Basics and points to note when processing stainless steel | What is the best processing method for each application?

Stainless steel is a material that is used in many industries due to its excellent corrosion resistance and strength. However, due to its difficulty in processing, proper processing techniques and know-how are required.This article explains the characteristics of stainless steel processing and appropriate countermeasures.

What is stainless steel?

Stainless steel is a metal material that is widely used as "rust-resistant steel (steel is an alloy whose main component is iron)."In Japan, it is classified under the symbol "SUS (Steel Use Stainless)" based on the JIS standard (Japanese Industrial Standards).

Stainless steel contains 10.5% or more chromium (Cr) in iron, which forms a thin oxide film called a passive film on the surface, preventing rust and corrosion.Furthermore, by adding elements such as nickel (Ni) and molybdenum (Mo), it is possible to improve properties such as corrosion resistance, heat resistance, and strength.Due to its excellent durability and ease of maintenance, it is used in a wide range of fields, including kitchen equipment, medical equipment, automotive parts, and construction materials.

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Stainless steel classification

Although it is simply called stainless steel, there are a wide variety of types. There are 65 types defined by the JIS standard alone, and it is said that there are over 200 types if overseas standards are included.

In the JIS standard, the name of stainless steel is written in the format "SUS + three-digit number + (symbol)".SUS (Steel Use Stainless) is a combination of the initials of the following English words:

The three-digit number indicates the chemical composition and properties of the stainless steel, and is broadly classified into three series: 3, 300, and 400. These major categories and characteristics are summarized in Table 600.

*Precipitation hardening systems are specified only after heat treatment

Austenitic

Austenitic stainless steels such as SUS304 and SUS316 are the most commonly used stainless steels.It contains a lot of chromium and nickel, giving it excellent corrosion resistance and workability. It is non-magnetic and suitable for cold processing, but it is prone to work hardening, so care must be taken when cutting or bending. It is widely used in food equipment, medical equipment, building materials, kitchen equipment, chemical equipment, etc.

Martensitic

Martensitic stainless steels such as SUS420J2 and SUS440C contain high carbon and achieve high hardness and strength when quenched.It is magnetic, has good machinability, and is highly resistant to wear, but is somewhat less resistant to corrosion. It is used for molds and tools such as blades, valves, pump shafts, nozzles, and instrument parts.

Ferrite

Ferritic stainless steel such as SUS430 is a low-carbon, magnetic material whose main component is chromium.It has excellent formability and high cost performance, and is widely used in household products and building materials. It is magnetic, corrosion-resistant, and suitable for tap water and indoor use, and is suitable for cold processing, but its weldability is somewhat inferior. It is used in household products such as kitchen sinks and washing machines, kitchen equipment, and building interior materials.

Two-phase system

Duplex stainless steels such as SUS329J1 have a multi-phase structure containing roughly equal amounts of austenite and ferrite, and offer an excellent balance of strength and corrosion resistance.It is magnetic and resistant to salt damage and stress corrosion, making it suitable for use in harsh environments. However, it tends to be somewhat inferior in formability and ductility. It is used in chemical plants, seawater pump shafts, water gates, flue gas desulfurization equipment, etc.

析出硬化系

Precipitation hardening stainless steel such as SUS630 is a type that can be strengthened by aging treatment by adding nickel, copper, etc.Because it is strengthened by solution treatment and aging treatment, it does not require normal quenching and tempering, and it has superior corrosion resistance compared to martensitic alloys.

It has a good balance of strength, corrosion resistance, and processability, and is used in engine valve springs, suspension springs, and aircraft structural parts.

Benefits of stainless steel processing

Although stainless steel is difficult to handle, it is a material that can offer many benefits when processed with the appropriate techniques.Here we will summarize the main benefits of choosing stainless steel.

1. Durability
2. Heat-resistant
3. Strength

Durability

Stainless steel is highly resistant to corrosion, chemicals, and oxidation, and does not require rust prevention treatment or painting, making it a material that requires little maintenance over the long term. Its smooth surface makes it difficult for dirt to adhere, and it is easy to clean.Ideal for locations where hygiene management is required, such as food equipment and medical facilities.

It is also resistant to chemicals, so it can be used safely in environments where chemicals are handled, such as pharmaceutical manufacturing facilities and laboratory equipment.

Heat-resistant

Austenitic and ferritic stainless steels have the property of maintaining stable performance even in high temperature environments of around 600°C.For this reason, it is widely used in applications requiring heat resistance, such as boilers, heat exchangers, and exhaust system components.

Austenitic steels in particular are resistant to thermal deformation and have excellent creep properties, making them suitable for structural parts that are used for long periods of time.In addition, because their expansion rate remains relatively constant with temperature changes, they have the advantage of easily maintaining dimensional stability when joined to dissimilar materials.

Strength

Precipitation hardening and duplex stainless steels have high tensile strength and yield strength, and are capable of withstanding higher loads and stress environments than general structural materials.Taking advantage of its lightweight and high strength characteristics, it is used in harsh conditions where conventional carbon steel or aluminum would not perform as well.

Furthermore, it has excellent fatigue and impact resistance, and demonstrates stable performance even in structural parts that are subjected to repeated loads and vibrations. As it is mainly austenitic and is less susceptible to low-temperature brittleness, it is used as a highly reliable material even in environments with large temperature changes, such as cold regions.

Main processing methods for stainless steel

To make the most of the diverse properties of stainless steel, it is essential to select the appropriate processing method.Here we will introduce some typical processing methods and their technical key points.

