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What makes a grade of steel "machinable"? The answer to that question has changed through the years as metal-cutting technology has stepped up, machining metals previously thought impossible. But another tributary has flowed into the innovation pool—advances in steelmaking. Metallurgists have tweaked steel's chemical makeup and microstructure to ease chip-breaking.

Yet easy chip-breaking often makes a rough road for other manufacturing processes up and downstream, like forming and welding. For instance, a high-level of manganese-sulfide inclusions can help chip-breaking, yet it can make life tough for the welder. So steelmakers perform a balancing act to meet the needs of the metal manufacturer.

According to Paul Dimitry, director of technical services for Macsteel, Jackson, MI, "The attributes people are looking for in machining are very simply the formation of small chips. Problems start with long stringy chips that bird nest and force you to stop and clear the machine before the next part."

The bottom line, he says, is that for a given depth of cut and speed, a steel's metallurgical makeup should make chip evacuation easy. For this, the ideal machinable material should have a pearlite, "blocky" microstructure.

The carbon in steel give strength and hardening characteristics. Lower carbon steels tend to produce parts of lower strength and are commonly used for highly formed applications. They tend to have a high level of ferrite and, hence, exhibit higher ductility, which creates those long, stringy chips. Cutting tools can remove low-carbon metal very efficiently, yet the chips produced can be large, long and difficult to evacuate. Steel with higher carbon content gives better strength, is suited for applications involving induction hardening, quenching or tempering, and has better chip breakage yet can be more difficult to machine.

Using higher-strength material takes more power to machine yet offers better chip-breaking; lower-strength steels don't put as much stress on the cutting tool, yet the process becomes more susceptible to bird-nesting. "In steelmaking," explains Dimitry, "the trick is to bridge those trade-offs."

INCLUSION ENGINEERING

One way to ease the trade-offs comes through inclusion engineering that results in small additions of alloying elements (i.e., micro-alloying). Depending on their make-up, inclusions can either be beneficial or detrimental to machining. Detrimental inclusions can be made of aluminum oxides and chemically complex inclusions like titanium carbonitrides; these act as an abrasive that can prematurely wear tooling. They originate from the deoxidation process in steelmaking, where products like aluminum and silicon-magnesium are added to reduce the oxygen content and improve the quality of the steel. These inclusions tend to be small, spherical or angular in shape. "They really act as an abrasive," Dimitry says. "The [inclusion's] hardness can be higher than the tooling's, so when the two come together, the harder element wears the softer element."

Beneficial inclusions can include manganese sulfides that, being soft and malleable, can be made into elongated "strings" in the steel's microstructure. These inclusions "act as a chip-breaker," Dimitry says, "so as tooling goes through the microstructure and they hit these [inclusions], they cause a fracture or breakage of the chip." They create smaller chips that, in turn, become easier to evacuate from the work area.

Bismuth and lead may also create inclusions that aid machinability. Regarding lead, during steelmaking the element strings out into long platelets that act as effective chip-breakers. Nevertheless, Dimitry says, lead does have some issues. For one, it has a very low melting point. So during manufacturing (for welding, in particular), fumes can emerge as temperature rises, which may create a health hazard in some circumstances, he says. The chips and other waste products may also require special handling. Also, "Lead is heavier than the steel," he says, "so it segregates quite dramatically, and can be difficult to get a uniform dispersion of lead throughout the steel."

The trick for machinablility involves maximizing the beneficial inclusions while minimizing the detrimental, all the while maintaining optimum chemistry for the steel. To reduce those detrimental aluminum and silicon-magnesium inclusions, steelmakers avoid adding them to reduce the oxygen content. Instead, they can now perform vacuum deoxidation, where the steel is exposed to a very low partial pressure that draws oxygen out of the steel. "We take the pressure down to 1- to 2-mm of mercury," says Dimitry.

"Atmospheric pressure is 760 mm of mercury. Take that down to 1 mm of mercury, and that's quite a vacuum."

In recent years, precise chemistry control has helped achieved optimum machinability as well. Some steelmakers no longer work to a generic spec but engineer grades specifically for a customer.

"We'll try to find the optimum chemistry," Dimitry says, "which involves analyzing the relationship between carbon, magnesium, sulfur and alloying elements to find a recipe that will prove optimum machinability."

A prime example involves steel made for constant-velocity joints, parts that used to be conventionally machined but today are hard-turned. "Years ago they could be made from a carburizing steel," Dimitry says. "Today, they are medium-carbon steel that's induction-hardened. Optimizing the elements allowed [steelmakers] to increase both the manganese and sulfur content to give more of those manganese-sulfide-type inclusions." And as mentioned, those inclusions greatly assist chip-breaking.

Precise heat-treatment also improves a steel's machinability. For low-carbon steel that tends to be soft and gummy, steel makers can provide a "normalizing" heat treatment, cooling precipitation-hardening elements to, basically, help increase the strength of the steel's ferrite structure. "We heat-treat the steel to the austenitic range, and air cool it to accelerate the cooling," Dimitry explains. "This cooling rate gives us that slightly higher hardness in the ferrite that helps improve machinablility."