stress corrosion cracking
DESCRIPTION
http://bzuiam.webs.comTRANSCRIPT
![Page 1: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/1.jpg)
![Page 3: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/3.jpg)
• Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Temperature is a significant environmental factor affecting cracking.
“OR” A structure under static tensile stress, much below the yield
stress, in contact with corrosive environment may fail due to SCC. Three conditions must be present simultaneously to produce SCC: a critical corrosive environment, a susceptible alloy and some component of tensile stress.
![Page 4: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/4.jpg)
Now the phenomena of “SCC” is shown in the following figures
![Page 5: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/5.jpg)
Cont’d
• SCC leads to the formation of a crack which would not have developed by the action of the stress or environment alone. It is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment.• The required tensile stresses may be in the form of directly
applied stresses or in the form of residual stresses.
![Page 6: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/6.jpg)
![Page 7: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/7.jpg)
Causes of SCC
• There are three main causes for SCC as given follows:
• Corrosive Environments• Tensile Stresses Above Threshold• A Susceptible Material
![Page 8: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/8.jpg)
![Page 9: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/9.jpg)
Cont’d• Common sources of tensile stresses are:1: External stresses caused by
• Centrifugal forces• Temperature variation
2: Residual stresses due to• Forming• Heat treatment• Welding• Machining• Grinding
![Page 10: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/10.jpg)
Cont’d
3: Concentration of stresses at the root of pre-existing cracks or notches
![Page 11: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/11.jpg)
Stages Involved In SCC
• The process of SCC consists of three stages:• Crack Initiation• Crack Propagation• Brittle Fracture
![Page 12: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/12.jpg)
Crack Initiation
• SCC is initiated by stress concentrations at defects on the material surface. The defect may be an existing material defect. The defect may also be a result of pitting corrosion, crevice corrosion, intergranular corrosion or local galvanic corrosion.
![Page 13: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/13.jpg)
![Page 14: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/14.jpg)
Crack Propagation
• The passivation film at the tip of the crack is broken due to plastic deformation. Pure and normally very active metal is exposed and will be attacked by corrosion. Grow rate will be a combination of corrosion and cracking. The crack starts to grow when the stress concentration at the end of the crack (KI) exceeds the threshold stress intensity factor for stress corrosion cracking (KISCC).
![Page 15: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/15.jpg)
Brittle Fracture
• When the stress concentration at the end of the crack exceeds the critical stress intensity factor (KI > KIC), there will be a rapid, unstable brittle fracture.
![Page 16: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/16.jpg)
Examples• Many materials, particularly high strength materials, are
susceptible to stress corrosion cracking when exposed to a specific environment. For example, cold worked brass, which is found in ammunition cartridges, is susceptible to stress corrosion cracking when exposed to an environment containing ammonia. In chloride containing environments, titanium alloys, aluminum alloys, and high strength stainless steels are susceptible and specific alloys, which are resistant to stress corrosion cracking, should be used. The stresses required to initiate and propagate cracking are often low and many failures occur due to residual stresses rather than applied stress.
![Page 17: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/17.jpg)
![Page 18: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/18.jpg)
Ways Of Propagation Of SCC
• SCC can proceed in either of two ways : • Intergranular Stress Corrosion Cracking• Transgranular Stress Corrosion Cracking
![Page 19: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/19.jpg)
Intergranular SCC• The cracks which propagate along the grain boundaries
called as intergranular stress corrosion cracking as shown in the following figures.
![Page 20: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/20.jpg)
Transgranular SCC
• The cracks which run through the individual grains called as transgranular stress corrosion cracking as shown in the following figures.
![Page 21: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/21.jpg)
Identification Of SCC
• Visual identification prior to failure is difficult due to the typical tightness of stress-corrosion cracks. A low-power hand lens will greatly aid determination. Detection of cracks can also be enhanced with ultrasonic, radiographic, or acoustic emission techniques.
![Page 22: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/22.jpg)
Preventive Measures• There are a number of different ways to control SCC. The method
used depends on the application and may involve changing the mechanical, metallurgical and environmental conditions.
• Mechanical• Avoid stress concentrators• Relieve fabrication stresses• Introduce surface compressive stresses• Reduce operating stresses
• Metallurgical• Change alloy composition• Change alloy structure• Use metallic conversion coating
![Page 23: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/23.jpg)
• Environmental• Add inhibitor• Modify temperature
![Page 24: Stress corrosion cracking](https://reader036.vdocuments.site/reader036/viewer/2022081421/554a4109b4c905293a8b50ab/html5/thumbnails/24.jpg)