Frequently Asked Questions
Weldability is a measure of how easy it is to make a weld in a particular parent material, without cracks, with adequate mechanical properties for service, and resistance to service degradation. It varies with many factors, as indicated in What is weldability?
For many practical purposes, the weldability of one material is a comparison with another, where the first material is one that has been welded successfully before. However, to quantify the weldability of a material, a series of tests - each specific to one aspect of weldability - is available.
For C-Mn and low alloy steels, the first concerns about weldability are those of fabrication cracking. These steels can be susceptible to solidification cracking, liquation cracking, hydrogen cracking, lamellar tearing and reheat cracking.
To assess the risk of solidification cracking, a 'circular patch' test can indicate susceptibility (see Fig.1). A circular test patch is removed from the centre of the test material and is bolted to a large restraining plate. The test material is positioned such that the gap around the test patch is constant, and is fillet welded in position on the restraining plate. A weld between the patch and the original plate is made, following the circumference of the patch. Centre-line and other solidification cracking can be observed, and the sample tested to determine whether any buried cracking exists.
In order to assess the risk of hydrogen cracking, a number of tests (and non-test methods) are available. EN 1011-2:2001 Annex C describes two methods for determining the welding parameters to avoid hydrogen cracking. This is available online in the Preheat toolkit. If testing is required, a number of options are available: A Tekken or Y-groove test is a highly restrained butt-weld test for HAZ (heat-affected zone) hydrogen cracking; the CTS (controlled thermal severity) test is a highly restrained fillet weld test. Test selection will depend on the material and joint configuration of interest, and details of these tests are in JIS Z 3158:1993(R2013) and BS 7363:1990 AMD2004. A number of conditions are selected in order to determine the cracking threshold, e.g. one preheat level, multiple heat inputs or multiple preheat levels with one heat input.
Large-scale testing, such as the scale butt test and full scale weld metal cracking tests may also be appropriate. It is important to know how much hydrogen is generated by the consumable too, this is described in 'How do I measure the diffusible hydrogen level in my ferritic steel weld'.
For lamellar tearing, the critical parameter is the inclusion content and type in the parent plate. A metallographic section to assess the inclusion type or a short transverse (through-thickness) tensile test will assess the risks. Low ductility in the short transverse direction is indicative of a possible problem with lamellar tearing.
There are no standardised reheat cracking tests, but a number of weld metal and HAZ tests have been proposed. If you are concerned about testing for reheat cracking, email contactus@twi.co.uk for details of the potential test methods.
For austenitic stainless steels, the principal concern is that of solidification cracking during welding. This can be tested using the 'circular patch' test, but selection of welding parameters to minimise this, along with filler metals which result in low ferrite content deposits rather than fully austenitic, can reduce the problem. See Weldability of materials - Stainless steel.
Aluminium is also susceptible to solidification cracking. Susceptibility can be measured by using the Houldcroft fishbone test, in which slots of increasing length are machined in the test-piece, reducing the strain as the weld progresses. It is principally for TIG welding, and can be used for other non-ferrous metals.
The tests outlined above are shown in Fig.1.