Workpiece materials

Workpiece material groups

The metal cutting industry produces an extremely wide variety of components machined from many different materials. Each material has its own unique characteristics that are influenced by alloying elements, heat treatment, hardness, etc. These, in turn, influence the choice of cutting tool geometry, grade and cutting data. To make this choice easier, workpiece materials are divided into six major groups, in accordance with the ISO-standard, and each group has unique properties regarding machinability:

ISO P – Steel is the largest material group, ranging from unalloyed to high-alloyed material and including steel castings and ferritic and martensitic stainless steels. Machinability is usually good, but differs a lot depending on material hardness, carbon content, etc.

ISO M – Stainless steels are materials alloyed with a minimum of 12% chromium. Other alloys may include nickel and molybdenum. Different conditions, such as ferritic, martensitic, austenitic and austenitic-ferritic (duplex), create a large range of materials. A commonality among all these materials is that the cutting edges are exposed to a great deal of heat, notch wear and built-up edge.

ISO K – Cast iron is, contrary to steel, a short-chipping type of material. Grey cast irons (GCI) and malleable cast irons (MCI) are quite easy to machine, while nodular cast irons (NCI), compact cast irons (CGI) and austempered cast irons (ADI) are more difficult. All cast irons contain SiC, which is very abrasive to the cutting edge.

ISO N – Non-ferrous metals are softer metals, such as aluminium, copper, brass etc. Aluminium with a Si-content of 13% is very abrasive. Generally, high cutting speeds and long tool life can be expected for inserts with sharp edges.

ISO S – Heat resistant super alloys include a great number of high-alloyed iron, nickel, cobalt and titanium based materials. They are sticky, create built-up edge, harden during working (work hardening), and generate heat. They are very similar to the ISO M materials but are much more difficult to cut, and reduce the tool life of the insert edges.

ISO H – This group includes steels with a hardness between 45-65 HRc, and also chilled cast iron around 400-600 HB. The hardness makes them difficult to machine. The materials generate heat during cutting and are very abrasive for the cutting edge.

O (Other): Non-ISO. Thermoplastics, thermosets, GFRP (Glass Fibre Reiforced Polymer/Plastic), CFRP (Carbon Fibre Reinforced Plastic), carbon fibre composites, aramid fibre reinforced plastic, hard rubber, graphite (technical). Various industries are now using composites to a greater extent, especially in the aerospace industry.

Workpiece material classification using MC codes

Merely dividing materials into six different groups does not provide enough information to select the correct cutting tool geometry, grade and cutting data. The material groups thus need to be broken down further into sub-groups. Sandvik Coromant has used the CMC-code system (Coromant Material Classification) to identify and describe materials from a variety of suppliers, standards and markets. With the CMC-system, materials are classified according to machinability, and Sandvik Coromant also provides suitable tooling and machining data recommendations.

In order to give even more specific recommendations to assist in improving productivity, Sandvik Coromant has generated a new material classification. It has a more detailed structure, includes more sub-groups, and has separate information on type, carbon content, manufacturing process, heat treatment, hardness, etc.

MC code structure

The structure is set up so that the MC code can represent a variety of workpiece material properties and characteristics using a combination of letters and numbers.

Example 1

The code P1.2.Z.AN
P is the ISO-code for steel
1 is the material group unalloyed steel
2 is the material sub-group for carbon content >0.25% ≤ 0.55 % C
Z is the manufacturing process: forged/rolled/cold drawn
AN is the heat treatment, annealed, supplied with hardness values

Example 2

N1.3.C.AG
N is the ISO-code for non-ferrous metals
1 is the material group aluminium
3 is the sub-group aluminium with Si content 1-13%
C is the manufacturing process: casting
AG is for the heat treatment: ageing

By describing not only the material composition, but also the manufacturing process and heat treatment, which doubtless influences the mechanical properties, a more exact description is available. This can then be used to generate improved cutting data recommendations.

Machinability definition

There are usually three main factors that must be identified in order to determine a material’s machinability, that is, its ability to be machined.

