1. Introduction
In the field of ferrous metal casting, the mechanical properties (such as hardness, toughness, and strength), machinability, and service life of castings directly depend on subsequent heat treatment processes. Among these, annealing, normalizing, quenching, and tempering—known in the industry as the "Four Heat Treatments"—are core techniques for regulating the internal structure of ferrous metals (mainly cast iron and cast steel), eliminating defects, and achieving performance customization. Whether it is engine blocks in the automotive industry, gear shafts in mechanical manufacturing, or wear-resistant parts in construction machinery, all require reasonable combination of the "Four Heat Treatments" to meet the requirements of actual working conditions. From the perspective of a professional casting manufacturer, this report will deeply analyze the process characteristics and core functions of the "Four Heat Treatments" and reveal the technical logic behind the performance improvement of ferrous metal castings.
2. The "First Heat Treatment" – Annealing: Eliminating Internal Stress and Improving Microstructural Uniformity
2.1 Process Definition and Core Parameters
Annealing is a heat treatment process in which ferrous metal castings after casting are slowly heated to a specific temperature (usually 20-50°C above Ac3, where Ac3 is the final temperature at which ferrite transforms into austenite during heating), held at that temperature for a certain period (generally 1-4 hours per 100mm of casting thickness), and then cooled to room temperature at an extremely slow rate (≤50°C/h). According to different objectives, annealing can be subdivided into full annealing, spheroidizing annealing, stress-relief annealing, etc. In casting production, "stress-relief annealing" and "full annealing" are the most widely used types.
2.2 Analysis of Core Functions
1. Eliminating Casting Internal Stress and Preventing Casting Cracking
During the casting process of ferrous metals, due to uneven heat dissipation of the mold and large differences in cooling rates between different parts of the casting (e.g., thick-walled and thin-walled parts), "thermal stress" is generated inside the casting. Meanwhile, the volume shrinkage of the casting during solidification is constrained by the mold, leading to the formation of "structural stress". If these internal stresses are not eliminated, the casting is prone to deformation or even cracking during storage, machining, or service (such as irregular cracks in cast iron manhole covers and post-welding cracking in cast steel parts). Through slow heating and cooling, annealing allows sufficient diffusion of atoms inside the casting, gradually releasing the stress. Tests have shown that the internal stress of castings can be reduced by 60%-80% after annealing, significantly improving dimensional stability.
0. Improving Microstructural Uniformity and Reducing Hardness to Optimize Machinability
The structure of cast blanks often has "compositional segregation" (e.g., local enrichment of carbon in cast steel) and "coarse grains" (excessive grain growth caused by slow cooling rates), which result in uneven hardness of the casting and rapid tool wear during cutting (e.g., "chipping" when machining cast iron parts). Full annealing can recrystallize the pearlite, ferrite (or cementite) structure inside the casting, forming fine and uniform equiaxed grains while reducing compositional segregation. Taking gray cast iron as an example, the hardness before annealing may reach HB220-250, making machining difficult; after annealing, the hardness can be reduced to HB180-200, increasing cutting efficiency by more than 30% and achieving a lower machined surface roughness (reduced from Ra6.3 to Ra3.2).
0. Laying the Foundation for Subsequent Quenching Processes
For cast steel parts that require quenching strengthening (such as pins in construction machinery), direct quenching of the coarse as-cast structure is likely to cause "quenching cracks" or "soft spots" (local insufficient hardness) after quenching. Through spheroidizing annealing (for cast steel with high carbon content), the flaky cementite inside the casting can be transformed into spherical cementite, reducing the austenitizing temperature and laying a foundation for uniform structural transformation during subsequent quenching, thereby reducing quenching defects.
3. The "Second Heat Treatment" – Normalizing: Refining Grains and Enhancing the Comprehensive Mechanical Properties of Castings
3.1 Process Definition and Core Parameters
Similar to annealing, normalizing also requires heating the casting to 30-50°C above Ac3 (or Accm, the final austenitizing temperature for hypereutectoid steel) and holding it for a certain period (usually 10%-20% shorter than that of annealing). However, the key difference lies in the cooling method: normalizing adopts "air cooling" (natural cooling in air), with a cooling rate 3-5 times faster than that of annealing.
3.2 Analysis of Core Functions
1. Refining Grains and Improving the Balance Between Strength and Toughness of Castings
Due to the faster cooling rate, austenite does not have enough time to grow fully during cooling in the normalizing process, resulting in a finer pearlite structure (or sorbite, troostite), with the grain size reduced from 50-80μm (after annealing) to 20-30μm. According to the "Hall-Petch Relationship", finer grains lead to higher metal strength (σs) and hardness (HB), while maintaining good toughness (αk). Taking Q235 cast steel as an example, the tensile strength before normalizing is approximately 380MPa, and the impact toughness is about 40J/cm²; after normalizing, the tensile strength increases to 420MPa, and the impact toughness rises to 55J/cm², fully meeting the stress requirements of ordinary mechanical parts.
