Slide 6 of 12
Weld metals used for overlays are alloyed in such a way as to yield different Ferrite Numbers or amounts of Ferrite in the overlay. The little magnets are representative of the magnetic response in these weld metals. The greater the ferrite number the greater the magnetic response and the greater the cracking resistance. 308 and 309 are probably the most popular grades used in joining, but they may be used for overlays. For highly alloy base plates or substrates, 309 is usually selected. You can think of ferrite as as garbage collector, collecting all the tramp elements that cause cracking. The more the overlay contains these garbage collectors, the more the alloy will resist cracking.