UNDERSTAND NORMALIZING, ANNEALING, QUENCHING AND TEMPERING (3)
Quenching, a heat treatment process for metals and glass. Alloy products or glass heated to a certain temperature, and then in water, oil or air rapid cooling, generally used to improve the hardness and strength of the alloy. Commonly known as “dip fire. “. Heat treatment of metal cooled in air or water or oil after holding for a period of time by reheating the quenched workpiece to an appropriate temperature below the critical point. Iron and steel workpiece after quenching has the following characteristics:
1. The results show that the microstructure of Martensite, Bainite and retained austenite is unstable.
2. There is large internal stress.
3. The mechanical properties can not meet the requirements. Therefore, the steel workpiece after quenching is generally through tempering.
7.The purpose of tempering
1. To improve the stability of the organization, so that the use of the workpiece in the process of no organizational change so that the workpiece geometric size and performance remain stable.
2. To eliminate the internal stress, in order to improve the use of the workpiece performance and stability of the workpiece geometry.
Adjust the mechanical properties of steel to meet the use requirements.
Tempering has these effects because, when the temperature rises, the atoms become more active, and the atoms of iron, carbon, and other alloy elements in steel can be diffused more quickly to achieve the rearrangement of the atoms Thus, the unstable and unbalanced organization is gradually transformed into a stable and balanced organization. The elimination of the internal stress is also related to the decrease of the mental strength when the temperature increases. Generally, the hardness and strength of steel decrease and the plasticity increases after tempering. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with a high content of alloying elements, when tempered in a certain temperature range, will precipitate some fine metal compounds, which will increase the strength and hardness. This phenomenon is called secondary hardening.
Tempering requirements: different use of the workpiece should be tempered at different temperatures to meet the use of requirements.
1. Cutting tools, bearings, carburizing and quenching parts, and surface quenching parts are usually tempered at low temperature below 250 °C. After tempering at low temperature, the hardness changes little, the internal stress decreases, and the toughness increases slightly.
2. High elasticity and necessary toughness can be obtained by tempering at 350 ~ 500 °C at medium temperature.
3. Parts made of medium carbon structural steel are usually tempered at 500 ~ 600 °C to obtain proper strength and toughness.
The heat treatment process of quenching and high-temperature tempering is called quenching and tempering.
When tempered at about 300 °C, the Brittleness of steel is often increased. This phenomenon is called temper Brittleness of the first kind. This temperature range should not normally be tempered. Some medium carbon alloy structural steels are also prone to embrittlement if they are slowly cooled to room temperature after high-temperature tempering. This phenomenon is known as Type II temper embrittlement. The addition of Molybdenum to the steel, or cooling in oil or water during tempering, prevents the second type of temper embrittlement. This brittleness can be eliminated by reheating the second temper Brittleness steel to the original temper temperature.
8. Annealing of the Steel
Concept: A process in which steel is slowly cooled after heating and holding to obtain a near-balanced microstructure.
1. Complete annealing
Process: heating AC3 above 30-50 °c → Holding → furnace cooling to below 500 °c → air-cooled room temperature.
Objective: to refine grain size, homogenize structure, improve plasticity and toughness, eliminate internal stress, and facilitate machining.
2. Isothermal annealing
Process: heating above AC3→ heat preservation → rapid cooling to pearlite transition temperature → isothermal residence → conversion to P → Air Cooling from the furnace;
Purpose: DITTO. But the time is short, easy to control, deoxidization, decarburization is small. (suitable for undercooling relatively stable alloy steel and large carbon steel parts).
3. Spheroidizing annealing
Concept: The process of spheroidizing the cementite in steel.
Object: Eutectoid and Hypereutectoid steels
(1) Isothermal spheroidizing annealing heating above ac120-30 °c → heat preservation → rapid cooling below AR120 °c → isothermal → furnace cooling to about 600 °c → Furnace Air Cooling.
(2) Common spheroidizing annealing heating above ac120-30 °c → holding → prolonged cooling to about 600 °c → Furnace Air Cooling. Long Cycle, low efficiency, not applicable.
Objective: to reduce the hardness, improve the plasticity and toughness, and facilitate the cutting process.
Mechanism: The plate or net cementite into granular (spherical)
Description: annealing heating, the organization is not entirely, so also known as incomplete annealing.
4. Stress Relief Annealing
Process: Heating to a temperature below AC1(500-650 degrees)→ holding → slow cooling to room temperature.
Objective: to eliminate the residual internal stress of casting, forging and welding parts, and to stabilize the size of the workpiece.
9. Tempering of the Steel
Process: The quenched steel is reheated to a temperature below A1 and then cooled (usually air cooled) to room temperature.
Objective: to eliminate the internal stress caused by quenching, stabilize the size of workpiece, reduce Brittleness, and improve the machinability.
Mechanical Properties: With the increase of tempering temperature, hardness, and strength decreased, plastic toughness increased.
1. Low-temperature tempering: 150-250 °C, m back, reduce internal stress and Brittleness, improve plastic toughness, have higher hardness, and wear resistance. Used for making measuring tools, cutting tools and rolling bearings, etc.
2. Medium temperature tempering: 350-500 °C, T tempering, with high elasticity, certain plasticity, and hardness. Used for making spring, forging die, etc.
3. High-temperature tempering: 500-650 °C, s tempering, with excellent comprehensive mechanical properties. Used for making gears, crankshaft, etc.