1.4406

Acid-resistant and stainless steels form the backbone of modern materials engineering. They are used where corrosion resistance, long-term durability and process safety are required — from chemical plants, through the food industry, to energy and medical applications. In practice, stainless steel is not a single material but a family of alloys with diverse properties determined by chemical composition and crystal structure. Among these, austenitic steels, which include 1.4406 / X2CrNiMoN17-11-2 / AISI 316LN, occupy a special place because of their combination of good ductility, strength and high resistance to many forms of corrosion.

Definition and characteristics of acid-resistant steel

“Acid-resistant steel” is a popular term for steels with enhanced resistance to aggressive chemical environments — including acids, chloride ions and other factors causing localised corrosion. The term is not synonymous with every type of stainless steel; it rather refers to alloys whose composition and microstructure provide particular chemical resistance. A key element of this resistance is the ability to form a thin, durable, self-healing chromium oxide layer — a passive film that protects the metal from further degradation.

Features and properties of austenitic stainless steels

Austenitic steels are characterised by an austenitic crystal structure (face-centred cubic lattice). From a user’s perspective this means:

• high ductility and formability, facilitating forming and bending;
• good weldability without the need for special heat treatments;
• a stable microstructure over a wide working temperature range (within certain limits);
• resistance to general corrosion and better resistance than ferritic/martensitic grades to pitting and crevice corrosion, especially when alloyed with elements such as molybdenum or nitrogen.

Austenitic stainless steels are non-magnetic in the annealed condition, which is useful in applications requiring low magnetic susceptibility.

Differences between acid-resistant and stainless steels

The distinction is subtle: most acid-resistant steels are typically stainless, but not every stainless steel meets the “acid-resistant” definition. Stainless steels form a broad group — austenitic, ferritic, martensitic, duplex — with different properties. The term “acid-resistant” relates more to the practical capability of an alloy to perform in chemically aggressive environments. The choice among them is determined by specific chemical composition and application requirements: resistance to chlorides, to oxidising acids or to high-temperature exposure.

Evolution of austenitic stainless alloys

The history of stainless steels is a story of seeking a compromise between corrosion resistance and mechanical properties. The origins date back to the early 20th century, when it was discovered that adding chromium to steel creates a passive oxide layer that protects the surface from further reaction. Over decades alloying elements were developed: nickel improved ductility and stabilised the austenitic structure, molybdenum increased resistance to pitting and crevice corrosion, and lowering carbon content (the “L” – low carbon – variants) prevented intergranular corrosion after heating.

In the second half of the 20th century modifications with added nitrogen appeared — an element that not only stabilises austenite but also increases strength and pitting resistance. This led to steels such as 316LN, which combined the advantages of classic 316 with the additional benefits conferred by nitrogen.

Origin and standardisation of 1.4406 / AISI 316LN

As the energy, chemical and marine industries developed, demand grew for steels with improved localised corrosion resistance and higher strength while retaining good weldability. The response was a modification of the 316 alloy — adding nitrogen and controlling carbon content. In the European system the alloy was designated 1.4406, in the German standard X2CrNiMoN17-11-2, and in the American system AISI 316LN. Standardisation has facilitated routine use of this alloy across industries and in material specifications.

Several parallel naming systems exist for stainless steels. Understanding the notation helps quickly identify material properties.

EN system – what the 1.4406 designation means

The designation 1.4406 is the European identification according to EN 10088-1/-2. This number identifies a group of alloys with a defined range of chemical composition and mechanical properties. The format simplifies material specification in technical documentation and orders, ensuring reproducibility of parameters across different suppliers.

German standard X2CrNiMoN17-11-2 – composition and notation

German designations present the composition in a shorthand:

• X – alloyed steel;
• 2 – approximate carbon content (around 0.02%);
• CrNiMoN17-11-2 – respectively: chromium ~17%, nickel ~11%, molybdenum ~2%, nitrogen ~0.1–0.2%.

The name is a clear abbreviation informing about the main alloying elements and its low-carbon, nitrogen-bearing character. Such notation helps an engineer quickly associate the material with its properties.

AISI classification and use of the 316LN designation

The AISI 316LN designation is widely used in technical documentation and manufacturer catalogues outside Europe. The letter “N” indicates nitrogen content, and “L” low carbon. In practice 316LN is equivalent to European 1.4406 and German X2CrNiMoN17-11-2, employed where high corrosion resistance in the presence of chlorides and greater strength than standard 316 are required.

