search.noResults

search.searching

dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
UDDEHOLM DIEVER


withinpowertrain has to last around 80-150K shots (depending on the design and press used) but in structural parts, this can be under 75K shots. For example, some types of shock tower dies have been known to produce under 30K shots due to their operation, design and complex geometry. Some have even been known to prematurely crack if the die has poor process control combined with insufficient material properties. But this poor performance is not just limited to shock tower dies as all structural parts are often complex in design and push the older die materials to the limit of what they can do. Why is this? If we look at this longitudinal bridge part and die (Figure3),


we can see that it has a very large surface area with many thin and thick sections. As these castings will make up the structural elements of the vehicle, it is important that the injection process is optimal to avoid porosity and other internal defects. Hence, gate speeds are often very high to fill the die as fast as possible and a typical structural part die has many more gates than the traditional powertrain die, 14 gates in this example! This means extra heat is generated in the gates and when you combine that with the general heating and cooling of the casting cycle, along with spraying of the die, you then get high levels of thermal fatigue or heat checking.


Cracks will appear very rapidly if the cooling channels in the die are inefficient and cooling is applied directly on the die’s work surface. This will result in a big temperature difference and stresses will result in cracks instantly occurring.


Figure 4 shows heat checking on the left under magnification and on the right as seen in a real production die


Above (Figure 4) we see the typical heat checking damage on a die surface under a microscope (100x) and from normal appearance to the eye on the right. To minimize the risk of this failure mechanism (heat checking) some superior material properties are an advantage. For example, better heat conductivity will result in less temperature difference and therefore less stress builds up in the material. Additionally, a good temper-back resistance is desirable, to prevent the surface of the material from losing hardness due to heat exposure. High ductility is also important in order to lower the risk of crack initiation. The material also needs a good toughness, both in room temperature and higher temperature, in order to reduce the rate of crack growth. H13 has a higher alloy content then H11 and therefore H13 will have a


Figure 3 we can see why the challenge, structural parts, big surface areas, thin sections, complex geometry


Many of these new structural parts are safety-critical and if the vehicle is to perform as designed in a crash situation, the panel must be clean of potential crack initiation marks. Heat checking damage would class as a potential crack initiation area and hence becomes a big problem on these structural parts. A trend is also for bigger and bigger parts and presses, this means that the inserts for the tool are also getting bigger. In some cases split lines and inserts are not allowed in the design which presents a problem as the bigger the insert the higher the risk of gross cracking! Now the die steel you select not only needs to have the ability to solve the main production problem of heat checking but also needs to be very tough and ductile in operation.


Heat Checking and the current tool steels In applications such as HPDC, there will be a large temperature difference on the tool’s work surface as the casting goes through its cycle. The difference in maximum and minimum temperature will create stresses in the material and eventually fatigue cracks will develop. A bigger temperature difference, coupled with full production, will increase the thermal fatigue resulting in a shorter die life. The heat checking pattern that forms on the die’s surface will also make marks on the castings that will lower the aesthetic and tolerance of the product.


slightly better temper- resistance and hot strength due to the precipitation of fine alloy carbides. This can be seen in some production results where heat checking resistance is slightly better over H11 but is not a significant advantage. H11 has a lower content of Vanadium, which lowers the risk of primary carbide formations thus promoting higher toughness and ductility. Therefore, it is a reverse of the result for H13 with H11 slightly better with toughness over H13. But again not a significant difference in both grades, not when compared to a tool steel grade like Uddeholm Dievar.


Uddeholm Dievar, a solution optimised for heat checking Is Uddeholm Dievar the solution to heat checking? Uddeholm customer feedback and case studies have reported that Dievar provides excellent results compared to H13 & H11 tool steels when heat checking is the main failure. Laboratory tests (Figure 5) have also shown that Uddeholm Dievar has better heat-checking depth resistance than premium H13 grades as we see in the below chart where we see the depth of crack is much greater in the H13 material than Dievar at the same hardness levels.


Figure 5 clearly illustrates that Uddeholm Dievar has far superior resistance to heat checking cracks over premium H13


www.internationalmetaltube.com IMT October 2019 19


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32