Let us consider two examples here:
g In the first example, let us say that the SAW process is being used to make a butt joint in a
1” thick plate using the following parameters: 600 A, 30 V, 18in/min travel speed, 100°C
preheat. Therefore, arc energy = 600 x 30 x 60 / 18 x 1000 = 60 kJ/in. Now, going to
Figure 5.27(a), we start at point A at 60 kJ/in at the bottom line and go up vertically to hit
the line for 1” thickness at point B; next, we go towards the right from point B until we
intersect the 100°C preheat line at point C. Now go up vertically again and at point D, read
the cooling rate as about 11°C/sec at 540°C (1000°F). The cooling rate is estimated at
540°C because the development of microstructures like ferrite and pearlite occurs in the
temperature range of about 500°C to 600°C during the relatively fast cooling of welds and it
is the cooling rate in this temperature range that is important.