1. Basic Concept of Steel Materials
Steel materials are among the most widely used material groups in mechanical design.
They are used for most core machine components such as frames, bases, shafts, gears, pins, brackets, blades, molds, and wear parts.
Steel materials are widely used for the following reasons:
They provide excellent strength relative to cost.
They can be processed through machining, welding, bending, and heat treatment.
Standard specifications and supply chains are well established.
Strength, hardness, toughness, and wear resistance can be adjusted through heat treatment.
Corrosion resistance, wear resistance, and fatigue strength can be improved through surface treatment.
However, not all steel materials are used for the same purpose.
In mechanical design, the appropriate steel material must be selected based on the part function, load condition, friction condition, manufacturing method, heat treatment requirements, and cost.
2. Major Classification of Steel Materials
Steel materials frequently used in mechanical design can be broadly classified as follows.
General structural steel
Used for frames, bases, brackets, welded structures, and general structural parts.Carbon steel
Used for shafts, pins, rollers, cams, and general machine element parts.Alloy steel
Used for high-load shafts, gears, spindles, power transmission parts, and high-strength fastening parts.Tool steel
Used for blades, molds, punches, dies, wear plates, and high-hardness parts.
These four groups are all iron-based materials, but they differ in carbon content, alloying elements, heat treatment capability, strength and hardness level, and typical applications.
3. General Structural Steel
3.1 Definition of General Structural Steel
General structural steel is a type of steel that emphasizes basic strength, weldability, bendability, machinability, and economy rather than special high-strength or high-hardness properties.
In mechanical design, it is widely used for parts that support the overall structure, such as frames, bases, brackets, supports, and welded structures.
Representative materials include:
SS400
S235
S275
S355
ASTM A36
SM400
SM490
3.2 Main Characteristics of General Structural Steel
The greatest advantage of general structural steel is its good structural performance relative to cost.
Main characteristics:
Relatively low material cost
Available in various forms such as plates, sections, round bars, and square bars
Easy to cut, weld, bend, drill, and machine
Suitable for welded structures
Advantageous for large frames and base structures
Not suitable for achieving high hardness through heat treatment
Vulnerable to corrosion, so painting, plating, or rust prevention treatment is required
General structural steel should be understood as a basic material for economically constructing machine structures rather than as a high-performance material.
3.3 Characteristics of Representative Materials
3.3.1 SS400
SS400 is one of the most commonly used general structural steels in Korean and Japanese mechanical design practice.
Main characteristics:
Low cost
Easy to obtain
Good weldability
Easy to machine
Suitable for general frames, brackets, and bases
Main applications:
Machine frames
Welded bases
General brackets
Cover supports
Machine stands
Non-precision structural parts
Design considerations:
Additional review is required for parts with strict strength requirements.
It is not suitable for high-load shafts, gears, or pins.
Surface treatment is required in corrosive environments.
It is generally not used for food-contact areas in food machinery.
3.3.2 S355 / SM490 Series
S355 or SM490 series steels are used for structures that require higher strength than SS400.
Main characteristics:
Higher yield strength than SS400
Suitable for large structures and medium-load frames
Can be used for welded structures
Useful for reducing frame thickness or securing structural rigidity
Main applications:
Medium-load machine frames
Large bases
Structural parts of lifting devices
Brackets subjected to relatively high loads
Industrial equipment structures
Design considerations:
Welding conditions and post-weld deformation must be considered.
As thickness increases, weld quality control becomes more important.
Machinability and cost may be slightly less favorable than SS400.
3.4 Main Applications of General Structural Steel
General structural steel is suitable for the following types of parts.
Frames
Frames form the skeleton of a machine.
They distribute loads over a wide area and provide fixed mounting positions for each module.
Applicable materials:
SS400
S355
SM490
Reasons for application:
Good weldability
Low material cost
Large structures can be manufactured economically
Post-machining and painting are easy
Base Plates
Base plates support equipment and provide mounting surfaces for motors, reducers, bearing units, guides, and other components.
