The effect of aging on material properties determines the optimal machining time.

Simon N. Lekakh and Von L. Richards, Missouri University of Science and Technology, Rolla, Missouri

(Click here to see the story as it appears in January's Modern Casting.)

There are various ways to improve casting machinability. Natural aging’s effect on cast iron

machinability differs based on its alloying elements, which are nitride-forming (titanium), carbide-

forming (chromium) and nitrogen mobility modifiers (manganese). Relationships between cast iron

aging and casting machinability have been verified in multiple laboratory and industrial tests, with

respect to parameters such as cutting force, tool wear, surface quality and dimensional accuracy. A

recent confirmation test verified the optimal aging time for a specific composition to improve gray iron

machinability.

Cast Iron Natural AgingUnderstanding how age strengthening affects machinability enables manufacturers to schedule the

optimal window for machining. Room-temperature aging phenomena has been documented for

different types of ferrous alloys, including cast irons and steels. In gray cast iron, tensile strength

increased by 5%-15% after 5-30 days of room-temperature aging.

Aging studies in quenched iron-based alloys indicated a three-stage precipitation process. In some

cases, a dip in strength is observed during the start of the aging process. Elevated temperature aging

kinetics in the cast iron revealed typical age strengthening curves obtained at different temperatures

(Fig. 1). An Arrhenius plot was constructed using the rate constants versus the reciprocal of the

absolute temperature (Fig. 2).

Effect of Alloying ElementsWhile elevated temperature aging is less dependent on alloy composition, cast iron chemistry strongly

affects room-temperature aging kinetics. From a practical perspective, the effect of variations in

manganese and sulfur on cast iron’s aging rate is important. In a study of cast iron with 0.8%-0.83%

manganese, aging was completed at 25 days, while this process needed only 15 days for cast iron with

0.51% manganese at similar 0.04%-0.06% sulfur levels.

To study the effect of alloying elements, aging kinetics of cast irons from six heats with variations in

manganese, nitrogen and sulphur were evaluated. Strength change curves typically had a

prestrengthening peak and a “relaxation valley” before achieving a full age strengthening.

Alloying with manganese affected both the time to prestrengthening and the full strengthening peak.

Cast iron from a heat with 0.53% manganese had the highest reaction rate. Iron with lower manganese

and especially higher manganese contents each had a longer aging reaction time.

Effect of Carbide/Nitride Forming ElementsNatural age strengthening of cast iron occurs in Fe-BCC (ferrite) by iron nitride precipitation. Carbide

forming elements such as chromium promote the decrease of free ferrite in cast iron and reduce the

total possible strengthening effect. An as-cast machinability test article produced from cast iron with

0.2% chromium did not show an improvement in machinability after aging.

Nitride forming elements such as titanium, aluminum and boron can fully suppress iron nitride

precipitate strengthening. Nitrogen, available at solidification to form metastable solid solution in

ferrite, affects the age strengthening of cast iron. Low soluble nitrogen left after titanium nitride

formation does not allow for the production of detectible age strengthening of cast iron. The

temperature range of super-saturation of ferrite lies from room temperature to 572F (300C), and

beyond this range the possibility of aging is limited according to thermodynamics.

Machinability of Aged Cast IronsCutting Tool Forces: The machinability test articles recommended by the American Foundry Society

were used for facing cuts on a computer numeric control (CNC) lathe. These test articles were

produced in a laboratory using nobake molds and in industrial metalcasting facilities using green sand

molds. Pearlite/ferrite cast irons with variations in carbon equivalent from 3.9% to 4.3% were tested in

as-cast condition and after 25 days of natural aging. In as-cast or in unaged condition, the cutting

forces increased with increasing hardness in irons having less carbon equivalent, which is typical and

expected. At the same time, a reverse type of dependency appeared in which the cutting force

decreased when the increasing hardness was due only to natural aging in each iron.

This unusual behavior could be explained by the energy requirement for chip formation. In unaged

cast iron, soft ferrite absorbs energy for significant plastic deformation. This effect results in edge

build-up on the tool tip, which also could promote increasing cutting force by enlarging the

deformation region (similar to tool wear). In contrast, when iron aging occurs as a result of Fe4N

precipitation in ferrite, it increases the iron’s strength and hardness and allows for chip formation with

a smaller amount of plastic deformation, which could decrease the cutting force.

Similar results were achieved in other cast irons having ferrite in metal matrix and different graphite

shapes. For example, aging decreased cutting forces after aging ductile iron with spherical graphite

and significant free ferrite.

