1. Basic Concept of Stainless Steel

Stainless steel is an alloy steel based primarily on iron and containing chromium, nickel, molybdenum, carbon, nitrogen, and other alloying elements to control corrosion resistance and mechanical properties.

The most fundamental characteristic of stainless steel is the thin and stable oxide film formed by chromium. This film is generally called a passive film, and when damaged, it can reform in the presence of oxygen.

Compared with ordinary carbon steel, stainless steel generally offers superior:

• Corrosion resistance

• Oxidation resistance

• Hygiene

• Surface cleanability

• Surface appearance

• Suitability for low- or high-temperature environments

• Long-term maintainability

However, not all stainless steels have the same properties.

The characteristics of stainless steel vary according to:

• Chromium content

• Nickel content

• Carbon content

• Molybdenum content

• Addition of nitrogen, titanium, niobium, and other elements

• Microstructure

• Heat treatment condition

• Degree of cold working

• Welding condition

• Surface treatment condition

Stainless steels commonly used in mechanical design can generally be divided into:

1. Austenitic stainless steel

2. Martensitic stainless steel

3. Ferritic stainless steel

These three groups differ not only in chemical composition, but also in the crystal structure retained at room temperature. Therefore, their mechanical properties, magnetism, heat treatment response, weldability, and machinability are also significantly different.

2. Difference Between Microstructure Names and Material Names

To understand stainless steel correctly, it is important to distinguish between a microstructure name and a material classification.

For example, ferrite originally refers to a type of crystal structure found in iron and steel. However, ferritic stainless steel is not the same material as ferrite in pure iron or low-carbon steel.

2.1 Meaning of Ferrite

At room temperature, pure iron has a body-centered cubic crystal structure, commonly called α-ferrite.

Ferrite generally has the following characteristics:

• Magnetic

• Relatively soft

• Very low carbon solubility

• Relatively good ductility and workability

• Difficult to harden significantly by heat treatment

• May show a ductile-to-brittle transition at low temperature

Ferrite is also one of the principal microstructures found in ordinary low-carbon steel.

2.2 Meaning of Ferritic Stainless Steel

Ferritic stainless steel is based on the same BCC crystal structure as ferrite in pure iron, but it contains approximately 10.5% to 30% chromium and may also contain molybdenum, titanium, niobium, and other alloying elements.

Therefore, ferritic stainless steel should be understood as:

A BCC iron alloy containing a high amount of chromium in solid solution.

In other words, its microstructure is ferritic, but the material itself is not pure iron.

When chromium atoms dissolve into the iron crystal lattice, they distort the lattice. This increases strength and hardness compared with pure iron. At the same time, chromium forms a passive film on the surface and improves corrosion resistance.

2.3 Difference Between Ferrite in Pure Iron and Ferritic Stainless Steel

Therefore, it is inaccurate to describe ferritic stainless steel simply as “a weak stainless steel close to pure iron.”

3. Major Alloying Elements and Their Effects

3.1 Chromium

Chromium is the most important element in stainless steel.

Main functions:

• Formation of a passive film

• Improvement of general corrosion resistance

• Improvement of oxidation resistance

• Improvement of high-temperature oxidation resistance

• Stabilization of ferritic structure

• Solid-solution strengthening

• Carbide formation

As chromium content increases, corrosion resistance and oxidation resistance generally improve.

However, chromium is also a strong ferrite stabilizer. Therefore, if nickel is insufficient, a ferritic structure tends to form.

3.2 Nickel

Nickel is a major austenite-stabilizing element.

Main functions:

• Stabilization of austenite at room temperature

• Improvement of ductility

• Improvement of toughness

• Improvement of low-temperature properties

• Improvement of weldability

• Improvement of formability

• Promotion of non-magnetic behavior

The excellent ductility and weldability of 304 and 316 stainless steels are largely due to nickel.

3.3 Molybdenum

Molybdenum is especially important in chloride-containing environments.

