The heat treatment requirements in mechanical machining are essentially technical specifications at the design stage.
Understanding material usage and processing changes forms a systematic knowledge structure. It’s not isolated but interconnected.
Primarily, we need to understand how many types of heat treatment methods are involved in the mechanical machining of parts. Given their frequency and importance, we focus on several key methods for analysis.
In mechanical machining, the most commonly used heat treatment methods are annealing, normalizing, quenching, and tempering.
01 Annealing Treatment
The definition of annealing treatment: A metal heat treatment process where metal parts are heated to a certain high temperature, held for a period, and then allowed to cool naturally.
Its main functions:
A. Reduce the hardness of the parts, improve machining performance;
B. Eliminate residual stresses in the parts, stabilize dimensions, and reduce the probability of deformation and cracking;
C. Refine grain structure, adjust the organization, eliminate material structure defects;
D. Uniform material structure and composition, improve material performance, or prepare for subsequent heat treatment processes.
02 Normalizing Treatment
The definition of normalizing treatment: A metal heat treatment process where metal parts are heated to a certain high temperature, held for a period, and then cooled in air using methods such as water spraying, misting, or blowing, unlike annealing treatment, its cooling rate is faster, resulting in finer material structure and improved mechanical properties.
Its main functions:
A. Remove internal stresses from the material;
B. Reduce material hardness and increase plasticity.
03 Quenching Treatment
The definition of quenching treatment: A heat treatment process where metal parts are heated to above the critical temperature Ac3 or Ac1, held for a period to transform all or part into austenite, and then rapidly cooled at a rate greater than the critical cooling rate to below Ms for martensitic transformation.
Its main functions:
A. Significantly increase the rigidity, hardness, wear resistance, and fatigue strength of the parts;
B. Meet the physical and chemical properties such as ferromagnetism and corrosion resistance of certain special steels.
04 Tempering Treatment
The definition of tempering treatment: A heat treatment method where steel, after quenching hardening or normalization treatment, is reheated to a temperature below the critical temperature for a period and then cooled at a certain rate to increase the material’s toughness.
Its main functions:
A. Eliminate residual stresses generated during quenching, prevent deformation, and cracking;
B. Adjust the hardness, strength, plasticity, and toughness of the workpiece to achieve better performance requirements;
C. Stabilize the structure and size to ensure accuracy;
D. Improve and enhance machining performance.
Attention, here comes the essence:
A. Annealing and normalizing can usually be used interchangeably, especially when the hardness of the parts obtained after treatment is not high (generally based on not affecting machining performance). We should prioritize normalizing treatment because it has a shorter processing cycle, and consequently lower costs.
B. Tempering generally needs to be used in conjunction with quenching or normalizing. Tempering is to “clean up” after quenching and normalizing because after these processes, the hardness of the parts will be too high, resulting in significant residual stresses, especially after quenching, when the parts exhibit significant brittleness. Tempering is usually necessary to “correct” and better meet our usage requirements.
As a mechanical design engineer, when our designed parts need heat treatment, the requirements are generally as follows:
A. To eliminate casting stress in materials, aiming to achieve more stable machining dimensions and accuracy;
B. To enhance the cutting performance of parts, aiming for higher machining efficiency, better machining quality, and lower machining costs;
C. To improve the rigidity, hardness, and wear resistance of parts.
Most of our heat treatment requirements for parts are designed around these three main aspects. Therefore, you only need to use the four heat treatment methods corresponding to your requirements.
Let’s take an example:
Suppose we design the bed of a vertical machining center, and we choose HT300 gray cast iron as the material. The process flow for machining it is roughly as shown in the below：
A. Upon receiving the casted blank, the first step is usually annealing.
The purpose of annealing is to eliminate residual internal stresses in the casting, improve the part’s machining performance. Some manufacturers may skip this step to save costs, opting instead to extend the cooling time during casting to partially relieve internal stresses, although this is a somewhat speculative approach. However, according to standard practices, annealing is essential for casted blanks.
B. Next is the rough machining process of the parts.
Since there are no strict requirements for the dimensions of the parts during rough machining, factories typically employ heavy cutting methods. During heavy cutting, the impact of the milling cutter on the part generates a certain degree of vibration, which serves as a stress-relieving process. However, this process also generates additional stress, prompting a secondary annealing treatment of the parts.
C. The purpose of the secondary annealing treatment is the same as the first one:
to stabilize the material structure, improve machining performance, and eliminate internal stresses in the parts. This is necessary to ensure that the dimensions and positional tolerances of the parts remain stable over time.
D. Semi-finished machining of the parts involves relatively small cutting volumes, so excessive machining stress is usually not a concern.
However, if the dimensional accuracy and positional tolerances of the parts are high, it is recommended to allow the parts to sit for some time before proceeding to finish machining. This allows the parts to naturally release some stress, ensuring the stability of the final product. Many overlook this process, preferring to streamline machining operations for apparent efficiency, but this can compromise quality.
E. During the precision machining process, after the parts have been allowed to sit for some time, the material becomes relatively stable.
Precision machining requires a high level of skill from the operator. Precision is not solely dependent on the accuracy of the machining equipment but can also be influenced by the clamping method. Especially for parts with poor strength and rigidity, careful attention must be paid to clamping to avoid deformation. Over-tightening the clamp can cause the part to deform, affecting machining precision. Therefore, clamping force is crucial during precision machining, a well-kept secret among experienced machinists.
The above illustrates the application of heat treatment processes involved in the mechanical machining of parts. There are many similar processes, such as quenching, carburizing, and nitriding, which require practical experience and accumulation in actual production. This capability serves as a testament to the excellence of a machining factory.
