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In the machining and application of acero para herramientas, heat treatment is often regarded as the process that breathes life into the steel. Drawing on the latest metallurgical data, we provide an in-depth analysis of a crucial step in the heat-treatment process: tempering. As our primary product is tool steel, our study is framed primarily from that perspective.
After undergoing hardening processes such as quenching or normalizing, steel enters a state of extreme tension. At this point, its internal structure is dominated by martensita. While steel in this condition exhibits exceptional hardness, it also becomes highly brittle, with poor toughness and significant internal stresses. Without subsequent treatment, steel in this state is highly susceptible to brittle fracture or cracking when subjected to impact or machining. Tempering is specifically designed to address this issue.
Tempering is not merely a heating process; it is a meticulous restructuring of the steel’s microstructure. The general procedure involves heating the material to a specific temperature and holding it there for a period of time. Essentially, it trades a minimal loss of strength for a significant increase in fracture resistance and ductility.
The Four Core Functions of Tempering
- Eliminate internal stresses to prevent cracking. Quenching generates significant thermal and structural stresses within the steel. Tempering effectively releases these stresses, preventing deformation and cracks during subsequent grinding or use.
- Enhance toughness and ductility. Tempering allows tool and die steel to maintain high hardness while gaining sufficient toughness to withstand impact loads, preventing chipping during use.
- Ensure dimensional stability. In untempered tool-and-die steel, retained austenita is unstable and will transform over time, causing volume changes in the tool or die. The tempering process stabilizes the microstructure, ensuring precision tools and dies maintain stable dimensions and shape.
- Customizable mechanical properties. By precisely controlling the tempering temperature and duration, the hardness can be tailored to the optimal range for specific applications.
The Process and Equipment for Tempering
Tempering is a process that reheats tool and die steel to a temperature below the lower critical point (Ac1). Since the temperature does not reach the phase transformation point, no structures such as bainite or pearlite are formed. Consequently, this process relieves stress only in the tool and die steel and refines its microstructure.
Four key variables affecting tempering effectiveness:
- Temperature. The tempering temperature for tool and die steels typically ranges from 175°C to 705°C. Low-temperature tempering aims to maintain high hardness, primarily for stress relief, and is commonly applied to aceros para herramientas de trabajo en frío like D2. High-temperature tempering sacrifices some hardness to achieve high toughness and ductility, frequently used for aceros para herramientas de trabajo en caliente such as H13. The higher the temperature, the more pronounced the decrease in hardness, but the greater the toughness.
- Soaking Time. Temperature and soaking time are interdependent. To ensure heat penetrates uniformly to the core of the workpiece, the industry-standard rule of thumb is: for every inch (approximately 25.4 mm) of material cross-sectional thickness, a minimum soaking time of 2 hours is required.
- Cooling rate. The cooling rate after tempering is equally essential. If cooling is too slow, “temper brittleness” may occur.
- The influence of alloy composition. Different alloying elements determine the steel’s resistance to softening.
After quenching, steel is loaded with internal stresses and remains in a precarious state, requiring immediate tempering. Many material fractures occur during the waiting period between quenching and cooling and the start of tempering. Never allow workpieces to cool to room temperature for extended periods after quenching, as the risk of cracking increases exponentially as temperature decreases.
Based on our experience, during actual heat treatment processes, if tempering cannot be performed immediately due to equipment scheduling constraints, we recommend temporarily holding the steel at 50–100°C (122–212°F). This simple step can effectively prevent tools and dies from being scrapped at the last minute.
The most common tempering equipment is the air convection furnace, which relies on circulating air for heating and is suitable for high-volume processing. Other methods include molten-salt baths and thermal-oil baths, which provide more uniform, rapid heating of materials. These are ideal for precision components requiring extremely high deformation control.
Microstructural Stages of Tempering in Steel
Tempering is not merely a process of heating; it is a phased evolution of steel’s internal structure as the temperature rises.
| Escenario | Approximate Temperature Range | Metallurgical Reaction | Property Effect & Change |
| Stage 1 | 0-250°C (32−480°F) | Epsilon carbide (ϵ-carbide, Fe2.4C) precipitation, loss of martensite tetragonality | Marginal decrease in hardness, but toughness significantly enhanced.. Volumetric contraction occurs |
| Stage 2 | 200−350°C (392−660°F) | Transformation of retained austenite (RA), typically to lower bainite, ferrite, and cementite/carbide mixtures | Slight volume expansion.. Occurs mainly in high-carbon steels |
| Stage 3 | 230−450°C (446−840°F) | Produces the softest structure with the highest ductility and best machinability.. Hardness reduces continuously and sharply | Volume contraction. Hardness continues to decrease, and toughness increases rapidly. |
| Stage 4 | 350−700°C (662−1292°F) | Coarsening and spheroidization of cementite particles in a ferrite matrix, along with recovery and recrystallization of ferrite. | Produces the softest structure with the highest ductility and best machinability.. Hardness reduces continuously and sharply. |
When the tempering temperature falls within the high-temperature range of approximately 500°C – 550°C, ordinary steel softens, but specific tool and die steels actually experience a rise in hardness rather than a decrease, sometimes even reaching peak values. This phenomenon is known as secondary hardening. At elevated temperatures, alloying elements such as molybdenum (Mo), vanadium (V), and chromium (Cr) in these tool and die steels precipitate into highly insoluble special alloy carbides.
Tempering Variations and Related Phenomena
We sometimes encounter a failure scenario in tool and die heat treatment where customers purchase high-quality D2 (1.2379) or H13 (1.2344) steel, but due to rushing to meet deadlines, they perform only a single tempering process. This results in the mold cracking prematurely during initial use. For high-speed steel or high-alloy tool and die steel (such as D2, H13), multiple tempering cycles are required. We also recommend 2 to 3 tempering cycles.
After quenching, a significant amount of unstable austenite (retained austenite) remains within the steel. The primary purpose of the first tempering is to transform this retained austenite into martensite. When the retained austenite transforms into martensite during the cooling phase of the first tempering, it forms fresh, untempered martensite. This newly formed structure remains highly stressed and extremely brittle. Therefore, a second tempering is essential to relieve these stresses and enhance toughness.
Tempering does not necessarily benefit from higher temperatures; if improperly controlled, it can actually cause steel to become brittle. In metallurgy, this phenomenon is known as temper embrittlement.
Although modern heat treatment shops are equipped with precision temperature control devices, experienced engineers can still determine the tempering temperature by observing the color of the oxide film on the steel surface when instruments are unavailable.
Below is a simplified comparison table (for reference only).
| Temperatura | Color of Oxides |
| 188°C (370°F) | Faint yellow |
| 210°C (410°F) | Dark straw |
| 232°C (450°F) | Purple |
| 265°C (510°F) | Light blue |