Main functions:

• Improvement of pitting corrosion resistance

• Improvement of crevice corrosion resistance

• Improvement of resistance to certain acidic environments

• Improvement of high-temperature strength

• Improvement of hardenability

316 and 316L are more suitable than 304 in saline environments because they contain molybdenum.

3.4 Carbon

Carbon strongly affects hardness and strength.

Main functions:

• Improvement of martensitic hardness

• Improvement of wear resistance

• Improvement of cutting performance

• Carbide formation

• Increase in tensile strength

Limitations:

• Reduction in ductility

• Reduction in impact toughness

• Reduction in weldability

• Increased risk of intergranular corrosion

• Increased cracking tendency

Carbon is beneficial in martensitic stainless steel, but in austenitic stainless steel it can reduce corrosion resistance in welded areas.

3.5 Nitrogen

Nitrogen contributes to austenite stabilization and solid-solution strengthening.

Main functions:

• Stabilization of austenitic structure

• Increase in yield strength

• Improvement of pitting corrosion resistance

• Partial replacement of nickel

3.6 Titanium and Niobium

Titanium and niobium are stabilizing elements that bind carbon and nitrogen.

Main functions:

• Suppression of chromium carbide formation

• Improvement of resistance to intergranular corrosion

• Improvement of weld-zone stability

• Stabilization of high-temperature microstructure

• Suppression of grain growth

These elements are important in ferritic grades such as 409 and 444.

4. Austenitic Stainless Steel

4.1 Definition

Austenitic stainless steel contains chromium and nickel as principal alloying elements and retains an austenitic microstructure at room temperature.

Representative grades:

• STS304

• STS304L

• STS316

• STS316L

• STS321

• STS310S

4.2 Microstructural Characteristics

Austenite has a face-centered cubic, or FCC, crystal structure.

Because the FCC structure provides many active slip systems at room temperature, austenitic stainless steel exhibits:

• High ductility

• High toughness

• Excellent formability

• Excellent weldability

• Low susceptibility to low-temperature brittleness

• Strong work hardening

• Generally non-magnetic behavior

4.3 STS304

Representative Chemical Composition

Relationship Between Composition and Properties

Chromium

The 18–20% chromium content forms a stable passive film and provides good corrosion resistance in ordinary water, food, humidity, and atmospheric environments.

Nickel

The 8–10.5% nickel content stabilizes the austenitic structure at room temperature.

As a result:

• Ductility is high.

• Toughness is good.

• Bending is easy.

• Press forming is possible.

• Weldability is good.

• Magnetism is generally very weak.

Low Carbon Content

The low carbon content contributes to good ductility and weldability. However, if the heat-affected zone remains within a certain temperature range for a long time, chromium carbides may precipitate.

Mechanical Characteristics

• Strength: Medium

• Ductility: Very high

• Toughness: Very high

• Hardness: Low to medium

• Wear resistance: Low to moderate

• Weldability: Excellent

• Formability: Excellent

• Work hardening: High

• Magnetism: Generally very weak

Main Applications

• Food-contact hoppers

• Tanks

• Piping

• Covers

• Frames

• Brackets

• Worktables

• Shafts

• Components exposed to wash water

4.4 STS304L

Representative Chemical Composition

Characteristics

304L has a lower carbon content to reduce chromium carbide precipitation during welding.

As a result:

• Better resistance to intergranular corrosion in welded areas

• Suitable for large welded structures

• Advantageous when post-weld heat treatment is difficult

• Similar formability and weldability to 304

Limitations

• Strength may be slightly lower than 304.

• Not suitable for parts requiring high hardness or wear resistance.

4.5 STS316

Representative Chemical Composition

Relationship Between Composition and Properties

Molybdenum

The 2–3% molybdenum content improves resistance to pitting and crevice corrosion.

Therefore, 316 is advantageous in:

• Saline environments

• Seafood processing

• Coastal environments

• Chloride-containing cleaning agents

• Certain acidic environments

• Structures where moisture remains for long periods

Nickel

The higher nickel content than 304 stabilizes the austenitic structure even with molybdenum present.

