With all the landmark advances in technology and materials, the key to improving the performance of cold work tooling still lies in exercising the same metallurgical discipline that has served the industry well for generations.
The toolmaker desiring better results will be on the right track if s/he will systematically re-examine three key areas of execution - (a) selection of the tooling material (b) heat treatment and (c) maintenance of surface integrity. Proper attention in these areas, combined with good design and build practices, will result in tooling that can achieve optimum performance.
On the surface, the broad range of alloy options may seem confusing. However, the ultimate choice should always depend on what combination of properties is desired in the tooling material. Usually, that choice for cold work tooling narrows down to finding the right tradeoff between wear resistance and toughness.
The first step in selecting the right material is to accurately predict the conditions that new tooling will encounter, or diagnose the symptoms displayed by tools currently in service. Judgment then can be based on this evaluation.
If a tool in use breaks or chips, for example, it is exhibiting a toughness problem that typically would indicate the need for a shock resistant tool steel like S7 (AISI S7). In another case, a tool that develops abrasion or galling problems would suggest the need for a wear resistant material like AISI Type D2 tool steel or even AISI M2 high speed steel. A2 tool steel is frequently chosen when the application requires a combination of wear resistance and toughness.
Frequently, cold work tooling fails due to a combination of factors. Then the alloy selection process becomes more challenging. Such is the case when microchipping occurs on the cutting edge of a punch. While this condition will lead to rapid wear along the punch shank, the problem should be recognized primarily as a toughness issue. To solve the problem, it might be necessary to slightly lower the hardness of the tool or select a material with higher toughness.
Today, a number of excellent advanced tooling materials are available to help meet the demands of difficult applications. Powder metal grades, for example, like Micro-Melt® M4 alloy and Micro-Melt® 10 alloy are highly alloyed materials that offer superior wear resistance compared with most conventional tool steels.
The Micro-Melt powder processing, however, results in a very uniform microstructure which enables these grades to also maintain good toughness characteristics. Other important benefits of powder processing include enhanced machinability and grindability, along with improved response to heat treatment.
A new premium P/M high speed steel known as Micro-Melt Maxamet® alloy takes wear resistance and hardness to new levels, bridging the gap between conventional tool steel and tungsten carbide. It offers a hardness capability of HRC 70 while maintaining better toughness than sintered carbide products.
For tooling requiring exceptional toughness, an advanced material known as AerMet®-for-Tooling alloy offers better fracture toughness than any conventional shock resistant tool steel. This grade may be considered where breakage or cracking have been seemingly unavoidable.
Assuming use of the correct tool steel, proper heat treatment then is critical to realizing the full potential of the chosen alloy. As an old shop toolmaker once put it, "Selecting the right steel is like putting bullets in your gun, and heat treatment is the same as pulling the trigger".
Consider that heat treatment today is less the art than it used to be. In the past, a large volume of tool steel was water- or oil-hardened. Heat treating was done in small furnaces, and results depended on the trained eye and learned skill of the heat treater.
Now the air hardening tool steels are most common, and heat treatment is usually done as a larger batch process using vacuum furnaces. Such equipment offers an advantage in that it allows a more closely controlled process that better protects the surface of the parts being run. This is all well and good, but not all vacuum hardening processes are created equal.
Results from vacuum heat treating can be disappointing for a variety of reasons. Problems can occur if the batch processing is not compatible with the time and temperature requirements of a particular part included in the load. Problems also can occur if the heat treater overloads his/her furnace, or if the equipment is not capable of quenching effectively.
Significant advances have been made in heat treating technology to provide furnaces that can solve many of the frequently encountered problems. For example, the newer 6-bar and 10-bar vacuum furnaces offer high pressure gas quenching capability which can have a profound effect on the metallurgical quality of hardened tool steels.
The downside of the new technology is that the equipment is expensive, often beyond the means of the smaller shops. Furthermore, a commercial heat treater with state-of-the-art equipment cannot run work at the same cost as a competitor with old equipment. However, in the interest of quality, it is imperative that tool and die shops support those heat treaters who have improved quality by making the investment in latest technology.
Equipment aside, many heat treatment issues still stem from the tendency of some practitioners to cut corners. It is relatively easy to get tool steel hard, but this often can be accomplished at the expense of an optimum metallurgical structure. The culprit is frequently the high priority given to fast turnaround. Heat treaters need to be allowed enough time to do the job right.
Careful heat treatment is extremely critical when it comes to the advanced tool materials. The highly alloyed grades, like Carpenter's CarTech Micro-Melt alloys, tend to be less forgiving when it comes to heat treat errors. These materials generally require triple tempering, which can take 12 hours or more. Undertempering can result in a significant loss of toughness.
Maintaining Surface Integrity
A tool that is made from the right material and properly heat treated can still fail if it is unintentionally damaged during subsequent finishing operations. Grinding burn, for instance, is a common affliction. If sufficient heat is developed during grinding, the surface of the tool will be tempered back, or even rehardened. This can seriously degrade the surface quality of the tool, sometimes causing low hardness and cracking. The result - premature tool failure!
Grinding problems can be minimized by using some of the new abrasive products that have become available for tool room use. CBN (cubic boron nitride) wheels, for example, are extremely effective for use on the tool steel grades that exhibit more difficult grindability.
Tools that are finished by EDM processing are not immune from surface problems either. If not properly controlled, the EDM process can also cause alteration of the microstructure at the tool surface, and in some cases a recast or "white layer" can be found. Depending on severity, these conditions can prove to be very detrimental to tool performance.
Significant improvements have been made in EDM technology. The newer machines have features that help minimize the potential for surface damage. As an example, tool makers can now easily utilize multiple, low power skim cuts as a means of improving surface quality and finish.
Obviously many factors can contribute to good tool performance. It is important to realize that these factors tend to work together like links in a chain. The end result, therefore, can be no better than the weakest link. Moreover, when a problem occurs, valuable time must be spent to determine which link is the culprit.
Today, a wide variety of coatings and special treatments are available that are supposed to enhance tooling performance. Some, such as the PVD thin film coatings, have established a credible track record. Such treatments should be considered, however, only for use on sound tools that have been made from the right material and properly heat treated. They should not be considered as a substitute for good craftsmanship, but regarded instead as "icing on the cake."