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Weldability of Stainless Steel

Notable for its corrosion resistance and widely used for food handling, cutlery, and many other applications, stainless steel has become one of the more popular metals in use today. The development of dozens of alloy variants has been a tremendous benefit to steel users. However, this variety has also made welding stainless steel more complicated than welding traditional carbon steel.

Don’t worry though, stainless alloys were once thought of as a major challenge to weld. But today they are described as “different” as opposed to “difficult” among most welders.

Stainless steels are high alloy steels that contain a minimum of 10.5% chromium and are usually alloyed with a number of other elements to improve and enhance heat resistance, mechanical properties, and/or fabricating characteristics. These alloying elements also modify and influence the weldability of the metal. To successfully weld this highly versatile material it is important to familiarize yourself with the various types of stainless steel and their properties. It can be divided into five main types: ferritic, martensitic, precipitation hardening, duplex, and austenitic.

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Types of Stainless Steel

AUSTENITIC

The 200 and 300 series are austenitic. They have high corrosion resistance and are highly formable but are prone to stress cracking. Also considered  the most easily weldable family of the stainless steels.

They exhibit good toughness because they are non-hardenable on cooling, and there is no need for pre or post-weld heat treatment. Austenitic alloys are normally welded with fillers with matching composition to the base material; in specific cases, type 308 filler is used for alloys 302 and 304 and type 347 filler used for 321.

Austenitic Stainless Available at Online Metals
301, 302, 303, 304, 316, 321, 347, N50 and N60

FERRITIC

All ferritic alloys are in the 400 family, but not all 400s are ferritic. They have lower ductility, are more brittle, prone to hot cracking, and posses lower corrosion resistance than the austenitic grades. But they offer higher resistance to stress corrosion cracking. This type is generally considered to have poor weldability.

Ferritic stainless steels become fully ferritic at high temperatures and undergo rapid grain growth. This leads to brittle, heat affected zones in the fabricated product. This is what gives this type its negative welding qualities.

When ferritic alloys are being welded, it is generally welded in thin sections less than 6mm thick. At this size, any loss of toughness is less significant. Thinker sections (0.25″+) have a higher risk of cracking during fusion. When welding the ferritic stainless steels, filler metals should be used which match or exceed the chromium level of the base alloy. 409 and 430 are commonly used as fillers, and austenitic types 309 and 312 for dissimilar joints.

Ferritic Stainless Available at Online Metals
430

MARTENSITIC

Martensitic stainless are 400 and 500 series. These alloys have higher strength, wear resistance and fatigue resistance than the austenitic and ferritic grades but a lower corrosion resistance. Martensitic steel becomes hard and brittle upon cooling, making it a great material for wear resistance but more difficult to weld as it has a tendency toward weld cracking on cooling. However, martensitic stainless steel can be successfully welded if careful precautions are taken to avoid cracking in the heat affected zone. The filler metals used should generally match the chromium and carbon content of base martensitic metal. Type 410 filler can be used to weld types 402, 410, 414 and 420 steels. Austenitic types 308, 309 and 310 are also used when welding martensitic steels to themselves or dissimilar metals.

Martensitic Stainless Available at Online Metals
410, 420, 440

PRECIPITATION-HARDENING

Precipitation-hardening stainless steels contain both chromium and nickel, which provide an optimum combination of the properties of martensitic and austenitic grades. Similar to martensitic grades, these steels are known for their ability to acquire high strength through heat treatment, while possessing the corrosion resistance of austenitic grades. Precipitation-hardening steels can be readily welded using procedures similar to those used for the 300 stainless steels. Grade 17-4 PH in particular is commonly used for welding (filler 17-7 PH is recommended), and can be successfully welded without preheating. As with many other alloys, achieving the same mechanical properties in the weld as in parent material is difficult in the precipitation-hardening series, even when utilizing a matching filler. Heat treating after welding can be used to help the weld metal achieve close to the similar properties as the parent metal.

Precipitaion-Hardening Stainless Available at Online Metals
455, 13-8, 15-5, 17-4

DUPLEX

Duplex stainless steels are “duplex” because they possess a two-phase microstructure consisting of grains of both ferritic and austenitic stainless steel. These steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades. They do, however, share a comparable corrosion resistance to the austenitic steels. Modern duplex steels are readily weldable, but the procedure, especially maintaining the heat input range, must be strictly followed. Thanks to the greater complexity of the material’s chemical composition, too much heat also adversely affects duplex stainless steels. Likewise, because Duplex alloys have portions of both austenitic and ferritic stainless, selecting a filler metal is slightly more challenging. Many types of duplex stainless base metals are not available as filler metals due to the fact that filler metal cools much more quickly than the base metal.

Processes

The procedure for welding stainless steel does not differ greatly from that of welding mild steel. Most all stainless steels can be joined through various types of welding. In order to help you find the best one for your material, here’s a breakdown of the weldability ratings and other fabrication properties for each type of stainless steel.

Common Stainless Weld Defects

Understanding common defects that may occur when welding is the first step to preventing them. Welding stainless steel is not much different from that required in welding standard carbon steel, with a few exceptions. First, you must exercise more care and control with regard to heating and cooling stainless steel. Second, it’s important to properly match filler metals with the material being welded. 

CRACKING

The most common welding imperfection for stainless steels is cracking. Even with austenitic stainless steels, the most readily welded of all stainless steels, has a risk of cracking. This is because austenitic steels lack ferrite, which dissolves harmful impurities that result in cracks. In order to prevent cracking in these metals, especially for fully austenitic structures, the choice of a filler containing ferrite is highly recommended. Ferritic stainless steels on the other hand, may crack during the welding process due to excessive grain coarsening which leads to poor toughness in the heat affected zone. When welding thin sections, no special precautions are necessary. In thicker material or highly restrained joints however, using a low heat input can minimize the size of the grain-coarsened zone and minimize the sensitivity to cracking. Likewise, using an austenitic filler may help to produce a tougher weld metal. Martensitic steels are very prone to cold cracking as a result of hydrogen, which is also experienced with low allow steels. The risk of cracking generally increases with the carbon content. The risk of cracking in martensitic stainless steels can be combated by utilizing a low hydrogen welding process such as TIG or MIG, or by using hydrogen controlled fillers. Furthermore, pre and post-weld treatments, especially for thicker sections and higher carbon materials, will help to toughen the structure, enable the hydrogen to diffuse from the weld metal, and reduce the risk of cracking.

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PREPARING TO WELD

As in any type of welding, it is important to clean stainless steel before welding it. What you may not realize, however, is how important it is to use tools, such as hammers and brushes, only on stainless steel because of how sensitive the material is to the presence of any carbon steel. For example, if you use a stainless steel brush to clean carbon steel, don’t use it again on any stainless steel. The same is true of stainless hammers and clamps. Why? Because trace amounts of carbon steel can become embedded in stainless steels, causing it to rust. Similarly, grinding carbon steel in proximity to stainless steel can result in problems. Carbon steel dust suspended in the air can land on nearby stainless steel and lead to rusting. This is why it’s a good idea to keep carbon steel and stainless steel work areas separate.

LEARN MORE ABOUT THE WELDABILITY OF METALS

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