Welding

Welding Stainless Steels

Stainless steel is renowned for its strength, durability, excellent corrosion resistance, and presents a versatile material for a wide range of applications. However, welding stainless requires careful consideration of pre-welding preparation, suitable welding techniques, and post-welding practices to preserve its mechanical properties and corrosion resistance. With various types of stainless steels available, understanding factors such as alloy composition, thermal expansion, and distortion is essential in achieving successful stainless steel welds.

Pre-welding preparation

Pre-welding preparation is critical to ensure favourable outcomes. Ensure a clean, dry, and uncontaminated weld area, free of carbon steel, zinc, paint, oil, grease, crayons, etc. This is especially important for low-carbon and stabilised grades and may require degreasing with a no-residue solvent.

Welding techniques

Several techniques are commonly employed for welding stainless steel, including:

Gas metal arc welding (MIG or GMAW).
Gas tungsten arc welding (TIG or GTAW), which is slower but more precise than GMAW, but, like MIG/GMAW,
requires freedom from drafts to maintain the gas shielding.
Shielded metal arc welding (SMAW or stick welding) which requires slag removal both between passes and after welding,
as well as regular replacement of the consumable "stick".
Submerged metal arc welding (SAW) can be used for plate - but heat input can be high enough to cause unwanted phase formation in highly alloyed areas.

Each welding process yields different results, and choosing the best welding technique for the application will generally depend on the thickness of the stainless steel used, speed and efficiency requirements, and the final appearance specified.

Welding consumables

Selecting the appropriate filler metal is crucial. Filler wires are specially produced to be slightly over-alloyed so that the cast microstructure of the weld bead will match or exceed the corrosion resistance of the wrought microstructure of the base or parent material. For austenitic stainless steels, the filler also minimises the risk of hot cracking by ensuring necessary levels of high-temperature ferrite are present. However, the welding technique must also control heat input and interpass temperature, as well as optimise the bead profile to avoid any concavity.

For duplex alloys, austenite forming nickel (and possibly manganese) in the consumable and nitrogen in the gas shield are standard techniques. It is important to avoid contamination and mix-up of filler wires when welding stainless steel. Fluxed electrodes must be kept dry in their original packaging or, less desirably, dried in an oven.

Post-weld surface treatments

Welds and the surrounding discoloured areas are treated to improve corrosion resistance and visual appearance. While surface mechanical treatments like wire brushing, buffing, or light abrasive blasting are often applied, it is important to note these methods do not reliably remove the low chromium, low corrosion resistance layer beneath the heat tint/weld scale. For optimal corrosion resistance, additional treatments such as pickling or passivation are usually required after mechanical treatment.

Special considerations for welding stainless steel

Welding stainless steel requires specialised knowledge and practices due to its unique attributes. Below are some important factors to keep in mind:

Passive film/passivity

Stainless steels rely on a very thin surface passive film for their corrosion resistance. It is vital to maintain and preserve the integrity of the passive film.

Avoid mechanical damage and contamination.
Repair any affected areas (e.g., high-temperature scale adjacent to a weld, mechanically damaged or ground or contaminated areas)
by pickling and, if exceptional corrosion resistance is required, an additional passivation.
Ensure a constant and sufficient availability of oxygen at the surface of the stainless steel.

Thermal conductivity

All stainless steels have much lower thermal conductivity than that of carbon (mild) steel (plain chromium grades approximately one-half and austenitic grades approximately one-third). This must be considered for any operation which involves high temperature, e.g., effects during welding (control of heat input) and longer times required for heating to attain a uniform temperature.

Expansion coefficient

Plain chromium grades have an expansion coefficient similar to carbon (mild) steels, but that of the austenitic grades is about one and a half times higher. The combination of high expansion and low thermal conductivity means that precautions must be taken to control distortion during welding, e.g., use low heat input, dissipate heat by use of copper backing bars and use adequate jigging. This factor must also be considered for components that use a mixture of materials, e.g., a heat exchanger with a mild steel shell and austenitic grade tubes.

Fabrication and installation

Adhering to best practices is essential in stainless steel fabrication and installation to ensure the passive film is restored after fabrication. These include:

Contaminant control - including carbon steel grinding dust on equipment or tooling/clamps used on carbon steel, etc.
Cutting techniques
Proper handling procedures
Storage
Installation and cleaning
Maintenance schedule to ensure long-term performance.

By understanding these key aspects and employing best practices, welders can harness the full potential of stainless steel while ensuring strong, durable, and corrosion-resistant joints.