420

Basic characteristics of martensitic steels

Martensitic steels are a group of stainless steels that, in the hardened condition, are characterised by a martensitic microstructure — a hard, needle‑like form of iron formed by rapid cooling from the austenitic temperature range. Unlike austenitic steels, martensitic grades contain a lower chromium content (typically 11–14%) and a relatively higher carbon proportion, which enables achieving high hardness after quenching and tempering cycles. Their features include:

• the ability to achieve high (tool) hardness,
• moderate corrosion resistance (better than carbon steels, worse than austenitic grades),
• good machinability and ease of heat treatment,
• relatively low impact toughness in the very hard condition — requiring a compromise between hardness and ductility.

Differences between 1.4021, X20Cr13, 2H13 and the AISI 420 standard

The nomenclature refers to the same material segment, but different standards use different designations: 1.4021 (EN/DIN number), X20Cr13 (German symbol describing C ~0.20% and Cr ~13%), 2H13 (2Х13 — Russian/Soviet system designation) and AISI 420 (American designation for the popular 420 grade / UNS S42000). In practice these names describe steel of similar chemical composition and properties, although tolerances and exact elemental percentage limits may vary depending on the standard and manufacturer specification. Differences mainly concern:

• ranges of carbon content (affecting maximum achievable hardness),
• allowable impurities and trace elements,
• requirements related to heat treatment and material documentation.

Classification criteria for stainless steels in the context of these grades

Classification of stainless steels is based on equilibrium microstructure and commonly divides materials into: ferritic, austenitic, martensitic, duplex and precipitation‑hardened (PH). Main criteria are:

• chromium content and other alloying elements,
• carbon ratio (influencing hardenability),
• microstructure after heat treatment (martensite vs ferrite vs austenite),
• technological purpose (e.g. tools, structural parts, implants).

1.4021/X20Cr13/2H13/AISI 420 are classified as martensitic — suitable where a combination of hardness and moderate corrosion resistance is needed.

Exact chemical composition determines machining possibilities and final service properties. Typical ranges of elements encountered in practice for the discussed grades are given below.

• Carbon (C): approximately 0.17–0.25% (in AISI 420 variants larger ranges are permitted, e.g. 0.15–0.40%). Carbon determines the ability to harden and the maximum hardness.
• Chromium (Cr): approximately 12–14%. Chromium provides corrosion resistance by forming a passive layer and increases hardness through formation of chromium carbides.
• Manganese (Mn): typically ≤1.0%. Improves machinability and affects strength.
• Silicon (Si): typically ≤1.0%. Added as a deoxidiser and strengthener.
• Phosphorus (P) and sulfur (S): typically ≤0.04% and ≤0.03% — impurities whose excess worsens ductility and resistance to cracking.
• Other elements (e.g. Ni, Mo, N) appear occasionally in modified variants, but the standard grade has a simple, predictable chemistry.

Carbon content and its influence on mechanical properties

Carbon is key. Higher carbon increases hardenability and the maximum hardness achievable after quenching — this translates into better ability to hold an edge or wear resistance. However, as carbon content rises, ductility and impact toughness decrease. In design practice this necessitates a compromise: very high hardness (e.g. for cutting tools) versus maintaining minimum material toughness.

Chromium as the principal alloying element — corrosion resistance and hardness

Chromium is important in two ways: it forms the passive layer responsible for corrosion resistance and it forms carbides (e.g. Cr7C3) which increase hardness and wear resistance. In grades with about 13% chromium acceptable corrosion resistance is achieved in atmospheric environments and in low‑aggression aqueous environments. However, in chloride‑rich environments or strongly oxidising media these steels yield to austenitic grades.

Other alloying elements — manganese, silicon, sulfur and their roles

• Manganese (Mn) increases strength and facilitates steel deoxidation during melting, while also influencing hardenability.
• Silicon (Si) acts as a deoxidiser and affects thermal stability of the alloy.
• Sulfur (S) and phosphorus (P) are usually limited, because while they improve machinability they worsen ductility and resistance to cracking.
• In modified technical grades microalloying (e.g. vanadium, niobium) is used to increase carbide stability and improve wear resistance, but such measures require precise process control.