Mechanical Characteristics

• Strength: Medium

• Ductility: High

• Toughness: High

• Corrosion resistance: Very good

• Chloride resistance: Better than 304

• Work hardening: High

• Weldability: Excellent

• Magnetism: Generally weak

4.6 STS316L

Representative Chemical Composition

Characteristics

316L maintains the molybdenum-based corrosion resistance of 316 while reducing carbon content.

Main advantages:

• Excellent resistance to intergranular corrosion after welding

• Good corrosion resistance in chloride environments

• Suitable for hygienic piping

• Suitable for CIP systems

• Suitable for pharmaceutical and chemical equipment

5. Martensitic Stainless Steel

5.1 Definition

Martensitic stainless steel contains chromium and carbon as principal alloying elements and can achieve high hardness and strength through quenching and tempering.

Representative grades:

• STS410

• STS420

• STS420J2

• STS440C

5.2 Microstructural Characteristics

Martensite forms when austenite is rapidly cooled from high temperature.

It has a body-centered tetragonal crystal structure supersaturated with carbon. Because of the severe lattice distortion, martensitic stainless steel exhibits high hardness and strength.

Main characteristics:

• High hardness

• High strength

• High wear resistance

• Magnetic

• Heat-treatable

• Low ductility

• Limited weldability

• Requires caution under impact loading

5.3 STS410

Representative Chemical Composition

Composition and Properties

Carbon

Because the carbon content is relatively low, the maximum achievable hardness is lower than that of 420 or 440C, but toughness is relatively better.

Chromium

Provides basic corrosion and oxidation resistance.

Low Nickel Content

Because austenite is not stable at room temperature, martensite forms readily after quenching.

Mechanical Characteristics

• Strength: High

• Hardness: Medium to high

• Toughness: Relatively good among martensitic grades

• Corrosion resistance: Limited

• Weldability: Limited

• Magnetism: Present

Applications

• Shafts

• Pins

• Valves

• Pump parts

• Bolts

• Nozzles

5.4 STS420 / STS420J2

Representative Chemical Composition

Composition and Properties

The higher carbon content than 410 increases hardness and wear resistance after quenching.

Advantages:

• High cutting performance

• High surface hardness

• Good edge retention

• Good wear resistance

Limitations:

• Reduced ductility

• Reduced toughness

• Reduced weldability

• Increased cracking risk

• Lower corrosion resistance than 304

Applications

• Food cutting blades

• Cutters

• Scrapers

• Valve seats

• High-hardness shafts

• Surgical instruments

5.5 STS440C

Representative Chemical Composition

Composition and Properties

The high carbon and chromium content forms a large amount of hard carbides and a high-hardness martensitic matrix.

Main characteristics:

• Very high hardness

• Very high wear resistance

• High compressive strength

• Good edge retention

• Relatively good corrosion resistance among martensitic grades

Limitations:

• Low impact toughness

• High cracking sensitivity

• Heat-treatment distortion

• Difficult machining and grinding

• Very poor weldability

Applications

• Stainless bearings

• Precision blades

• Bearing balls

• High-hardness valve parts

• Wear pins

• Measuring tools

6. Ferritic Stainless Steel

6.1 Definition

Ferritic stainless steel uses chromium as its principal alloying element and contains very little or almost no nickel.

It retains a BCC ferritic microstructure at room temperature and is magnetic.

Representative grades:

• STS409

• STS430

• STS434

• STS444

6.2 Mechanical Position of Ferritic Stainless Steel

Ferritic stainless steel is neither as soft as pure iron nor as hard as martensitic stainless steel.

Its mechanical characteristics can be summarized as follows:

• Medium strength

• Low to medium hardness

• Lower ductility than austenitic grades

• Stiffness similar to ordinary carbon steel

• Low thermal expansion

• Higher thermal conductivity than austenitic stainless steel

• Magnetic

• Cannot be hardened by conventional heat treatment

• Limited weldability

• Reduced impact toughness at low temperature

Ferritic stainless steel can therefore be described as:

A magnetic stainless steel that balances the structural characteristics of carbon steel with the corrosion resistance of stainless steel, while providing economy and thermal stability.

