Physical and Mechanical Properties and Cutting Performance of Typical Difficult-to-Machine Materials
With the advancement of materials science and technology and the continuous improvement of material preparation techniques, a large number of new materials have emerged in the production field. Most of these materials have relatively good overall physical and mechanical properties, but extremely poor metal cutting performance, and are therefore referred to as difficult-to-machine materials in production. The unique mechanical properties of difficult-to-machine materials present many difficulties in cutting, especially in ultra-slender, deep-hole machining, which has become a technical bottleneck in production. This chapter focuses on the physical and mechanical properties and cutting performance of several typical difficult-to-machine materials.
3.1 Basic Concepts of Metal Machinability
3.1.1 Metrics for Measuring the Machinability of Metal Materials
The machinability of metal materials refers to the ease with which a metal can be cut under certain cutting conditions. The degree of machining difficulty varies depending on the conditions and requirements of the cutting process. The machinability of metal materials is a relative concept, and is generally measured using four indicators: surface quality, tool durability, specific cutting force, and chip control. 1. Machining surface quality
Generally, the surface roughness of mechanical parts is used to measure the machinability of metal materials. The smaller the surface roughness of the part after machining, the higher its machinability. For precision parts with special requirements, the depth of the metamorphic layer on the machined surface, the residual stress and the degree of hardening are used to measure its machinability. This is mainly because the depth, residual stress and degree of hardening of the metamorphic layer directly affect the stability of the part's shape and size, magnetic permeability, electrical conductivity and creep resistance.
2. Tool durability
The tool durability is used to measure the machinability of metal materials. Tool durability refers to the total cutting time of a newly sharpened tool from the start of cutting until the wear reaches the tool blunting standard. Tool durability indicators can be divided into three situations:
(1) Under the same tool durability conditions, the allowable cutting speed for cutting the workpiece material is examined. This index is a commonly used index to measure machinability. The allowable cutting speed is expressed as Vr, which means: when the tool life is T (min, the cutting speed value allowed for cutting the metal material. The larger the vy value, the better the machinability of the workpiece. In general, T = 60min; for difficult-to-cut materials, T = 30min or T = 15min. If T = 60min, the cutting speed vy can be expressed as v60.
(2) Under the same cutting conditions, the value of the tool life of the workpiece material is examined. The larger the tool life value, the better the machinability of the workpiece.
(3) Under the same cutting conditions, the volume of metal removed when the workpiece material reaches the tool blunting standard is examined. The larger the metal cutting volume, the better the machinability of the workpiece.
3. Unit cutting force When the machine tool power is insufficient or the machine tool-fixture-tool-workpiece system is insufficiently rigid, the unit cutting force is often used to measure the machinability of the workpiece.
4. Chip Control Difficulty
Effective control of chip flow and reliable chip breaking indicate good machinability, such as in deep-hole drilling, deep-hole boring, and deep-hole trepanning. Conversely, poor machinability indicates poor machinability. In actual production, relative machinability is often used to measure the machinability of the metal being worked. The V60 value of 45 steel (hardness 170-229 HB, strength θ = 0.637 GPa) is used as a benchmark, denoted as (V60). The ratio k of the V60 value of other metals being worked to (V60) is called relative machinability: ky = V60/(V60)j.
The relative machinability of commonly used metal materials can be divided into eight levels, as shown in Table 3.1.

3.1.2 Factors Affecting the Machinability of Metal Materials
The machinability of metal materials is related to their physical and mechanical properties, chemical composition, heat treatment status, metallographic structure, machining requirements, and processing conditions.
1. Hardness
1) Influence of Workpiece Material Hardness at Room Temperature
Generally speaking, materials of the same type with higher hardness have lower machinability. When the material hardness is high, the contact length between the chip and the rake face decreases. Consequently, the normal stress on the rake face increases, and frictional heat is concentrated on the smaller tool-chip contact surface, leading to higher cutting temperatures and increased wear. Excessively high workpiece hardness can cause tool tip burnout and chipping. Figure 3.1 shows the relationship between hardness and machinability of carbon steel.

