EDM 2013
Böhler Edelstahl GmbH & Co KG
EDM 2013
Böhler Edelstahl GmbH & Co KG
Grinding, polishing and EDM
of BOHLER Tool Steels
Customer Seminar
Technical Service (ET-TS)
2013
Kundenseminar 2013
Böhler Edelstahl GmbH & Co
KG
Grinding and polishing of tool steels 1 - 16
EDM of tool steels 17 - 42
General
failure investigations
Recommendations 43 - 48
Content of the presentation
Topics – Overview
Kundenseminar 2013
Böhler Edelstahl GmbH & Co
KG
Grinding and polishing
EDM 2013
Böhler Edelstahl GmbH & Co KG
Res
idu
al S
tres
s, N
/mm
2 2
Tension
Compression
0
+
-
Depth below Surface, mm 0
Abusive Grinding
Conventional Grinding
Electro Polishing
Gentle Grinding
Abusive Grinding + Shot Peening
Residual Surface Stress due to Surface Grinding AISI D2 (1.2379) material
EDM 2013
Böhler Edelstahl GmbH & Co KG
a= 100 m, Vt = 2.4 m/min
600
1000
800
400
200
100 200 300 400 600 500 700 80
0
900 1000 0
Distance from Machined Surface, (m)
Mic
ro H
ard
ness (
HV
0.0
5)
SiC; no coolant
SiC; with coolant
CBN; no coolant
CBN; with coolant
Grinding of hardened tool steels Surface hardness at grinding (AISI D2)
EDM 2013
Böhler Edelstahl GmbH & Co KG
Grinding of hardened tool steels Grinding cracks
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Böhler Edelstahl GmbH & Co KG
Abusive Grinding - Cracks and Grinding Marks
Grinding cracks
Grinding burns
Grinding of hardened tool steels Grinding cracks and burns
EDM 2013
Böhler Edelstahl GmbH & Co KG
Burn marks
Grinding of hardened tool steels Burn marks along the cutting edge
EDM 2013
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1.2379 (AISI D2)
HT: HT: 1040°C
TT: 2x300°C
Hot etching: 50% HCl
Temperature = 70°C
20µm
Grinding of hardened tool steels Grinding cracks
EDM 2013
Böhler Edelstahl GmbH & Co KG
Grinding of hardened tool steels Grinding cracks - CD mould
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Böhler Edelstahl GmbH & Co KG
Thermal influenced surface
Thermal influence surface with two micro cracks
Metallographic specimen
Grinding of hardened tool steels Tablet punch made of S1 material: Heavy thermal influence on the
grinded surface (high stress potential) which is full of micro cracks.
EDM 2013
Böhler Edelstahl GmbH & Co KG
Tool blank heat treated and ground
Ground surface and cracks
Grinding cracks after hot etching
Grinding cracks in transverse direction
Grinding of hardened tool steels Grinding cracks – bone file
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Böhler Edelstahl GmbH & Co KG
Grinding of hardened tool steels
EDM 2013
Böhler Edelstahl GmbH & Co KG
10 µm
Polishability and Homogeneity Polishing defects (comets) on a glossy finished mould surface,
caused by 5 to 8 µm large Al-Oxide-inclusions
(ASTM E45 D thin)
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Böhler Edelstahl GmbH & Co KG
DIN
1.2316
• Improved polishability
M303
EXTRA
1.2316
• Poor polishability
Improved polishability with Bohler M303
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Böhler Edelstahl GmbH & Co KG
EDM of tool steels Sink EDM and wire cutting
EDM 2013
Böhler Edelstahl GmbH & Co KG
High precision in thin strips or keyway areas
Surface influence
Process reliability
High cost effectiveness
Geometry freedom (back taper)
(lower) surface influence
EDM of tool steels EDM technology vs CAD/CAM milling technique HSC
EDM 2013
Böhler Edelstahl GmbH & Co KG
Wire cutting is a variant of the EDM. It was introduced
commercially in 1969 for the first time. A metal wire electrode is
used in this process.