6.3 Why Ferritic Stainless Steel Is Stronger Than Pure Iron

Ferritic stainless steel contains a large amount of chromium dissolved in the iron crystal lattice.

Because chromium atoms differ in size from iron atoms, they distort the lattice. This distortion impedes dislocation movement, increasing strength and hardness.

This mechanism is called solid-solution strengthening.

Therefore, ferritic stainless steel has higher:

• Yield strength

• Tensile strength

• Hardness

• High-temperature oxidation resistance

• Corrosion resistance

than pure iron.

6.4 Strength

Ferritic stainless steel is stronger than pure iron and very low-carbon steel.

General trend:

Ferritic stainless steel is more suitable as a sheet-metal, exterior, or thermally stable structural material than as a high-strength material for heavily loaded shafts or gears.

6.5 Hardness and Wear Resistance

Ferritic stainless steel is harder than pure iron, but cannot achieve high hardness through quenching.

Therefore, it is generally unsuitable for:

• Blades

• Gear tooth surfaces

• Bearing contact surfaces

• Punches

• Dies

• High-wear sliding parts

• High-hardness shafts

Ferritic stainless steel is used for its corrosion resistance, magnetism, low thermal expansion, and economy rather than for hardness.

6.6 Ductility and Formability

Ferritic stainless steel can be bent and press-formed, but its formability is lower than that of austenitic stainless steel.

This is because the BCC structure has less active slip behavior at room temperature than the FCC austenitic structure.

Practical tendencies:

• Simple bending: Good

• Shallow press forming: Possible

• Deep drawing: Less suitable

• Complex shapes: Increased cracking risk

• Small bending radii: Require caution

• Repeated bending: Requires caution

6.7 Toughness and Impact Properties

Ferritic stainless steel may lose toughness at low temperatures and in thick sections.

The BCC structure can undergo a ductile-to-brittle transition as temperature decreases.

Conditions requiring caution:

• Low-temperature environments

• Impact loading

• Thick plate

• Sharp notches

• Highly constrained structures

• Weld heat-affected zones

• Repeated thermal shock

Therefore, austenitic stainless steel is generally more suitable for cryogenic tanks or highly impact-loaded structures.

6.8 Elastic Modulus and Stiffness

The elastic modulus of ferritic stainless steel is similar to that of ordinary carbon steel.

For the same geometry and thickness:

• It deflects less than aluminum.

• It has bending stiffness similar to carbon steel.

• It differs only slightly from austenitic stainless steel and may be slightly stiffer.

However, the actual stiffness of sheet-metal covers depends strongly on:

• Sheet thickness

• Bent geometry

• Ribs

• Beads

• Support spacing

• Fastening positions

6.9 Fatigue Properties

Ferritic stainless steel can be used under ordinary repeated loading, but it is not considered an exceptionally high-fatigue-strength material.

Factors affecting fatigue life:

• Welds

• Notches

• Surface scratches

• Corrosion

• Press-forming marks

• Repeated thermal expansion

• Stress concentration

STS409 is used in automotive exhaust systems not because of very high static strength, but because of the combined advantages of:

• Low thermal expansion

• High-temperature oxidation resistance

• Stability under repeated thermal cycling

• Formability

• Economy

6.10 Thermal Expansion

Ferritic stainless steel has a lower coefficient of thermal expansion than austenitic stainless steel.

General trend:

• Carbon steel: Low

• Ferritic stainless steel: Low

• Martensitic stainless steel: Relatively low

• Austenitic stainless steel: High

This makes ferritic stainless steel suitable for:

• Exhaust pipes

• Heat shields

• Heat exchangers

• High-temperature covers

• Structures subjected to repeated heating

• Assemblies exposed to large temperature changes

6.11 Thermal Conductivity

Ferritic stainless steel has higher thermal conductivity than austenitic stainless steel.

Advantages:

• Reduced local heat concentration

• Potentially improved heat exchange

• Reduced temperature gradients

• Reduced thermal stress

However, it is not necessarily more thermally conductive than carbon steel or aluminum.