Chemical composition is key to understanding alloy behaviour in different environments. Below are typical, indicative composition ranges for 1.4406/316LN — values may vary slightly depending on the standard, manufacturer and product form.

• Fe: remainder (balance);
• Cr (chromium): approximately 16.5–18.5% — responsible for forming the passive oxide layer, crucial for corrosion resistance;
• Ni (nickel): approximately 10–13% — stabilises austenite, improves ductility and resistance to general corrosion;
• Mo (molybdenum): approximately 2.0–3.0% — enhances resistance to localised and crevice corrosion, particularly in the presence of chlorides;
• N (nitrogen): approximately 0.08–0.20% (often around 0.10–0.15%) — increases strength, stabilises austenite, improves pitting resistance and reduces the risk of intergranular corrosion;
• C (carbon): maximum 0.03% (the “L” – low carbon – version) — low carbon content limits carbide precipitation and thus reduces susceptibility to intergranular corrosion;
• Si (silicon): ≤1.0% — contributes to oxidation resistance at high temperatures and assists in casting processes;
• Mn (manganese): ≤2.0% — commonly present as a deoxidant and stabiliser;
• P (phosphorus) and S (sulphur): minimal amounts, in accordance with the standard, so as not to impair mechanical properties and corrosion resistance.

It is worth emphasising that nitrogen plays a special role in 316LN: it acts as a strengthening agent without unduly reducing ductility, and it also aids resistance to pitting, making the alloy particularly valuable in chloride-containing environments.

Chromium, nickel and molybdenum contents and their roles

• Chromium (Cr): the foundation of passivity — the chromium oxide layer forms a “skin” that protects the steel from further oxidation. A minimum of about 12–13% is needed for stainless behaviour, and the higher chromium content in 316LN (around 17%) provides enhanced chemical resistance.
• Nickel (Ni): acts as an austenite stabiliser, raises toughness and resistance to cracking. Nickel allows the alloy to retain ductile properties even at low temperatures.
• Molybdenum (Mo): crucial for improving resistance in the presence of chloride ions and for resistance to pitting and crevice corrosion. Even modest additions of Mo significantly increase the alloy’s suitability for aggressive environments.

The importance of nitrogen for mechanical properties

Nitrogen serves as a strengthening element — it increases yield and tensile strength while maintaining good ductility. It also positively affects resistance to pitting corrosion. Nitrogen is preferred over increasing nickel content because it can achieve similar effects in stabilising austenite and increasing strength at potentially lower cost. In 316LN its presence also improves material behaviour at low temperatures.

Influence of trace elements and stabilising additions

Trace elements such as titanium (Ti), niobium (Nb) or additional nitrogen may be added in controlled amounts to stabilise microstructure and limit carbide precipitation. Excessive phosphorus, sulphur or oxygen can degrade mechanical properties and corrosion resistance. In practice manufacturers carefully control these contents to ensure stability and consistency of material properties.

Understanding mechanical and physical properties is essential when designing components and determining service conditions.

Tensile strength, ductility and hardness

316LN steels have a favourable combination of strength and ductility. Typical characteristics in the annealed condition (indicative values depending on processing and product form):

• tensile strength (Rm): on the order of several hundred MPa (limiting values depend on form and treatment);
• yield strength (Rp0.2): higher than standard 316L due to nitrogen, which translates to greater load-bearing capacity for the same cross-section;
• elongation at break: good, typically above 30–40% in the annealed condition, indicating considerable plastic deformation before fracture;
• hardness: moderate; the material is amenable to machining but shows work-hardening during plastic deformation.

Nitrogen provides increased strength without significant loss of toughness, which is advantageous in designing thin-walled structures and components subject to dynamic loads.

Fatigue resistance and temperature effects

316LN exhibits good fatigue resistance under normal operating conditions, although, as with any austenitic steel, attention is required when designing components with stress concentrators and in the presence of aggressive environmental factors. Across low to moderately high temperatures, 316LN retains structural stability. Prolonged exposure to very high temperatures can lead to precipitation of undesirable phases (e.g. sigma phase), which may reduce impact toughness and corrosion resistance; therefore, for temperatures above certain limits special heat treatment procedures and alternative material selection are applied.

At low temperatures the austenitic microstructure and the presence of nickel ensure the material maintains good ductility and impact toughness.