1. Classification of the workpiece material from a metallurgical/mechanical point of view.
2. The cutting edge geometry to be used, on the micro and macro level.
3. The cutting tool material (grade) with its proper constituents, e.g. coated cemented carbide, ceramic, CBN, or PCD, etc.

The selections above will have the greatest influence on the machinability of the material at hand. Other factors involved include: cutting data, cutting forces, heat treatment of the material, surface skin, metallurgical inclusions, tool holding, and general machining conditions, etc.

Machinability has no direct definition, unlike grades or numbers. In a broad sense it includes the ability of the workpiece material to be machined, the wear it creates on the cutting edge and the chip formation that can be obtained. In these respects, a low alloyed carbon steel is easier to cut compared to the more demanding austenitic stainless steels. The low alloyed steel is considered to have a better machinability compared to the stainless steel. The concept “good machinability”, usually means undisturbed cutting action and a fair tool life. Most evaluations of machinability for a certain material are made using practical tests and the results are compared to the results of another type of material under approximately the same conditions. In these tests, other factors, such as micro-structure, smearing tendency, machine tool, stability, noise, tool-life, etc. are taken into consideration.

ISO P steel

• MC codes for steels
• Unalloyed steel – P 1.1-1.5
• Low alloyed steel – P 2.1-2.6
• High alloyed steel – P 3.0-3.2

What is ISO P steel?

• Steel is the largest workpiece material group in the metal cutting area
• Steels can be non-hardened, or hardened and tempered with a common hardness up to 400 HB. Steel with a hardness above approx. 48 HRC and up to 62-65 HRC belong to ISO H
• Steel is an alloy with iron as the major component (Fe-based)
• Unalloyed steels have a carbon content lower than 0.8%, and are composed solely of iron (Fe), with no other alloying elements
• Alloyed steels have a carbon content lower than 1.7 % and contain alloying elements such as Ni, Cr, Mo, V and W
• Low alloyed steels contain alloying elements less that 5%
• High alloyed steels contain more than 5% alloying elements

Machinability in general

• The machinability of steel differs, depending on alloying elements, heat treatment and manufacturing process (forged, rolled, cast, etc.)
• In general, chip control is relatively easy and smooth
• Low carbon steels produce longer chips that are sticky and require sharp cutting edges
• Specific cutting force k_c1: 1400-3100 N/mm
• Cutting forces, and thus the power required to machine them, remain within a limited range

Alloying elements

C influences hardness (higher content increases abrasive wear). Low carbon content <0.2%, increases adhesive wear, which will lead to built-up edge and bad chip breaking.

Cr, Mo, W, V, Ti, Nb (carbide formers) – increase abrasive wear.

O has a large influence on machinability; it forms non-metallic, oxidic and abrasive inclusions.

Al, Ti, V, Nb are used as fine-grained treatment of steel. They make the steel tougher and more difficult to machine.

P, C, N in ferrite, lowers ductility, which increases adhesive wear.

Positive effect

Pb in free machining steel (with low melting point) reduces friction between chip and insert, lowers wear and improves chip breaking.

Ca, Mn (+S) form soft lubricating sulphides. High S-content improves machinability and chip breaking.

Sulphur (S) has a beneficial effect on machinability. Small differences, such as those between 0.001% and 0.003% can have substantial effects on machinability. This effect is used in free machining steels. Sulphur content of around 0.25% is typical. Sulphur forms soft manganese sulfide (MnS) inclusions that will form a lubricating layer between the chip and the cutting edge. MnS will also improve chip breakage. Lead (Pb) has a similar effect and is often used in combination with S in free machining steels at levels of around 0.25%.

Both positive and negative

Si, Al, Ca form oxide inclusions that increase wear. Inclusions in steel have an important influence on the machinability, even though they represent a very small percentages of the total composition. This influence can be both negative and positive. For example, aluminium (Al) is used to deoxidize the iron melt. However, aluminium forms hard abrasive alumina (Al2O3), which has a detrimental effect on machinability (compare the alumina-coating on an insert). This negative effect can, however, be counteracted by adding Calcium (Ca), which will form a soft shell around abrasive particles.

