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The wide world of welding stainless steel

Proper weld prep, filler metals make all the difference

person welding stainless steel

People often are surprised to learn that there’s more than one type of stainless steel. The three most common types found in standard fabrication shops are austenitic, martensitic, and ferritic. The technique required to weld any of these steels isn’t much different than that required to weld carbon steel, with a few exceptions.

The development of nonrusting 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.

People often are surprised to learn that there’s more than one type of stainless steel. The original stainless steel, introduced by Harry Brearley in 1913, was far more corrosion-resistant than standard carbon steel, but at the cost of lower ductility. In the years since, metallurgists experimenting with different amounts of alloying materials have enhanced stainless steel performance in various ways.

The technique required to weld stainless steel is not much different from that required to weld standard carbon steel, with two exceptions. First, you must exercise more care and control with regard to heating and cooling stainless steel. Second, it’s more important to properly match filler metals with the material being welded.

Types of Stainless Steel

Five types of steel, each with numerous variations, fall under the stainless steel umbrella. All are categorized based on their microstructure—the result of both chemistry and the way in which the steel is heated and worked. Microstructure has a large influence on steel’s strength, ductility, and other physical and chemical attributes.

Three types of stainless are most commonly found in standard fabrication shops. Austenitic stainless steel is probably the most widely used, especially in typical machining and fabrication applications. Hard martensitic stainless steel is frequently used in high-wear applications such as hardfacing. Ferritic stainless steel is less expensive than other forms of stainless, making it a favorite for such consumer products as automotive exhaust components.

A fourth type, duplex stainless steel, is a combination of austenite and ferrite microstructures, making it stronger than either of its components but also more difficult to work with. Finally, precipitation-hardening stainless steels include other alloying elements—niobium, for example—that increase both strength and cost. Both duplex and precipitation-hardening stainless are specialty types used primarily in high-performance applications, such as aerospace and process industries, and we will not go into detail about them.

Prepping the Stainless Steel 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.

The other important factor in preparing to weld is making sure you have the proper filler material, which means knowing what type of base material you are welding. In many cases, it is as easy as using filler metal with the same number as the base metal. For example, if you were joining two pieces of 316L base metal, you would use 316L filler metal.

There are, of course, situations where it is not so simple, like when you are joining dissimilar metals or when you are doing an overlay.

welding stainless steel

Use hand tools that are dedicated to cleaning and prepping stainless steel. This material is extremely sensitive to the presence of carbon steel, and even trace amounts can cause the stainless to rust.

Austenitic Stainless Steel

As the most common type of stainless being used in fabrication shops, austenitic stainless steel is identified as the 300 series. While these base materials do not require preheating, they do have a maximum interpass temperature. Once the base metal reaches 350 degrees F, such as when you’re making multiple passes, you need to stop welding and let the material cool down.

Some of the 300 series stainless steels are referred to as fully austenitic—310, 320, and 330 steels. You have to manage these carefully to prevent cracking by using a low-heat-input process and by making welds that are convex. If you make a flat or concave weld on these materials, it will be susceptible to cracking.

Another parameter to consider is composition of the base material and filler metal. Consider 316L stainless. A grade with an “L” in the name is usually limited to temperatures of 800 degrees F or less in most applications, but the L doesn’t mean low temperature. It designates low carbon content, typically 0.03 percent carbon.

As mentioned earlier, the proper filler metal to use with this base metal has the same designation, 316L. However, don’t think that matching the base metal number is sufficient. You might have some 316L filler metal on hand, but that doesn’t mean you can use it to weld 316H base metal. The “H” stands for high carbon content, and even though it would weld on 316L base metal just fine, it will not hold up once the welded piece is put into service.

The most popular austenitic stainless grade is 304, but selecting a filler metal to use with this base metal is a little less straightforward, because there is no 304 filler metal. Instead, the filler metal to use in this case is 308L. It has a slightly different chemistry that allows the filler metal to undergo the rapid solidification and cooling associated with welding without cracking.

Another example is 321 base metal, which includes small amounts of titanium. However, any titanium in a filler metal would get burned up in the process of welding. In this case, the proper filler is 347, which has a chemistry similar to 321 but the titanium is replaced with niobium.

Fortunately, most often the filler metal and base metal grades are the same. If you have questions about which to use, welding supply companies always are willing to help you confirm your selection or determine the more unfamiliar combinations.

Martensitic Stainless Steel

The martensitic types of stainless steel are used less for joining than as overlays and for building up wear-resistant material. They generally have a minimum interpass temperature.

One common application for this type of material is rejuvenating the steel rolls used in continuous casting mills. Once the rolls wear beyond a certain point, they are resurfaced using a martensitic wire. Before welding begins on the roll, a torch or resistance heater is used to heat the roll to 400 to 600 degrees F. Once the welding begins, the temperature cannot be allowed to drop below that preheat temperature. Martensitic stainless gets very hard and brittle upon cooling, which is great for wear resistance but tough on welds as they are being made. Staying above the minimum interpass temperature keeps the area around the weld from cooling down too quickly.

