Welding Metallurgy & Heat Treatment Flashcards

In aluminum welding, the 'heat-affected zone softening' occurs because:

Answer: Heat dissolves strengthening precipitates in precipitation-hardened alloys

In age-hardened (precipitation-hardened) aluminum alloys, welding heat dissolves or overages the strengthening precipitates in the HAZ, significantly reducing hardness and strength.

What is the significance of the Ac1 and Ac3 temperatures in welding a carbon steel?

Answer: Ac1 = lower critical temp (start of austenite on heating); Ac3 = upper critical (all austenite)

Ac1 is the temperature at which austenite begins to form on heating, and Ac3 is the temperature at which the transformation to austenite is complete; the HAZ regions between these temperatures are intercritical zones.

Reheat cracking (stress relief cracking) in low-alloy steel welds is most likely to occur in which zone?

Answer: Coarse-grained HAZ during PWHT or elevated-temperature service

Reheat cracking occurs in the coarse-grained HAZ when carbide precipitation during PWHT strengthens the grain interiors more than the boundaries, causing intergranular cracking under residual stress.

What does a 'weld thermal cycle' describe?

Answer: The time-temperature history experienced by a point in the weld or HAZ

The weld thermal cycle is the complete time-temperature profile (rapid heating, peak temperature, and cooling) experienced by any specific point in the weld metal or surrounding base metal.

Which welding process characteristic most directly controls the width and severity of the heat-affected zone?

Answer: Heat input (energy per unit length of weld)

Heat input (J/mm = voltage × amperage × 60 / travel speed) determines how much thermal energy is deposited per unit length, directly controlling HAZ width, peak temperature, and cooling rate.

What is the primary function of delta ferrite in austenitic stainless steel weld metal?

Answer: Reduces hot cracking susceptibility by dissolving impurity segregates

Delta ferrite (3–8 FN) in austenitic weld deposits is beneficial because it can dissolve low-melting impurities (S, P) that would otherwise segregate to austenite grain boundaries and cause hot cracking.

During cooling of a weld in low-carbon steel, which transformation product forms first from austenite at the highest temperature?

Answer: Grain boundary ferrite (allotriomorphic ferrite)

As austenite cools, ferrite nucleates first at prior austenite grain boundaries (allotriomorphic or grain boundary ferrite) before lower-temperature products like Widmanstätten ferrite or pearlite form.