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Machinability

The most important properties of stainless

steel are corrosion resistance and tensile

strength, but the alloying materials thatproduce these properties often adversely

affect the machinability. In theory, further

modifications to the alloy mixture could

fine tune the properties and produce an

ideal alloy with all the advantages and

none of the drawbacks, but such recipes

always generate problems of their own. It

would be easy, for example, to improve the

machinability by adding sulphur, which is

in fact done in certain free cutting steels -

but poorer corrosion resistance inevitably

results. And in practice, current material

standards place clear restrictions on the useof chemical additions to modify the

machinability of stainless steels.

But there is plenty of scope within the

machining process variables themselves for

improving the machinability, as long as the

choices are made carefully.

Potential problems when machining stainless steel

Type of

stainless steel

Built-up edge

area

Temperature Strain hardening Burr formation Abrasive wear

Austenitic/

DuplexHigh High High High Low

Ferritic/

Martensitic andPH steel

Low High Low Low High

Built-up edge

A very common problem when machining

austenitic or duplex steels, is the tendency

for the chips to become welded onto the

cutting edge and onto the workpiece. This

unavoidably leads to reduced tool life, since

the welded on chips wear away flakes ofthe coating, and even parts of the substrate,

when they are ultimately torn off the cut-

ting edge. An unacceptable surface finish to

the workpiece is another very likely result.

When edge build-up takes place it is recom-

mended that the cutting speed be increased,

in order to obtain a temperature above the

so-called built-up edge area shown in

cutting data charts, and that a sharp cutting

edge together with a PVD coated grade be

used. This combination will reduce the ten-

dency of the chips to become welded onto

the edge.

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Temperature

The heat conduction capacity plays an

important role in metal cutting since mostof the heat from the cutting zone is carried

away with the chips. However, stainless

steel has poorer heat conduction capacity

and leaves more heat in the cutting zone.

These higher temperatures increase both

the tendency for tool wear, and the risk of

plastic deformation of the cutting edge,

particularly with continuous cutting

processes like turning.

Limiting the cutting forces can controlthese high temperatures, eg. by using a

CoroMill concept with positive angles of

inclination, combined with the very posi-

tive rake angles in inserts adapted for

machining stainless materials (e.g. ML).

Furthermore certain insert grades are par-

ticularly resistant to the effects of high tem-

peratures, e.g. GC2030 with an especially

hard substrate, or GC2040 which is coatedwith an insulating layer of aluminium oxide

(Al2O3).

Low productivity Built-up edge area High productivity

Working areaProduced quantity

vc

m/min101 126 156 201 251 314

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Strain hardening of the machined surface

Stainless steels with a high austenitic con-

tent often have a tendency to strain harden.

Most susceptible are the austenitic and

super austenitic types, the duplex varieties

less so. This strain hardening effect leads to

changes in the structure of the material and

markedly increases the hardness of the

material at the surface, which duly increases

wear on the cutting edge at the cutting

depth.

This effect can be reduced both by

strengthening the main cutting edge, and by

varying the cutting depth so that the wear

is distributed over a larger proportion of

the main edge. It is normally most advanta-

geous to choose a cutting depth and a feed

which ensure that the cutting edge in ques-

tion is driven into the material below the

hardened zone. In addition the cutting

speed could be reduced, since the strain

hardened zones generate considerably

higher temperatures.

Burr formation

Burr formation is normally the result of

negative insert angles, often in the strength-

ening chamfer. The problem can be reduced

or even eliminated by using a positive,

sharper cutting edge, more suitable for this

range of materials. (This is the reason why

the substantially stronger, but negative,

geometries, developed for milling in steel

are often unsuitable for the milling of stain-

less steel.)

Novel machining methods

The traditional approaches to problem

solving when machining are usually

founded on simple, well known and under-

stood principles, such as:

choose a tougher grade that can cope

with built-up edge better

reduce the temperature by reducing thecutting speed and using coolant.

