Quenching, Tempering, and Quality Control of Forgings from Hot Hammer Forging Dies

Before quenching, inspect and remove machining defects such as knife marks from hot hammer forging dies. To prevent oxidation and decarburization, use a protective atmosphere or heat in a box for protection.

      Before quenching, hot forging dies should be inspected and any machining defects such as tool marks should be removed. To prevent oxidation and decarburization, protective atmospheres or packed-box heating methods should be used.
During quenching heating, hammer forging dies should undergo either one or two preheating stages. During preheating, it is crucial to ensure that the core of the die is thoroughly heated through, so as to avoid excessive thermal stresses when the temperature rises to the quenching range. The conventional quenching temperature for hammer forging dies is typically selected within the temperature range where austenite grains do not significantly grow, thereby ensuring high impact toughness. Since hot-forging dies are subjected to substantial impact loads, they must possess excellent toughness. Increasing the quenching heating temperature can promote the formation of a larger proportion of lath martensite, which in turn enhances the fracture resistance of the forging die and reduces the likelihood of cracking. For example, the most commonly used hammer forging die steels—5CrMnMo and 5CrNiMo—exhibit peak strength, toughness, and ductility after being quenched at 900°C and tempered twice at 420–550°C. In other words, these two steels achieve their optimal combination of strength, toughness, ductility, and thermal stability under these specific heat treatment conditions. Of course, the prerequisite for raising the quenching heating temperature of forging dies is that the heating temperature must not cause austenite grain growth; otherwise, properties such as impact toughness will deteriorate.
When heating hammer forging dies in a box-type electric furnace, the heating rate is 2–3 minutes per millimeter; when heating in a salt-bath furnace, the rate is 1 minute per millimeter. For larger hammer forging dies, after quenching and cooling down to 780–800℃, they should be oil-quenched. The pre-cooling time for small modules is approximately 3–5 minutes, while for large modules it is about 5–8 minutes.
When cooling hammer-forged dies with hot oil, the oil temperature should be kept below 80℃. If the oil temperature is too high, uneven cooling may occur, leading to insufficient hardness or quenching soft spots. When the oil has cooled down to between 150 and 200℃, remove the dies from the oil and allow them to air-cool to prevent cracking caused by excessive cooling. It’s also crucial not to remove the dies from the oil too early during cooling. If removed too soon, although the surface may achieve the required hardness, the core will fail to harden properly. In this case, tensile stresses will develop between the hardened surface layer and the unhardened core. Moreover, since the core remains unhardened and unable to provide adequate support for the surface, the die is prone to cracking during service.
The forging die consists of two main parts: the cavity and the dovetail. The hardness requirements for the dovetail and the die surface of the forging die differ. For large- and medium-sized dies, the cavity area requires a hardness of 36–42 HRC; whereas the dovetail portion, which bears impact loads, requires a hardness of 30–35 HRC. The dovetail region is often a location where machining stresses are concentrated. To prevent cracking during cooling, it is best to immerse the entire die in oil for 3–5 minutes, then briefly expose the dovetail portion above the oil surface to allow its temperature to rise before reimmersing it in oil again. This process—repeatedly exposing and reimmersing the dovetail three to five times—can achieve self-tempering of the dovetail and effectively prevent cracking.
After quenching, forging dies should be tempered at different temperatures depending on the required hardness.
After quenching, the forging die for hammers should be immediately placed in the furnace for tempering. Before heating it up to the tempering temperature, it must first undergo a preheating stage and be thoroughly heated through. Given the large size of forging dies, if they are directly heated to the tempering temperature without prior preheating, a significant temperature difference will develop, leading to thermal stresses. The tempering time for hammer forging dies must be sufficient; otherwise, the core of the die may exhibit excessive hardness, resulting in cracking. Tempering should be carried out twice. The second tempering temperature should be approximately 10℃ lower than the first tempering temperature, and the holding time can be shortened by 20% to 25%. After tempering, oil cooling is preferred. This not only helps prevent the occurrence of Type II temper embrittlement but also avoids oxidation of the die surface during air cooling after tempering, thereby maintaining its surface roughness.