1.4105

Definition and classification of ferritic steels

Ferritic stainless steels are a group of iron-based alloys with a dominant ferrite microstructure (body-centred cubic, BCC), typically containing about 10.5–30% chromium and very low carbon content. They are magnetic, have good thermal conductivity and lower ductility compared with austenitic stainless steels. Their basic features:

• Microstructure: ferrite (BCC) in the as-supplied condition, with no retained austenite.
• Carbon content: low, which limits tempering effects and prevents formation of chromium carbides.
• Chromium: the key element responsible for forming the passive oxide layer.
• Magnetism: most ferritic grades are magnetic.

Historically, ferritic steels were developed in the mid-20th century as an attempt to obtain materials with good corrosion resistance while reducing cost compared with nickel-rich austenitic steels. Additions such as molybdenum or nitrogen have increased their resistance to pitting and to aggressive environments.

Characteristics of superferritic steels

The term “superferritic” refers to high-chromium variants of ferritic steels which, thanks to increased chromium content (typically above 20%) and additions of molybdenum and possibly nitrogen, achieve significantly improved corrosion resistance, including resistance to pitting and crevice corrosion. Features of superferritics:

• More robust passivation due to the high chromium content.
• Better resistance to halide attack (depending on Mo and N additions).
• Lower susceptibility to stress corrosion cracking in chloride-containing environments compared with austenitics.
• Technological challenges: greater risk of forming brittle phases (e.g. sigma) with improper heating and cooling; control of alloy cleanliness is critical.

Superferritic grades are used where resistance beyond that of standard ferritics is required while avoiding the cost and limitations associated with nickel-bearing austenitic steels.

Differences between ferritic and austenitic stainless steels

Differences between these two main groups of stainless steel are fundamental and influence material selection for specific applications:

• Microstructure: ferritic — BCC (magnetic); austenitic — FCC (non-magnetic).
• Composition: austenitics contain significant amounts of nickel and often higher chromium; ferritics typically contain no nickel.
• Corrosion resistance: austenitics are generally better in aggressive environments overall, but ferritics are resistant to stress corrosion cracking in chloride solutions.
• Heat treatment: ferritics do not harden by quenching to the same extent as carbon steels; heat treatment is mainly used to reduce stresses and for recrystallisation.
• Cost: ferritics are usually cheaper due to the absence of nickel.

Understanding these differences is fundamental to selecting the correct grade for operating requirements.

1.4105, also labelled X6CrMoS17, is a ferritic stainless steel made freely machinable by the addition of sulphur. The chemical composition and the role of individual elements determine its functional properties.

Principal alloying elements and their functions

A typical composition (indicative values, dependent on supplier and standard) for X6CrMoS17 might be:

• Carbon (C): approximately 0.04–0.08% — low content to limit chromium carbides and to improve resistance to intergranular corrosion.
• Chromium (Cr): approximately 16–18% — essential for forming and maintaining the passive oxide layer; influences corrosion resistance and oxidation tolerance.
• Molybdenum (Mo): small amounts, e.g. 0.3–0.8% — improves resistance to pitting and crevice corrosion in the presence of chloride ions.
• Sulphur (S): controlled content, typically in the range 0.02–0.15% — added as a machinability aid; forms sulphide inclusions that facilitate chip breaking.
• Silicon (Si) and manganese (Mn): minor additions up to about 1% — affect deoxidation and casting/strengthening behaviour.
• Phosphorus (P): trace, maximum around 0.04% — limited because it can cause embrittlement.

It should be emphasised that the above values are indicative. Exact specification should come from the supplier’s certificate in accordance with the relevant standards (e.g. EN).

Influence of chromium, molybdenum and sulphur on properties

• Chromium: the primary “stainless” element. It forms a passive Cr2O3 film that protects the steel from general corrosion. Higher chromium content improves resistance to oxidation and corrosive environments, but excessive chromium can increase the tendency to form intermetallic phases (e.g. sigma) if heat treatment is unfavourable.
• Molybdenum: important for pitting and crevice corrosion resistance, particularly in chloride-containing environments. Even small additions improve the PRE (pitting resistance equivalent) and increase durability in corrosive conditions.
• Sulphur: acts as a machinability aid — it forms sulphide inclusions which facilitate chip breaking during turning or milling. Unfortunately, sulphur reduces local corrosion resistance and can adversely affect mechanical properties at high temperatures.

The use of sulphur is a compromise: manufacturers obtain better machinability at the expense of some corrosion resistance. Therefore 1.4105 with sulphur is intended for applications where mechanical machining is important and corrosion requirements are not extreme.

