Ferritic stainless steel refers to a category of stainless steel alloy that typically contains over 12% chromium and low carbon content. As a result of this chemical composition, the material has a metallurgical structure composed of ferritic grains, and thus differs from other forms of stainless steel that are comprised of either austenitic or martensitic grains.
Ferritics are nonhardenable by heat treating and are always magnetic. Typical applications for ferritic stainless steels include heat exchangers, uses in petrochemical manufacturing, automotive exhaust systems and trim, furnaces, home appliances and food processing equipment.
Ferritic Stainless Steel Properties
Ferritic stainless steels are not hardenable by heat treatment, and are only marginally hardenable by cold working.
Mechanical properties are impaired as a consequence of embrittlement, which is due to inceased grain size and martensitic phase formations in heat affected zones subjected to temperatures above approximately 2000°F/1100°C.
Heat generated by welding typically alters the Ferritic stainless steel base metal adjacent to the fusion zone—the HAZ or Heat Affected Zone. Areas subjected to temperatures above approximately 2000°F/1100°C will exhibit embrittlement and subsequent impairment of the metal’s mechanical properties. This is due to increased grain size and martensitic phase formations.
A consequence of the grain growth is increased notch sensitivity and raising of the ductile–brittle transition temperature to above room temperature. As a result, after the weld cools, the final joint exhibits brittleness and increased tendency to crack. Control of the weld heat input, (such as using an electron beam or laser beam procedure) and use of a post weld heat treating cycle may mitigate some of the harmful effect of welding. Such measures do not mitigate problems or improve the joint properties in the HAZ, but can contribute to reduction of residual welding stresses and provide some HAZ softening. A possible draw back to the use of post-weld heat treating is the possibility of the formation of an embrittling intermetallic phase (Sigma phase) in some specific alloys.
Ferritic Stainless Steel Grades
Certain Ferritic stainless grades have been developed to address the welding limitations associated with conventional ferritic stainless. These utilize minor modifications of alloy com‐ position to reduce enhance interstitial elements, or close control of the chromium–nickel ratio to provide a mixed ferrite–martensite or ferrite–austenite microstructure. An example is Ferritic Type 409, which has minor amounts of titanium added to enhance weldability and improve ductility to allow for post weld cold forming. This alloy is commonly found in stainless steel car exhaust systems.
Type 442 is another ferritic grade suitable for automotive exhaust and heat exchanger components. 442 is classed as “dual stabilized,” containing minor amounts of titanium and columbium, in order to minimize the occurrence of titanium stringers. The addition of titanium and niobium stabilization enhances weldability as well as oxidation and corrosion resistance.
Welding Ferritic Stainless Steels
Welding with conventional electric arc fusion processes require multiple passes, which generate excessive heat input and result in metallurgical impairment described above. Use of electron beam or laser welding technology can minimize impairments and allow development of more applications for ferritic grade stainless steel, especially where resistance to high temperature oxidation and low coefficients of thermal expansion are needed.
Fixturing
Electron Beam and Laser welding of Ferritic stainless steels requires a precise joint in order to maintain permissible gap and avoid mismatch. Good weld fixturing is necessary so that welds can be accurately placed, minimizing HAZ areas.
Joint Types
- Butt Weld:
- A fit-up tolerance of 15% of the material thickness is desirable.
- Sheared edges are acceptable provided they are straight and square.
- Misalignment and out-of-flatness of parts should be less than 25% of the material thickness.
- Lap Weld (burn-through or seam weld):
- Air gaps between pieces to be Lap Joint welded severely limit weld penetration and/or feed speed.
- For round welds in stainless, no gap can be tolerated unless inert gas coverage can be maintained over the entire weld area.
- Fillet Weld:
- Square edges and good fit-up are also necessary.