CWI Welding Process Principles 5 — Questions and Answers
Question 1: What is the primary metallurgical reason that austenitic stainless steels can experience 'sensitization' during welding?
- Hydrogen absorption from the shielding gas
- Precipitation of chromium carbides at grain boundaries, depleting adjacent areas of chromium (Correct answer)
- Formation of ferrite stringers in the weld metal
- Oxidation of the weld pool surface
Correct answer: Precipitation of chromium carbides at grain boundaries, depleting adjacent areas of chromium
When heated in the sensitization range (about 800–1600°F), carbon combines with chromium at grain boundaries to form chromium carbides, creating chromium-depleted zones susceptible to intergranular corrosion.
Question 2: Which condition is MOST likely to cause 'incomplete fusion' in a weld?
- Excessive preheat
- Insufficient amperage or improper electrode angle preventing the arc from melting the base metal sidewall (Correct answer)
- Too slow a travel speed
- Excessive shielding gas flow rate
Correct answer: Insufficient amperage or improper electrode angle preventing the arc from melting the base metal sidewall
Incomplete fusion occurs when the weld metal fails to fuse with the base metal or adjacent weld passes, most commonly due to insufficient heat input, poor technique, or incorrect electrode angle.
Question 3: What does 'heat input' (kJ/in or kJ/mm) quantify in welding?
- Total weld bead length produced per electrode
- Energy transferred per unit length of weld, calculated from voltage, amperage, and travel speed (Correct answer)
- Maximum interpass temperature allowed by the WPS
- Thermal efficiency of the welding power source
Correct answer: Energy transferred per unit length of weld, calculated from voltage, amperage, and travel speed
Heat input = (Amps × Volts × 60) / (Travel Speed in in/min), expressed in kJ/in; it governs cooling rates, grain growth, and mechanical properties of the weld and HAZ.
Question 4: In friction stir welding (FSW), the joint is made by:
- Melting the base metal with a high-energy laser beam
- A rotating non-consumable tool that generates frictional heat and mechanically intermixes the plasticized material (Correct answer)
- Resistance heating at the faying surface under hydraulic pressure
- An arc between a consumable electrode and the workpieces in an inert gas envelope
Correct answer: A rotating non-consumable tool that generates frictional heat and mechanically intermixes the plasticized material
FSW uses a rotating shoulder-and-pin tool to generate frictional heat that plasticizes the base metal without melting it; the pin mechanically stirs the softened material to form a solid-state weld.
Question 5: When welding with an E7018 electrode, the '18' suffix designates:
- The electrode diameter in millimeters
- A low-hydrogen coating with iron powder, usable in all positions with DCEP or AC (Correct answer)
- The minimum yield strength in ksi
- The maximum preheat temperature in degrees Fahrenheit
Correct answer: A low-hydrogen coating with iron powder, usable in all positions with DCEP or AC
Per AWS A5.1, the last two digits '18' denote a low-hydrogen iron powder covering suitable for all positions with DCEP or AC, providing excellent mechanical properties and crack resistance.
Question 6: Which process variable has the GREATEST effect on depth of penetration in SAW?
- Flux type
- Welding current (amperage) (Correct answer)
- Travel speed
- Electrode extension (stick-out)
Correct answer: Welding current (amperage)
In SAW, welding current (amperage) is the primary variable controlling penetration depth; higher current produces a more forceful arc that drives deeper into the base metal.
Question 7: A CWI reviewing a welding procedure specification (WPS) notices that the listed process is FCAW-G with 75% Ar / 25% CO2 shielding gas. What concern would replacing this with 100% CO2 raise?
- 100% CO2 would eliminate all spatter
- 100% CO2 increases oxidizing potential and spatter, and may affect mechanical properties, requiring requalification (Correct answer)
- 100% CO2 is required for all FCAW-G electrodes
- 100% CO2 provides cleaner welds with improved appearance
Correct answer: 100% CO2 increases oxidizing potential and spatter, and may affect mechanical properties, requiring requalification
100% CO2 is more oxidizing and produces higher spatter levels compared to Ar/CO2 blends; changing shielding gas composition is an essential variable in most codes and typically requires WPS requalification.
CWI Welding Process Principles 5
What is the primary metallurgical reason that austenitic stainless steels can experience 'sensitization' during welding?