Delrin breaks down by hydrolysis in hot water, so it's a no-go with steam. Go with PEEK, polyimide (kapton), polysulfone PSU or polyphenyl sulfone PPSU or polyphenylene oxide (PPO or Noryl) for steam. Teflon works to, but it's soft. And all plastics will have expansion coefficients about 20-50 times larger than typical metals.
But for anything up to 180 F, Delrin (acetal) is lovely stuff, it machines so much better than other plastics (PEEK does too, but it's so dang expensive!).