Anatomy Of Free Machining Steel

In some ways, life in a metalcutting shop would be much simpler if there were only one choice for each kind of material. Think about it: You order and stock steel, brass and aluminum . . . period. All process parameters could be optimized around these limited choices.

Well that of course is not the way it is, and truly, it's a good thing we have choices. Engineered materials, designed to give predictable performances, are arguably key variables in the manufacture of precision turned parts. One company that knows the connectivity among various metalcutting parameters is Ispat Inland Bar Products (East Chicago, Indiana).

The machine tool, cutting tools and machining fluids all play critical roles in the metalcutting process. However, it's the material being cut that determines how the machine tool is set for speeds and feeds, the geometry and type of the cutting tools, and the formulation of the metalworking fluid. Material calls the process tune.

Different formulations in the manufacture of steel bar provide different machinability characteristics in the machine tool. By understanding some of the basic metallurgy behind free machining steel, shops can make better recommendations to customers or designers about specifying the right steel for the job, with an eye toward efficient manufacturability.

What Is Machinability?

Defining machinability is difficult. There are as many interpretations as there are tests to determine machinability.

For example, machinability tests can consist of tool life, surface finish, cutting temperature, power consumption and cuttability (in this the rate of penetration of a drill under constant feed is measured). Because machining consists of so many variables, it's very difficult measure to "machinability" overall.

For our purposes let's think about machinability as basically a benchmark for a specific process combination of material, cutting data, cutting tool, machine tool and cutting fluid. Once the process is running, the idea becomes how to optimize each variable to increase the overall machinability of the process.

A Primer On Steel

Steel is an amazing material. It is malleable, strong and versatile enough to be applied in numerous ways, and it's relatively inexpensive.

The principal raw materials in steel are iron ore, coal (coke) and limestone. When melted together these raw materials produce pig iron, which is the foundation of steel.

Steelmaking, then, is the process of refining pig iron, as well as iron and steel scrap, to remove unwanted elements from the mix and to add measured amounts of elements that produce desirable properties for various performance specifications.

Part of that processing in most steelmaking includes the combining of carbon and oxygen into the melt. This results in a gas. If the dissolved oxygen is not removed prior to pouring the steel, it will continue to react in steel as it cools toward solid. A deoxydizer is used to remove the gas.

The degree of deoxidation in a heat can affect some of the properties of the steel. To what degree a given heat of steel is deoxidized and how that affects machinability for your application are good questions to ask your supplier.

There are basically two types of steel—carbon and alloy. In general, steel is classified as carbon when its manganese, silicon and sulfur contents are within specified maximum percentages. With the exception of deoxidizers, there are no other intentionally added alloying elements. Alloy steel covers pretty much everything else including stainless and tool steels.

Variables In Machining The System

The construction of a machining process starts with the material selection. For steel, the basic criteria include its composition, microstructure, physical and mechanical properties, and geometry.

Once the material is selected, the rest of the process parameters fall into place. Cutting data, for example, is determined by the kind of material a shop is running. Speed, feed and depth of cut for various operations on the workpiece blank are calculated based on the material. Cut data is also the production rate target as it determines cycle time.

The cutter itself also impacts cycle time. The type of tools, material, geometry and coating are critical process variables. Another cutter selection factor is the condition of the machine tool on which it will run.

The machine tool's rigidity influences chatter and therefore the cutting data. A machine tool's accuracy and repeatability directly impact tolerance specifications. The amount of power available on the machine is also directly related to production performance.

Finally the cutting fluid is a machining variable that must be part of the systemized approach to the machining process. Shops must look at the type of coolant needed for an application, its flow, concentration, temperature and viscosity.

The entire machining process works together as a system of variables. Changing one has positive or negative impact on others. Systematically reducing or eliminating variability in material, cutting tool, machine tool and coolant is key to process stability.

Stepping Up To The Bar

Most of the engineering characteristics for steel are well known and documented. SAE standards and AISI standards are easily accessible to designers for specifying the correct steel formula for a given application.

Steel mills such as Ispat Inland manufacture hot rolled steel to various formulations that conform to SAE and AISI engineering specifications. This material is then shipped to cold finishers around the country where it is processed further, generally cold drawn, into the bars that are eventually delivered to shops.

Increasingly, shops are demanding better and better consistency from one batch of bar to the next. As shops automate processing for parts with an eye toward unattended or lightly attended operations, eliminated variability in the material is critical to successfully running "lights-out" operations.

A key service provided by steel suppliers is dimensional accuracy of the bar. After the bar is cold drawn, it can be turned, ground or polished to a tight tolerance. As shops try to gain productivity by unattended operation, having the OD of one batch of bar identical in size to the next eliminates the need of intervention for adjustments to the workholding and rough turning operations and reduces size as a material variable.

