Seamless Steel Pipe Austempering Precautions

Austempering is an important process in the production of seamless steel pipe. In the process of isothermal quenching, what are the precautions?

Isothermal quenching is one of the effective quenching methods for reducing the deformation and cracking of steel tubes, but how to correctly adopt this method is reasonable according to the requirements of the material, size, hardness, toughness, deformation, and specific working conditions of the seamless steel tubes. In the following, the following issues to be noted during isothermal quenching are summarised below for reference by heat treatment operators.

1. If isothermal quenching is performed on steels with lower hardenability such as carbon seamless steel tubes, the quenching heating temperature should be appropriately increased. H is to increase the stability of austenite, and to avoid and prevent the occurrence of isothermal processes. High temperature non-bainite transformation.
2. Austempering is limited by the size of steel and seamless steel tubes. In the case of isothermal quenching, the basic principle is to ensure that seamless steel tubes do not undergo a high-temperature transformation during the cooling process. Therefore, the type and size of seamless steel tubes directly affect the quality of isothermal quenching. The general requirements are carbon steel seamless tubes. The effective thickness should not exceed 5mm, and the effective thickness of alloy steel should be less than 30mm. From the viewpoint of carbon content, carbon steel at 0.4% to 0.6% is not suitable for isothermal quenching, and carbon steel above 0.6% can be isothermally quenched.
3. The temperature and time of isothermal quenching should be determined by referring to the C curve of the steel according to the performance requirements of the steel pipe. The principle is that all austenite is transformed into lower bainite, and the cooling medium is usually a salt bath solution. To ensure the stability of temperature, a cooling device should be added to the medium to prevent the increase of the medium temperature and affect the quenching quality. In addition, the length of isothermal time should be based on the end of the transformation of the seamless steel pipe. If it is too long, it will reduce the operating efficiency. Therefore, it must be taken seriously.
4. Some of the seamless steel tubes subjected to isothermal quenching may not be tempered. However, for partially quenched workpieces, the internal retained austenite will be transformed into martensite in the subsequent air-cooling process. Therefore, tempering must be performed in order to eliminate the brittleness and stability of the seamless steel pipe. The tempering temperature should be lower than the isothermal temperature.