1.4570

Austenitic steels are a subgroup of stainless steels in which the room-temperature structure is austenitic — similar to the γ iron phase. This austenite stability is achieved mainly by appropriate nickel content and alloying additions, which results in characteristic features: good ductility, resistance to hydrogen embrittlement and high corrosion resistance in many conditions.

Basics of the microstructure of austenitic steels

• Austenite (γ-Fe) — a face-centred cubic (FCC) lattice, which makes these materials more ductile than ferritic or martensitic alloys.
• Austenite stability at low temperatures is obtained by the addition of nickel and other elements (e.g. nitrogen), which lower the transformation temperature.
• In practice, the austenitic microstructure is not hardened by conventional quenching and tempering, so these steels are not used to obtain high hardness by heat treatment; their strengthening is achieved by cold work.

This structure brings tangible benefits: resistance to corrosion in oxidising environments, excellent ductility and machinability, making austenitic steels popular in the food, chemical and valve industries.

Features of acid resistance and corrosion resistance

Acid resistance and general corrosion resistance stem primarily from the presence of chromium, which forms a thin, self-healing layer of chromium oxides on the surface — passivation. In austenitic steel the interaction of chromium with nickel and other additions (e.g. copper) determines the ability to resist:

• atmospheric rusting,
• corrosion in oxidising environments,
• pitting corrosion in chloride environments (here molybdenum, nitrogen and microstructure have a decisive influence).

Steels from the 18/8 group, to which 1.4570 belongs, offer universal resistance in many conditions, although in strongly reducing environments or those containing chlorides their resistance is lower than that of grades with molybdenum (e.g. 316).

Origin and standard designations of steel 1.4570

The designation 1.4570 comes from the European numbering system for stainless steels. Alternative names are:

• X6CrNiCuS18-9-2 — classic notation by composition: X6 (approx. 0.06% C), Cr 18%, Ni 9%, added Cu and S (sulfur) added to improve machinability,
• AISI 303Cu — the American designation for this copper-alloyed version of 303.

The grade was developed as a variant of the popular 303 stainless steel, which, thanks to additions of copper and controlled sulfur content, improves machinability and in certain chemical environments shows better resistance.

Historically, the development of 303-type steels and their variants dates back to the first half of the 20th century, when industry required grades easy to machine, intended for complex parts with threads, screws and precision machine elements. The addition of sulfur as an element improving chip breakage is an engineering idea that revolutionised mass production of small stainless-steel items.

Differences between X6CrNiCuS18-9-2 and other stainless steels

1. Machinability — X6CrNiCuS18-9-2 contains sulfur and copper, which significantly improve chip breakage and the dynamics of turning, milling and threading compared with, for example, 304, which is harder to machine.
2. Corrosion resistance — compared with 304 and 316, 1.4570 has similar atmospheric resistance but lower resistance than 316 in chloride environments due to the lack of molybdenum.
3. Weldability — the presence of sulfur worsens welding properties; 1.4570 is not recommended where extensive welding is required without affecting the structure and corrosion resistance.
4. Applications — the choice of 1.4570 is motivated mainly by cost and the need for high machining efficiency rather than maximal corrosion resistance.

For grades such as 1.4570 it is important to look at each alloying element, because even small amounts of elements added for technological reasons change the material’s behaviour.

Typical composition (approximate values, dependent on standard and manufacturer):

• Carbon (C): ~0.06% (the X6 symbol means approx. 0.06% C)
• Chromium (Cr): ~17–19%
• Nickel (Ni): ~8–10%
• Copper (Cu): ~0.5–2% (in the 303Cu variant)
• Sulfur (S): ~0.15–0.35% (to improve machinability)
• Manganese (Mn), Silicon (Si), Phosphorus (P) — small amounts, below 1–2% depending on specification

Below is a discussion of the roles of the main elements.

Chromium content and its effect on anti-corrosion properties

Chromium is the central element of stainless steels. At about 17–19% it forms a passive layer of chromium oxides that:

• provides resistance to atmospheric and chemical corrosion in oxidising environments,
• limits the rate of oxidation and formation of rust stains,
• regenerates spontaneously after damage, provided the environment is not too aggressive.

In practice, 18% Cr is a compromise between resistance and good ductility; this is why grades of this class are widely used as construction materials in the food, medical and chemical industries.

The role of nickel and copper in the structure and resistance of the steel

• Nickel (Ni) — an austenite stabiliser. Its presence maintains the FCC structure over a wide temperature range, which results in excellent ductility, impact resistance and lack of hardenability. Nickel also affects corrosion resistance, especially in acidic environments.
• Copper (Cu) — the introduction of copper in the 303Cu grade has a twofold significance:

– it improves resistance to certain acids, particularly dilute sulphuric acid, and increases resistance in reducing environments,
– it contributes to improved machinability and can slightly modify the alloy’s structure. Copper, however, does not replace nickel or chromium in the key functions for corrosion resistance.

The importance of sulfur addition and its effect on machining

Sulfur added to 1.4570 occurs as sulfide inclusions (mainly MnS), which act as “break points” in the metal. Because of this:

• chips during turning and milling break into short segments, which facilitates production of threads and external parts;
• cutting forces on tools are reduced, which increases machining speeds and efficiency in mass production.

