What to Know about Machining Hard Metals

The hardness of a metal is influenced by its chemical composition, grain structure, and any heat treatments or alloying processes it has undergone. For instance, precipitation-hardened stainless steels and quenched-and-tempered tool steels exhibit high hardness due to microstructural transformations. These characteristics enhance wear resistance and strength but also demand greater cutting forces during machining.

Higher hardness typically correlates with lower machinability. This results in accelerated tool wear, slower cutting speeds, and the need for advanced tooling and coolant strategies. Machinists often rely on carbide or ceramic tools, optimized tool paths, and high-pressure coolant to maintain productivity and surface finish. Understanding how hardness affects machinability helps engineers make informed decisions during material selection and part design.

Not all commonly machined metals rank equally in hardness. Aluminum alloys are relatively soft and easy to machine, while stainless steels and nickel-based alloys like Inconel are much harder. Comparing these materials using scales like Rockwell or Brinell allows engineers to anticipate machining challenges and design parts that balance performance with manufacturability.

Key Factors that Affect Hardness

While tensile and compressive strength describe how a material responds to distributed forces, hardness specifically measures resistance to localized deformation under concentrated stress. This property is critical for applications involving contact surfaces, cutting tools, and components subject to wear.

Several factors affect a metal’s hardness, including composition, heat treatment, grain size, and work hardening.

Composition

A metal’s resistance to localized plastic deformation is primarily determined by its chemical makeup. Alloying elements contribute by forming hard secondary phases or disrupting the crystal lattice, increasing hardness. Techniques like precision hardening further enhance this by generating fine, distributed particles that hinder dislocation movement and stabilize the microstructure.

Heat Treatment

Heat treatment alters a material’s hardness by modifying its microstructure through controlled thermal cycles. In steels, processes such as quenching and tempering transform softer phases into martensite or tempered martensite, significantly boosting hardness and wear resistance.

Grain Size

Smaller grain sizes increase hardness by introducing more grain boundaries, which obstruct dislocation movement and strengthen the material. This Hall-Petch effect means that grain refinement—achieved through cold working or precise heat treatments—can lead to substantial hardness gains. Conversely, larger grains reduce boundary density, making deformation easier and lowering hardness.

Work Hardening

Work hardening, or strain hardening, increases hardness by inducing plastic deformation, which raises dislocation density within the crystal lattice. These dislocations impede further movement, enhancing both hardness and strength. This is especially relevant in ductile metals like copper, aluminum, and low-carbon steel. Processes such as rolling, drawing, or bending can significantly improve surface hardness, though excessive work hardening may reduce ductility and increase the risk of cracking.

How Hardness Impacts Machining Operations

Hardness plays a critical role in CNC machining, affecting tool longevity, cycle times, surface finish, and dimensional accuracy.

Because hard metals resist cutting forces, they generate more heat and wear tools faster. These effects must be managed to maintain part quality and control costs. Material selection, tolerance setting, and machining partner choice all influence machining effectiveness and final part quality.

Tool Wear

Machining harder materials intensifies tool wear due to increased cutting forces and elevated temperatures at the tool-workpiece interface. These conditions accelerate wear mechanisms like abrasion, adhesion, and diffusion, which degrade carbide and high-speed steel tools. Engineers often use coated tooling, refined geometries, and advanced toolpath strategies to mitigate wear—though these solutions can raise tooling costs and extend cycle times.

Surface Finish

Material hardness also affects surface finish. Harder metals can produce smoother surfaces if the tooling and setup are optimized. However, inadequate fixturing or tooling can lead to chatter and microcracking, compromising surface integrity. In precision-critical industries like aerospace and medical manufacturing, surface quality often determines whether a part passes inspection or is scrapped.

Machinability

Machinability reflects how easily a material can be cut, with hardness being a key factor. Moderately hard materials offer a good balance of strength and machinability, while extremely hard alloys—such as hardened tool steels or titanium aluminides—require advanced tooling and slower cutting speeds. These constraints affect machining costs and limit achievable geometries and tolerances.

Hardness should be viewed as a machining constraint, cost driver, and performance variable. Engineers who understand its impact on cutting mechanics are better equipped to make strategic decisions about tooling, feeds and speeds, and process planning.

How Commonly Machined Metals Rank

The table below ranks the most commonly used metals in CNC machining from softest to hardest, with ranges accounting for the variety of alloys you see in a given metal (e.g., Aluminum 6061, Aluminum 6082, and Aluminum 7075).

In the instance where a traditionally hard metal won’t suit your project, there are options for making a low-hardness metal harder, if that particular material fits your project better. For example, solution annealed stainless steel 17-4 is less hard than the same alloy that’s been heat treated.

Mild steels, carbon steels, alloy steels, and aluminum alloys are commonly treated to make them harder. See the table below:

For engineers, understanding how hardness affects machining operations is essential for optimizing process efficiency, ensuring part quality, and controlling costs. It informs everything from RFQ evaluations to tool selection and CAM programming.