Overcoming Stainless-Steel Welding Challenges

June 1, 2018

Welding stainless steel can be challenging. The weld pool can be sluggish compared to welding mild steels—making low heat input low a critical success factor.

When a component must withstand corrosion and extreme temperatures—as low as -100 F and as high as 1800 F—manufacturers often turn to stainless steel. While commonly used in manufacturing, stainless steel can present welding challenges. Keeping heat input low plays a critical role, since stainless steel is less conductive to heat steel. This, along with selecting the filler metal and following proven best practices, is critical for optimal weld results.

Understanding Types

Understanding the differences between the five commonly used grades of stainless steel helps to ensure selection of the right grade. Across all five grades, chromium and nickel represent the main alloy materials to varying degrees. Three of the most common types: austenitic, ferritic and martensitic. As the material cools, the result will be one of these types, or a combination of types, depending on how the metal is cooled and what temperature it reaches during cooling.

As for weldability, all stainless-steel alloys share a common challenge: sluggishness of the weld pool. Using electrodes with boosted silicon content can help address this.

Strength varies across the alloys. If the material classification has an L-grade, such as 308L, this designates a lower carbon level, which can mean slightly lower tensile strength.

Choosing the right filler metal for each type of stainless steel depends on the characteristics of the base material, the properties required for the finished welds and the environment to which the welds will be exposed.

The five types of stainless steel:

Austenitic alloys have chromium content from 16 to 25 percent, and nickel from 8 to 20 percent. Additional alloying elements include silicon, manganese, nitrogen and molybdenum. Austenitic stainless steels do well in highly corrosive environments and are commonly used for medical equipment and kitchen equipment, such as mixers and dishwashers. The most common austenitic steels are 304, 308, 309 and 316. For 304 and 308, a Type 308 filler metal can be used. For a 309 base material, there are numerous options, including 308, 309 or 316 filler metals. For Type 316 base materials, use a Type 316 filler metal. Pre- and post-weld heat requirements aren’t typically an issue with austenitic stainless steels. If there is a need to perform post-weld heattreat, avoid the temperature range of 1200 to 1650 F, as carbide formation occurs rapidly in this range and causes weld embrittlement.

Ferritic alloys have a chromium-content range of 10.5 percent to greater than 25 percent, and typically have the best corrosion resistance. With tensile strengths of 55 to 65 ksi, they generally aren’t as strong as austenitic and martensitic stainless steels, and find use in automotive exhaust systems, chemical processing, and the pulp and paper industries. Common grades include 409 and 430, with matching filler metals of 409 and 430. Generally limited to service temperatures below 750 F due to their tendency to form embrittling phases, weld-solidification cracking also can be an issue with ferritic-base materials, making it important to use a filler metal with stabilizing alloys such as titanium or niobium, which help lower solidification cracking susceptibility.

Martensitic alloys, commonly used for steam and gas pipes, turbine blades and other applications that may encounter steam and moisture buildup, provide a good combination of high tensile strength and corrosion resistance. Keep in mind that materials with high tensile strength tend to have lower ductility. Martensitic stainless steels typically have an 11.5 to 18-percent chromium range and higher levels of carbon and other alloying elements that promote the formation of martensite. Common martensitic stainless steels, 410 and 420, can be matched with 410 and 420 filler metals with similar characteristics. Susceptible to hydrogen-induced cracking, controlling the heat input through proper preheat, interpass and post-weld temperature requirements minimizes cracking as does reducing the amount of restraint on the weld. A post-weld heattreatment, when used on these types of stainless steels to temper the martensite formed, will impact the hardness, tensile strength and ductility of the weld.

Precipitation-hardening (PH) stainless steels go through heattreatments to obtain their strength and hardness. Grades of PH stainless steels can have strengths of more than 200 ksi, making them the strongest types of stainless steel. A common PH stainless steel, 17-4, finds use in applications needing high strength and corrosion resistance, such as missile-launch tubes, aircraft frames and high-pressure bottles, and matches well with a 17-4 PH filler metal or 630 filler metal. Because it goes through a controlled cooling to achieve its properties, bringing a 17-4 PH stainless steel to a solution-treated condition before welding is recommended. This requires a temperature usually within the range of 1650 to 1800 F for 1 to 2 hr., followed by a quench. Post-weld heattreating brings the material back to the desired properties after welding. Contact the supplier of the PH stainless steel to get recommendations on post-weld heating temperatures and times to achieve the desired properties.

Duplex alloys, designed to have a microstructure of 50-percent ferrite and 50-percent austenite in their finished form, have a service temperature range of about -40 to 535 F and strengths above 60 ksi, which provide a mix of abrasion and corrosion resistance. Used in a range of applications, including oil and gas pipelines, use of the newer duplex filler metals is growing.

There are five commonly used grades of stainless steel. Understanding the differences between them helps ensure use of the right one in a welding application. Across all five types, credit chromium and nickel as the material’s main alloys in varying degrees.

Best Practices

In addition to proper filler-metal selection, successful stainless-steel welding requires following these best practices:

Increase silicon levels to help with weld, pool flow and fluidity. The more sluggish weld pool of stainless steel in the filler metal, due to less fluid, can cause some issues, especially for welders not familiar with the material. If the less-fluid weld pool causes concern, choose a filler metal with more silicon in the classification, such as ER308LSi versus a standard ER308L filler metal. Increased silicon levels help with weld-pool flow and fluidity.
Use faster travel speeds to help keep heat input low. Slow travel speed increases heat input, which can burn alloying elements out of the metal and impact weld properties including strength, ductility and corrosion resistance. While a travel speed of 3 to 8 in./min. is typical with other materials, welding stainless steel with flux-cored or metal-cored wires calls for travel speeds of 8 to 11 in./min. Consider, too, the final appearance of the weld. A weld bead using flux-cored or metal-cored wires will have a distinct gold color or rainbow sheen. Gas-tungsten-arc welds should not have this bead appearance.
Avoid contamination of the weld using a dedicated stainless-steel brush to clean stainless welds. Using the same brush to clean stainless steel and mild steel can cause cross-contamination and result in rust. Similarly, don’t use a brush for stainless steel to clean aluminum, or vice versa.
Use proper safety systems and protective gear. Some filler metals produce higher levels of weld fume than others, so it’s important to have proper ventilation or weld-fume-source capture in place when using them. In some applications, the welder may want to wear a helmet equipped with a respirator. MF