The accurate statement is:

Ferritic stainless steel has better thermal conductivity than austenitic stainless steel.

6.12 Heat Treatment Response

Ferritic stainless steel cannot generally be hardened by conventional quenching and tempering.

Reasons:

• Low carbon content

• It does not fully transform to austenite during heating

• Insufficient martensite forms during rapid cooling

Possible effects of heat treatment:

• Stress relief

• Recrystallization

• Softening

• Grain growth

• Adjustment of precipitates

Properties that are difficult to obtain:

• Blade-level hardness

• Bearing-grade hardness

• Gear-tooth hardness

• Martensitic-level wear resistance

6.13 Welded-Zone Characteristics

Ferritic stainless steel is prone to grain growth during welding.

Coarse grains may cause:

• Reduced toughness

• Lower impact resistance

• Increased brittleness

• Lower fatigue strength

• Increased cracking tendency

If carbon and nitrogen are high, carbides and nitrides may form and cause:

• Intergranular corrosion

• Weld-zone embrittlement

• Reduced ductility

• Reduced corrosion resistance

Methods used to improve performance include:

• Lower carbon content

• Lower nitrogen content

• Titanium addition

• Niobium addition

• Molybdenum addition

• Minimization of welding heat input

7. Representative Ferritic Grades

7.1 STS409

Representative Chemical Composition

Composition and Properties

Chromium

Provides basic high-temperature oxidation resistance and corrosion resistance.

Titanium

Binds carbon and nitrogen and improves weld stability and high-temperature performance.

Mechanical Characteristics

• Relatively low strength

• Good formability

• Low thermal expansion

• Magnetic

• Good resistance to thermal fatigue

• Limited appearance-related corrosion resistance

Applications

• Automotive exhaust systems

• Mufflers

• Heat shields

• High-temperature ducts

• Combustion equipment covers

7.2 STS430

Representative Chemical Composition

Composition and Properties

Chromium

The 16–18% chromium content provides suitable corrosion resistance in indoor and mildly corrosive environments.

Low Nickel

Lowers cost and maintains ferritic structure.

As a result:

• Magnetic

• Lower ductility than 304

• Lower thermal expansion

• Relatively economical

• Cannot be heat-treatment hardened

Mechanical Characteristics

• Strength: Medium

• Hardness: Low to medium

• Formability: Moderate

• Toughness: Lower than 304

• Thermal expansion: Low

• Magnetism: Present

• Corrosion resistance: Good in mild environments

Applications

• Exterior covers

• Kitchen appliances

• Home appliances

• Decorative panels

• Protective panels

• Dry-environment brackets

• Magnetically attachable covers

7.3 STS444

Representative Chemical Composition

Composition and Properties

High Chromium

Provides high general corrosion resistance and oxidation resistance.

Molybdenum

Improves resistance to pitting and crevice corrosion.

Low Carbon and Nitrogen

Reduces weld embrittlement and the risk of intergranular corrosion.

Titanium and Niobium

Stabilize carbon and nitrogen and improve:

• Weld-zone corrosion resistance

• Microstructural stability

• High-temperature performance

• Grain growth control

Mechanical Characteristics

• Strength: Medium

• Corrosion resistance: High among ferritic grades

• Thermal expansion: Low

• Stress-corrosion cracking resistance: Good

• Magnetism: Present

• Weldability: Better than 430

• Heat-treatment hardening: Not possible

Applications

• Heat exchangers

• Hot-water tanks

• Water-heating equipment

• Cleaning equipment

• Heat-resistant piping

• Corrosion-resistant structures

8. Overall Comparison of the Three Groups

9. Material Selection by Component

9.1 Tanks and Hoppers

Recommended materials:

• STS304

• STS304L

• STS316

• STS316L

Selection criteria:

• General food processing: 304

• Welded structures: 304L

• Saline environments: 316

• Saline and welded structures: 316L

9.2 Shafts

Recommended materials:

• STS304

• STS316

• STS410

• STS420

Selection criteria:

