CWI Metallurgy and Weld Discontinuities Questions and Answers — Questions and Answers
Question 1: In the heat-affected zone (HAZ) of a single-pass carbon steel weld, which region is characterized by the largest grain size and is typically located immediately adjacent to the fusion boundary?
- Grain-coarsened region (GCHAZ) (Correct answer)
- Grain-refined region (FGHAZ)
- Partially transformed region
- Tempered region
Correct answer: Grain-coarsened region (GCHAZ)
The grain-coarsened region (GCHAZ) experiences the highest temperatures in the HAZ, just below the melting point of the material. This high heat input allows for significant austenitic grain growth, which results in a coarse grain structure upon cooling. This region is located directly next to the fusion line and often exhibits the lowest toughness within the HAZ.
Question 2: A welder completes a groove weld on a thick section of high-strength, low-alloy (HSLA) steel using an SMAW electrode that was not stored in a proper rod oven. An inspection performed 48 hours later reveals a crack in the heat-affected zone. What is the most likely type and cause of this discontinuity?
- Solidification crack caused by high sulfur content.
- Lamellar tear caused by through-thickness strain.
- Hydrogen-induced crack caused by moisture in the electrode. (Correct answer)
- Crater crack caused by improper weld termination.
Correct answer: Hydrogen-induced crack caused by moisture in the electrode.
The combination of three factors points to hydrogen-induced cracking (also called delayed or cold cracking): a susceptible microstructure (the HAZ of HSLA steel), sufficient residual stress (from welding), and the presence of diffusible hydrogen (from moisture in the improperly stored electrode). The delayed nature of the crack appearing 48 hours later is also a classic indicator of this mechanism.
Question 3: During fabrication, a T-joint is made by welding a plate perpendicularly onto the surface of another, thicker plate. After welding, a crack is found in the base material of the thicker plate, running parallel to the plate surface and having a step-like appearance. This discontinuity is best described as:
- A fatigue crack.
- A hot crack.
- A lack of fusion discontinuity.
- A lamellar tear. (Correct answer)
Correct answer: A lamellar tear.
Lamellar tearing is a specific type of cracking that occurs in the base material, beneath the weld. It is caused by high through-thickness strains acting on a material with poor ductility in that direction, often due to the presence of flattened, non-metallic inclusions. The T-joint configuration, which induces high stress perpendicular to the plate surface, and the crack's location and step-like appearance are classic indicators of lamellar tearing.
Question 4: A welding procedure for a thick-walled chrome-moly pressure vessel requires post-weld heat treatment (PWHT). What is the primary metallurgical reason for performing this step?
- To increase the ultimate tensile strength of the weld.
- To refine the as-welded grain structure of the weld metal.
- To temper brittle microstructures and reduce residual stresses. (Correct answer)
- To remove entrapped slag from within the weld deposit.
Correct answer: To temper brittle microstructures and reduce residual stresses.
PWHT is primarily used to relieve the high residual stresses induced by welding and to temper any hard, brittle microstructures (like martensite) that may have formed in the weld and HAZ during cooling. This process improves ductility, toughness, and resistance to brittle fracture. It does not typically increase tensile strength (it often slightly reduces it) or refine the grain structure.
Question 5: Solidification cracking, also known as hot cracking, is most likely to occur in a weld metal that has:
- A high concentration of deoxidizers like silicon and manganese.
- A very low carbon equivalent and slow cooling rate.
- A fully austenitic microstructure with low residual stress.
- A wide solidification temperature range and high levels of impurities like sulfur and phosphorus. (Correct answer)
Correct answer: A wide solidification temperature range and high levels of impurities like sulfur and phosphorus.
Hot cracking occurs at high temperatures as the weld metal solidifies. It is promoted by a wide solidification temperature range, which creates a prolonged period where weak, low-melting-point liquid films exist between solidifying grains. Tensile stresses from shrinkage can then easily pull these grains apart. Impurities like sulfur and phosphorus exacerbate this by forming these weak liquid films.
Question 6: Which of the following is the LEAST likely cause of porosity in a Gas Metal Arc Welding (GMAW) weld?
- Inadequate shielding gas flow rate.
- Contaminants such as oil or moisture on the base material.
- Low travel speed with appropriate heat input. (Correct answer)
- An excessively long electrode extension (stickout).
Correct answer: Low travel speed with appropriate heat input.
Low travel speed generally allows more time for gases to escape the molten weld pool, thus reducing the likelihood of porosity. Inadequate shielding gas flow, contaminants on the base material, and excessive electrode extension are all very common causes of gas entrapment (from the atmosphere or from the contaminants themselves) which directly leads to porosity.
CWI Metallurgy and Weld Discontinuities Questions and Answers
In the heat-affected zone (HAZ) of a single-pass carbon steel weld, which region is characterized by the largest grain size and is typically located immediately adjacent to the fusion boundary?