Corrosion resistant steels
Machining stainless steels has traditionally
been regarded as a problematic task, the
perceived wisdom being that their poor
heat conduction and inherently ‘sticky’
nature generate a large amount of heat during the cutting operation. This reasoning is
often based on prior bad experiences of
using a continuous process like turning or
drilling, but, when an intermittent process
such as milling is employed the machining
of stainless steel becomes a different proposition. The latest milling tools definitively
disprove the arguments that this material is
difficult to machine, or that poor productivity and tool-life are inevitable problems.
Stainless steel alloys
Stainless steel is high-alloy steel where the
most important alloying materials are
chrome (Cr) and nickel (Ni) together with
small quantities of carbon. Small amounts
of other alloying materials may also be
added to obtain specific properties.
A common feature of all stainless steels is
their great resistance to chemical attack.
This property is sometimes characterised as
‘passivity’ and is obtained when 11–13% of
chrome is added. A continuous, thin,
impervious layer consisting mainly of
chromic oxide is formed on the surface of
the steel, which protects the underlying
material against attack.
Stainless steels can be classified in accordance with their microstructures – which to
a large extent affect their machinability –
into the following main groups:
- Martensitic
- Austenitic
- Ferrite-austenitic (Duplex)
- Precipitation hardened (PH)
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Machinability
The most important properties of stainless
steel are corrosion resistance and tensile
strength, but the alloying materials that
produce 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 use
of 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
Austenitic/
Duplex
Ferritic/
Martensitic and
PH steel
Built-up edge
area
Temperature
Strain hardening
Burr formation
Abrasive wear
High
High
High
High
Low
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 of
the coating, and even parts of the substrate,
when they are ultimately torn off the cutting edge. An unacceptable surface finish to
the workpiece is another very likely result.
When edge build-up takes place it is recommended that the cutting speed be increased,
in order to obtain a temperature above the
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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 tendency of the chips to become welded onto
the edge.
Low productivity
Built-up edge area
Produced quantity
High productivity
Working area
101
126
156
201
251
314
vc
m/min
Temperature
The heat conduction capacity plays an
important role in metal cutting since most
of 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.
peratures, e.g. GC2030 with an especially
hard substrate, or GC2040 which is coated
with an insulating layer of aluminium oxide
(Al2O3).
Limiting the cutting forces can control
these high temperatures, eg. by using a
CoroMill concept with positive angles of
inclination, combined with the very positive rake angles in inserts adapted for
machining stainless materials (e.g. –ML).
Furthermore certain insert grades are particularly resistant to the effects of high tem77
Strain hardening of the machined surface
Stainless steels with a high austenitic content 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.
is distributed over a larger proportion of
the main edge. It is normally most advantageous to choose a cutting depth and a feed
which ensure that the cutting edge in question 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.
This effect can be reduced both by
strengthening the main cutting edge, and by
varying the cutting depth so that the wear
Burr formation
Burr formation is normally the result of
negative insert angles, often in the strengthening 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 stainless steel.)
Novel machining methods
The “traditional” approaches to problem
solving when machining are usually
founded on simple, well known and understood principles, such as:
●
choose a tougher grade that can cope
with built-up edge better
●
reduce the temperature by reducing the
cutting 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
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cases, be more advantageous than turning,
and may open up alternative solutions
which could potentially be employed.
For example, the traditional way of avoiding BUE-wear has been to reduce the cutting speed and apply coolant, aiming to
reduce the temperature below the “built-up
edge 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 deformation on the cutting edges often makes it
impossible to increase the machining
parameters sufficiently to avoid the builtup edge area altogether.
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 causes
the 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.
But in a milling operation the cutting environment 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 in
cut but rather passes in and out, means that
the cutting speed can in fact be increased
above the built-up edge area without automatically 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 temperature 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
Produced quantity
156
201
251
314
vc
m/min
79
Consequently, a general recommendation
from Sandvik Coromant is to use the newly
developed products for the milling of stainless steel, with a cutting speed of approximately 200 m/min without the use of
coolant.
1. Positive tool concepts such as different
CoroMill tooling systems.
There are a number of factors that make
this possible:
3. Grades that are able to cope with higher
temperatures, having a hard substrate or
a heat protecting layer.
2. Positive insert geometries that provide a
good chip flow, low cutting forces and
have sharp edges that cut easily even in
“sticky” and strain hardened materials.
Importance of tool precision
With those stainless steels that have a great
propensity to strain hardening, parameters
such as run-out in the cutter body are of
great significance and can be very disadvantageous. If there is a feed per tooth of
0,15–0,20 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
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rather rub against the surface and produce
heat. This will inevitably increase the strain
hardening in the component.
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 finegrained substrate and PVD coating is preferred. This is because a fine-grain structure 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 buildup - 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 abrasively 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.
GC2040
GC2040 has its toughness in the substrate
as a result of its relatively high proportion
of binder (Co) and a wear resistant, heat
resistant coating which enables the grade to
retain its hardness even at high temperatures. CVD inserts are coated with TiCN +
Al2O3+TiN, where the TiCN layer provides abrasive wear resistance and Al2O3
acts 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.
GC2030
Inserts in grade GC2030 have a mediumhard, very fine-grained substrate that is
PVD coated with TiAlN + TiN. It is possible to grind this grade to form sharp edges
on the inserts, which make them light-cutting and thereby suitable for “sticky” materials. PVD coated inserts often have advantages over CVD coated ones which can be
of great benefit in certain types of operations demanding toughness, e.g. 90° milling
of stainless steel.
