DESTRUCTIVE TESTING B

DESTRUCTIVE TESTING ON WELDING TEST COUPEN- PART:B

HARDNESS TESTING

·         Hardness of the material is the resistance to plastic deformation.

·         If hardness is above the specified limit then the material may get crack.

·         There are three methods of hardness testing.

1.      Vickers hardness test

2.      Rockwell hardness test

3.      Brinell hardness test

·         Both Vickers and Brinell method are suitable for carrying out hardness testing.

·         For hardness testing requires indenter to measures the hardness in the base metal (on both side of the weld) in the weld metals across Heat affected zone.

·         Hardness values are on the test reports are as follows

·         220 HV10: hardness 220 by Vickers method of 10 kg indenter load.

·         30 HRC: hardness 30, Rockwell method, diamond cone indenter (C)

·         230 HBW: Hardness 230, Brinell method, tungsten ball indenter.          

  

 

CRACK TIP OPENING DISPLACEMENT

·         CTOD test measures material fracture toughness.

·         This measured data is essential for making an appropriate decision when a crack is discover during inspection of equipment that is in service.

  

TEST SPECIMEN

·         It is a rectangular or square shape bar cut transverse to the axis of but weld.

·         A v- notch is machined at the center of the bar which will be coincident with the test position-weld metal or HAZ.

·         A shallow saw cut is then put into the bottom of the notch and the specimen is then put into the machine that induces a cyclic bending load until a shallow fatigue crack initiate from the saw cut.

TEST METHOD

·         The specimen is tested on a temperature below the ambient temperature.

·         Specimen temperature is control by immersion into the bath having liquid that is cooled at required temperature.

·         A load is applied to the specimen cause bending and induce a concentrated stress at the tip of the crack.

·         Clip gauges attached to the specimen across the mouth of the machined notch.

·         It gives reading of the increase in the width of the mouth of the crack as the load is gradually increased.

·         Fracture toughness is expressed as the distance that the crack tip is opens without initiating of the brittle crack.

·         The clip gauges enable a chart to be generated to showing the increase in width of the crack mouth against the applied load from which a CTOD value is calculated.

                      

ACCEPTANCE CRITERIA

·         An application standards or clients specifications of a minimum CTOD value that indicates ductile tearing.

·         A very tough steel weldment will allow the mouth of the crack to open widely by ductile tearing at the tip of the crack whereas a very brittle weldment will tends to fracture when the applied load is quite low and without any extension at the tip of the crack.

·        CTOD value is expressed in mm.