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Metallic materials consist of a microstructure of small crystals called “grains” or crystallites. The nature of grains is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of diffusion and the rate of cooling within the microstructure. Heat treatment is often used to alter the mechanical properties of a metallic alloy, manipulating properties such as the hardness, strength, toughness, ductility, and elasticity.
There are two mechanisms that may change an alloy’s properties during heat treatment: the formation of martensite causes the crystals to deform intrinsically, and the diffusion mechanism causes changes in the homogeneity of the alloy.
Automotive OEM manufacturers always look for the most suitable material to fulfill desired engine applications. Cast irons (eg. QT700-2), non-quenched steels (eg. 48MnV), and/or tempered steels are their major choices over long-term research and development. These materials are less expensive, easier to machine and simultaneously, the material performance matches the requirements of most street cars.
Examples of typical properties you can expect:
| MATERIAL | Rm (MPa) | Rel (MPa) | A (%) | Z (%) |
|---|---|---|---|---|
| QT700-2 (cast iron) | ≧700 | ≧400 | ≧2 | - |
| 48MnV (non-quenched steel) | ≧689 | ≧400 | ≧13 | ≧26 |
The racing and high performance stock market relentlessly pursue the highest horsepower, torque, and RPM. While the excellent strength performance material is very important, and the superior microstructure through correct heat treatment process become essential.
Heat Treatment, when combined with the right choice of material, can produce desired mechanical properties and steel structures for different applications. For example, for our AISI4340 raw steel bars, we only choose Hot Rolling Annealing status. This steel bars’ status ensures that the internal structure reaches or approaches the equilibrium state, performs well during machining, and is well prepared for quenching and tempering.
Quenching, is the process by heating steel to a temperature above Ac3 (critical point temperature of austenite transformation), keeping the temperature for a certain length of time, and cooling it down at the appropriate cooling rate, resulting in a martensite
structure. Quenching improves hardness, strength, and wear resistance, however, it turns the steel brittle. This brittleness can be eliminated through timely tempering.
Tempering, is the process of keeping quenched steel at a specific temperature for a certain length of time, then allowing it to cool down. Its main purpose is to reduce brittleness while simultaneously releasing the internal stress accumulated while quenching. We can get desired plasticity and toughness through the tempering process. In order to meet different performance requirements for various components, hardness can be adjusted through tempering – reducing brittleness and obtaining the required strength, toughness, and plasticity.
Core Hardening normally includes quenching followed by temper-
ing. After Core Hardening, the material will have the tempered sorbite structure, which offers superior mechanical properties over normalized sorbite structure under the same hardness.
At BW, we do Core Hardened 100% in-house, ensuring specifications:
| MATERIAL | Rm (MPa) | Rel (MPa) | A (%) | Z (%) | KU2 (J) |
|---|---|---|---|---|---|
| AISI 4340 | ≧980 | ≧785 | ≧12 | ≧55 | ≧78 |
According to the empirical formula of quenching heating time: T = a*K*D
The quenching parameters need to be adjusted according to different component dimensions (related to D) and different furnace loading (related to K), to achieve the best material performance. At BW, we quench using a small heating furnace in order to fine-tune parameters, matching every individual component dimension.
provides stable re-assembling accuracy