Martensitic stainless steel grades are basically alloys of iron and chromium. The chromium level typically ranges from 12% up to 17%. In contrast to ferritic stainless, which does not use carbon, the composition of martensitic grades will have higher nominal carbon content, which ranges between 0.10% (Type 410) up to 1.2% (Type 440).
Due to the carbon levels, martensitic stainless steels undergo significant hardening after exposure to high welding heat, followed by rapid cooling in air or quenching in oil or water. The rapid cooling results in transformation to a high hardness, low ductility, brittle microstructure, commonly known as martensite. This occurs both in the weld deposit and heat affected zone. After cooling, a post weld heating cycle (tempering) can be used to reduce brittleness and hardness, and impart some ductility. For thicker material, and to manage residual stress from shrinkage, preheating can also be used to reduce the cooling rate and achieve a lower hardness.
Martensitic Stainless Grades and Welding Concerns
Because the susceptibility to cracking increases with the hardenability, which increases with the carbon content, higher-carbon grades stainless steels, such as Type 420 and the Type 440 series, are not generally specified for welding. However, to improve the weld toughness and reduce the susceptibility to cracking, an austenitic stainless steel or nickel based alloy filler material can introduced into the weld. This can be accomplished using a pre-placed shim, or with filler wire feeding. Note that use of filler can benefit the weld fusion region but cannot reduce the brittleness in the heat-affected zone.
Grades which are in lower carbon range can be alloyed and heat treated to provide combinations of mechanical properties suitable for mechanical components, such as those found in pumps, valves and shafts, which also require good resistance to corrosion and oxidation. To impart high temperature properties, such as creep resistance, for applications in heat exchangers or turbines, minor alloy additions, such as Boron, Cobalt, Niobium or Titanium, are used.
A special Martensitic stainless grade known as 410-Ni Mo has been developed to mitigate the welding issues mentioned above. The alloy composition is modified by adding controlled levels of Nickel and molybdenum, which reduce the hardenability and provide a provide a more weld-friendly martensite or ferrite–austenite microstructure. 410-Ni Mo alloy is commonly found in large, stainless steel castings used in hydro-electric generation equipment.
When wear resistance is desired, higher carbon levels can provide properties suitable for use on surgical blades, plastic injection molds, cutlery. Special compositions have been developed for use as hardfacing and are deposited by cladding over a lower hardness substrate. Laser cladding can provide deposits with superior properties with minimal levels of dilution.
Some grades of martensitic stainless steel, such as Type 416, 416Se, 420F and 440F, are referred to as “Free-Machining,” and are very prone to solidification cracking due to the presence of elements such as selenium, sulfur and/or lead, which are added to enhance machinability. Because of their crack sensitivity, welding of these materials is not recommended. However, when welding cannot be avoided, use of an austenitic filler metal, such as grade 308, or 309, or preferably a high-ferrite grade 312, may be used to mitigate solidification defects. When using laser or electron beam weld processes, the reduced heat input of these weld beam can also minimize dilution of the weld metal with the elements from the free-machining base metal.
Welding Martensitic Stainless Steels
Pre-weld material preparation processes, such as EDM or laser cutting, can result in the generation of a narrow zone known as “recast layer.” This layer can negatively affect weld quality and needs to be removed. Filing or abrasive grinding are commonly used. When grinding, silicon carbide abrasives should not be used due to risk of introducing residual level of silicon carbide, which can decompose when exposed to welding conditions. Use of aluminum oxide abrasives is preferred.
Final pre-weld cleaning is also critical for high quality electron beam or laser welding. All foreign material which can cause contamination or porosity must be removed. This is best performed with a stainless steel wire brush, followed by wiping with a suitable clean solvent.
When welding with laser, it is critical to provide an inert gas (usually high purity welding grade argon) to cover the fusion zone, as well as backup shielding for protection of underside of the weld joint. The presence of air will contribute to excessive oxidation, porosity and increased risk of cracking. The EB welding process, which occurs in a vacuum, avoids these concerns.
Fixturing
Electron beam and laser welding of martensitic 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 heat affected zones, which can have high brittleness.
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 laser 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.