1.4718

History and development of the grade

The roots of heat-resistant steels date back to the 19th and early 20th centuries, when the power and metallurgical industries required materials capable of withstanding hot flue gases and the oxidising atmospheres of furnaces. The development of alloy steels—primarily through the addition of chromium—enabled the formation of protective oxides that slow further oxidation of the metal. In the second half of the 20th century ferritic and then martensitic grades were refined to increase strength and hardness of components exposed to wear and high temperatures.

AISI 422 evolved as a grade that combines high temperature resistance with the mechanical properties typical of tool and spring steels. The addition of silicon (Si) improves the adhesion and structure of the protective oxide scale; chromium (Cr) is responsible for passivation; an appropriate carbon (C) level enables the formation of a hard, martensitic microstructure after quenching. The names and symbols (AISI, X…, 1.47xx) originate from different classification systems used in the USA and Europe, reflecting the international nature of the steel’s applications.

Industrial and everyday applications of the steel

AISI 422 has found wide application where high hardness, resistance to high temperature exposure and moderate corrosion resistance are simultaneously required. Practical examples:

• furnace components, flue tubes and shields in industrial boilers and furnaces;
• components of exhaust systems and turbochargers in the automotive sector;
• machine parts exposed to wear at elevated temperature (e.g. guides, cams);
• tooling elements and cutting fixtures operating at raised temperatures;
• industrial furnace components, grates, meshes and heating elements.

In everyday industrial practice AISI 422 serves as a “workhorse” where a compromise combination of hardness and oxidation resistance is required.

Understanding the chemical composition is the starting point for predicting material behaviour under specific service conditions. Note that exact values may vary slightly between manufacturers and standards; below are typical ranges of constituents and their roles.

Major chemical constituents and their role

• Carbon (C): typically around 0.40–0.50%. Carbon increases hardenability and hardness after heat treatment by enabling martensite formation. Higher C improves wear resistance but reduces ductility and weldability.
• Chromium (Cr): typically about 8.5–10.5%. Chromium is responsible for forming protective chromium oxides that enhance oxidation and corrosion resistance at elevated temperatures.
• Silicon (Si): a significant addition, usually in the 1–3% range (hence the “Si” symbol in the name). Si improves formation and adhesion of the oxide scale, increases resistance in oxidising atmospheres and acts as a deoxidiser during melting.
• Manganese (Mn): typically below 1% — serves as an austenite stabiliser during melting and improves machinability.
• Sulfur (S) and phosphorus (P): trace amounts (S, P < 0.03–0.04%) — excessive contents degrade ductility and strength at elevated temperatures.
• Molybdenum (Mo), nickel (Ni): in most AISI 422 variants their contents are minimal or zero; they are not characteristic elements of this grade.

Note: the additional symbols in the title (H9S2) and the European name X45CrSi9-3 reflect the significant proportions of these elements—mainly carbon, chromium and silicon—though specific numbers may be interpreted differently across nomenclature systems.

Comparison with other heat-resistant steel grades

• Compared with austenitic heat-resistant steels (e.g. AISI 310, AISI 309), AISI 422 has a higher carbon content and less nickel, which results in a harder, martensitic microstructure after heat treatment but lower corrosion resistance in aggressive environments (e.g. chlorides).
• Compared with high-Cr ferritic steels (e.g. 1.4512), AISI 422 has higher hardenability and better wear resistance due to carbon content, but may be more susceptible to embrittlement if heat treatment is improper.
• Compared with other martensitic steels (e.g. AISI 420), AISI 422 is characterised by a larger silicon addition and often better oxidation resistance, making it more suitable for higher temperature applications.

The mechanical behaviour of this steel depends strongly on heat treatment, degree of hardening and tempering. Below are typical ranges and phenomena of practical importance.

Tensile strength and hardness

• Tensile strength (Rm): after appropriate quenching and tempering typical values can range widely, usually from 600 to 1100 MPa depending on heat treatment condition.
• Yield strength (Rp0.2): typically 400–900 MPa.
• Hardness: after quenching and appropriate tempering hardnesses of roughly 40–55 HRC can be achieved, depending on carbon content and tempering parameters. In the as-delivered condition (e.g. softened) hardness will be significantly lower.

