Poor heat treating of tool steel has unintended consequences, significant costs
November 7, 2011
Proper heat treatment is essential to optimize tool steel properties. This entails not only selecting the appropriate time and temperature parameters for the grade involved, but also equipment fully capable of doing the job at hand. Toolmakers should talk with their heat treat facilities to ensure that when it comes to heat treating requirements, everyone is on the same page.
In many aspects of manufacturing, the little things count. This has never been truer in today’s stamping environment than in the quest for good tooling performance. While few would argue that many variables contribute to tool performance, one of the most basic is the correct heat treatment of tooling materials. Yet it seems that this has become a problematic area that adds significant, unintended costs.
Although considered a black art by some, the science of heat treatment entails carefully conducted thermal processing to bring about specific changes in the metallurgical structure (see Figure 1). The process includes a hardening cycle (also referred to as austenitizing and quenching) followed by tempering, which restores toughness to the newly obtained martensitic microstructure.
The hardening commonly is done in vacuum furnaces, which keep tools clean and provide accurate temperature control (see Figure 2 and Figure 3). Quenching requires forced gas cooling, and newer equipment often uses high pressures that can significantly enhance the quality of the heat treatment. Subsequent tempering usually requires multiple steps (double or triple draws) that lock the desired changes into the tool steel. A full hardening and tempering process takes time, often 12 or more hours of furnace time.
Proper heat treatment is essential to optimize tool steel properties. This entails not only selecting the appropriate time and temperature parameters for the grade involved, but also equipment fully capable of doing the job at hand. Newer vacuum furnaces have improved performance, particularly in regard to quenching, which significantly benefits a tool steel’s metallurgical structure. At the same time, heat-treating organizations must be conscious about how they fixture parts, batch parts, and build loads to achieve the best results.
While the simple grades generally have a standard recipe, the more advanced tool steel grades (needed for the most demanding applications) require careful processing tuned to the intended application. Because of relatively high alloy content, advanced tool steels tend to be less forgiving when it comes to heat-treat parameters. And the familiar hardness check unfortunately is not a definite indicator of proper processing. There are many ways a tool steel can end up with a less than ideal microstructure yet still produce the expected hardness results. For example, a high-alloy, premium-tool-grade material may be undertempered—and hence have low toughness—but the hardness may be perfectly satisfactory.
Ultimately, the correct metallurgical microstructure will provide the best combination of wear, toughness, and strength properties, and the hardness test—though quick and simple—does not provide absolute verification. Shortchanging the process often results in reduced toughness, which leaves tooling prone to chipping and fracture.
Many changes have affected the dynamics associated with the business of heat-treating tools. In years gone by most toolmaking apprenticeship programs taught metallurgy basics; heat treating was considered a basic of the toolmaking trade.
The increased use of higher-alloy, air-hardening tool steel grades has made it less practical to conduct tool steel heat treatment in-house, which is why most modern toolrooms outsource the operation to commercial shops that have made the investment in the more sophisticated equipment now required. At the same time, all manufacturers obviously face intense pressure to control cost and reduce turnaround time. The toolmaker with little appreciation for the science can easily take aim at his heat-treat source as part of his effort to save a penny or expedite work.
In the absence of other criteria, a toolmaker may direct work to the source that can get tools back to him quickly and cheaply. Unfortunately, many times this does not favor the heat-treater who has invested in the latest technology, insists on making proper batching decisions, does not overload equipment, and will not shortchange a cycle, even if the tool steel’s hardness seems to be satisfactory. Conversely, a growing number of quality-conscious toolmakers now send work to top heat-treat shops, even if it means shipping out of state or allowing a week for turnaround.
Further complicating matters, it has become difficult for toolmakers to find answers to tool steel-related questions. In the past they could rely on the producing mills for the needed instruction about how best to process the materials. Today tool steel is a commodity to the extent that the various grades often come to market through channels that offer minimal technical support. Consequently, heat-treating mistakes can occur and problems can get out of hand simply because the needed information is not available.
Consider one Midwest stamping operation that had difficulties fineblanking a part from high-strength, low-allow (HSLA) material. The shop ultimately upgraded its tool steel to one with increased toughness compared to the D2 previously used. After a successful trial, several sets of production tools were made with high expectations. However, the first set put into service failed immediately despite having the correct hardness.
Metallurgical investigation reveal-ed a significantly overheated microstructure and little evidence of tempering. It turned out that to save time, the toolmaker had sent tools to an ill-equipped local heat-treating facility. To make matters worse, the tools were run (for convenience’ sake) with some other work in a cycle with a too-high hardening temperature, which damaged the tools beyond repair. Not only was the cost of the tooling lost, the stamping operation suffered significant downtime. Because the backup toolset was also heat-treated in the same batch, the entire job had to wait until a new set of tools could be made.
So what can be done to gain control to ensure heat-treatment issues do not become a detriment to successful tooling performance? Much depends on the nature of the relationship between the toolmaker and the heat-treater, as well as the tool steel supplier. Often a toolmaker has never even paid a visit to his heat-treater’s facility and has no idea how his material was really processed. Similarly, the tool steel supplier can conveniently disappear from the scene when problems are afoot. Good communication between all parties is obviously essential. And remember, price is not everything.
It often seems toolmakers may not feel entirely confident judging their heat-treat source because they don’t understand the process. However, much can be accomplished from a practical standpoint by simply visiting the shop and auditing its methods. The heat treatment of tool steel is fundamentally no more difficult than other bulk processing of production parts, but it does require a certain attention to detail and careful handling. Is your heat-treater properly equipped and willing to take the time needed to process your parts properly, or is it more geared to high-volume, bulk processing?
What, then, are some simple steps that toolmakers can take to make certain their costly, carefully manufactured tooling components are properly heat-treated? To start, acquire all the needed information and data from the tool steel supplier. This information should indicate the specific optimal hardness and recommended heat-treat procedure based on the intended application. This is essential for the new higher-alloy, high-performance materials, but is also good to review for familiar tool steel grades.
After carefully selecting a heat-treater, work with the company to develop a heat-treat recipe for the job at hand. This can include some “wiggle room” so you don’t incur large, special lot charges, but should be specific enough to guarantee consistent results. After this, schedule tooling production to allow ample time for heat treatment. Maintain part traceability, and require that the heat-treater provide documented certification of the process.
Finally, perform an occasional audit to make certain things are in order. This preferably includes metallurgical evaluation of a sample part, a task that could possibly be performed by the tool steel supplier or by an outside lab at a reasonable cost. An on-staff metallurgist is a bonus.
All this boils down to paying the same attention to heat treatment as you do to other operations that affect tooling performance. Great care is required to design, manufacture, handle, and maintain critical tool and die components. A little effort can ensure that heat treatment does not become the weak link in the chain.