View into the working zone example
Electrode
tool
Dielectric
Electricity
EDM of tool steels Wire cutting
EDM 2013
Böhler Edelstahl GmbH & Co KG
The Electro erosion reproduced
the shape of an electrode in a
metal workpiece by means of
sinking.
Example:
surface area: 45 cm2
final surface: Ra 0,32 µm
EDM time: 8 hours
EDM of tool steels Sink eroding
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Böhler Edelstahl GmbH & Co KG
EDM of tool steels Process
EDM 2013
Böhler Edelstahl GmbH & Co KG
1.2080 (AISI D3)
EDM of tool steels Surface influence
EDM 2013
Böhler Edelstahl GmbH & Co KG
20 30 40 50 60 10
600
500
400
300
200
100
HV
re-hardened
zone
tempered zone unaffected
material
1.2379 (D2)
material
1723 1270 820 680 450 350 310
Maximum re-heating temperature (°K)
Distance from melted zone µm
EDM of tool steels Surface hardness traverse
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AISI P20
(1.2312)
EDM of tool steels Influence of Sulphur
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Micro cracks Void
White layer
Heat affected zone
Uninfluenced structure
Reducing white layer 3 cuts and micro shot peening
EDM of tool steels Surface influence
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Böhler Edelstahl GmbH & Co KG
Fatigue Resistance Effect of Surface Quality in Hard Machining
Hard Milling Hard Milling+ Extrudehone Hard Milling+ Hand Polishing
Damaged zone ~30 µm
Damaged zone ~15 µm
Damaged zone ~2 -3 µm
• Hard milling of tool steels results in remarkable damage of tool surface
• Only small improvement by automated finish machinig technologies
• Hand Polishing of tool surfaces offers best quality up to now
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Böhler Edelstahl GmbH & Co KG
Grade Electrode
C (%)
center
C (%)
surface
Cu (%)
center
Cu (%)
surface
W302
(1.2344) Copper 0,37 1,81 0,13 1,40
W302
(1.2344) Graphite 0,37 3,10 0,13 0,13
K110
(1.2379) Copper 1,51 1,98 0,07 1,25
K110
(1.2379) Graphite 1,51 3,14 0,07 0,07
EDM of tool steels Electrode influence
EDM 2013
Böhler Edelstahl GmbH & Co KG
Polished surface after EDM
Tool: Tablet die
Problem: Holes after polishing
Reason: Stray field electrolysis
Cause: Increased electric
conductance of
electrolyte
EDM of tool steels Stray field electrolysis
EDM 2013
Böhler Edelstahl GmbH & Co KG
EDM of tool steels Surface corrosion caused by stray field electrolysis
EDM 2013
Böhler Edelstahl GmbH & Co KG
Material: W 300 (H11)
Hardness: 54 HRc
Sequence:
milling – HT- grinding – EDM of the gearing area
Reason:
Residence time in electrolyte, Post processing (cleaning)
EDM of tool steels Pittings on a die insert at the spark eroded gearing area
EDM 2013
Böhler Edelstahl GmbH & Co KG
Depth of penetration ( µm )
Str
ess (
N/m
m2
)
Depth of penetration ( µm )
Stress relieving before EDM: 4h @ 650°C furnace cooling
Rough machining Fine machining
EDM of tool steels Residual stress after EDM – Din 56NiCrMoV7 material
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Böhler Edelstahl GmbH & Co KG
EDM of tool steels Stress related cracking
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Te
nsile
str
ess
Co
mp
ressiv
e
str
ess
( + )
( - )
Stress condition
after hardening
and surface grinding
Stress - induced
crack formation
Pre-machining in the
as annealed
condition serves to
reduce the stresses
caused by
heat treatment
EDM of tool steels Stress related cracking
EDM 2013
Böhler Edelstahl GmbH & Co KG
Tempering temperature (°C)
Hard
ness (
HR
c)
Tem
perin
g
Working
hardness
Working
hardness
Low tempering temp.
High internal stresses
no transformation of retained
austenite
High Tempering temp.
