Welding and Metal Working Flashcards

When welding high-strength low-alloy (HSLA) steel with a carbon equivalent (CE) of 0.45, which preheat temperature range is most appropriate to prevent hydrogen-induced cracking?

Answer: 300°F–400°F (149°C–204°C)

A carbon equivalent of 0.45 places HSLA steel in a moderate-to-high susceptibility range for hydrogen-induced cracking (HIC). AWS D1.1 and metallurgical practice call for a preheat of roughly 300°F–400°F for this CE value to slow the cooling rate, allow hydrogen to diffuse out, and prevent brittle martensite formation in the heat-affected zone. Lower preheats are insufficient; the very high range (500°F+) is reserved for higher CE or thicker sections.

A welder notices porosity concentrated at the start of each GMAW bead but not in the middle or end. The most likely cause is:

Answer: Inadequate shielding gas coverage due to slow pre-flow purge time

Start porosity in GMAW is a classic symptom of insufficient pre-flow time: when the arc strikes before shielding gas has purged the nozzle and established proper coverage, atmospheric oxygen and nitrogen are trapped in the initial weld pool. The middle and end of the bead are unaffected because gas flow is fully established. Wire feed speed and voltage issues would cause more uniform or travel-speed-related defects, and base metal contamination would produce distributed porosity.

Which grain structure change occurs in the heat-affected zone (HAZ) of a carbon steel weld immediately adjacent to the fusion line, and what mechanical property is most adversely affected?

Answer: Coarse-grained austenite forms and transforms to coarse martensite or bainite; toughness (impact strength) decreases

Immediately adjacent to the fusion line, the base metal is heated above the grain-coarsening temperature (~1100°C for carbon steel). Austenite grains grow rapidly at these temperatures. Upon cooling, this coarse austenite transforms to coarse martensite or bainite, which is hard but brittle. Toughness — measured by Charpy impact tests — drops significantly in this coarse-grained HAZ region. This is why post-weld heat treatment (stress relief or normalizing) is often required for critical applications.

When using the plasma arc cutting (PAC) process to cut stainless steel, nitrogen is often preferred over straight compressed air as the plasma gas primarily because:

Answer: Nitrogen prevents chromium carbide precipitation and oxidation of the cut face, preserving corrosion resistance

Stainless steel's corrosion resistance depends on its passive chromium oxide layer. Using compressed air introduces oxygen and nitrogen together, but the oxygen causes heavy oxidation and chromium depletion on the cut edge, and can promote sensitization (chromium carbide precipitation at grain boundaries). Nitrogen or nitrogen-hydrogen mixtures shield the molten metal from oxidation, producing a cleaner cut face that retains corrosion resistance and requires less post-cut cleanup. Kerf width and cost are secondary considerations that do not justify the choice metallurgically.

A maintenance technician uses an oxyacetylene torch to straighten a bent structural steel beam by applying heat. After heating the convex side and allowing it to cool, the beam curves MORE in the wrong direction. The most probable error was:

Answer: Heat was applied to the convex (outside of the bend) surface instead of the concave (inside) surface

Flame straightening works by creating localized thermal expansion followed by constrained contraction. To straighten a bend, heat must be applied to the concave (inside, shorter) side of the curve. Heating this side causes it to expand plastically, then contract upon cooling — pulling the bow straight. Heating the convex (outside, longer) side does the opposite: it causes that side to contract further, increasing the bend. This is a fundamental error in flame straightening technique and produces results opposite to the goal.

In resistance spot welding of dissimilar-thickness aluminum sheets (1.0 mm and 2.5 mm), the weld nugget consistently forms offset toward the thinner sheet rather than centered at the faying surface. The most effective corrective adjustment is:

Answer: Use offset electrodes with a larger dome radius on the thinner-sheet side to increase electrical resistance and heat concentration on that side

In resistance spot welding, heat is generated at points of highest electrical resistance. The thicker sheet has greater bulk resistance and thermal mass, so the nugget naturally migrates toward the thinner, lower-resistance sheet. Compensating by using a larger dome-radius (or smaller contact area) electrode on the thinner-sheet side concentrates current density there, increasing contact resistance and localized heat on that side — effectively shifting the nugget back toward the interface. Reducing current simply makes a smaller nugget without fixing its location; hold time and flat-face electrodes do not address the resistance imbalance.