1.4335

What are austenitic steels?

Austenitic steels are a group of iron-based alloys whose equilibrium microstructure is formed by the austenite phase — a crystal lattice with a face-centred cubic (fcc) structure. They are characterised by:

• high chromium and nickel content (and often additions such as nitrogen, molybdenum, niobium),
• corrosion resistance in many environments owing to a passive oxide layer on the surface,
• good ductility and toughness, including at low temperatures,
• non-magnetic behaviour in the annealed condition (though cold working may increase the proportion of magnetic phases).

Austenite is stabilised by elements such as nickel, nitrogen or manganese, which lower the austenite–ferrite transformation temperature.

Differences between acid-resistant and stainless steels

In practice the terms “acid-resistant steel” and “stainless steel” are sometimes used interchangeably, but there is a subtle difference:

• stainless steel: a general term for steels with significant corrosion resistance due to formation of a passive chromium oxide layer; it includes ferritic, austenitic, martensitic, duplex and other groups,
• acid-resistant steel: a subgroup of stainless steels that exhibit enhanced resistance to aggressive acidic environments and chemicals; they typically contain higher amounts of chromium and additions that increase resistance to specific chemical attack (e.g. molybdenum against chloride ions, nitrogen improving general resistance).

In engineering practice the choice between them is based on the service environment — presence of chloride ions, temperature, aggressiveness of chemicals and the need for oxidation resistance.

Basic mechanical and chemical properties

Austenitic steels combine the following features:

• high ductility and elongation, which facilitate forming and stamping,
• good impact toughness at low temperatures, making them suitable for cryogenic applications (with appropriate alloy selection),
• inability to be hardened by heat treatment (they do not gain significant strengthening by martensitic transformation); they can, however, be strengthened by cold work,
• corrosion resistance dependent on chromium, nickel, molybdenum and nitrogen content — higher amounts of these elements give better protection against general, pitting and crevice corrosion,
• good oxidation resistance at elevated temperatures for alloys with increased chromium and nickel content, although prolonged exposure to extreme temperatures may lead to oxidation and spallation of the layer.

In technical practice each application requires analysis of the specific environment and service conditions, because even steel with high Cr and Ni can corrode in the presence of strong reducing agents or chloride ions at certain temperatures.

1.4335, designated in the EN system as X1CrNi25-21, is a high-quality austenitic steel with significant chromium and nickel content. The code name suggests approximate proportions of elements: about 25% chromium and 21% nickel, with very low carbon content (the X1 designation indicates low carbon), which minimises carbon supersaturation and carbide precipitation.

Analysis of alloying elements and their functions

The composition of 1.4335 can be described by the main constituents and their roles:

• Chromium (Cr, ~24–26%): the principal element ensuring formation of a stable passive oxide layer; increases oxidation resistance and general corrosion resistance,
• Nickel (Ni, ~20–22%): an austenite stabiliser, improves ductility, formability and resistance to brittle fracture; increases high-temperature resistance,
• Carbon (C, very low, typically ≤0.03%): limited content prevents chromium carbide precipitation and sensitisation during welding,
• Manganese (Mn), silicon (Si): in smaller amounts they affect machinability and austenite stabilisation,
• Nitrogen (N, optionally): may be added to improve strength and resistance to pitting; nitrogen is a strong austenite stabiliser,
• Small additions of niobium (Nb) or titanium (Ti) may occur in stabilised grades, protecting against carbide precipitation.

Each element has a specific role: nickel maintains austenite, chromium forms the anti-corrosion barrier, nitrogen further strengthens the structure and improves localised corrosion resistance.

Crystal structure and its influence on material properties

The microstructure of 1.4335 is predominantly austenitic (fcc). This structure provides:

• excellent ductility and formability,
• good impact resistance,
• microstructural stability over a wide temperature range, although at very high temperatures and long exposure times grain growth and intergranular precipitations may occur.

The absence of ferritic or martensitic phases in the as-delivered condition minimises magnetism and preserves a uniform mechanical character.

Comparison with other austenitic steels

Compared with standard steels such as X5CrNi18-10 (AISI 304), 1.4335 stands out for:

• significantly higher Cr and Ni contents, translating to better oxidation resistance and higher operating temperatures,
• lower carbon content, which improves resistance to intergranular corrosion after welding,
• better resistance to oxidising media and higher stability under redox conditions.

