Historical data on forging-die life is well established. A study of over 120 different cavity forms from 20 different forgers showed that more than 70% of all forging-die failures are associated with abrasive wear; more than 25% are due to mechanical fatigue and gross cracking; and thermal fatigue cracking and plastic deformation account for the rest. However, this information only provides a focus for material development and a starting point for material selection. It is the understanding of these phenomena and what to do about them that can lead to proper development and use of new die material grades.
In this article we look at new developments in tool steels for these key forging applications while also providing a blueprint for enhancing the characteristics of these grades in their applications.
Steps in the Development of a New Tool SteelA methodical approach to improving tooling performance is essential to success. A brief summary of some of the more important steps in tooling analysis is as follows:
- Choose one or two areas of poor performance.
- Carefully determine tool failure mechanisms. A full failure analysis is needed.
- Review the past performance of the current tool material. Determine what the material is and how it was manufactured, thermally treated, coated, machined, etc. Run simulations.
- Review the production environment and factors that could affect tooling performance.
- Proceed with trials: decide number of tooling sample tests per trial; get detailed heat-treatment charts for each trial; get detailed coating parameters, provide samples for coating; and establish manufacturing parameters.
- Run and follow up on trials – track tool performance and photograph results regularly.
- Implement results on immediate tooling.
- Implement results on tool groups.
Die Material PropertiesThe properties profile for forging-die tool steels has a number of general characteristics that will always be required in forging operations.
- The steel should have improved physical properties over conventional tool steels.
- The steel must show sufficient hardness and ability to retain this hardness at elevated temperatures.
- The steel must have an enhanced level of hot tensile strength.
- The steel must have good toughness and ductility at low and elevated temperatures.
- The steel must have sufficient hardenability and retain wear resistance and thermal properties as dies are resunk.
- The steel must have adequate fatigue resistance.
It is important to understand that chemistry alone does not tell the complete story of a steel’s capabilities, which is why it is important to ask questions about how the steel is processed. If the steel is not remelted, it is important to stipulate the minimum forging reduction ratio and to know if a homogenizing heat treatment has been performed to get good properties from a conventionally produced grade.
If the steel has been remelted, find out how and in what type of unit. Not all remelted steels are produced using the same updated technology. For example, grades like Uddeholm’s Premium H13 (Orvar Superior), Dievar and Bohler’s Premium H13 are remelted through controlled solidification in a steel bath and under a protective atmosphere, giving better homogeneity of structure and steel cleanliness than in previous ESR furnaces. Bohler’s VMR grades are melted in vacuum arc remelting furnaces, obtaining extremely high cleanliness levels. An example of the toughness enhancements by going to cleaner steelmaking practices is shown in Figure 1.
Tool-Steel Property EnhancementsPhysical Properties – Thermally induced stresses are proportional to the thermal expansion coefficient, the elastic modulus and the temperature differences during the forging process. Temperature difference and maximum temperature can be reduced by increasing the thermal conductivity, which can also reduce the temperature gradient and the resulting stresses on the tool. An advantage of the newly engineered tool steels is that their thermal conductivity is improved over that of an AISI H13 chemistry, as shown in Figure 2.
Application ResultsExamples of positive results with new tooling materials have been significant. In one case, a screw press was used to manufacture a wheel rack of 1.2312 steel at a forging temperature of 1150-1180°C (2100-2156°F). The die was 280 mm x 150 mm x 65 mm, and the part weighed 640 grams. The previous die was made from H13 steel, hardened to 50-52 HRC and nitrided. Severe wear was obvious after 3,000 parts produced. A change to Bohler W360 Isobloc steel at a hardness of 56 HRC without nitriding was able to obtain an improvement of over 120% from previous results and four reworkings.
We conducted one study to improve not just the tooling material but the heat treatment as well. A die (Figure 7) was originally produced of standard H13 material. The die material was kept at H13, but the steel source was changed. That, along with an improvement in heat-treatment control, increased the die life average from 3,000 parts to 3,700 parts. When Dievar was introduced, that average increased to over 7,000 parts. Then, to further improve the situation, the outer die was replaced with Dievar and the inner die with W360 Isobloc. Die-life improvement went to 20,000 parts with Dievar and 10,000 parts with the W360 Isobloc.
Our last example is a punch nose (Figure 8). It is interesting to look at the hot wear at the surface of the H13. In actuality, the tool damage is a function of heat checking as well as wear. A change of the material to a more heat-resistant tool steel, type 1.2367, more than doubled the punch life.
ConclusionsTo obtain the best tooling economy in forging it is important to follow a methodology for evaluating new tooling materials and related processes. This article has examined several key material properties and shown how recent improvements in tool-steel development have created new grades that can address some of the key concerns in many forging applications.
This article was adapted from a paper presented at the International Forging Congress held in Chicago, September 2008, by author Patricia Miller, senior technical manager, Bohler-Uddeholm, Elgin, Ill. She may be reached at 800-652-2520 x8732; or at Patricia.Miller@bucorp.com
All photos and graphics courtesy Bohler-Uddeholm Corporation