Why is an Impact Toughness Test required in Welding


Charpy V-notch test is used to determine the toughness of the weld output by measuring the energy to fracture a notched test sample. As toughness of steel for specific application must be adequate enough to prevent any brittle fracture loss but low alloy steels may lose their toughness at quiet lower temperature. So after welding the specimen is tested at the specific temperature to determine the relative impact toughness and consequently evaluate the weld materials. The test is applied to assess the quality of weld output. It don’t give the fracture toughness that is entailed in service estimations. 

What is toughness?
Toughness can be defined in two ways.

  • Resistance to brittle failure in a material. A tougher material will show more resistance to brittle fracture and cracks.
  • Energy required to cause a brittle failure. So more energy will be required to get brittle fracture in a tougher material.

Why impact toughness test?
In carbon and low alloy steels, change in fracture toughness is associated with the change in temperature. With the decrease in temperature, these materials turn to brittle from ductile nature. Plot of impact energies against the temperature gives ‘S’ curve, which is divided into two shelves; upper shelf and lower shelf. Above mentioned steel types show ductile behavior at the upper shelf and switch to brittle nature when the temperature falls to the lower shelf. The temperature at which the transition of ductile to brittle occurs, is called transition temperature. 
From the above discussion it is important to have a sound knowledge of temperature at which the change in fracture behavior may occur and for weld structure to perform at the upper shelf temperature. 

Specimens
Codes and standards demonstrate the dimensions required for impact test specimens. Normally they are 10mm x 5-7.5mm x 55mm. Energy values may tend to vary for the same test so three specimens are taken for each test and an average is calculated from the energy values. The location of specimens within the weld and even the position of notch on the specimen is quiet an important factor in evaluating the impact energies. 

Test Machine
Impact testing apparatus has an ‘I’ shape appearance and consists of anvil at the lower portion and scale assembly (graduated scale with a moving pointer) at the upper end. A pendulum hammer swings from a specific height and strikes the specimen placed on the anvil. 

Standards
Standards for Charpy V impact testing are BS EN ISO 148-1 and ASTM E23.

Procedure
First the specimen with notch is allowed to cool at the specific temperature by immersing in liquid/gas container. After the temperature is stabilized, the specimen is quickly shifted to machine and the hammer is impacted behind the notch. The height through which a hammer swings, is the measure of energy.  The energy in joules, utilized to break the specimen is recorded on the machine scale by the movement of a pointer on that scale.
                After taking average, a comparison is made between the test results and the values specified in codes and standards. The comparison tell us whether the specific requirements have been met or not. On the other hand this test also tell us about some toughness attributes which are;

  • Crystallinity – If fractured surface has crystalline appearance, the fracture will be brittle. More the crystallinity more the brittle fracture and vice versa.
  • Expansion – If the fracture halves from behind the notch are expanded, specimen will have more toughness. It is better to say that more the expansion more will be the ductile fracture. On the other hand less the lateral expansion more will be the brittle fracture.

Last Comments
For impact testing, correct adjustment of welding parameters is important, as values may be changed due to welding consumables, heat input, post treatments, composition and many other factors.
Aluminum alloys and austenitic stainless steel (300 series) are used in cryogenic applications because they don’t exhibit fracture behavior change as the temperature falls.

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