case study: spline profiles (1) established within the growth project grd1-1999-10748...

Case study: spline profiles (1)

Established within the GROWTH project GRD1-1999-10748 "Improvement of Service Life and Reliability of Cold Forging Tools with respect to Fatigue Damage due to Cyclic Plasticity (COLT)", funded by the European Community.

COLT Design Manual for Cold Forging Dies

Case study: Cold forging of spline profilesCase study: Cold forging of spline profiles

• Die material: Vanadis 23, 60 HRc

• Prestressing system: ID 50 mm, OD 140 mm

Net-shape forged spline tap




Typical failure modeTypical failure mode

• Fatigue cracks in the grounds of the spline geometry

• The cracks start in the fillets in axial direction and propagate on the top in radial direction.




FE model of the tooling systemFE model of the tooling system

2D model of the die, here prestressed by STRECON E+

Stripwinding and casingE=205 GPa, =0.3, elastic

Carbide winding coreE=540GPa, =0.23, elastic

Die, E=225GPa, =0.3elastic-plastic

process load:1000 MPa

A) conventional container with double ring0.6% interference

B) STRECON® Basic, 0.8% interference

C) STRECON® E+, 0.5% interference




A) Prestressing by a double stress ring at 0.6% interferenceA) Prestressing by a double stress ring at 0.6% interference

Tangential stress distributionat max. process load

risk of early crack initiation due to tensile stressesat maximum process load and large stress range

565 MPa

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

-1.5 -1 -0.5 0 0.5 1

Total strain in %S

tre

ss

in M

Pa

=2850 MPa

570

380

190

0

-190

-380

-570

-760

-950

-1140

-1330

MPa

Tangential stress-strain responsein the fillet (for one load cycle)




-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

-1.5 -1 -0.5 0 0.5 1

Total strain in %

Str

es

s in

MP

a

B) Prestressing by STRECONB) Prestressing by STRECON®® Basic at 0.8% interference Basic at 0.8% interference


40 MPa =2840 MPa

570

380

190

0

-190

-380

-570

-760

-950

-1140

-1330

MPa


Enhanced fatigue strength due to the elimination of tensile stresses, but still the same stress range as the conventional double stress ring




Risk of over-prestressingRisk of over-prestressing

190 MPa

200

180

160

140

120

100

80

60

40

20

0

Condition of the die after prestressing by 0.8% and subsequent disassembly (e.g. for separate surface treatment).

If the die is prestressed by a high interference level and taken out from the prestressing system, cracks may occur in subsequent treatments.

Equivalent plastic strains

0.2%

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

%Residual stresses




-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

-1.5 -1 -0.5 0 0.5 1

Total strain in %

Str

es

s in

MP

a

=2480 MPa

C) Prestressing by a STRECONC) Prestressing by a STRECON®® E E++ at 0.5% interference at 0.5% interference

-125 MPa

570

380

190

0

-190

-380

-570

-760

-950

-1140

-1330

MPa

Optimum fatigue strength due to the elimination of tensile stresses and the minimization of the stress range






Further optimization: change of the diameter ratioFurther optimization: change of the diameter ratio

The closer the carbide winding core to the inner contour of the die insert,the more effective the stiffness of the STRECON E+ container.

Design D: die OD ø 40 mmprestressed by STRECON E+

Design C: die OD ø 50 mmprestressed by STRECON E+

ø 50 ø 40




-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

-1.5 -1 -0.5 0 0.5 1

Total strain in %

Str

es

s in

MP

a

=2240 MPa

Design D: Prestressing by modified STRECONDesign D: Prestressing by modified STRECON®® E E++ container container (ID reduced from 50 to 40 mm) at 0.45% interference(ID reduced from 50 to 40 mm) at 0.45% interference

-210 MPa

570

380

190

0

-190

-380

-570

-760

-950

-1140

-1330

MPa

Optimum fatigue strength due to the elimination of tensile stresses and the minimization of the stress range (from 2850 down to 2240 MPa)






Comparison of elastic expansion under process loadComparison of elastic expansion under process load

0.06

0.08

0.1

0.12

0.14

0.16

0.18

A)conventional

B) STRECONBasic

C) STRECONE+ (ID 50)

D) STRECONE+ (ID 40)

Dia

met

er c

han

ge

in m

m

Reduced elastic expansion due to stiffness of STRECON E+ container

Diameter change under process loadElastic expansion under process pressure

contour under prestressing

contour under process load

1

2

1 1 1 12 2 2 2




Recommendations for the designRecommendations for the designof dies for the cold forging of spline profilesof dies for the cold forging of spline profiles

Recommended die material:

• Powder-metallurgical tool steel provides an excellent fatigue strength

Recommended prestressing system:

• STRECON® E+

reduces the stresses and cyclic plastic strains in the fillets,leads to improved fatigue life

The outer diameter of the die insert should be close to the inner contour (better effectivity of the stiff carbidewinding core).

For variable adjustment of the inner diameter of the die(to compensate for wear and manufacturing tolerances),a STRECON® VARI-FIT is recommended, see case study.

case study: spline profiles (1) established within the growth project grd1-1999-10748...

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