Applicable materials:
SS400
S355
Reasons for application:
Thick plates are readily available.
Milling, drilling, and tapping can be performed.
Good stiffness relative to cost.
Design considerations:
If precision is required, stress relief and surface machining may be necessary after welding.
Rust removal and surface preparation before painting are important.
Brackets
Brackets are used to mount sensors, covers, motors, cylinders, guides, and other components.
Applicable materials:
SS400
SPHC
S45C
STS304
Cases where general structural steel is used:
General fixing brackets with low load
Non-food-contact areas where painting is possible
Structural parts where cost reduction is important
Design considerations:
If the bracket is exposed to wash water in food machinery, STS304 should be reviewed first.
For thin sheet-metal brackets, bending stiffness must be considered.
3.5 Advantages and Limitations of General Structural Steel
Advantages:
Low cost
Easy availability
Good weldability
Suitable for large structures
Easy to machine and modify
Easy to apply surface treatments such as painting and plating
Limitations:
Low corrosion resistance
Difficult to achieve high hardness through heat treatment
Not suitable for precision, high-wear, or high-load machine elements
Rust occurs when surface treatment is damaged
Not suitable for food-contact areas where hygiene is important
4. Carbon Steel
4.1 Definition of Carbon Steel
Carbon steel is a steel material mainly composed of iron and carbon.
Compared with general structural steel, its strength and heat treatment capability as a machine component are more important.
Depending on carbon content, carbon steel can be classified as:
Low-carbon steel
Medium-carbon steel
High-carbon steel
In mechanical design, medium-carbon steels are used most frequently, with S45C being the most representative example.
4.2 Main Characteristics of Carbon Steel
Carbon steel can achieve higher strength and hardness than structural steel, and its mechanical properties can be adjusted through heat treatment.
Main characteristics:
Higher strength than general structural steel
Hardness and wear resistance can be increased through heat treatment
Suitable for machine element parts such as shafts, pins, rollers, and cams
Relatively good machinability
Lower cost than alloy steel
Low corrosion resistance
Weldability decreases as carbon content increases
4.3 Representative Carbon Steel Materials
4.3.1 S45C
S45C is the most widely used medium-carbon steel in mechanical design.
It is used for parts that require somewhat higher strength and wear resistance than ordinary structural parts.
Main characteristics:
Good balance between strength and machinability
Hardness can be improved after heat treatment
Suitable for shafts, pins, rollers, cams, and similar parts
Easy to obtain and relatively economical
Main applications:
General shafts
Pins
Rollers
Spacers
Cams
Gear blanks
Jig components
Rotating parts
Machined mechanical parts
Design considerations:
Surface treatment is required in corrosive environments.
For high-load gears, SCM-series alloy steel may be more suitable.
It is often not suitable for welded structures.
Deformation may occur after heat treatment.
For precision parts, grinding after heat treatment should be considered.
4.3.2 S50C
S50C has a slightly higher carbon content than S45C, allowing higher hardness and strength to be achieved.
Main characteristics:
Easier to obtain higher hardness than S45C
Wear resistance improves after heat treatment
Used for mold bases, plates, and machine parts
Main applications:
Plates
Mold bases
Machine parts exposed to wear
High-strength jigs
Cam components
Design considerations:
Machinability and weldability are somewhat less favorable than S45C.
Heat treatment deformation must be considered.
Toughness must be reviewed for parts subjected to high impact.
4.4 Heat Treatment of Carbon Steel
Carbon steel properties can be adjusted through heat treatment.
The heat treatments commonly considered in mechanical design are as follows.
Quenching
Quenching is a process in which the material is heated to a high temperature and then rapidly cooled to increase hardness.
Purpose:
Hardness improvement
Wear resistance improvement
Surface damage resistance improvement
Design considerations:
Deformation and cracking may occur.
Subsequent tempering is required.
Precision parts may require grinding after heat treatment.
