1.4438

Stainless steels were developed in response to the need for materials resistant to corrosion that at the same time retain mechanical properties and workability. The history begins in the early 20th century — the discovery of stainless steel is often attributed to Harry Brearley (1913), and the development of austenitic steels and nickel-containing alloys followed in subsequent decades. The gradual addition of elements such as chromium, nickel and molybdenum led to the formation of a wide range of stainless steels adapted to different applications.

Austenitic steels are the largest group of stainless steels and are valued for their corrosion resistance, high ductility and toughness at low temperatures. In industrial nomenclature standards such as the German Werkstoffnummer system (e.g. 1.4438), European designations X-CrNiMo… and American AISI/ASTM (e.g. 317L) are used.

Basic properties of austenitic steels

• Crystal structure: austenitic (regular FCC — face-centred cubic lattice), which corresponds to high ductility and good toughness, also at low temperatures.
• Corrosion resistance: thanks to chromium content (>10.5%) a surface passive layer of chromium oxides forms, protecting the material against further oxidation.
• Good weldability: austenitic alloys tolerate welding well without the need for intensive preheating; low carbon content (the “L” versions) reduces the risk of carbide precipitation and sensitisation.
• Ductility and toughness: austenitic steels are very ductile, easy to form and show good impact strength even at sub-zero temperatures.
• Lower thermal conductivity and greater elongation compared with ferritic and martensitic steels, which affects behaviour under thermal loading.

Differences between acid-resistant and stainless steel

The terms “stainless steel” and “acid-resistant steel” are sometimes used interchangeably, but they have different nuances:

• Stainless steel is a broad category of steels which, thanks to chromium content, form a passive layer and exhibit resistance to general atmospheric and chemical corrosion. Examples include 304 and 316.
• Acid-resistant steel is a subgroup whose composition and microstructure provide increased resistance to attack by aggressive acids (e.g. sulfuric, phosphoric) and to the corrosive action of chloride ions. The addition of molybdenum and sometimes a higher nickel level makes these steels better able to withstand acidic and halogen-aggressive environments.

In practice each grade has its application conditions: not every stainless steel will be “acid-resistant” in the context of resistance to specific media and temperatures. 1.4438 / AISI 317L sits among alloys with higher chemical resistance thanks to the presence of molybdenum, often being chosen where 304 would be insufficient.

1.4438 (X2CrNiMo18-15-4, AISI 317L) is a low-carbon austenitic steel with molybdenum addition and relatively high nickel content. In practice 317L denotes the low-carbon version (Low Carbon), which limits carbide precipitation at grain boundaries during heat treatment and welding.

History and application of naming standards

Nomenclature systems evolved alongside the steel industry:

• AISI (American Iron and Steel Institute): classification with simple symbols (e.g. 304, 316, 317L), widely used globally.
• EN / Werkstoffnummer (e.g. 1.4438): the European numeric system and an expanded chemical description system (X2CrNiMo18-15-4), where the numbers refer to the approximate content of key elements: 18% Cr, 15% Ni, 4% Mo, and X2 suggests limited carbon content.
• Such parallel designations facilitate commercial and technical exchange between markets while indicating conformity with production and quality requirements.

Basic mechanical and chemical properties

Mechanical and chemical properties of 1.4438 vary depending on processing condition (annealed, hardened, cold worked). Typical values for the delivery condition (annealed and stress-relieved) are as follows:

• Density: approximately 7.9–8.0 g/cm³.
• Melting temperature: approximately 1370–1400 °C.
• Tensile strength (Rm): typically 500–700 MPa, depending on cold work and specimen thickness.
• Yield strength (Rp0.2): typically 180–320 MPa.
• Elongation at break (A5): usually >40% for the annealed condition, indicating high ductility.
• Hardness: low to moderate in the annealed state; values depend on heat treatment and cold working.
• Impact resistance: high, also at low temperatures, thanks to the austenitic structure.

The above values are indicative — manufacturers and standards provide detailed data for specific supplies and material conditions.

Typical chemical composition of 1.4438 / AISI 317L (indicative values; results depend on specification and manufacturer):

• Carbon (C): ≤ 0.03% (L — low carbon version)
• Chromium (Cr): ~17.5–19.5% (approx. 18%)
• Nickel (Ni): ~13.0–16.0% (depending on batch; X2CrNiMo18-15-4 nominally 15%)
• Molybdenum (Mo): ~3.0–4.5% (approx. 3–4%)
• Manganese (Mn): ≤ 2.0%
• Silicon (Si): ≤ 1.0–1.5%
• Phosphorus (P): ≤ 0.045%
• Sulfur (S): ≤ 0.015%
• Nitrogen (N): up to approx. 0.11% (depends on the alloy)
• Iron (Fe): remainder

The ranges above show which elements are responsible for the key properties of 1.4438. Below we discuss the role of the most important additions.