Analysis of standards and specifications for 1.4105 X6CrMoS17

Major standards and documents specifying requirements for stainless steels include:

• EN 10088 — a series of European standards covering classification, chemical and mechanical requirements and the intended use of stainless steels.
• DIN — German material designations and detailed specifications; the symbol 1.4105 is a number in the DIN/Werkstoff catalogue.
• Supplier certificates — each lot should be supplied with documentation confirming chemical composition and mechanical properties.

When purchasing 1.4105 pay attention to declared S, Mo values and component tolerances, as they determine suitability for a given application.

Production of stainless steel is a complex process involving melting, refining, casting, rolling and finishing. For ferritic grades such as 1.4105, control of impurities and microstructure is important.

Melting and forming methods

Key production stages:

• Melting: most commonly in electric arc furnaces (EAF) with refining in vacuum furnaces or converters. Control of carbon content and residual non-metals is important.
• Refining and degassing: processes that reduce dissolved gases and control sulphur and phosphorus levels.
• Casting: continuous casting into slabs, blooms or billets. The quality and cleanliness of castings influence final properties.
• Hot rolling (and cooling): forming semi-finished products; rolling parameters affect grain size and inclusion distribution.
• Finishing rolling and shaping: cold rolling achieves required thicknesses and improves surface finish.

Control of microstructure and alloy cleanliness is critical, particularly for steels containing Mo and S. Appropriate cooling parameters limit the formation of undesirable intermetallic phases.

Heat treatment and its effect on microstructure

Ferritic steels are not heat-treated in the same way as carbon steels to obtain hardening, but heat treatment plays important roles:

• Tempering/normalising annealing: used to reduce stresses, obtain a uniform microstructure and improve ductility.
• Stress-relief annealing after cold working: restores plasticity after plastic deformation and removes internal stresses.
• Avoiding temperature ranges that promote brittle phases: prolonged exposure to temperatures around 350–900°C can lead to precipitation of intermetallic phases (e.g. sigma), worsening ductility and corrosion resistance. Therefore heat treatments and welding are carefully controlled.

A key issue is preserving the ferritic microstructure without excessive grain growth or formation of unwanted phases.

Mechanical processing technologies – challenges and advantages

Thanks to its sulphur content, 1.4105 is relatively easy to machine. Using appropriate parameters enables efficient production of precision components. Practical aspects:

• Turning, milling, drilling: correct tool geometry and cooling reduce material welding to the tool; sulphur helps chip breaking.
• Bending and cold forming: ferritics have lower ductility than austenitics, which requires careful procedures and larger bend radii.
• Surface finishing: polishing and decorative finishes are possible, but sulphide inclusions may affect surface smoothness.

Advantages include good machinability and dimensional stability at elevated temperatures compared with austenitic steels.

Understanding mechanical and physical properties is essential for designing components from 1.4105.

Tensile strength and hardness

Typical mechanical values for ferritic steels (with exact specifications depending on delivery condition):

• Yield strength (Rp0.2): on the order of 200–350 MPa (depends on cold working and annealing).
• Tensile strength (Rm): around 400–700 MPa depending on material condition and processing.
• Elongation at break (A5): usually 15–30% for ferritic grades in the as-produced or annealed condition.
• Hardness: values on the HB or Rockwell scales depend on processing; generally moderate, enabling machining.

In practice these properties should always be verified against the delivery certificate and mechanical tests of the batch.

Corrosion resistance and environmental factors

• General corrosion resistance: good in alkaline and neutral conditions due to chromium. Sulphur lowers localised resistance.
• Pitting resistance: improved by molybdenum, but lower than the best austenitics with high Mo and N additions.
• Stress corrosion cracking (SCC): ferritics are less susceptible to SCC in chloride environments than austenitics.
• High-temperature resistance: good up to certain temperatures, but with a risk of increasing brittle phase formation if specific temperature ranges are exceeded.

In practice 1.4105 is used where corrosion requirements are not extreme while good machinability is desired.

Thermal conductivity and linear expansion

Ferritic stainless steels have better thermal conductivity than austenitics and a lower coefficient of thermal expansion. For example:

• Thermal conductivity: typically higher than that of austenitics, which facilitates heat dissipation during processing and service.
• Coefficient of linear expansion: lower than austenitics, resulting in better dimensional stability under temperature cycles.

These properties make ferritics desirable where dimensional stability with temperature change and efficient heat dissipation are important.

1.4105 is used across many industries, particularly where a compromise between corrosion resistance and good machinability is required.

