21-Mat-A5 Phase Transformations and Thermal Treatment · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2013 — 10-Met-A5, Mechanical Behaviour and Fracture of Materials. Three hours, closed book, any non-communicating calculator permitted. Eight questions of 20 marks each; the rubric states that five questions constitute a complete paper and that only the first five appearing in the answer book are marked. All eight are answered here. Several questions ask explicitly for essay-format answers, and the marking scheme rewards clarity and organisation, so the answers below are written as structured prose rather than as note form.
The printed exam header reads 10-Met-A5, Mechanical Behaviour and Fracture of Materials. The paper has no phase-transformation or heat-treatment question in the classical (TTT/CCT diagram, hardenability, tempering-curve) sense; the syllabus actually examined is deformation, strengthening, creep, fatigue, fracture, toughening, deformation processing and environmental degradation. The answers below are written to the printed subject.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
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.
All four are forms of environmentally assisted cracking: in each, a material that would be perfectly ductile in air, and perfectly stable in the environment without stress, fails in a brittle manner when the two act together. What distinguishes them is the nature of the aggressive species, whether the stress is static or cyclic, and where the damage is done — and each leaves a recognisable fracture surface.
Conditions. SCC requires three things simultaneously: a susceptible alloy, a specific chemical environment, and a sustained tensile stress. Remove any one and cracking stops. The alloy–environment pairing is remarkably specific: austenitic stainless steel cracks in hot chloride solutions but is immune in ammonia; $\alpha$-brass cracks in ammonia (the “season cracking” of cold-drawn cartridge cases) but not in chlorides; carbon steel cracks in caustic, in nitrates and in carbonate–bicarbonate; high-strength aluminium alloys crack in chloride solutions in the short-transverse direction. The stress need not be applied — residual stress from welding, cold forming or a shrink fit is the usual culprit, which is why SCC so often appears in a structure that is nominally unloaded. There is a threshold stress intensity, $K_{ISCC}$, below which cracks do not propagate, and the operative temperature range is often narrow (chloride SCC of stainless steel essentially requires above about 60 °C).
Appearance. Macroscopically the failure is brittle: no necking, no measurable general corrosion, often a bright fracture on a component that looks new. The signature is branching — a main crack that divides repeatedly into a tree-like network, best seen on a metallographic cross-section, with corrosion product inside the crack. The path may be intergranular (sensitised stainless steel, where chromium-depleted zones flank the grain-boundary carbides; caustic cracking of steel) or transgranular (chloride SCC of austenitic stainless steel, which produces fan-shaped, cleavage-like facets with fine river markings and feather-like arrest features). The distinguishing point for the examiner is that the crack is slow and steady, its growth rate over a wide range of $K$ is nearly independent of stress intensity (the plateau region), and it is driven by the environment rather than by the load.
Conditions. HIC needs a source of atomic hydrogen, a susceptible high-strength or hard microstructure, and a tensile stress or triaxial constraint. Hydrogen enters as an atom, not as H$_2$: from cathodic reactions during corrosion or cathodic protection, from acid pickling and electroplating, from welding with damp consumables, and above all from wet H$_2$S service, where the sulphide ion poisons the hydrogen-recombination reaction and drives almost all of the discharged hydrogen into the steel instead of into gas bubbles. Susceptibility rises steeply with strength: quenched-and-tempered steels above roughly 1000 MPa tensile strength become highly vulnerable in almost any hydrogen-charging environment, and in sour service the threshold is lower still — the basis of the 22 HRC (about 790 MPa tensile) hardness cap for carbon and low-alloy steels in NACE MR0175/ISO 15156. Two further features distinguish it from SCC. The damage is delayed — failure occurs hours or days after loading, while the hydrogen diffuses to the triaxially stressed region ahead of the crack tip — and it is reversible before cracking begins, so a low-temperature bake (for example 190 °C for several hours after plating) drives the hydrogen out and restores the ductility. Susceptibility is worst near room temperature: too cold and the hydrogen cannot diffuse, too hot and it does not stay.
