The Importance of Following the Advised Parameters when Welding Stainless Steels.

By Peter Stones CEng IWE/EWE IEng MWeldI

Adhering to advised welding parameters, such as heat input, (related to Volts, Amps and travel speed) and interpass temperatures, is crucial when welding stainless steels due to their specific metallurgical characteristics and susceptibility to various defects. Generally, the key benefits of stainless steels and the reasons why they are utilised are the corrosion resistance and the mechanical strength. The integrity and performance of stainless steel welds heavily depend on the parameters used, and have a large impact on the mechanical properties, corrosion resistance, and overall quality of the weldments.

Heat Input

Heat input, a measure of the energy introduced into the material per unit length of the weld, significantly impacts the microstructure and hence the properties of stainless steel welds. Stainless steels, particularly duplex grades, have precise thermal properties and melting points. High heat input can lead to excessive grain growth, resulting in a coarse-grained microstructure that reduces toughness and ductility. It can also promote the formation of detrimental phases, such as sigma phase or carbide precipitates, which can compromise both the material's corrosion resistance and mechanical properties.

Conversely, too low heat input might result in insufficient fusion and poor weld penetration, leading to weak joints and increased susceptibility to cracking. Maintaining the advised parameters for Volts, Amps and travel speed, ensures a balanced thermal profile, promoting optimal grain size and preventing the formation of unwanted phases. This balance helps achieve desired mechanical and corrosion resistant properties and remember that these parameters have been proven to work.

Interpass Temperature

Interpass temperature is the temperature of the weld area between successive welding passes. Controlling interpass temperature is essential to prevent overheating, which can cause adverse metallurgical changes. High interpass temperatures, (welding the second and subsequent weld beads on top of a previous weld without allowing time for the metal to cool down to the advised temperature,) can lead to the accumulation of heat in the material, exacerbating the issues associated with high heat input, such as grain coarsening and phase transformations.

In practice, this occurs when the welder wants to get an additional pass in the joint before the end of shift, so ignores the advised interpass temperature. In the other extreme, for the same reason, the welder wants to weld an additional pass, but taking heed of the interpass temperature, cools the joint with water. Rapid cooling creates large grain growth in the microstructure with fewer grain boundaries and consequently the metal loses its impact toughness and becomes brittle.

For duplex stainless steels, in particular, controlling interpass temperature is critical for maintaining the correct balance of austenite and ferrite phases. High interpass temperatures can shift this balance, leading to a higher ferrite content, which reduces the toughness and corrosion resistance of the weld. In contrast, maintaining the recommended interpass temperature helps ensure that the microstructure remains stable, and the properties of the weld are preserved.

Corrosion Resistance

One of the key benefits from using stainless steels are their corrosion resistance, which depends on the formation of a passive chromium oxide layer on their surface. Improper welding parameters can disrupt this layer, leading to localized corrosion such as pitting or crevice corrosion. High heat input and excessive interpass temperatures can cause chromium carbide precipitation at grain boundaries, depleting chromium in those regions and creating sites for intergranular corrosion. Adhering to the advised welding parameters helps maintain the integrity of the passive layer, ensuring long-term corrosion resistance.

Stress Corrosion Cracking (SCC)

Stainless steels, particularly austenitic grades, are susceptible to stress corrosion cracking under certain conditions. Improper heat input and interpass temperature control can introduce residual stresses and thermal gradients, increasing the likelihood of SCC. By following recommended parameters, welders can minimize residual stresses and reduce the risk of SCC, thereby enhancing the reliability of the welded structures.

Practical Considerations

In addition to the metallurgical benefits, adhering to advised welding parameters ensures compliance with industry standards and specifications. This adherence is crucial for achieving certification and meeting the stringent requirements of applications in industries such as oil and gas, nuclear power, and food processing. It also facilitates consistency in weld quality, reducing the need for costly repairs and rework.

Conclusion

In summary, adhering to advised welding parameters for heat input and interpass temperatures is vital for achieving high-quality welds in stainless steels. Proper control of these parameters ensures optimal microstructure, preserves mechanical properties, and maintains corrosion resistance. It also minimizes the risk of defects such as cracking and phase transformations, thereby enhancing the performance and longevity of welded structures. Ensuring these parameters are followed is essential for the success and reliability of stainless steel welding projects.