Food and chemical industry

The food and chemical industries require materials that meet hygiene requirements and resist chemical agents. 1.4105 is used for:

• parts of packaging and food-processing machinery where ease of machining and reasonable resistance to neutral environments are needed;
• components not in direct contact with aggressive chemicals or for short-term contact with salt and low-concentration acids.

Note that sulphur reduces corrosion resistance, so for exposure to aggressive solutions this is not the optimal choice.

Manufacture of catering equipment and pipelines

Because it combines general corrosion resistance and good machinability, 1.4105 is used in:

• catering equipment components not exposed to intensive salt or chloride attack;
• pipeline components for less aggressive media;
• decorative and structural elements in equipment where material economy and ease of manufacture are important.

In practice manufacturers choose 1.4105 where cost offsets more expensive steel grades and corrosion demands are moderate.

Other sectors using ferritic steels

• Automotive: trims, decorative and functional parts, exhaust systems in some applications.
• Domestic appliances: housings and internal parts where temperature resistance and aesthetics are important.
• Construction: fittings, facade elements, profiles not exposed to extremely aggressive conditions.
• Energy and petrochemical: certain heat exchanger parts or pipes in less aggressive conditions; here superferritics or other Mo-rich grades are more commonly used.

Selection of 1.4105 is always the result of analysing cost, machinability and environmental conditions.

Advantages of superferritic steels over ferritic grades

Superferritic grades bring additional benefits due to increased chromium, molybdenum and sometimes nitrogen:

• Higher resistance to localised corrosion — especially pitting and crevice corrosion.
• Better resistance to chemical erosion in chloride-rich environments and seawater.
• Lower risk of stress corrosion cracking in chloride environments than with austenitics.
• Longer service life in harsher conditions, translating to lower maintenance costs.

However, superferritics can be more expensive and more demanding in production control, since higher alloy contents favour formation of brittle phases under poor heat treatment.

Use of superferritics in demanding conditions

Superferritic steels are applied where standard ferritics fail:

• Heat exchangers in marine/chemical installations — resistance to chlorides and salty vapour.
• Components in desalination plants — exposed to chloride ions.
• Structural elements in chemically aggressive installations, where long-term resistance is needed at a relatively lower cost compared with high-nickel austenitics.

Choosing superferritics is a strategic design decision where cost-effectiveness and corrosion are key factors.

Understanding corrosion mechanisms enables appropriate use of 1.4105 and prediction of its behaviour in a given environment.

Corrosion resistance mechanisms in ferritic steels

• Passivation: formation of a thin, compact chromium oxide (Cr2O3) film on the steel surface that limits further metal reaction with the environment.
• Resistance to stress corrosion cracking: ferritics, due to microstructure and low/no nickel content, are generally less prone to SCC in chloride environments.
• Influence of inclusions and additives: high alloy cleanliness and control of inclusions are key to maintaining resistance to localised corrosion — sulphide inclusions aid machinability but may, under certain conditions, initiate pitting.

These mechanisms interact; external factors such as temperature, aggressor concentration and the presence of mechanical stresses are also important.

Effect of acidic and alkaline environments on X6CrMoS17

• Acidic environments (e.g. acid solutions): depending on concentration and temperature, 1.4105 can show limited resistance; in the presence of strong acids and oxygen-consuming conditions, grades with higher Mo content or other stainless steel families are preferable.
• Alkaline environments: usually less aggressive for ferritics, although specific chemical combinations can present problems.
• Presence of chloride ions: a key risk factor for pitting; Mo additions help, but 1.4105 does not match the performance of the best superferritics or austenitics with high Mo and N.
• Temperature: increased temperature often accelerates corrosion processes and promotes precipitation of undesired phases; controlling operating temperature and avoiding prolonged exposure in critical ranges is necessary.

Selection of 1.4105 requires analysis of the specific medium and operating conditions, taking into account durability and maintenance plans.

Welding ferritic steels requires specific approaches to avoid embrittlement, formation of intermetallic phases and cracking.

Welding techniques suitable for ferritic steels

• MIG/MAG and TIG: the most commonly used techniques; selection of shielding gases and filler wires/electrodes is critical.
• Use of ferritic fillers or matched austenitic fillers: often fillers with composition close to the base material are used or, where improved ductility is needed, austenitic fillers. Choice depends on service requirements and the need to maintain corrosion resistance.
• Heat input control: limiting heat input is important; preheating is rarely used and excessive heating leads to grain growth and unwanted phase formation.

Good practice is to prepare welding procedures (WPS) based on qualification tests for the specific material and joint configuration.