Compared with alloys containing molybdenum, 1.4335 may be less resistant to chloride-induced corrosion unless molybdenum or nitrogen has been added as an adjustment.

AISI 310L is a widely used low-carbon variant of 310 stainless steel, known for its oxidation resistance and stability at high temperatures. The additional suffix “NAG” appearing in some market offerings typically refers to a commercial variant or a specific modification of the production process. In practice this may mean, for example, increased nitrogen content (Nitrogen-Augmented Grade) or other microstructural improvements; interpretation of this abbreviation depends on the manufacturer.

Main chemical constituents and their role

A typical composition of AISI 310L includes:

• Chromium (Cr, ~24–26%): resistance to oxidation at high temperatures and primary corrosion protection,
• Nickel (Ni, ~19–22%): austenite stabiliser and enhancement of high-temperature resistance,
• Carbon (C, low, specifically controlled in the L variant): limits carbide precipitation, improves weldability and reduces sensitisation,
• Nitrogen (N) — in “NAG” variants the content may be increased: nitrogen improves yield strength and resistance to localised corrosion,
• Small amounts of Mn, Si and trace elements.

In practice AISI 310L combines oxidation protection with structural properties at temperatures around 1000–1150°C.

Mechanical properties and corrosion resistance

AISI 310L offers:

• good ductility and resistance to spalling and cracking at elevated temperatures,
• excellent resistance to oxidation and oxidative wear in hot gas atmospheres,
• moderate resistance to pitting corrosion in the presence of chloride ions; variants with molybdenum or increased nitrogen content improve this trait,
• good weldability thanks to the low carbon level, which prevents carbide formation in the heat-affected zone.

Production technologies used for AISI 310L NAG

Production of higher-quality 310L alloys often employs advanced metallurgical methods:

• refining processes (VIM, AOD) improve compositional homogeneity and reduce dissolved gas content,
• controlled nitrogen injection during melting can produce “NAG” variants with enhanced strength and corrosion resistance,
• homogenising anneals and cooling control achieve a fine-grained, stable microstructure.

Such technologies directly affect the repeatability of properties and reliability of the material in critical applications.

Comparing the two alloys facilitates design decisions. Both serve as high-temperature and corrosion-resistant materials, but differences in composition and production translate into specific advantages for different applications.

Corrosion resistance in various environments

• Oxidising environments (e.g. hot flue gases, oxygen-containing atmospheres): both steels show very good resistance due to high Cr and Ni contents. 1.4335, with its reduced carbon content and high Ni, provides durable surface passivation. AISI 310L also performs well here, and NAG versions with added nitrogen may exhibit similar or better properties.
• Environments containing chloride ions (pitting, crevice corrosion): generally pitting resistance depends on molybdenum and nitrogen. If 1.4335 does not contain molybdenum, it may be outperformed by Mo-bearing special alloys in very aggressive chloride environments. AISI 310L without Mo likewise has limited pitting resistance; nitrogen-bearing (NAG) variants improve this characteristic.
• Acidic environments: the choice depends on the type of acid and its concentration. In strongly oxidising environments high-chromium alloys perform better.

High-temperature and oxidation resistance

• 1.4335 and AISI 310L are designed for high-temperature service; both types exhibit good oxidation resistance within ranges typical for furnace applications, heat exchangers and flue ducts.
• 1.4335, with higher nickel content, may retain structural stability better under prolonged thermal loading. AISI 310L is renowned for oxidation resistance above 1000°C; low-carbon NAG variants have advantages in welding and prolonged exposure to thermal cycling.

Mechanical strength and ductility

• Both materials feature good ductility and impact properties. Differences in strength may result from specific nitrogen doping or other alloying additions.
• Nitrogen, when present (e.g. in NAG variants), raises yield strength and tensile resistance, which is important in structures subjected to stresses from temperature changes.
• In practice 1.4335 may offer slightly higher yield strength at constant operating temperature due to austenite stabilisation by nickel.

Austenitic steels require a specific approach during processing to preserve their advantages and avoid adverse transformations.