In practice this means AISI 422 can be used where a compromise between strength and resistance to high temperature is required, for example in loaded parts exposed to wear.

Resistance to elevated temperatures

• The steel shows good oxidation resistance in oxidising atmospheres up to about 600–800°C. Silicon improves the structure and adhesion of the oxide scale, slowing further oxidation.
• Prolonged exposure above 600°C leads to gradual degradation of mechanical properties due to carbon diffusion, carbide precipitation and recrystallisation phenomena. For critical applications continuous service temperatures are typically designed below 600°C.
• Under short-term or cyclic conditions good performance can be obtained even at higher temperatures, provided the temperature change dynamics and the atmosphere are controlled.

Corrosion and oxidation resistance

• Oxidation resistance: thanks to chromium and silicon content the steel forms a protective oxide layer that reduces the rate of further oxidation. Alloying elements and scale thickness and adhesion also play a strong role.
• Corrosion resistance in aqueous and chloride-containing environments is moderate — inferior to austenitic steels with high Cr and Ni contents. In applications exposed to aggressive media alternatives or additional surface protections should be considered.
• In reducing atmospheres (sulphur compounds, CO, H2) the steel may undergo faster chemical corrosion processes, including sulfidation and surface carburisation.

Microstructure is key to understanding material behaviour in service.

Types of microstructural phases

• Martensite: after austenitisation and rapid cooling (quenching) the main phase responsible for high hardness. It is a supersaturated solution of carbon in iron with characteristic sharp needle-like morphology.
• Carbides: mainly M23C6 and other chromium carbides that precipitate at grain boundaries and in the matrix; their amount and distribution depend on C content and the thermal history. Carbides strengthen the material but excessive precipitation can lead to brittle behaviour.
• Oxide scale: a surface layer of Cr2O3 and silicates, particularly important in protecting against further oxidation.
• Possible embrittling phases: with prolonged exposure at certain temperature ranges and improper heat treatment intermetallic phases or excessive carbide precipitation can occur, reducing impact toughness.

Influence of microstructure on material performance

• A dense, fine distributed carbide morphology and a well-formed martensite provide high wear resistance and increased strength.
• Irregular carbide precipitates at grain boundaries promote crack initiation, especially under fatigue conditions or at elevated temperatures.
• Controlling microstructure through proper heat treatment is decisive to maintain a favourable compromise between hardness and toughness.

Production and subsequent processing determine the final component quality.

Melting and rolling methods

• Melting: most often in electric arc furnaces (EAF) with further refining in a vacuum induction furnace (VIM) or ingot casting with cleaning processes. Precise dosing of alloying elements and degassing are important due to high sensitivity of properties to small composition changes.
• Hot rolling: produces a recrystallised structure and appropriate grain size; rolling temperature and cooling method after rolling determine carbide size and distribution.
• Cold working: used for parts requiring tighter assembly tolerances or improved surface properties; can increase strength through work hardening.

Heat treatment – annealing, quenching and ageing

• AUSTENITISATION: typical austenitisation temperatures for martensitic steels with C ~0.4–0.5% lie in the 900–1050°C range. The process aims to dissolve carbides and obtain a homogeneous austenite phase.
• QUENCHING: rapid cooling (oil, air, depending on required hardenability) causes the transformation of austenite to martensite; this allows attainment of high hardness.
• TEMPERING: tempering in the range 150–600°C enables achieving the desired compromise of hardness, toughness and resistance to cracking; higher tempering temperatures reduce hardness but improve ductility and resistance to brittle fracture.
• AGEING: long-term service at elevated temperatures causes carbide precipitation and property changes — this should be considered when selecting heat treatment parameters.

A well-designed heat cycle is key to avoid excessive carbide precipitation and property degradation.