Tempering
Tempering is performed after quenching to reduce brittleness and recover toughness.
Purpose:
Reduction of brittleness
Improvement of toughness
Relief of internal stress
Stabilization of mechanical properties
Design considerations:
The balance between hardness and toughness varies depending on tempering temperature.
If tempered at too high a temperature, hardness may decrease.
Induction Hardening
Induction hardening rapidly heats and cools only the surface to increase surface hardness.
Purpose:
Improvement of surface wear resistance
Maintenance of internal toughness
Surface strengthening of shafts, pins, and rollers
Main applicable parts:
Shafts
Pins
Rollers
Cams
Sliding contact areas
Design considerations:
Hardening depth must be specified according to design conditions.
Deformation after heat treatment must be considered.
Only selected contact surfaces can be hardened.
4.5 Main Applications of Carbon Steel
General Shafts
S45C is widely used for general shafts.
Reasons for application:
Torsional strength can be secured.
Machining is easy.
Keyways, threads, steps, and shoulders can be machined.
Surface hardness can be increased by induction hardening if necessary.
Design considerations:
In corrosive environments, plating, black oxide, or rust preventive oil is required.
For shafts exposed in food machinery, STS304 or STS420 should be considered.
Pins
Pins may be subjected to shear load, wear, and repeated impact.
Applicable materials:
S45C
SCM440
SUJ2
STS420
Reasons for applying S45C:
Good strength relative to cost
Surface hardness can be secured through heat treatment
Suitable for general hinge pins or locating pins
Design considerations:
SCM440 should be considered for high-load or high-impact applications.
For precise positioning, grinding and surface hardness are important.
Rollers
Rollers are components that experience rotation, contact, and wear at the same time.
Applicable materials:
S45C
SCM440
STS304
Urethane coating
Rubber lining
Reasons for applying S45C:
Easy to machine
Surface wear can be reduced through induction hardening
Suitable for general conveying rollers
Design considerations:
For food transfer rollers, STS304 or food-grade coatings should be considered.
Surface roughness and hardness must be specified according to friction conditions.
4.6 Advantages and Limitations of Carbon Steel
Advantages:
Higher strength than general structural steel
Hardness and wear resistance can be improved through heat treatment
Good machinability
Lower cost than alloy steel
Easy to apply to machine element parts
Limitations:
Low corrosion resistance
Weldability is often poor
Alloy steel may be more suitable for high-load or high-fatigue parts
Deformation may occur after heat treatment
Use in food-contact areas should be limited
5. Alloy Steel
5.1 Definition of Alloy Steel
Alloy steel is steel that contains alloying elements such as chromium, molybdenum, nickel, and manganese in addition to iron and carbon.
Compared with ordinary carbon steel, it offers better strength, toughness, fatigue strength, wear resistance, and heat treatment performance.
In mechanical design, alloy steel is used for critical machine elements subjected to high load, high-speed rotation, repeated load, impact, and wear.
Representative materials:
SCM415
SCM420
SCM435
SCM440
SNCM220
SNCM439
5.2 Main Characteristics of Alloy Steel
Alloy steel allows mechanical performance to be designed at a higher level than general carbon steel.
Main characteristics:
Suitable for high-strength parts
High strength and hardness can be obtained after heat treatment
Excellent fatigue strength
Strong resistance to impact and repeated load
Suitable for gears, shafts, spindles, bolts, and power transmission parts
More expensive than carbon steel
Heat treatment conditions and post-processing management are important
Alloy steel should not simply be understood as “strong steel.”
It is a material that maximizes design performance when used together with proper heat treatment.
5.3 Representative Alloy Steel Materials
5.3.1 SCM415 / SCM420
SCM415 and SCM420 are chromium-molybdenum steels widely used for carburizing.
Main characteristics:
Suitable for carburizing treatment
Can create a hard surface and tough core
Suitable for gears, cams, pins, and sprockets
Provides both surface wear resistance and internal toughness
Main applications:
Gears
Sprockets
Cams
Power transmission parts
Wear pins
Gear shafts
Reducer components
Design considerations:
Carburizing depth must be specified.
