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Tool steels for tough tasks.

Tool steels are in a state of continual improvement-evolving to take on the most difficult machining and hot-and cold-forming operations. Alloying and processing developments continue to add structural properties to the growing database of tool-steel selection, processing, and performance capabilities.

That's good, because production demands continue to increase. To maximize the ultimate tool's performance, deeper understandings of tool-steel wear and fracture mechanisms are required, along with insight into alloying structures and processing relationships .

Tool steels aren't cheap. The newest alloys can command major premiums over more conventional grades, but as any toolmaker knows, the lion's share of tool cost is toolmaking, not tool material. With material cost typically only 10% to 20% of finished tool cost, that portion can be reduced even further when the cost of tool repair, regrinding, and lost production time are considered.

In one example cited by Uddeholm, tool life of a forming and coining punch of its vanadium alloy (Vanadis 4) was seven times that of a prior D2 punch, but when all costs were considered, the small premium paid in tooling cost ($530 per tool versus $425 for the D2 tool) resulted in a cost savings of 35 times the difference in material price. PM is doing well

Although powder-metal (PM) tool-steel alloys have been available for nearly two decades, the big increase in their use has been in just the past five years. Says Crucible Steel's Harry O'Brien, tool-steel marketing manager, "We offer the CPM (Crucible Particle Metallurgy) product as the solution to tooling problems-primarily wear and edge-chipping, along with toughness, in some cases."

In the CPM process, pre-alloyed molten steel is atomized into a spray of micro ingots of uniform size and composition, and isostatically pressed to 100% density. The PM benefits of tailored composition permit going beyond the capabilities of conventional alloying. Comparing the same grades of PM with conventional alloys can show increases of 2 to 2.5 times in toughness. Secondary benefits are improvements in grindability and machinability resulting from better control of carbide size and better stability against distortion during heat treatment.

PM tools tend to hold coatings better because of their very fine and uniform carbide structures. For plastic-mold use, the key benefit is improved wear resistance to combat the cold-working of PVC and the abrasion of glass-filled resins. (CPM (or any PM) is not for every application, O'Brien admits. "If you're doing only short runs, or don't need more wear resistance or toughness, i.e., don't have any tooling problems, then there's no great benefit." Presently 10% of their tool-steel business and increasing faster than any other category, CPM is taking business away from conventional tool steels.

Alloys other than CPM that are doing well for Crucible are L6, an oil-hardening grade for roll-forming applications, and Marlok, a new managing diecast steel that offers improved resistance to heat-checking (the primary failure mode for diecasting dies) and some machining advantages over premium-quality H13 grades.

Application areas

With the expiration of some of Crucible's basic PM patent position last year, Carpenter Technology Corp, Reading, PA, is aggressively contesting for this business with their own array of high-speed powder alloys and seeing similarly good response to basic PM benefits.

Key application areas, says Eric Lamm, commercial manager, tool steels, are broaches, endmill cutters, reamers, drills, taps, and dies. Not only are there different alloys for different situations, but different alloys for the same basic tool to get different properties. "In the diecasting area, 883 Plus and Extendo Die, premium-melted hot-work alloys, are seeing much broader acceptance. It has been a question of getting spec'ed on diecasters' prints. Once people use it and find it to be very good for them, they come back."

For Crucible, the three key application areas are: cold work stampings, knives, slitters), plastic molds, and die casting. "In plastics tooling, we've introduced a stainless holder-block series called Maxel 400FM, that has done very well. It's a maintenance-free mold base material. If you run just a few parts, and put that mold on the shelf, you would normally have to clean up the rust when you use it next time. The only way to keep that block from corroding was to chrome-plate it, which is not easy to do. "In contrast, Maxel alloy holds up very well and requires minimal maintenance. The material is extremely machineable and eliminates going outside for plating.

"We also have a new grade that we've applied for patents on, called Maxel 400 Super, that provides more corrosion resistance than most stainless mold steels, and is tougher and much more weldable. Machining trials indicate that it is as machineable or better than competitive materials."

Coatings on PM?

Coatings have cut into the tool-steel market by providing major tool-life benefits to users, O'Brien notes. But when people try to save money by hard coating a low-alloy grade, like A2 or D2, the PVD process can temper that material into the mid-50s Rc. "You get a hard coating on a very soft substrate and thus, not much compressive support," he notes. "In contrast, CPM materials, primarily M4 and 10V, will hold 62 to 64 Rc under those same PVD coating-process conditions, and that substrate hardness is there when you wear through the coating. So, the tool holds up a lot longer than an A2 or D2 tool, and you don't end up ruining it.

"Although CPM alloys work very well uncoated, in those applications where some improvement is needed, we will recommend coating a CPM M4, for example."

Domestic tool steels hanging tough

The US tool-steel market generally has been off about 15% from last year, with imports stable at about 32%, not gaining market share. However, if the VRAs come off in March, we could have a whole new ball game," says Carpenter Technology's Eric Lamm. The biggest recent change in market was in 1981, when tool steel sales dropped from 120,000 tons to 95,000. That, he feels, was due to a big shift toward imported finished products-primarily automobiles and appliances-that cut into the tool and die business. "In the last two years, I haven't seen any differences in the level of imported finished dies or the ability of our domestic customers to compete with that imported product." The new applications for tool steels, he feels, are offsetting any loss in traditional applications. Crucible's O'Brien agrees, noting that his company is doing as well as ever. "I see no major increase in what people are buying overseas. In fact, some tell us that they're bringing their tooling back to the US. We export CPM products all over the world." With the competitor mix stable-no new players in such a capital-intensive industry-the domestic suppliers are strengthening their focus on product areas. This should bode very well for tool-steel users, in terms of better service, improved quality levels, and greater involvement in JIT programs.
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:includes related article
Author:Sprow, Eugene
Publication:Tooling & Production
Date:Dec 1, 1991
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