by Frank Babish
Selecting and using the right filler metal
The process of choosing a stainless filler metal involves matching it to the base materials, selecting a shielding gas and often calculating ferrite content and total heat input. The best choice might not be apparent, especially for those not familiar with the metallurgical nuances. For example, joining 304L to 316L (a common application) presents several filler metal options:
- 316L (19% Cr, 12% Ni, 3% Mo, .4% Si). A perfectly good choice, but the puddle will be comparatively sluggish.
- 316LSi, which has the same composition as above, but with .9% silicon, which improves wetting and makes the puddle easier to manipulate.
- 309LSi (24% Cr and 13% Ni) can enhance corrosion resistance by “over-matching” chromium content compared to the other options.
- Generally, the higher alloyed of the two base metals should be used. In this example, 308L or 308LSi could be used but it is not an optimum choice.
You also have to understand service conditions (corrosive environment, temperature and required mechanical properties). For example, in nitric acid, a 310L filler (which has no Molybdenum) will perform better than a 316L or 310LMo due to selective attack of Mo.
Cost also influences filler metal selection. Fabricators use 309L to join 400 Series ferritic alloys, but the high nickel content of 309L adds cost.
A modified 307 filler, with 8% Ni and 7% Mn, might be a lower cost alternative because manganese costs less and acts as an austenitizing agent to create a suitable crack resistant microstructure in the weld metal.
Fabricators often try to use the same shielding gas as they do for MIG or flux-cored welding carbon steel. It won’t work with stainless because shielding gases with a carbon dioxide content of 5% or more add too much carbon to the weld metal, degrading its corrosion resistance.
Recommendations for short circuit MIG welding stainless include “tri-mix” gases that contains a blend of 85 to 90% He, up to 10% Ar and 2 – 5% CO2; alternatively Ar + 1-2% CO2 gives good results. A common choice for spray transfer MIG is Ar + 1-2% O2. Options include Ar + (1-2% CO2); Ar + 30% He + 1-2% O2; and Ar + 30% He + 1-2% CO2. Adding O2 gives the puddle better fluidity and provides arc stabilization; helium gives better weld penetration and increases fluidity of the weld pool and CO2 increases and broadens penetration.
Welding standard austenitic stainless steels requires keeping the ferrite level high enough so it resists cracking but is low enough to prevent transforming to a brittle phase or possible selective attack. To determine balance between the austenite and ferrite forming elements in iron, engineers use one of three constitution diagrams: Schaeffler (the original), DeLong or WRC-1992 (now preferred).
Static charts are available, but online and mobile applications allow users to switch between diagram options, enter variables (e.g., percentage of carbon, chromium, molybdenum, nickel, nitrogen, etc.) and obtain results.
To preserve metallurgical properties, some applications require calculating heat input, which may be a maximum, minimum, or both. For example, with a 2205 duplex alloy, welding below the minimum heat input can result in the wrong phase balance after welding, while excess heat can also affect the corrosion resistance or embrittle the metal.
Measured in Joules per inch (or kJ/mm), heat input requires knowing current, voltage, weld length and arc time. The formula is Heat Input = (60 x Amps x Volts) / (1,000 x travel speed in in/min) = KJ/in. Over-welding often causes distortion and excess heat input. Rather than reducing volts or amps, a better solution could be increasing travel speed to yield a more ideal bead profile.
New stainless filler metal options are being developed to meet changing metallurgies, more extreme service conditions and cost constraints. As a result, even experienced fabricators can benefit by consulting a filler metal expert and leveraging resources such as filler metal data books and online applications. SMT
Frank Babish is an application specialist for Exaton, an ESAB brand.