Metal flame quenching
Metal flame hardening is a heat treatment process that uses a gas flame to rapidly heat the metal surface and then quickly cool it to achieve surface hardening. The core principle is to heat the metal surface to the austenitizing temperature through a high-temperature flame, and then quickly cool it by spraying water or mist, so that the surface is transformed into martensite structure, thereby obtaining high hardness and wear resistance, while the core still maintains its original toughness. This process has a long history, simple equipment and flexible operation. It is particularly suitable for large, complex-shaped or single-piece small-batch production of metal parts, such as machine tool guides, rollers, gears, etc. For example, after flame quenching, the surface hardness of the guide rails of large machine tools is significantly improved, and they can withstand long-term friction and loads. At the same time, the toughness of the core ensures that the guide rails will not break due to impact, greatly extending the service life of the equipment.
The metal flame hardening process primarily involves pretreatment, flame heating, cooling, and quality inspection. Pretreatment requires cleaning the metal surface to remove impurities such as oil, rust, and scale to ensure uniform heating and high-quality quenching. Flame heating is a critical step, typically using an oxy-acetylene flame, though mixtures such as oxy-propane or oxy-natural gas may also be used as needed. The flame temperature can reach over 3000°C, rapidly heating the metal surface to the desired temperature. During operation, the flame nozzle is maintained at a distance from the workpiece surface (typically 5-15mm) and moved at a moderate speed to ensure uniform heating and minimal overheating. The heating time depends on the workpiece material, thickness, and desired hardened layer, generally ranging from a few seconds to tens of seconds.
The effectiveness of the cooling process directly affects the quality of quenching. The commonly used cooling medium is water, and sometimes oil or brine is also used. There are two cooling methods: continuous cooling and spray cooling. Continuous cooling is suitable for flat or simply curved parts. It cools the heated workpiece directly by immersing it in water. Spray cooling is suitable for parts with complex shapes. Cooling water is atomized through a nozzle and sprayed onto the heated surface, making the cooling more uniform. The cooling rate must be fast enough to ensure the transformation of austenite to martensite, but it is also necessary to avoid excessive cooling that may cause cracking in the workpiece. For materials prone to cracking, such as high-carbon steel, segmented cooling can be used, first air cooling for a period of time and then spraying water to reduce internal stress.
Flame quenching of metals offers advantages such as low equipment investment, flexible operation, and a wide range of applications, but it also has some limitations. Its main advantages include: simple equipment, requiring only basic equipment such as a flame spray gun, oxygen cylinders, and acetylene cylinders, resulting in low cost; the ability to perform localized quenching on large parts without the need for large heating equipment; and flexible operation, with the flame nozzle’s movement path and speed adjustable according to the part’s shape. However, flame quenching is difficult to precisely control the heating temperature, making overheating and uneven heating prone to occur, resulting in large fluctuations in surface hardness. Furthermore, the surface roughness after quenching is high, sometimes requiring subsequent processing. Furthermore, flame quenching requires high operator skill and extensive experience to ensure quality.
With the development of industrial technology, metal flame quenching technology is also constantly improving and perfecting. The research and development of new flame nozzles has improved the uniformity and efficiency of heating. For example, multi-head nozzles can heat multiple parts at the same time, improving production efficiency; the application of automatic flame quenching equipment realizes the automatic control of the heating and cooling process, reduces the influence of human factors, and improves the stability of quenching quality. In addition, by combining with computer simulation technology, the temperature field distribution during the flame heating process can be simulated in advance, and process parameters such as nozzle movement speed and flame power can be optimized to further improve the quenching effect. In the future, metal flame quenching technology will continue to develop in the direction of automation and precision. While maintaining its flexibility and low-cost advantages, it will continuously improve processing quality and reliability, providing the machinery manufacturing industry with better surface hardening solutions.
