What Is Temper Brittleness in Steel and How Can It Be Prevented?
Temper brittleness is a metallurgical phenomenon in which quenched steel loses toughness after tempering under certain temperature conditions. Although tempering generally reduces hardness and improves toughness, some steels show a drop in impact toughness within specific temperature ranges. This reduction in toughness can negatively affect the performance of dies, molds, and structural components subjected to heat treatment.
Understanding the causes and prevention methods of temper brittleness is important for improving steel reliability, especially in tooling and die-casting mold applications.
1. What Is Temper Brittleness?
When quenched steel is tempered, hardness typically decreases while toughness increases as the tempering temperature rises. However, in many steels, the relationship between tempering temperature and impact toughness is not completely linear. Two brittleness valleys may appear during tempering:
- Low-temperature temper brittleness: approximately between 200°C and 400°C
- High-temperature temper brittleness: approximately between 450°C and 650°C
In these temperature ranges, increasing the tempering temperature may actually reduce impact toughness instead of improving it.
Alloy steels with a martensitic structure are particularly sensitive when tempered between 450°C and 600°C. In some cases, steel tempered at 650°C and then slowly cooled through the range of 350°C to 600°C can also become brittle. Because toughness can often be restored by reheating the brittle steel to about 650°C and then cooling it rapidly, this phenomenon is commonly referred to as reversible temper brittleness.
2. Causes of Temper Brittleness
Temper brittleness is mainly caused by the segregation of impurity elements at prior austenite grain boundaries. These impurities weaken the grain boundary strength and make intergranular fracture more likely, resulting in a loss of toughness.
3. Mechanism of High-Temperature Temper Brittleness
High-temperature temper brittleness is closely related to the segregation behavior of impurity and alloying elements. Elements such as phosphorus (P), antimony (Sb), tin (Sn), nickel (Ni), and chromium (Cr) can concentrate at prior austenite grain boundaries, often within a very thin atomic layer.
The main characteristics of this mechanism include:
- Impurity elements accumulate at prior austenite grain boundaries and reduce grain boundary fracture strength.
- Nickel and chromium not only segregate themselves, but also promote the segregation of impurity elements.
- If the steel is quenched but not tempered, or tempered outside the embrittlement range, this type of segregation is generally not observed.
- Molybdenum (Mo) can suppress impurity segregation at grain boundaries and reduce embrittlement sensitivity.
- Carbon also influences the embrittlement process.
In general, plain carbon steels are less sensitive to high-temperature temper brittleness, while alloy steels containing chromium, manganese, nickel, and silicon are more sensitive. The degree of sensitivity depends on the type and amount of alloying elements present.
4. How to Prevent Temper Brittleness
Several practical methods can be used to reduce the risk of temper brittleness in steel:
(1) Improve Steel Purity
Reducing impurity elements in the steel is one of the most effective ways to minimize temper brittleness. Cleaner steel with lower levels of phosphorus, tin, antimony, and similar impurities shows better toughness stability after tempering.
(2) Add Beneficial Alloying Elements
Adding suitable alloying elements such as molybdenum (Mo) and tungsten (W) can help reduce embrittlement sensitivity. Molybdenum is especially effective because it suppresses impurity segregation at grain boundaries. However, the alloy composition must still be optimized carefully, and high steel purity remains essential for long-term performance.
(3) Use Rapid Cooling After Tempering
For parts with relatively small size and simple geometry, rapid cooling after tempering can reduce the likelihood of impurity segregation during cooling.
(4) Apply Subcritical Quenching
Subcritical quenching below the A1–A3 temperature range can help refine the grain structure and reduce impurity segregation at grain boundaries, thereby improving toughness.
(5) Use High-Temperature Deformation Heat Treatment
Thermomechanical treatment at high temperature can refine the grain size. A finer grain structure increases grain boundary area and reduces the concentration of segregated impurity elements at any single boundary.
(6) Quench After High-Temperature Tempering
After high-temperature tempering, oil cooling or water cooling can be used to suppress impurity segregation at grain boundaries and help maintain better toughness.
5. Considerations for Die-Casting Molds
For die-casting molds and other hot work tooling, tempering or stress-relief annealing should be avoided as much as possible within the high-temperature brittleness range. Selecting the correct heat treatment window is critical for maintaining mold toughness, crack resistance, and service life.
Conclusion
Temper brittleness is an important heat treatment issue in alloy steels, especially in tooling and mold materials that require both strength and toughness. By improving steel purity, selecting suitable alloying elements, refining heat treatment processes, and controlling cooling methods, manufacturers can significantly reduce the risk of brittleness and improve long-term material performance.