Abstract: In addition to the main failure modes of die-casting molds under the influence of molten metal erosion and thermal stress on the surface, thermal fatigue damage is also caused. In the actual production process, the micro structure and electrical discharge machining of die materials will also cause cracking failures of die-casting molds.
Keywords: HPDC ; surface cracking; electrical discharge machining
H13 steel and other hot work die steels are widely used in die-casting dies, forging dies and extrusion dies that require high toughness and resistance to cold and hot fatigue due to their excellent harden ability, harden ability and good high temperature properties. At present, most of the aluminum alloy die-casting mold molds use H13 steel and its improved steel types Dievar, 1.2344ESR, etc., but early cracks will occur in different degrees in actual use. This article analyzes the possible causes of early cracks and preventive measures from a set of molds in the actual production process through the mold material composition, micro structure and EDM.
1.Chemical composition
From the chemical composition of Table 1, the die material belongs to the hot work die steel with low Si and high mo. The main function of reducing Si content in steel is to reduce segregation, refine austenite grains and improve the plastic toughness of steel. The main advantages of increasing Mo content in steel are improving harden ability, inhibiting carbide precipitation and bainite transformation at grain boundary, improving toughness and hot cracking resistance. The steel with low Si and high Mo is not easy to appear component super cooling, dendrite or cellular columnar crystal during solidification, so as to avoid serious dendrite segregation. However, Mo and V elements can form alloy carbides with more stable thermodynamic properties, such as VC, MOC and Mo2C. These alloy carbides precipitate at 500 ~ 600? and keep fine and dispersive morphology, which is an important guarantee for high temperature materials to keep hot hardness. According to the measured chemical composition, the die steel should have good micro structure and crack resistance. However, in the actual production process, the die steel often cracks in the early stage, which has to be studied from its micro structure.
2. Micro structure
(1) Unused mold material after vacuum quenching and tempering
It can be seen from the metallo graphic picture 1 that the substrate has an uneven structure after heat treatment.
Observed at a low magnification with a metallo graphic microscope, there are light-colored areas dispersed in Figure 1a. After zooming in, you can see that more granular carbides are precipitated in these areas, and the carbides in this area are larger than those in the surrounding normal tissue. The size of the carbide particles is larger, which is still segregation in nature. Due to the precipitation of more carbides and alloy carbides in these light-colored areas, the surrounding carbon and alloy elements are depleted. During normal quenching and heating, due to the large carbide particles in the segregation area, it is not easy to dissolve, it is still in a state of lean carbon and lean alloy, and it is easy to generate overheating during quenching to obtain a coarse marten site structure. Fire causes the work piece to be brittle, and the strength and toughness of the material is severely weakened, which increases the tendency of the material to undergo brittle cracking. Observe the annealed steel before vacuum quenching.
Observed under a metallo graphic microscope at low magnification, Figure 2a
There are also uneven structures. After zooming in, it can be seen in Figure 2b that there are larger particles of carbides in this segregated region. It shows that there is segregation in the structure of the raw material, and this structure shape has a great influence on the uniformity of the material. However, this segregation cannot be improved in the subsequent vacuum quenching process and is retained, which will definitely have an important impact on the service life of the mold.
(2) The mold material after use
The mold material is put into actual production, and after only using 30,000 mold times, cracks occur on the surface of the mold, as shown in FIG. 3. Figure 3a is a single crack, which is basically fractured along the crystal; Figure 3b is a pit formed after long-term erosion and peeling at the intersection of multiple cracks; the surrounding area of the crack also has segregation structure and obviously overheated grain boundaries, such as Figures 3c and 3d. The metallurgical defects of the raw materials cannot be improved during the spheroidizing annealing and vacuum quenching stages. Under the action of liquid metal erosion and cyclic thermal stress on the surface, these segregated areas are the most vulnerable and most prone to occur. Cracked.
3. Preventive measures
(1) Examine the metallo graphic structure of raw materials. Ordinary mold manufacturers do not perform metallo graphic inspection on the raw materials provided by the suppliers, whether in the annealed or vacuum quenched and tempered states, which cannot ensure that the mold materials meet the requirements. For die-casting molds, you can choose to sample at the position of the material sleeve, so that the core part of the mold core is not damaged, and the quality of the mold material can be researched.
(2) Remove the hot remelting zone caused by EDM. The hot remelting zone has high hardness and brittleness, and it is easy to cause micro cracks.
Especially in the case of ordinary flame-baking molds. This requires removing the hot remelting zone after EDM and tempering the mold to reduce the residual stress of the heat-affected layer.