Weldability and joining techniques

• Both steels are readily weldable thanks to low carbon content (especially “L” and “X1” types), which reduces the risk of sensitisation and intergranular corrosion. Recommended techniques include TIG, MIG/MAG and gas-shielded arc welding.
• For critical structures it is important to use filler materials chemically compatible with the base steel, control heat input in the heat-affected zone and apply appropriate post-weld annealing procedures for long-term high-temperature service.
• High-nitrogen variants require attention: nitrogen can increase susceptibility to hydrogen-induced cracking under specific conditions. Welding parameters and preheating must follow the manufacturer’s recommendations.

Cold and hot forming methods

• Cold working: austenitic steels work-harden strongly during cold deformation; this allows improvement of mechanical properties by forming. However, excessive hardening can make subsequent processing difficult; intermediate anneals are performed.
• Hot working: forging and rolling in appropriate temperature regimes restore ductility and enable forming of complex shapes.
• Machining and turning: require tools with suitable geometry and cooling; austenitic steel tends to produce long, gummy chips and to adhere to tool edges.

Processing temperature ranges and their significance

• Homogenising annealing typically takes place at temperatures appropriate for the specific alloy — it aims to reduce stresses and dissolve unwanted precipitates.
• Low-carbon variants (L, X1) allow more flexible heat treatment without the risk of severe sensitisation.
• During hot working it is necessary to control temperature to avoid excessive grain growth, which impairs resistance to brittle fracture and reduces strength.

Both materials are used where a combination of corrosion resistance, high-temperature stability and good weldability is required.

Chemical and petrochemical industry

• Reactors, heat exchangers, pipelines and valves are often made from materials with high Cr and Ni contents. 1.4335, due to low carbon and high Ni content, performs well in installations requiring long-term oxidation resistance. AISI 310L is used where aggressive media are present at elevated temperatures, provided there is no excessive exposure to chloride ions without additional protection.
• In processes involving acid mixtures and high temperatures, selection often relies on operational data and life-cycle costs.

Food and pharmaceutical industry

• Food-processing and pharmaceutical equipment uses low-carbon steels with excellent surface cleanliness. AISI 310L, thanks to low C and good polishability, is used where hygiene and resistance to cleaning and disinfectants are required.
• 1.4335 can be used in specialised laboratory equipment and apparatus where resistance to aggressive, oxidising environments is needed.

Power industry and high-temperature applications

• Furnace elements, combustion chambers, flue ducts, boiler and turbine parts — these are natural environments for alloys such as 1.4335 and AISI 310L. Their oxidation resistance and structural stability at high temperatures provide long component lifetimes.
• In power generation control of composition and production methods is crucial — materials must meet fatigue and corrosion requirements for long-term service.

Compliance with standards and certification processes are crucial when selecting material for critical applications.

International standards concerning 1.4335 and AISI 310L

• The European standard EN 10088 classifies stainless and acid-resistant steels, assigning numbers such as 1.4335 (X1CrNi25-21). These standards define chemical composition, mechanical properties and test methods.
• ASTM and AISI (American standards) define equivalents such as AISI 310L for specifications widely used in North America and internationally.
• Additional standards for plates, tubes and rolled products (e.g. ASTM A240) set requirements for surface finish, mechanical tests and dimensional tolerances.

Quality certification and material control

• Certificates such as 3.1 or 3.2 in accordance with EN 10204 confirm conformity of a batch of material with the declared composition and mechanical properties.
• Control processes include mechanical testing (tensile tests, impact tests), corrosion tests (e.g. pitting resistance tests), chemical composition analysis (spectrometry) and metallographic examinations.
• In critical applications additional attestations are often required, e.g. NACE compliance for sour service (requirements for use in acidic environments in the oil and gas industry).

Industry requirements and safety standards

• Sectors such as petrochemicals, power generation, pharmaceuticals and food impose specific requirements regarding cleanliness, trace contaminants and welding and inspection methods.
• These standards aim to minimise failure risk and ensure long service life — compliance is often contractually required.

Material durability results from the interaction of alloy composition with surrounding chemical, mechanical and thermal conditions.

Corrosion in the presence of chemical factors

• Chloride ions are among the most dangerous factors for austenitic steels — they cause pitting and crevice corrosion. Materials without additional molybdenum have limited resistance in such conditions, so in chloride environments alloys with Mo or increased nitrogen are preferred.
• Chromic acid and oxidising environments are generally well tolerated by high-chromium austenitics. In reducing environments or those containing sulphides and fluorides material behaviour may differ and requires assessment.

