To make a sound weld, without hot cracking, it is generally desirable to have a small amount of ferrite in the weld metal, approximately 3-5%. This ferrite presence may be sufficiently high enough to cause the weld joint to be slightly magnetic.
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2. When do you use overmatching welding products?

During welding, elements can be lost through the arc and the resulting weld chemistry may not match the base metal chemistry, thereby lowering critical corrosion resistant elements such as chromium, molybdenum and nitrogen. There may be loss of other critical characteristics, such as impact resistance. Consequently, it may be necessary to weld a joint with welding products, richer in these critical elements to compensate for these losses. These situations exist when welding the duplex stainless steels or when welding corrosion resistant alloys where the molybdenum content needs to be maintained above a certain concentration e.g. welding super-austenitic alloys (6% Mo) with filler metals with 9-16% Mo.
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3. Why do I need to use Type 309 or 310 filler metal when welding the 300 series stainless steel to carbon steel?

Carbon steel is a ferritic alloy, while the 300 series stainless steels are austenitic. In making a weld, small quantities of each base metal are melted and contribute to the weld. (In the case where no filler metal is used – autogenous welding, the weld will have 50% ferrite and 50% austenite, which can result in a brittle structure of mixed phases i.e. martensite and ferrite). Adding a fully austenitic , overmatching welding product, such as 309 or 310, which may constitute 50% of the molten weld pool while smaller percentages are supplied from the base metals being welded, the resulting weld chemistry will remain predominantly austenitic, with a smaller varying quantity of ferrite. Each dissimilar weld combination should be evaluated on their own merit to determine the best selection of filler materials and acceptable levels of ferrite in the final weld.  
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4. What is TIG and MIG welding?

The acronyms stand for tungsten inert gas (TIG) welding and metal inert gas (MIG) welding. These processes can be manual, semi-automatic or fully automatic processes e.g orbital welding using the TIG process is fully automatic.  back to main menu

5. What is SMAW and SAW welding?

The acronyms stand for shielded metal arc welding (SMAW) using coated electrodes, and submerged arc welding (SAW), which uses fluxes for the protection of the arc and the molten metal. back to main menu

6. What is the Schaeffler diagram?

The is a graph plotting the nickel equivalence (austenitic elements Ni, N, Mn) and the chromium equivalence (ferritic elements, Cr, Si, Mo) weld metal chemistry to predict the theoretical phase components for the final weld.  back to main menu

7. How can you weld duplex stainless steels and ensure that the weld joint retains equivalent properties?

The duplex stainless steels have typically 50% ferrite and 50% austenitic structures, allowing strength and corrosion resistance. To ensure that the weld has comparable properties, filler metals with overmatching chemistries (higher nitrogen, nickel and molybdenum) are selected to ensure the phases are balanced in the final weld structure. In somne cases, higher alloyed austenitc welding products are used to ensure that the duplex stainless steel welds have comparable or better corrosion resistance than the base metals.  back to main menu

8. Why do stainless steel weld joints have to be prepared for welding?

The tightly adherent oxide associated with stainless steel has a higher melting point than the base metal. This would require higher heat imputs than desired or recommended to melt these oxides. Consequently, oxides should be removed from the adjacent surfaces to the weld, before welding, to ensure sound welds without oxide being taken into the weld pool and resulting in oxide inclusions.  back to main menu

9. Do I have to remove heat tint after welding?