21-Mat-B6 Ceramic Materials · December 2018
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.
This is a textbook austempering treatment. The strapping is quenched rapidly enough from the austenitizing temperature to bypass pearlite formation entirely, then held ISOTHERMALLY in the $380^{\circ}\text{C}$ lead bath — a temperature that lies below the pearlite "nose" of the TTT curve (roughly $550^{\circ}\text{C}$ for a plain-carbon steel in this composition range) but above the martensite-start temperature $M_s$ for this medium-carbon (0.32 wt% C) grade. Held at $380^{\circ}\text{C}$ for long enough, the austenite transforms isothermally and completely to bainite — specifically, given the relatively high isothermal hold temperature within the bainite range, an UPPER-bainite morphology of feathery ferrite laths with interlath carbide, rather than the more acicular lower bainite that would form closer to $M_s$. Because the transformation is already complete before the strapping is finally cooled to ambient temperature, that last cooling step from $380^{\circ}\text{C}$ produces no further phase change (there is no austenite left to convert to martensite). The resulting bainitic structure is valued for continuous strapping production because it combines good strength with better toughness than tempered martensite of comparable hardness, and because the isothermal, lower-thermal-gradient nature of austempering greatly reduces quench distortion and cracking risk compared with a direct martensitic (water/oil) quench.
Long, elongated ("stringer") ferrite grains, aligned with the strapping's rolling/processing direction, are the signature of inherited COMPOSITIONAL BANDING (segregation) from the original cast slab that was never fully removed by homogenization. During solidification, substitutional alloying and residual elements — particularly manganese, phosphorus and sulphur — segregate interdendritically, leaving alternating carbon-lean and carbon/Mn-richer bands. Subsequent hot rolling into strip/strapping mechanically elongates these bands into thin, parallel stringers running along the rolling direction. When the strapping is later austenitized and cooled (including during the austempering treatment itself, if the austenitizing hold was not long/hot enough to fully homogenize), the locally carbon-LEAN bands behave like a lower-carbon steel with a somewhat higher $A_3$ and lower local hardenability, so they preferentially nucleate and grow as proeutectoid FERRITE first, tracing out the original segregation bands as elongated stringers, while the carbon/Mn-richer bands transform to bainite. This banded structure is detrimental because it makes the material sharply ANISOTROPIC: the soft, continuous ferrite stringers — the weaker, more ductile phase embedded lengthwise in an otherwise stronger bainitic matrix — provide an easy, elongated path for void nucleation and crack propagation under transverse or through-thickness load, lowering the effective strength of the strapping even though the bulk composition and nominal heat treatment are correct. The underlying cause is insufficient homogenization (time/temperature) at the austenitizing step to diffusively erase the inherited casting segregation before the isothermal bainitic transformation locks the banded structure in.