Appearance. In a high-strength quenched-and-tempered steel the fracture is typically intergranular along prior-austenite grain boundaries, with a characteristically smooth, faceted, “rock candy” appearance and often fine hairlines on the facets; there is no ductility and no reduction of area. In lower-strength line-pipe steel the manifestation is different and has its own names: hydrogen collects at elongated MnS inclusions and at banded segregation, recombines to molecular H$_2$, and generates enormous internal pressure, producing planar blisters and stepwise, ladder-like internal cracking parallel to the plate surface with no external stress at all. The mechanisms invoked are hydrogen-enhanced decohesion (HEDE) at the boundaries and hydrogen-enhanced localised plasticity (HELP) at the crack tip.
Conditions. Corrosion fatigue requires a cyclic stress in a corrosive environment, and its defining characteristic is that no specific alloy–environment couple is needed. Any environment that corrodes the metal at all — seawater, humid air, condensate, produced water — will reduce the fatigue life of any alloy in it. This is the practical distinction from SCC, which is confined to particular pairs, and it makes corrosion fatigue the more insidious of the two. Two consequences follow for design. First, the endurance limit disappears: a steel that shows a flat S–N curve at $10^{7}$ cycles in air shows a continuously falling curve in seawater, so there is no safe stress amplitude and design must be for finite life. Second, the damage is strongly frequency-dependent — lower frequencies are far more damaging, because the environment has more time to act per cycle — whereas inert fatigue is essentially frequency-independent. Cathodic protection helps, but overprotection substitutes hydrogen embrittlement for corrosion fatigue.
Appearance. Initiation is at corrosion pits, which act as the stress concentrators that persistent slip bands would otherwise have had to create, so the initiation stage — usually most of the life in air — is largely bypassed. The fracture surface is macroscopically that of a fatigue failure (a flat, beach-marked region growing from an origin, with a final fast-fracture zone), but it is heavily corroded and oxide-covered, so the striations are blunted or obliterated and the beach marks are hard to read. A cross-section typically shows multiple parallel cracks initiating from a line of pits rather than the single dominant crack of ordinary fatigue, and the cracks are usually transgranular and unbranched — the absence of branching is the most reliable metallographic discriminator from SCC.
Conditions. LME requires physical wetting of a stressed solid metal by a specific liquid metal, and a tensile stress. Like SCC, the couples are specific: zinc on steel and on austenitic stainless steel, mercury on brass, on aluminium and on titanium, cadmium on steel and on titanium, lead and bismuth on steel, gallium on aluminium, solder on copper. The liquid must wet the solid, which usually means the oxide film must be disrupted — by deformation, by a flux, or by galvanising practice. The classic industrial cases are: a galvanised structural member being welded or flame-cut, where molten zinc runs into a stressed region; resistance spot welding of galvanised advanced high-strength steel, where the zinc coating melts at the weld's periphery under the electrode's tensile load; a cadmium-plated fastener heated in service; and mercury spillage onto an aluminium heat exchanger. The effect requires no time for diffusion or electrochemistry — a component can fail in milliseconds — and it occurs at or just above the melting point of the embrittling metal, disappearing again if the liquid solidifies or evaporates.
Appearance. The fracture is dramatically brittle in a material that is otherwise fully ductile: a tensile specimen that would show 40 per cent reduction of area shows almost none, and the load drops to zero without warning. The path is normally intergranular, and the giveaway on a metallographic section is a film or wedge of the embrittling metal penetrating along the grain boundaries ahead of and behind the crack, detectable by energy-dispersive analysis in the electron microscope. Cracks are typically short, multiple and open at the surface where the liquid contacted it. The accepted mechanism is adsorption-induced reduction in cohesion: atoms of the liquid metal adsorb at the crack tip and lower the cohesive strength of the bond across the boundary, so the tip stays atomically sharp instead of blunting by plasticity.
A failure analyst separates them by four questions. Was the stress static or cyclic? Cyclic points to corrosion fatigue, static to the other three. Was the environment specific to this alloy, or merely corrosive? A specific couple indicates SCC or LME; general corrosivity indicates corrosion fatigue. Was there a liquid metal present, or a source of atomic hydrogen? These identify LME and HIC respectively. Does the crack branch? Branching is characteristic of SCC, its absence of corrosion fatigue. The metallographic path — intergranular for HIC and LME, either for SCC, transgranular for corrosion fatigue — and the presence of a foreign element on the fracture surface then confirm the diagnosis.