However, aging does not always improve cast iron machinability. For example, aging cast iron

containing carbide forming elements produced a completely opposite effect on casting machinability.

There was a visible and statistically significant increase of the average normal cutting forces for aged

samples versus unaged samples. The ratio of passive to normal cutting forces is used as an indicator

of tool wear, because as a tool loses sharpness, it has an increasing passive reaction force. This ratio

increased more significantly when cutting aged gray iron with carbide-promoting element content. The

microstructure in this case was pearlitic with some steadite and free carbide but no free ferrite.

To verify the effect of microstructure on cast iron machinability, castings from the same heat were

tested further after ferritizing/resolutionizing heat treatment. This treatment transformed pearlite to

ferrite and produced a resolutionizing effect, which allowed repeating the natural aging. The effect

observed was opposite to the previously discussed test of cast iron with pearlite matrix and steadite

phase, in that aging of ferritized/resolutionized gray iron improved machinability. The cutting forces

were decreased at all cutting speeds studied (Fig. 3). It can be concluded from these tests that all gray

iron showing improved machinability in the aged condition contained some amount of free ferrite,

while gray iron showing increased cutting forces after aging had no free ferrite but was entirely

pearlitic with cementite/steadite phases.

This differing behavior of aged cast irons depending upon metal matrix relates to the energy of chip

formation. Although gray cast iron is a brittle material in tension, chips can experience significant

plastic deformation because the stress state during machining is dominated by compression and

shear. If chip formation is assumed to be a plastic strain to fracture event, then changes in fracture

toughness would logically affect machining behavior. Fracture work during tensile testing was

estimated from the stress-displacement curve. In the pearlitic iron, the work of fracture and cutting

forces increased after aging.

On the contrary, iron ferritized by heat treatment showed decreased work of fracture and cutting

forces due to aging.

Tool Wear and Industrial Machining MeasurementsTool wear is lower when machining gray cast iron aged at room temperature because aged iron

requires less work input from the machining center to form and break off chips. The decrease in

required work has been demonstrated by tool force measurements and by testing amperage drawn

while machining unaged and aged iron. The least power was required to machine castings aged for 3-6

days versus iron aged for 1, 9 and 20 days. At that optimal aging time, machined castings had better

surface quality (less roughness), but all aged iron had better surface finish than unaged iron.

Other tests were performed with industrial face machining of brake discs for a passenger car.

Excessive tool wear produced changing tool geometry and increased cutting forces, which promoted

elastic deformation of casting with increasing tilt and destroying required tolerance on

perpendicularity (“tilt”). Tilt data from the machining of industrial castings were compared in two

ways. The machining of the 50 unaged castings required two tool position changes. Tool position

changes were not required during machining of aged castings after 50 or 200 castings, indicating more

consistent dimensions and reduced downtime for tool position corrections. Figure 5 gives a comparison

of measured tool wear for different operations. Aging decreased tool wear significantly in most of the

operations.

Industrial Recommendation for Improving Cast Iron Machinability by Aging

Three possible scenarios exist for changes in machinability of gray iron during natural aging (Table 1).

First scenario:  Aging does not occur and therefore, has no influence on machinability. Lack of aging

effects in the iron can be caused by elevated nitride forming elements (particularly titanium) relative

to nitrogen. Additions of nitrogen to iron are possible and can enhance aging. Thermodynamic data

can be applied to determine if there is enough “free nitrogen” to age a cast iron. A simplified criterion

might be:  If %N < (0.15-0.20) %Ti, aging will not occur.

Second scenario:  If cast iron exhibits aging, this phenomenon can be used for improving casting

machinability. Aging is accompanied by decreasing cutting forces and tool wear. These irons have

enough free nitrogen to promote age strengthening.

Decreased cutting forces and increased mechanical properties were proven in laboratory castings

having different carbon equivalents. These irons had some free ferrite and no free cementite or

steadite. Optimal aging time depends upon particular “free manganese” content and could be

evaluated. Decreasing aging time for improving machinability could be done by warm (slightly

elevated temperature) aging.

Third scenario:  Gray iron has elevated concentrations of carbide forming elements such as chromium

in addition to a large percentage of phosphorus. These combinations of chemistry with a particular

cooling rate could promote steadite/cementite formation in fully pearlitic matrix. If this iron has

negligible free ferrite, aging will increase cutting forces in this iron. Effective inoculation and chemistry

control will affect casting machinability interaction with aging in these cast irons. However, in this

scenario, “fresh” castings might prove more machinable.

