456A Metallurgy & Heat Treatment 2 — Questions and Answers
Question 1: What is the difference between 'normalizing' and 'annealing' heat treatments for carbon steel?
- Normalizing uses air cooling to produce a fine pearlite microstructure; annealing uses furnace (slow) cooling to produce a softer, coarser structure (Correct answer)
- Normalizing heats above Ac3; annealing heats below Ac1 only
- Annealing is used to harden steel; normalizing is used to soften it
- They are identical processes with different names
Correct answer: Normalizing uses air cooling to produce a fine pearlite microstructure; annealing uses furnace (slow) cooling to produce a softer, coarser structure
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?
- 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 (Correct answer)
- The iron-carbon compound (Fe3C) that forms hard, brittle zones
- The body-centred cubic iron phase stable at room temperature
- A fine mixture of ferrite and cementite formed during slow cooling
Correct 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
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?
- Notch toughness (energy absorbed during fracture) at specified temperatures, indicating resistance to brittle fracture (Correct answer)
- Tensile strength and yield point of the weld metal
- Hardness of the HAZ across the weld
- Fatigue life under cyclic loading conditions
Correct answer: Notch toughness (energy absorbed during fracture) at specified temperatures, indicating resistance to brittle fracture
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?
- 250 HV maximum (Correct answer)
- 350 HV maximum
- 300 HV maximum
- No hardness limit is specified for HAZ
Correct answer: 250 HV maximum
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?
- Cracking from the combined effect of tensile stress and a corrosive environment; austenitic stainless steel in chloride-containing environments is the most common concern (Correct answer)
- Cracking from mechanical fatigue in corrosive media; most common in carbon steel exposed to water
- Hydrogen blistering from wet H2S; most common in sour crude oil pipelines
- Galvanic corrosion at the weld toe of dissimilar metal joints
Correct 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
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.
Question 6: What is 'carbon migration' (carbon diffusion) in dissimilar metal welds between carbon steel and stainless steel?
- Carbon migrates from the carbon steel into the stainless steel weld metal at elevated service temperatures, creating a decarburized zone in the carbon steel and a carburized zone in the stainless (Correct answer)
- Carbon from the stainless steel deposits onto the carbon steel surface during welding, causing surface hardening
- Carbon precipitates at the weld interface as cementite, forming a brittle layer
- Carbon diffuses only during PWHT, not during welding or service
Correct answer: Carbon migrates from the carbon steel into the stainless steel weld metal at elevated service temperatures, creating a decarburized zone in the carbon steel and a carburized zone in the stainless
In dissimilar metal welds between carbon steel and austenitic stainless steel operating at elevated temperatures (above ~400°C), carbon migrates from the lower chromium (carbon steel) side toward the higher chromium (stainless) side, where it is more thermodynamically stable. This creates a soft, weak decarburized zone in the carbon steel HAZ and a hard, brittle carburized zone in the stainless weld metal, reducing creep and fatigue resistance.
456A Metallurgy & Heat Treatment 2
What is the difference between 'normalizing' and 'annealing' heat treatments for carbon steel?