Machining and welding capabilities

• Machining: AISI 422 is machined relatively well, but its hardenability requires appropriate selection of tools and cutting parameters. Cemented carbide tools with coatings, cooling and reduced feed rates are used to maintain surface quality.
• Welding: due to the high carbon content weldability is limited. Recommendations:

– use low-carbon filler materials or special wires for heat-resistant steels;
– preheating to reduce the risk of cold cracking;
– control of overheating and post-weld heat treatment (PWHT) — tempering after welding — to reduce stresses and the risk of embrittlement.

• Proper welding techniques and careful edge preparation are essential for durable joints.

In practice the steel is assessed not by a single parameter but by performance in specific conditions — in oxidising, reducing atmospheres and under cyclic heating.

Behaviour in oxidising and reducing atmospheres

• Oxidising atmospheres (presence of oxygen, combustion gases, carbon dioxide): chromium and silicon additions promote formation of a stable, adherent oxide layer. In such conditions AISI 422 exhibits good oxidation resistance, particularly in the temperature range up to approximately 600–700°C.
• Reducing atmospheres (presence of CO, H2, sulphur-containing gases): resistance decreases. Processes such as sulfidation and surface carburisation can destabilise the oxide layer and accelerate degradation.
• The content of contaminants in flue gases (e.g. sulphur compounds, chlorides) significantly affects component longevity — in the presence of sulphur local scale detachment and rapid degradation often occur.

Reactions to cyclic temperature changes

• Cyclic heating and cooling cause expansion and contraction of scale and substrate. If oxide adhesion is poor, cracking and spallation of the scale occur, exposing fresh substrate and accelerating corrosion.
• AISI 422 has relatively good resistance to cyclic oxidation due to Si content, which improves oxide adhesion. Nevertheless the number of cycles and temperature amplitude determine longevity — intensive thermal cycling with large temperature differences requires additional protections or alloys with higher spallation resistance.
• In applications with rapid heating to very high temperatures materials with better cyclic-change resistance are recommended, while AISI 422 is selected where the compromise between cost and resistance is acceptable.

This steel is used in many branches of industry where durability at elevated temperatures combined with hardness is required.

Power and boiler industry

• Furnace components, flue pipes, grates and boiler shields: AISI 422 is used for parts exposed directly to fire and flue gases. Its oxidation resistance and the ability to achieve high hardness make it suitable for components that also undergo abrasive wear.
• Components of converters and heat exchangers operating at moderately high temperatures.

Automotive and machine parts production

• Exhaust system and turbocharger components: where a hot medium is present and resistant properties are required while maintaining mechanical performance.
• Machine parts exposed to wear at high temperatures, such as guides or bushings in industrial systems.

Furnace components and heat-resistant equipment parts

• Burners, combustion chambers, fastening and structural elements of industrial furnaces.
• Shields and heating elements and structures in tunnel and flame furnaces.

A practical example: a grate element in a cement furnace must simultaneously support load and resist abrasion, and not wear rapidly in contact with hot, oxidising gases — here AISI 422 is often an optimal choice.

Understanding the symbolism and related standards facilitates material selection and certification.

International and Polish standards

• AISI/SAE: AISI 422 is the designation widely used in Anglo-Saxon countries, corresponding to a family of martensitic heat-resistant steels.
• EN: number 1.4718 is the designation according to the European (EN) system, used in material documentation and technical orders.
• The name X45CrSi9-3 refers to the EN naming convention where elements of the designation indicate carbon content and main alloying elements (Cr, Si).
• Additional standards: EN 10088 (stainless steels), documents regulating welding procedures, non-destructive testing and material certifications.

In technical documentation and orders it is advisable to specify both the EN number and the AISI equivalent to avoid misunderstandings.

Significance of material symbols and designations

• AISI 422: AISI/SAE system — quick recognition of the group (422 = martensitic heat-resistant steel).
• X45CrSi9-3: a format popular in Europe where “X” denotes an alloy steel, “45” — carbon content ~0.45%, “Cr9” — about 9% chromium, “Si9-3” — presence of silicon (symbolically indicating level, interpreted in the given standard).
• 1.4718: the steel number according to EN/ISO system; it is the most precise identifier in technical documentation.

