Mechanical Properties

Transcription

1 Mechanical Properties Elastic deformation Plastic deformation Fracture Fatigue Environmental crack growth

2 Crack Instabilty ß σ T The critical crack length for given σ a a c = Q 2 K Ic σ a 2 a r ß a Sources of the critical crack Manufacturing defects Crack growth in service Fatigue Corrosion (H-embrittlement)

3 Crack Growth to Failure Initial feature Crack growth Unstable crack propagation Crack growth mechanisms Fatigue (cyclic load) Corrosive crack growth (hydrogen) Characteristic pattern: Initiating flaw Defect or corrosion pit Nucleated defect (fatigue) Crack growth to critical size Identify by characteristic fracture mode Corrosion: often intergranular Fatigue: beach marks, striations Final failure at critical size Crack length a = a c Crack mechanism = expected unstable mode Usually ductile fracture

4 Example: Failure of a High-Strength Steel Spring in Seawater ductile intergranular Initiation at a corrosion pit Significant 2nd stage growth Final fracture at expected ac pit Intergranular mode Ductile mode

5 Causes of Environmental Cracking Aqueous corrosion Anodic cracking: stress corrosion cracking Cathodic cracking: hydrogen embrittlement Gaseous embrittlement Hydrogen H 2 S Liquid embrittlement Acid Caustic (base) Liquid metal

6 Corrosion Cracking of Steel Anodic cracking (stress corrosion cracking) Progressive cracking of passive films Anodic dissolution Passive film prevented by, for example, Cl - Green rust Hydrogen embrittlement Hydrogen generated at crack tip Cathodic cracking (hydrogen embrittlement) Hydrogen charging Plating (bake-out required) Cathodic protection Mg, Zn, Al creates Fe cathode H liberated if acidic solution or oxygen depletion H migrates to crack tip, caues embrittlement

7 Environmental Fracture Toughness: 4340

8 Crack Growth Velocity Cr-Mo-V steel at RT

9 Crack Growth Velocity in Hydrogen

10 Fracture Surface SEM SMB-30 (tested sea water with Mg, fractured at 30lb after 42mins) Load

11 Fracture Surface SEM SMB-30 (tested sea water with Mg, fractured at 30lb after 42mins) Area B Area C Area A Area B Area A Area C

12 Fractographs from 432 Intergranular fracture Corrosion pit?

13 Crack Propagation Mode Note that fracture is mixed mode High fraction of intergranular Significant fraction of transgranular Transgranular has feathery appearance Need to appreciate fracture mode

14 Fractographs of 12Ni-0.25Ti after heat treatment and hydrogen charging A Quenched and tempered at 450 C for 300 hrs.: brittle intergranular B As-quenched: brittle transgranular C Single spike reversion to austenite: mixed mode D Double spike reversion to austenite, tempered at 450 C for 300 hrs: ductile rupture

15 Transgranular Hydrogen Embrittlement Intergranular fracture sources Contaminants on prior austenite grain boundaries Hydrogen attracted to free surfaces Transgranular embrittlement Clean or roughened prior γ boundaries Hydrogen promotes fracture across prior γ boundaries Actual path: martensite lath boundaries Hydrogen accumulates on lath boundaries Crack follows boundaries across grain

16 Lath Martensitic Steel Packet Block boundary Aligned substructure Prior Austenite grain 20µm

17 Crystallographic Alignment in Lath Martensitic Steel 500nm 500nm

18 Transgranular Hydrogen Embrittlement in Lath Martensitic Steel Profile micrograph of transpacket crack after hydrogen embrittlement Crack follows packet boundaries, boundary cracks connect with short shear segments 5.5Ni steel, QT condition

19 Fractographs of 12Ni-0.25Ti after heat treatment and hydrogen charging A Quenched and tempered at 450 C for 300 hrs.: brittle intergranular B As-quenched: brittle transgranular C Single spike reversion to austenite: mixed mode D Double spike reversion to austenite, tempered at 450 C for 300 hrs: ductile rupture

20 Delayed Fracture Hydrogen introduced into defect-free specimen Defect-free means no K>K IH Hydrogen sources Electroplating (particularly Cd-plating) Cathodic protection (particularly Mg) Environment Hydrogen diffuses to stress concentration Common stress concentrations Notches Inclusions Grain boundaries Combination of stress and hydrogen produces K>K IH Crack nucleation and propagation to embrittlement

21 Time to Failure after Charging

22 Corrosion Fatigue Corrosion fatigue in X42 line pipe (low strength) In nitrogen gas (a) fatigue striations In H 2 (b): mixed transgranular, intergranular No obvious fatigue striations

23 Corrosion Fatigue Corrosion fatigue in sea water Sample from exemplar nut Room temperature Al protection ΔK ~ 25 ksi in Fracture mode Transgranular feathery Secondary cracking

24 Corrosion Fatigue Corrosion fatigue in sea water Sample from exemplar nut Room temperature Al protection ΔK ~ 18 ksi in Fracture mode Transgranular feathery Strong intergranular component

25 Candidates: Corrosion Fatigue Crack branching Corrosion fatigue A553B pressure vessel steel Fracture mode Transgranular Branching from inclusions - Wu and Kanada, NIMS, Tsukuba, Japan

26 Corrosion Fatigue Corrosion fatigue of a hip implant Compacted corrosion products on fracture surface Fatigue striations

27 Corrosion fatigue in a subsea structure Thumbnail crack and beach marks on 432 Corrosion product dense on thumbnail crack surface