Final product quality is determined not only by chemical composition but also by melting practice, plastic working and heat treatment.

Steelmaking and rolling methods

The following production methods are used for stainless steels:

• EAF (Electric Arc Furnace) — arc furnace steelmaking from scrap; common in stainless steel production.
• AOD (Argon Oxygen Decarburisation) — a refining process that reduces carbon and oxygen content, important for stainless steel quality.
• Vacuum Degassing (VD) — vacuum degassing, lowering soluble gas content and improving mechanical properties.

The steel is then hot‑rolled, often with additional reduction and calibration operations to obtain the desired grain structure and geometric precision. For specialised products forging and finish forging are also used.

Quenching and tempering — achieving the correct microstructure

A typical heat treatment cycle for martensitic steels includes three stages:

• Homogenising/normalising anneal (if required) to uniformise the microstructure and dissolve excessive carbides.
• Heating to the austenitisation range (usually about 980–1040°C for steels with ~13% Cr), followed by rapid cooling (quenching) — martensite is formed.
• Tempering at a controlled temperature (depending on required hardness/toughness compromise), which reduces brittleness and stabilises the structure.

Selection of temperatures and times is critical: excessive austenitisation leads to grain growth, overly aggressive cooling may cause cracking.

Precipitation hardening — principle of operation and application

Precipitation hardening (PH) is based on dissolving alloying elements in the matrix during high‑temperature treatment and then ageing to produce fine, dispersed precipitates that obstruct dislocation motion. Characteristics:

• PH allows achieving high strength while maintaining moderate ductility.
• In stainless steels PH is used in specialised alloys (e.g. 17‑4PH) that contain elements promoting precipitation (Cu, Al, Nb).
• This method differs from classical quenching because it does not rely solely on carbides/cementite but on controlled dispersion of small particles.

In practice 1.4021/AISI 420 are not PH steels, but comparing both technologies helps understand material choice: when extreme hardness is needed — choose martensitic; when a combination of high strength with good machinability and dimensional stability is required — consider precipitation‑hardened steels.

Microstructure determines what the material can do — from strength to wear resistance.

Description of 1.4021 microstructure and its effect on strength

In the quenched condition the microstructure of 1.4021 is characterised by a needle‑like martensitic arrangement with dispersed chromium carbides. This feature results in:

• high hardness and wear resistance,
• low plasticity at maximum hardening,
• susceptibility to impact cracking if appropriate tempering is not performed.

In engineering practice a combination suitable for the application is sought: e.g. sharp cutting tools will be harder and less tough, structural parts requiring fatigue strength will be tempered to increase ductility.

Advantages and limitations of precipitation hardening

Advantages of PH:

• possibility to achieve very high strength while retaining good impact toughness,
• repeatability and control of properties via ageing parameters,
• better maintenance of dimensional tolerances than with aggressive quench and temper cycles.

Limitations:

• requirement to use special alloys with additional elements,
• processes are more complex and costly than classical quenching,
• in some applications reduced resistance to high service temperatures.

Comparison of hardness and ductility in different treatment variants

For AISI 420/1.4021 typical hardness and strength values (indicative):

• in the annealed state: hardness about 170–220 HB, high ductility,
• after quenching and tempering: hardness from ~40 HRC to over 50 HRC depending on carbon content and treatment parameters,
• higher hardness is associated with lower impact resistance and reduced ductility.

In PH steels similar or higher strength values can be obtained with better toughness, important where impact resistance is required.

Corrosion resistance defines the limits of application. Martensitic stainless steels have a moderate character.

Range of corrosion resistance in acidic and alkaline environments

Steels with about 13% chromium show good resistance to:

• atmospheric environments,
• weak chemical solutions and low‑aggression alkaline solutions,
• some foodstuffs with low chloride content.

They are less resistant in:

• chloride‑rich environments (e.g. seawater, brines) — risk of pitting and intergranular corrosion,
• strongly oxidising and acidic solutions, where austenitic alloys will perform much better.

