【导语】 Best practices contribute to robust joints and maintain its corrosion resistance.
Stainless steel isn’t necessarily difficult to work with, but welding it requires careful attention to detail. It doesn’t dissipate heat as well as mild steel or aluminum, and it can lose some of its corrosion resistance if too much heat is put into it. Best practices can contribute to maintaining its corrosion resistance. Images: Miller Electric
The corrosion resistance of stainless steel makes it an attractive choice for many critical tube and pipe applications, including high-purity food and beverage, pharmaceutical, pressure vessel, and petrochemical uses. However, the material doesn’t dissipate heat as well as mild steel or aluminum do, and poor welding practices can decrease its ability to resist corrosion. Applying too much heat input and using the wrong filler metal are two culprits.
Following some best practices for stainless steel welding can help improve results and ensure that the metal maintains its corrosion resistance. Furthermore, upgrading the welding process can deliver productivity benefits without impacting quality.
Tip 1: Select a low-carbon filler metal
In stainless steel welding, filler metal selection is crucial to controlling carbon levels. The filler metal used for stainless steel tube and pipe welding should enhance the weld properties and meet the application requirements.
Look for filler metals with an “L” designation, such as ER308L, because these provide a lower maximum carbon content, which helps retain corrosion resistance in low-carbon stainless alloys. Welding a low-carbon base material with a standard filler metal can increase the carbon content of the weld joint and thereby increase the risk of corrosion. Avoid filler metals with an “H” designation, since these provide higher carbon content designed for applications that require greater strength at high temperatures.
When welding stainless steels, it’s also important to choose a filler metal with low trace (also called tramp) elements. These are residual elements—including antimony, arsenic, phosphorus, and sulfur—in the raw materials used to make filler metals. They can affect the material’s corrosion resistance substantially.
Tip 2: Pay attention to weld preparation and proper fit-up
Because stainless steel is so sensitive to heat input, joint preparation and proper fit-up play key roles in controlling the heat to maintain the material’s properties. With gaps or uneven fit-up between the parts, the torch must stay in one spot longer, and more filler metal is needed to fill those gaps. This results in heat buildup in the affected area, which can overheat the part. Poor fit-up also can make it harder to bridge gaps and get the necessary weld penetration. Take care to ensure part fit-up is as close to perfect as possible with stainless steel.
Cleanness also is very important with this material. Very small amounts of contaminants or dirt in the weld joint can cause defects that reduce strength and corrosion resistance in the final product. To clean the base material before welding, use a dedicated brush specifically for stainless steel that has not been used on carbon steel or aluminum.
Tip 3: Control sensitization with temperatures and filler metal
In stainless steel, sensitization is the primary cause of the loss of corrosion resistance. It can occur when weld temperatures and cooling rates fluctuate too much, changing the microstructure of the material.
This OD weld on a stainless steel pipe, welded using GMAW and Regulated Metal Deposition (RMD) without a back purge for the root pass, is similar in appearance and quality to welds made with GTAW with a back purge.
A key part of corrosion resistance in stainless steel is chromium oxide. But if carbon levels in the weld are too high, it forms chromium carbides. These tie up the chromium and prevent the formation of the needed chromium oxide that gives stainless steel its corrosion resistance. Without enough chromium oxide, the material doesn’t have the desired properties, and corrosion can set in.
Preventing sensitization comes down to filler metal selection and controlling heat input. As stated previously, it’s important to choose a low-carbon filler metal for stainless steel welding. However, sometimes carbon is needed to provide strength for certain applications. When it’s not possible to choose a low-carbon filler metal, controlling the heat is especially important.
Minimize the time the weld and the heat-affected zone are held at high temperatures— generally considered to be 950 to 1,500 degrees F (500 to 800 degrees C). The less time a weld spends in this range, the less heat can build up. Always check and adhere to interpass temperatures in the welding procedure for the application.
Another option is to use filler metals designed with alloying ingredients such as titanium and niobium that prevent the formation of chromium carbides. Because these ingredients also affect strength and toughness, these filler metals can’t be used in all applications.
Tip 4: Understand how shielding gas impacts corrosion resistance
Using gas tungsten arc welding (GTAW) for the root pass is the traditional method for welding stainless steel tube and pipe. This typically requires a back purge of argon gas to help prevent oxidation on the back side of the weld. However, using wire welding processes is becoming more common with stainless steel tube and pipe. In these applications, it’s important to understand how the various shielding gases affect the material’s corrosion resistance.
Mixtures of argon and carbon dioxide, argon and oxygen, or three-gas mixtures (helium, argon, and carbon dioxide) have traditionally been used when welding stainless steel with the gas metal arc welding (GMAW) process. Often, these mixes contain mostly argon or helium and less than 5% of carbon dioxide since carbon dioxide can contribute carbon to the weld pool and increase the risk of sensitization. Pure argon isn’t recommended for GMAW on stainless.
Flux-cored wires for stainless steel are designed to run with a traditional mixes of 75% argon and 25% carbon dioxide. The flux contains ingredients designed to prevent the carbon in the shielding gas from contaminating the weld.
Tip 5: Consider different processes and waveforms
As GMAW processes have evolved, they have simplified stainless steel tube and pipe welding. While some applications may still require the GTAW process, advanced wire processes can offer similar quality and much better productivity in many stainless applications.
An ID weld on stainless steel, made with GMAW RMD, is similar in quality and appearance to the corresponding OD weld.
Using a modified short-circuit GMAW process such as Regulated Metal Deposition (RMD) from Miller for the root pass eliminates the back purge in certain austenitic stainless steel applications. The RMD root pass can be followed by pulsed GMAW or flux-cored arc welding fill and cap passes—a change that saves time and money compared to using GTAW with back purging, especially on larger pipes.
RMD uses a precisely controlled short-circuit metal transfer that creates a calm, stable arc and weld puddle. This provides less chance of cold lap or lack of fusion, less spatter, and a higher quality root pass on pipe. Precisely controlled metal transfer also provides uniform droplet deposition and makes it easier to control the puddle and, therefore, heat input and welding speeds.
An unconventional process can increase welding productivity. When using RMD, welding speed can be from 6 to 12 in./min. Because the process allows an increase in productivity without putting additional heat into the part, it helps maintain the properties and corrosion resistance of stainless steel. The reduced heat input of the process also helps control distortion of the base material.
This pulsed GMAW process provides a shorter arc length, narrower arc cone, and less heat input compared with traditional spray pulse transfer. Since the process is closed-loop, arc wandering and variations in tip-to-work distances are virtually eliminated. This provides easier puddle control for in-position and out-of-position welding. Finally, coupling pulsed GMAW for fill and cap passes with RMD for the root pass permits welding procedures with one wire and one gas, eliminating process changeover time.