corrosion resistance are combined with acceptable material costs.

Power industry – boilers and industrial furnaces

One of the main application areas for H25T is the power industry:

• components of steam boilers and combustion chamber parts where oxidation resistance and high‑temperature performance are required,
• parts of industrial furnace constructions, tubes and shields exposed to hot flue gases,
• heat exchangers operating in dry, oxidising atmospheres.

Thanks to stability at high temperatures and oxidation resistance H25T extends component life and reduces maintenance frequency.

Automotive and aerospace – engine and turbine components

In automotive and some aerospace applications H25T can be used for parts operating in hot sections of exhaust systems, turbines or exhaust conduits where use of austenitic steels would be costly or unnecessary. In aerospace its use is limited to applications where extreme thermal stresses are not present and where oxidation resistance is required but extreme creep resistance is not.

Chemical and petrochemical industry – pipelines and vessels

Under elevated temperature and moderately aggressive atmospheres H25T is used in:

• pipelines conveying hot gases,
• shields and process structures,
• vessels and ducts exposed to temperature in combination with dry, oxidising atmospheres.

In environments containing aggressive chemicals such as acids or halides a full resistance analysis and possible use of alternative alloys or protective coatings are necessary.

Choosing between H25T and other grades depends on service conditions, cost and durability requirements.

Differences in composition and properties

• Compared with high‑nickel austenitic steels, H25T is cheaper and does not require nickel, but may offer lower corrosion resistance in reducing or sulphur‑containing environments.
• Compared with ferritics with lower chromium, H25T provides much better oxidation protection thanks to 25% Cr, allowing operation at higher temperatures.
• Compared with heat‑resistant steels alloyed with molybdenum or aluminium, which are designed for specific aggressive conditions, H25T may not match chemical corrosion resistance, but compensates with weldability stability and resistance to intergranular corrosion due to titanium stabilisation.

Advantages and limitations in different service conditions

Advantages:

• high oxidation resistance owing to high chromium content,
• improved stability against intergranular corrosion due to titanium,
• favourable property‑to‑cost ratio compared with nickel alloys.

Limitations:

• limited creep resistance at very high temperatures compared with the best austenitic and creep‑resistant alloys,
• susceptibility to attack in environments containing halides or flue gases with high sulphur content, depending on process conditions,
• requirement for appropriate filler materials and welding procedures to preserve post‑processing properties.

Selection of H25T should be preceded by analysis of operating conditions, anticipated temperature cycles and the type of contacting medium.

Compliance with applicable standards and quality documentation is crucial when designing and procuring industrial materials.

International and national quality standards

For the grade designated 1.4746 references are most commonly made to European (EN) and German (DIN) standards. Key normative areas include:

• standards on chemical composition and classification of stainless and heat‑resistant steels (e.g. EN 10088 for stainless steels – various parts regulate grades, tolerances and tests),
• standards on mechanical testing, weld testing and welding procedures,
• standards on oxidation resistance tests and corrosion tests under high‑temperature conditions.

In practice supplier documentation should include a certificate of conformity with the relevant EN/DIN standard and test results confirming composition and properties.

Requirements for material documentation

For critical components the following documents should be required:

• material certificate 3.1 or 3.2 (according to EN 10204) confirming chemical composition and mechanical test results,
• reports of non‑destructive tests of welds and material (UT, RT, PT, MT) where required,
• manufacturer’s technical data sheet with guidance on heat treatment, welding and use,
• certificates concerning quality control and, if necessary, compliance with additional industry‑specific standards (e.g. power, aerospace).

Complete and transparent documentation simplifies component qualification and ensures compliance with purchaser requirements.

Effective diagnostics and control enable detection of production and process defects and monitoring of property changes during service.

