# HOW TO WELD STAINLESS STEELS

Stainless steels belong to a family of steels that have a series of special characteristics and uses in industry. The main characteristic of these steels is their good corrosion resistance, which is due to their 12% chromium content.

Chromium is primarily used in steel manufacturing applications to provide corrosion resistance and a bright finish. Other representative alloying elements in stainless steels include: nickel, molybdenum, niobium, and titanium; as well as deoxidizers such as manganese and silicon.

Classification:

Stainless steels have a wide range of applications; they are classified according to their chemical composition and microstructure, and are thus known as austenitic, ferritic, and martensitic stainless steels, among the most common. Duplex, super duplex, and precipitation-hardening grades also exist.

Austenitic stainless steels.

This group of steels is referred to as austenitic because it contains nickel as an alloying element, which promotes the formation of an austenitic microstructure. This group of steels is widely used across different industrial sectors.  

Their weldability is straightforward; they can be welded using any welding process depending on the level of quality required. They can be welded using SMAW, GMAW, GTAW, and FCAW processes.

Among their most representative characteristics are:

• They are non-magnetic.
• They cannot be hardened by heat treatment; they can only be hardened by cold working.
• They have greater corrosion resistance, hence their use under severe corrosion service conditions (for example, in the chemical industry).
• Due to their nickel content (which promotes austenite formation).
• The classification of these steels is covered in various tables of the American Iron and Steel Institute (AISI); the most relevant in this group belong to the 300 and 200 series, for example: 201, 202, and Series 3XX: 301, 302, 304, 314, 316, 316L, etc.

Martensitic stainless steels.

This group of steels is called martensitic due to its high hardenability, which makes them highly corrosion resistant and gives them very good mechanical strength. They are chromium alloys with approximately 17% chromium and other alloying elements; their weldability requires careful process control to prevent cracking. When a sound welding procedure is established, they can be welded using all processes, including SMAW, GMAW, GTAW, and FCAW.   

• Corrosion resistance
• Excellent hardenability to achieve a martensitic microstructure
• They are magnetic
• Resistance to softening at elevated temperatures
• Useful in high-quality knives, bearing balls, and valves
• High mechanical strength and low ductility as the metal becomes harder.
• The most representative grades are: 410, 416, 420, 431, 440.

 Ferritic stainless steels.

This group of stainless steels includes some grades from the 400 series according to the American Iron & Steel Institute (AISI); their microstructure is stable ferritic from room temperature up to the melting point. They are steels manufactured with 12% to 27% chromium, with carbon content controlled to the lowest practical percentage in order to minimize its detrimental effect on corrosion resistance. These stainless steels contain little or no nickel. The absence of nickel results in a lower cost compared to austenitic stainless steels; ferritic stainless steels are widely used in the automotive market. These steels are practically non-hardenable by heat treatment. Their weldability is somewhat complex, but as with martensitic steels, a welding procedure must be established in order to achieve an excellent joint; all welding processes can also be used, including SMAW, GMAW, GTAW, and FCAW.

• Not hardenable by heat treatment.
• Good corrosion resistance
• They are magnetic
• They are relatively economical.
• Good mechanical strength
• The most common grades are 405, 430, 442, and 446.

Recommended procedure for welding stainless steels

As with any welding process, the procedures for welding stainless steels must be strictly followed to achieve the best results.

Stainless steels owe their corrosion resistance to their chromium content.  When this type of steel is heated to a temperature between 430°C and 820°C, carbon in excess of 0.02% combines with chromium at the grain boundaries to form chromium carbides. This can cause chromium to lose its corrosion-resistant properties, and when the material is exposed to a corrosive environment, “intergranular corrosion” may occur. For this reason, the use of low-carbon alloys is recommended. These electrodes are identified by the letter L, which denotes an alloy with a low carbon content.

Other challenges when welding stainless steels include:

• Their coefficient of thermal expansion is approximately 50% greater than that of carbon steel, which causes a tendency to warp.
• Their low thermal conductivity leads to serious distortion problems and a tendency to burn through thin sheet.
• Their high electrical resistivity causes electrodes to overheat and glow red. Oxyfuel welding is not recommended; however, if no other alternative is available, low-melting-point alloys may be used, such as silver alloy (silver brazing).

• As a recommendation, cleaning to remove carbon contamination from the surface (degreasing) must not be carried out using hydrocarbons; water and soap or a suitable pickling agent may be used instead.

• For joint preparation with a bevel, use an angle grinder or a stainless steel wire brush. The grinder disc and wire brush must be kept exclusively for use on stainless steel; using these accessories on other materials can cause contamination.

Before proceeding with the welding process, the quality requirements of the joint to be welded must be assessed. The most recommended process for welding stainless steel and ensuring weld quality is GTAW (TIG). This process allows heat input to be controlled and produces excellent weld quality. As recommendations for making the joint, take the following factors into account:

• Use the smallest diameter electrode possible.
• Apply with minimal weaving.
• Use a high travel speed.
• Use heat sink backing bars (copper) at the joint overlap.