These empirical approaches can often be

based on experience of continuous metal

cutting such as turning, but not all such

experience can be directly transferred to

milling because the conditions are different.

Milling in stainless steel can, in certain

cases, be more advantageous than turning,

and may open up alternative solutions

which could potentially be employed.

For example, the traditional way of avoid-

ing BUE-wear has been to reduce the cut-

ting speed and apply coolant, aiming to

reduce the temperature below the built-upedge area of the temp vs vc charts. This

approach stems, logically enough, from

experience of built-up edge while turning

or drilling, where the risk of plastic defor-

mation on the cutting edges often makes it

impossible to increase the machining

parameters sufficiently to avoid the built-

up edge area altogether.

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Produced quantity

156 201 251 314

vc

m/min

But in a milling operation the cutting envi-

ronment of the inserts is very different, and

these empirical principles do not apply. The

intermittent nature of the process, in which

each individual insert is not constantly incut but rather passes in and out, means that

the cutting speed can in fact be increased

above the built-up edge area without auto-

matically increasing the likelihood of insert

damage.

Similarly, the addition of more coolant

alongside such an increase in cutting speed,

another seemingly logical step, is in fact

precisely the wrong course. In milling,

when the inserts go into cut their tempera-ture increases very quickly and the coolant

actually has very little effect: the coolant

does not enter the cutting zone because the

chips block the way. However, the coolant

does subsequently reach the cutting edge

and cools it effectively, particularly on the

surface of the insert, as soon as the insert

emerges from cut. So for each revolution of

the milling cutter this repeated cycle causesthe difference in temperature between the

surface and the centre of the insert to rise,

which in turn creates large thermal stresses

that soon lead to cracks in the cutting edge.

Therefore dry machining should be

adopted when the cutting speed is

increased.

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With those stainless steels that have a great

propensity to strain hardening, parameterssuch as run-out in the cutter body are of

great significance and can be very disadvan-

tageous. If there is a feed per tooth of

0,150,20 mm/tooth and a run-out in the

cutter body of 0,1 mm (in certain tools

there may be much more), this means that

some inserts will not cut at all but will

rather rub against the surface and produce

heat. This will inevitably increase the strainhardening in the component.

Consequently, a general recommendation

from Sandvik Coromant is to use the newly

developed products for the milling of stain-

less steel, with a cutting speed of approxi-

mately 200 m/min without the use of

coolant.

There are a number of factors that make

this possible:

1. Positive tool concepts such as different

CoroMill tooling systems.

2. Positive insert geometries that provide a

good chip flow, low cutting forces and

have sharp edges that cut easily even insticky and strain hardened materials.

3. Grades that are able to cope with higher

temperatures, having a hard substrate or

a heat protecting layer.

Importance of tool precision

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Two grades with different characteristics for machining stainless materials

As a general principle, if the material is

"sticky" such as austenitic, super duplex

and duplex, an insert grade with a fine-

grained substrate and PVD coating is pre-

ferred. This is because a fine-grain struc-

ture enables a sharp edge to be ground, and

this can then be maintained if it is protected

with a PVD coating. Furthermore a PVD

layer reduces the tendency for edge build-

up - and in those cases where such built up

edges are created and torn off anyway, the

fine-grained substrate copes better than the

more coarse-grained ones, since the damage

to the edge is not as severe.

If, on the other hand, the material is abra-

sively wearing, as for example precipitation

hardened, ferritic/martensitic steel, or has a

casting skin, then a grade with a tough bulk

and a wear resistant coating has clear

advantages.

As well as these two extreme cases, there

are naturally overlapping areas where both

grade types can be used, e.g. cast austenitic

stainless steel and titanium stabilised

austenitic stainless steel.