• Cast steel has a rough surface structure, which can include sand and slag, and places a high demand on the toughness of the cutting edge
• Rolled steel exhibits a fairly large grain size, which makes the structure uneven, causing variations in the cutting forces
• Forged steel has a smaller grain size and is more uniform in structure, which generates fewer problems when cut

MC codes for steels

Steels are, from a machinability point of view, classified into unalloyed, low alloyed, high alloyed and sintered steels.

Unalloyed steel – P 1.1-1.5

Definition

In unalloyed steels, the carbon content is usually only 0.8%, while alloyed steels have additional alloying elements. The hardness varies from 90 to 350HB. A higher carbon content (>0.2%) enables hardening of the material.

Common components

Predominant uses include: constructional steel, structural steel, deep drawn and stamped products, pressure vessel steel, and a variety of cast steels. General uses include: axles, shafts, tubes, forgings and welded constructions (C<0.25%).

Machinability

Difficulties in chip breaking and smearing tendencies (built-up edge) require special attention in low carbon steels (< 0.25%). High cutting speeds and sharp edges and/ or geometries, with a positive rake face and thin coated grades, will decrease the smearing tendencies. In turning, it is recommended that the depth of cut remains close to or bigger than the nose radius to improve chip breaking. In general, machinability is very good for hardened steels. However, they tend to generate relatively large flank wear on the cutting edges.

Low alloyed steel – P 2.1-2.6

Definition

Low alloyed steels are the most common materials currently available in metal cutting. The group includes both soft and hardened materials (up to 50 HRc).

Common components

Mo and Cr-alloyed pressure vessel steels are used for higher temperatures. General uses include: axles, shafts, structural steels, tubes and forgings. Examples of components for the automotive industry are: con rods, cam shafts, cv-joints, wheel hubs, steering pinions.

Machinability

Machinability of low alloyed steels depends on the alloy content and heat treatment (hardness). For all materials in the group, the most common wear mechanisms are crater and flank wear.

Hardened materials produce greater heat in the cutting zone and can result in plastic deformation of the cutting edge.

High alloyed steel – P 3.0-3.2

Definition

High alloyed steels include carbon steels with a total alloy content of over 5%. This group includes both soft and hardened materials (up to 50 HRc).

Common components

Typical uses of these steels include: machine tool parts, dies, hydraulic components, cylinders and cutting tools (HSS).

Machinability

In general, machinability decreases at higher alloy contents and hardness. For example, at 12-15% alloying elements and hardness up to 450 HB, the cutting edge needs good heat resistance to withstand plastic deformation.

ISO M stainless steel

• MC codes for stainless steel
• Ferritic and martensitic stainless steel – P5.0-5.1
• Austenitic and super-austenitic stainless steel – M1.0-2.0
• Duplex stainless steel – M 3.41-3.42

What is ISO M stainless steel?

• An alloy with the element iron (Fe) as the major constituent
• Has a chrome content which is higher than 12%
• Has a generally low carbon content (C ≤ 0.05 %)
• Various additions of Nickel (Ni), Chromium (Cr), Molybdenum (Mo), Niobium (Nb) and Titanium (Ti) supply different characteristics, such as resistance towards corrosion and strength at high temperatures
• Chrome combines with oxygen (O) to create a passivating layer of Cr2O3 on the surface of the steel, which provides a non-corrosive property to the material

Machinability in general

The machinability of stainless steels differs depending on alloying elements, heat treatment and manufacturing processes (forged, cast, etc.) In general, machinability decreases with a higher alloy content, but free-machining or machinability improved materials are available in all groups of stainless steels.

