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Selecting Filler Metals for Stainless Steel Welding
Q: We're a mid-size job shop that manufactures a wide variety of products from various types of metal. Currently we have a project that requires welding 304L to 316L stainless steel and welding both of these alloys to A572 Grade 50 steel. Some of the welds are structural and some are used in corrosive-environment applications. What type of welding consumable would work for the entire job?
A: Unfortunately, there is no such thing as a one electrode fits all. Ideally, the design engineer for the project should have specified which filler metal to use based on the in-service conditions. However, there are a few things to consider before settling on a filler metal.
It's important to understand that stainless steels come in many types, and often more than one option is available. The material is used in applications primarily for its corrosion resistance, but that is just one reason that is used. Certain types of stainless help prevent contamination in food and pharmaceutical applications, have great cryogenic toughness or elevated-service-temperature properties and have an appealing, unpainted finished appearance.
What makes stainless steel "stainless" is the addition of 11 percent minimum chromium. Generally speaking, ferritic and martensitic stainless steels, or 400 series which mainly contain carbon (C) and chromium (Cr), are used in applications requiring good strength and possible elevated temperatures. Austenitic, or 300 series, contain nickel (Ni) in addition to Cr and C; are nonmagnetic; have excellent low-temperature toughness; and, used with the proper precautions, are the most readily weldable stainless. These are the three main types of stainless steel. Two other groups are duplex and precipitation hardened, which are less common and used in specialized applications.
The primary goal when welding stainless steel to itself or to carbon steel is to control the amount of ferrite in the final weld, which is generally in the 5 to 10 percent range. Ferrite is a microstructure that affects the performance of the weld at various concentrations. Ferrite content that is too low can lead to issues with hot cracking during welding, while ferrite content that is too high can lead to reduced ductility, poor corrosion resistance, and cracking at elevated temperatures.
It is important to know the in-service application for various combinations of base materials, but here are some general recommendations. Note: The L attached to the stainless type designator stands for low carbon, which minimizes excessive carbide precipitation. The Si stands for increased silicon, which helps with weld puddle fluidity.
For welding 304 to itself, use a 308/308L filler metal that offers a matching chemistry. Additionally, 309/309L is recommended for welding 304 to carbon steel or for the barrier layer for overlays on carbon steel.
The 316/316L types of stainless steels contain molybdenum (Mo) and are commonly used in more corrosive applications than the 304 type. Therefore, the filler metal needs to match the base material, providing the additional Mo for those corrosive conditions.
Again, to ensure proper filler metal selection, verify the base material combinations and their in-service applications with the customer or design engineer.
This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.