Böhler Edelstahl GmbH & Co KG EDM 2013 · AISI D2 (1.2379) material . EDM 2013 ... Böhler Edelstahl GmbH & Co KG AISI P20 (1.2312) EDM of tool steels Influence of Sulphur . EDM

EDM 2013

Böhler Edelstahl GmbH & Co KG

EDM 2013


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


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


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


Grinding of hardened tool steels Grinding cracks

EDM 2013


Abusive Grinding - Cracks and Grinding Marks

Grinding cracks

Grinding burns

Grinding of hardened tool steels Grinding cracks and burns

EDM 2013


Burn marks

Grinding of hardened tool steels Burn marks along the cutting edge

EDM 2013


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


Grinding of hardened tool steels Grinding cracks - CD mould

EDM 2013


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


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

EDM 2013


Grinding of hardened tool steels

EDM 2013


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)

EDM 2013


DIN

1.2316

• Improved polishability

M303

EXTRA

1.2316

• Poor polishability

Improved polishability with Bohler M303

EDM 2013


EDM of tool steels Sink EDM and wire cutting

EDM 2013


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


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


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

EDM 2013


EDM of tool steels Process

EDM 2013


1.2080 (AISI D3)

EDM of tool steels Surface influence

EDM 2013


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

EDM 2013


AISI P20

(1.2312)

EDM of tool steels Influence of Sulphur

EDM 2013


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

EDM 2013


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

EDM 2013


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


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


EDM of tool steels Surface corrosion caused by stray field electrolysis

EDM 2013


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


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

EDM 2013


EDM of tool steels Stress related cracking

EDM 2013


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 2013


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


Insufficiently

tempered

structure

White Layer

M = 500 :1

Micro cracks

EDM of tool steels Wire cutting D2 material – poorly tempered

EDM 2013


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 2013


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 2013


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


Dimension:

ø 182mm

Harden:

1220°C

Temper:

550°C

Hardness:

63 HRc


EDM 2013


Defect:

Stress cracking

Reason:

Raw material selection

Heat treatment

EDM parameters


EDM 2013


EDM of tool steels Stress related cracking during wire cutting

EDM block

EDM 2013


EDM of tool steels Plastic mould steel

Hardening temperature: 1050°C (overheated) EDM layer: up to 40µm

EDM 2013


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

EDM 2013


Remove large volume before heat treatment.

Look at a balanced condition of the work piece.

EDM of tool steels - Recommondations 2. Mechanical pre-machining

EDM 2013


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

EDM 2013


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


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

EDM 2013


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


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


Thank you!

Documents

Böhler Edelstahl GmbH & Co KG EDM 2013 · AISI D2 (1.2379) material . EDM 2013 ... Böhler Edelstahl GmbH & Co KG AISI P20 (1.2312) EDM of tool steels Influence of Sulphur . EDM