456A Metallurgy & Heat Treatment 2

Question 1

What is the difference between 'normalizing' and 'annealing' heat treatments for carbon steel?

Accepted Answer

Normalizing uses air cooling to produce a fine pearlite microstructure; annealing uses furnace (slow) cooling to produce a softer, coarser structure

Answer

Both normalizing and annealing heat carbon steel above Ac3 (austenite region) to refine grain structure and relieve stresses. Normalizing uses air cooling, producing a fine pearlite microstructure with moderate strength. Annealing uses very slow furnace cooling, producing a coarser, softer microstructure with maximum ductility. Normalizing is used to refine grain and improve toughness; annealing is used to maximize machinability or ductility.

Question 2

What does 'austenite' mean in the context of carbon steel metallurgy?

Accepted Answer

The face-centred cubic (FCC) iron phase stable at high temperatures (above approximately 727°C for pure iron) that can dissolve large amounts of carbon

Answer

Austenite is the high-temperature FCC phase of iron (gamma iron) that is stable above approximately 727°C (the A1 temperature). Its FCC crystal structure can accommodate up to 2.14% carbon in solid solution. During welding, the base metal adjacent to the fusion line is heated into the austenite region; the resulting microstructure after cooling depends on the cooling rate and composition.

Question 3

What is the Charpy impact test used to measure in welded joints?

Accepted Answer

Notch toughness (energy absorbed during fracture) at specified temperatures, indicating resistance to brittle fracture

Answer

The Charpy V-notch impact test measures the energy (in joules) absorbed when a notched specimen is fractured by a swinging pendulum hammer. For weld qualification, specimens are taken from the weld metal and HAZ and tested at the specified minimum design temperature. The test quantifies toughness — resistance to brittle fracture — which is critical for structural applications in cold climates.

Question 4

What is the Vickers Hardness (HV) limit typically specified for the HAZ in sour service (H2S) environments per NACE MR0175/ISO 15156?

Accepted Answer

250 HV maximum

Answer

NACE MR0175/ISO 15156 (materials for sour service) specifies a maximum HAZ hardness of 250 HV (approximately 22 HRC) for carbon and low-alloy steels to prevent sulfide stress cracking (SSC). Harder zones are more susceptible to hydrogen embrittlement from H2S environments. This limit drives preheat, heat input, and PWHT requirements for sour service welding.

Question 5

What is 'stress corrosion cracking' (SCC) and which material-environment combination is most relevant for Canadian pressure piping welding?

Accepted Answer

Cracking from the combined effect of tensile stress and a corrosive environment; austenitic stainless steel in chloride-containing environments is the most common concern

Answer

SCC requires three simultaneous conditions: susceptible material, tensile stress (residual or applied), and specific corrosive environment. Austenitic stainless steel in chloride environments (including some process streams and marine atmospheres) is the classic SCC combination. Residual welding stresses are a major contributor; PWHT or post-weld stress relief can reduce susceptibility.

Red Seal Welder 456A Practice Test

Red Seal Welder 456A Practice Test

456A Metallurgy & Heat Treatment 2