1. Working Conditions and Performance Requirements of Hot Extrusion Dies and Medium and Small Forging Dies

During operation, hot extrusion dies are subjected to compressive stress, bending stress, and tensile stress from mold release. The impact load is smaller than that of hammer forging dies, but the contact time with the hot metal is longer, and the operating temperature is higher than that of hammer forging dies. The temperature rise also varies with different metals being extruded, reaching as high as 800-850℃. The thermal stress caused by rapid cooling and heating is also greater than that of hammer forging dies, and the friction is more intense. The main failure modes of hot extrusion dies are excessive plastic deformation of the die cavity, fatigue failure caused by repeated heating and cooling, thermal wear, and surface oxidation corrosion.

The working conditions of medium and small forging dies are similar to those of hot extrusion dies, except that the impact load is greater. Therefore, the failure modes of medium and small forging dies are also similar to those of hot extrusion dies. Therefore, hot extrusion dies and medium and small forging dies are required to have higher hot fatigue resistance, thermal stability, and better wear resistance than hammer forging dies, as well as higher high-temperature strength and sufficient toughness.

2. Steels for Hot Extrusion Dies and Medium and Small Forging Dies

Commonly used steels for hot extrusion dies and medium and small forging dies include tungsten-based hot work tool steels and chromium-based hot work tool steels, as well as new hot work tool steels such as chromium-molybdenum-based, tungsten-aluminum-based, and chromium-platinum-tungsten-based steels, and matrix steels.

(1) Tungsten-based hot work tool steel The representative steel grade of this type of steel is the traditional 3Cr2W8V steel. Due to its poor hot fatigue resistance, its application in hot extrusion dies will gradually decrease, but it is widely used in die-casting dies, so it will be introduced in detail in die-casting die steels.

(2) Chromium-based hot work tool steel Representative steel grades of chromium-based hot work tool steels include 4Cr5MoSiV, 4Cr5MoSiV1, and 4Cr5W2VSi. The first two are equivalent to the American H11 and H13 steels, and 4Cr5W2VSi is derived from 4Cr5MoSiV steel, with W=2% replacing Mo=1%. The mass fraction of carbon in these three steels is all around 5%, belonging to medium-carbon medium-chromium steel.

The common characteristics of this type of steel are:

① Due to the high chromium content, it has high hardenability. For example, a 150mm thick 4Cr5MoSiV1 steel part can be oil-quenched and hardened. Moreover, the undercooled austenite of this type of steel has high stability between 400-600℃, and can be kept at a constant temperature for a long time without transformation, so it is suitable for graded quenching.

② Good hot fatigue resistance, this is because chromium and silicon improve the oxidation resistance of the steel, so chromium-based steel can better adapt to the working conditions of rapid cooling and heating.

③ High tempering stability. For example, the quenched hardness of 4Cr5MoSiV and 4Cr5W2VSi steel reaches the maximum value when quenched at around 1070℃ and 1200℃ respectively. After tempering, the hardness remains almost unchanged with increasing tempering temperature, and a secondary hardening peak appears at 500-550℃, after which it decreases rapidly.

④ Compared with tungsten-based hot work tool steel, it has higher toughness, but insufficient high-temperature strength and slightly lower heat resistance. The working temperature generally does not exceed 650℃.

⑤ The types and quantities of carbides contained in this type of steel are roughly the same, so the overheating sensitivity is also roughly the same.

⑥ Chromium-based hot work tool steel has higher hot plasticity, small deformation resistance, and small tendency to crack during forging, but the forging temperature range is slightly narrow, and the forging temperature must be strictly controlled.

(3) Chromium-molybdenum steel and chromium-tungsten-molybdenum steel This type of steel includes 4Cr3Mo3SiV (H10), 3Cr3Mo3VNb (HM3), 3Cr3Mo3W2V (HMI), 5Cr4W5Mo2V (RM2), 4Cr3Mo3W4VNb (GR), etc.

The following is a brief introduction to the performance characteristics and applications of some steel grades.

1) 3Cr3Mo3VNb (HM3) steel: This steel is characterized by a lower carbon content and the addition of a small amount of molybdenum, so it has higher hot fatigue resistance and toughness, good tempering stability, and other excellent process properties. It is suitable for manufacturing pressure-forming dies, roll forging dies, and small hammer forging dies with strong water cooling, and its service life is significantly higher than that of dies made of 5CrNiMo, 4Cr5W2VSi, and 3Cr2W8V steel.

2) 5Cr4W5Mo2V (RM2) steel: This steel has a carbon mass fraction of about 0.5%, and the total mass fraction of alloying elements is 12%. In the service state, it contains more carbides, mainly M6C. Therefore, this steel has high tempering resistance and thermal stability. The thermal stability at a hardness of 50HRC can reach 700℃, and the wear resistance is also good. It is suitable for making small-section hot extrusion, high-speed forging dies, and roll forging dies.

