Heat Treatment of Quenched and Tempered Steel: Core of Process and Key Points of Quality Control
As a key structural material in the field of mechanical manufacturing, the performance of quenched and tempered steel highly depends on the quenching and high-temperature tempering heat treatment process. This process changes the internal metallographic structure of the steel, achieving the optimal matching of strength and toughness, and providing reliable mechanical performance guarantees for core components such as gears and shafts.
Basic Process Flow and Core Parameters
The standard process of heat treatment for quenched and tempered steel includes five key links: preheating, austenitizing heating, heat preservation, quenching cooling, and high-temperature tempering. Taking the commonly used 40Cr steel as an example, its typical process parameters are as follows: after preheating at 500-600℃ in a box-type heat treatment furnace, it is heated to 840-860℃ for austenitizing heating. The holding time is controlled at 1-1.5 minutes per millimeter of the effective thickness of the workpiece to ensure that carbides are fully dissolved and evenly distributed. Then, it is quickly transferred to oil for quenching (the cooling rate must be sufficient to obtain martensite structure). After the workpiece is cooled to below 150℃, it is heated to 550-600℃ for high-temperature tempering, and air-cooled after holding for 2-4 hours.
There are significant differences in process parameters among different steel grades. For high-strength quenched and tempered steels containing nickel and molybdenum (such as 40CrNiMoA), the austenitizing temperature needs to be increased to 860-880℃ to promote the dissolution of alloying elements. The tempering temperature is adjusted according to performance requirements - about 500℃ is selected when high hardness is required, and it can be increased to 600℃ when high toughness is pursued.
Key Points of Quality Control and Solutions to Common Problems
Temperature uniformity during the heat treatment process is the core to ensuring the stable performance of quenched and tempered steel. If the temperature difference in the furnace exceeds ±10℃, it is easy to cause the hardness deviation of different parts of the workpiece to exceed 3HRC. The use of box-type furnaces with multi-zone temperature control or continuous heat treatment production lines, combined with multi-point monitoring of thermocouples, can effectively control temperature fluctuations.
The selection of quenching medium directly affects the cooling rate and workpiece deformation. For thin-walled parts with complex shapes, using fast quenching oil at 20-30℃ can reduce the risk of cracking; for large forgings, water-oil double-liquid quenching is commonly used, which first cools quickly to about 300℃ with water, then transfers to oil for slow cooling to balance the depth of the hardened layer and the amount of deformation. Special attention should be paid to eliminating internal stress during the tempering process. Through stepwise heating (at a rate of 100-150℃ per hour) and furnace cooling after the end of heat preservation, the residual stress of the workpiece can be reduced by more than 60%.
Among common quality defects, "soft spots" are mostly caused by insufficient heating or uneven cooling, which can be solved by extending the holding time and cleaning the oxide scale on the surface of the workpiece; temper brittleness can be inhibited by avoiding the brittle temperature range of 400-500℃ or by rapid water cooling after tempering.
Technological Development and Application Expansion
With the penetration of intelligent manufacturing technology, the heat treatment of quenched and tempered steel is moving towards precision and intelligence. An AI adaptive control system introduced by a heavy industry enterprise can automatically adjust the heating power and quenching timing according to the real-time monitored furnace temperature curve and workpiece temperature feedback, improving the performance consistency of mass-produced gear blanks by 25%. The application of vacuum heat treatment technology has effectively solved the problem of oxidation and decarburization in traditional processes, reducing the surface roughness of precision bearing steel to below Ra0.8μm.
In the field of new energy equipment, the heat treatment process of quenched and tempered steel is constantly innovating. For 42CrMo steel used in wind power main shafts, the developed "intercritical quenching + segmented tempering" process, while ensuring the tensile strength is ≥900MPa, has increased the impact toughness to more than 80J, meeting the service requirements of -40℃ low-temperature working conditions and promoting the application expansion of quenched and tempered steel in extreme environments.
Your message must be between 20-3,000 characters!