Introduction to quenching
Date:2025-06-05
I. Core purpose
Obtain high hardness and high wear resistance: This is the main purpose of quenching. Through rapid cooling, the high-temperature austenite structure is transformed into a high-hardness martensite (or bainite) structure.
Improve strength and yield limit: The martensite structure itself has a very high strength.
Prepare the structure for subsequent tempering: The martensite obtained by quenching is usually hard and brittle, with large internal stress, and cannot be used directly. Tempering must be used to adjust its hardness and toughness to obtain the required comprehensive mechanical properties.
II. Basic principles
The core of quenching is to use non-equilibrium cooling to inhibit the diffusion-type transformation of high-temperature austenite to equilibrium structure (pearlite, bainite), so that it undergoes a diffusion-free phase transformation when cooled to a lower temperature (below the martensite start transformation temperature Ms point) to form martensite.
Austenitization: Heating the steel to a critical temperature (Ac3 or above Ac1) and keeping it warm, so that the original structure (such as ferrite and cementite) is completely transformed into a single-phase austenite structure with uniform composition and fine grains.
Rapid cooling: Cooling the austenitized workpiece rapidly at a rate greater than the critical cooling rate of the material, so that the austenite does not have time to undergo diffusion-type decomposition (to form pearlite or bainite).
Martensitic transformation: When cooled below the martensitic transformation temperature Ms, austenite is instantly transformed into martensite in a shear manner (without carbon atom diffusion). Martensite is a supersaturated solid solution of carbon in α-Fe with a body-centered tetragonal lattice structure.
Characteristics: Martensite is the hardest and strongest structure in quenched steel, but it is very brittle and has extremely poor plastic toughness.
Transformation characteristics: Martensitic transformation only occurs between Ms and Mf (martensitic transformation termination temperature), which is an incomplete transformation (even if cooled below Mf, there is still a small amount of residual austenite). The transformation is completed instantly, and the amount of transformation mainly depends on the temperature reached by cooling (the more below the Ms point, the greater the transformation amount), which has nothing to do with the holding time. The transformation is accompanied by volume expansion, resulting in large structural stress and thermal stress inside the workpiece.
III. Quenching process
Pretreatment:
Cleaning: Remove impurities such as oil and scale on the surface of the workpiece to ensure uniform heating and prevent surface defects.
Preheating (optional): For workpieces with complex shapes, large cross-sectional changes or high-alloy steel, preheating (below the critical point temperature) can be performed first to prevent deformation and cracking caused by heating too quickly.
Heating:
Temperature selection: The heating temperature depends on the chemical composition of the steel (mainly the carbon content) and the required structural properties.
Hypoeutectoid steel: Usually heated to 30-50°C above Ac3 (complete austenitization).
Eutectoid steel and hypereutectoid steel: Usually heated to 30-50°C above Ac1 (incomplete austenitization, retaining undissolved carbide particles, which helps to improve wear resistance and refine grains). Excessive heating temperature will lead to coarse austenite grains, coarse martensite after quenching, and brittle performance.
Equipment: Commonly used box furnaces, pit furnaces, salt bath furnaces, controlled atmosphere furnaces, vacuum furnaces, etc.
Insulation:
Keep enough time at the quenching temperature to make the temperature inside and outside the workpiece uniform, and complete the homogenization of austenite composition and grain growth (need to control to avoid overheating).
The insulation time depends on factors such as the effective thickness of the workpiece, the amount of furnace loading, the heating medium, and the type of steel.
Cooling:
Core step: Rapidly immerse the heated and insulated workpiece into the selected quenching medium for cooling.
Key requirements: In the temperature range where austenite is most unstable (about 650-550°C, near the "nose" of the C curve), the cooling rate must be greater than the critical cooling rate of steel (Vc) to avoid the formation of non-martensitic structures (pearlite, bainite).