GC2030

Inserts in grade GC2030 have a medium-

hard, very fine-grained substrate that is

PVD coated with TiAlN + TiN. It is possi-

ble to grind this grade to form sharp edges

on the inserts, which make them light-cut-

ting and thereby suitable for sticky mate-

rials. PVD coated inserts often have advan-tages over CVD coated ones which can be

of great benefit in certain types of opera-

tions demanding toughness, e.g. 90 milling

of stainless steel.

GC2040

GC2040 has its toughness in the substrate

as a result of its relatively high proportionof binder (Co) and a wear resistant, heat

resistant coating which enables the grade to

retain its hardness even at high tempera-

tures. CVD inserts are coated with TiCN +

Al2O3+TiN, where the TiCN layer pro-

vides abrasive wear resistance and Al2O3acts as a heat barrier in the coating.

GC2040 has a medium-large grain size in

the substrate and is used particularly with

inserts that do not have any need of sharp

cutting edges, since the edges must be

rounded off before coating in order to

avoid flaking. The tough substrate of

inserts in GC2040 has been developed to

cope with applications requiring toughness,

such as components with a casting skin,

workpieces with hard particles that are

abrasively wearing on cutting edges, or

cases where vibration problems demand

bulk toughness.

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MM/2040 ML/2040 MM/2040 ML/2040

ML/2040 ML/2040 MM/2040 ML/2040

MM/2030 ML/2030 MM/2040 MM/2030

MM/2030 ML/2030 MM/2040 MM/2030

ISO M geometries for milling in stainless steel

Some general requirements are applicable

for the machining of stainless materials.

More positive than steel geometries

which means:

minimum burr formation

reduced tendency to edge build-up

less heat development

less risk of strain hardening of the work-

piece

Open chipbreakers:

generate less heat

facilitate chip evacuation

Periphery ground inserts (ML for all

CoroMill cutters and MM for CoroMill

245) provides:

good tolerances

minimal run-out

sharp cutting edges with minimal ER

There are two different geometries for use

when machining in stainless materials,

depending on the type of operation that is

to be undertaken.

ML geometries are the first choice for:

Precipitation hardened (PH) steel

Ferritic/martensitic steels

Finish milling of all types of pre-treated

stainless steels

MM geometries are the first choice for:

Rough milling in austenitic steels

Rough milling in all cast stainless steels

For operations requiring a tough cutting

edge, e.g. components with a casting skin

or gas cut components.

Roughing Finishing Roughing Finishing

Type of stainlesssteel

Ferritic/Martensitic

PH-steel

Austenitic

Duplex

ConditionPre-machined, rolled Cast

Insert recommendations

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Trouble shooting for milling in stainless materials

The importance of fully appreciating the

interplay between the tool concept, grade,

insert geometry and cutting data in order to

obtain a good result is vital when machin-

ing stainless steel, a very distinctive material

with its own particular characteristics. A

half-hearted implementation of the recom-

mendations, or an attempt to combine them

with more traditional and familiar methods,

is very likely to result in failure.

Some of the obvious problems which canarise, and some suggested solutions, are

listed in the following table.

Problem Suggested Insert

geometry

Alteration to

cutting data

Temperature

Suggested insert

grade

CoroMill cutter

GC2030 hardness

GC2040 Al2O3thermal barrier

ML positive rake

angle

Positive machining

angles

Abrasive wear GC2040-CVD

coated

GC2040-CVD

coated

Built-up edge GC2030-PVD

coating

ML/MM-sharp

edges

High cutting speed

Strain hardening ML-sharp edge with

positive rake angle

Minimal run-out If ap >1 mm

increase feed/tooth

Burr formation ML-sharp edge with

positive rake angle

Positive machining

angles

Reduce feed/tooth

Austenitic stainless steels, especially grade

AISI 316, are by far the most common

group of stainless steels and represent

approximately 60% of the total machined,

largely because of their superior corrosion

resistance. The most familiar type is the

18/8 type (18% Cr 8% Ni), which repre-

sents a basic level of corrosion resistance

within the group.