Confirmation TestFive AFS 5J, 10-in. diameter test articles were poured into nobake molds from one 200-lb. induction

furnace heat. The cast iron chemistry is shown in Table 2. Microstructure was mostly pearlitic with

approximately 5%-10% ferrite. Measured hardness in the middle section of the test article was 200-

210HB in the as-cast condition (unaged). The as-cast surface layer (1/8 in.) was removed in

preliminary machining to avoid the effects of cast surface structure, mold-metal interaction and

geometry variance on test results. Test articles were face CNC machined at day 0, day 5, day 9, day 15

and day 22 with measurement of cutting forces.

Eight cuts (30 min. total machining time) were performed from each disc, using a new tool insert each

time. The thickness of the test article produced eight duplicate cuts and each test was repeated twice.

The test results are shown in Fig. 7.

These test results were compared to the predictions according to suggested methodology.

Step 1—Evaluation of the possible age strengthening:  Nfree = N-0.20Ti = 0.01-0. 2*0.008 = 0.0084

wt.% or 84 ppm; total %N and %Ti leads one to expect approximately 0.14 wt. % Fe4N. Age

strengthening will occur.

Step 2—Control microstructure:  In a matrix without free carbide/steadite having a small amount of

free ferrite around flake graphite, age strengthening can improve casting machinability according to

the second scenario (Table 1).

Step 3—Aging time:  Full aging time is 15-17 days and prestrengthening time is 7-9 days. The tool

force dropped significantly during the first five days and was also low at 15 days, roughly

corresponding to the expected times for room-temperature age strengthening.

The predictions based on the previous studies were confirmed. A significant decrease in cutting force

and standard variation were observed after 9-15 days of natural aging, which is between predicted

prestrengthening and full aging time. Regarding other machinability parameters, tool wear not only

depends on the average value of cutting force but also the stability of cutting process, and tool wear

continued to decrease up to the full aging time.

These rules can assist in determining the optimal machinability window for aged cast iron:

Estimate free nitrogen based on total nitrogen and concentration of titanium as %N >0.2 %Ti,

but not high enough to form gas porosity in order to have age strengthening.

Check microstructure concerning ferrite/pearlite content without steadite/carbides. If no free

ferrite is present, particularly with all pearlite and some carbide or steadite, the machinability

might be better with fresh castings given a composition that will age strengthen. If free ferrite

is present, age strengthening will provide a corresponding improvement in machinability.

Estimate room-temperature aging time based on free manganese left after sulfide formation.

Aging acceleration is possible with a low temperature bake.

This article is based on a technical paper (12-026), “Aging and Machinability Interactions in Cast Iron,”

presented at the American Foundry Society’s 116th Metalcasting Congress in 2012.

Machining Cast Iron ComponentsOne of the main advantages provided by cast iron components is their ability to incorporate many

design features that often must be “added-on” in other manufacturing methods. As a result, a cost

savings “down the line” in manufacturing processes, such as machining, can be achieved if the proper

design and manufacturing procedures are understood and followed during the component’s conception

and initial development.

Opportunities for cost savings also exist in the subsequent processing of components themselves. In

machining, these cost reduction opportunities are available with an understanding of the material’s

metallurgy and its place in the overall machining system.

The three primary components of a machining system are:

machine tool, whose stiffness affects the duration of machining required;

machining parameters, which include cutting tools (materials, coatings and geometry), speeds,

feed rates, depths of cut and cutting fluids (types and concentration, focusing on reducing the

total cost per part and an increase in production rate). Several factors, such as tool cost, tool

life, and practical limits of cutting speeds and feeds play an important role in the selection of

tool material as well as reliability and predictability of performance. In addition, the most

satisfactory tool usually will be the one to perform the machining at the minimum cost;

workpiece parameters, which include microstructure characteristics, material hardness,

chemical composition and mechanical properties.

 

These components, and their interaction with the iron casting’s material, determine a component’s

machinability.

Cast Iron MachinabilitySince it is not an absolute material property, machinability means different things to different people.

Machinability is the relative ease or difficulty of removing metal in transforming a raw material into a

finished product with the desired dimensional requirements at the best cost. The four major aspects of

machinability are:

tool life;

surface finish;

power or cutting force required;

the form of chips produced during a machining operation.