81
ISO M geometries for milling in stainless steel
Some general requirements are applicable
for the machining of stainless materials.
Open chipbreakers:
●
generate less heat
More positive than “steel” geometries
which means:
●
facilitate chip evacuation
●
minimum burr formation
●
reduced tendency to edge build-up
●
less heat development
●
less risk of strain hardening of the workpiece
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:
MM geometries are the first choice for:
●
Precipitation hardened (PH) steel
●
Rough milling in austenitic steels
●
Ferritic/martensitic steels
●
Rough milling in all cast stainless steels
●
Finish milling of all types of pre-treated
stainless steels
●
For operations requiring a tough cutting
edge, e.g. components with a casting skin
or gas cut components.
Insert recommendations
Type of stainless
steel
82
Condition
Pre-machined, rolled
Roughing
Finishing
Roughing
Cast
Finishing
Ferritic/Martensitic
MM/2040
ML/2040
MM/2040
ML/2040
PH-steel
ML/2040
ML/2040
MM/2040
ML/2040
Austenitic
MM/2030
ML/2030
MM/2040
MM/2030
Duplex
MM/2030
ML/2030
MM/2040
MM/2030
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 machining stainless steel, a very distinctive material
with its own particular characteristics. A
half-hearted implementation of the recommendations, 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 can
arise, and some suggested solutions, are
listed in the following table.
Problem
Suggested insert
grade
Suggested Insert
geometry
CoroMill cutter
Temperature
GC2030 – hardness
GC2040 – Al2O3
thermal 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
Alteration to
cutting data
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
Machining example
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 represents 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 2–4 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 <= 1 mm, a
feed per tooth of approximately 0,1
mm/tooth and a pre-treated surface will
alleviate any problem of excessive heat and,
if the surface finish so requires, coolant can
be used without forfeiting too much in tool
life.
83
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 operations. But only if both elements are used
together. 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
84
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 builtup edge area will be passed and the tool life
of the insert, as well as the surface finish of
the workpiece, will be improved considerably.
Feed recommendations
Insert for:
CoroMill 245
Feed, fz (mm/tooth)
Max chip thickness,
hex (mm)
Insert
geometry
Starting
value
(min.–max.)
Starting
value
(min.–max.)
E-ML
K-MM
0,14
0,23
(0,08–0,21)
(0,10–0,28)
0,10
0,16
(0,06–0,15)
(0,07–0,20)
CoroMill 390
Size 11
11
17
17
18
E-ML
M-MM
E-ML
M-MM
M-MM
0,10
0,13
0,10
0,15
0,18
(0,05–0,15)
(0,08–0,20)
(0,05–0,15)
(0,08–0,20)
(0,08–0,25)
0,10
0,13
0,10
0,15
0,18
(0,05–0,15)
(0,08–0,20)
(0,05–0,15)
(0,08–0,20)
(0,08–0,25)
CoroMill 390
long edge
Size 11
11
11
E-ML
M-MM
M-MH
0,10
0,13
0,16
(0,05–0,15)
(0,08–0,20)
(0,08–0,22)
0,10
0,13
0,16
(0,05–0,15)
(0,08–0,20)
(0,08–0,22)
18
M-MM
0,18
(0,08–0,25)
0,18
(0,08–0,25)
0,8
0,8
2,0
2,0
E-ML
M-MM
M-ML
M-MM
0,10
0,16
0,12
0,17
(0,08–0,15)
(0,10–0,20)
(0,08–0,15)
(0,10–0,20)
0,10
0,16
0,12
0,17
(0,08–0,15)
(0,10–0,20)
(0,08–0,15)
(0,10–0,20)
E-ML
M-MM
0,21
0,28
(0,08–0,28)
(0,10–0,42)
0,15
0,20
(0,06–0,20)
(0,07–0,30)
For D3
10
12
15
20
25
E-MM
E-MM
E-MM
E-MM
E-MM
0,10
0,15
0,20
0,25
0,30
(0,10–0,50
(0,10–0,50)
(0,15–0,50)
(0,20–0,50)
(0,20–0,50)
0,10
0,10
0,10
0,10
0,10
(0,07–0,20)
(0,07–0,20)
(0,07–0,20)
(0,07–0,20)
(0,07–0,20)
CoroMill Ball
Nose
Size 10
12
16
20
25
30
32
40
50
E-M
M-M
M-M
M-M
M-M
M-M
M-M
M-M
M-M
0,05
0,05
0,08
0,10
0,12
0,15
0,15
0,20
0,25
(0,05–0,10)
(0,05–0,10)
(0,08–0,15)
(0,10–0,20)
(0,15–0,25)
(0,15–0,35)
(0,15–0,35)
(0,20–0,40)
(0,25–0,40)
0,05
0,05
0,08
0,10
0,12
0,15
0,15
0,20
0,25
(0,05–0,10)
(0,05–0,10)
(0,08–0,15)
(0,10–0,20)
(0,15–0,25)
(0,15–0,35)
(0,15–0,35)
(0,20–0,40)
(0,25–0,40)
CoroMill 331
Size
04–05
08–14
H-ML
H-ML
0,15
0,18
(0,08–0,22)
(0,08–0,22)
0,10
0,12
(0,05–0,15)
(0,05–0,15)
CoroMill 290
rε =
rε =
rε =
rε =
CoroMill 200
CoroMill 300
85