Fatigue Properties of a S45C Steel Subjected with

Ultrasonic Nano-crystal Surface Modification

Ri-ichi Murakami

The University of Tokushima, Dept. of Mechanical Engineering,2-1 Minami-josanjima-cho, Tokushima, 770-8506 Japan

2

An-AC

A

n-X

③

Hybrid nanocrystalline

Based on ②

Background

B

A

A

n-B

Coating and deposition

①

PVDCVDElectroplatingElectrodeposition……

A

An-A

②

Surface self-nanocrystalline

Shot peeningWater peeningRoller burnishingLaser shock peeningUltrasonic nano-crystal surface modificationNonequilibrium thermodynamics ……

Methods to achieve nano-crystal surface layer

3

100µm100µm

dd

AfterBefore

Machining direction

Fig. 1(a) TEM image shows surface layers as an amorphism(b) Microstructures “before” and “after” UNSM treatment

Size of crystal structure on surface layer

After50~200 nm

Before20 µm

Depth of deformed nano surface 100 µm

(a) (b)

Deformation of crystal structure

SKD61

4

Comparison

Tech. vs. Character SP LSP LPB, DR USP UNSM

Energy source Powder injection Laser heat Static load

Ultrasonic dynamic

load

Static load + Ultrasonic

dynamic load

Compressive residual stress/depth 1 5 4 2 4

Hardness & depth 1 4 3 2 5

Nano structure 2 1 3 4 5

Surface topology 2 5 1 3 4

Surface Roughness 2 1 4 3 5

SP: Shot Peening, LSP: Laser Shock peening, LPB : Low Plasticity Burnishing, DR: Deep Rolling, USP : Ultrasonic Shot Peening, UNSM: Ultrasonic Nanocrystal Surface Modification

Effect of various peening technologies on mechanical properties

5: Best

~1: Worst

5

UNSM Device

Fig. 2 Configuration of the UNSM device Fig. 3 Distribution of stress and amplitude

680004500034000Number of vibration strike(/mm2)

3Load(kg)

30Amplitude(μm)

2.38Tip diameter(mm)

WCTip material

UNSM UnitEquipment

C3C2C1Type of specimen

Table 1 UNSM Treatment Condition

6

Test procedure

Fracture Surface Investigation (SEM)

Fatigue Test (Dual-spindle rotating bending)

Surface Residual stress Test (XRD)

Surface Roughness Test

Micro-Vickers hardness Test

7

Micro-Vickers Hardness

Fig. 4 Micro-Vickers Hardness Distribution of S45C

8

Surface Roughness

Table 2 Surface Roughness of Specimen

Group Surface (100×) Surface (500×) Roughness(Ra)

No treatment 2.25μm

UNSM C1 1.49μm

UNSM C2 1.46μm

UNSM C3 1.00μm

9

Microstructures of S45C

pearlity

ferrite

Fig. 5. (a) Untreated specimen

10

Surface Nano-crystal layer

pearlity

ferrite

Fig. 5. (b) UNSM C3

Nano-crystallayer

11

Nano-crystal layer by SEM

Fig. 5. (c) UNSM C3

Nano-crystal layershows as a series ofparallel lines

12

Surface feature of UNSM

Fig. 6 Surface feature of UNSM

Tip trace direction

45°

Inter-granular micro cracks

13

Residual stress

Table 3 Residual Stress (MPa)

Depth Polished C1 C2 C3

20μm -20.5 -162.8 -244.1 -366.7

α-Fe, (211)plane, K=-323.17MPa/(°)

( ) ( ) ( )( )

2 2

24 2

2 sin 2 sin

sin sini i i i

i i

nM

n

θ ψ θ ψ

ψ ψ

− ×=

−

∑ ∑ ∑∑ ∑

Isoclinic method (sin2Ψ)

N N’

O σx

Ψ0

Ψηη

14

Fatigue

Fig. 8 S-N curve of S45C

15

Fracture Surface

Fig. 7 Fracture Surface of S45C after UNSM

(All the cracks initiates from surface)

a) C1, 450MPa, 1.45×105; b) C2, 450MPa, 8.16×105; c) C3, 450MPa, 2.26×107; d) C1, 450MPa, 1.45×105; e) C2, 450MPa, 8.16×105; f ) C3, 450MPa, 2.26×107.

2μm 12μm 30μm

16

Crack initiation

Fig. 9 Observation of the crack (UNSM C3, 474MPa, 2.3075×105)

a) 1.70×105; b)1.74×105; c) 2.28×105.

70μm

17

Crack initiation

Fig. 10 Crack growth speed

18

Tearing adhesive plaster model

cos2

Sr rBG rBF a

ρ

⎛ ⎞− × ⎜ ⎟×⎝ ⎠= =

The least efficiency ρ of nano-crystal layer to restrain the crack initiation is:

For UNSM C3, it is at least 48%.

Fig. 11. Fatigue fracture math model of S45C after UNSM:a) Simplification of surface originating fracture; b) Fatigue fracture math model of S45C after UNSM.

r

19

Conclusions

Surface hardness: no treatment<C1<C2<C3.

Surface roughness: no treatment<C1<C2<C3.

Surface residual stress: no treatment<C1<C2<C3.

Fatigue life: no treatment<C1<C2<C3.

Crack initiated from the surface of specimens, because of the tip trace and inter-granular cracks UNSM process. But nano-crystal layer can delay the crack initiation speed.

20