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Pitt scientists get the lead out of machined steel

Monday, July 19, 1999

By Byron Spice, Science Editor, Post-Gazette

No doubt somewhere amid the lore of early steelmaking is an explanation for why someone first mixed lead into steel. Whether by design or by serendipity, lead became a key ingredient in steel that could be readily machined without rapidly wearing out cutting tools. The subsequent failure of metallurgists to figure out why lead had this effect has for decades been an obstacle to getting the lead out of machining steel.

 
Anthony DeArdo, left, and Isaac Garcia, both of the Basic Metals Processing Research Institute at Pitt, set up a consortium in 1994 to help finance their research on removing toxic lead from steel.  

Lead, a toxic metal that can hamper brain development in even small doses, gave up its secret only after a pair of University of Pittsburgh materials scientists, Anthony DeArdo and Isaac Garcia, stripped apart steel samples, analyzing them atom by atom. In the process, they learned how they could replace lead with environmentally benign tin, creating a new machining steel.

A patent on the new steel, dubbed "green steel" because of its environmental benefits, is to be issued this summer and Ford Motor Co. already has ordered a shipment of power-steering parts to be made with the lead-free steel.

"It's looking very promising," said Robert Squire, president of Curtis Screw Co. in Buffalo, one of the nation's largest machining companies and a member of an industry consortium that fronted $500,000 for the Pitt research. "It competes well against leaded steel, the best machining steels there are."

Leaded steel is a small percentage of all steels. The American Iron and Steel Institute doesn't keep track of leaded steel as a category, though Squire estimates about 3 million tons of leaded steel, mainly a grade known as 12L14, are used by the nation's 700 machine shops each year. Last year, total U.S. steel production was about 98 million tons.

"It's a very profitable steel," said DeArdo, noting that worldwide the market may approach $1 billion. Under a car or in an engine, for instance, threaded fittings and all the long rods with machined ends are made of machining steel.

But lead is problematical, particularly for steel mills that must spend money on environmental controls, said Nancy Gravatt, a spokeswoman for the American Iron and Steel Institute. A lead-free steel could save steelmakers money.

Leaded steel doesn't pose a problem for recyclers such as auto shredders because the overall lead content of the steel in a vehicle is low, said Greg Crawford, operations vice president for the Steel Recycling Institute.

But scrap dealers who normally pay for steel chips from machine shops draw the line at leaded scrap, said Squire of Curtis Screw. "They do you a favor to haul it away," he added.

DeArdo and Garcia and their research team at the Basic Metals Processing Research Institute became involved in the lead issue almost 10 years ago, performing analyses on behalf of Bethlehem Steel that focused primarily on how to improve leaded steels.

But they ran into the same frustrations as previous researchers. Machinability of leaded steels varied considerably from company to company, sometimes even between batches. Yet the chemical composition -- carbon, manganese, lead, sulfur, phosphorus -- was the same. Electron microscope analysis showed no differences. None of the standard metallurgical tests were of any help.

And there was the problem of lead's function. The most popular notion was that lead, which has a relatively low melting point at 621 degrees, served as a lubricant for cutting tools. But DeArdo and Garcia didn't find it a satisfying answer.

"What we did surmise was that there was something beyond what we could see," DeArdo recalled. Knowing they would have to undertake more detailed, more expensive studies, he and Garcia in 1994 established a consortium, with members ranging from steel producers to scrap dealers, to finance the research. Members include Laurel Steel of Ontario and Saarstahl Steel of Germany; USS/Kobe Steel also has supported the research.

One set of studies analyzed the type of fracturing that occurs at different temperatures. Machining can result in a smooth finish, but it's actually controlled fracturing -- chipping off tiny hunks of steel. Steel is composed of individual crystals of iron-containing ferrite and can fracture in only two ways: either the ferrite grains break apart from each other, a process called grain boundary fracturing, or the grains themselves split, a process called ductile fracturing.

At room temperature, they found, the bonds between ferrite grains are stronger than the grains, so leaded steel undergoes ductile fracturing. But machining doesn't occur at room temperature; friction from the cutting tools can raise temperatures to between 400 and 1,100 degrees. At those temperatures, DeArdo and Garcia found, the bonds between grains somehow weakened and the steel underwent grain boundary fracturing.

"Once we saw this, we knew that lead was acting as a grain boundary embrittler," DeArdo explained. At even higher temperatures, ductile fracturing once again occurred. By happy coincidence, lead's embrittling effects, which make steel easier to chip, occur at temperatures coinciding with machining temperatures.

So the lead was acting at the grain boundaries, but in what form? Lead or some lead compound? To answer that question, they turned to an instrument called an atom probe field ion microscope -- one of three that were built for U.S. Steel and then donated to Pitt when USS closed its Fundamental Laboratory in Monroeville in the late 1960s. The instrument strips atoms off metal samples, layer by layer, allowing researchers to determine its atomic composition and structure.

DeArdo said some lead no doubt is bound up in the ferrite, but the lead at the grain boundaries turned out to be pure lead, not a compound.

With that knowledge, he and Garcia were able to select a suitable substitute for lead.

Tin shares lead's tendency not to dissolve in steel, but doesn't share its toxicity.

"This is not just a simple recipe that any Tom, Dick or Harry could go out and do," DeArdo said, noting that the steel has to be heat-processed in a precise manner so that the tin remains at the grain boundaries. "If cooling is too quick, you don't get segregation of the tin along the grain boundaries."

Heat processing, he guesses, may also explain the variability in leaded steels. Different cooling practices at different mills may result in too little or too much lead being deposited at the grain boundaries.

One further insight the Pitt scientists uncovered is that the deposition of tin along the grain boundaries is reversible. Heat processing a piece after it's been machined will cause the tin to migrate from the boundaries and into the ferrite, eliminating the brittleness that improves machining but may weaken the piece's performance.

Garcia and DeArdo are applying this insight to stainless steels and to forms such as gear steels and structural steels that are prized for their strength and hardness, not their machinability. Adding tin to the mix would allow them to be machined; subsequent heat treating would then restore their resilience.

Alan Cramb, assistant director of Carnegie Mellon University's Center for Iron and Steelmaking Research, said Pitt's new grade of steel has many properties that should make it attractive, though he cautioned that new grades often must undergo a long process of customer acceptance.

In Buffalo, Squire said Curtis Screw has "cut" only about 15,000 pounds of the green steel thus far "and in our business, that's not a heck of a lot."

But whereas previous lead-free steels quickly displayed problems -- eating up cutting tools, or producing long slivers instead of tiny chips -- the tweaking thus far required to use the new steel has been typical of what usually occurs when adjusting for the normal variances in batches of leaded steel.

"I think it will just take off like gangbusters," said Squire, who has 90 tons of green steel and is expecting delivery of another 125 tons next month. "I'm literally getting calls from all over the world on this thing."



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