Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 105

Adequately wide ranges of current and welding speed are available for practical
use. With some coated steels, however, the limiting factor is the tendency for
cracking to occur in the weld or in the heat-affected zone. Interrupted current
programmes, for example 2 cycles on and 2 cycles off, give a lower tendency
for cracking and consequently a wider welding range. 155 The use of interrupted
currents also reduces the tendency for surface brassing of the weld and for wire
breaks.
An alternative method for coated steels is narrow seam welding, as illustrated
in Figure I. 104, In this method, scraping devices and high pressure profile rollers
are used continuously to clean and profile the wheel electrodes. Such machines
are used for welding petrol tanks and exhaust systems. Welding speeds of up to
5 metres per minute may be achieved and narrow welds in the range 1.5-3.5 mm
may be produced. These welds are stronger than the parent steel and are suitable
for watertight and airtight applications. 155

Laser seam welding

A laser provides a uniquely concentrated form of energy and is now being used
for the manufacture of tailor-made welded blanks. Sheets of different strength,
thickness or coating type are welded together prior to pressing the component.
The method produces very narrow welds (~,1 mm wide), free from distortion,
with a minimal heat-affected zone. The welded sheets may be used to form
complex pressings and when used with metallic-coated steels, there is only
minimal damage to the coating and galvanic protection of the weld.
It has been found 156 that by using a 5 kW laser, satisfactory welds can be
achieved at speeds of 10 metres per minute for 0.7 mm strip, dropping to 5 metres

End view

Scraper

device

High-pressure

proflle roller

Sheq

r- ~'z F.:.'-'.'-'-.: ..... -:::-'.-'.'-'.'~

"~" High-pressure

e miler

~.= ~,~;~ Scraper

/,,~k device

I" ........... i

Figure 1.104 Schematic detail of electrode wheel showing scraper and profiled friction
drive wheel 155