0. Simplifying Processes and Reducing Production Costs
Compared with the slow cooling of annealing, the air cooling of normalizing does not require special cooling equipment (such as a slow-cooling chamber in an annealing furnace), and the holding time is shorter, shortening the production cycle by 20%-30%. For mass-produced small and medium-sized castings (such as brackets for automobile chassis), normalizing can replace annealing as a "pre-treatment process". While ensuring machinability (with a hardness usually of HB190-220, slightly higher than that after annealing), it reduces the heat treatment cost per unit product, which is the core reason for the wide application of normalizing in the automotive casting industry.
0. Eliminating the "Widmanstätten Structure" Defect in Castings
Some cast steel parts (especially low-carbon cast steel) tend to form a "Widmanstätten structure" (ferrite precipitating along austenite grain boundaries in a needle-like distribution) when the cooling rate during casting is between that of annealing and quenching. This structure significantly reduces the toughness of the casting and makes it prone to brittle fracture. Through rapid cooling in normalizing, the precipitation of needle-like ferrite can be inhibited, and the Widmanstätten structure can be transformed into a fine pearlite + ferrite structure, restoring the impact toughness of the casting to a normal level (increasing by 40%-60%).
4. The "Third Heat Treatment" – Quenching: Achieving a Leap in Casting Hardness and Wear Resistance
4.1 Process Definition and Core Parameters
Quenching is a process in which the casting is heated to 30-50°C above Ac3 (for hypoeutectoid steel) or Ac1 (for hypereutectoid steel, the initial temperature at which pearlite transforms into austenite during heating), held at that temperature to fully austenitize the structure, and then rapidly immersed in a cooling medium (such as water, oil, or brine) for rapid cooling (the cooling rate must be higher than the "critical cooling rate" to prevent pearlite precipitation), resulting in a martensitic (or bainitic) structure. Key parameters include: heating temperature (usually 850-950°C for cast steel and 950-1050°C for high-chromium cast iron), holding time (to ensure sufficient structural transformation), and cooling medium (the cooling rate of water is approximately 200°C/s, and that of oil is about 50°C/s).
4.2 Analysis of Core Functions
1. Significantly Improving Casting Hardness and Wear Resistance
The core of quenching is to prevent austenite from decomposing into pearlite through rapid cooling, instead transforming it into a martensitic structure. Martensite is a supersaturated solid solution with a dense atomic arrangement and extremely high hardness (the hardness of cast steel can reach HRC50-60 after quenching, and that of high-chromium cast iron can reach HRC60-65). For castings requiring wear resistance (such as liners for crushers and bucket teeth for excavators), the wear resistance of quenched castings is 3-5 times that of untreated castings. Taking high-chromium cast iron liners as an example, the wear loss before quenching is approximately 0.5g/h, while after quenching, it can be reduced to less than 0.1g/h, significantly extending the service life.
0. Reserving High-Strength Potential for Subsequent Tempering
Although the martensitic structure after quenching has high hardness, it has two major defects: first, extremely high internal stress (the "frozen" atomic arrangement caused by rapid cooling prevents stress release), making it prone to cracking; second, extremely poor toughness (with an impact toughness of only 5-10J/cm², making it prone to brittle fracture), so it cannot be used directly. However, the high-strength characteristic of the martensitic structure provides an "adjustment space" for the subsequent tempering process—through tempering, most of the hardness can be retained while eliminating internal stress and improving toughness, achieving a precise balance between "hardness and toughness".
0. Meeting High-Strength Requirements for Special Working Conditions
In fields such as construction machinery and mining machinery, some castings (such as columns of hydraulic supports and tool holders of shield machines) need to withstand extremely high pressure and impact force, which ordinary normalized parts cannot meet. Through "modulation treatment" (quenching + medium-temperature tempering), castings can achieve a combination of high strength (tensile strength ≥800MPa) and relatively high toughness (impact toughness ≥40J/cm²), ensuring no deformation or fracture under extreme working conditions.
5. The "Fourth Heat Treatment" – Tempering: Eliminating Quenching Defects and Customizing the Final Performance of Castings
5.1 Process Definition and Core Parameters
Tempering is a process in which quenched castings are reheated to a specific temperature below Ac1 (divided into low-temperature tempering at 150-250°C, medium-temperature tempering at 350-500°C, and high-temperature tempering at 500-650°C according to performance requirements), held at that temperature for a certain period (usually 2-4 hours), and then cooled to room temperature by air cooling or oil cooling. Tempering cannot be carried out independently and must be combined with quenching to form a "quenching + tempering" combined process, which is crucial for the performance customization of ferrous metal castings.
