Class test - Destructive testing Flashcards
Explain why hardness tests are carried out in a welding – fabrication environment. [8]
To determine if the weld metal matches that of the parent metal and if a cross section of a joint is subjected to hardness tests, the relative values in the weld, the heat affected zone and the parent metal can be investigated.
High values in the heat affected zone will indicate a fast quench condition and may suggest the necessity of changes in the welding technique in order to increase the heat input and reduce the quenching effect and hardness.
The results may demand a change in either parent metal or electrode
Define hardness. [2]
The ability to withstand indentation
List three types of hardness testing
Brinell, Rockwell and vickers
explain why charpy impact test are employed in a welding/fabrication environment and how is the test carried out?
Impact tests are employed to determine the behaviour of materials when subjected to shock loads.
For most engineering purposes it is desirable that a metal should possess some measure of toughness, eg, that it should absorb some energy before it fractures.
The test applied is one in which a notched specimen is broken by a swinging hammer and the amount of energy required to break the specimen is recorded. The notch in the specimen will be on the opposite side to the swinging hammer
Fully describe how the thermal severity test is carried out
consists of two plates, one square and the other rectangular. The square plate, 3” on a side, is fastened by a ½” bolt through its centre to the rectangular plate and anchored by welding.
The test weld at the right side of the specimen is called the bithermal weld because the plates supply two paths for heat flow, one in each plate.
The test on the left side is called the trithermal weld since heat can flow in one direction in the top plate and in two directions in the bottom plate, or a total of three paths.
The contacting surfaces are ground before assembly to ensure efficient heat flow. After the specimen is assembled and has cooled to room temperature, the bithermal weld is deposited,
the specimen is all allowed to re – cool and the trithermal weld is deposited.
After a hold for 72 hours at room temperature, the degree of cracking is determined by measuring the crack length of three metallographic specimens cut from cross – sections of the weld.
Fully explain CTOD testing.
The test is performed by placing the specimen into three point bending and measuring the amount of crack opening. This is done by means of a strain gauge attached to a clip placed between two accurately positioned knife edges at the mouth of the machined notch.
As bending proceeds, the crack tip plastically deforms until a critical point is reached when the crack has opened sufficiently to initiate a cleavage crack.
As a rule of thumb, a CTOD value of between 0.1mm and 0.2mm at the minimum service temperature is regarded as demonstrating adequate toughness.
Name three improvement techniques with regards to fatigue. [3]
a) Toe grinding
b) Peening the surface of a weld
c) Machining the surface of a weld to make the weld concave
Describe the purpose of the following mechanical tests. [12]
a) Tension tests
b) Bend tests
c) Fillet weld tests
d) Notch toughness tests
a Tension tests are used to determine the ultimate strength of groove welded joints.
b) Bend teats are used to determine the degree of soundness and ductility of groove weld joints
c) Fillet weld tests are used to determine the size, contour and degree of soundness of fillet welds
d) Notch toughness tests are used to determine the notch toughness of the weldment
What is the purpose of the nick break test?
T he fracture surface will reveal defects such as slag inclusions, porosity, lack of fusion etc.
- List the three types of bend tests and what type of defects will they reveal. [6]
Root , face and side bends.
Type of defects – slag inclusions, porosity, lack of fusion