Thermal stability,
Low internal stresses
transformation of retained
austenite
EDM of tool steels Heat treatment – Tempering and EDM
EDM 2013
Böhler Edelstahl GmbH & Co KG
Insufficiently
tempered
structure
White Layer
M = 500 :1
Micro cracks
EDM of tool steels Wire cutting D2 material – poorly tempered
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Böhler Edelstahl GmbH & Co KG
Cutting punch – HSS M2 182mm,
thickness 70mm, annealed max. 280
HB for blanking of sheet metal
Indicated
heat treatment parameter:
hardening temperature
1150°C / 5min – cooling rate 5 bar
Tempering:
540°C / 120 min
550°C / 120 min
500°C / 120 min
required hardness: 63 HRC
Stress crack
(area of sample
preparation)
Reason for investigation: Cracking
during wire cutting
EDM of tool steels Stress related cracking
EDM 2013
Böhler Edelstahl GmbH & Co KG
1st Reason for cracking:
EDM
8µm thick white layer and 3µm
heat affected zone, caused by too
intensive metal removal.
1000x
2nd Reason for cracking:
Incorrect heat treatment (does not
confirm with parametern mentioned)
hardening temperature too high
incorrect tempering
high retained austenite content
of 30,6%
500x
EDM of tool steels Stress related cracking
EDM 2013
Böhler Edelstahl GmbH & Co KG
High energy input when wire cutting
structure does not confirm with
parameters stated
(High retained austenite content and
hardness on work piece – 60HRc)
What does it mean for the customer?
HT - parameter to be verified
EDM - parameter must be modified
EDM of tool steels Stress related cracking – Investigation summary
EDM 2013
Böhler Edelstahl GmbH & Co KG
Dimension:
ø 182mm
Harden:
1220°C
Temper:
550°C
Hardness:
63 HRc
EDM of tool steels Stress related cracking
EDM 2013
Böhler Edelstahl GmbH & Co KG
Defect:
Stress cracking
Reason:
Raw material selection
Heat treatment
EDM parameters
EDM of tool steels Stress related cracking
EDM 2013
Böhler Edelstahl GmbH & Co KG
EDM of tool steels Stress related cracking during wire cutting
EDM block
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EDM of tool steels Plastic mould steel
Hardening temperature: 1050°C (overheated) EDM layer: up to 40µm
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Böhler Edelstahl GmbH & Co KG
MICROCLEAN PM-steels are well capable for spark eroding
and wire cutting because on the basic of high homogeneity
and excellent cleanliness.
Select steels with secondary hardness maxima, because these
steels are offering lower internal stress conditions.
ESR remelted steels are featuring improved homogeneity
and therefore are more moderate in spark eroding and wire
cutting than conventional melted materials.
EDM of tool steels - Recommondations 1. Choice of tool steels
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Böhler Edelstahl GmbH & Co KG
Remove large volume before heat treatment.
Look at a balanced condition of the work piece.
EDM of tool steels - Recommondations 2. Mechanical pre-machining
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Especially complex shaped components require
correct austenitising temperature and multiple
tempering treatments.
Steel grades with high tempering resistance
are essential.
As a result the combined stress level is lower.
EDM of tool steels - Recommondations 3. Heat treatment
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Böhler Edelstahl GmbH & Co KG
Energy input
high medium low
flashing – discharge duration - amperage- short circuit formation
Parameter
EDM of tool steels - Recommondations 4. Spark erosion with suitable parameters and conditions
EDM 2013
Böhler Edelstahl GmbH & Co KG
Stress relieving at a temperature
approximately 30°C below the last tempering
temperature to reduce the surface hardness
without influencing the core hardness.
EDM of tool steels - Recommondations 5. Stress relieving at tempering temperature
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Polishing – Micro shot peening
With the target to remove the surface influences
from the EDM process.
EDM of tool steels - Recommondations 6. Mechanical machining
EDM 2013
Böhler Edelstahl GmbH & Co KG
Legal Statement
The data contained in this presentation backup is merely for general information and
therefore shall not be binding on the company. We may be bound only through a
contract explicitly stipulating such data as binding. Measurement data are laboratory
values and can deviate from practical analyses.
EDM 2013
Böhler Edelstahl GmbH & Co KG
Thank you!
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