Non‑destructive and destructive tests

Applied methods include:

• Visual inspection (VT) and dimensional and surface checks.
• Non‑destructive testing (NDT): ultrasonic (UT), radiographic (RT), magnetic particle (MT) and penetrant (PT). These are key for weld quality assessment and detection of cracks or inclusions.
• Destructive tests: tensile tests, hardness, impact (Charpy), metallographic examinations and microstructure analysis to detect precipitates, elemental segregation or other irregularities.
• High‑temperature resistance tests, including oxidation tests in controlled atmospheres and creep tests for components operating long‑term at high temperatures.

Methods for monitoring property changes during service

In service it is important to detect early signs of degradation:

• periodic non‑destructive inspections, especially at locations exposed to the greatest thermal and chemical stresses,
• analyses of material samples taken during overhauls (metallography, composition analysis in oxide layers),
• monitoring thickness loss due to oxidation and erosion,
• assessment of changes in mechanical properties on selected samples to detect loss of load‑bearing capacity.

Systematic monitoring and planned preventive actions extend component life and reduce the risk of failure.

The durability of heat‑resistant steel depends not only on its composition but also on the service environment – the nature of the medium, temperature, presence of contaminants and thermal cycling are crucial.

Resistance to aggressive atmospheres and chemical factors

H25T performs best in dry, oxidising atmospheres where its oxide layer forms an effective protective barrier. In environments containing sulphur, chlorides or halogen compounds, corrosion can accelerate degradation. Combinations of high temperature and presence of sulphur or salts are particularly hazardous, potentially leading to accelerated oxidation, sulfidation or enhanced localised corrosion.

In applications involving contact with liquid chemicals or aggressive gases, consideration should be given to additional protective coatings, alternative materials or appropriate operating procedures to reduce the risk of premature failure.

Response to temperature changes and thermal stresses

Cyclic heating and cooling introduce thermal stresses that can lead to thermal fatigue and crack formation. H25T has favourable ferritic properties in terms of structural stability, but component design should minimise stress concentration zones and account for thermal expansion. At locations with large temperature differences it is advisable to use design solutions that reduce stresses and prevent local overheating.

For components exposed to rapid temperature changes regular inspections, use of thermal shields and proper start‑up and shut‑down procedures are recommended to avoid sudden thermal shocks.

Heat‑resistant materials continuously evolve under industrial demands and technological progress. H25T serves as a starting point for further modifications and optimisation.

Innovations in composition and production

Development trends include:

• optimisation of composition using trace elements and microalloying to improve creep strength and corrosion resistance without significant cost increases,
• improvement of alloying and refining technologies (e.g. better contaminant control, precise dosing of Ti), leading to higher homogeneity and reproducibility of properties,
• application of advanced metallurgical methods and modelling to predict steel behaviour under given service conditions and adapt composition to specific needs.

At the same time the area of coatings and surface modifications is developing, which combined with H25T can significantly extend the life of components operating in extreme conditions.

Potential new applications in advanced technologies

Given H25T’s properties, possible application directions include:

• components of thermal energy concentration systems and technologies related to renewable energy sources, where temperature resistance and material cost are important,
• parts in advanced engines and turbines with moderate thermal loading, especially where weight and cost reduction compared with nickel alloys is required,
• future automotive industry applications, for example in exhaust systems and turbochargers operating at higher temperatures while keeping material costs reasonable.

Progress in metal additive manufacturing may also open new possibilities for shaping H25T components, enabling production of complex geometries and optimisation of cooling or heat‑flow paths.

H25T X8CrTi25 1.4746 is a heat‑resistant steel characterised by high chromium content and a titanium stabilising addition, which results in good oxidation resistance, reduced susceptibility to intergranular corrosion and reasonable mechanical properties across typical industrial service temperatures. Users value it for its favourable property‑to‑cost ratio, ease of processing and applicability across a wide spectrum of applications – from boilers and furnaces, through industrial pipelines, to exhaust system components. Selection of H25T, however, requires a deliberate approach: analysis of service conditions, selection of welding procedures and quality control, and consideration of limitations related to creep and exposure to aggressive media. Further development of the material and production technology creates prospects for expanding applications, particularly in industries seeking a compromise between performance and cost‑effectiveness.