1. Cutting
2. Welding
3. surface treatment

Cutting

Austenitic stainless steel is prone to work hardening and tool wear and seizure, so the use of carbide or coated tools is effective.Furthermore, optimization of cutting conditions such as low speed and high feed rate is required.

The selection and supply of cutting oil and coolant is also important to suppress heat generation. Machine rigidity and precision of feed control are key to suppressing burrs, and the use of NC control and 5-axis machining centers can achieve both precision and mass productivity.

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Welding

When welding stainless steel, it is essential to understand the thermal effects of each material.Austenitic steels have excellent weldability, but ferritic and martensitic steels require preheating and post-heat treatment to prevent cracking.

TIG, MIG, and laser welding can cause issues such as discoloration of the heat-affected zone (HAZ) and reduced corrosion resistance, so post-treatment such as pickling and passivation is effective. Introducing automated welding also makes it possible to achieve both quality and productivity.

surface treatment

Stainless steel undergoes a variety of surface treatments depending on the environment and purpose of use.

• Pickling: Removes oxide scale and burns, improving appearance and corrosion resistance
• Buffing: A glossy, smooth finish, useful for applications where aesthetics are required
• Electrolytic polishing: Finely smooths the surface to prevent microbial adhesion (for medical and food applications)
• Design finish: Combining functionality and design, such as hairline

Sanwa Needle Bearings for stainless steel processing

Sanwa Needle Bearings performs ultra-precision machining of stainless steel.We can provide stable quality by selecting the optimal processing method, such as cutting, grinding, or polishing, depending on the product application and design requirements.

In particular, in fields where high quality is required, such as design parts and parts related to medical and food products, it is important to manage materials and applications appropriately.We achieve ultra-precision machining of ±0.0001mm, and provide optimal support according to our customers' quality requirements.

Why is stainless steel difficult to process?

Stainless steel is highly valued in many fields due to its excellent material properties, but it faces many challenges when it comes to processing.Here we will explain the main reasons why stainless steel processing is considered difficult.

1. Work hardening occurs easily, causing large amounts of tool wear and heat generation
2. Low thermal conductivity makes it prone to burning and discoloration
3. Poor chip evacuation can easily cause burrs and deformation

Work hardening occurs easily, causing large amounts of tool wear and heat generation

Stainless steel has a high work hardening property, and hardens locally during cutting and rolling.This increases cutting resistance, causing severe tool wear and heat generation, leading to reduced tool life and poor machining accuracy.

Furthermore, cutting into hardened areas can become unstable, which can lead to tool breakage and poor finish quality.

Low thermal conductivity makes it prone to burning and discoloration

Stainless steel has a low thermal conductivity, so processing heat tends to concentrate on the material and tools, making it prone to seizure and discoloration.In particular, welding and grinding can easily cause changes in the properties of the heat-affected zone (HAZ) and poor appearance.

Such discoloration and oxidation scale directly lead to a decrease in corrosion resistance, so post-processing such as pickling or electrolysis may be required after processing.

Poor chip evacuation can easily cause burrs and deformation

When cutting stainless steel, the chips are thin and tend to get tangled and wrap around the tool or workpiece, causing problems such as poor discharge, damage to the machined surface, and increased tool load.

In particular, when machining deep holes or complex shapes, chip disposal is difficult and re-cutting can easily result in burrs or deformation.This is important to improve drainability.

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Precautions when processing stainless steel

To process stainless steel with high precision, it is necessary to take into account the material's characteristics and consider appropriate processing conditions, tool selection, and even post-processing.Below are some key points to note and their solutions.

1. Prevents seizure and burrs
2. Optimization of tool materials and cutting conditions
3. Prevents discoloration and corrosion after processing

Prevents seizure and burrs

Stainless steel is prone to seizure and burrs due to heat generation and wear, so optimizing cutting conditions is essential.In particular, conditions such as low speed and high feed rate enable efficient machining while minimizing heat generation.

In addition, coolant with excellent cooling performance contributes to suppressing heat concentration and extending tool life. Furthermore, adjusting the cutting edge shape and relief angle of the tool can also minimize the occurrence of burrs.

Optimization of tool materials and cutting conditions

When processing stainless steel, the selection of tools is important.Carbide alloyceramic-TiAlN coated tools have excellent wear resistance and heat resistance, and suppress work hardening and thermal degradation.

In addition, it is necessary to adjust the cutting speed, feed rate, depth of cut, etc. depending on the material and shape, and optimization through actual machine testing is effective. At the same time, it is essential to ensure machine rigidity and chucking accuracy to suppress vibration and chatter.

Prevents discoloration and corrosion after processing

In heat processing, burns and discoloration can cause corrosion, so post-processing by pickling or de-burning is essential, which removes the oxide film and restores corrosion resistance.

Furthermore, selecting a material with high pitting corrosion resistance depending on the usage environment, such as seawater or an acidic atmosphere, is effective in ensuring long-term durability.The key to maintaining quality is to combine materials and treatments that take into account the product's intended use and installation environment.

Summary | Understand the characteristics of stainless steel processing and choose the optimal processing method

While stainless steel has excellent corrosion resistance and strength, it is also a material that presents many challenges in processing, such as work hardening, the effects of heat, difficulty in chip disposal, etc. In order to properly address these challenges, it is necessary to understand the material properties of stainless steel and then combine the optimal processing method, tool selection, and processing conditions.

Sanwa Needle Bearing has extensive experience and know-how in stainless steel processing, based on its many years of experience in ultra-precision processing.With our integrated in-house production system, we are able to handle a wide range of technologies, including ultra-precision machining with an accuracy of ±0.0001mm.If you have any problems with stainless steel processing, regardless of the material or lot size, please feel free to contact us.