“The Gym for Metals!” — How Heat Treatment Makes Parts Stronger, More Stable, and More Durable

In the world of precision parts manufacturing, heat treatment stands alongside surface finishing as one of the most critical processes.
Its purpose is not just to make materials “harder,” but to stabilize the internal structure of the metal, enhance mechanical performance, extend service life, and ensure that machining precision is maintained during later assembly.
For CNC machining, heat treatment forms the hidden foundation of quality —without it, even the finest machining accuracy cannot be sustained over time.

🌡️ Why Is Heat Treatment Necessary?

After casting, forging, or cutting, metals often retain residual stresses caused by high temperatures.
If these stresses are not relieved, parts may develop:

• Deformation or dimensional drift after machining

• Surface cracks or fatigue failures

• Insufficient hardness leading to rapid wear

• Strength degradation over long-term use

Heat treatment modifies the grain structure and carbon distribution within the metal,achieving an ideal balance between hardness, toughness, and stability.
In other words, it is a structural reorganization inside the metal,transforming the material from within to make it stronger and more reliable.

⚙️ Common Heat Treatment Methods and Characteristics

🧩 How Heat Treatment Affects Dimensions

While heat treatment enhances strength and hardness, it also causes changes in size and shape.
During quenching, for example, rapid cooling creates large temperature differences between the inner and outer layers, leading to uneven stress — and thus deformation or cracking.

To prevent this, engineers apply three key strategies during the design and machining stages:

1. Machining Allowance
   Dimensions are intentionally offset before heat treatment (e.g., +0.02–0.05 mm on diameters) to allow post-grinding corrections.

2. Shape Fixing with Jigs
   Fixtures are used to prevent distortion in long shafts or thin-walled components during stress relief.

3. Stepwise Heat Treatment
   For example, annealing (to remove stress) is performed before quenching and tempering to minimize deformation.

🧠 Heat Treatment Strategies for Different Materials

Each material requires a specific heat treatment approach — the wrong one can lead to brittleness or reduced strength.

• Carbon Steel (S45C, SCM440) → Quenching + Tempering to balance hardness and toughness.

• Stainless Steel (SUS420, SUS440C) → Vacuum heat treatment to prevent oxidation and maintain a bright surface.

• Tool Steel (SKD11, SKH51) → High-temperature tempering to ensure high hardness and wear resistance.

• Aluminum Alloys (A6061, A7075) → T6 treatment for precipitation hardening to boost strength and ductility.

Professional engineers design heat treatment conditions and temperature curves based on part function, tolerance requirements, and operating environment,
then verify the results through hardness testing and microstructure analysis to ensure stable and consistent properties.

🔍 Practical Example

Take a high-strength pin shaft as an example.
Without heat treatment, its tensile strength is only about 80 kgf/mm²,making it prone to wear and breakage under continuous load.
After carburizing and tempering, the surface hardness reaches HRC 58,while the core remains tough, tripling its wear resistance.
To minimize dimensional changes, machining allowances are planned in advance,and post-heat-treatment grinding keeps dimensions within ±0.005 mm.
This ensures that the component performs reliably even under heavy loads.