Deformation may occur after heat treatment.
For gear teeth, post-heat-treatment grinding or finishing should be considered.
It may be excessive in cost for simple structural parts.
5.3.2 SCM440
SCM440 is a representative alloy steel widely used for high-strength shafts, bolts, pins, and spindles.
Main characteristics:
Excellent strength and toughness
Stable mechanical properties can be obtained after quenching and tempering
Suitable for high-load shafts and power transmission parts
Better fatigue strength and impact resistance than S45C
Main applications:
High-load shafts
Spindles
High-strength bolts
Pins
Crank components
Coupling components
Hydraulic cylinder rods
Power transmission shafts
Design considerations:
Material and machining costs are higher than S45C.
Heat treatment conditions must be clearly specified.
Precision shafts may require grinding after heat treatment.
For exposed parts in food machinery, corrosion risk must be reviewed.
5.3.3 SNCM Series
SNCM-series steels are high-strength alloy steels containing nickel, chromium, and molybdenum.
They are used for parts requiring higher toughness and impact resistance than SCM-series steels.
Main characteristics:
Very high strength and toughness
Suitable for large high-load parts
Strong against impact and repeated load
Used for high-grade machine elements
Main applications:
Large shafts
High-load gears
Aerospace and heavy equipment components
High-strength spindles
Power transmission parts subjected to high impact load
Design considerations:
High cost
Heat treatment control is important
May be an excessive specification for general industrial machinery
Application necessity should be reviewed based on actual load conditions
5.4 Heat Treatment of Alloy Steel
Alloy steel should be designed together with heat treatment.
Rather than specifying only the material name, it is important to specify heat treatment conditions and target hardness together.
Quenching and Tempering
Quenching and tempering are used to achieve a balanced combination of strength and toughness.
Main purposes:
Strength improvement
Toughness securing
Fatigue strength improvement
Internal stress stabilization
Main applicable materials:
SCM440
SNCM439
Main applicable parts:
Shafts
Bolts
Pins
Spindles
High-load structural parts
Carburizing
Carburizing is a process in which carbon is diffused into the surface of low-carbon alloy steel and then heat treated to harden the surface.
Main purposes:
Surface hardness improvement
Wear resistance improvement
Maintenance of internal toughness
Gear life improvement
Main applicable materials:
SCM415
SCM420
SNCM220
Main applicable parts:
Gears
Cams
Sprockets
Gear shafts
High-wear power transmission parts
Nitriding
Nitriding is a surface hardening process that increases hardness by diffusing nitrogen into the surface.
Main purposes:
Surface hardness improvement
Wear resistance improvement
Minimization of deformation
Fatigue strength improvement
Main applicable parts:
Precision shafts
Gears
Mold components
Spindles
Sliding parts
Design considerations:
Hardened depth is often shallower than carburizing.
Surface layer quality control is important.
It is advantageous for precision parts, but cost may be high.
5.5 Main Applications of Alloy Steel
High-Load Shafts
High-load shafts are subjected to torsion, bending, and fatigue loads.
Applicable materials:
SCM440
SNCM439
Reasons for application:
High fatigue strength
Strength and toughness can be secured through quenching and tempering
Advantageous under repeated loads
Gears
Gears are subjected to contact stress, bending stress, and wear at the same time.
Applicable materials:
SCM415
SCM420
SCM440
SNCM220
Reasons for application:
Surface hardness can be increased.
Wear resistance and fatigue strength can be secured.
Suitable for power transmission parts.
Design considerations:
Tooth surface hardness and core toughness must be considered together.
Heat treatment deformation and tooth profile accuracy are important.
For high-precision gears, grinding after heat treatment should be considered.
High-Strength Bolts and Pins
High-strength fastening parts and pins may be subjected to tensile load, shear load, impact, and repeated load.
