What Is Ferrite Testing?
Ferrite Testing — also referred to as Ferrite Number (FN) measurement — is the quantitative
determination of the ferrite content in austenitic stainless steel and duplex stainless steel weld
deposits. It is performed using a calibrated ferritescope or Feritscope instrument that measures
the magnetic permeability of the weld metal and converts this to a Ferrite Number (FN) value.
Ferrite content in austenitic stainless steel welds is a carefully controlled parameter because:
- Too little ferrite (< ~3 FN): increases susceptibility to hot cracking (solidification cracking)
during welding — a serious fabrication defect risk
- Too much ferrite (> ~10 FN for standard austenitic grades): reduces toughness, increases
susceptibility to sigma phase embrittlement at elevated temperatures, and can impair
corrosion resistance in some environments
- For duplex stainless steels: ferrite-to-austenite phase balance (typically 40–60% ferrite
target) is critical to the mechanical properties and corrosion resistance that are the defining
characteristics of duplex grades