Some typical machining operations on AISI

316 might include rough machining. Here

suitable cutting parameters could, for

example, be a cutting depth (ap) of 24 mm,

a feed per tooth of approximately 0,2

mm/tooth, and a non machined surface, e.g.

with a casting skin still on the workpiece.

With finish machining the parameters will

be different. A cutting depth of

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A combination of increased cutting speed

and dry machining can often provide a way

to avoid problems of built-up edge on the

cutting inserts, particularly in milling oper-

ations. But only if both elements are usedtogether. If an austenitic steel is wet milled,

for example AISI 316 with a cutting speed

of 120 m/min, and then the coolant is

removed, the heat will increase, and the

process will end up in the built-up edge

area. Equally, if the cutting speed is

increased to 150 m/min and the coolant is

retained, the working temperature will

again rise undesirably, the machining will

enter the built up edge area and the results

will still be poor. However, if the cutting

speed is increased to around 200 m/min and

dry machining carried out, then the built-

up edge area will be passed and the tool lifeof the insert, as well as the surface finish of

the workpiece, will be improved considerably.

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Feed recommendations

E-ML 0,14 (0,080,21) 0,10 (0,060,15)K-MM 0,23 (0,100,28) 0,16 (0,070,20)

E-ML 0,10 (0,050,15) 0,10 (0,050,15)M-MM 0,13 (0,080,20) 0,13 (0,080,20)E-ML 0,10 (0,050,15) 0,10 (0,050,15)M-MM 0,15 (0,080,20) 0,15 (0,080,20)M-MM 0,18 (0,080,25) 0,18 (0,080,25)

E-ML 0,10 (0,050,15) 0,10 (0,050,15)M-MM 0,13 (0,080,20) 0,13 (0,080,20)M-MH 0,16 (0,080,22) 0,16 (0,080,22)

M-MM 0,18 (0,080,25) 0,18 (0,080,25)

E-ML 0,10 (0,080,15) 0,10 (0,080,15)M-MM 0,16 (0,100,20) 0,16 (0,100,20)M-ML 0,12 (0,080,15) 0,12 (0,080,15)M-MM 0,17 (0,100,20) 0,17 (0,100,20)

E-ML 0,21 (0,080,28) 0,15 (0,060,20)

M-MM 0,28 (0,100,42) 0,20 (0,070,30)

E-MM 0,10 (0,100,50 0,10 (0,070,20)E-MM 0,15 (0,100,50) 0,10 (0,070,20)E-MM 0,20 (0,150,50) 0,10 (0,070,20)E-MM 0,25 (0,200,50) 0,10 (0,070,20)E-MM 0,30 (0,200,50) 0,10 (0,070,20)

E-M 0,05 (0,050,10) 0,05 (0,050,10)M-M 0,05 (0,050,10) 0,05 (0,050,10)M-M 0,08 (0,080,15) 0,08 (0,080,15)M-M 0,10 (0,100,20) 0,10 (0,100,20)M-M 0,12 (0,150,25) 0,12 (0,150,25)M-M 0,15 (0,150,35) 0,15 (0,150,35)M-M 0,15 (0,150,35) 0,15 (0,150,35)M-M 0,20 (0,200,40) 0,20 (0,200,40)M-M 0,25 (0,250,40) 0,25 (0,250,40)

H-ML 0,15 (0,080,22) 0,10 (0,050,15)H-ML 0,18 (0,080,22) 0,12 (0,050,15)

Insert for:Insertgeometry

Startingvalue (min.max.)

CoroMill 245

CoroMill 390

CoroMill 390long edge

CoroMill 290

CoroMill 200

CoroMill 300

CoroMill BallNose

CoroMill 331

Startingvalue (min.max.)

Feed, fz(mm/tooth) Max chip thickness,hex(mm)

Size 1111171718

Size 111111

18

r= 0,8

r= 0,8

r= 2,0

r= 2,0

For D31012152025

Size 101216202530324050

Size04050814