The role of chromium in corrosion resistance

Chromium is the foundation of stainless steels. At contents above about 10.5% a passive surface layer of chromium oxides forms, which:

• protects the steel from oxidation and further corrosion,
• enables self-repair of the passive layer in the presence of oxygen,
• determines resistance to general corrosion in atmospheres, water and many chemicals.

In 1.4438 chromium content of around 18% ensures stable protection, and together with molybdenum enhances resistance to localised corrosion.

The importance of nickel in the austenitic structure

Nickel stabilises the austenitic (FCC) structure at room temperature. Its effects include:

• maintaining ductility and impact resistance,
• improving formability and suitability for plastic forming,
• influencing corrosion resistance in certain environments.

Higher nickel content in 1.4438 (approx. 15%) improves general corrosion resistance and microstructure stability, especially in combination with molybdenum.

The function of molybdenum in increasing acid resistance

Molybdenum is a key element in 1.4438. Its roles:

• significantly increasing resistance to pitting and crevice corrosion caused by the presence of chloride ions,
• improving resistance in acidic environments (particularly organic acids and some inorganic acids),
• raising the overall durability of the passive layer in aggressive conditions.

Thanks to molybdenum, 317L is used where 304 or 316 might experience localised forms of corrosion.

Other elements and their influence on material properties

• Carbon (C): in the L version limited carbon content minimises carbide precipitation during welding, reducing the risk of intergranular corrosion (sensitisation).
• Manganese (Mn): austenite stabiliser, influences mechanical properties and helps limit oxidation during melting.
• Silicon (Si): influences oxidation resistance at high temperatures and improves casting properties; excess can reduce ductility.
• Nitrogen (N): in moderate amounts increases strength and resistance to stress corrosion cracking.
• Phosphorus and sulfur (P, S): present in trace amounts; excess worsens ductility and corrosion resistance.

In the design intent, the choice of chemical composition is a compromise between the cost of elements, corrosion resistance requirements and mechanical properties.

Understanding mechanical and physical properties is key for designing components and determining their service life.

Tensile strength and yield strength

1.4438 features relatively high tensile strength while retaining good toughness. Approximate values:

• Tensile strength (Rm): 500–700 MPa (depending on processing condition).
• Yield strength 0.2% (Rp0.2): 180–320 MPa.

In practice higher Rm and Rp0.2 values are achieved by cold working; cold-worked material gains hardness and strength at the expense of ductility.

Ductility and impact behaviour — performance at different temperatures

• The austenitic structure provides high ductility: elongation at break often exceeds 40% in the annealed state.
• Impact toughness remains high even at low temperatures, making 1.4438 suitable for applications requiring deformation at low temperatures.
• The material can be heavily cold formed, enabling production of thin-walled components and pipes.

Wear resistance and fatigue behaviour

• Wear resistance: moderate; austenitic steels are generally not ideal where intensive abrasion is required without additional surface treatment.
• Fatigue resistance: good when components are properly designed (avoidance of sharp edges and stress concentrators). However in corrosive environments corrosion fatigue can shorten component life — hence correct material selection and surface protection are necessary.

Corrosion resistance is one of the main reasons for choosing 317L. Due to its composition this steel shows good resistance to both general and localised forms of corrosion, but there are limitations depending on the environment.

General versus localised corrosion (crevice and pitting)

• General corrosion: 1.4438 copes well with oxidising environments and many chemicals; the surface undergoes slow degradation in harsh conditions, but the passive layer significantly slows the process.
• Localised corrosion (pitting and crevice corrosion): molybdenum improves resistance specifically to these forms. In the presence of chloride ions 317L is more resistant than 304, and often better than 316, although at very high chloride concentrations and elevated temperatures local damage can occur.

Behaviour in acidic and alkaline environments

• In acidic environments (e.g. sulfuric solutions, some organic acids) 317L shows better resistance than e.g. 304 or 316, especially at lower concentrations and moderate temperatures.
• In alkaline environments general resistance is also good; however each case requires analysis, since corrosion rates depend on concentration, temperature and presence of aggressive ions.

Effect of temperature and environment on corrosion resistance

• Temperature: increasing temperature usually increases the aggressiveness of the corrosive environment. Pitting and crevice corrosion intensify at higher temperatures. Therefore in high-temperature applications or warm chloride-containing environments, materials with even higher resistance are recommended (e.g. superaustenitic or duplex grades).
• Environment: presence of oxygen, chlorides, sulphides, sulphur compounds and organic substances affects degradation rate. System design should consider the nature of the media and operating conditions (temperature cycles, presence of stagnation, etc.).