3) 4Cr3Mo2W4VTiNb (GR) steel: This steel is obtained by adding a small amount of niobium to tungsten-molybdenum-based hot work tool steel, thus obtaining high tempering stability and high hot strength. Its hot fatigue resistance, thermal stability, wear resistance, and high-temperature strength are significantly higher than those of 3Cr2W8V steel. After oil quenching at 1160-1200℃ and tempering at 630-600℃ twice, each time for 1h, its hardness can reach 50-55HRC, tensile strength can reach 1880MPa, and impact toughness is 17J/cm². This steel has good hardenability and cold and hot workability, and is suitable for manufacturing hot extrusion, precision forging, and high-speed forging dies.

4) Matrix steel Among matrix steels, there are many steel grades that can be used as both cold work tool steel and hot work tool steel, such as 6W8Cr4VTi (LM1), 6Cr5Mo3W2VSiTi (LM2), and 6Cr4Mo3Ni2WV (CG-2), etc. Among them, 5Cr4Mo3SiMnVAI (012A1) steel is widely used in hot extrusion dies, such as bearing hot extrusion punches and transmission rod hot extrusion dies, and its service life is significantly improved compared with the traditional hot work tool steel 3Cr2W8V.

3. Material Selection for Hot Extrusion Dies and Medium and Small Forging Dies

When selecting materials for hot extrusion die processing, the primary consideration should be the type of metal being extruded and its extrusion temperature. Secondary factors include extrusion ratio, extrusion speed, and lubrication conditions to enhance die service life. Table 3-1 shows the selection of materials for hot extrusion dies. Material selection for medium and small forging dies primarily considers the type of forging material and production volume. Secondary considerations include the impact of die size, deformation speed, and lubrication conditions on die life.

4. Heat Treatment of Hot Extrusion Dies and Medium and Small Forging Dies

The manufacturing process route for these dies is generally: blanking * forging, pre-heat treatment * machining, quenching, tempering, and finishing.

The following analyzes the process characteristics of each thermal processing step.

(1) Forging Process: Steels used for hot extrusion dies and medium and small forging dies are mostly high-alloy steels, so the blanks require good forging. Especially for hot work die steels containing aluminum, care should be taken to control the forging heating temperature and holding time to avoid severe decarburization leading to premature die failure.

(2) Preliminary Heat Treatment

1) Annealing: The annealing process for hot extrusion dies and medium and small forging dies mainly involves correctly selecting the annealing temperature, maintaining sufficient holding time, and cooling at an appropriate cooling rate. In addition, to ensure good wear resistance, a certain number of carbides must be retained after quenching. Since the shape of the carbides has a great impact on the toughness of the steel, the shape of the carbides after annealing should also be noted. Generally, round and fine carbides are desired.

2) High-Temperature Tempering: To improve the mechanical properties (especially fracture toughness) of the forged blanks, a post-forging tempering method is often used for blank pretreatment. This heat treatment method involves heating the forged die blank to a high temperature for quenching and then high-temperature tempering.
This treatment allows for uniform carbide distribution and a round and fine shape, not only improving the steel's properties but also shortening the pretreatment cycle. The quenching heating temperature for tempering treatment can be determined according to different steel grades, such as 3Cr3Mo3 W2V steel at 120090, which is similar to the conventional quenching temperature. The high-temperature tempering temperature is generally between 700 and 750℃.

3) Normalizing after Forging: For blanks with obvious intergranular chain-like carbides after forging, they must be normalized to eliminate them before spheroidizing annealing, because these chain-like carbides are difficult to eliminate by direct annealing.

4) Quenching and Tempering: For commonly used hot extrusion die steel and medium and small forging die steel, when selecting the quenching temperature, the main considerations are the size of the austenite grain size and the impact toughness, and the working conditions, structural shape, and failure mode requirements of the die.
The selection of the quenching holding time mainly considers the completion of the phase transformation, allowing carbon and alloying elements to be fully dissolved to ensure high tempering resistance and hot hardness. The quenching holding time coefficient is generally 0.5-1 min/mm for salt bath furnaces, with smaller sizes having larger coefficients.

Hot extrusion die steel and medium and small forging die steel belong to high-alloy steel with good hardenability. Quenching cooling can use oil cooling or air cooling. Isothermal quenching or stepped quenching can also be used for dies with small deformation requirements. The correctness of the tempering process has a very important effect on the failure mode of the die. The principle of selecting the tempering temperature is to improve the hardness of the die as much as possible without affecting the die's anti-brittle ability. This requires determining the tempering parameters based on the specific failure mode of the die. The quenched dies should be tempered as soon as possible, especially for dies with complex shapes. Tempering should be carried out when the die surface temperature is below 80℃. To avoid residual stress, both heating and cooling during tempering should be slow. Tempering is generally carried out twice, and the tempering time can be calculated at 3 min/mm, but it should not be less than 2 hours. The second tempering temperature can be 10-20℃ lower than the first.