Welding problems and minimisation strategies

• Formation of brittle phases (e.g. sigma): avoid prolonged exposure in temperatures that encourage precipitation; use rapid cooling and control interpass temperatures.
• Microstructural changes and HAZ brittleness: minimise heat input, use low heat input techniques and control heat flow.
• Heat-affected zone cracking: select fillers and welding parameters that reduce stresses; if necessary apply stress-relief treatments.
• Corrosion in the weld area: choose fillers that give similar or better corrosion resistance; clean and passivate welded surfaces.

Correct preparation and qualification of welding procedures are key to durable and safe assemblies.

Ensuring compliance with standards is important for safety, legality and predictable product quality.

European and international standards

• EN 10088 (series): regulates classification, chemical and mechanical properties of stainless steels. Requirements for ferritic grades are defined within this standard.
• DIN/Werkstoff: the number 1.4105 is used in material catalogues in countries with German material traditions and corresponds to a specific composition and usage.
• ISO: international standards supplementing and describing test methods.

Manufacturers and suppliers should provide declarations of conformity and material certificates (e.g. a 3.1 certificate in accordance with EN 10204) confirming chemical composition and mechanical properties.

Certificates confirming quality and intended use

• Material certificate: a detailed document containing results of chemical analyses and mechanical tests.
• Compliance with industry standards: e.g. for food applications hygienic requirements and certificates permitting contact with food.
• Audits and quality management systems: suppliers following ISO 9001 and other quality standards offer greater confidence in parameter repeatability.

Purchasers should require material documentation and specify required standards when placing orders.

Correct use and appropriate maintenance of ferritic steel structures extend their lifetime and minimise the risk of failure.

Operational and maintenance recommendations

• Regular inspections: visual checks, non-destructive testing in critical locations (e.g. welds, areas exposed to chlorides).
• Cleaning and passivation: removing contaminants and salt deposits; chemical passivation to restore or strengthen the Cr2O3 layer.
• Avoidance of stagnant water: standing water promotes pitting and crevice corrosion.
• Control of operating temperature: avoid prolonged exposure in ranges where intermetallic phases may precipitate.

Regular maintenance and proper protective measures significantly extend component life.

Common operational problems and prevention

• Pitting and crevice corrosion: use appropriate materials for the chemistry of the medium; design to eliminate crevices and provide suitable seals.
• Stress corrosion cracking: avoid combining high stresses with chloride environments; where possible use more resistant materials or reduce stresses via processing.
• Loss of aesthetics and staining: regular cleaning and removal of deposits, especially on decorative surfaces.

Prevention and careful design minimise the risk of failures and reduced service life.

Materials continue to evolve; development of new technologies and alloys creates opportunities to increase performance and durability.

Modern production and processing technologies

• Improved refining and degassing methods: reduce inclusions and impurities, enhancing corrosion resistance and strength.
• Precise microstructure control: advanced rolling and heat treatment techniques enable uniform and stable microstructures.
• Coatings and surface modifications: anti-corrosion, passivation and PVD/CVD technologies to increase resistance and service life.

Modern production processes allow development of ferritics with improved properties while preserving cost effectiveness.

Use of new alloys and protective coatings

• New superferritic variants with optimised composition (higher Cr, Mo additions and minimal N) improve pitting resistance.
• Functional coatings: enhance corrosion and wear resistance; allow ferritics to be used in environments previously reserved for more expensive steels.
• Composites and hybrid materials: combining a ferritic substrate with a chemically resistant coating or internal liner for specialised uses.

Innovations focus on raising the utilitarian value of the material while keeping its economic advantages.

Material selection is a decision process based on analysis of the operating environment, mechanical requirements, machining costs and durability. When considering 1.4105 bear in mind the following points:

• Nature of the medium: if the environment contains significant chlorides, consider a superferritic grade or a grade with higher Mo/N additions.
• Machining requirements: where high machinability is important, 1.4105 can be advantageous due to sulphur additions.
• Temperature conditions: avoid prolonged exposure in temperatures that promote precipitation of undesired phases; design welding and heat treatment processes with these limits in mind.
• Cost and availability: 1.4105 often offers an attractive cost/property balance, but each project should consider total life-cycle costs.
• Standards and certificates: request documentation confirming compliance with EN/DIN and the purchaser’s specification.

Selection should be a compromise between material properties and service requirements — a well-designed component in 1.4105 can combine economy and durability where its use is justified.

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The article presents 1.4105 X6CrMoS17 in the context of properties, production, applications and limitations. Knowledge of chemical composition, typical technological processes and behaviour in various operating conditions enables informed material selection for specific engineering tasks. Every design decision should be based on supplier documentation, environmental analysis and tests tailored to actual service conditions.