Production and processing of 1.4438 include standard metallurgical methods as well as specific heat and mechanical processing that influence microstructure and final properties.

Metallurgical methods and forming of semi-finished products

• The steel is produced in converter or electric arc furnaces (EAF), then refined and cast.
• Semi-finished products such as sheets, pipes, bars and profiles are produced by hot and cold rolling. Austenitic steels roll well, enabling production of thin sheets and pipes while maintaining a uniform structure.

Heat treatment: annealing and its effect on microstructure

• Solution annealing (wyżarzanie roztwórcze): typically carried out at 1050–1100 °C, followed by rapid cooling (e.g. water) to dissolve carbides and restore a homogeneous austenitic microstructure.
• Annealing removes internal stresses and restores ductility, which is particularly important after cold working.
• Improper prolonged exposure to intermediate temperatures (approx. 600–900 °C) can lead to precipitation of brittle phases (e.g. sigma), which degrades toughness and corrosion resistance.

Mechanical methods and rolling of the steel

• Hot rolling: forming billets into semi-finished products; affects grain size and directional properties.
• Cold rolling: improves surface finish and increases strength by work hardening; subsequent annealing is required to recover ductility.
• Plastic forming (bending, stamping, drawing): austenitic steels are easy to form, allowing manufacture of complex components, e.g. thin-walled tanks or decorative elements.

Austenitic steels are among the most weldable groups of stainless steels, and the L version further simplifies welding procedures due to reduced carbon content.

Types of joining permitted for this alloy

• Arc welding (SMAW, TIG, MIG/MAG) — commonly used; TIG (GTAW) provides the best joint quality and aesthetics.
• Gas and plasma welding: used in specific applications.
• Resistance welding (e.g. spot welding) — useful for thin sheets.
• Welding techniques are chosen depending on thickness, technological requirements and joint quality.

Recommendations for filler materials

• It is recommended to use wires and electrodes matched to 317L, e.g. electrodes/wires type ER317L or similar, to maintain comparable chemical composition and corrosion resistance.
• In the absence of 317L filler wires, 316L is often used as filler, however in critical applications fillers matched for molybdenum content are preferred.
• Low carbon content in the base material reduces preheat requirements; typically preheat is not necessary, but this depends on component size and construction.

Typical defects and ways to avoid them

• Intergranular corrosion (sensitisation): carbide precipitation at grain boundaries in the heat-affected zone; minimised in the L version, but solution annealing can be applied if necessary.
• Pitting and crevice corrosion: minimised by selecting suitable filler material and designing to avoid media traps.
• Formation of brittle phases (sigma): avoid prolonged heating in the 600–900 °C range; control thermal cycles and consider solution annealing.
• Insufficient cleaning and surface preparation: cleanliness of the weld and removal of contaminants prevents inclusions and weld weakening.

Good practice includes weld quality control (penetrant testing, radiography, UT) and welding procedures compliant with standards.

1.4438 is used where a combination of chemical resistance, durability and weldability is required. Below are example industries and applications.

Chemical and petrochemical industry

• Process pipelines, tanks and apparatus in contact with acids (especially when chlorides and sulphides are present), heat exchangers and pump components.
• Equipment for transport of aggressive liquids where resistance to pitting and crevice corrosion is required.

Food and pharmaceutical industry

• Processing components requiring cleanliness, easy cleaning and resistance to aggressive cleaning agents: pipes, tanks, fittings.
• In pharmaceuticals and food production hygiene standards often mandate use of stainless steels with low impurity levels and good weldability.

Energy and marine technologies

• In energy: heat exchangers, chemical installations in steam circuits, components in environments with sulphur compounds.
• In marine technologies: coastal and offshore installation components, e.g. certain fittings and equipment, although in very aggressive marine conditions specially alloyed superduplex or superaustenitic grades may be chosen.

Construction and sanitary equipment

• Sanitary elements, fittings, parts of valves and water treatment items; a good surface appearance combined with durability makes 1.4438 suitable for demanding applications.

Selection of 1.4438 should be based on analysis of the working environment, temperature, presence of chloride ions and budgetary requirements.

Comparing with other popular grades helps understand the advantages and limitations of 317L.

Comparison with AISI 304 and 316 – strength and resistance

• Compared with AISI 304:

– 1.4438 has significantly better resistance to pitting and crevice corrosion thanks to molybdenum.
– 304 is cheaper and often adequate in less aggressive environments.