5.2 Analysis of Core Functions
1. Eliminating Quenching Internal Stress and Preventing Casting Cracking
The internal stress of the martensitic structure after quenching can reach more than 1000MPa. If used or machined directly, it is highly prone to cracking (such as cracks appearing during grinding of tools that have not been tempered after quenching). Through low-temperature tempering, some carbon atoms inside the martensite can precipitate to form fine carbides, reducing the internal stress by 50%-70% while basically retaining the high hardness after quenching (for example, the hardness of tools can still reach HRC58-60 after low-temperature tempering at 200°C, with a significant reduction in internal stress).
0. Adjusting the Balance Between Hardness and Toughness to Match Actual Working Conditions
The tempering temperature is a core variable for regulating the performance of castings:
· Low-Temperature Tempering (150-250°C): Its main function is to eliminate internal stress, with minimal hardness reduction (≤HRC2) and a slight improvement in toughness (impact toughness increased from 5J/cm² to 10-15J/cm²). It is suitable for parts requiring high hardness and high wear resistance (such as cutting tools, measuring tools, and bearing rings).
· Medium-Temperature Tempering (350-500°C): The martensitic structure is transformed into "tempered troostite", with the hardness reduced to HRC35-45 and the toughness significantly improved (impact toughness ≥30J/cm²). At the same time, it has a high elastic limit, making it suitable for spring-like castings (such as automobile suspension springs and valve springs) to prevent permanent deformation of the spring under repeated stress.
· High-Temperature Tempering (500-650°C): The martensite is completely transformed into "tempered sorbite", with the hardness reduced to HRC25-35 and the toughness greatly improved (impact toughness ≥50J/cm²), while maintaining a relatively high strength. This process is called "modulation treatment" and is suitable for parts bearing alternating loads and impact loads (such as crankshafts, gear shafts, and connecting rods), which can effectively prevent fatigue fracture of the parts.
0. Stabilizing the Casting Structure and Dimensions to Improve Long-Term Reliability
The martensitic structure after quenching is in a "metastable state". During long-term service or temperature changes, structural transformation (such as the decomposition of martensite into pearlite) may occur, leading to dimensional deformation and performance degradation of the casting. Through tempering, the structure can be transformed into a stable tempered structure (troostite, sorbite), improving the dimensional stability by more than 80%. For example, after quenching + high-temperature tempering, the shape and position error (such as cylindricity) of machine tool spindles can be controlled within 0.005mm during long-term service, ensuring machining accuracy.
6. Synergistic Effects of the "Four Heat Treatments" and Practical Application Cases
In actual casting production, the "Four Heat Treatments" are not used independently but are combined according to the casting material and performance requirements. Taking the crankshaft of an automobile engine (made of 45# cast steel) as an example, its process route is: cast blank → normalizing (refining grains, with a hardness of HB200-220, facilitating rough machining) → rough machining → quenching + high-temperature tempering (modulation treatment, with a tensile strength ≥800MPa and impact toughness ≥50J/cm²) → finish machining. Among them, normalizing lays the structural foundation for subsequent quenching, and quenching + tempering achieve the final performance—neither step can be omitted.
Another example is high-chromium cast iron wear-resistant parts (such as ball mill liners), with the process route: casting → annealing (eliminating internal stress to prevent quenching cracking) → quenching (heating at 950°C, cooling in brine, with a hardness of HRC60-65) → low-temperature tempering (at 200°C, eliminating internal stress and retaining high hardness). In this case, annealing serves as a "preliminary guarantee" for quenching, and low-temperature tempering acts as a "subsequent optimization" for quenching. The three processes work together to ensure the high wear resistance and crack resistance of the liner.
7. Conclusion
As the core heat treatment processes in ferrous metal casting, the "Four Heat Treatments" each play an irreplaceable role: annealing serves as the "basic guarantee" to eliminate internal stress and optimize the structure; normalizing acts as the "efficient choice" to refine grains and balance performance; quenching functions as the "strength core" to achieve a leap in hardness; and tempering serves as the "performance customization" method to eliminate defects and match working conditions. For casting manufacturers, only by accurately mastering the process parameters and synergistic logic of the "Four Heat Treatments" can they customize high-quality ferrous metal castings according to customer needs (such as wear resistance, impact resistance, and high precision), promoting the product upgrading of equipment manufacturing, automotive, construction machinery, and other fields. In the future, with the application of intelligent heat treatment equipment, the process accuracy of the "Four Heat Treatments" will be further improved, injecting stronger impetus into the high-quality development of the ferrous metal casting industry.