• Long-chipping material
• Chip control is fair in ferritic/martensitic materials, becoming more complex in the austenitic and duplex types
• Specific cutting force: 1800-2850 N/mm
• Machining creates high cutting forces, built-up edge, heat and work-hardened surfaces
• Higher nitrogen (N) content austenitic structure increases strength and provides some resistance against corrosion, but lowers machinability, while deformation hardening increases
• Additions of Sulphur (S) are used to improve machinability
• High C-content (>0.2%) provides relatively large flank wear
• Mo and N decrease machinability. However, they provide resistance to acid attacks and contribute to high temperature strength
• SANMAC (Sandvik trade name) is a material in which machinability is improved by optimizing the volume share of sulphides and oxides without sacrificing corrosion resistance

MC codes for stainless steel

Identification of workpiece material group

The microstructure that a stainless steel attains depends primarily on its chemical composition, in which the main alloy components Chromium (Cr), and Nickel (Ni) are most important (see diagram). In reality, the variation can be wide due to the influence of other alloy components that strive to stabilize either the austenite or the ferrite. The structure can also be modified by heat treatment or, in some cases, by cold working. Precipitation hardening ferritic or austenitic stainless steel have an increased tensile strength.

Austenitic steels

Austenitic-ferritic (duplex) steels

Ferritic chromium steels

Martensitic chromium steels

Ferritic and martensitic stainless steel – P5.0-5.1

Definition

From a machinability point of view, ferritic and martensitic stainless steels are classified as ISO P. Normal Cr content is 12-18%. Only small additions of other alloying elements are present.

Martensitic stainless steels have relatively high carbon content, which make them hardenable. Ferritic steels have magnetic properties. Weldability is low for both ferritic and martensitic and medium to low resistance against corrosion, which increases with a larger Cr content.

Common components

Often used in applications that place a limited demand on corrosion resistance. The ferritic material is relatively low cost due to the limited Ni content. Examples of applications are: shafts for pumps, turbines steam and water turbines, nuts, bolts, hot water heaters, pulp and food processing industries due to lower requirements on corrosion resistance.

Martensitic steels can be hardened and are used for edges in cutlery steel, razor blades, surgical instruments, etc.

Machinability

In general, machinability is good and very similar to low alloyed steels. Therefore it is classified as an ISO P material. High carbon content (>0.2%) enables hardening of the material. Machining will create flank and crater wear with some built-up edge. ISO P grades and geometries work well.

Austenitic and super-austenitic stainless steel – M1.0-2.0

Definition

Austenitic steels are the primary group of stainless steels; the most common composition is 18% Cr and 8% Ni (e.g.18/8-steels, type 304). A steel with better resistance to corrosion is created by adding 2-3% molybdenum, which is often called “acid-proof steel” (type 316). The MC group also includes super-austenitic stainless steels with a Ni content over 20%. The austenitic precipitation hardening steels (PH) have an austenitic structure in the heat treated condition and a Cr content of >16% and a Ni content of >7%, with approx. 1% aluminium (Al). A typical precipitation hardened steel is 17/7 PH steel.

Common components

Used in components where good resistance against corrosion is required. Very good weldability and good properties at high temperatures. Applications include: chemical, pulp and food processing industries, and exhaust manifolds for aeroplanes. Good mechanical properties are improved by cold working.

Machinability

Work hardening produces hard surfaces and hard chips, which in turn lead to notch wear. It also creates adhesion and produces built-up edge (BUE). It has a relative machinability of 60%. The hardening condition can tear coating and substrate material from the edge, resulting in chipping and bad surface finish. Austenite produces tough, long, continuous chips, which are difficult to break. Adding S improves machinability, but results in lowered resistance to corrosion. Use sharp edges with a positive geometry. Cut under the work hardened layer. Keep cutting depth constant. Generates a lot of heat when machined.

Duplex stainless steel – M 3.41-3.42

Definition

By adding Ni to a ferritic stainless Cr-based steel, a mixed base structure/matrix will be formed, containing both ferrite and austenite. This is called a duplex stainless steel. Duplex materials have a high tensile strength and maintain a very high corrosion resistance. Designations such as super-duplex and hyper-duplex indicate higher content of alloying elements and even better corrosion resistance. A Cr content between 18 and 28%, and a Ni content between 4 and 7% are common in duplex steels and will produce a ferritic share of 25-80%. The ferrite and austenite phase are usually present at room temperature at 50-50% respectively.