The cost of this benefit is reduced ductility and impact toughness in some conditions and worsened weldability and local corrosion resistance, since sulfides create local anodic sites in electrochemical corrosion processes.

Mechanical properties depend on the material condition: annealed, cold-worked or partially hardened. Below is a description of typical characteristics and their practical significance.

Tensile strength and impact toughness

• Tensile strength — in the annealed condition 303-type austenitic steels exhibit moderate strength, typically in the range of several hundred MPa. Strengthening by cold work can significantly raise yield strength and tensile strength.
• Impact toughness — sulfur addition lowers impact toughness compared with standard 18/8 grades, especially in zones with inclusions. However, in normal applications 1.4570 retains sufficient resistance to cracking under typical dynamic loads.

Exact mechanical values should be read from supplier material data sheets, since they depend on rolling, annealing and the degree of cold work.

Ductility and susceptibility to plastic forming

• Austenitic steels are characterised by excellent ductility; despite the presence of sulfur 1.4570 remains amenable to bending, pressing and forming, although local inclusions can affect deformation uniformity.
• For parts requiring high ductility and a smooth surface it is worth considering finishing operations and appropriate control of the pressing process to avoid defects forming around sulfide inclusions.

Wear resistance and environmental factors

• Wear resistance of 1.4570 is moderate; in mechanical applications exposed to high friction it is advisable to use coatings or material pairings with higher hardness.
• In chemical environments the steel shows good resistance in the absence of chlorides and strong reducing agents; in the presence of chlorides it is better to use grades with molybdenum (e.g. 316).

Knowledge of processing specifics allows full use of this steel’s advantages, especially in the mass production of small and medium-sized parts.

Cutting, welding and forming

• Cutting: Thanks to good machinability, parts made from 1.4570 can be cut quickly using standard turning and milling tools. For best results use carbide tools and cooling to reduce heating and tool wear.
• Welding: Weldability is limited by the presence of sulfur. Welding performed without appropriate procedures can lead to brittleness in the heat-affected zone and reduced corrosion resistance. Practical recommendations:

– avoid welding if possible; design mechanical joints,
– if welding is necessary, use suitable techniques: choose appropriate filler materials (e.g. 308/309 wires), limit arc temperature and control cooling,
– after welding carry out passivation treatments and surface quality checks.

• Forming: Good ductility allows bending and pressing. For thin-walled parts take into account the risk of local cracking at points of concentration of inclusions.

Heat treatment and its effect on properties

• Austenitic steels such as 1.4570 are not hardened in the classic sense; heat treatments are used mainly to remove work hardening and restore structure:

– Solution annealing in the range 1000–1120°C, then rapid cooling (e.g. in water) to prevent carbide precipitation at grain boundaries.
– Stabilisation — not always necessary in low-carbon grades, but used where intergranular corrosion prevention is required.

• Heat treatment affects hardness, ductility and corrosion resistance; therefore in mass production it is important to maintain repeatable procedures.

Specialist processing techniques for corrosion-resistant materials

• Electropolishing and chemical passivation — improve surface corrosion resistance by removing contaminants and smoothing micro-topography.
• Protective coatings — in mechanical or chemical applications ceramic coatings, PVD or organic coatings are used to improve wear resistance and aesthetics.
• Cold work with controlled deformation — allows higher strength parameters without losing ductility, but requires precise control to avoid cracking due to inclusion concentration.

Comparisons help translate chemical data into practical design decisions.

AISI 304 and 316 – differences in composition and applications

• AISI 304 (EN 1.4301):

– composition: approx. 18% Cr, 8% Ni,
– advantages: excellent versatility, good weldability, wide use in kitchens, fittings and decorative elements,
– limitations: more difficult to machine than 303/303Cu.

• AISI 316 (EN 1.4401 / 1.4404):

– composition: approx. 16–18% Cr, 10–14% Ni, 2–3% Mo,
– advantages: better resistance to chloride pitting and corrosion in marine and chemical environments,
– limitations: more expensive, harder to machine than 303Cu.

• 1.4570 (AISI 303Cu):

– composition: similar to 304, with added sulfur and copper,
– advantages: excellent machinability, appropriate resistance in general environments,
– limitations: poorer weldability and lower resistance in chloride environments than 316.

In practice the choice between these grades is a compromise: 1.4570 for economical mass production of machine elements, 316 where strong chloride corrosion is present, and 304 where good weldability and general corrosion resistance are required.

Use of sulfur additions in AISI 303Cu versus other grades

Sulfur as a technological addition is also present in other grades to improve machinability, but its presence is a compromise. By comparison:

• 303/303Cu offers the highest machinability without special coatings and processes.
• 304/316 without sulfur require slower machining and more advanced tooling.
• In applications where aesthetics, weldability and surface corrosion resistance are important, non-sulfur grades are a better choice, and 1.4570 appears where production efficiency dominates.

Grade 1.4570 finds wide practical use where fast and inexpensive production of complex stainless-steel parts is needed.