Importance of surface treatment and passivation

Surface finish and chemical treatments are crucial for durability:

• nitric passivation or nitric acid treatment improves and renews the passive layer,
• polishing and surface finishing reduce the risk of initiation of localised corrosion,
• PVD coatings, nitriding or electroplating increase wear and corrosion resistance in specific applications.

Without appropriate surface treatment, even grades with nominally good resistance will rapidly degrade in harsh conditions.

Examples of environments in which the steel remains durable

• food industry where hygiene is maintained and salt content is low,
• medical instruments (e.g. surgical tools) — after sterilisation and passivation,
• machine parts and tools in dry or mildly humid industrial conditions.

In marine environments or where aggressive chloride solutions occur, consider austenitic steels or special corrosion‑resistant alloys.

The combination of hardenability and moderate corrosion resistance makes these grades versatile.

Tools and machine parts requiring durability and resistance

• industrial knives and cutting tools,
• injection moulds for plastics in non‑corrosive applications,
• pump and valve components in moderately aggressive environments,
• springs and shafts operating in dry conditions.

Structural elements in the food and medical industries

• surgical instruments (e.g. scalpels, forceps) — after meeting surface finish and passivation requirements,
• components of food‑processing equipment where contact with products is transient and hygiene is controlled.

In medical applications lower carbon variants are often preferred to improve corrosion resistance and sterilisation capability.

Use in automotive and aerospace industries

• components requiring hardness and wear resistance that are not exposed to strongly corrosive conditions,
• internal engine and auxiliary mechanism parts where low weight is not the primary requirement,
• in aviation martensitic steels are used in auxiliary components, though alloys with higher strength‑to‑weight ratios predominate.

Choice of steel type always depends on compromise between cost, mechanical properties and corrosion resistance.

Differences compared with ferritic and austenitic steels

• Austenitic (e.g. 304, 316): excellent corrosion resistance, good ductility, not hardenable by heat — cannot be hardened thermally, require mechanical work for strengthening. Higher material cost.
• Ferritic: high chromium content, magnetic, good corrosion resistance and low hardenability. Usually cheaper but less ductile.
• Martensitic: can be hardened, offer high hardness, poorer corrosion resistance than austenitic grades. A cost compromise where hardness is required.

Mechanical and chemical properties versus price and availability

Martensitic steels are often cheaper than advanced PH alloys or high‑alloy austenitics. Availability is good, making them attractive for tool production and mass production of parts. However, in applications requiring long‑term exposure to aggressive media economics may favour more expensive but more durable solutions.

Advantages and disadvantages of choosing martensitic steels for specific applications

Advantages:

• ability to achieve very high hardness and wear resistance,
• good machinability and predictable heat treatment,
• favourable cost/performace ratio for mechanical applications.

Disadvantages:

• limited corrosion resistance in aggressive environments,
• need for precise tempering to avoid brittleness,
• welding difficulties (pre‑ and post‑weld heat treatments may be necessary).

Understanding standards facilitates correct material selection and communication between suppliers and buyers.

Description of PN, EN and ASTM standards for these grades

• PN‑EN: In Europe these materials appear in EN 10088 (stainless steels) and correspond to specific EN/DIN numbering, e.g. 1.4021. Standards specify composition ranges, mechanical requirements and tests.
• ASTM/AISI: AISI 420 corresponds in American systems to UNS S42000; ASTM standards may define requirements for flat products, bars and forged parts.
• PN/ISO: additional guidelines, material certificates and test methods are set out in ISO standards and industry documents.

Steel equivalents in DIN, JIS and other systems

Equivalents between naming systems are common in practice but not always identical in tolerances. Typical equivalents:

• DIN/EN: 1.4021 = X20Cr13,
• AISI/UNS: AISI 420 = UNS S42000,
• Russian: 2Х13 (2H13) is often indicated as equivalent to X20Cr13; sometimes 40Х13 is encountered as a higher‑carbon variant.

Always check material documentation and manufacturer declarations when ordering or designing to avoid misunderstandings.