Applicable materials:
SCM435
SCM440
Reasons for application:
High tensile strength can be secured.
Stable properties can be obtained after quenching and tempering.
Strong resistance to repeated load.
Design considerations:
Tightening torque and fatigue failure must be reviewed together.
Hydrogen embrittlement caused by surface treatment should also be considered.
5.6 Advantages and Limitations of Alloy Steel
Advantages:
High-strength design is possible.
Excellent mechanical properties after heat treatment.
Good fatigue strength and wear resistance.
Suitable for critical parts such as gears, shafts, and spindles.
Service life can be greatly increased when combined with surface hardening.
Limitations:
More expensive than general carbon steel.
Performance varies greatly depending on heat treatment conditions.
Heat treatment deformation and post-processing costs must be considered.
It may be excessive for simple structural parts.
Corrosion resistance is lower than stainless steel.
6. Tool Steel
6.1 Definition of Tool Steel
Tool steel is a high-hardness steel used under severe conditions such as cutting, forming, wear, high temperature, and repeated impact.
It is mainly used for molds, blades, punches, dies, cutting parts, and wear parts rather than general machine structures.
Tool steel can achieve very high hardness and wear resistance through heat treatment.
Representative materials:
SKD11
SKD61
SKH51
SKS3
STD11
6.2 Main Characteristics of Tool Steel
Tool steel has a different purpose from ordinary structural materials.
Hardness, wear resistance, thermal stability, cutting performance, and toughness are more important than simple strength.
Main characteristics:
High hardness can be achieved.
Excellent wear resistance.
Suitable for molds and cutting parts.
Dimensional stability after heat treatment is important.
Some grades retain hardness even at high temperatures.
They may become brittle or weak against impact.
Machining and heat treatment costs are high.
6.3 Representative Tool Steel Materials
6.3.1 SKD11
SKD11 is a cold work tool steel used for parts requiring high hardness and wear resistance.
Main characteristics:
Very high hardness can be obtained after heat treatment.
Excellent wear resistance.
Suitable for cold dies, punches, dies, and blades.
Relatively good dimensional stability.
Main applications:
Cutting blades
Punches
Dies
Cold work molds
Wear plates
Slitter knives
Forming mold components
Design considerations:
It may be vulnerable to impact.
Grinding is often required after heat treatment.
Sharp edges may create a risk of cracking.
When used as a blade in food machinery, corrosion resistance must be reviewed.
6.3.2 SKD61
SKD61 is a hot work tool steel suitable for molds and parts used in high-temperature environments.
Main characteristics:
Good high-temperature strength and thermal shock resistance
Used for hot work molds and die-casting molds
Good hardness retention at high temperatures
More suitable than SKD11 for hot working conditions
Main applications:
Die-casting molds
Hot forging dies
Extrusion dies
High-temperature jigs
Hot forming parts
Design considerations:
SKD11 may be more suitable for ordinary cutting blades.
Heat treatment conditions and surface treatment conditions should be reviewed together.
Crack control is important under repeated thermal shock.
6.3.3 SKH51
SKH51 is a high-speed steel used for cutting tools and high-speed friction conditions.
Main characteristics:
Maintains hardness even at high temperatures
Suitable for cutting tools
Excellent wear resistance and hot hardness
High cost
Main applications:
Drills
End mills
Cutting tools
Cutters
High-speed cutting blades
Punches
Design considerations:
Because of its high cost, it should be applied only where necessary.
Fracture risk must be reviewed under high-impact conditions.
Heat treatment and grinding quality are important.
6.4 Heat Treatment of Tool Steel
Heat treatment determines the performance of tool steel.
Even with the same material, hardness, toughness, wear resistance, and dimensional stability vary greatly depending on heat treatment conditions.
Quenching
Quenching is the basic heat treatment used to obtain high hardness.
Purpose:
Hardness improvement
Wear resistance
Cutting performance improvement
Design considerations:
Cracks may occur due to rapid cooling.
Complex-shaped parts may deform significantly.
