Die Steel Oil Quenching

---

Die steel can be roughly divided into three types: cold-rolled die steel, hot-rolled die steel and plastic die steel for forging, stamping, cutting, die-casting and so on. Due to the different uses of various molds and complicated working conditions, the steel for molds should have high hardness, strength, wear resistance, sufficient toughness, and high hardenability and quenching according to the working conditions of the molds they manufacture. Hard and other process performance. Due to the different uses of these types and the complicated working conditions, the performance requirements for steel for molds are also different.

In recent years, vacuum heat treatment technology has developed rapidly, and the manufacturing and popularization of domestic vacuum furnaces is also very strong. With the rapid promotion of vacuum heat treatment technology, it provides a platform for upgrading, cleaning production and improving the quality of mold products for the traditional . Now I will introduce how to select equipment and discuss the key points in the process technology to facilitate the selection of furnace type and improvement of the process, but the specific parameters need to be verified by experiments according to actual requirements.

1. Preparation before mold heat treatment
First of all, the material (or mold) of the material to be heat-treated should be fully understood. The supplier (supplier or steel mill) should be required to obtain the necessary technical data, such as composition, supply state (annealing) hardness, purity, impact. Toughness values, original microstructure, metallurgical defects, etc. If you are eager to install the furnace and neglect the inspection of the original module cracks, until the quenching of several pieces, you will think of checking the raw materials, which will cause unnecessary trouble and loss.
Reasonable design and selection of the final hardness after heat treatment of the mold also affects the performance and service life of the mold, so it is necessary to unify the opinions with the client and the tester. The final hardness of the mold after heat treatment is related to its size, complexity, and inherent quality of the material. It is recommended that the small mold complex parts are (46~48)HRC, the simple parts are (48～52)HRC; the medium mold complex parts are (42～44)HRC, the simple parts are (44～46)HRC; the large mold quality materials are (44 ~ 46) HRC, common materials are (42 ~ 44) HRC, complex parts are (40 ~ 42) HRC.

2. H13 steel vacuum annealing
2.1. Purpose, process and technical requirements of annealing
The purpose of module annealing is to soften it and reduce the risk of distortion and cracking. Figure 1 shows the general annealing process for H13 steel modules. Slowly heat the part to 870 °C at ≤60 °C / h. The holding time (2~4) h is determined according to the effective size of the module. It can also be maintained at 0.8min/mm after the furnace temperature reaches the temperature. In order to ensure that oxidation and decarburization do not occur, WZT series single-chamber vacuum furnace (extreme vacuum degree 0.1Pa) can be used, and the pressure in the heat preservation stage is controlled at (0.1-10) Pa. When cooling, it can be cooled under vacuum. When the temperature is lower than 500 °C, it can be filled with 1×105Pa high-purity nitrogen or high-purity nitrogen gas and other reducing gas mixture to ensure no oxidation and no coloration. . The annealed module has a hardness of < 235HB and is organized into pearlite + uniformly distributed granular carbide.

Figure 1 vacuum annealing process of h13 steel

2.2, the advantages and necessity of vacuum annealing
H13 steel vacuum annealing has the following advantages over annealing in air and atmosphere furnaces:
(1). It is easier to achieve less oxidation, no decarburization annealing than a controlled atmosphere. This is because measuring and controlling pressure is easier than measuring and controlling carbon potential.
(2). It can realize the adjustment of the rising and cooling speeds of different temperatures, which is beneficial to quality control and efficiency.
(3). In addition to the measurement, display and control of the furnace temperature, the complexity of the quality and shape of the visual module, a number of auxiliary thermocouples are added to understand the actual temperature changes of various parts of the module to facilitate comprehensive control.
(4). It is convenient and accurate to perform isothermal annealing on the module. For molds that are heat treated in muffle furnaces and ordinary annealing furnaces, because the furnace heat capacity is too large, it is impossible to achieve rapid cooling at the required speed. However, the vacuum annealing furnace is equipped with a heat shield structure and a fan, a cooler, etc., so that the heat capacity is small and the cooling capacity is strong.
Therefore, H13 steel has uniform microstructure after vacuum isothermal annealing, short cycle, high efficiency and bright surface. Depending on the selected transition temperature, the resulting tissue has different thickness profiles and different hardness depending on the transition temperature. The isothermal annealing process can be referred to other articles in the vacuum technology network.

3, H13 steel vacuum quenching
3.1, vacuum heating and austenite alloying should pay attention to the problem
The standard heating method of H13 steel in vacuum furnace is three-stage heating, as shown in Figure 2. That is, the module is heated to 700 ° C at a rate of 200 ° C / h, then heated to 870 ° C at 165 ° C / h, and then heated to 1025 ° C as soon as possible. The first two sections are preheating sections, and one or two sections may be selected as needed, for the purpose of dilating. The latter section is ventilated plus heat preservation (alloying). When heating in a single-chamber vacuum furnace, it is easier to achieve accurate, uniform and efficient temperature by means of an auxiliary thermocouple. Difficulties in a two-chamber vacuum furnace can only be judged by the experience of the operator observing the heating chamber. TS is the surface temperature, TC is the core temperature, and TS-TC < 90 °C in the first stage of preheating before entering the second stage. When the second stage is warmed up, TS-TC < 40 °C, before entering the third stage. The third stage of heating should make the heart reach the temperature set by the equipment as soon as possible, and the end of the fire is completed, and the alloying is continued. The blistering time is too long, reducing the efficiency, but in actual production, most cases are insufficient for osmotic. The normal temperature range of austenitic alloying is (1010~1030) °C, and the alloy is alloyed for 30 min after the core is warmed. If the holding time is too long, the workpiece distortion will increase and the crystal grains will grow up. The grain size should be controlled at least above 7 levels. The pressure of the equipment used is (10-1 ～10-2) Pa, and the leakage rate is 5 × 10-1 Pa/h. Excessive pressure will cause excessive evaporation of alloying elements on the surface of the module.

Figure 2 Heating method of H13 steel in vacuum furnace

3.2, when to use vacuum oil quenching
For H13 steel, the higher the quenching temperature, the earlier the carbide precipitation time (before the formation of pearlite and bainite) during cooling, so the toughness decreases as the quenching temperature increases. At this time, grain growth is not the only factor affecting toughness, because H13 steel contains 1% V, which is insensitive to grain growth. Tests with different cooling rates have shown that the lowest cooling rate results in the lowest impact toughness. Low impact toughness values ​​are associated with the precipitation of grain boundary carbides in steel. Therefore, in order to obtain the highest impact toughness of H13 steel, high quenching temperature and high cooling speed should be properly combined. Oil cooling is much faster than air cooling and air cooling. From this point of view, the impact resistance is optimal when oil is cooled.
Figure 3 is a cooling curve for H13 steel oil quenching. As can be seen from Fig. 3, when the steel bar having a diameter of <100 mm is quenched in oil, the central cooling rate curve is just tangent to the carbide precipitation start line, which has almost no adverse effect on the impact toughness of the steel. Therefore, the first choice of H13 steel workpiece quenching of this size should be vacuum oil quenching, WZC series double chamber vacuum oil quenching air cooling furnace can be selected. However, when 1 effective size is >100mm, 2 pairs of dimensional accuracy requirements are strict, and when there is no need for large quenching deformation, vacuum oil quenching should be abandoned, and other cooling methods should be selected.

Figure 3 Cooling curve of H13 steel during oil quenching

Vacuum Heat Treatment Furnace