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Special Topic
Welding Filler Materials
Stainless steel is used for many of the welded components in a nuclear facility , including piping and pressure vessels . For operators new to GTAW of stainless alloys , they will comment that the weld pool is difficult to manipulate – “ it behaves like chewing gum !” – compared to low alloy or mild steel . The concept of over-alloying may be new as well . For those who ask , “ why is it harder to work with stainless ?”, read on .
By Peter Stones , Ieng MWeldl IWE / EWE
The addition of chromium and nickel to stainless steel welding filler metals has a significant impact on the behavior of the weld pool , often resulting in a sluggish or slow-moving weld pool . This phenomenon occurs due to several key factors related to the chemical and metallurgical properties of these alloying elements . Chromium is one of the primary alloying elements in stainless steel , known for its excellent corrosion resistance and high-temperature strength .
When chromium is present in the welding filler metal , it forms a stable oxide layer on the surface of the weld pool , commonly referred to as the passive layer . This passive layer acts as a protective barrier , preventing the base metal and filler metal from being exposed to corrosive environments . While this oxide layer is beneficial in terms of corrosion resistance , it also contributes to the sluggishness of the weld pool .
When choosing a grade of stainless steel filler metal , note that the added silicon in grades such as 308LSi , 309LSi , and 316LSi make the weld pool more fluid and easier to control .
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The passive layer formed by chromium oxide has a higher melting temperature compared to the surrounding weld pool . As a result , the weld pool containing chromium-rich filler metal requires more heat input to reach the desired molten state . This higher melting temperature increases the energy required to maintain a fluid weld pool , making it more difficult for the weld pool to flow and merge smoothly with the base metal .
Similarly , the addition of nickel to stainless steel filler metals also plays a role in making the weld pool less fluid . Nickel enhances the mechanical properties and toughness of stainless steel , making it suitable for a wide range of applications . However , nickel has a lower thermal conductivity compared to other elements in stainless steel , such as iron and chromium .
This lower thermal conductivity causes the weld pool to retain heat for a longer duration , resulting in slower cooling rates and reduced fluidity . Moreover , nickel has a higher solidification temperature , which further contributes to the sluggishness of the weld pool . As the weld pool solidifies , the nickel-rich regions tend to solidify last , forming dendritic structures that impede the fluid flow of the weld pool . These dendrites act as barriers , inhibiting the movement of liquid metal and making the weld pool sluggish .
In addition to the chemical and metallurgical factors , the weld pool ’ s sluggishness can also be influenced by welding parameters such as heat input , travel speed , torch angle and shielding gas composition .
Adjusting these parameters can help mitigate some of the sluggishness associated with chromium and nickel-rich filler metals , but it is important to keep within the heat input and interpass temperature required in the weld qualification .
For some grades of stainless steel such as 308L , 309L and 316L , there are modified grades that have a slightly higher silicon content and are referenced as 308LSi , 309LSi and 316LSi . Whilst all of the corrosion resistive and mechanical properties are the same as the ‘ L ’ grade , the added silicon has the effect of making the weld pool more fluid and making it easier to weld . There is even one grade of duplex stainless steel ( 2209 ) that has added silicon for exactly the same reason , 22.8.3 . LSi .
Over-alloying the weld filler metal grade for stainless steel can provide several benefits in certain welding applications . This process involves increasing the concentration of alloying elements , such as chromium and nickel , beyond the levels present in the base metal . Here are some advantages of over-alloying the weld filler metal :
1 . Enhanced corrosion resistance : Stainless steel is known for its corrosion-resistant properties , and over-alloying the filler metal can further enhance this characteristic . By increasing the chromium content , the passive oxide layer formed on the weld surface becomes thicker and more protective . This offers superior resistance to various corrosive environments , including high-temperature and highly corrosive applications .
10 Stainless Steel World Americas - June 2024 | www . ssw-americas . com