Common components

Used in machines for the chemical, food, construction, medical, cellulose and papermaking industries and in processes that include acids or chlorine. Often used for equipment related to the off-shore oil and gas industry.

Machinability

Relative machinability is generally poor, 30%, due to high yield point and high tensile strength. Higher content of ferrite, above 60%, improves machinability. Machining produces strong chips, which can cause chip hammering and create high cutting forces. Generates a lot of heat during cutting, which can cause plastic deformation and severe crater wear.

Small entering angles are preferable to avoid notch wear and burr formation. Stabilty in tool clamping and workpiece fixing is essential.

ISO K cast iron

• MC codes for cast iron
• Malleable Cast Iron (MCI) K 1.1-1.2 and Grey Cast Iron (GCI) K 2.1-2.3
• Nodular Cast Iron (NCI) K 3.1-3.5
• Compacted Graphite Iron (CGI) K 4.1-4.2
• Austempered Ductile Iron (ADI) K 5.1- 5.3

What is ISO K cast iron?

There are 5 main types of cast iron:

• Grey Cast Iron (GCI)
• Malleable Cast Iron (MCI)
• Nodular Cast Iron (NCI)
• Compacted Graphite Iron (CGI)
• Austempered Ductile Iron (ADI)

Cast iron is a Fe-C composition with a relatively high percentage of Si (1-3%). Carbon content is over 2%, which is the maximum solubility of C in the austenitic phase. Cr (Chromium), Mo (Molybdenum) and V (Vanadium) form carbides, which increase strength and hardness, but lower machinability.

Machinability in general

• Short-chipping material with good chip control in most conditions. Specific cutting force: 790 – 1350 N/mm
• Machining at higher speeds, especially in cast irons with sand inclusions, creates abrasive wear
• NCI, CGI and ADI require extra attention due to the different mechanical properties and the presence of graphite in the matrix, compared to normal GCI
• Cast irons are often machined with negative types of inserts as these provide strong edges and safe applications
• The carbide substrates should be hard and the coatings should be of thick aluminium oxide types for good abrasive wear resistance
• Cast irons are traditionally machined dry, but can also be used in wet conditions, mainly to keep the contamination of dust from carbon and iron to a minimum. There are also grades available that suit applications with coolant supply

Influence of hardness

• The influence of hardness related to machinability for cast irons follows the same rules as for any other material
• For example, ADI (austempered ductile iron) and CGI (compacted graphite iron) as well as NCI (nodular cast iron) have hardnesses up to 300-400 HB
• HB. MCI and GCI average 200-250 HB
• White cast iron can achieve a hardness over 500 HB at rapid cooling rates where the carbon reacts with the iron to form a carbide Fe3C (cementite), instead of being present as free carbon. White cast irons are very abrasive and difficult to machine

MC codes for cast iron

From a machinability point of view, cast irons are classified into malleable, grey, nodular, compacted graphite iron (CGI) and austempered ductile iron (ADI) types. Some of the higher hardnesses can be found in nodular cast irons and the ADIs.

The austempering heat treatment converts ductile iron, (NCI), into austempered ductile iron (ADI).

Malleable Cast Iron (MCI) K 1.1-1.2 and Grey Cast Iron (GCI) K 2.1-2.3

Definition

Malleable cast iron is produced from a close to white iron matrix, which is then heat treated in two steps, producing a ferrite + perlite + tempered carbon structure, leading to irregular graphite grains compared to the more fracture-inducing lamellar structure in the grey cast iron. This means that the malleable material is less sensitive to cracking and its values for rupture strength and elongation are higher.

Grey cast iron has the graphite in typical flake form, and the main characteristics are: low impact strength (brittle behavior); good thermal conductivity, less heat when the engine operates and low heat in cutting process; good dampening properties, absorbs the vibrations in the engine.

