This sounds too good to be true, so what’s the problem? Welding is usually required to make almost anything from one of the more than 150 types of stainless steel. Welding stainless steel is a complex task. Some of these issues include the presence of chromium oxide, how to control heat input, which welding process to use, how to handle hexavalent chromium and how to do it right.
Despite the difficulties of welding and finishing this material, stainless steel remains a popular and sometimes the only option for many industries. Knowing how to use it safely and when to use each welding process is critical to successful welding. This can be the key to a successful career.
So why is stainless steel welding such a difficult task? The answer starts with how it was created. Mild steel, also known as mild steel, is blended with at least 10.5% chromium to produce stainless steel. The added chromium forms a layer of chromium oxide on the surface of the steel, which prevents most types of corrosion and rust. Manufacturers add varying amounts of chromium and other elements to steel to change the quality of the final product, and then use a three-digit system to differentiate grades.
Commonly used stainless steels include 304 and 316. The cheapest of these is 304, which contains 18 percent chromium and 8 percent nickel and is used in everything from car trim to kitchen appliances. 316 stainless steel contains less chromium (16%) and more nickel (10%), but also contains 2% molybdenum. This compound gives 316 stainless steel additional resistance to chlorides and chlorine solutions, making it the best choice for marine environments and the chemical and pharmaceutical industries.
A layer of chromium oxide can ensure the quality of stainless steel, but this is what makes welders so upset. This useful barrier increases the surface tension of the metal, slowing down the formation of a liquid weld pool. A common mistake is to increase the heat input, as more heat increases the fluidity of the puddle. However, this can adversely affect stainless steel. Too much heat can cause further oxidation and warp or burn through the base metal. Combined with sheet metal used in large industries such as automotive exhaust, this becomes a top priority.
Heat destroys the corrosion resistance of stainless steel perfectly. Too much heat is used when the weld or the surrounding heat affected zone (HAZ) turns iridescent. Oxidized stainless steel produces amazing colors ranging from pale gold to dark blue and purple. These colors make for a nice illustration, but may indicate welds that may not meet some welding requirements. The most stringent specifications do not like weld coloration.
It is generally accepted that gas-shielded tungsten arc welding (GTAW) is best suited for stainless steel. Historically, this has been true in a general sense. This is still true when we try to bring those bold colors into artistic weaving to meet the highest quality standards in industries such as nuclear power and aerospace. However, modern inverter welding technology has made gas metal arc welding (GMAW) the standard for stainless steel production, not just automated or robotic systems.
Since GMAW is a semi-automatic wire feed process, it provides a high deposition rate, which helps to reduce heat input. Some pros say it’s easier to use than GTAW because it relies less on the skill of the welder and more on the skill of the welding power source. This is a moot point, but most modern GMAW power supplies use pre-programmed synergy lines. These programs are designed to set parameters such as current and voltage, depending on the filler metal entered by the user, material thickness, gas type and wire diameter.
Some inverters can adjust the arc throughout the welding process to consistently produce an accurate arc, handle gaps between parts, and maintain high travel speeds to meet production and quality standards. This is especially true for automated or robotic welding, but also applies to manual welding. Some power supplies on the market offer a touch screen interface and torch controls for easy setup.
Welding stainless steel is a complex task. Some of these issues include the presence of chromium oxide, how to control heat input, which welding process to use, how to handle hexavalent chromium and how to do it right.
Choosing the right gas for GTAW usually depends on the experience or application of the welding test. GTAW, also known as tungsten inert gas (TIG), in most cases uses only an inert gas, usually argon, helium, or a mixture of both. Improper injection of shielding gas or heat can cause any weld to become excessively domed or rope-like, and this will prevent it from mixing with the surrounding metal, resulting in an unsightly or unsuitable weld. Determining which mixture is best for each weld can mean a lot of trial and error. Shared GMAW production lines help reduce wasted time in new applications, but when the most stringent quality is required, the GTAW welding method remains the preferred method.
Welding stainless steel poses a health hazard to those with a torch. The greatest danger is posed by fumes released during the welding process. Heated chromium produces a compound called hexavalent chromium, which is known to damage the respiratory system, kidneys, liver, skin and eyes and cause cancer. Welders must always wear protective equipment, including a respirator, and ensure that the room is well ventilated before starting welding.
The problems with stainless steel don’t end after welding is complete. Stainless steel also requires special attention in the finishing process. Using a steel brush or polishing pad contaminated with carbon steel may damage the protective chromium oxide layer. Even if damage is not visible, these contaminants can make the finished product susceptible to rust or other corrosion.
Terrence Norris is Senior Applications Engineer at Fronius USA LLC, 6797 Fronius Drive, Portage, IN 46368, 219-734-5500, www.fronius.us.
Rhonda Zatezalo is a freelance writer for Crearies Marketing Design LLC, 248-783-6085, www.crearies.com.
Modern inverter welding technology has made gas GMAW the standard for stainless steel production, not just automatic or robotic systems.
WELDER, formerly called Practical Welding Today, represents the real people who make the products we use and work with every day. This magazine has been serving the welding community in North America for over 20 years.
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