Magnetic properties and thermal conductivity

• magnetism: in the annealed condition austenitic steels are essentially non-magnetic; however plastic deformation and the presence of certain phases can introduce some local ferromagnetism;
• thermal conductivity: lower than carbon steels, which affects the design of heat dissipation systems and heat treatment processes;
• thermal expansion: relatively high for steels, which must be considered when designing joints and seals in equipment operating over a wide temperature range.

The ability of 316LN to operate in aggressive environments stems from a combination of composition and microstructure.

Chemical corrosion resistance – acids and aggressive media

Thanks to chromium and molybdenum content, 316LN shows good resistance to a wide range of acids and chemical solutions. In oxidising environments dominated by nitric or dilute sulphuric acid, the steel remains passive. In industrial applications where mixtures of aggressive media occur, 316LN is valued for its combination of corrosion resistance and strength, making it a first-choice material for pipes, heat exchangers and process tanks.

Resistance to pitting and intergranular corrosion

• pitting: molybdenum and nitrogen increase resistance to the formation of localised corrosion in the presence of chloride ions; 316LN performs better than 304, and in many conditions better than standard 316;
• intergranular corrosion: low carbon content limits formation of chromium carbides during heat treatment; nitrogen further stabilises austenite and reduces susceptibility to sensitisation, resulting in higher intergranular corrosion resistance even without special post-weld heat treatment.

Nevertheless, in highly chloride-bearing environments and at elevated temperatures there remains a risk of crevice corrosion, so susceptibility is assessed using indicators such as the PREN (pitting resistance equivalent number).

Use of 316LN in marine and chloride-containing environments

Marine environments rich in chloride ions are a true test for materials. Thanks to Mo and N additions and low C content, 316LN offers better pitting and crevice corrosion resistance than classic 304 or 316 steels. Hence its widespread use in shipbuilding components, desalination systems, coastal installations and equipment operating in saline atmospheres. High resistance and durability translate into lower operating costs and longer intervals between inspections.

Effective use of 316LN requires knowledge of appropriate forming, cutting and welding techniques, as well as awareness of the effects of heat treatments on microstructure.

Forming and cutting techniques

• cold forming: austenitic steel deforms well but tends to work-harden under large strains; appropriate tool selection and lubrication are important;
• bending and stamping: 316LN retains ductility, facilitating manufacture of thin-walled components and precision parts;
• machining: the ductility of the material can cause issues during cutting — tools with suitable geometry and cooling are used; relatively low thermal conductivity requires efficient heat removal;
• cutting: possible methods include laser, plasma, waterjet and, under special conditions, oxy-fuel cutting; mechanical cutting with shears or saws is common for commercial thicknesses.

In practice the choice of method depends on thickness and edge-quality requirements.

Welding principles for 316LN – selection of methods and filler materials

Welding 316LN is relatively straightforward compared with some other steels, but several rules should be followed:

• methods: TIG (GTAW), MIG/MAG (GMAW), MMA (covered electrodes) — all are used; TIG is preferred where high quality is required;
• filler materials: use wires and electrodes matching 316L/316LN or with similar chemistry; matching composition ensures consistent weld properties;
• pre- and post-treatments: preheating is usually unnecessary; however, heat input control and rapid cooling limit the risk of undesirable phase precipitation; where sigma phase formation has occurred or where restoration of corrosion resistance is required, solution annealing is recommended at about 1040–1120°C followed by rapid cooling;
• avoiding contamination: iron oxides, carbon from consumables and difficult-to-remove inclusions can weaken corrosion resistance; use of clean shielding gases and proper surface preparation is crucial.

Thanks to nitrogen the alloy maintains a stable austenitic structure after welding, which helps preserve mechanical and corrosion properties of welds.

Effect of heat treatment on microstructure and properties

Heat treatment affects the distribution of alloying elements in the matrix and the occurrence of intermetallic phases. Key aspects:

• solution annealing: restores a homogeneous microstructure, removes carbides and brittle phases;
• prolonged exposure to intermediate temperatures may lead to precipitation of phases such as sigma, which reduce toughness and corrosion resistance;
• control of cooling rate is important to prevent sensitisation and to preserve corrosion properties.

Design and manufacturing processes must take these aspects into account to ensure long-term reliability.

The versatility of 316LN means it is used across many industry sectors, often where corrosion and mechanical requirements are combined.

Chemical and petrochemical industry

• process tanks and pipelines for chloride-containing compounds;
• heat exchangers and pressure equipment;
• structural components of installations exposed to aggressive chemical environments.

