Improved Corrosion Resistance through Inductive Heat Treatment

Transcription

1 Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment Materials Science Engineering (MSE) 2014 Authors: Dipl.-Ing. Alexander Tump, Dr.-Ing. Timm Bauschke [Mubea Fahrwerksfedern GmbH] Prof. Dr. Robert Brandt [Universität Siegen Lehrstuhl für Werkstoffsysteme und Fahrzeugleichtbau] Datum:

2 Contents Reviewed Material Fatigue strength of the material Correlation fatigue limit and hardness Surface Layer Modification (SLM) Concept Mechanisms behind the Surface Layer Modification Fatigue life results of tested components (coil springs) Summary 2

3 Reviewed Material Properties Tensile strength 1700 MPa up to 2200 MPa Silicon-Chromium-Steel Tempered by Inductive-Heat-Treatment Usage High dynamically stressed parts Environmental conditions (e.g. salt-water) coil spings for automotive application 3

4 Fatigue strength of the material tension residual stress σ 30 µm Wire cross section depth wire not grinded III I Fatigue strength is influenced by compressive residual stresses at the wire surface through shot-peening (positive) cracks and decarburization at the surface layer (I) non metallic inclusions (III) corrosion starting at the surface layer (I) compression non metallic inclusion corrosion pit 500 µm 500 µm 4

5 Fatigue strength of the material Fatigue strength is influenced by tension residual stress σ compression depth SLM wire grinded HPP2 250 µm III II I compressive residual stresses at the wire surface through shot-peening (positive) cracks and decarburization at the surface layer (I) non metallic inclusions (III) corrosion starting at the surface layer (I) Enhancement of the stress capacity removal of surface cracks and surface decarburization in (I) avoiding crack initiation (I) reduction of the crack propagation rate (II) optimized residual stresses in (III) SLM Surface Layer Modification (SLM) allows to enhance the stress capacity by optimization of the surface layer 5

6 Correlation fatigue limit and hardness Rotational bending test (smooth, unnotched specimen) tensile strength MPa* maximum fatigue limit coil springs Requirements: - high fatigue life - sagloss resistance (high tensile strength) source: Garwood et al., 1951 Enhancement of the fatigue limit with increased hardness Higher tolerable stress level Critical hardness value due to the decrease of fracture toughness * DIN EN ISO

7 Surface Layer Modification (SLM) Concept Target Enhancement of the stress capacity Concept: Surface Layer Modification (SLM) Reduction of hardness in the surface layer Elevated hardness in the core area Creation of the hardness gradient Hardness in HV Soft layer Core SLM Standard Distance from surface in mm Adjustment of the core hardness in the tempering process T 1 Adjustment of the surface hardness via an additional inductive heat treatment process T 2 Surface Hardness = 520 HV (R m ~ MPa*) tempering SLM cooling station T 1 T 2 H 2 O wire-ø Tensile Test R m = MPa Core Hardness = 650 HV (R m ~ MPa*) * DIN EN ISO

8 Mechanism behind the Surface Layer Modification Fracture toughness and tensile strength fracture toughness KIc [MPa m] 80 material: 54SiCr6 / SAE tensile strength MPa induktive tempered /SENB piece tempered / CT (source: Holland, D.; Dahl, W. 1992) Improvement of fracture toughness K IC The fracture toughness increases significantly with decreasing tensile strength 8

9 Mechanism behind the Surface Layer Modification Correlation inclusion size to failure inclusion size to failure in µm Analysis of fatigue life tests (dry conditions) on coil springs N=268 Material: 54SiCr6 / SAE 9254 Inductive heat treated material tensile strength of the used wire MPa Failure caused by inclusions Inclusions become more critical in materials with higher tensile strength 9

10 Mechanism behind the Surface Layer Modification Crack propagation rate and crack initiation 1,E-05 material: 54SiCr6 / SAE 9254 ## R = 0,2 crack propagation rate da/dn[m] (log) 1,E-06 1,E-07 1,E-08 1,E-09 1,E-10 increasing crack propagation rate delayed crack initiation 2200 MPa 2100 MPa 2000 MPa 1900 MPa 1700 MPa cyclic loading K [MPa m] (log) Enhancement of the crack threshold K th Lower tensile-strength tends to higher K th -values (crack initiation) Reduction of crack propagation rate da/dn The crack propagation rate increase with higher tensile strength (parallel shift of the paris-line) 10

11 Results of tested components Dynamic-Fatigue-Corrosion-Test (coil springs) standard spring-steel n = 4 n = standard spring-steel n = 4 corrosion resistant spring-steel n = cycles N cycles N Standard: Integral: 580 HV Standard: Integral: 620 HV 0 Standard: Integral: 580 HV SLM: Integral: 580 HV Surface:515 HV Core: 590 HV SLM: Integral: 620 HV Surface:515 HV Core: 640 HV SLM: Integral: 580 HV Surface:515 HV Core: 590 HV Surface Layer Modification (SLM) enhances the fatigue life (stress capacity) SLM can be used with different materials A substitution of expensive materials (e.g. corrosion resistant materials) is possible 11

12 Summary SLM is based on... reduced hardness of the surface layer. elevated hardness level in the core area. SLM... avoids a supercritical hardening at the surface. increases the fracture toughness of the surface layer. decreases the crack propagation rate in the surface layer. reduces the risk of fracture induced by inclusions. enhances the fatigue life of dynamically loaded components SLM offers... potential to enhance the stress capacity (mass reduction is possible). SLM-mechanism is investigated on... specimen with different hardness but homogenous structure. parts with applied SLM-Technology (coil springs). 12

13 THANK YOU Special thanks to the ˮEuropäischer Fonds für regionale Entwicklung (EFRE) and Europäischer Sozialfonds (ESF) for supporting the project. Special thanks to the ˮLehrstuhl für Materialkunde und Werkstoffprüfungˮ (Prof. Dr.-Ing. H.-J. Christ) for supporting the fracture mechanical investigations. 13