Effect of temperature and thermal cycles

• Prolonged exposure to high temperatures can lead to grain growth and precipitation of intermetallic phases, weakening strength and resistance to cracking.
• Thermal cycles (rapid cooling and heating) can induce thermal stresses, and combined with aggressive environments can cause stress-corrosion cracking.
• Controlled annealing and appropriate alloy selection (low-carbon, stabilised) minimise these risks.

Preventive measures and maintenance

• Surface passivation (chemical pickling and formation of Cr2O3) improves corrosion resistance.
• Regular inspections, cleaning and monitoring of the service environment (e.g. chloride concentration) are essential.
• Use of protective coatings, cathodic protection or corrosion inhibitors is applied where the base material cannot be changed.

Materials continuously evolve — new alloy technologies and production processes open up new possibilities.

New alloying technologies and composition modifications

• Growing importance of nitrogen as an alloying addition: nitrogen improves strength and resistance to pitting, sometimes allowing reduction of costly nickel.
• New combinations of elements, including controlled amounts of molybdenum, niobium or vanadium, achieve desired properties at optimised costs.
• Work on “lean” stainless steels — with reduced nickel content while maintaining resistance through nitrogen additions and composition optimisation.

Advanced surface processing and finishing methods

• Electropolishing and plasma coating improve corrosion resistance and finishing properties.
• Laser surface modification and plasma treatments controlled at the microstructural level can produce layers with increased wear and corrosion resistance.
• Metal 3D printing and additive manufacturing technologies bring new possibilities for producing complex austenitic components.

Sustainability and material recycling

• Due to rising raw material costs, especially nickel, and environmental requirements, stainless steel recycling is becoming strategic. High-alloy steels are well recovered and reprocessed.
• Development of alloys with a smaller carbon footprint combined with efficient recycling logistics aligns with global sustainable development goals.

Choosing between 1.4335 and AISI 310L NAG requires analysis of technical criteria, costs and service conditions.

Selection criteria depending on application

• High-temperature and oxidising applications: if the main requirement is oxidation resistance and stability at temperatures above 1000°C, AISI 310L (especially the NAG variant) is often preferred due to its track record in furnaces and flue ducts.
• Chemically aggressive environments (chlorides, acids): if there is a risk of localised corrosion, consider 1.4335, especially when high overall resistance is required; however, in chloride-rich environments Mo-bearing alloys or specially enhanced 310 versions may be better.
• Welding processes and hygienic constructions: low-carbon AISI 310L is beneficial for weldability and hygienic surface finishes.

Costs and material availability

• Price depends on nickel and other costly element contents. High-Ni alloys will be more expensive but can offer operational advantages.
• Availability depends on local markets and manufacturers offering nitrogen-enhanced variants or special certifications. Consider long-term maintenance and inspection costs.

Assessment of long-term performance and maintenance

• Material selection should be based on expected service conditions: temperature, chemical aggressiveness, likelihood of mechanical stresses and thermal cycling.
• Long-term operating costs (maintenance, downtime, component replacement) often outweigh savings from a lower material price; investing in material better matched to service conditions usually pays off.

• EN 10088 — Stainless steels. Technical delivery conditions for stainless steels for general purposes.
• ASTM A240/A240M — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications.
• Sedriks, A. J., Corrosion of Stainless Steels, 2nd ed., Wiley-Interscience — a classic work on corrosion mechanisms of stainless steels.
• ASM Handbook, Volume 1 & 4 — contains detailed data on metals, alloys and their processing.
• Totten, G. E. (ed.), Steel Heat Treatment Handbook — information on heat treatment and steel properties.
• Technical materials and product data sheets from stainless steel manufacturers (particularly concerning 310L variants and high-nickel alloys).
• Industry articles on high-temperature applications and passivation and protective coating methods.
• Standards and guidelines for quality control and certification: EN 10204, NACE MR0103/ISO 15156 (requirements for sour service — application in the oil and gas industry).

Each of the listed sources provides additional, detailed metallurgical and operational data that should be consulted when designing and selecting materials for conditions specific to a given installation.