The weld bead formed by laser welding of steel is hard and narrow compared with that produced by arc welding processes, making application of standard weld procedure qualification tests impractical and/or inappropriate. In particular, Charpy testing of laser welds has often proved impractical because of the phenomenon of Fracture Path Deviation (FPD) in the specimen. At very low temperatures, a Charpy specimen notched in the centre of the weld bead will fail in the plane of the notch in the normal way. As the test temperature is increased, so there is an increased tendency for the plane of failure of the specimen to fall outside the welded region, so that the energy absorbed in the process is no longer representative of the toughness of the weld (see Fig.1).
The occurrence of FPD in laser welds has been recognised for many years and is generally considered to be related to:
- the hardness overmatch of the weld metal (the higher the ratio of weld metal hardness to parent metal hardness, the greater is the tendency to FPD),
- the width of the weld zone (the narrower the weld zone, the greater is the tendency to FPD),
- the inherent toughness of the weld (for a given weld, behaviour changes with increasing test temperature, from 'in-plane' to FPD); it seems reasonable to assume that the actual toughness of the weld is simultaneously increasing, but it cannot be measured
Since qualification of weld procedures is often dependent on meeting some target value of Charpy energy, various attempts have been made to modify the standard Charpy test for use with laser welds (and with other narrow-profile welds such as electron beam and orbital friction welds). Modifications which have been attempted in order to constrain the fracture path to the weld metal include:
- side-grooving or side-slitting of the specimen,
- use of a fatigue crack in place of a V-notch,
- sandwiching of the narrow weld between two side welds (supporting welds), for test purposes only,
- notching the specimen perpendicular to the plane of the weld, so that FPD is impossible (in this case, % crystallinity is measured instead of absorbed energy)
- use of an austenitizing heat treatment to harden the parent material, thus reducing the hardness overmatch between the parent and weld metal (the hardened material would be used for test purposes only).
- use of miniature test specimens
There are difficulties in interpreting test results on either standard or modified Charpy test results. Some practitioners rely on avoiding FPD, whilst others believe that FPD is indicative of reasonable toughness and thus need not be prevented. In critical cases, it may be necessary to use alternative techniques such as fracture mechanics testing or structural testing, and expert advice should be sought.
Hadley, I: 'Testing the waters - shipbuilding laser welds face toughness trials'
, Bulletin, Vol. 41, No. 5, September-October 2000. (Only available to Industrial Members of TWI.)
Hadley, I: 'Charpy Testing of Laser Welds - the Significance Of Fracture Path Deviation', Bulletin, September-October 2001, 75-78. (Only available to Industrial Members of TWI.)
Hayes B, Norris I M and Towers O L, 1986: 'Preliminary investigation of a modified Charpy test for the assessment of narrow welds', TWI Industrial Members Report No. 308, July 1986. (Only available to Industrial Members of TWI.)
Kristensen, J K and Borggreen, K: 'Evaluation of laser welds in structural steels', International Journal for the Joining of Materials, 1996, Vol. 8, No.1, pp 48-54
Misawa, T, Takasa, S, Nakano, Y and Yasuda, K, 1996: 'Ductile-brittle transition evaluation of laser welded steel metal by means of small specimen impact test', Steel and Copper, 1996, 82/8, (in Japanese)
Sumpter, J D G: 'Fracture toughness evaluation of laser welds in ship steels', European Symposium of Assessment of Power Beam Welds (ASPOW), GKSS, Geesthacht, Germany, 4-5 Feb 1999