2) The Effect of Workpiece Material High-Temperature Hardness on Machinability
The higher the high-temperature hardness of a workpiece material, the lower its machinability. As cutting temperatures increase, the hardness of the tool material decreases, and the ratio of tool hardness to workpiece hardness also decreases, increasing tool wear. For example, high-temperature nickel-based alloys and heat-resistant steels, which have relatively high high-temperature hardness, have extremely low machinability.
3) The Effect of Hard Spots in the Workpiece Material on Machinability
The sharper and more widely distributed the hard spots in the workpiece material, the lower its machinability. Hard particles affect tool wear in two ways: First, their high hardness can cause abrasions on the tool; second, the microscopic hard particles at the workpiece's grain boundaries increase the material's strength and hardness, increasing its resistance to shear deformation during cutting and reducing the material's machinability.
4) The Impact of a Material's Work Hardening Properties on Machinability
The higher the work hardening properties of a workpiece material, the lower its machinability. For example, austenitic stainless steel exhibits significant surface hardening after cutting, with its surface microhardness 1.4 to 2.2 times higher than the original substrate hardness. Increased work hardening properties increase cutting forces and temperatures, leading to abrasions on the tool caused by hardened chips, causing boundary wear on the secondary flank, and increasing tool wear.
2. Strength
Workpiece material strength includes both room-temperature strength and elevated-temperature strength. Higher room-temperature strength increases cutting forces, higher cutting temperatures, greater tool wear, and poorer machinability. Generally, machinability decreases as metal strength increases. For example, at room temperature, the σ of 20CrMo alloy steel is slightly lower than that of 45 steel (650 MPa). However, at 600°C, 20CrMo alloy steel's σ is actually higher than that of 45 steel (180 MPa), reaching 400 MPa. Therefore, the machinability of 20CrMo alloy steel at high temperatures is poorer than that of 45 steel.
3. Plasticity and Toughness
For materials with high plasticity, plastic deformation increases due to the expansion of the plastic deformation zone. For materials with high toughness, the plastic zone may not expand during plastic deformation, but the absorbed plastic deformation work does increase. Although the causes are different, increased plasticity and toughness both lead to increased plastic deformation work. For workpiece materials with the same strength, greater plasticity results in greater plastic deformation and greater plastic deformation work consumption. Consequently, when cutting such workpiece materials, cutting forces and temperatures are higher, and adhesion to the tool is more likely to occur, increasing tool wear and increasing machined surface roughness. Therefore, the greater the plasticity of the workpiece material, the worse its machinability. The plasticity of the workpiece material is too small, which shortens the contact length between the tool and the chip. The cutting force and cutting heat are concentrated near the tool edge, which will also increase tool wear. Excessive or insufficient plasticity (or brittleness) in the workpiece material can reduce machinability. The greater the workpiece's toughness, the more work energy and cutting forces are consumed during cutting. Toughness also has a significant impact on fracture, so the tougher the workpiece, the worse its machinability.
4. Thermal Conductivity of the Workpiece Material
The higher the thermal conductivity of the workpiece material, the more heat is carried away by the chips and conducted through the workpiece, which promotes low cutting zone temperatures and improves machinability. For example, stainless steel and high-temperature nickel-based alloys have very low thermal conductivity, only 1/3 to 1/4 that of 45 steel, resulting in poor machinability. However, workpiece materials with high thermal conductivity experience high temperature rises during machining, making it difficult to control machining dimensions.
5. Chemical Composition
The strength and hardness of steel generally increase with increasing carbon content, while its plasticity and toughness decrease. High-carbon steel has higher strength and hardness, resulting in greater cutting forces and increased tool wear; low-carbon steel has higher plasticity and toughness, resulting in greater deformation during cutting. Carbon steel is difficult to break and produces rough machined surfaces. Medium carbon steel falls somewhere in between, with better machinability. To improve the performance and machinability of steel, alloying elements such as chromium (Cr), nickel (Ni), vanadium (V), molybdenum (Mo), tungsten (W), manganese (Mn), silicon (Si), and aluminum (Al) can be added. Cr, Ni, V, Mo, W, and Mn increase the strength and hardness of steel, while Si and Al tend to form hard particles such as silicon oxide and aluminum oxide, increasing tool wear. Low levels of these elements (generally limited to 0.3%) have little effect on the steel's machinability. However, exceeding 0.3% negatively impacts the steel's machinability.

Adding trace amounts of sulfur (S), selenium (Se), lead (Pb), bismuth (Bi), and calcium (Ca) to steel can form inclusions, embrittle the steel, or act as a lubricant, reducing tool wear and improving the machinability of the workpiece. While adding phosphorus (P) increases the strength and hardness of steel, its toughness and plasticity decrease significantly, making it more susceptible to chip breakage. Figure 3.2 shows the effects of different elements on the machinability of structural steel.

6. Metallographic Structure
Different metallographic structures have a direct impact on machinability. Generally, the ratio of ferrite to pearlite in steel affects machinability. Ferrite has high plasticity, pearlite has high hardness, and martensite is harder than pearlite. Therefore, a lower pearlite content increases the allowable machining speed, increases tool durability, and improves machinability. Conversely, a higher martensite content results in poor machinability.