Preheating, cooling conditions, and subsequent tempering are important.
Tempering
Tempering is performed after quenching to reduce brittleness and secure the required toughness.
Purpose:
Reduction of brittleness
Internal stress relief
Securing toughness
Dimensional stabilization
Design considerations:
Tempering temperature should be specified according to target hardness and operating conditions.
For cutting edges, the balance between hardness and toughness is important.
Surface Treatment
Tool steels may be additionally surface-treated when necessary.
Representative surface treatments:
Nitriding
TiN coating
DLC coating
PVD coating
Oxidation treatment
Purpose:
Wear resistance improvement
Friction reduction
Prevention of seizure
Service life extension
Design considerations:
The combination of base material hardness and coating layer is important.
Coating peeling must be considered.
In food machinery, the risk of foreign material contamination due to coating peeling must be reviewed.
6.5 Main Applications of Tool Steel
Cutting Blades
Cutting blades require high hardness and edge retention.
Applicable materials:
SKD11
SKH51
STS420
Special stainless blade steels
Reasons for applying tool steel:
Good cutting performance
Excellent wear resistance
Long cutting edge life
Design considerations:
In food machinery, rust risk and cleanability must be reviewed.
Tool steel may have lower corrosion resistance than stainless steel.
For food-contact blades, corrosion resistance, hardness, and hygiene must be reviewed together.
Mold Components
Molds are subjected to repeated compression, shearing, friction, and heat conditions.
Applicable materials:
SKD11
SKD61
SKH51
Reasons for application:
High hardness and wear resistance can be secured.
Suitable for repeated forming conditions.
Service life can be improved when combined with surface treatment.
Wear Plates
Wear plates are replaceable parts that absorb wear caused by repeated contact with other parts.
Applicable materials:
SKD11
Induction-hardened S45C
Heat-treated SCM440
Ceramics
UHMW-PE
Reasons for applying tool steel:
Strong resistance to metal-to-metal contact wear
Long service life
Suitable for high-load conditions
Design considerations:
The mating material may be damaged, so material pairing must be reviewed.
Lubrication conditions are important.
Fracture risk must be reviewed in areas with high impact.
6.6 Advantages and Limitations of Tool Steel
Advantages:
High hardness can be achieved.
Excellent wear resistance.
Suitable for cutting, molding, and wear parts.
Service life can be significantly extended when combined with surface treatment.
Grades can be selected according to application, such as cold work, hot work, or cutting.
Limitations:
High cost.
Difficult to machine.
Risk of deformation and cracking after heat treatment.
May be vulnerable to impact.
Corrosion resistance is often lower than stainless steel.
In food machinery, rust, coating peeling, and cleanability must be reviewed.
7. Comparison of the Four Steel Material Groups
7.1 Comparison by Function
Category | Main Purpose | Representative Parts | Key Characteristics |
|---|---|---|---|
General structural steel | Structural support | Frames, bases, brackets | Weldability, economy, structural strength |
Carbon steel | General machine elements | Shafts, pins, rollers, cams | Strength, machinability, heat treatment capability |
Alloy steel | High-load machine elements | Gears, high-strength shafts, spindles | High strength, fatigue strength, wear resistance |
Tool steel | Cutting, forming, and wear | Blades, molds, punches, dies | High hardness, wear resistance, cutting performance |
7.2 Comparison from a Design Perspective
Item | General Structural Steel | Carbon Steel | Alloy Steel | Tool Steel |
|---|---|---|---|---|
Cost | Low | Medium | High | High |
Weldability | Good | Limited | Limited | Poor |
Machinability | Good | Good | Moderate | Difficult |
Heat treatment effect | Low | Good | Very good | Very good |
Wear resistance | Low | Moderate to good | Good | Very good |
Toughness | Moderate | Moderate | Good | Depends on grade |
Corrosion resistance | Low | Low | Low | Low to moderate |
Main use | Structures | General machine parts | High-load parts | Molds, blades, wear parts |
8. Material Selection Guide by Part
8.1 Frames
Recommended materials:
SS400
S355
SM490
Reasons for selection:
Good weldability
Economical
Easy to manufacture large structures
For food machinery:
If the part is an internal structure not exposed to wash water, painted steel may be considered.
