Understand normalizing, annealing, quenching and tempering (1)
The microstructure obtained by high-temperature tempering is tempered sorbite. The main purpose of tempering is to eliminate quenching stress and get the required structure. According to the different tempering temperature, tempering can be divided into a low temperature, medium temperature, and high temperature. Tempered Martensite, troostite, and sorbite were obtained.
The quenching and tempering heat treatment is called quenching and tempering heat treatment, which aims to obtain the strength, hardness, and plasticity, toughness is better comprehensive mechanical properties. Therefore, widely used in automobiles, tractors, machine tools and other important structural parts, such as connecting rods, bolts, gears and shafts. Hardness after tempering is generally HB200-330.
The main aim of annealing is to make the internal structure of the metal reach or close to the equilibrium state and to prepare for the subsequent processing and the final heat treatment. De-stress annealing is a kind of annealing process to eliminate the residual stress caused by plastic deformation processing, welding, and so on. There is internal stress in the workpiece after forging, casting, welding, and cutting. If it is not eliminated in time, it will deform the workpiece and affect the precision of the workpiece. It is vital to reduce the internal stress in the machining process by stress relief annealing. The heating temperature of the stress-free annealing is lower than the phase transformation temperature, so there is no microstructure transformation during the whole heat treatment process.
Internal stress is mainly through the workpiece in the thermal insulation and slow cooling process of natural elimination. To eliminate the stress inside the workpiece more thoroughly, the heating temperature should be controlled when heating. Generally, it is low temperature to enter the furnace, then with 100 °C / h or so heating speed heating to the specified temperature. The welding parts shall be heated at a temperature slightly above 600 °C. The holding time is usually 2 ~ 4h, depending on the case. The holding time of stress-relief annealing is limited, the cooling rate is controlled at (20 ~ 50) °C / H, and the air-cooling of casting cannot be finished until the temperature is below 300 °C. Aging treatment can be divided into natural aging and artificial aging two kinds of natural aging is to place the casting in the open field for more than half a year, then it occurs slowly, so as to eliminate or reduce the residual stress Artificial aging is the stress-relief annealing of castings heated to 550 ~ 650 °C. IT saves time and removes residual stress more thoroughly than natural aging.
3.What’s the Tempering?
Tempering is a heat treatment process which heats the hardened metal or parts to a specific temperature and cools them in a certain way after holding them for a particular time. Tempering is an operation carried out immediately after quenching. It is usually also the last step of heat treatment of the workpiece, so the combined process of quenching and tempering is called the final heat treatment. The primary purposes of quenching and tempering are:
1) reduce the internal stress and reduce the Brittleness; there is a lot of stress and Brittleness, such as not timely tempering often produce deformation and even cracking.
2) adjust the mechanical properties of the workpiece, the workpiece after quenching, high hardness, Brittleness, in order to meet the different performance requirements of various parts, can be adjusted through tempering, hardness, strength, plasticity, and toughness.
3) to stabilize the size of the workpiece. By tempering, the microstructure can be stabilized to ensure that it will not deform in future use.
4) improving the cutting properties of some alloy steels.
In production, often according to the performance requirements of the workpiece. According to the different heating temperature, the tempering is divided into low-temperature tempering, medium temperature tempering, and high-temperature tempering. Quenching and subsequent high-temperature tempering combined with heat treatment process known as quenching and tempering, that is high strength at the same time, good plastic toughness. It is mainly used for processing machine structural parts with a heavy load, such as machine tool spindle, automobile rear axle half axle, strong gear and so on.