In these applications resistance to pitting and crevice corrosion, enhanced by molybdenum and nitrogen in 316LN, is important.

Energy sector and high-temperature installations

• boiler, heat exchanger and plant components in power stations;
• turbine and plant parts operating in steam and aggressive condensate environments.

Low carbon content and a stable microstructure make the material a common choice where corrosion resistance and mechanical stability at elevated temperatures are required.

Food and medical sectors

• process equipment, piping and tanks free from contamination;
• apparatus requiring easy cleaning and resistance to disinfectants.

In medicine 316LN is used for components with mechanical and corrosion demands, though specialised variants such as 316LVM are often preferred for implants.

Machinery construction and structural elements

• bolts, rivets, fasteners and structural elements exposed to corrosion;
• marine parts, pumps and valves.

Thanks to higher strength compared with 316L, 316LN enables weight reduction of structures while meeting load-bearing requirements.

Comparisons help select the right material for a given application.

Differences in chemical composition and service properties

• 304 vs 316LN: 304 contains no molybdenum and has lower pitting resistance. 316LN offers better resistance in chloride-containing environments and higher strength due to nitrogen.
• 316 vs 316L: 316L has lower C than 316, reducing sensitisation; 316LN additionally contains nitrogen, which increases strength and pitting resistance.
• 316LN vs duplex: duplex alloys (ferritic–austenitic mixtures) generally have higher strength and greater resistance to chloride-induced corrosion, but are less ductile and more difficult to weld. 316LN offers better formability and easier processing, making it preferable where forming and welding are required.

Benefits and limitations of using 316LN

Benefits:

• combination of higher strength with good ductility;
• improved resistance to localised corrosion compared with standard austenitic steels lacking nitrogen;
• good weldability and applicability across many industries.

Limitations:

• in extremely aggressive chloride environments or at very high temperatures, alloys with even higher resistance, such as superaustenitic or duplex grades, may be required;
• raw material costs can be higher than for simpler alloys like 304.

In practice selecting 316LN is a compromise: it provides a broad spectrum of advantages without excessive cost increase.

Compliance with standards is a prerequisite for material acceptance in many industrial and construction applications.

EN, ISO and ASTM standards for stainless steels

• EN 10088-1/-2: classification and chemical composition of stainless steels; 1.4406 is included within these standards;
• ISO: international standards concerning properties and testing of materials;
• ASTM/ASME: American standards applied to specific products (e.g. plates, pipes, bars), such as ASTM A240 (plates, sheets), ASTM A276 (bars), ASTM A182 (forgings and forged parts for pressure equipment) — equivalent standards from different systems are often cited in project documentation.

Familiarity with standards enables correct material selection, quality control and compliance with procurement requirements.

Material certification and quality control

Material certification typically includes documents type 2.1 or 3.1 according to EN 10204, confirming conformity with chemical and mechanical requirements. In practice important elements are:

• chemical analysis and mechanical testing for deliveries;
• non-destructive testing (ultrasonic, radiographic) for pressure parts and critical components;
• weldability assessments and fatigue crack testing for specialised products.

Attention to certificates and tests minimises the risk of failure and is a condition for material use in regulated sectors.

The future of stainless steels is linked to seeking better properties while reducing costs and environmental impact.

New alloy technologies and their impact on material properties

• increasing nitrogen content and the development of high-nitrogen steel systems allow achieving greater strength while reducing nickel content, which lowers costs and stabilises raw-material prices;
• development of superaustenitic alloys with higher molybdenum content and nitrogen additions offers even better pitting resistance;
• processing technologies such as selective laser melting (SLM) and metal 3D printing enable production of complex shapes with precise microstructural control, opening new applications for 316LN and its modifications.

Sustainability and stainless steel recycling

Stainless steel has one of the highest recycling rates in the materials industry. In the context of resource conservation and CO2 emission reduction, developing alloys with reduced proportions of critical raw materials such as nickel, and increasing the share of recycled steel, becomes a priority. Additionally, optimisation of production processes and selection of longer-life alloys contribute to a lower overall environmental footprint.

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1.4406 X2CrNiMoN17-11-2 AISI 316LN is an example of a mature, versatile alloy which, through considered alloying — in particular additions of molybdenum and nitrogen — achieves a balance between corrosion resistance, mechanical properties and manufacturability. Its applications range from the chemical industry and energy sector to marine and food industries. Knowledge of its composition, processing possibilities and application limits enables the design of safe, durable and economical engineering solutions.