Guidance on choosing the appropriate standard for projects

• For international projects select a standard with clear mechanical and chemical requirements (e.g. EN 10088) and communicate these to the supplier.
• In technical documentation specify not only the grade name but also heat treatment requirements, allowable surface treatments and tests.
• For critical components request a material test certificate (MTC) and chemical analysis by OES.

Reliable control is the guarantee of safety and durability.

Hardness, tensile and impact testing

• Hardness: Rockwell (HRC) or Brinell (HB) tests depending on range. Hardness after heat treatment must be checked on representative samples.
• Tensile strength (Rm): mechanical testing in accordance with EN/ASTM; Rm values provide information on design limits.
• Impact toughness (Charpy): especially important for parts subject to dynamic loads; low impact toughness signals risk of cracking.

Corrosion resistance tests — methods and significance

• Salt spray test (ASTM B117): useful for comparative assessments, although it does not always directly translate to performance in specific conditions.
• Pitting potential measurements and tests in chloride solutions — help assess resistance to pitting corrosion.
• Passivation and testing its effectiveness: measurement of passive layer quality.

Microstructure control — techniques and equipment

• Optical microscopy after chemical etching to assess carbide distribution and martensitic structure.
• SEM (scanning electron microscopy) and EDS analysis to identify phases and contaminants.
• Studies of carbide dispersion and grain size analysis — key for fatigue and corrosion properties.

Test documentation should be an integral part of the delivery certificate, particularly in critical applications.

Advances in metallurgy open new possibilities to improve properties without radically increasing costs.

New alloy compositions improving mechanical properties

• Micro‑alloying (V, Nb, Ti) to control carbides and stabilise microstructure, translating into improved resistance to cracking.
• Nitrogen additions (N) increasing strength and hardenability without significantly worsening corrosion resistance.
• Development of variants with controlled carbon content for optimal compromise between hardness and corrosion resistance.

Advances in heat treatment and surface‑treatment techniques

• Precise control of quench/temper cycles using programmable vacuum furnaces.
• Surface treatments by laser, plasma and ion techniques to modify the top layer, improving surface hardness and wear resistance.
• Advanced coatings (PVD, DLC) offering extreme wear resistance while maintaining corrosion performance.

Sustainability and environmental impact

• Recycling of stainless steel is well developed, reducing demand for primary resources and CO2 emissions.
• Search for substitutes for critical elements and optimisation of melting processes to lower carbon footprint.
• Development of longer‑life materials reduces life‑cycle costs and environmental impact.

Material choice depends on application context — operating conditions, mechanical requirements and budget.

Criteria for selecting a steel grade depending on application

• If priority is hardness and wear resistance: choose a martensitic steel with higher carbon content and plan appropriate quenching/tempering.
• If better corrosion resistance is required: consider austenitic grades (e.g. 304/316) or PH steels with higher resistance.
• For welded parts: take into account welding difficulties of martensitic alloys — preheating and post‑weld heat treatment may be necessary.

Proper storage and maintenance rules

• Store in a dry, well‑ventilated area; avoid contact with moisture and aggressive substances.
• Finished parts should undergo passivation and be stored in packaging that minimises scratches.
• Regular cleaning and surface maintenance (removal of deposits, crack inspection) extend service life.

Common mistakes and how to avoid them in service

• Excessive hardening without appropriate tempering leads to cracking — always plan heat treatment cycles according to intended use.
• Using 1.4021 in marine environments without surface treatment accelerates corrosion — use chloride‑resistant materials instead.
• Incorrect welding without preheating can produce a brittle HAZ — use welding procedures adapted for martensitic steels.

Applying this knowledge in design and production practice allows maximising benefits from using 1.4021/X20Cr13/2H13/AISI 420 while minimising risks stemming from their limitations.

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This article combines description of chemistry, microstructure, production processes and practical guidance on the use of martensitic stainless steels and outlines the essence of precipitation hardening as an alternative technology. The content should help in choosing an appropriate grade depending on functional requirements and service conditions.