Is the heat treatment process the same for different metal materials?

一, The basis for differentiating process concepts
The main idea behind metal heat treatment is to use temperature fields to regulate the movement of atoms and the change of phases. However, various metals have quite diverse ways of arranging their atoms and changing phases.
Iron-based alloys (steel): According to the iron-carbon phase diagram, processes like austenitization and martensitic transformation make the material stronger. For instance, to fully austenitize the structure of 45 # steel, it needs to be heated to 840–860 °C and then cooled quickly to make martensite, which has a hardness of HRC50–55.
Aluminum alloy: depends on solid solution strengthening and aging strengthening mechanisms. For example, when working with 6061 aluminum alloy, solution treatment means holding it at 530–540 °C for 4–6 hours to completely dissolve the Mg ₂ Si phase. Then, it is quenched in water to make a supersaturated solid solution, and finally, it is aged at 175 °C to make nanoscale strengthening phases. It is possible to raise the tensile strength from 180MPa to 310MPa.
For titanium alloys, heat treatment in the beta phase area controls the microstructure. By air cooling TC4 titanium alloy after solid solution treatment in the β phase region at 980–1020 °C, it can get a layered α+β structure with a tensile strength of 1100 MPa. If isothermal treatment is done at 850 °C, an equiaxed alpha phase can be formed, and the elongation rate can be raised to 15%.
二, Different ways of doing typical processes
Different types of metal have developed their own heat treatment process methods because they have different performance needs.
1. Different ways to work with steel materials
Quenching and tempering: To get high hardness (HRC62-65) and wear resistance, high carbon tool steel (like T10A) needs to be quenched at 1000–1050 °C and tempered at 200 °C. To get a tempered martensite structure, 42CrMo quenched and tempered steel needs to be quenched at 850 °C and tempered at 550 °C. This structure combines strength (σ b ≥ 1080MPa) and toughness (ak ≥ 39J).
Unique process: Cryogenic treatment (-196 °C liquid nitrogen cooling) can turn leftover austenite into martensite, making 9Cr18Mo bearing steel 1-2HRC harder; induction heating surface quenching can make a 5mm thick hardened layer on the gear's surface while keeping the core tough.
2. Processes that are only for non-ferrous metals
T6 treatment (solid solution+artificial aging) is the typical way to make 6000 series aluminum alloys stronger. To balance strength and stress corrosion resistance, 7075 aluminum alloy needs T74 two-stage aging (120 °C/24h → 160 °C/8h).
Copper alloy: Beryllium bronze (QBe2) forms the γ 'phase when it is aged at 320–340 °C, and it has a hardness of HRC38–42. Brass (H62) is stress-relieved at 300–350 °C to get rid of cold work hardening.
Magnesium alloy: After being treated in a solution at 415 °C, AZ91D magnesium alloy forms the β - Mg ₁₇ Al ₁₂ phase by aging at 175 °C. This makes the yield strength 30% stronger.
3. Unique needs for refractory metals
Tungsten alloy: needs recrystallization annealing at 1400–1600 °C to get rid of cold work hardening, and the grain size must be kept at ≤ 50 μm to keep the strength at high temperatures.
Molybdenum alloy: To fix radiation damage and bring back the performance of its nuclear reactor structural materials, it is annealed at a high temperature of 1800–2000 °C.
三, Laws that are common to process design
Even if the process parameters are very diverse, the heat treatment design for different types of metal materials follows these basic rules:
The phase diagram is the main idea behind all process designs. The critical temperature (Ac ₁, Ac ∝, Ms, etc.) is used to choose the heating temperature. For instance, to completely dissolve the carbides in 304 stainless steel, it needs to be heated to 1050–1100 °C.
Control of the cooling rate: Choose the right medium (water, oil, polymer, etc.) to control the cooling pace and get the tissue to change in a certain way. Water quenching can turn high-carbon steel into martensite, whereas oil quenching can keep low-alloy steel from breaking.
Stress relief annealing (for example, steel at 300–400 °C and aluminum alloy at 150–200 °C) is used to get rid of processing stress and keep things from bending or cracking. Instead of heat treatment, vibration aging (VSR) is widely used to reduce stress in aircraft aluminum alloy parts.
Collaborative surface modification: Using chemical heat treatment (carburizing, nitriding) and surface quenching together to get different levels of performance. For instance, gears are carburized with 20CrMnTi (930 °C × 8h), quenched, and tempered at low temperatures. This gives them a surface hardness of HRC58-62 and keeps them tough at the core.
四, The leading direction of process improvement
As materials science has advanced, heat treatment techniques have exhibited the following innovative trends:
Laser/electron beam local heat treatment is a precise way to control the temperature of small areas of tissue. It is used to make the gas film holes surrounding aircraft engine blades stronger.
Smart process system: It can forecast how tissue will change and improve parameters based on a digital twin heat treatment process simulation. For instance, GE uses ProCAST software to keep the turbine disk from deforming more than 0.1mm during quenching.
Technology for making things in a green way: Low pressure carburizing (LPC) substitutes traditional gas carburizing to cut down on CO₂ emissions. Quenching oil replacement technology (such PAG polymer) cuts down on VOC emissions.