Basics of Ferrous Materials Used in Machining! Types and Features of Machining Materials Explained

Basics of Ferrous Materials

Many of the materials we use in our daily lives that we refer to as "iron" are actually not pure iron, but alloys containing carbon and other elements. When using machine tools for machining, the selection of materials has a significant impact on machining quality and productivity. Let's begin by gaining a deeper understanding of the basics of ferrous materials.

Differences Among Iron, Steel, and Cast Iron

Ferrous materials are broadly divided into three categories based on their carbon content: "pure iron," "steel," and "cast iron."

"Pure iron" is a material with a carbon content of less than 0.02%. It is characterized by a glossy white color, rather than the typical gray. However, it is rarely ever used in industrial products due to its low strength and tendency to rust.

"Steel" is a material with a carbon content of 0.02% to 2.14%. Many of the materials we commonly refer to as "iron" fall into the steel category. It is the most widely used because it offers moderate strength and machinability, and its properties can be adjusted through heat treatment. Typical examples include structural steel (SS400) and carbon steels for machine structural use (S45C).

"Cast iron" is a material with a high carbon content, ranging from 2.1% to 6.67%. Its relatively low melting point makes it suitable for casting, which involves melting the material and pouring it into a mold. It is used to manufacture parts with complex shapes, such as manhole covers and mechanical parts.

Units of Metal Hardness

There are several standards for units to express metal hardness, which differ by measurement method.
For example, "Brinell hardness (HB)" measures hardness by pressing a steel ball into the material and evaluating the size of the resulting indentation.

"Vickers hardness (HV)" is measured by pressing a diamond in the shape of a square pyramid into the material. This method allows for high-precision measurements and can also be used to measure the hardness of thin materials and surface-treated layers.

"Rockwell hardness (HRC)" is a method of measuring hardness by pressing a diamond indenter against the material and recording the depth of the indentation. Measurement is relatively easy, with minimal operator-induced error.

How to Read Material Standards

Ferrous materials are designated based on the JIS standards using combinations of letters and numbers that indicate their material properties and characteristics.
For example, in the case of carbon steel material for machine structures, "S45C" stands for:

S: Steel
45: 0.45% carbon content
C: Carbon steel

For stainless steel, "SUS304" stands for:

SUS: Steel Use Stainless (stainless steel)
304: Grade number indicating the combination of materials in its composition

By understanding these material codes, you can assess the basic properties of each material. However, even when using the same standard, there may be slight differences in properties depending on the manufacturer. We recommend checking the details of each material individually.

What Is Heat Treatment?

Heat treatment is a factor that greatly affects the properties of metals. By applying heat, the properties of a material can be changed to suit your purpose.

There are several methods of heat treatment, some of which are introduced here.

Heat treatment is an important process for greatly altering the properties of materials. It requires fine control of setting conditions, such as temperature and cooling rate. Because improper heat treatment may degrade material properties instead, selecting the appropriate treatment conditions based on your intended purpose and material is essential.

Major Ferrous Materials Commonly Used in Machining

This section explains the characteristics and suitable applications of typical ferrous materials commonly used on the machining workfloor.

Carbon Steels for Machine Structural Use

Carbon steels for machine structural use is one of the most commonly used materials for machining.
Typical carbon steels for machine structural use, such as S45C and S50C, offer a good balance of strength and machinability, leading to their use in a wide range of applications. Another major feature is that their properties can be adjusted by heat treatment.

Main applications

• General mechanical parts
• Shafts
• Gears
• Various types of fastenings, etc.

When machining, one key consideration with carbon steels for machine structural use is their relative susceptibility to rust. To prevent rust, it is important to use anti-rust cutting fluid during machining and apply surface treatments as needed after machining.

Rolled Steel for General Structure

Rolled steel for general structure (such as SS400) is a material that is both economical and easy to machine. It is primarily used for frames of building structures and industrial equipment, and is characterized by its excellent weldability. For example, SS400 is a structural steel with a tensile strength of at least 400 N/mm2, and is widely used as a general structural steel material.

Main applications

• Railway vehicles
• Bridges
• Columns, etc.

As a material, it cannot be hardened by quenching and is susceptible to rust, requiring anti-rust treatment. It is generally used after treatment, such as coating or plating.

Stainless Steel

Used for parts requiring resistance to corrosion and heat, stainless steel is also a material commonly used in machining.
A typical example is "SUS304," which is widely used in food machinery and medical equipment. However, it is characterized by significant tool wear during machining, and is prone to changes in hardness and magnetization.

Main applications

• Food machinery parts
• Medical apparatus
• Parts for outdoor use, etc.

Note that SUS304 is a material prone to work hardening, which makes heat dissipation difficult and shortens tool life. Along with choosing tools with superior wear resistance, it is important to avoid increasing the cutting speed or cutting depth more than necessary. Additionally, to prevent heat-induced hardening during machining, make sure to apply adequate cutting fluid and maintain an appropriate cutting speed.

Chromium Molybdenum Steel

Chromium molybdenum steel (such as SCM435) is a material that offers an excellent balance of strength and toughness. It is easy to harden by quenching and retains stable mechanical properties after heat treatment. Among chromium molybdenum steels, SCM435 achieves a tensile strength of at least 930 N/mm² after heat treatment and is also characterized by its high fatigue strength.

Main applications

• High-strength bolts
• Automotive parts
• High-load shafts
• Aircraft parts, etc.

Chromium molybdenum steel maintains its strength even in high-temperature environments of up to approximately 500°C, making it suitable for high-temperature, high-pressure parts. It is also resistant to acids and saltwater, and is recognized as a long-lasting ferrous material.

Machining Ferrous Materials According to Their Characteristics Is Essential

Ferrous materials exhibit a wide range of characteristics depending on their carbon content and alloying elements. Selecting the appropriate material for your application is therefore essential. Moreover, another feature of ferrous materials is that they can be processed in combination with heat treatment to obtain the mechanical properties suited to your purpose.

To that end, it is important to have a thorough understanding of the properties required of the product and to approach the manufacturing process with a holistic perspective—from material selection to machining condition settings.