One of the most exciting challenges to metallurgists over the last century has been to increase the

strength of materials with a minimum increase in cost. This has been achieved by cold working,

alloying, the use of phase transformations, and the refinement of grain size and microstructure. The

result is that machining has become more dependent upon material microstructures.

Cast iron, which is an alloy with 1.8-4.5% carbon (C) content, is one of the most free-machining ferrous

materials. Consistent microstructure, however, is the key to optimum cast iron machinability because

cast iron can show a wide range of machining behavior depending upon composition and

microstructure.

The key to differentiating between types of cast iron is the size and shape of the graphite particles.

The microstructure features of cast iron, such as particle composition and dispersion, particle

population density, and aspect ratios, significantly affect machinability. Cast iron’s mechanical

properties are enhanced with additives such as silicon (Si), magnesium (Mg), chromium, molybdenum

(Mb) and copper (Cu).

Following is a closer look at cast irons and their machinability.

Gray Iron—Gray cast iron is characterized by randomly oriented graphite flakes, which develop

brittleness and poor ductility in the material. It is used widely in the automotive industry for engine

blocks, brake disks, brake drums and housings. Gray iron has excellent machinability with superior

wear resistance characteristics and damping capability.

Ductile Iron—Ductile (or nodular) iron is popular for wheel parts, crankshafts and camshafts. In ductile

iron, the graphite particles, due to the injection of a small amount of Mg in the melt, exist in spherical

shapes that provide superior ductility and high strength and toughness. In general, ductile iron (such

as grade GGG40) is easy to machine but produces built-up edges on the cutting tool due to its higher

ferrite content. Machining certain grades of ductile iron (such as GGG60) will result in rapid insert wear

due to pearlite content.

Compacted Graphite Iron (CGI)—In CGI, graphite particles are randomly oriented and elongated similar

to gray iron, but they are thicker, shorter and have shorter edges. The interconnected compacted

graphite provides slightly higher thermal conductivity and more damping capacity. With the evolved

process control technologies, CGI use is growing in the automotive and heavy truck industries with

components that are prone to have simultaneous mechanical and thermal loading.

Austempered Ductile Iron (ADI)—Developed by adding alloying elements such as Cu, Mb  and nickel

(Ni) to ductile iron and then performing a special heat treatment (austempering), ADI has increased

ductile iron’s ductility to 12-22%. Thus, ADI is a stronger and tougher material than conventional

ductile iron. The required mechanical properties of ADI are achieved by controlling the heat treatment

parameters. In machining, ADI, which contains bainite, is more prone to work hardening and built-up

edge than straight ductile iron.

High Alloy/White Iron—White iron is produced via rapid casting solidification and provides high

compressive strength and excellent wear resistance. White iron contains large quantities of hard

carbides that are difficult to machine and are responsible for high tool-wear rates. In addition, high-

alloy irons (including high alloy white, gray and ductile iron) have extreme abrasive wear, heat and

corrosion resistance, low thermal expansion and non-magnetic properties.

Since every type and grade of cast iron is unique, machining cast iron components depends upon the

material’s graphite structure, microstructure of the metal matrix, temperature-to-time history of the

castings and the distribution of C that remains in the metal matrix.

With all of these different variables, machining guidelines are dependent upon the make-up of the

material. General “rules of thumb” to follow in regard to cast iron machinability include:

a reduced C content results in a coarser-graphite structure and lower machinability;

higher Si content in the iron results in a lower tendency to built-up-edge and better

machinability;

increased pearlitic graphite content makes pearlite or white grades harder and stronger and

more demanding on the cutting tool;

a high content of fine-grained pearlites is troublesome for machining as the cutting tool needs

to work harder and under hotter conditions to cut through the hardest particles.

Tool RequirementsThe ideal cutting tool material for machining cast iron should have high strength and hardness in

addition to high fracture toughness. Although this combination of properties is impossible to achieve in

practice since high strength and low fracture toughness are synonymous, the selection of the proper

cutting tool is important for machining various types of cast irons.

When machining cast irons, the most common problems are flank wear, crater wear, notch wear and

built-up-edge caused by abrasion, adhesion and diffusion. Thus, the basic requirements for the cutting

tool material for machining cast iron are:

resistance to adhesive and abrasive wear caused by the variable microstructures;

sufficient toughness for the material to be machined;

the capability to machine at high speeds and feed rates.

The best performing carbide inserts used to machine cast irons are coated with alumina, usually with

one or more layers of titanium carbide (TiC) and titanium nitride (TiN). The alumina provides the high

hardness needed for abrasive resistance and excellent chemical stability.

