Identification of residual stress phenomena based on the hole drilling method in explosively welded steel-titanium composite

Aleksander Karolczuk*, Mateusz Kowalski*, Krzysztof Kluger*, Fabian Żok**

* Opole University of Technology

XII International Symposium on Explosive Production of New Materials: Science, Technology, Business, and Innovations, May 25-30, 2014, Cracow

The Project was financed from a Grant by National Science Centre (Decision No. DEC-2011/03/B/ST8/05855)

** Z.T.W Explomet S.J., Opole, Poland

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The plan of presentation

• Introduction• Experimental research• Residual stress calculations• Results analysis• Summary

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Introduction

1) Sources of residual stresses

Manufacturing process

The heat treatment

Flattening process

2) Consequences of residual stresses

Failure of joining process

Material strength under monotonic and fatigue loadings (negative or

positive)

Stress corrosion cracking (suppressed under compressive

stresses)

3) The aim of the paper

Determination of the influence of the heat treatment on residual stress state in titanium layer of Steel-Titanium bimetal

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Experimental research

Basic information concerning the analyzed bimetal:

Flayer plate: Titanium Grade 1 (6 mm)Basic plate: S355J2+N steel (40 mm)

Steel S355J2

Chemical element: C Si Mn P S Cu

Maximum content, % weight: 0,22 0,55 1,60 0,025 0,025 0,45

Titanium Grade 1

Chemical element: C Fe H N O Ti

Maximum content, % weight: 0,10 0,20 0,015 0,03 0,18 99,5

Table 1. Chemical composition steel S355J2+N (EN 10025-2:2004) and titanium Grade 1

MaterialMechanical properties

ReH, MPa Rm, MPa E, GPa , - A5, % , 1/KS355J2+N 382-395* 598-605* 206 0,27-0,30 24-34* 13,010-6

Grade 1 189-215 (Rp02)* 308-324* 100 0,37** 43-56* 8,610-6

*- manufacturer certificate , **- own research (titanium after explosive welding

Table 2. Mechanical properties of the steel S355J2+N and titanium Grade 1

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Experimental research

Draft of welded plate with ignition point and specimen locations.

- 3 plates (210 x 180 x 46 mm) without the heat treatment

- 3 (210 x 180 x 46 mm) plates after the heat treatment

The heat treatment:soaking in 600oC for 90 minutes

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Experimental research

Residual stress measurements were performed using the hole drilling method that consists of strain measurements (relaxation) around the drilled hole

Manofacturer: TML TokyoSokkiKenkyujo Co., Ltd.Type: FRS-2Dimensions : gauge length: 1.5 mm

width: 1.3 mm outer diameter: 9.5 mm

Centerline diameter: 5.14 mmNominal resistance: 120 0.5 Gauge factor: 2.0

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Experimental research

Diameter: 1.5 mm (drill) Speed : 6000 rpm

Two points of measurement for each plate

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Experimental research

Strain history registered in titanium during the drilling process

Measured stabilized strain values in titanium layer, registered in three directions: A, B, C

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Experimental research

Influence of the drilling process on the measured strains

Titanium plate after the heat treatment:

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Residual stress calculations

Residual stress calculations were performed according to:

- The strain gauge manufacturer’s prescriptions (TML)

- The ASTM (E837-08) prescriptions (ASTM)

TML ASTM

Single hole depth equal to 1.2 of hole diameter

Takes into account the strain measured for several hole depths (more accurate)

Assumptions:

- Uniform stress distribution – averaged residual stress state - Plane stress state- Isotropic materials

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Residual stress calculations

PlatePoint 1 Point 2

1 2 (1) (P) 1 2 (1) (P)

I 235 227 -50 -12 269 213 -34 -20

II 243 210 -35 174 373 339 -125 99

IV 313 248 -2 -100 310 220 -43 -130

Example results:

TML - the specimens without the heat treatment.

Where: (1) - angle measured clockwise from direction A to 1; (P) - angle

measured clockwise to A direction from detonation direction.

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Result analysis

The mean values of the principal stress σ1 , σ2 and standard deviations calculated according to the TML and ASTM prescriptions for the specimens without and after the heat treatment.

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Result analysis

Maximum principal stress direction and direction of detonation ?

Directions of the maximum principal stresses related to detonation directions. Calculations according to the TML and ASTM methods.

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Result analysis

1) The tensile stresses in the titanium layer – introduced during explosive welding process

2) Compressive stresses in the titanium layer - appearing after the heat treatment are the result of different thermal expansion coefficients of welded materials

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Summary

1) The heat treatment changes the residual stress state in titanium. The stress state in specimen without the heat treatment is tensile and after the heat treatment is compression.

2) Direction of the maximum principal stress does not coincide with direction of detonation wave.

3) Calculation shows inhomogeneous residual stress state. The stresses change depending on the hole depth.