Fig. Dissimilar metal weld joint testing
Testing
Destructive and non destructive testing
Testing of welds is required for qualification, procedure establishment and design purposes. Tensile, bend, impact, fatigue, stress-rupture, and fracture toughness tests are very useful for welded joints. Metallographic examination is required for microstructure analysis and failure investigation.
Cold cracking tests like CTS, implant tests and hot cracking tests are required to establish weldability under severe conditions.http://www.onlinelibrary.wiley.com
Corrosion tests are done for checking the service performance under different corrosive media.
NDE (Non destructive examination) is used for production quality control:Visual,Liquidpenetrant,Magneticparticle,Ultrasonic,Radiography testing are widely used.http://www.ndt.org
Load testing,Hydro static testing,Pneumatic testing,Leak testing are used for assessment of product quality.
Welding inspection procedure
1.Drawings,Instructions,specifications
2.Quality assurance,Quality control
3.Welding procedures and personnel qualification
4.Production test samples
5.Test results
6.Roprts and records
7.Safety procedures.
8.Total quality management
Welding discontinuities
Manual metal arc welding faults
The defects commonly encountered in MMAW are:
• weld cracking
• porosity
• slag inclusions
• lack of fusion
• insufficient or excessive penetration
• contour faults
• undercut
• excessive spatter
• stray arcing.
NDE for defect analysis
1.DPI(Liquid penetrant inspection)
Used for defects open to surface: Blow holes,Surface cracks
2.MPI(Magnetic particle inspection)
Used for surface cracks and near to sub surface cracks
3.Radiography
Used for internal defects:Porosity,Cracks,Slag inclusions,Tungsten inclusions,Incomplete penetration
4.Ultrasonic inspection:
Used mainly for internal defects: Cracks,Incomplete penetration,
Lack of fusion etc.
Hot cracking susceptability
Varestraint Test.
The varestraint test is one of the most commonly employed to evaluate hot cracking. This test utilizes external loading to impose plastic deformation in a plate while an autogenous weld bead is deposited by fusion welding on the long axis (longitudinal) or short axis (transverse) of the plate. The severity of deformation is varied by changing the bend radius. The magnitude of strain that causes cracking is an indicator of the hot cracking susceptibility.
Upon cooling, the weld is examined for cracks using a low power microscope. Three criteria—the cracking threshold, the maximum crack length, and the total combined crack length—are used to evaluate test data.
Controlled Thermal Severity Test(Hydrogen induced cracking)
The controlled thermal severity (CTS) test is used to evaluate susceptibility to cracking in carbon, carbon-manganese, and low-alloy steels. It is used to evaluate the effect of welding consumables, welding heat input, and heat treatments on the crack susceptibility of the heat-affected zone.
In this test, a plate is bolted and anchor welded to a second plate with two fillet (lap) welds, The fillet located at the plate edges is bithermal in that it has two paths of heat flow. The lap weld located near the middle of the bottom plate is trithermal as it has three paths of heat flow, thus inducing faster cooling. Further control of the cooling rate is possible by varying the plate thicknesses or using preheat.
When the specimen has cooled to ambient temperature, it is sectioned and inspected for cracks, and hardness is measured in the weld metal and the heat affected zone.
Creep rupture test
Two types of tests are typically employed to define creep-rupture properties. The creep test is used to determine the amount of deformation as a function of time. The rupture test measures the time for fracture to occur. These test methods are often combined into a creep rupture test in which both types of data are obtained. The creep rupture test measures time to failure at a given applied load, although final elongation and reduction of area are also determined. Testing is performed at a constant tensile force and constant temperature.
The test specimen used to determine creep rupture strength is a smooth, unnotched specimen that may be an all-weld-metal specimen or a transverse weld specimen. However, transverse creep tests are seldom conducted because the variations in elongation in each region of the welded joint render total elongation measurements.
The testing machine must apply the constant load even as the specimen elongates. Thus, load control must be used with universal tension testing machines. In addition, shock loads, torsion, and bending (e.g., from nonaxially aligned specimens and grips) must be avoided. The apparatus must provide constant temperature throughout the test period.The time to failure of each test is plotted against the applied stress, and the best straight line depicting the lower limit of stress is drawn for each test temperature. The time to failure at a constant temperature increases with decreasing stress level and decreases with increasing temperature. The linear relationship between the logarithm of stress and the logarithm of time is reliable provided metallurgical reactions do not occur dur- ing the life of the test.