New developments in tool steel

Tool steel selection typically isn’t an issue until it becomes an issue.

When tooling starts breaking in the press because it can’t stand up to a new material or a change in part design, that’s when engineers start looking around for an alternative. Everyone knows they have a problem, and they need a new solution.

Until that emergency occurs, however, engineers and toolroom personnel typically stay with what has worked for them in the past. No one is actively pushing the question, “Do you think we should be looking at new tool steels for this project?” because everyone is focused on meeting production deadlines, maintaining quality levels, and keeping costs down wherever possible. Tool steels such as D2 and M2 are popular choices for a reason: They do the job quite well for most applications.

That doesn’t mean other tool steels haven’t emerged as viable alternatives for challenging metal forming jobs. Perhaps metal stampers just don’t know what they are. That’s why two companies shared their latest developments with conference attendees at the 2012 FABTECH® in Las Vegas.

Prehardened Tool Steels

Heat treating of tool steel can cause problems for metal stampers. As they seek to create hardened tooling, the extreme temperatures—reaching nearly 2,000 degrees F in some cases—can alter the tool steel makeup, potentially resulting in tooling that can’t stand up to the rigors of some aggressive stamping jobs. Those in the toolroom who are responsible for quickly reacting to remedy broken tooling often are faced with the need to replace tooling as quickly as possible, which is difficult to do because they know the tooling needs heat treating to exhibit the long-life trait that is often expected; as a result, they are left with an unenviable choice: quickly deliver tooling that hasn’t been heat-treated and risk it lasting only for a short time, or extend the downtime for the stamping press as the tooling is sent out to the heat treater so that it can be hardened for longterm use.

A prehardened tool steel from Europe can help to resolve both of these problems, according to Tom Schade, executive vice president, International Mold Steel. Toolox® 44, developed by SSAB, Oxelösund, Sweden, is a quenched and tempered, prehardened tool steel that delivers a hardness of 45 HRC. The material has a low-carbon profile, much like SSAB’s other products Hardox® and Weldox®, which helps to impart a toughness and ease of machinability

“When Toolox first came out, it wasn’t thought of for die steel. It was for die casting and forging dies,” Schade said. “But people started to experiment with it. It had great impact strength, but the question was whether the 45 HRC was hard enough and strong enough to support a coating for surface engineering. The answer that came up in a lot of cases was yes.

“Now people can buy the preheated steel at 45 HRC, finish-machine their tool, coat it, and they’re in business.”

To illustrate the point about the tool steel’s impact strength, Schade pointed to an automotive stamping application in England: the die kept breaking while forming a U-shaped bracket made out of 2.4-mm-thick mild carbon steel. The stamper was getting only 10,000 hits out of the tooling made from D2 steel before it broke; the die design had features such as sharp corners that exposed stressed areas of the heat-treated tool steel and led to the breakage. The stamper tried several different tool steels and found that Toolox 44 with a nitriding surface treatment met its goal of extended service life, eventually reaching 254,000 hits with the same die insert.

“For stampings, Toolox is being adopted more quickly in Europe, but in the U.S. it’s in its infancy,” said Paul Britton, national sales manager, International Mold Steel. The tool steel was introduced to European metal manufacturers in 2003.

One of the biggest areas of interest for this tool steel is tooling repair, according to Schade. By keeping the tool steel handy, tool- and diemakers can prepare a replacement tool very quickly, knowing the tool steel doesn’t need to be sent out for heat treating because it is already hard enough to stand up to the rigors of the press. It’s a matter of hours, not days. Also, when the press is shut down for an extended period, the replacement tool insert can be sent out for surface treatment, turning the die from a temporary fix into production tooling. As long as the coating process doesn’t exceed 1,094 degrees F, the tool steel properties remain unchanged.

Schade added that hot stamping looks to be a promising area for use of the prehardened tool steel. Many stamping companies have been using H13 tool steel in these types of dies because it is viewed as having good thermal conductivity to help draw the heat out of the formed steel part during the quenching process. Schade said Toolox 44 has 50 percent better thermal conductivity than H13.

“We can dramatically reduce the cycle time while you’re quenching because the heat can get sucked out of the part because of the higher thermal conductivity,” he said.

PM Tool Steels Exclusively for Metal Stamping

Although most in the stamping industry are very familiar with M2 tool steel, M4 is another choice. It differs from D2 in that it is usually manufactured via powder metallurgy, a process in which metal powders are pressurized in a container to create semifinished products or near-net-shape components. This process is useful in creating high-alloy steels, such as those used in tool- and diemaking. The PM process also eliminates the brittleness that sometimes appears when tool steels with high alloy content are made in a traditional steel mill process.

M4 has a slightly higher alloy content than M2. It does a better job of resisting the abrasion and impact associated with long running or heavy-duty stamping operations.

The problem is that these tool steels are not necessarily designed only for stamping applications, said Ed Tarney, customer technical support manager, Erasteel Inc.

“M4 and M2 can be overkill for the metal forming market,” he said. “They were designed to be cutting tools for drills, milling cutters, taps, and reamers. They all have properties that help the steel to hold their edges at high temperatures—around 1,000 degrees F.”

When forming metal parts, Tarney said, the tooling rarely gets over a few hundred degrees Fahrenheit.

“With M2 and M4, you’re paying for alloy content contributing to high-temperature resistance, and typically the higher the alloy content, the lower the toughness of a tool steel,” he added.

That led Erasteel to introduce ASP® 2005, a modified PM M4 tool steel that has a lower alloy content than standard M4 material (ASP 2004). The lower alloy content results in greater toughness and in a steel designed specifically for cold- forming applications, rather than high-temperature cutting tool applications. It offers a hardness range of 60-65 HRC with improved impact toughness over PM M4 across the full hardness range.

“It was designed to be used as a substitute for M4 for anyone making metal forming tools for nearly any application,” Tarney said.

While stamping companies might be interested in the reduced alloy surcharge for ASP 2005 when compared to ASP 2004, they will be more likely to adopt the new PM tool steel if their engineers are looking for something with better toughness traits, Tarney said. Tooling cost is definitely more of an issue in today’s competitive marketplace, but for many stampers it still boils down to performance—keeping those presses running.

“New tool steels in this market are adopted most readily when the old ones don’t work anymore or have obvious limitation,” Tarney said. “The appearance of the high-strength alloys, especially in automotive, has driven some of the latest rounds of tweaking in materials or coming up with new materials. They have created a situation where the standard existing steels weren’t performing as well as desired.”

Tool steel-makers are taking notice of the metal forming market. Now they just need metal stampers to show some interest in their new tooling developments.