• General corrosion resistance: 304

• Saline environment: 316

• Strength and hardness: 410

• Higher wear and hardness: 420

9.3 Cutting Blades

Recommended materials:

• STS420J2

• STS440C

• Special stainless blade steels

Selection criteria:

• General cutting: 420J2

• High hardness and long life: 440C

• Balance of impact resistance and corrosion resistance: Special blade steel

9.4 Exterior Covers

Recommended materials:

• STS304

• STS430

304 is suitable when:

• Water washing is used

• Humidity is high

• Hygienic appearance is required

• Cleaning conditions are severe

430 is suitable when:

• The environment is dry

• Cost reduction is important

• Simple sheet-metal construction is used

• Magnetic attachment is required

• There is no direct exposure to wash water

9.5 High-Temperature Structures

Recommended materials:

• STS409

• STS430

• STS310S

409 is suitable when:

• Exhaust structures are involved

• Repeated heating occurs

• Heat shielding is required

• Economy is important

10. Application in Food Machinery

10.1 Food-Contact Areas

Commonly used materials:

• STS304

• STS304L

• STS316

• STS316L

Selection considerations:

• Salt content

• Acidic food

• Cleaning chemicals

• Moisture retention

• Welded zones

• Surface roughness

• Drainability

10.2 Cutting Areas

Commonly used materials:

• STS420J2

• STS440C

• Special stainless blade steels

Items to review:

• Target hardness

• Material being cut

• Impact load

• Blade thickness

• Regrinding

• Rust risk

• Drying after washing

10.3 Non-Contact Exterior Areas

Conditions suitable for 430:

• Dry environment

• No direct wash-water exposure

• Cost reduction

• Magnetism required

• Simple exterior panels

Conditions favoring 304:

• Wet environment

• Water washing

• Salt exposure

• Cleaning chemicals

• Long-term corrosion resistance

11. Drawing Specification Requirements

11.1 Material Designation

Examples:

• STS304

• STS316L

• STS420J2

• STS430

• STS444

11.2 Applicable Standards

Examples:

• KS D 3698 STS304

• JIS G 4303 SUS420J2

• ASTM A240 Type 316L

• ASTM A276 Type 440C

11.3 Heat Treatment

Martensitic examples:

• STS420J2, quenched and tempered, HRC 52–56

• STS440C, quenched and tempered, HRC 58–60

11.4 Surface Treatment

Examples:

• 2B

• BA

• No.4

• HL

• Ra 0.8 μm max.

• Weld polishing

• Pickling

• Passivation

12. Final Summary

Austenitic stainless steel is stabilized by nickel in an FCC austenitic structure.

This produces:

• High ductility

• High toughness

• Excellent weldability

• Excellent formability

• Generally weak magnetism

• No heat-treatment hardening

• Strong work hardening

Martensitic stainless steel contains chromium and carbon and forms martensite through quenching.

This produces:

• High hardness

• High strength

• High wear resistance

• Magnetism

• Heat-treatment hardening

• Low ductility

• Limited weldability

Ferritic stainless steel has the same basic BCC structure as ferrite in pure iron, but it is not the same material as pure iron.

A large amount of chromium is dissolved in the iron crystal lattice, producing:

• Higher strength and hardness than pure iron

• Better corrosion resistance than ordinary carbon steel

• Magnetism

• Low thermal expansion

• Better thermal conductivity than austenitic stainless steel

• Medium structural strength

• No heat-treatment hardening

• Limited weldability

• Possible loss of toughness at low temperature or under impact

Therefore, the three groups are best distinguished as follows:

• Corrosion resistance, hygiene, and weldability: Austenitic

• Hardness, cutting performance, and wear resistance: Martensitic

• Economy, magnetism, low thermal expansion, and high-temperature stability: Ferritic

Ferritic stainless steel should not be understood as a weak material similar to pure iron. It is more accurately described as a magnetic stainless steel that uses chromium solid-solution strengthening and passivation to achieve a balance of structural performance, corrosion resistance, thermal stability, and economy.