API 653 Welding & Metallurgy 3 — Questions and Answers
Question 1: Which metallurgical phenomenon occurs when carbon migrates from the base metal to the weld deposit during welding of dissimilar steels?
- Sensitization
- Carbon migration (decarburization/carburization) (Correct answer)
- Weld decay
- Sigma phase formation
Correct answer: Carbon migration (decarburization/carburization)
Carbon migrates from the lower-alloy steel toward the higher-alloy weld deposit or vice versa at elevated temperatures, creating a decarburized zone susceptible to creep.
Question 2: Per API 653, when is a welding procedure qualification test required before making repairs to a tank in service?
- Only when welding on tanks built before 1970
- When the repair involves a new base metal, filler metal, or process not previously qualified (Correct answer)
- Only when the tank stores flammable liquids
- Only for repairs exceeding 12 inches in length
Correct answer: When the repair involves a new base metal, filler metal, or process not previously qualified
API 653 requires a qualified welding procedure specification (WPS) supported by a procedure qualification record (PQR) whenever the combination of variables falls outside previously qualified ranges.
Question 3: What is 'lamellar tearing' and what plate orientation makes it most likely to occur?
- Surface cracking caused by rapid cooling; most likely in vertical welds
- Subsurface cracking caused by through-thickness tensile stress in plates with low short-transverse ductility (Correct answer)
- Heat-affected zone cracking due to hydrogen; most likely in overhead welds
- Weld centerline cracking due to solidification shrinkage
Correct answer: Subsurface cracking caused by through-thickness tensile stress in plates with low short-transverse ductility
Lamellar tearing occurs when restrained welded joints place through-thickness tensile stress on plates containing elongated MnS inclusions with low ductility in that direction.
Question 4: In API 653 tank inspection, what is the significance of a 'heat affected zone' (HAZ) in the context of corrosion?
- The HAZ is always more corrosion resistant than base metal
- The HAZ can have altered microstructure and residual stresses that increase susceptibility to certain corrosion mechanisms (Correct answer)
- The HAZ is unaffected by welding heat and behaves identically to base metal
- The HAZ has higher carbon content than the weld metal
Correct answer: The HAZ can have altered microstructure and residual stresses that increase susceptibility to certain corrosion mechanisms
The thermal cycle during welding can sensitize stainless steels, harden carbon steels, and introduce residual stresses in the HAZ, all of which can accelerate corrosion under certain conditions.
Question 5: Which electrode classification would be selected when making low-hydrogen welds on A516 Grade 70 carbon steel in accordance with AWS A5.1?
- E6010
- E6013
- E7018 (Correct answer)
- E7024
Correct answer: E7018
E7018 is a low-hydrogen electrode (H8 designation) suitable for structural steels with higher strength and/or thickness, where hydrogen-induced cracking is a concern.
Question 6: What is the primary reason API 653 limits the maximum hardness of a completed repair weld (including HAZ) in a tank shell?
- High hardness indicates poor fusion and must be rejected
- Excessive hardness indicates a martensitic microstructure susceptible to hydrogen stress cracking and brittle fracture (Correct answer)
- High hardness reduces the weld's tensile strength below minimum requirements
- Hardness limits ensure the weld is softer than the base metal for cathodic protection purposes
Correct answer: Excessive hardness indicates a martensitic microstructure susceptible to hydrogen stress cracking and brittle fracture
Hardness above approximately 200 HBW (Brinell) in carbon steels indicates a martensitic structure that is susceptible to hydrogen-induced cracking and exhibits reduced toughness.
Question 7: During a hot tap on an in-service tank shell, what is the critical metallurgical concern that dictates the minimum shell thickness acceptable for the procedure?
- Ensuring adequate material for threading the fitting
- Preventing burn-through due to insufficient heat sink on thin material in contact with flammable product (Correct answer)
- Maximizing joint efficiency of the attached fitting
- Avoiding distortion of the shell plate during cooling
Correct answer: Preventing burn-through due to insufficient heat sink on thin material in contact with flammable product
Thin shell plates have insufficient mass to conduct heat away from the weld, risking burn-through into the tank contents; API 653 specifies minimum thickness limits for hot tapping.
API 653 Welding & Metallurgy 3
Which metallurgical phenomenon occurs when carbon migrates from the base metal to the weld deposit during welding of dissimilar steels?