When welding martensitic stainless steel, it is imperative that you hit an accurate preheat temperature and maintain the minimum interpass temperature the entire time you’re welding. Otherwise, you’re likely to end up with cracks.

As with many other stainless varieties, if you are joining martensitic base metals you will probably use a filler metal with the same number. In some applications, an austenitic stainless steel filler metal can be used for joining. For overlays, which are frequently placed on carbon steel, 410 is a standard filler metal choice. But regardless of the type of work, the keys to success in welding with martensitic filler metal are proper preheat and slow cooling afterward.

Ferritic Stainless Steel

Automotive applications use the most ferritic stainless steel. The two most common grades used in this application are 409 and 439. Ferritic stainless usually comes in thicknesses of ¼ in. or less, so most welding with this material is done in a single pass. This is good because welding ferritic stainless steel is most successful with low heat input, and maximum interpass temperature is 300 degrees F.

If you violate that guideline, you’ll know it soon enough. At high heat inputs, the material begins to experience grain growth and can quickly lose strength. In the less common application of welding thicker ferritic stainless, be especially careful to limit the heat input. Aside from that, match the filler material grade to the base metal grade and your welds should turn out well.

Duplex Stainless Steel

Too much heat also adversely affects duplex stainless steels, thanks to the greater complexity of the material’s chemical composition. Remember, this kind of stainless steel has portions of both austenitic and ferritic stainless, which also makes selecting a filler metal a bit more difficult. Many types of duplex stainless base metals are not available as filler metals due largely to the fact that filler metal cools much more quickly than the base metal. Adjusting the chemistry slightly allows the weld to have similar strength and material properties as the base metal.

One example is 2205 base metal, a duplex stainless that includes some nickel. The filler metal you would use with that is 2209, because it produces a weld with similar amounts of ferrite and austenite as the base metal, heading off welding issues. Another example is 2507 base metal, which you would weld with 2594 filler metal.

Welded production parts generally require some type of welding procedure, including test welds and approvals that spell out in detail how to make the weld. The procedure details not only the filler metal to be used, but also any preheat and interpass temperature requirements. However, when you find yourself welding a project using duplex stainless, you might need some guidance in filler metal selection.

Welding Mixed or Unknown Base Metals to Stainless Steel

Sometimes you may need to weld dissimilar metals or unknown base metals, such as when making field repairs. Fortunately, filler metals have been developed with chemistries that are specifically designed for such situations. It’s not uncommon, for example, to want to join 304L stainless steel and carbon steel. In that case, consider 309L filler material, which is likely to be a good choice for dissimilar metals up to about 750 degrees F.

If you’re uncertain about the base metal composition, consider one of the electrodes developed specifically for repairs, such as the 312 stainless electrode. Marketed under various trade names that tout their all-around compatibility, these weld-all electrodes feature chemistries with high strength, corrosion resistance, and good ductility. They also are compatible with most types of base metals. And the fact is, the 312 electrodes and others like them do the job. The downside is they might cost three to four times as much as standard gas metal arc welding (GMAW) wire. However, when performance counts and you want to count on success, it’s well worth the price.

When welding stainless steel, remember to first look for a filler metal that is the same as the base metal. If you run into difficulty finding an exact match, consult with your welding supply company to find an appropriate material. And when there are questions about base metal composition but you still need to ensure the ability to make good welds, know that specialty repair electrodes are available that will help you do the job and do it well.

Temperature and the Stainless Steel Weld

When you are welding stainless steel, it’s important to monitor the temperature for both the weld metal and base metal. If you fail to stay within the specified temperature ranges, you’ll probably experience performance problems. There are three ways you can check the steel’s temperature when you’re welding:

  1. Temperature-indicating sticks have a long track record of accurately confirming the temperature. However, they have limited ranges, and different sticks are required for each target temperature.
  2. Electronic infrared thermometers sense the steel’s surface temperature quickly and from a distance. They require a clear line of sight, which usually is not a problem. Shiny surfaces and other light-related conditions can lead to false readings, as can variations in the distance from the surface. But most people who use this type of device have learned to accommodate such quirks.
  3. Electronic surface temperature probes offer a third means of monitoring temperature. They are available with handles in different lengths that allow you to touch the metal for a reading. Some also can be mounted on the workpiece. This is an ideal setup when running a test weld because it allows you to monitor the temperature continuously and even print a graph of the metal temperature throughout the duration of the welding.
About the Author

Mike Barrett

Applications Engineer

22801 St. Clair Ave.

Cleveland, OH 44117

216-481-8100