21-Mat-B6 Ceramic Materials · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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.
Hardenability is a measure of how DEEP a steel can be hardened — i.e. how far below the quenched surface a substantially martensitic structure can be produced — for a given quench severity; it describes the EASE with which martensite forms throughout a section, not the maximum hardness that martensite itself can reach. Two steels can have identical maximum (surface) hardness after quenching yet very different hardenability, if one transforms to non-martensitic products (pearlite/bainite) just a few millimetres below the surface while the other stays martensitic through its full cross-section. Hardenability is conventionally quantified by the Jominy end-quench test (hardness plotted against distance from the water-quenched end of a standard bar).
Temper embrittlement (TE) is a loss of impact toughness (a rise in the ductile-to-brittle transition temperature) that develops in certain alloy steels — typically those containing Mn, Cr and/or Ni together with trace residual impurities such as P, Sb, Sn or As — when they are tempered within, or slow-cooled through, roughly the 350–575 °C range, WITHOUT any corresponding loss of hardness or tensile strength. Two related forms are recognized: "one-step" embrittlement, which develops rapidly on tempering directly in that range and is fully reversible by re-austenitizing; and the classically named (two-step, "reversible") temper embrittlement, which develops more slowly on prolonged holding or slow cooling through the range and is likewise reversible by a fast re-quench from above the range followed by RAPID cooling through it (avoiding a slow re-transit). The most commonly accepted mechanism is equilibrium segregation of the trace impurity elements to prior-austenite grain boundaries, a process thermodynamically promoted by the presence of Mn, Ni and Cr in solid solution (which lower the free energy of segregation for the impurity atoms). The segregated impurity layer weakens grain-boundary cohesive strength, so that when fracture eventually occurs it switches from the normal transgranular/ductile mode to INTERGRANULAR fracture along the embrittled prior-austenite boundaries — an intergranular fracture surface in an otherwise normally hard, strong steel is the classic diagnostic signature of temper embrittlement.
Martensite is a diffusionless, supersaturated interstitial solid solution of carbon trapped in a body-centred TETRAGONAL (BCT), not cubic, iron lattice — the carbon atoms that would normally have partitioned into cementite during a diffusional transformation are instead frozen in octahedral interstitial sites, and they distort the surrounding lattice anisotropically, stretching it along the tetragonal $c$-axis (the $c/a$ ratio rises roughly linearly with wt% C). This lattice distortion creates strong local strain fields that strongly impede dislocation motion (interstitial solid-solution strengthening), so hardness rises with the amount of trapped carbon. More carbon in the parent austenite means more carbon trapped in the resulting martensite and a larger $c/a$ distortion, hence higher hardness — up to the practical carbon limit for fully martensitic structures (beyond roughly 0.6–0.8 wt% C the benefit saturates and retained-austenite content starts to work against it).
