© COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON V The amount of ferrite in austenitic stainless steel weld metal may be measured by magnetic devices because the ferrite is magnetic.  A small amount of ferrite in austenitic stainless weld metal is good, because it prevents weld cracking.  If the weldment is to be in very low temperature service, however, large amounts of ferrite should be avoided because ferrite is not tough at low temperatures.  Also, if the weldment is to be used in high temperature (higher than 1000°F) service, the ferrite should be maintained at low levels because the ferrite becomes brittle at those temperatures. 5.5   CALCULATION OF FERRITE CONTENT IN
Several simple, yet accurate, methods have been developed for determining the balance between the austenite and ferrite forming elements in iron.  When the chemical composition of the weld metal is known, the Schaeffler or WRC-1992 diagrams can be used.  See Figures 9 and 10. The purpose of these diagrams is to calculate the nickel and chromium equivalent of the weld metal in question and plot the point on the appropriate diagram.  The nickel equiva- lent is the sum of the nickel content and all other austenite formers, multiplied by coefficients representing their austenite forming effect as compared to that of nickel.  The chromium equivalent is calculated in the same manner.  In both diagrams, the nickel equivalent is the vertical axis, and the chromium equivalent is the horizontal axis.  The WRC-1992 diagram has an advantage since it also takes the nitrogen content into consideration.  Nitrogen is a power- ful austenite forming element.  If the nitrogen content is not known, we assume 0.06% for GTAW and SMAW electrodes and,  0.08% for GMAW and FCAW filler metals. When chemical composition is not available, two common instruments can also be used to determine ferrite content.  Since ferrite at room temperature is magnetic and austenite is not, a relationship between magnetic response and ferrite content can be established.  The more magnetic response to the instrument, the more ferrite present in the metal.  The two commercially available instruments that use this principal to measure ferrite content are the Magne gage and the Severn gage.  The Magne gage is a laboratory instrument, while the Severn gage is a pocket-size instrument designed for on-site readings. In the past, ferrite was expressed as a volume percent of the metal.  However, because of non-standard calibration, conflicting and inaccurate results often occurred.  To eliminate this problem, the ferrite volume percent was changed to a standardized expression known as the ferrite number (FN) and has been adopted by the Welding Research Council (WRC), the American Welding Society (AWS), and other agencies.  Ferrite numbers (FN) are

Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals
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