Stainless steel is one of the most popular materials when corrosion resistance is important. Its high amounts of chromium and other alloying elements along with the structural properties of carbon steel make it an extremely useful material for many projects. However, not all stainless steels are easily welded and it can be argued that some stainless steels grades are not weldable at all. So to help, this article will highlight some of the best grades for welding stainless steel.
Austenitic Stainless Steels
Austenitic stainless steels can be welded together using many different welding processes. Some are more preferred for welding than others, such as 304, 308, 316, 321, and 347 which are all austenitic grades that are weldable.
It should be noted, however, that these grades can be subject to intergranular corrosion because of their relatively high amount of carbon. Intergranular corrosion occurs when the chromium in stainless steel combines with the carbon instead of creating a protective chromium oxide layer. These chromium carbides will allow the area around the weld to be subject to corrosion as time passes.
To combat the risk of intergranular corrosion, the alloys 304, 308, and 316 are also available in a low carbon form. This is designated by an “L” suffix, such as 304L. The lower amount of carbon in these grades allows the chromium to form a protective chromium oxide layer rather than getting tied up with carbon atoms. Other options include grades like 321 or 347 which are stabilized. This means that additions of titanium or niobium are used so that the carbon combines with them before it is able to combine with the chromium. These two forms of intergranular corrosion prevention allow austenitic stainless steels to be welded with a reduced risk of weld or heat affected zone failure.
Ferritic Stainless Steels
In general, ferritic stainless steels are the most readily weldable stainless steel. While ferritic grades still have chromium and other alloying elements, the reduced amount of these elements compared to austenitic stainless steel make ferritic stainless steels more weldable. Intergranular corrosion is not so much of a concern due to the lower amounts of chromium in ferritic stainless steels. This means that maintaining corrosion resistance will most likely not be as high of a concern when compared to an application using austenitic grades.
The lower amounts of alloying elements also reduce the risk of hot cracking during the welding process. If excessive heat input is used in the welding process, ferritic stainless steels can be subject to excessive grain growth in the heat affected zone. If this happens, a loss of toughness and ductility will occur. Ferritic stainless steels such as 407 and 430 are excellent choices for ferritic stainless steels in welding applications.
Martensitic Stainless Steels
Martensitic stainless steels are more difficult to weld than ferritic or austenitic because of the higher carbon content. The increased carbon content in conjunction with the other alloying elements found in stainless steels increase the chance of a brittle microstructure formation. This can cause weld cracking. In order to prevent cracking, several precautions must be taken. Hydrogen brought into the weld from the welding process must be kept to a minimum to reduce the risk of hydrogen cracking. Also, preheat and post-weld heat treatment should be used in order to reduce the brittleness of the weld joint and heat-affected zone.
Martensitic grades such as 403, 410, and 420 are a few of the grades that can be welded when proper heat treating is performed and proper filler metals are selected. However, there are martensitic grades of stainless steel available that are considered near impossible to weld. Care should be taken to avoid these when selecting a martensitic stainless steel for a welding application.
Other Types of Stainless Steel
Duplex stainless steel is part austenite and part ferrite in composition. To achieve this hybrid makeup, complex chemical compositions are used. They generally have lower nickel and higher amounts of chromium than other stainless steels. They can be welded, but different filler metals have to be used for different types of duplex stainless steels. For example, grade 2205 needs to be welded with a grade 2209 filler metal.
Precipitation hardening stainless steels derive most of their strength and hardness from the intermetallic precipitates that block dislocations in the stainless steel microstructure. These precipitates are formed by a special form of heat treating. When the heat from welding is applied to these materials, it can compromise the original mechanical properties of the precipitation hardening stainless steel. These materials must be heat treated again after welding following the material manufacturer’s guidelines. Commonly welded precipitation hardening stainless steels include 17-4PH and 17-7PH. It is important to select the correct filler metal for all stainless steels, including these grades.
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