If the part is exposed to wash water or requires a hygienic exterior, STS304 should be considered.
8.2 General Shafts
Recommended materials:
S45C
SCM440
STS304
STS420
Selection criteria:
General load: S45C
High load or repeated load: SCM440
Corrosive or washdown environment: STS304 or STS420
Severe surface wear: induction hardening or surface treatment
8.3 Gears
Recommended materials:
SCM415
SCM420
SCM440
S45C
POM
MC Nylon
Steel material selection:
Low-load general gears: S45C
High-load power transmission gears: carburized SCM415 or SCM420
Gears subjected to high impact and repeated load: SCM440 or SNCM series
8.4 Blades
Recommended materials:
SKD11
SKH51
STS420
Special stainless blade steels
Selection criteria:
Cutting life priority: SKD11 or SKH51
Corrosion resistance and hygiene priority: STS420 or stainless blade steel
Food machinery application: hardness, corrosion resistance, cleanability, and rust risk must be reviewed together
8.5 Wear Plates
Recommended materials:
SKD11
Heat-treated SCM440
Induction-hardened S45C
UHMW-PE
Ceramics
Selection criteria:
High-load metal-to-metal wear: SKD11 or SCM440
General wear: induction-hardened S45C
Low-friction food transfer wear: UHMW-PE
Extreme wear: ceramics
9. Application of Steel Materials in Food Machinery Design
In food machinery, steel material selection must not be based only on strength.
Corrosion, hygiene, cleanability, and the possibility of foreign material contamination must also be reviewed.
9.1 Application of General Structural Steel
In food machinery, general structural steel can mainly be used for non-contact structural areas.
Possible applications:
Internal bases
Non-exposed frames
Motor bases
Reducer supports
Support structures in dry environments
Design considerations:
Corrosion may occur when exposed to wash water.
Paint peeling may cause foreign material contamination.
As a rule, it should not be used for food-contact areas.
9.2 Application of Carbon Steel
Carbon steel can be used for machine element parts such as shafts, pins, and rollers.
Possible applications:
Non-contact shafts
Internal drive shafts
Hinge pins
Roller shafts
Cams
Design considerations:
Rust may occur.
It may be unsuitable for food-contact areas or positions exposed to wash water.
Surface treatment and rust prevention management are required.
9.3 Application of Alloy Steel
Alloy steel is suitable for high-load drive components.
Possible applications:
Internal gears of reducers
Drive shafts
High-load pins
Sprockets
Power transmission parts
Design considerations:
Corrosion protection is required for externally exposed parts.
Stainless steel materials should be considered first for food-contact areas.
Lubrication and rust prevention management are required.
9.4 Application of Tool Steel
Tool steel can be used for food cutting, forming, and wear parts.
Possible applications:
Cutting blades
Cutters
Scrapers
Wear plates
Punching parts
Design considerations:
Corrosion resistance may be insufficient.
Drying after washing is required.
Food-contact blades should be compared with stainless blade steels.
If coating is applied, peeling risk must be reviewed.
10. Steel Material Selection Procedure
When selecting steel materials, it is useful to review them in the following order.
10.1 Confirm the Function of the Part
First, identify the function of the part.
Does it support a structure?
Does it transmit rotational power?
Is it subjected to friction and wear?
Does it perform a cutting function?
Is it subjected to impact load?
Is it exposed to food or wash water?
10.2 Confirm Load Conditions
Next, review the load conditions.
Static load
Repeated load
Impact load
Bending load
Torsional load
Contact load
Shear load
If the load is simple and structural, general structural steel may be suitable.
If repeated load and rotational load are significant, carbon steel or alloy steel should be considered.
