What is the quenching and tempering of seamless pipes?

What is the quenching and tempering of seamless pipes?

Quenching and tempering refer to heat treatment processes used to improve the comprehensive mechanical properties of seamless pipes. That is, high temperature tempering treatment is performed after quenching. The purpose is to achieve a good combination of strength and toughness for seamless pipes, which not only have high strength, but also have excellent toughness, plasticity and cutting properties. Some alloy steel pipes or low alloy steel pipes must be quenched and tempered to obtain good overall properties. These steel pipes can also be called quenched and tempered steel pipes. After ductile iron is quenched and tempered, it can not only improve the strength, but also improve the hardness and wear resistance.

seamless pipe quenching and tempering steps and principles

Quenching is the first step. The heating temperature depends on the composition of the steel pipe. The quenching medium is selected according to the hardenability of the steel and the size of the steel pipe. After quenching, the internal stress of steel is very large and it is very brittle, so it must be tempered to eliminate stress, increase toughness, and adjust strength. Tempering is the most important process that ultimately determines the mechanical properties of quenched and tempered steel pipes. The curve of the mechanical properties of various steel pipes changing with tempering temperature, also known as the steel pipe tempering curve, can be used as the basis for selecting the tempering temperature. For high-temperature tempering of some alloy quenched and tempered steel pipes, attention should be paid to preventing the occurrence of the second type of temper brittleness to ensure the performance of the steel pipe.

Quenched and tempered steel

The so-called quenched and tempered steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%. Parts made of this type of steel are generally required to have good comprehensive mechanical properties, that is, they have good plasticity and toughness while maintaining high strength. Therefore, it is customary to call this type of steel quenched and tempered steel. It should be noted that sometimes quenched and tempered steel also refers to the process of adding manganese and silicon during smelting steel, so pay attention to the difference.

Heat treatment process of seamless pipe

The heat treatment process of seamless pipes includes annealing, normalizing, quenching, tempering and surface heat treatment. Tempering includes high temperature tempering, medium temperature tempering and low temperature tempering.

Reheating the quenched steel pipe to a certain temperature and then cooling it using a certain method is called tempering. Its purpose is to eliminate the internal stress generated by quenching, reduce hardness and brittleness, and achieve expected mechanical properties.

Quenching of seamless steel pipe

The process of heating the steel pipe to 30-50℃ above Ac3, keeping it warm for a period of time, and then cooling it rapidly to obtain M and B structures. M generally has high hardness and brittleness, and is the product of rapid cooling of A. In addition to M and B, the structure of the steel pipe after quenching also has a small amount of A residue, which are unstable structures. After quenching, thermal stress and structural stress will be generated inside the steel pipe, which can generally be eliminated and improved by tempering. Quenching and tempering (Q+T) can greatly improve the comprehensive performance of steel.

Several commonly used quenching methods for seamless steel pipes include:

Low-temperature quenching: Low-temperature quenching is a heat treatment method in which the seamless steel pipe is heated to a low temperature (usually 800-900°C) and then slowly cooled. This method can reduce the overheating phenomenon on the surface of the seamless steel pipe, thereby reducing the risk of deformation and cracking.

Medium-temperature quenching: Medium-temperature quenching is a heat treatment method in which the seamless steel pipe is heated to a medium temperature (usually 1000-1200°C) and then slowly cooled. This method can enhance the hardness and wear resistance of seamless steel pipes, but it also leads to an increased risk of deformation and cracking. High temperature quenching: High temperature quenching is a heat treatment method in which seamless steel pipes are heated to high temperatures (usually 1200-1500°C) and then rapidly cooled. This method can improve the hardness and wear resistance of seamless steel pipes, but the heating time and cooling rate need to be strictly controlled to avoid deformation and cracking of seamless steel pipes. Air cooling: Air cooling is a heat treatment method in which seamless steel pipes are slowly cooled in the air. This method can reduce overheating on the surface of seamless steel pipes, thereby reducing the risk of deformation and cracking. Oil cooling: Oil cooling is a heat treatment method in which seamless steel pipes are slowly cooled in oil. This method can improve the hardness and wear resistance of seamless steel pipes, but attention should be paid to the temperature and purity of the oil to avoid contamination or quality problems of seamless steel pipes. Seamless steel pipe quenching treatment

Process: Heat the steel part to the quenching temperature, keep it for a period of time, and then quickly cool it in water, brine or oil (individual materials in air). Purpose:

① To obtain higher hardness and wear resistance of steel parts;

② To make steel parts obtain some special properties after tempering, such as higher strength, elasticity and toughness.

Seamless steel pipe quenching category:

a. Single liquid quenching

Process: Heat the steel parts to the quenching temperature, keep them warm and cool them in the quenching agent. Single liquid quenching is only suitable for carbon steel and alloy steel parts with simple shapes and low technical requirements. During quenching, for carbon steel parts with a diameter or thickness greater than 5-8mm, use salt water or water cooling: alloy steel parts use oil cooling.

b. Double liquid quenching of seamless steel pipe

Process: Heat the steel parts to the quenching temperature. After keeping warm, quickly cool them in water to 300 ~ 400℃, and then move them into oil for cooling.

c. Flame surface quenching of seamless steel pipe

Process: Spray the flame of acetylene and oxygen mixture on the surface of the parts, so that the parts are quickly heated to the quenching temperature, and then immediately spray water on the surface of the parts. Flame surface hardening is suitable for single-piece or small-batch production. Large medium carbon steel and medium carbon alloy steel parts with hard and wear-resistant surfaces, such as crankshafts, gears, guide rails, etc., can withstand impact loads.

d. Surface induction heating quenching

Process: Put the steel part into the inductor, the inductor generates a magnetic field under the action of a certain frequency of alternating current, and the steel part generates an induced current under the action of the magnetic field, so that the surface of the steel part is quickly heated (2~10min) to the melting temperature. At this time, water is immediately sprayed on the surface of the seamless steel pipe. The surface of the seamless steel pipe quenched by surface induction heating is hard and wear-resistant, while the core maintains good strength and toughness. Surface induction heating quenching is suitable for medium carbon steel and alloy steel pipes with medium carbon content.

Note: Quenching and tempering usually refers to the heat treatment process of quenching + high-temperature tempering to obtain tempered troostite.