Automotive industry and precision engineering

• Fasteners and screws — 1.4570 is often chosen for the production of threaded parts, screws and nuts where precision and rapid machining are priorities.
• Shafts and bushings used in mechanisms with moderate loads, where resistance to atmospheric corrosion is important but harsh chloride conditions do not occur.
• Precision parts in electronic devices and mechanisms — due to ease of turning and low tendency to create burrs.

Corrosion-resistant fittings and chemical installations

• 1.4570 is used in valves, connectors and components of installations, particularly where good machinability and resistance in environments not aggressive to chlorides are required.
• In installations for transporting non-aggressive or mildly aggressive media (without high chloride concentrations) it is used for its favourable price-to-performance ratio.

Consumer goods and food-industry applications

• Small domestic appliances, parts of catering equipment and kitchen accessories produced in series thanks to ease of machining.
• In food applications an advantage is resistance to typical detergents and cleaning agents, however with strongly acidic products or those containing halides it is necessary to apply appropriate passivation procedures and quality control.

Working conditions define the long-term durability of components made from 1.4570; key factors are the presence of chlorides, operating temperatures and mechanical loads.

Corrosion resistance in acidic and alkaline environments

• In reducing acidic environments steel with copper addition may show better resistance than standard 303, but everything depends on concentration and temperature of the medium.
• In alkaline and neutral environments the steel retains good resistance provided aggressive anions (e.g. Cl-) are absent.
• In practice the decision to use 1.4570 should take into account the specific medium, temperature and exposure time.

Pitting and stress corrosion phenomena – prevention and monitoring

• 1.4570, like other austenitic steels without molybdenum, is more susceptible to chloride pitting in the presence of tensile stress and elevated temperature.
• Prevention:

– avoid chloride-rich environments or use higher-resistance materials (e.g. 316),
– apply passivation and electropolishing of surfaces,
– control stresses by thermal relaxation or appropriate design.

• Monitoring:

– non-destructive testing (e.g. penetrant, ultrasonic),
– regular inspections of critical points,
– analysis of the composition and cleanliness of the service environment.

Industrial use requires compliance with standards and quality control procedures.

European and international standards

• EN 10088 — the series of standards for stainless and acid-resistant steels; 1.4570 appears in steel product catalogues as a grade defined by composition and properties.
• International and American equivalents: AISI 303Cu, UNS S30300 (or similar catalogues), ASTM may contain specifications for bars, pipes and plates.
• Standards cover both chemical composition and mechanical requirements as well as test methods.

Quality requirements and verification tests

• Chemical composition tests (spectroscopy),
• Mechanical tests: tensile, impact, hardness,
• Corrosion tests: pitting tests, tests in chloride solutions,
• Weldability tests and inspection of heat-affected zones,
• Surface inspections and passivation requirements.

Suppliers and manufacturers should provide material certificates compliant with standards, confirming batch conformity with requirements.

Material development proceeds in two main directions: improving functional properties and balancing environmental impact and cost.

New alloying additions and their effect on material properties

• Nitrogen (N) as an alternative to part of the nickel: increases strength and corrosion resistance while reducing content of expensive elements.
• Controlled additions of copper and other microelements allow tailoring resistance in specific media (e.g. acids) without significantly degrading other properties.
• Development of sulphur-free grades with comparable machinability through innovative metallurgical techniques, which may in the future reduce the need for sulphide inclusions.

Trends in ecological processing and recycling of stainless steel

• Stainless steel is a highly recyclable material; emphasis is placed on recovery and reuse with minimal loss of quality.
• Future alloy designs consider lower content of critical raw materials, energy required for production and CO2 emissions.
• Use of digital technologies (Industry 4.0) to monitor production and quality control reduces waste and improves repeatability of alloy parameters.

• Characteristics: 1.4570 (X6CrNiCuS18-9-2, AISI 303Cu) is an austenitic steel with copper and sulfur additions, designed for excellent machinability while maintaining the corrosion resistance typical of 18/8 grades.
• Composition: approx. 17–19% Cr, 8–10% Ni, small amounts of C, Mn, Si, with additions of Cu and S; composition may vary depending on standard and manufacturer.
• Mechanical properties: good ductility and moderate strength, can be strengthened by cold working; lower impact toughness than non-sulphur grades.
• Processing: excellent machinability due to sulphide inclusions; weldability is limited and requires care; heat treatment limited to solution annealing and stabilising treatments.
• Applications: fasteners, screws, precision parts, valves, domestic and industrial appliance components where rapid, economical production with atmospheric and moderate corrosion resistance is a priority.
• Limitations: lower resistance to chloride pitting than molybdenum-bearing grades (e.g. 316), worsened weldability and reduced resistance in very aggressive environments.
• Prospects: development of grades using nitrogen, optimisation of composition for better ecology and recycling, and improvement of processing methods that may minimise the compromises introduced by sulphur addition.

Steel 1.4570 is an example of a practical compromise: an engineering trade-off in which production economy, ease of machining and sufficient corrosion resistance come together to enable efficient manufacture and durable use in industrial and consumer applications.