Page 39 - MetalForming-Jun-2018-issue

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 fer- ritic-base materials, making it impor- tant 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 resist- ance. 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 marten- site. Common martensitic stainless steels, 410 and 420, can be matched with 410 and 420 filler metals with sim- ilar characteristics. Susceptible to hydrogen-induced cracking, control- ling the heat input through proper pre- heat, interpass and post-weld temper- ature requirements minimizes cracking as does reducing the amount of restraint on the weld. A post-weld heat- treatment, 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 heattreat- ments 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 stain-
There are five commonly used grades of stainless steel. Understanding the differ- ences 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.
less steel, 17-4, finds use in applications needing high strength and corrosion resistance, such as missile-launch tubes, aircraft frames and high-pres- sure 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, bring- ing a 17-4 PH stainless steel to a solu- tion-treated condition before welding is recommended. This requires a tem- perature 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 heat- ing 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 fin- ished 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 resist- ance. Used in a range of applications, including oil and gas pipelines, use of the newer duplex filler metals is growing.
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 con- cern, 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 resist- ance. 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 alu- minum, or vice versa.
• Use proper safety systems and pro- tective gear. Some filler metals produce higher levels of weld fume than others, so it’s important to have proper venti- lation or weld-fume-source capture in place when using them. In some appli- cations, the welder may want to wear a helmet equipped with a respirator. M F
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Fabrication: Welding