Ferrite test for duplex stainless steel

First, I instruct about welding requirements and additional test for PQR. Because welding and heat input control, welding consumable are affected to ferrite content after welding.

Welding requirements

2 codes are mostly used for qualification of duplex stainless steel.

  • Structural steel: AWS D1.6
  • Piping: ASME Section IX

Additional procedure qualification requirements

A maximum heat input of 2.5kJ/mm should be acceptable for the duplex steels and 2.0kJ/mm maximum for super duplex. 

Consumables and as-welded deposits:

Duplex: 

  • Nitrogen: 0.14% (Min.)
  • Nickel: 8.0% (Min)
  • Molybdenum: 3.0% (Min)

Super duplex: 

  • Nitrogen: 0.2% (Min.)
  • Nickel: 9.0% (Min)
  • Molybdenum: 3.5% (Min)

Shieding gas/ Backing gas composition shall be argon or nitrogen with argon mix are essential variable.

Oxygen content of the back purging gas shall max. 0.50% (5000 ppm).

Preheat and interpass temperature recommend are shown in API RP 582 2016 table 5.

Hardness test: max. 320 HV10 for DSS, max. 350 HV10 for SDSS.

Ferrite content: AWS A4.2M

  • Weld metal: 30 to 60%
  • HAZ: 40 to 65 %
  • Base metal: 40 to 60%

Ferrite content down to 25 % may be acceptable for the weld metal if corrosion or other tests are satisfactory to the purchaser. When a nickel alloy consumable is used, the ferrite content in the weld deposit is not required to be tested.
To convert from ferrite % to FN, the following can be used: For 22 % Cr duplex stainless steel, ferrite % = 0.7 × FN; For 25 % Cr duplex and super duplex stainless steels, ferrite % = 0.65 × FN.

Corrosion test: ASTM G48 or ASTM 923

Impact test: follow specification

The lateral expansion shall be minimum 0.38 mm. 

Welding consumables

Selection welding consumables refer to:

  • API RP 582 2016 table A.3 - Suplex Stainless Steel or
  • AWS D1.6 Annex F - Suggested Filler Metal for Various Combinations of Stainless Steels and Other Ferrous Base Metals .

Visual inspection

Cleaning and surface after welding.

NORSOK standard M-601 Annex B - Acceptable oxidations of welds in stainless steels 

Test procedure

There are 2 units of ferrite test: Fe% and FN

Ferrite convert refer to API RP 582 2016 Para 11.3.4.3.

Ferrite Machine: FERITSCOPE FMP30

  • Connect the probe with the ferrite meter and then turn on the instrument.
  • Select value to be expressed (Fe% or FN (Ferrite Number)).
  • The probe has to be placed vertically on the surface of the measuring object to perform a measurement.
  • Record the measured value to be displayed on screen.
  • After the measurement appears in the display, the probe can be lifted again and the equipment is ready to measure next.
  • A minimum of six ferrite readings for each location shall be taken at random different locations on the finished surface along the longitudinal axis of the weld bead or in the designated locations of parent metal / clad layers.

Sample for calibration before testing (within 2 hours working time)

Calibration on sample 59.8% (as result 59.4% are acceptable)

Test on flange: 55.1 %

Test on pipe: 49.6%


Test on weld metal: 47.5%


Test results recorded

ACCEPTANCE CRITERIA

For Duplex Stainless Steel, Production welds shall be checked on the cap along the weld with a calibrated ferrite gauge and shall not exceed 60% ferrite. The measurement spacing along the weld length shall be the lesser of three equidistant locations or 300mm (12"). A minimum of six (6) measmements shall be taken at each location. The surface shall be prepared as recommended by the manufacturer of the testing instrument. Acceptance shall be based on the average of the measurements at each location. Of the six (6) local measurements, the highest and lowest values may be discarded and the average of the remaining three reported as the final value.

Ferrite content shall be within the following ranges:

  • Weld metal—30 % to 65 %,
  • HAZ—40 % to 65 %,
  • Base metal—40 % to 60 %. 

Above guidance and photos for references. The specification requirements are essential variable and must strictly follow.

Nguyen Duy Bien
 

Enjoy learning and researching, reading book. Knowledge welding inspection painting inspection.

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