May 2021 Volume 3

MATERIALS

Forging Die Steels – FAQ By Richard Polenick

Why is it important to maintain a proper die temperature in production? Die steels perform best when maintained within a temperature range at a pre-heating stage and while in the press for production. This requirement is related to the steel’s toughness or ability to absorb impact forces. A hammer forging process delivers high impact forces to the heated billet but also to the die tooling. Die steels undergo a metallurgical phenomenon known as the Ductile- To-Brittle-Transition process or DBTT. What this means is that above a certain temperature the steel maintains a ductile or flexible condition vs. a brittle or inflexible condition at lower temperature. So if a forge shop chooses to operate a die below the proper prescribed temperature for that steel alloy, they are running the risk of a crack developing or even a catastrophic sudden failure. This applies primarily to hammer and mechanical press operations. Shop floor conditions vary widely in the forging industry, as do the methods and equipment used to maintain die temperature. Anything from open flames to internal heater rods are used for this important task. Die steel suppliers can advise to what temperature their steel alloys must be maintained. It should be noted that there are die steels that are designed to be operated at lower temperatures to accommodate shop conditions but these tend to be higher alloy grades that drive higher steel prices. These grades also are the preferred materials for non-heated components of the press tooling structure such as bolsters, sow blocks, and spacer plates. What causes die surface cracks or heat checks? Surface cracks or heat checks in the impressions of a die are caused by thermal fatigue. Just as the wire of a paper clip breaks after repeated back-and-forth flexing, the surface of a forging die is exposed to repeated physical expansion and contraction of very shallow surface regions of the die. The expansion happens due to exposure to the hot billet material being placed in contact and then flowing across the steel during deformation. And then, depending upon the shop practice, a lubricant or coolant may be applied to the die surfaces. This cooling action causes that same surface to undergo a contraction. All the while deeper into the die steel the temperature remains relatively stable and resists the surface motions. Over time and repeated production cycles this sets up internal stresses at the die steel surface that eventually exceed the strength of the steel and a crack develops. These cracks may get deeper or longer until they become a risk to propagating a crack failure.

A major aspect of any closed die forging operation is the design, build, and performance of the tooling used to manufacture forged parts. Forging dies can be simple or complex, low or high volume, inexpensive or costly. But the process requires some type of die tooling and these items are predominately made from a steel alloy. The steel grades can vary from low alloy pre-hardened types to higher alloy grades that require heat treatment and finishing. So what are some frequently asked questions about forge die steels and how to work with them? What die steel should we use for this job? This question usually is the starting point for a whole series of follow- up questions to better understand the scope of the project and then be able to select a proper die steel to do the job. Steel selection is largely dependent on the type of forging process, the die design, and the material being forged. Die performance is commonly a measure of the wear resistance of the steel and its ability to resist surface cracks or catastrophic fractures. But those two factors can be a delicate balance to achieve. The best steel for wear may not be the most crack resistant, and vice versa. The type of forging process is a key factor. Hammer vs. mechanical press vs. hydraulic press all bringdifferent aspects todie performance. Basically, the velocity of the applied forces and the length of contact time of die steel to heated billet are the issues. Hammers apply fast, high forces with short contact times versus mechanical and hydraulic types at slower force application and longer die contact. Longer die contact drives higher die steel exposure temperatures and the risk of losing steel surface hardness. Lower hardness leads to die wear or deformation. Process control is also important. Consistent billet size and temperature, quantity of surface scale, die lube practice, die temperature control, and part production rate all should be considered in the steel selection process as they also can affect long- term die performance. Finally, the die design is the focus. Standard buster, blocker, finisher forming sequence? Shallow or deep geometry? Tight radii? Changes in cross-section? Generous draft? All to be considered. An experienced die designer can look at a part drawing or CAD model and recognize what areas of a part will be a challenge. But of course the part must be made to the product drawing! Maybe with some customer negotiation? With answers to the above questions, a proper die steel alloy and working hardness can be selected to offer a reasonable length of service and lowmaintenance.

FIA MAGAZINE | MAY 2021 17