What's New In Steel?

Much of the newest formulations in steel bar have been in the area of free-machining steel. While free-machining steel has been around as a good workable material for some time, there have recently been some advances and substitutions in the main free-machining ingredient, which historically has been lead.

Unleaded gas and unleaded paint are part of a campaign to get lead out of circulation. Likewise, unleaded steel is being used increasingly by screw machine shops as environmental expenses associated with disposal of chips with lead become more burdensome.

The benefits of adding lead to steel are many. In the cut, lead acts as a lubricant between the cutting tool and the workpiece, resulting in longer tool life. This lubricity reduces friction in the secondary shear zone as the chip slides over the face of the cutter. It also helps create more manageable chip configurations by helping the chip to curl and break rather than string out into birds' nests.

Lead has a melting point around 621º F—well below steel. As heat builds in the cut, the lead is able to leech out of the steel and lubricate the cutter/part interface. This reduces the heat generated in all three shear zones and leads to longer tool life and the possibility of increased speeds and feeds.

Steelmakers have worked hard to find a substitute for lead in free-machining steels. In the case of Ispat Inland, the company's brand of Incut steels uses bismuth as the substitute for lead. Machinability tests conducted comparing lead and bismuth additives are said to be equivalent.

The free-machining properties between bismuth and lead are similar because the two elements behave alike in the melt. Both are insoluble in steel and give lubrication in the cut. Bismuth actually distributes better in the melt than lead. It has a slightly lower melting temperature, 520º F. Bismuth has the environmental advantage over lead of not being toxic. Use of bismuth as a lead substitute in steel is proprietary to Ispat Inland.

According to Ispat Inland, leaded steel is still the overwhelming choice for designers specifying material to be manufactured. However, designers are generally not faced with the disposal restrictions that machine shops must include in the quote.

Because manufacturability is a key factor in part design, shops often have the opportunity to influence equivalent material substitutions. Understanding how materials such as steel are made and modified to different performance criteria can help shops make more informed recommendations about machinability that can possibly save money for both parties.

What It Means To You

Getting a job to run well is a combination of art and science, experience and knowledge. Achieving efficient and profitable machinability of a job is a combination of many interlocked factors.

Today's competitive manufacturing environment dictates that shops that are going to flourish need to understand each machining variable and its impact on the process. The bottom line is that using good raw material up front is probably the most critical selection that a shop makes in setting up a job.

 

Machinability

These charts show various elements that act as machinability enhancers and detractors to base steel. Elements that are added for deoxidation and common strengtheners have a negative effect on machinability. The second column shows the percentage machinability increases when various elements and combinations are added to the base steel.

Factors That Influence Machinability

A. Base Steel Machinability
Hardness Microstructure
   C, Mn, Ni, Cr, Mo, S
   .02/.04%

B. Machinability Detractors
Deoxidizers
   Si, Al, Ti, Zr
Strengtheners
   Cd, V, B

C. Machinability Enhancers
Traditional
   S, Pb, Bi, Te, Se
Non-Traditional
   Ca

Machinability Improvement Additive

+50% to 80% : .26/.35 S + Pb or Bi + Te or Se
+40% to 60%:  .26/.35 S + Pb or Bi or Te or Se or Ca
+30% to 50%:  .26/.35 Sor .08/.13 S + Pb or Bi or Te or Se or Ca
+20% to 30%:  .08/.13 S or .15/.30 Pb or .08/.15 Bi or Ca
+10% to 15%:  .06/.08 S or Ca

Base Steel Machinability

In The Cut

The intersection of a cutting tool and workpiece material is a violent and very hot place. A cutting tool is designed to attack a material in a way that lifts a chip. The energy that is put into the cutter, feed and speed is output in the form of heat. The more energy that is put into the cut, the more heat is produced.

Chips are actually cut from the material slightly ahead of the tool. This area is referred to as the primary shear zone. It's where the pressure and forces are sufficient to create plastic deformation, which allows the material to yield to the cutter.

Also in the cut, a secondary shear zone forms along the face of the cutting tool. This is where the chip actually slides up the face of the cutter. The friction in this area of the cut can raise temperatures to 1,2000 C.

Finally, there is a third shear zone that is located under and slightly behind the leading edge of the cutter. This zone is formed by springback material at the bottom of the cut that was depressed as the primary shear zone yielded.

Because steel is produced as a liquid and because there are numerous elements suspended in that fluid, how they are distributed in the solid material you're cutting affects how smoothly and consistently this chip formation process occurs. Think, for example, if the cutter is merrily traversing a workpiece and hits an inclusion of very hard carbide, the yield of the material will change dramatically. It could possibly break the edge off the tool. For steelmakers, achieving heat-to-heat consistency, which reduces "surprises" in the cut, is the ongoing technological challenge.