If wear and cutting are the key requirements, tool steel should be considered.
10.3 Confirm Environmental Conditions
Environmental conditions strongly affect material selection.
Items to review:
Indoor or outdoor use
Dry or wet conditions
Exposure to wash water
Food-contact requirements
Use of oil or lubricant
High-temperature or low-temperature environment
Exposure to corrosive gases or chemicals
Most steel materials have low corrosion resistance, so stainless steel or surface treatment should be considered together in corrosive environments.
10.4 Confirm Manufacturing Method
Materials should be selected together with the manufacturing method.
Items to review:
Will the part be welded?
Will it be machined?
Will it be bent as sheet metal?
Will it be heat treated?
Is grinding required?
Will surface treatment be applied?
For example, general structural steel is suitable for welded frames.
S45C or SCM440 is suitable for precision shafts.
SKD11 or SKH51 is suitable for high-hardness blades.
10.5 Decide Heat Treatment and Surface Treatment
The performance of steel materials varies greatly depending on heat treatment and surface treatment.
Items to review:
Induction hardening
Quenching and tempering
Carburizing
Nitriding
Black oxide
Zinc plating
Electroless nickel plating
Chrome plating
PVD coating
DLC coating
Design considerations:
Deformation after heat treatment must be considered.
Surface treatment thickness may affect dimensions.
In food machinery, coating peeling and plating peeling risks must be reviewed.
11. Notes on Specifying Materials in Drawings
When specifying materials on drawings, it is better to specify the required conditions together with the material name rather than writing only the material name.
11.1 Material Name Specification
Examples:
SS400
S45C
SCM440
SCM415
SKD11
SKH51
11.2 Heat Treatment Specification
Examples:
S45C, induction hardened
SCM440, quenched and tempered
SCM420, carburized
SKD11, heat treated to HRC 58–62
Depending on the part, the target hardness, hardening depth, and heat treatment area should be specified on the drawing.
11.3 Surface Treatment Specification
Examples:
Black oxide
Zinc plating
Electroless nickel plating
Hard chrome plating
Nitriding
DLC coating
Design considerations:
Check whether dimensions change after surface treatment.
Surface treatment thickness must be considered for fits, bearing assembly areas, and threaded areas.
11.4 Notes for Food Machinery Drawings
In food machinery, not only the material name but also the application location and surface condition are important.
Items to review:
Food-contact or non-food-contact area
Exposure to wash water
Surface roughness
Weld polishing requirements
Risk of plating or coating peeling
Possibility of contact with lubricant
Possibility of corrosion
For food-contact areas, stainless steels such as STS304, STS316, and STS420 are generally considered before ordinary steel materials.
12. Summary
In mechanical design, general structural steel, carbon steel, alloy steel, and tool steel each have clearly different roles.
General structural steel is an economical structural material used to form the skeleton of a machine.
It is suitable for frames, bases, brackets, and welded structures, but has low corrosion resistance and wear resistance.
Carbon steel is suitable for general machine element parts.
Centered around S45C, it is used for shafts, pins, rollers, and cams, and its strength and wear resistance can be improved through heat treatment.
Alloy steel is suitable for critical parts subjected to high load, repeated load, and fatigue load.
SCM415, SCM420, and SCM440 are representative examples, and they are used for gears, high-strength shafts, spindles, and high-load pins.
Tool steel is a high-hardness material specialized for cutting, forming, and wear conditions.
SKD11, SKD61, and SKH51 are representative examples, and they are used for blades, molds, punches, dies, and wear plates.
When selecting steel materials, strength alone must not be the only criterion.
The function of the part, load condition, friction condition, operating environment, heat treatment, surface treatment, machinability, cost, and maintainability must all be reviewed together.
Especially in food machinery, corrosion, cleanability, hygiene, and the possibility of foreign material contamination may be more important than the strength of the steel material itself.
Therefore, for food-contact areas, stainless steel or food-grade materials should generally be considered before applying ordinary steel materials directly.