What is the purpose of quenching carbon structural steel?

What is the purpose of quenching carbon structural steel?
The quenching process for carbon structural steel involves heating the steel to the austenitizing temperature (typically 30-50°C above Ac3, requiring complete austenitization for hypoeutectoid steel), holding the steel at this temperature for a period of time to allow the microstructure to fully transform into austenite, and then rapidly cooling it in a cooling medium such as water or oil. Its core purpose is to achieve a "breakthrough" improvement in mechanical properties by altering the steel's internal microstructure (transforming austenite into non-equilibrium structures such as martensite or bainite), particularly for strength and hardness requirements. This process can be categorized into the following four core objectives:
1. Significantly improve steel hardness and wear resistance
The equilibrium microstructure of carbon structural steel (e.g., pearlite + ferrite) has a relatively low hardness (e.g., the hot-rolled hardness of Q235 steel is approximately HB150-180, and that of 45 steel is approximately HB170-210). This makes it unsuitable for applications requiring wear and scratch resistance (e.g., wear-resistant surfaces on mechanical parts and tool edges). During quenching, the austenite rapidly cools (the cooling rate must exceed the "critical cooling rate"). The atoms don't have time to diffuse and form equilibrium pearlite, and are instead forced to form martensite. Martensite is a supersaturated solid solution of carbon in α-Fe, with significant lattice distortion and numerous internal dislocations. This significantly increases hardness:
Medium-carbon structural steel (such as 45 steel) can reach a hardness of HRC55-60 after quenching, far exceeding that of hot-rolled steel.
High-carbon structural steel (such as 65 steel) can even reach a hardness of HRC60-65 after quenching, approaching the level of ordinary tool steel.
This high hardness effectively improves the wear resistance of parts and reduces wear during use (e.g., quenching machine tool guideways and gear tooth flanks).
2. Significantly Improves Steel Strength and Deformation Resistance
In addition to hardness, quenching significantly increases the tensile strength, yield strength, and deformation resistance of carbon structural steel. Ferrite in the equilibrium structure has high plasticity but low strength, while pearlite has moderate strength. Martensite, however, can achieve a 50%-100% increase in strength due to lattice distortion and dislocation strengthening.
For example, 45 steel has a hot-rolled tensile strength of approximately 600 MPa. After quenching (without tempering), the tensile strength rises to over 1000 MPa, and the yield strength increases from 355 MPa to over 800 MPa.
For parts that bear loads, resist impact, or resist deformation (such as shafts, connecting rods, and bolts), the high strength after quenching ensures that the parts do not deform or fracture under complex operating conditions, thus meeting structural load-bearing requirements. 3. Lays the foundation for the subsequent "tempering" process, achieving a "strength-toughness balance."
Although carbon structural steels achieve extremely high hardness and strength after quenching, they have significant drawbacks: high internal stress in the martensite structure, poor ductility, and extremely low toughness (prone to brittle fracture, with impact energy absorption typically only 5-10J). Direct use can easily lead to failure due to minor impacts or stress concentrations. Therefore, quenching is rarely used as a "final heat treatment," but rather as a precursor to the tempering process. Its core function is to provide a "high-hardness, high-strength foundational structure" for subsequent performance adjustments.
Subsequent tempering at different temperatures (low-temperature tempering 150-250°C, medium-temperature tempering 350-500°C, and high-temperature tempering 500-650°C) can eliminate internal martensite stress and improve toughness while retaining most of the quenched strength.
For example, after quenching and high-temperature tempering (quenching and tempering) of 45 steel, the hardness is reduced to HRC 28-32, while the tensile strength remains above 800 MPa and the impact energy absorption is increased to over 50J. This achieves an ideal balance of "high strength and high toughness," making it the most commonly used comprehensive performance optimization solution in machinery manufacturing. 4. Improving Local Performance and Meeting "Local Strengthening" Requirements
Some carbon structural steel parts do not require overall high strength (to avoid overall brittleness) and only require strengthening of critical working areas (such as wear surfaces and areas of concentrated stress). In these cases, quenching can achieve "local performance improvements" through a "local heating + local cooling" approach.
For example, machine tool guide rails require surface wear resistance (high hardness), but the guide rail matrix must maintain toughness (to avoid impact fracture). This can be achieved through "induction heating surface quenching." This involves heating only the guide rail surface to austenitization and rapidly cooling it, forming quenched martensite (hardness HRC 50-55) on the surface while maintaining the matrix's original equilibrium structure (maintaining toughness).
For another example, the head or threaded portion of a bolt requires shear and wear resistance. Local quenching can achieve critical performance while avoiding overall brittleness and reducing the risk of part failure.