Question 4 of 7: Heat-Treated Microstructures in 0.45%C Steel; Season Cracking and Dezincification in Brass; Weld Decay

Question 4: Heat-Treated Microstructures in 0.45%C Steel; Season Cracking and Dezincification in Brass; Weld Decay (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. (a) A 0.45% plain-carbon (hypoeutectoid) steel; four target microstructures. (b) Cartridge brass CA260 (70Cu–30Zn), susceptible to season cracking (SCC) and dezincification. (c) An austenitic stainless steel weldment showing intergranular attack in a band near the weld ("weld decay").

Find. (a) The heat-treatment route to each named microstructure. (b) Mechanisms of season cracking and dezincification, and how each is minimized. (c) Why weld decay occurs, and how to minimize it.

Approach

Each part is answered from the governing transformation diagram or corrosion mechanism: (a) uses the TTT/CCT diagram for 0.45%C steel (which cooling path/isothermal hold produces which product); (b)–(c) are electrochemical/metallurgical corrosion mechanisms, each paired with the standard industrial mitigation.

1. (a) Ferrite $+$ pearlite. Austenitize above $A_3$ ($\approx840$–$860^\circ$C for this hypoeutectoid composition), then slow-cool (furnace-cool/anneal, or air-cool/normalize) through the transformation range. Pro-eutectoid ferrite nucleates first at prior austenite grain boundaries as $T$ drops below $A_3$; the remaining, carbon-enriched austenite then transforms to pearlite (alternating ferrite/cementite lamellae) on crossing $A_1$ ($\approx727^\circ$C). On a CCT diagram this is any cooling curve slow enough to pass through the ferrite-then-pearlite "nose" region.
2. (a) All martensite. Austenitize, then quench rapidly (e.g. water quench) fast enough that the cooling curve bypasses the nose of the CCT diagram entirely (missing the ferrite/pearlite and bainite noses), continuing below $M_s$ down through $M_f$. The diffusionless, shear-type transformation converts all remaining austenite directly to supersaturated, body-centred-tetragonal martensite — no time is available for any carbon-diffusion-controlled product to form.
3. (a) All spheroidite. Starting from a pearlitic (or martensitic) structure, hold isothermally at a temperature just below $A_1$ ($\approx700^\circ$C) for a long time (many hours). At that temperature, the lamellar (or fine tempered-martensite) cementite is unstable relative to spherical particles, which minimize interfacial energy, so the cementite slowly coarsens into globular particles dispersed through a continuous ferrite matrix — the softest, most machinable structure available for this steel.
4. (a) All bainite. Austenitize, then quench rapidly to an intermediate temperature that lies between the pearlite nose and $M_s$ (roughly $250$–$550^\circ$C for this steel), and hold isothermally there (austempering) until the isothermal transformation is complete before finally air-cooling to room temperature. This produces bainite (fine ferrite laths/plates with dispersed carbides) — a structure distinct from both pearlite's lamellar morphology and martensite's supersaturated plate structure.
5. (b) Season cracking (stress corrosion cracking). SCC in brass requires the simultaneous presence of (1) a tensile stress — commonly a residual stress left over from cold-forming operations like deep drawing that were never stress-relieved — and (2) a specific aggressive environment, classically an ammoniacal atmosphere (also amines or mercury compounds), which attacks brass along grain boundaries; the crack propagates intergranularly with little visible bulk ductility, and failure can be delayed and sudden. Minimization: a low-temperature stress-relief anneal ($\approx250$–$300^\circ$C, well below the recrystallization temperature so cold-worked strength is largely retained) after forming, avoidance of ammonia-bearing environments, or selection of a lower-zinc, more SCC-resistant alloy.
6. (b) Dezincification. This is a form of selective leaching in which zinc is preferentially dissolved out of the brass, leaving a porous, mechanically weak, copper-rich residual structure — either a broad "layer-type" attack or localized "plug-type" attack — often with little visible change to the part's outward geometry despite a drastic strength loss. Susceptibility increases with zinc content (higher-Zn alloys like this 30%-Zn CA260 are more prone than low-Zn red brasses) and is worst in stagnant, low-flow, warm, or chloride-rich water. Minimization: use an inhibited brass (small As, Sb, or P additions — "arsenical"/Admiralty-type brass — that suppress the selective-dissolution mechanism), switch to a lower-zinc or dezincification-resistant alloy, or apply cathodic protection/water treatment.
7. (c) Why weld decay occurs. During welding, the heat-affected zone (HAZ) adjacent to the fusion line — not the weld metal, which cools too quickly, and not the base metal far from the weld, which never gets hot — is held transiently in the sensitizing range ($\approx450$–$850^\circ$C) long enough for chromium carbides (Cr$_{23}$C$_6$) to precipitate at the austenite grain boundaries. Because chromium diffuses to the growing carbide faster than it can diffuse in from the grain interior to replenish the boundary region, a narrow zone right at the boundary is left depleted below the $\approx12\%$ Cr needed to sustain the passive oxide film; this Cr-depleted zone becomes anodic relative to the Cr-rich grain interiors and corrodes preferentially in an intergranular pattern — located specifically in the HAZ band, not the fusion zone itself.
8. (c) Minimizing weld decay. Use a low-carbon grade (304L, 316L; carbon below $\approx0.03\%$ sharply slows Cr$_{23}$C$_6$ nucleation/growth kinetics), use a stabilized grade (321 with Ti, 347 with Nb, which preferentially form TiC/NbC and keep chromium in solid solution), or perform a post-weld solution anneal (heat above $\approx1000$–$1050^\circ$C to redissolve any carbides, then quench rapidly through the sensitizing range to prevent reprecipitation).