Welding Stainless Steel
Stainless steel is widely used in many industries and applications, due to its corrosion resistance, strength, and ability to be sanitized. Yet stainless has a reputation for being tricky to weld. With an understanding of the specific application and the alloy to be welded, stainless steel actually can be one of the best metals for welding.
Stainless steel includes a minimum of 10% chromium, a highly reactive metal. The chromium on the surface of a piece of stainless oxidizes (rusts) quickly in the presence of oxygen or water molecules in the atmosphere. These oxidized chromium molecules form a thin, tight film, called a passivation layer that acts as a barrier against the surrounding air, preventing any further oxidation of the steel.
During the welding process, the passivation layer is breached. But instead of reforming, chromium in the stainless will combine with carbon. This can leave areas more susceptible to rust, and no longer “stainless.”
Prevent warping
Stainless steel has a low thermal conductivity, which means that a lot of heat can build up quickly in one spot. During the welding process, this can cause warping – affecting both the shape and strength of the final piece. For this reason, it’s important to keep the temperature of the workpiece within the recommended range. It also helps to choose a low-carbon stainless grade, such as 304L.
One way to prevent overheating is lining the work pieces by clamping an aluminum or copper “chill bar” along both lengths. The aluminum has a much faster heat absorption rate, so will tend to draw heat quickly away from the stainless. If the workpiece is a small-diameter tube or an enclosed piece with no outside access, an argon gas purge can be used to cool it down.
In addition to these measures, using the correct settings on the welding machine and moving the welding torch along at a good pace will help prevent overheating and warping.
When preparing a stainless steel workpiece for welding, use a stainless steel wool or a stainless wire brush to remove any oxides on the surface. Keep in mind that any wire brushes or other tools that have been used on regular carbon steel should never be used on stainless, because particles of carbon will react with the chromium.
Welding Different Types of Stainless Steel
Different types of stainless steel contain different alloying materials in different proportions. Each of these affect the weldability of the alloy. The five basic types of stainless include austenitic, ferritic, duplex, martensitic, and precision hardening. Each of these types has a different internal microstructure, which affects their welding properties. Even different alloys within groups require different welding procedures.
Austenitic Stainless
The most common type of stainless steels are the chromium-nickel austenitic alloys (300 series), including 304 and 316 stainless. Of these, Grade 304 is the most versatile and most widely used, with excellent forming and welding characteristics.
Grade 304L, the low carbon version of 304, prevents carbide precipitation which could act as an inroad to corrosion. It does not require post-weld annealing and so it is extensively used in heavy gauge components (typically over 6mm).
Grade 316 is the standard molybdenum-bearing grade, second in importance to 304 amongst the austenitic stainless steels. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304. It has excellent forming and welding characteristics.
Austenitic stainless steels can be welded fairly easily. However, magnetic clamps can’t be used to hold workpieces during welding since austenitic stainless is non-magnetic.
In general, austenitic stainless should be welded using a metal filler containing ferrite, which can help prevent cracking of the cooled weld. For 304 stainless, choose 308 or 308L filler. For 316 stainless, use 316 filler. When joining an austenitic stainless piece to a carbon steel piece, choose 309L filler. However, check with the filler manufacturer for specific recommendations.
Ferritic Stainless
Ferritic stainless alloys have little or no added nickel but have a high percentage of chromium. This makes them more likely to crack when heated during the welding process, especially for workpieces thicker than 6 mm. They also can become brittle in the heat-affected zones. Less heat should be applied in order to minimize cracking.
Filler metals appropriate for welding ferritic stainless include 409 and 430. When welding joints of dissimilar metals, types 309 or 312 can be used.
Duplex
These hybrid alloys are called duplex because the composition is about 50% austenitic and 50% ferritic stainless with grains of both types. Duplex alloys can be welded successfully, as long as the amount of heat applied remains within the recommended boundaries.
Martensitic
Like ferritic alloys, martensitic stainless contains a higher percentage of chromium, plus a significant amount of molybdenum. These alloys are susceptible to cold cracking due to the presence of hydrogen bubbles trapped in the weld. Heating the weld after laying it down should help the hydrogen diffuse from the metal before it cools and cracks. A low-hydrogen welding process, such as GMAW or GTAW, also can help prevent this, as well as the use of hydrogen-controlled metal fillers.
For the best results, choose a metal filler that closely matches the chromium and carbon percentages of the workpiece metal. For example, use filler type 410 with Grade 402, 410, 414, and 420 martensitic stainless. Types 308, 309, and 310 can also be used.
Precipitation Hardening (P-H)
This Martensitic type stainless has been made even stronger by adding aluminum, copper, and niobium. P-H alloys are heat-treated in a precipitation hardening process to boost their overall strength, while retaining high corrosion resistance similar to Austenitic types.
The metal filler used should be closely matched to the alloy. Heat treating welds of P-H steels can improve the weld’s performance.
Industrial Metal Supply stocks a full range of stainless steel, as well as welding equipment and supplies. Contact us today or visit one of our six locations in Southern California and Arizona.
