Factors Affecting Machinability of Steel and Cast Iron in CNC Machining

The machinability of steel and cast iron is influenced by several important material properties. Understanding these factors helps improve cutting performance, extend tool life, and achieve better surface quality during CNC machining and mold manufacturing.

1. Chemical composition of steel
The chemical composition of steel has a major effect on machinability. In general, the higher the alloy content, the more difficult the material is to machine. As carbon content increases, metal cutting performance usually decreases, making the material harder on cutting tools.

2. Material structure
The internal structure of the steel also plays an important role in cutting performance. Common structures include forged, cast, extruded, rolled, and machined forms. Forged and cast materials often have surface conditions that are more difficult to machine than rolled or more uniform materials.

3. Hardness
Hardness is one of the most important factors affecting metal cutting performance. As a general rule, the harder the steel, the more difficult it is to machine. High-speed steel tools are typically suitable for materials up to about 330-400 HB. High-speed steel tools with TiN coating can machine materials up to around 45 HRC. For harder materials in the range of 65-70 HRC, carbide, ceramic, cermet, and cubic boron nitride tools are generally required.

4. Non-metallic inclusions
Non-metallic inclusions usually have a negative effect on tool life. For example, aluminum oxide is highly abrasive and can accelerate tool wear during machining.

5. Residual stress
Residual stress in the workpiece can also reduce machinability and cause machining instability. For this reason, stress relief treatment is often recommended after rough machining and before finishing operations.

Machinability of cast iron
In general, the higher the hardness and strength of cast iron, the lower its machinability and the shorter the expected insert and tool life. Even so, many common cast iron grades used in production still offer good machining performance when matched with the correct tool and cutting conditions.

Cast iron machinability is closely related to its microstructure. Hard pearlitic cast iron is more difficult to machine. Flake graphite cast iron and malleable cast iron usually have very good cutting performance, while ductile iron is generally more difficult to machine.

Main wear types when machining cast iron
The main wear mechanisms encountered during cast iron machining are abrasive wear, adhesion wear, and diffusion wear.

Abrasive wear is mainly caused by hard carbides, sand inclusions, and hard cast skin on the material surface. Adhesion wear and built-up edge tend to occur at low cutting temperatures and low cutting speeds. The ferrite phase in cast iron is particularly likely to adhere to the cutting edge, but this can often be reduced by increasing cutting speed and cutting temperature.

Diffusion wear is temperature dependent and usually occurs at high cutting speeds, especially when machining high-strength cast iron grades. These materials resist deformation and generate higher cutting temperatures. In such cases, ceramic and cubic boron nitride tools may be used at high cutting speeds to achieve better tool life and improved surface finish.

Tool requirements for cast iron machining
Tools used for machining cast iron generally need high hot hardness and good chemical stability. At the same time, tool selection must also consider the machining process, workpiece condition, and cutting parameters, since the cutting edge may also need sufficient toughness, resistance to thermal fatigue, and edge strength.

Evaluation of cutting performance
Good cutting performance in cast iron machining depends on how the cutting edge wears over time. Rapid edge dulling can lead to thermal cracks, edge chipping, premature tool failure, damage to the workpiece, poor surface quality, and excessive waviness. In contrast, balanced and gradual flank wear with a stable sharp cutting edge is the preferred condition for reliable machining.