Weldability of Stainless Steel

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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 non-rusting stainless steels with distinct attributes that include varying degrees of corrosion resistance, strength, and workability has been a tremendous benefit to steel users. This development, however, also has made welding stainless steel more complicated than welding traditional carbon steel. Stainless steels, along with other corrosion resistant alloys, were once thought of as a major challenge to weld. Today, welding stainless steel is 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 stainless steel. To successfully weld this highly versatile material it is important to familiarize yourself with the various types of stainless steels and their properties. Stainless can be divided into four main types: ferritic, martensitic and precipitation hardening, duplex, and austenitic.

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Types of Stainless Steel

Stainless steels are created from the addition of alloys, namely chromium and nickel, to steel. Depending on their composition, stainless steels are classified in to different grades that fall into four main types: ferritic, martensitic and precipitation hardening, duplex, and austenitic. Nickel plus carbon, manganese and nitrogen forms austenite stainless steel. Chromium plus silicon, molybdenum and niobium forms ferrite stainless steel. The structure of welds in stainless steels can be largely predicted based on their chemical composition. Because of their different microstructures, the alloy groups have both different welding characteristics and are susceptible to different welding defects.

AUSTENITIC

Austenitic stainless steels are 200 and 300 series stainless. They have high corrosion resistance and are highly formable but are prone to stress cracking. They are considered to be the most easily weldable family of the stainless steels. Comparatively little trouble is experienced in making satisfactory welded joints in their physical characteristics and mechanical properties are given proper consideration. Austenitic alloys are also commonly used for welded fabrications because they can be readily welded using any of the arc welding processes. They exhibit good toughness because they are non-hardenable on cooling, and there is no need for pre- or post-weld heat treatment. Austenitic stainless steels 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: sheet  302: ballssheet  303: hex bar, rectangle bar, round bar, square bar  304: round bar, channelsheet/plate, expanded sheetperforated sheet, textured sheet, tread sheet, threaded rod, rectangle tube, round tube, square tube  304/304L: angle, hex bar, rectangle bar, round bar, square bar, piperound tube  316: balls, round bar, threaded rod, rectangle tube  316/316L: angle, hex bar, rectangle bar, round bar, square bar, pipe, sheet/plateround tube, square tube  321: hex bar, round bar  347: hex barround bar  N50: round bar  N60: round bar

FERRITIC

Ferritic stainless steels are 400 series. They have lower ductility and lower corrosion resistance than the austenitic grades but offer high resistance to stress corrosion cracking. The ferritic stainless steels are generally considered to have poor weldability when compared to the austenitic stainless steels, as the brittleness and poor ductility of these materials limit their applications in the welded condition. Ferritic stainless steels become fully ferritic at high temperatures and undergo rapid grain growth, which leads to brittle, heat affected zones in the fabricated product. They possess reduced formability, susceptibility to embrittlement, susceptibility to hot cracking, and an adverse effect on their mechanical properties (toughness and ductility) when welded. If welding, ferritic stainless steels are generally welded in thin sections, most less than 6mm in thickness where any loss of toughness is less significant. Thinker sections (>1/4 inches) 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; types 409 and 430 are commonly used as fillers, and austenitic types 309 and 312 for dissimilar joints.

Ferritic Stainless Available at Online Metals
430: sheet

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: sheet  420: rectangle barsquare bar  440: balls  440C: rectangle barround barsquare bar

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: round bar  13-8: round bar  15-5: hex bar, round barsheet  17-4: round barrectangle barsquare barsheet

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.

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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.

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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.

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