Common components

Components manufactured from MCI include: axle bearings, track wheels, pipe fittings and high strength gears. Components manufactured form GCI include: frying pans, engine blocks, cylinders for compressors, gears and gear box cases.

Machinability

Malleable cast iron has a higher tensile strength than GCI, and resembles NCI in its machinability, but both generally have excellent machining properties. In general, cast iron with a perlitic struture increases the abrasive wear, while ferritic structures increase the adhesive wear.

Grey cast iron has low impact strength, generates low cutting forces and the machinability is very good. Wear is created in the cutting process only by abrasion; there is no chemical wear. Grey cast iron is often alloyed with Cr in order to improve the mechanical properties. The higher strength will then result in decreased machinability.

Nodular Cast Iron (NCI) K 3.1-3.5

Definition

Nodular cast iron has spherically shaped graphite, and the main characteristics are good stiffness (Young’s module); good impact strength = tough material, not brittle; good tensile strength; bad damping properties, does not absorb the vibrations in the engine; bad thermal conductivity, higher heat in cutting process. In comparison to GCI, the graphite in NCI appears in the form of nodules, which contributes to higher tensile properties and toughness.

Common components

Hubs, tubing, rollers, exhaust manifolds, crankshafts, differential housings, bearing caps, exhaust manifolds, bedplates, turbo charger housings, clutch plates and fly wheels.

Turbo-charger housings and exhaust manifolds are often made of SiMo alloyed cast iron, which is more resistant to heat.

Machinability

Nodular cast iron has a strong tendency to form a 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.

Compacted Graphite Iron (CGI) K 4.1-4.2

Definition

CGI is a material that can meet both the increasing demands for strength and weight reduction and still retain reasonable machinability. The thermal and damping characteristics of CGI are between NCI and GCI. Resistance to metal fatigue is twice that of grey iron. The graphite particles in CGI are elongated and randomly oriented, as in grey cast iron, but they are shorter, thicker and have rounded edges. The coral-like morphology in CGI, together with the rounded edges and irregular bumpy surfaces of the graphite particles, provides strong adhesion between the graphite and the iron matrix. This is why the mechanical properties are so improved in CGI, relative to grey cast iron. CGI with a perlitic content below 90% is most common.

Common components

CGI is well suited for engine manufacturing, where lighter and stronger materials that can absorb more power are needed. The engine block weight alone can be reduced by approx. 20 percent compared with one made from GCI. Other examples are cylinder heads and disc brakes.

Machinability

From a machinability point of view, compacted graphite iron is between grey and nodular cast iron. With two to three times the tensile strength of grey cast iron and lower thermal conductivity, machining of CGI generates higher cutting forces and more heat in the cutting zone. An increased content of titanium in the CGI material influences tool life negatively.

The most common machining operations are face milling and cylinder boring. Instead of cylinder boring, a change of method to circular milling can improve both tool life and productivity.

Austempered Ductile Iron (ADI) K 5.1- 5.3

Definition

Austempered ductile iron forms a family of heat-treated cast irons. The austempering heat treatment converts ductile iron to austempered ductile iron (ADI), whose characteristics include excellent strength, toughness, and fatigue characteristics. ADI is stronger per unit weight than aluminium and as wear resistant as steel. Tensile and yield strength values are twice those of standard ductile iron. Fatigue strength is 50% higher, and it can be enhanced by shot peening or fillet rolling.

Common components

ADI castings are increasingly displacing steel forgings and castings, welded fabrications, carburised steel, and aluminium due to its superior performance. Its dominant uses are in the automotive industry, where it is used for suspension and transmission parts, etc. It is also used in the power/energy and the mining and construction sectors.

Machinability

A 40-50% reduction in tool-life compared to NCI can be expected. Tensile strength and ductility of ADI are near to steel, but the chip formation process classifies ADI as a ductile iron (segmented chip formation). The micro hardness of ADI is higher, when compared to steels of comparable hardness. Higher ADI grades contain hard particles in the micro-structure. High thermal and mechanical loads, due to high strength and ductility, will concentrate wear near the cutting edge due to the segmented chip formation process, and wear on the top rake. Hardening during chip formation results in high dynamic cutting forces. The cutting edge temperature is a strong factor for determining wear.

