Solving Die Failure with Special Tool Steels
Using the metaphor of old, less-efficient cars compared with modern vehicles, the authors suggest that using age-old tool-steel grades may not measure up to the performance needed from modern forging die steels in certain applications.
We all try to use our time and money wisely. Who wants to waste time or throw money away?
Many people today travel to work over long distances, and many of us select a car with the most efficient properties. These more-efficient cars are often more expensive, but you work out the breakeven point when the extra cost is paid back.
If we suggested that a 50-year-old car design was the best and most efficient way you could travel to work, you would be wise not to listen to the advice. Yet this type of thinking can often be what forging companies use when selecting a die tool steel if they are not careful.
This article will discuss why this is often the case and what needs to change in the forging industry so that the correct die steel is selected to solve the most costly problems in the supply chain. Although our focus will be on press forging, which is Uddeholm’s main market, the principles apply to all hot applications.
It is very important to make your tooling as efficient as possible. Modern forging is a very competitive industry and must survive in a globalized world where a few percent on the cost of a part can be the difference between making a profit and sustaining a loss. The bulk of the industry’s customers are large automotive companies that often demand price reductions on each project and often review this throughout a project’s life span.
The aerospace and special-alloy forging industries are a good example in which die life is often much lower than in the automotive segment. However, they persist in selecting die steels based on price rather than looking at what is causing the poor die life in the first place and how to solve it. To understand how best to do this, we must first understand the main failures in forging where big improvements are needed in die life.
Maintain Your Dies Well
Before we start with the tool steel, it is important to mention that good die maintenance is critical to any tool steel working at its best. Preheating of the die is one of the most important parts of maximizing die life.
During the forging operation, preheated billets are formed between the two dies. The high temperature of the workpiece material and the low temperature of the die will result in high levels of stress on the die surface. If the stresses become too high, cracks will start to form and will soon lead to failure. To reduce the number of stress buildups in the die, the temperature difference needs to be reduced. This can be done by preheating the die to around 150-220°C. The preheated die will also result in higher toughness and ductility, which will lower the growth rate of cracks and crack initiation.
What is the failure? Are you solving it with your die steel?
If we asked press forging companies what is the main type of die failure that prevents them from maximizing die life, the majority would say “abrasive wear.” The second could be “plastic deformation,” but sometimes you can get “cracking,” which can end production very fast. The most common die steels used today are listed in Figure 3. We have to ask ourselves, do these solve the biggest problem of abrasive wear, and how do they rate with the other two problems?
The answer for the main problem of abrasive wear is no, the chemistries of 1.2344 (also known as H13), 1.2343 (also known as H11) and 1.2714 are all lacking in this area, or they have compromises that we will explore in the next section. At this point we will discard 1.2714.
If you want to solve abrasive wear and plastic deformation, then this steel is like an old low-technology car that may get you from points A to B, but it does so with a very high running cost. This tool steel (1.2714) is very good at being tough, but that is offset with terrible abrasive wear and plastic-deformation properties, which is the compromise one makes for good toughness. You often see this grade no higher than the 40/44HRC range, which makes it good for hammer forging in large dies where toughness is the most desirable property.
Now, continuing with the car analogy, we move to the 1970s YUGO 45 based on an even older car. Our steels here would be H13 and H11. These tool steels trace their roots back to the pre-1940 era, but – even with process improvements – they have not changed much to this day. In its day, the YUGO 45 was useful at the cheap price it was sold for, like the VW Beetle from the 1930s, but not today as a modern, efficient car!
This mirrors the basic principles of forging tool-steel selection if you do not focus on the true areas of high cost, such as the lifetime running costs of the tooling. AISI H11 and H13 are cheap, easy to buy and they work OK. But is just OK really what you want? Furthermore, how do these steels compare to modern special-chemistry die steels currently available?
How to Solve Your Problems with Modern Tool Steels
To solve these problems of abrasive wear and plastic deformation, we need to look at modern tool steels. For example, Uddeholm Unimax uses high hardness combined with good toughness from its chemistry and the electro slag refining (ESR) process to achieve this. The ESR process results in increased cleanliness, homogeneity and more-or-less-equal properties in all directions of the block or round bar of tool steel.
The ESR process is characterized by taking the conventional cast ingot and remelting it through a slag bath, which then forms a new purified ingot with these superior properties. It then goes through forging and other processes to give even more superior properties.
Again using the car theme, we could say that Unimax is like a modern, efficient plug-in hybrid electric vehicle (PHEV) that uses all the good parts of the old car’s technology, like a combustion engine, and combines that with electric motors and batteries (new chemistry and ESR process).
Unimax has a higher carbon content than H13/H11, which allows it to reach the high hardness of 56/58 HRC. However, hardness is just one step toward achieving better abrasive wear resistance. You also need to have great temper-back resistance, which is achieved with almost double the amount of molybdenum. This gives excellent hot strength to resist temper-back effect and the resulting plastic deformation from this drop in hardness.
We see from Figure 5 that Unimax takes a long time to temper back because of its ability to reach higher hardness levels than H13, H11, Dievar and QRO90. Unimax maintains this higher level of hardness all the way through the production cycle until over 10 hours, at which point QRO90 starts to outperform it. The benefit is in the first 10 hours, where you gain big advantages in extra wear resistance from the high level of hardness.
This performance is repeated in customer case studies in high-wear applications. Please notice that the grades from Uddeholm, Vidar Superior (modified H11) and Orvar Supreme (H13), while good steels, prove the points made in the first part of this section – that H11 and H13 chemistries do not have the properties to resist temper-back effect like the new steels, such as Uddeholm Unimax, Dievar and QRO90 Supreme.
Different tool steels can solve different problems, as we see in the case study charted in Figure 6. Today, you may face problems in production relating to your die steel.
Just as the cars produced decades ago are no match for modern, more-efficient vehicles, so too are modern tool steels more efficient in certain applications than older traditional tool-steel formulations. We suggest you step away from your old YUGO 45 (H13/H11) and consider the more modern tool steels as a solution to die-life problems.