• Compared with AISI 316 / 316L:

– 317L typically has higher molybdenum and nickel contents (depending on specification), which translates into better resistance in acidic environments and against chloride attack.
– 317L is usually more expensive than 316L due to higher molybdenum and nickel content.

The choice between 316L and 317L depends on the specific medium: in highly aggressive environments 317L performs better, but in many applications 316L remains an economical and sufficient alternative.

Advantages and limitations compared with ferritic and martensitic steels

• Ferritic (e.g. 430): cheaper, good oxidation resistance, but poorer ductility, worse weldability and generally lower corrosion resistance in many environments than austenitics. Not suitable where high ductility and impact resistance are required.
• Martensitic (e.g. 410, 420): can be heat treated to high strength and hardness, but at the cost of corrosion susceptibility and toughness; used in tools and blades, not where chemical resistance is expected.
• Austenitic (e.g. 317L): provide the best compromise of corrosion resistance, ductility and weldability, although more expensive.

Even the best materials require attention in service. Knowing hazards and applying proper maintenance extend the life of installations.

Factors accelerating corrosion and protection methods

Factors accelerating corrosion:

• presence of chloride ions and temperatures that increase environmental aggressiveness,
• stagnation of fluids promoting crevice formation and local deoxygenation,
• surface contamination (weld residues, steel swarf),
• galvanic contact with materials of different potentials (e.g. copper, carbon steel) in the presence of an electrolyte.

Protection methods:

• design without crevices and liquid traps,
• passivation (e.g. nitric acid pickling) after machining and welding,
• use of inhibitors and anti-corrosion agents in the process fluid,
• cathodic protection where applicable,
• regular cleaning and removal of contaminants.

Surface care and cleaning rules

• Regular washing with water and a neutral-pH detergent removes deposits and biofilm.
• Avoid strongly alkaline or chlorinated cleaners without consultation, as some may increase chloride aggressiveness.
• After welding and mechanical processing passivation and pickling are recommended to restore the passive layer.
• Removal of steel swarf and ferromagnetic contaminants — e.g. dry brushing — prevents localised corrosion sites.

Monitoring condition of the steel and damage prevention

• Regular visual inspections and thickness measurements (ultrasonic),
• Electrochemical tests (e.g. potential measurements, pitting tests),
• Maintaining material records and monitoring operational parameters (temperature, concentrations),
• Corrosion coupons and corrosion baskets to assess the working environment in real time.

These actions help detect early signs of degradation and take remedial measures before critical damage occurs.

Steel technologies continue to evolve to meet rising environmental, economic and technological demands.

Alloy composition modifications for better sustainability

• Searching for solutions with reduced nickel content (to lower costs and reliance on raw material markets) and alternative austenite stabilisers (e.g. nitrogen).
• Developing “lean” alloy versions and new combinations of molybdenum and nitrogen to retain corrosion resistance with lower shares of expensive elements.
• Focusing on alloys with lower environmental impact across the life cycle — from melting to recycling.

Use of advanced surface treatment methods

• PVD coatings, ceramic sputtering and plasma technologies improve surface resistance and wear properties.
• Electropolishing and passivation using optimised (environmentally safer) processes increase component longevity.
• Laser and plasma surface treatments to modify surface state and improve resistance to pitting and corrosion fatigue.

Trends in processing and recycling of stainless steels

• Growing importance of stainless steel recycling: these materials are among the most recycled metal alloys, aligning with circular economy goals.
• Melting technologies using scrap and scrap segregation allow maintaining alloy quality with lower consumption of primary raw materials.
• Development of additive manufacturing (metal 3D printing): although 316L is most commonly used in metal printing, progress in powders and process parameters opens the way for use of alloys similar to 317L in specialist applications.

These innovations lead to materials better adapted to the complex challenges of modern industrial service.

1.4438 X2CrNiMo18-15-4 (AISI 317L) is an example of a well-balanced engineering solution: it combines chemical resistance, ductility, good weldability and machinability, making it suitable across many industries. Facing increasing demands for installation durability and environmental protection, alloys such as 317L remain an important element of engineers’ material toolkit.

The future of this type of steel is linked to further composition optimisation, development of surface processing technologies and integration with recycling and low-emission production processes. As engineers tackle ever more specialised tasks, 1.4438 and related grades will be adapted — sometimes replaced by alloys with even more specialised properties, other times modified to meet new challenges. In practice, material selection remains contingent on understanding working conditions, corrosion resistance requirements and cost constraints; 317L often proves to be a sensible and durable choice.