Question 6 of 8: Heat Treatments for 0.45% C Steel; Brass and Stainless-Steel Corrosion

Question 6: Heat Treatments for 0.45% C Steel; Brass and Stainless-Steel Corrosion (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. 0.45% plain-carbon steel (hypoeutectoid); CA260 (70Cu-30Zn) cartridge brass; austenitic stainless steel weldments.

Find. (a) Heat-treatment routes to each named microstructure. (b) Season cracking and dezincification mechanisms and mitigation. (c) Weld decay mechanism and mitigation.

Approach

Part (a) maps each named microstructure onto the cooling path (from austenite) needed to produce it, read against a schematic CCT curve. Parts (b) and (c) are electrochemical corrosion mechanisms specific to two named alloy families; each is explained by identifying the local electrochemical or metallurgical condition that creates a corrosion cell, then the practical fix that removes that condition.

1. (a) Ferrite + pearlite. Austenitize above the upper critical temperature, then cool slowly (furnace cool / full anneal, or a somewhat faster air-cool / normalize for a finer pearlite spacing) so the transformation tracks the C-curve's high- temperature nose region — proeutectoid ferrite forms first, followed by pearlite at the eutectoid.
2. All martensite. Austenitize, then quench rapidly enough (e.g. water quench) that the cooling curve misses the nose of the C-curve entirely, reaching the martensite-start temperature $M_s$ before any diffusional transformation can begin; the diffusionless shear transformation to martensite then proceeds on further cooling through $M_s$ down to $M_f$.
3. All bainite. Austenitize, then quench rapidly to a temperature between the nose of the C-curve and $M_s$, and hold isothermally (austempering, typically in a salt bath) until the transformation to bainite is complete, then air-cool to room temperature.
4. All spheroidite. Hold the steel for a long time at a temperature just below the eutectoid (sub-critical anneal), or equivalently temper an already-formed martensite/pearlite structure for a long time at high temperature; the driving force to minimize interfacial (cementite/ferrite) surface energy slowly spheroidizes the cementite into discrete spherical particles in a ferrite matrix.
5. (b) Season cracking (stress corrosion cracking) in CA260 brass. This is intergranular cracking that requires the simultaneous presence of (i) a residual or applied tensile stress (commonly locked in by cold work such as deep drawing, without a subsequent stress relief) and (ii) a specific corrosive environment, classically ammonia or amines (moist air near ammonia sources, fertilizer or cleaning-agent vapors) that selectively attacks stressed grain boundaries in the zinc-rich brass. Minimized by: a low-temperature stress-relief anneal after cold forming (removes the residual tensile stress without significantly softening the part), avoiding ammoniacal/amine environments, or using a lower-zinc brass composition.
6. Dezincification (selective leaching). Zinc, the less noble constituent of the Cu-Zn solid solution, preferentially dissolves out of the brass (particularly in stagnant, slightly acidic, or chloride-containing waters), leaving behind a porous, mechanically weak, copper-rich (often redeposited as spongy copper) layer with the brass's original shape but little of its strength. Minimized by: adding a small amount of arsenic, antimony, or phosphorus (an "inhibited" brass, e.g. admiralty brass), or using a low-zinc (<15% Zn) brass / red brass composition, or cathodic protection in aggressive service.
7. (c) Weld decay in austenitic stainless steel. During welding, the heat-affected zone adjacent to the weld passes through the sensitization range (roughly 450–850°C) long enough for chromium carbides ($\text{Cr}_{23}\text{C}_6$) to precipitate at grain boundaries; because chromium diffuses far more slowly than carbon, the carbide growth locally depletes the adjacent grain-boundary region of chromium below the roughly 12 wt% needed to sustain the passive oxide film, creating a narrow, chromium-depleted, anodic path that corrodes preferentially — intergranular attack in a band paralleling the weld. Minimized by: using a low-carbon grade (304L, 316L, carbon kept below the level needed for significant carbide precipitation), a stabilized grade (321 with Ti, or 347 with Nb, which preferentially tie up carbon as $\text{TiC}$/$\text{NbC}$ instead of chromium carbide), or a post-weld solution-anneal-and-quench that redissolves the carbides and re-homogenizes the chromium before it can re-precipitate.