Let's introduce the heat treatment process for carbon structural steel.
The core of the heat treatment process for carbon structural steel is to adjust the internal microstructure (such as ferrite, pearlite, martensite, etc.) of the steel by controlling the heating, holding, and cooling processes, thereby optimizing its mechanical properties (strength, hardness, ductility, and toughness) to suit different applications. Because carbon structural steel has an extremely low alloying element content (containing only basic elements such as C, Si, and Mn, with no or minimal other alloying elements), its heat treatment processes are characterized by simplicity, economy, and targeted strengthening. They are primarily categorized into two main types: bulk heat treatment and surface heat treatment. The following is a detailed introduction to these common process types:
1. Bulk Heat Treatment: Changing the Overall Microstructure and Properties of Steel
Bulk heat treatment involves heating, holding, and cooling the entire steel material. It is suitable for parts requiring optimized overall mechanical properties (such as shafts, gears, bolts, etc.). The most commonly used bulk heat treatment processes for carbon structural steel include annealing, normalizing, and quenching and tempering.
2. Surface Heat Treatment: Strengthens Only the Steel Surface, Maintaining Core Toughness
Surface heat treatment of carbon structural steel is primarily targeted for applications requiring high hardness/wear resistance on the surface while maintaining core plasticity/toughness (e.g., gears and crankshafts). Since it contains no alloying elements, surface strengthening relies on surface composition adjustment or rapid surface heating and cooling. Common processes include case hardening and carburizing (for low-carbon steel).
1. Surface Hardening (High-Frequency/Medium-Frequency Induction Heating Surface Hardening)
Core Principle: Using high-frequency (100-500kHz) or medium-frequency (500Hz-10kHz) induction current, the steel surface is rapidly heated to above Ac3 (the core temperature remains below Ac1, preserving the original microstructure). This is followed by immediate cooling by water spray, achieving a "hardened surface while maintaining a soft core." Process Features:
Fast heating speed (a few seconds to tens of seconds), small heat-affected zone (hardening only the surface 0.5-5mm), minimal part deformation;
No change in the surface chemical composition is required, strengthening the surface solely through rapid phase transformation;
Applicable to medium-carbon steel (such as 45# steel and 50# steel); surface quenching of low-carbon steel is less effective (the hardened layer is low hardness).
Applications: Crankshaft main journals, gear tooth surfaces, machine tool guideways, and other parts requiring surface wear resistance and core impact resistance.
2. Carburizing and Quenching (Specialized for Low-Carbon Structural Steel)
Core Principle: Low-carbon steel (such as Q235 and 20# steel) has a low carbon content (≤0.25%) and cannot achieve high hardness through direct quenching. Instead, carburizing is required to increase the surface carbon content to 0.8%-1.2% (forming a high-carbon surface layer). This is followed by quenching and low-temperature tempering, ultimately achieving a "highly hard and wear-resistant surface with a low-carbon, soft and tough core." Typical Process:
Carburizing: Place the part in a carburizing furnace (using a carburizing medium such as natural gas or propane) and hold it at 900-950°C for 4-10 hours to allow carbon atoms to penetrate the surface.
Quenching: After carburizing, cool directly to 820-850°C, hold, and then quench in water or oil (to form martensite on the surface).
Low-Tempering: Hold at 150-200°C for 1-2 hours to eliminate quenching stress and stabilize surface hardness.
Performance and Applications: Surface hardness can reach HRC58-62 (high wear resistance), while core hardness ≤250HB (high toughness). Suitable for parts subject to impact and surface wear (such as transmission gears and universal joint forks). III. Key Considerations for Heat Treatment of Carbon Structural Steel
Poor Hardenability: Avoid Quenching Large Sections:
Carbon structural steel does not contain alloying elements (such as Cr, Ni, and Mo), resulting in extremely low hardenability. Only small-section parts (such as a 45# steel shaft with a diameter ≤20mm) can be fully hardened through water quenching. Quenching large-section parts can easily form a "non-martensitic structure" (such as pearlite) in the core, leading to uneven internal and external properties and even cracking. Therefore, large-section parts are often treated with "normalizing" or "high-temperature tempering" instead of quenching. Processing Differences Between Low-Carbon and Medium-Carbon Steels:
Low-carbon steel (C ≤ 0.25%, such as Q235 and 20#): Has good plasticity but low strength. It is generally not quenched alone. Annealing/normalizing is used to improve workability, or carburizing and quenching are used to strengthen the surface.
Medium-carbon steel (C = 0.25%-0.6%, such as 45# and 55#): It is primarily suitable for quenching and tempering. The "hardness-toughness" balance can be achieved by adjusting the tempering temperature. It is a core material for mechanical parts.
Cooling Medium Selection:
Cooling rate directly determines quenching quality:
Water quenching: Fast cooling rate, suitable for medium-carbon steel (such as 45#), can achieve high hardness, but is prone to internal stress (which requires timely tempering to eliminate).
Oil quenching: Slower cooling rate than water, suitable for parts with slightly larger cross-sections, can reduce the risk of cracking, but has slightly lower hardness than water quenching.
Air cooling/furnace cooling: Used only for annealing and normalizing, not for quenching. Stress relief is key:
Residual stress remains within parts after quenching and welding. If not relieved, these parts are susceptible to deformation and cracking during processing or use. Therefore, low-temperature tempering (after quenching) or stress relief annealing (after welding) is necessary to release these stresses and stabilize part dimensions.
Summary
The heat treatment process for carbon structural steel focuses on low cost and targeted application. Annealing/normalizing achieves softening, stress relief, and improved workability; quenching and tempering enhances mechanical properties; and case quenching/carburizing achieves surface-to-core performance differentiation. In practical applications, the appropriate process must be selected based on the steel's carbon content (low carbon/medium carbon), part cross-sectional dimensions, and application requirements (wear resistance/impact resistance/ease of machining) to maximize the cost-effectiveness of carbon structural steel.