جدول المحتويات
In the production and processing of tool steel, annealing is an absolutely indispensable core step. Simply put, annealing softens the metal material. Without this process, forged steel retains immense internal stress and extreme hardness, rendering it unworkable for machining or processing upon delivery to the customer. Through annealing, we aim to reduce material hardness—typically controlled below <255HB—enhancing its machinability and laying the groundwork for subsequent precision manufacturing.
Successful annealing is not merely a matter of heating steel; it is a process involving thermal cycles. This process consists of three critical stages: heating, soaking, and cooling.
Why Annealing Is Essential: Objectives and Purposes
The primary purpose of annealing is to soften the metal, thereby significantly enhancing its workability and machinability. However, for high-alloy tool steels like د2, 1.2379, and ح13, annealing serves a far more multifaceted role—it is a multidimensional quality assurance process.
1. Softening and Ductility Restoration.
Steel that has undergone forging or rolling often becomes extremely hard and brittle internally due to cold working or severe deformation. Without annealing, the material not only becomes difficult to machine but is also prone to fracture during processing. The annealing process effectively reduces hardness, restoring the material’s ductility and toughness. This is akin to giving a taut muscle a thorough massage, allowing it to relax and preparing it for subsequent milling and turning operations.
2. Stress Relief.
During casting, forging, rolling, and even welding, steel accumulates significant internal stresses. If these stresses remain trapped inside the material, they act like a ticking time bomb. One of the key functions of annealing is to release these residual stresses. If this step is not properly executed, the steel becomes highly susceptible to deformation, warping, or even cracking during subsequent wire-cutting or heat-treatment processes. At Aobo Steel, our emphasis on “Dimensional Stability” stems from our understanding that thoroughly eliminating these hidden dangers during the annealing process ensures the steel maintains its shape stability throughout subsequent processing.
3. Microstructural Control.
For high-end mold steel, annealing helps us achieve the desired microstructure. More importantly, through diffusion annealing at elevated temperatures, we can refine grain size and improve the uniformity of alloy element distribution, minimizing chemical segregation. A uniform microstructure enables tools to achieve a significant leap in service life.
The Metallurgical Mechanism: Recovery, Recrystallization, and Grain Growth
Many professional clients often press us with the question: “What exactly happens inside the steel during those dozens of hours of heating in the furnace?” Simply put, forging infuses steel with energy and pressure, while annealing is the process of releasing that energy.
When tool steels such as D2 or H13 are placed in an annealing furnace, the internal structure of the steel undergoes the following three key changes as temperature and time progress:
1. Recovery.
This marks the initial stage of annealing. Our forged tool steel contains numerous microscopic defects known as “dislocations,” which store substantial amounts of energy. During the low-temperature phase, these micro-defects begin to rearrange themselves into more stable, lower-energy states. At this stage, the steel’s hardness and strength show little change, but internal residual stresses begin to dissipate, and physical properties such as electrical conductivity return to pre-forging levels.
2. Recrystallization.
This is the most critical step in the annealing process, marking the moment when softening truly occurs. When the temperature rises above the recrystallization temperature, the old grains—previously distorted, elongated, and filled with stress—are completely replaced. A new generation of strain-free, equiaxed grains begins to form and displace the old microstructure. The tool steel’s ductility is significantly restored, and hardness decreases substantially. For our high-hardness steels like D2 and د3, this step determines whether the tool steel can ultimately be machined smoothly.
3. Grain Growth
If the annealing temperature is too high or the holding time is too long, the newly formed grains begin to devour each other, growing increasingly larger. Coarse grains are a major drawback for tool steel. They cause the steel to become brittle and may even lead to surface defects during subsequent deep processing, such as the commonly observed “orange peel effect.” Precise control of furnace temperature and time ensures immediate transition to controlled cooling after recrystallization is complete, strictly preventing excessive grain growth. This guarantees a fine-grained microstructure and robust, tough material properties.
Specific Types of Annealing Processes
Annealing treatments fall into several broad categories defined by the maximum temperature reached and the cooling method used.
