λ = 0.82;
ρ = 1810;
cp = 1000;
To = -10;
d = 0.4;
αext = 20;
αint = 5;
Texter = To;
Texint = 20;
rce = ρ*cp*D[T[x, t], t] == λ*
            D[T[x, t], {x, 2}];
poc = T[x, 0] == To;
okrleft = -λ*
              D[T[x, t], x] == αext*(Texter - T[x, t]) /. 
          x -> 0;
okrright = -λ*
              D[T[x, t], x] == αint*(-Texint + T[x, t]) /. 
          x -> d;
tmax = 3600*10;
res = NDSolve[{rce, poc, okrleft, okrright}, 
            T[x, t], {x, 0, d}, {t, 0, 
              tmax}][[1]];
teplota = T[x, t] /. res;
Plot3D[teplota, {x, 0, d}, {t, 0, tmax}, 
      AxesLabel -> {"x[m]", "t[s]", "T[°C]"}]