Accurate reading of markings requires reference to the specific standard, but the combination of several designations allows clear specification of expected composition and properties.

Material selection is an art of compromises — below is a comparison with several common grades.

AISI 422 vs AISI 310 and AISI 309

• AISI 310 / 309 — austenitic steels with high chromium and nickel contents, intended for work at very high temperatures (up to 1000°C and above in some applications). They exhibit excellent oxidation and corrosion resistance.
• AISI 422 — a martensitic steel with higher hardenability and wear resistance, but inferior corrosion resistance and shorter life at extreme long-term temperatures.
• Advantages of AISI 422: better mechanical strength, hardness and wear resistance; lower cost than high-alloy Ni-rich alloys.
• Limitations: reduced suitability for very aggressive chemical environments and prolonged extreme temperatures — in such cases AISI 310/309 are preferred.

Adapting to specific service conditions

• When the criterion is oxidation resistance at very high temperatures — high-alloy austenitic steels are often chosen.
• When high hardness and wear resistance at moderately elevated temperatures are required — AISI 422 is often the more economical and effective choice.
• In design practice the decision is based on analysing: operating temperature, atmosphere character (oxidising vs reducing), mechanical loads, temperature cycling, budget and manufacturability.

Component longevity depends on correct operation, inspection and repair.

Recommended operating conditions

• Continuous operation in oxidising atmospheres: recommended temperatures below ~600°C for long-term service; in short-term applications higher temperatures are possible if cyclic behaviour is controlled.
• Avoid environments rich in chlorides and sulphides without additional protection — such conditions accelerate degradation.
• Limit rapid temperature cycles or use coatings and shields to protect against direct flame action and aggressive flue gas constituents.

Methods to control material condition and prevent damage

• Regular surface inspections — check for scale cracking, spallation and local corrosion.
• Non-destructive testing (NDT): dye penetrant, ultrasonic and magnetic tests for crack detection; spectroscopy and analysis of oxide layers to assess oxidation.
• Inspection of welded joints — monitor stresses and possibly temper after welding.
• Restoration of protective oxide scale through appropriate tempering procedures and application of surface coatings (e.g. ceramic coatings, chrome plating, thermal spraying) in extreme conditions.

Preventing failures involves combining regular diagnostics with correct selection of operating parameters and appropriate maintenance.

Engineering materials are an area of continuous research and improvement. AISI 422 also finds a place in development directions.

Modern production technologies

• Improvement of melting processes: use of vacuum and electrochemical cleaning to minimise impurities and control carbide distribution.
• Additive manufacturing technologies (metal printing): initial work on powder printing of heat-resistant steels allows creating complex shapes and repairing worn components. Challenges remain in controlling microstructure and eliminating porosity.
• Coatings and surface regeneration: thermal spraying, laser cladding and other repair techniques extend the service life of components operating in extreme conditions.

Research on improving strength and resistance

• Alloy modifications: research into small additions of titanium, vanadium or rare-earth elements to control carbide size and stability and improve oxide adherence.
• Optimisation of heat treatment: developing quenching and tempering cycles optimised to minimise unwanted precipitates and maximise toughness at required hardness.
• Computer modelling and simulation: use of CALPHAD methods and FEM simulations to predict phase formation and material behaviour in service, allowing faster testing of composition and process combinations.

These innovations aim to extend the application range of AISI 422 where users today face compromises between cost and durability.

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AISI 422 H9S2 X45CrSi9-3 1.4718 is a material with a well-defined profile: a martensitic steel with significant carbon content and additions of chromium and silicon, offering a favourable combination of hardness, oxidation resistance and capability for elevated temperature service. Its use, however, requires deliberate selection of processing and service parameters to avoid typical pitfalls related to brittleness, weldability and long-term exposure to aggressive atmospheres. In engineering practice AISI 422 remains a widely used material and a subject of dynamic research and manufacturing improvements.