Frequently Asked Questions
Because of the large weld pools and high welding speeds often associated with submerged arc welds, solidification or 'hot cracking' may be encountered and is usually found along the centreline of the weld.
Solidification cracking is controlled by the composition of the weld, its solidification pattern and the strain on the solidifying weld metal. The problem is aggravated by the presence of phosphorus, sulphur and carbon and if these elements are known to be present in the parent material in higher amounts than usual, a change should be made to a wire with a higher manganese content and steps taken to minimise dilution and ensure good weld bead profiles. The most dangerous element is carbon which, if other considerations allow, can be kept low in the weld by use of high silica fluxes, i.e. manganese and calcium silicate types. Alternatively, if the carbon level is not too high, a basic flux would be more preferable as this can help to reduce weld metal sulphur levels. Sometimes useful improvement to the weld metal composition can be obtained by selecting a wire that is particularly low in carbon, sulphur and phosphorus, so as to reduce the risk of cracking.
The weld bead shape also has a critical effect. Deep narrow welds, with high depth to width ratios, are prone to centreline cracking, Fig.1. Mushroom-shaped beads as shown in Fig.2 should also be avoided.
Fig.1. Form factor for SA weld beads:
a) W > d giving tendency for surface cracks;
b) W < d giving tendency for centreline cracking;
c) W/d ≈ 3/2 giving sound welds
Fig.2. Mushroom shaped weld penetration resulting from high voltage combined with low speed
A formula has been developed to predict the cracking tendency of SAW weld metal composition. ( Ref. 1 ). The crack susceptibility, in arbitrary units known as units of crack susceptibility (UCS) has been related to the composition of the weld metal (in weight%) as follows:
230C + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1
This formula is valid for weld metal containing the following:
C 0.03 to 0.23 (NOTE: Contents of less than 0.08% to be taken as equal to 0.08%)
S 0.010 to 0.050
P 0.010 to 0.045
Si 0.15 to 0.65
Mn 0.45 to 1.6
Nb 0 to 0.07
Alloying elements and impurities in the weld metal up to the following limits do not exert a marked effect on values of UCS:
In the above formula, values of less than 10 UCS indicate a high resistance to cracking and above 30 a low resistance. Within these approximate limits the risk of cracking is higher in weld runs with a high depth/width ratio, made at high welding speeds or where fit-up is near the maximum allowable.
For fillet weld runs having a depth/width ratio of about 1.0, UCS values of 20 and above indicate a risk of cracking whilst for butt welds the values of about 25 UCS are critical. Decreasing the depth/width ratio from 1.0 to 0.8 in fillet welds may increase the allowable UCS by about 9. However, very low depth/width ratios, such as are obtained when penetration into the root is not achieved, also promote cracking.
Cracking is normally only a problem in root runs, as in Fig 3a, where dilution of parent plate into the weld is high giving excessive carbon contents. Long and deep weld pools as in Fig.3b or welds made at high welding speeds or with high restraint and large gaps as in Fig.3c, accentuate the problem. Conversely, a combination of high arc voltage and slow welding speed can produce a mushroom shaped weld bead with solidification cracks at the weld bead sides.
Occasionally a groove may be found on the surface running along the centre of the weld. This may be caused by shrinkage and although it is sometimes mistaken for incipient solidification cracking it is actually only superficial.
Ref.1 BS EN 1011-2: 2001 'Welding - Recommendations for welding of metallic materials - Part 2: Arc welding of ferritic steels'.