Gray iron is prone to built-up-edge at low cutting speeds, and the tools also are susceptible to abrasive

(flank) wear. It is recommended to machine at higher cutting speeds and, at the same time, flush the

workpiece with abundant coolant. For high-production jobs, a chemical vapor deposition-coated tool for

wear resistance at high surface speed must be chosen. Silicon nitride (SiN) has been proved to be one

of the preferred tool materials for high-speed machining of gray iron. Its relatively high fracture

toughness and high hardness at high cutting temperatures, in addition to its insensitivity to thermal

shocks, accounts for its excellent performance during gray iron machining.

For machining ductile iron, SiN tools perform poorly, however the coated tools show superior

performance. In the past, tools used for machining ductile cast iron consisted of tungsten carbide (6%

cobalt) substrate with multiple layers of TiCN/Al2O3/TiN coatings. The newly developed tool combines

a 6% cobalt substrate with a medium temperature TiCN/Al2O3/TiN coating. At higher speeds, the TiCN

coatings soften and the effect of the Al2O3 coating becomes predominant. The results have indicated

that deposition of the new multi-layered coated carbide results in a 40% reduction in flank wear at

speeds of 200 m/min when compared to conventional coated carbide. Also, the new grades achieve a

25% increase in tool life at 300 m/min.

Since ADI is more prone to work hardening and built-up-edge than ductile iron, the TiN-coated tools are

the best choice for machining because TiN reduces friction, work hardening and built-up-edge. When

deep hole making is part of the design of ADI parts, lower manganese and Mb additions must be

specified for the cast component to reduce the amount of retained austenite, which will work harden

more than bainite. Higher amounts of Cu and Ni then must be specified to obtain the required degree

of hardenability.

Dry MachiningWith the continued development of advanced tool coatings, high-speed dry machining of cast iron has

become possible. The key is to balance between advanced machining strategies, special tooling and

machine tool specifications. It has been observed that a combination of high feed rates and high

spindle speeds (in place of increasing the forces) reduces the thrust force against the workpiece.

One such application using HSK-63A tooling with high pressure, through spindle air running up to

14,000 revolutions/min and feed rates up to 1575 in./min showed that thrust forces substantially

decreased (75% in some instances). At higher cutting speeds and feeds, the intense heat generated in

the vicinity of the cutting edge of the tool elevates the metal’s temperature in the affected zone up to

1112-1292F (600-700C) and plasticizes the iron. Most of the heat is removed later with the chips due

to high feed rates, which makes the workpiece thermally stable and dimensionally accurate.

Heat dissipation without coolant requires high-performance tool coatings, heat-resistant tool materials

and high pressure through spindle air. For high-speed dry machining of cast iron, the tools must have:

high hardness at high operating temperatures to resist abrasive wear;

high structural strength to resist cutting forces at high chip loads and high operating

temperatures;

high fracture toughness, resistance to thermal shocks and chemical stability with respect to

the workpiece.

As a result, dry machining requires either coated tools or ceramic/cubic boron nitride cutting tool

materials to withstand the intense heat generated by the process. The coatings with a low friction

coefficient and low thermal conductivity work best at isolating a tool from heat and TiAlN-based

coatings are recommended for dry machining of cast ferrous materials, including cast irons.   

This article, written by Anil K. Srivastava and Michael E. Finn, originally ran in Engineered Casting

Solutions.

Nodular Cast Iron (NCI)

-material classification: K3.xNodular cast iron has spherically shaped graphite and the main characteristics are good stiffness (Young’s modulus); good impact strength = tough material, not brittle; good tensile strength, higher heat in cutting process.

Nodular cast iron has a strong tendency to form built-up-edge. This tendency is stronger for the softer NCI materials with higher ferritic contents. When machining components with high ferritic contents and with interrupted cuts, adhesion wear is often the dominating wear mechanism. This can cause problems with flaking of the coating. The adhesion problem is less pronounced with harder NCI materials that have a higher perlitic content. Here, abrasive wear and/or plastic deformation are more likely to occur.

Grades to use are 3210 or 3215. The preferred geometries are -KF, -KM, -KR and -KRR. When finishing, consider wiper geometry -WMX.

ASM Specialty Handbook: Cast Irons

Metal Cutting Theory and Practice By David A. Stephenson, John S. Agapiou

Materials Handbook: A Concise Desktop Reference By François Cardarelli