ISO N non-ferrous materials

What are ISO N non-ferrous materials?

• This group contains non-ferrous, soft metals with hardnesses under 130 HB, except for high strength bronzes (>225HB)
• Aluminium (Al) alloys comprising less than 12-13% silicon (Si) represent the largest part
• MMC: Metal Matrix Composite: Al + SiC (20-30%)
• Magnesium based alloys
• Copper: electrolytic copper with 99.95% Cu
• Bronze: copper with Tin (Sn) (10-14%) and/or aluminium (3-10%)
• Brass: copper (60-85%) with Zinc (Zn) (40-15%)

Machinability of aluminium

• Long-chipping material
• Relatively easy chip control, if alloyed
• Pure Al is sticky and requires sharp cutting edges and high v_c
• Specific cutting force: 350–700 N/mm
• Cutting forces, and thus the power required to machine them, are low
• The material can be machined with fine-grained, uncoated carbide grades when the Si content is below 7-8%, and with PCD tipped grades for aluminium with higher Si content
• Over eutectic Al with higher Si content > 12% is very abrasive

Common components

Engine block, cylinder head, transmission housings, casings, aerospace frame components.

MC codes for N-materials

ISO S HRSA and titanium

• MC codes for S materials
• HRSA materials – S 1.0-3.0
• Titanium– S 4.1-4.4

What is ISO S HRSA and titanium?

• The ISO S group can be divided into heat resistant super alloys (HRSA) and titanium
• HRSA materials can be split into three groups: Nickel-based, iron-based and cobalt-based alloys
• Condition: annealed, solution heat treated, aged, rolled, forged, cast
• Properties: increased alloy content (Co more so than Ni), results in better resistance to heat, increased tensile strength and higher corrosive resistance

Machinability in general

= Stainless steels

= Heat treated (aged)

= Solution treatment (annealed)

• The physical properties and machining behaviour of each varies considerably, due to both the chemical nature of the alloy and the precise metallurgical processing it receives during manufacture
• Annealing and aging are particularly influential on subsequent machining properties
• Difficult chip control (segmented chips)
• Specific cutting force: 2400–3100 N/mm for HRSA and 1300–1400 N/mm for titanium
• Cutting forces and power required are quite high

Aging

In order to achieve higher strength, heat-resistant alloys can be “precipitation hardened”.

By treating the material at elevated temperatures, i.e. aging treatment, small intermetallic particles are precipitated in the alloy. These particles will hinder movement in the crystal structure and, as a result, the material will be more difficult to deform.

MC codes for S-materials

From a machinability point of view, HRSA steels are classified into iron, nickel and cobalt-based materials. Titanium is divided into commercially pure alpha-alloys and near alpha-alloys, alpha/beta alloys and beta-alloys.

HRSA materials – S 1.0-3.0

Definition

High corrosion-resistant materials which retain their hardness and strength at higher temperatures. The material is used at up to 1000°C and is hardened through an aging process.

• The nickel-based version is the most widely used - over 50% of the weight of an aeroplane engine. Precipitation hardened materials include: Inconel 718, 706 Waspalloy, Udimet 720. Solution strengthened (not hardenable) include: Inconel 625
• Iron-based material evolves from austenitic stainless steels and has the poorest hot strength properties: Inconel 909 Greek Ascolloy and A286
• Cobalt-based materials have the best hot temperature performance and corrosion resistance, and are predominantly used in the medical industry: Haynes 25 (Co49Cr20W15Ni10), Stellite 21, 31
• Main alloying elements in HRSA materials
  Ni: Stabilizes metal structure and material properties at high temperatures
  Co, Mo, W: increase strength at elevated temperatures
  Cr, Al, Si: improve resistance to oxidation and high temperature corrosion
  C: increases creep strength

Common components

Aerospace engine and power gas turbines in the combustion and turbine sections, oil and gas marine applications, medical joint implants, high corrosion resistant applications.

