[ Welding ]
[ Welding ]
upon cooling . Precautions must be taken to avoid cracking in the HAZ , especially in thick section components . High hardness in the HAZ makes this type of steel very prone to hydrogen cracking . When the joint cools to room temperature , the weld is completely transformed to untempered martensite . The risk of cracking increases with the carbon content and is combated using hydrogen-controlled fillers . For thicker sections and higher carbon materials , pre- and post-weld treatments are carried out to reduce the risk of cracking [ 2 ] . The filler metals are selected to match the chromium and carbon content of the martensitic steel .
Figure 1 . Different categories of stainless steel- their chemical composition
aluminium , titanium , copper , niobium or molybdenum . It is a special alloy produced by vacuum melting . Different categories of stainless steels with their chemical compositions are depicted in Figure 1 .
Mechanism of welding of stainless steel It is essential to acquire details of the composition and properties of stainless steel to comprehend the welding mechanism , which depends upon :
•• Carbon content
•• Alloying elements
••
Grain coarsening at high temperature
•• Type of filler and its composition
•• Pre- or post weld heat treatment
••
Hardening effect of heat on the welded joint As the composition of stainless steel changes from one category to another , the welding mechanism changes accordingly .
Ferritic stainless steel Ferritic stainless steel is predominantly single-phase and non-hardenable and is readily fusion welded . It undergoes rapid grain growth at high temperatures , leading to brittle , heataffected zones ( HAZ ), and it therefore has poor weldability . Usually , ferritics are welded in thin sheets or sections less than 6 mm thick . Filler metals that match or exceed the chromium level of the base alloy should be used . Ferritic stainless steel may crack during welding due to excessive grain coarsening , leading to poor toughness in the heat-affected zone . Caution is required for welding thin sections . With thicker sections , a low heat input can minimise the size of the grain coarsened zone and , therefore , sensitivity to cracking [ 1 ] . Ferritic stainless steel is generally welded using the TIG process , offering high quality , versatility and longevity . The low heat input of the TIG process makes it ideal for thin material .
Martensitic stainless steel There are two types of martensitics : lowcarbon ( between 0.05 to 0.35 %) high strength and high-carbon ( between 0.60 and 1.5 %) high hardness . Martensitics are vulnerable to cool cracking and do not weld easily . Low-carbon martensitic stainless has high strength and good weldability , whereas high-carbon martensitic stainless has low weldability and toughness and increased carbides . This steel becomes hard and brittle
Austenitic stainless steel Austenitics have a crystalline structure and a face-centred cubic lattice structure . It has high weldability , ductility , toughness and formability . Austenitic stainless steel is readily welded using any arc welding process , namely TIG , MIG , MMA or SA . Since it is non-hardenable on cooling , it exhibits good toughness , and it does not require pre- or postweld heat treatment . Normally , fillers with a matching composition to the base material are used . This steel is a single-phase FCC material ; it can be strengthened by cold working and cold solution strengthening .
Duplex ( austenitic + ferritic ) stainless steel Duplex is an almost equal mixture of ferrite ( bcc ) plus austenite ( fcc ), combining the toughness and weldability of austenite with the strength and resistance to localised corrosion of ferrite . It has higher tensile and yield strengths and better weldability and formability than austenitic and ferritic stainless steels . Duplex stainless steel ’ s weldability is better than ferritic stainless steel ’ s and not as good as austenitic stainless steel . Duplex is welded with a relatively high heat input and low interpass temperatures . Welding temperatures should be chosen carefully , as too much heat can compromise the structural integrity of the steel . Weld filler is selected more carefully since filler metal cools much more quickly than the base metal .
Precipitation hardening stainless steel PH stainless steel is a combination of martensitic and austenitic stainless steel . While its weldability is not on par with
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