1. Full Annealing (Supercritical Annealing)
Full annealing (supercritical annealing) is the cornerstone of mold steel production. Its objective is clear: to achieve the lowest possible hardness and strength in the steel, thereby securing optimal machinability.
We need to heat the steel to 30–50°C above the material’s “upper critical temperature (Ac3)”. This ensures complete austenitization of the steel’s internal structure. Simply put, it involves dismantling the steel’s original chaotic internal structure and preparing it for recrystallization.
Full annealing requires extremely slow cooling rates. Instead of pulling the steel out for air cooling, we shut off the heating power and leave the steel inside the furnace chamber, allowing it to cool extremely slowly as the furnace temperature naturally decreases. This process is highly time-consuming, but it ensures the steel undergoes the most thorough microstructural transformation during the prolonged cooling period.
After prolonged cooling, tool steel develops a mixed microstructure composed of ferrite and coarse pearlite. For hypereutectoid steels like D2 (1.2379), temperature control must be strictly managed. If the heating temperature is excessively high (far exceeding Aسم) and cooling is improperly controlled, carbides will form a brittle network structure at grain boundaries. This renders the material extremely brittle, severely compromising subsequent mechanical properties.
2. Process Annealing (Subcritical Annealing)
Process annealing is a highly efficient and economical heat treatment method. It is not intended to alter the final properties of steel, but rather serves as an intermediate step between processing operations. Its purpose is to restore the material’s ductility, enabling further processing.
The most distinctive feature of process annealing lies in its precise temperature control. The heating temperature is strictly maintained 10-20°C below the lower critical temperature (Ac1). At temperatures below Ac1, no phase transformation occurs within the steel, and it does not convert to austenite. By avoiding complex microstructural changes, this process not only conserves heating energy but also reduces processing time. Typically, heated steel can be cooled in still air without the lengthy furnace cooling required for full annealing. This effectively eliminates work hardening caused by cold-working processes such as cold-rolling or cold-drawing, restoring the steel’s ductility and flexibility.
3. Spheroidizing Annealing (Soft Annealing)
It is a crucial heat treatment step for medium- and high-carbon steels. Steels like D2 or D3, with extremely high carbon content, are internally filled with hard and brittle carbides. Without spheroidizing annealing, customers would find machining extremely difficult—not only would feed rates be challenging, but tool breakage would be highly likely. The core objective of spheroidizing annealing is to achieve the maximum theoretical softness and optimal ductility possible for the metal material. Before spheroidizing annealing, carbides in steel typically appear as flakes, networks, or needle-like forms, making machining and cutting difficult. Through spheroidizing annealing, we transform the carbide morphology into spherical particles and distribute them uniformly within the ferritic matrix. The resistance encountered by spherical particles is significantly lower than that encountered with network structures.
Spheroidizing Annealing is performed at temperatures near or slightly below the lower critical temperature (Ac1). To gradually “round” the carbides, extended holding times are required. This entails higher electricity costs and longer lead times, but we believe it is absolutely worthwhile to ensure smooth subsequent machining for our customers.
4. Isothermal Annealing
Simply put, isothermal annealing involves rapidly cooling tool steel to a specific temperature range, holding it there for a period, and then cooling it further. It offers a faster alternative to traditional slow cooling.
Some customers worry: “Does faster mean lower quality?” Based on data and our experience, this is not the case. The most significant advantage of isothermal annealing lies in significantly reduced time costs and nearly consistent results.
Similar to full annealing, the steel is first heated to transform its internal structure entirely into austenite. The material is then rapidly cooled to the temperature range of the pearlite transformation zone and held at this temperature for a period. During this isothermal holding, the austenite undergoes a complete transformation into a mixture of ferrite and carbides, known as pearlite. Once the transformation is complete, the material can be removed from the furnace for accelerated cooling.
خاتمة
Many non-specialists believe annealing is merely simple heating, but in our view, annealing is an art of heat treatment that precisely controls microstructure.
Tool steel has transformed from a hard, brittle state back into a soft, ductile state. It is no longer tense but fully prepared for your next operation—without complex CNC machining or precision wire cutting.