= Solution
For mild <steel>, the corresponding bounds are
$$
\boxed{t_{\max,\rm mild}=\frac1\pi
\left(\frac{K_{IC,\rm mild}}{Y\sigma_{y,\rm mild}}\right)^2,
\qquad t_{\rm yield,\rm mild}\simeq\frac{7\,\mathrm{MPa\,m}}{\sigma_{y,\rm mild}}.}
$$
A relatively low yield <stress> increases the thickness needed to carry pressure, but a favorable toughness-to-yield-strength ratio can give a much larger leak-before-break upper bound. This tradeoff often makes a tough structural <steel> more attractive than choosing the highest strength alone.
For Arctic service, the <ductile-to-brittle transition> is crucial. A room-temperature toughness cannot be assumed to persist below the transition, and welding, constraint and loading rate affect the comparison. A suitably qualified low-temperature <steel> can be practical; the word mild alone does not establish that qualification.
\b[TODO: numerical thickness cannot be completed without the unavailable 2018 NST Data Book values.] The <temperature> and state of the tabulated toughness must also be identified before making a numerical service claim.
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