Machinability

Machinability of HRSA-materials increases in difficulty according to the following sequence: iron-based materials, nickel-based materials and cobalt-based materials. All the materials have high strength at high temperatures and produce segmented chips during cutting which create high and dynamic cutting forces.

Poor heat conductivity and high hardness generate high temperatures during machining. The high strength, work hardening and adhesion hardening properties create notch wear at maximum depth of cut and an extremely abrasive environment for the cutting edge.

Carbide grades should have good edge toughness and good adhesion of the coating to the substrate to provide good resistance to plastic deformation. In general, use inserts with a large entering angle (round inserts) and select a positive insert geometry. In turning and milling, ceramic grades can be used, depending on the application.

Titanium– S 4.1-4.4

Definition

Titanium alloys can be split into four classes, depending on the structures and alloying elements present.

• Untreated, commercially pure titanium
• Alpha alloys – with additions of Al, O and/or N
• Beta alloys – additions of Mb, Fe, V, Cr and/or Mn
• Mixed a+ß alloys, in which a mixture of both classes is present

The mixed α+β alloys, with type Ti-6Al-4V, account for the majority of titanium alloys currently in use, primarily in the aerospace sector, but also in general purpose applications. Titanium has a high strength to weight ratio, with excellent corrosion resistance at 60% of the density of steel. This enables the design of thinner walls.

Common components

Titanium can be used under very harsh environments, which could cause considerable corrosion attacks on most other construction materials. This is due to titanium oxide, TiO2, which is very resistant and covers the surface in a layer which is approx. 0.01 mm thick. If the oxide layer is damaged and there is oxygen available, the titanium rebuilds the oxide immediately. Suitable for heat exchangers, de-salting equipment, jet engine parts, landing gears and structural parts in the aerospace field.

Machinability

The machinability of titanium alloys is poor compared to both general steels and stainless steels, which places special demands on the cutting tools. Titanium has poor thermal conductivity; strength is retained at high temperatures, which generates high cutting forces and heat at the cutting edge. Highly-sheared, thin chips, with a tendency for galling create a narrow contact area on the rake face, generating concentrated cutting forces close to the cutting edge. A cutting speed that is too high produces a chemical reaction between the chip and the cutting tool material, which can result in sudden insert chippings/breakages. Cutting tool materials should have good hot hardness, low cobalt content, and not react with the titanium. Fine-grained, uncoated carbide is usually used. Choose a positive/open geometry with good edge toughness.

ISO H hardened steel

What is ISO H hardened steel?

• This group of materials contains hardened and tempered steels with hardnesses >45 – 68 HRC
• Common steels include carburizing steel (~60 HRc), ball bearing steel (~60 HRc) and tool steel (~68 HRc). Hard types of cast irons include white cast iron (~50 HRc) and ADI/Kymenite (~40 HRc). Construction steel (40–45 HRc), Mn steel and different types of hardcoatings, i.e. stellite, P/M steel and cemented carbide also belong to this group
• Typically hard part turning fall within the range of 55–68 HRC

Machinability

• Hardened steel is the smallest group from a machining point of view and finishing is the most common machining operation. Specific cutting force: 2550–4870 N/mm. The operation usually produces fair chip control. Cutting forces and power requirements are quite high
• The cutting tool material needs to have good resistance to plastic deformation (hot hardness), chemical stability (at high temperatures), mechanical strength and resistance to abrasive wear. CBN has these characteristics and allows turning instead of grinding
• Mixed or whisker reinforced ceramic are also used in turning, when the workpiece has moderate surface finish demands and the hardness is too high for carbide
• Cemented carbide dominates in milling and drilling applications and is used up to approx. 60 HRc

Common components

Typical components include: transmission shafts, gear box housings, steering pinions, stamping dies.

MC codes for hardened steel