[ Welding ]
on small and large projects . This technology uses an electrical current to heat frayed metal edges and seal them together . It is useful on metal with a low melting point because it can be adjusted to prevent the distortion of metal . Resistance or spot welding is used on sheet metal products as the contacting metal surface points are joined by the heat obtained from resistance to electric current .
• Mig / gas metal arc welding Mig offers a strong joining of two pieces of stainless steel . It is a semi-automatic process utilising an argon-rich shielding gas and a solid wire electrode . Other gas mixtures , including those with helium , oxygen and carbon dioxide , are often used to stabilise the arc and improve weld quality . A pulsed current supply makes it easier to weld tough-toreach spots on complex stainless steel projects .
In addition to having a minimum of 10.5 % chromium , stainless steel is alloyed with other elements to improve heat resistance , mechanical properties and fabricating characteristics . These elements also modify and influence weldability .
Ferritic stainless steels Ferritic stainless steels are considered to be predominantly single-phase and non-hardenable and can be readily fusion welded . At high temperatures , they undergo rapid grain growth leading to brittle heat-affected zones and have poor weldability . They are welded usually in thin sheets or sections of less than 6 mm thickness . Filler metals should match or exceed the chromium level of the base alloy . Ferritics are very suitable for use in welding especially in heat exchanger tubing because of their high thermal conductivity and low thermal expansion . Ferritics may crack during welding process due to excessive grain coarsening leading to poor toughness in the heat effected zone . No precaution is required for welding thin sections , however with thicker sections a low heat input can minimise the size of the grain coarsened zone and sensitivity to cracking 2 .
Martensitic stainless steels Martensitics are vulnerable to cool cracking and do not weld easily as they may become hard and brittle upon cooling . They can be successfully welded provided precautions are taken to avoid cracking in the HAZ , especially in thick section components and highly restrained joints . High hardness in the HAZ makes this type of stainless steel prone to hydrogen cracking . The weld gets completely transformed to untempered martensite by allowing the joint to cool to room temperature . Such steels should be welded with careful precautions . The risk of cracking generally increases with the carbon content and it can be combated by utilising a low hydrogen welding process such as TIG or MIG , or by using hydrogen-controlled fillers . The preand post-weld treatments , especially for thicker sections and higher carbon materials , may help to toughen the structure , enable the hydrogen to diffuse from the weld metal , and reduce the risk of cracking . 3 The filler metals should match the chromium and carbon content of base martensitic metal . Type 410 filler is used to weld types 402,410,414 and 420 steels . However , some martensitic grades of stainless steel are considered near impossible to weld .
Austenitic stainless steels These steels can be readily welded using any of the arc welding processes ( TIG , MIG , MMA and SA ). As they are non-hardenable on cooling , they exhibit good toughness with no need for pre- or post-weld heat treatment . Austenitics fall in the 200 and 300 series of stainless steel alloys and are highly resistant to corrosion and extremely malleable . Normally fillers with matching composition to the base material are used .
Duplex ( austenitic-ferritic ) steels The weldability and welding characteristics of duplex stainless steels are better than those of ferritic steels , although not generally as good as austenitic steels . Modern duplexes with significant nitrogen content are readily weldable . During welding one must be very careful with temperatures since too little or too much heat can compromise the structural integrity of the steel . Selecting a filler metal is slightly more challenging . Many types of duplex stainless base metals are not available as filler metals since filler metal cools much more quickly than the base metal .
Precipitation hardening stainless Precipitation-hardening stainless steels are a combination of martensitic and austenitic stainless steels . They are resistant to corrosion and capable of being hardened through heat . They possess above-average weldability . They can be hardened through heat treatment to levels comparable to martensitic steel while having similar corrosion resistance to austenitics . Although the weldability of precipitation-hardening stainless steel is not on par with the weldability of austenitic stainless steel , it is very good . They can be welded without
24 Stainless Steel World November 2022 www . stainless-steel-world . net