To investment-cast stainless steel, carve the part in wax, build a ceramic shell of fine silica flour and a colloidal silica binder in 6-8 layers dried 24 hours each, burn out the wax, and pour at 1,550-1,600 °C. That is 6-8 days of drying before the pour, and the pour temperature is about 600 °C above the bronze the corpus’s lost-wax article is built for, so the shell and the furnace must be proven on a test casting first.
Part of Stainless Steel Production
Lost-wax casting of stainless steel for complex shapes such as valve bodies, pump housings and fittings.
The Shell Is The Hard Part, Not The Wax
The carrier gives the sequence: carve the part in wax, coat with a ceramic shell of fine silica flour and colloidal silica binder built up in 6-8 layers, dry 24 hours between layers, burn out the wax at 700 °C, pour molten stainless at 1,550-1,600 °C, then cool, break the shell and clean.
The wax and gating work is the same as in Lost Wax Casting: sprue, runners and vents attached to the pattern. But that article is written around bronze at 950 °C and a clay-slurry investment, with the mould preheated to 200-400 °C, and it lists cast iron at 1,200 °C as needing a very hot mould. Stainless is above all of those: the pour at 1,550-1,600 °C is 600-650 °C hotter than the bronze at 950 °C. A clay shell that is fine for bronze is untested for steel, so build test shells from the refractory clays in Refractory Materials and pour a scrap-metal trial before committing a stainless melt. I recall that steel shells are usually preheated far hotter than bronze ones; I cannot source the figure.
The binder, colloidal silica, has no corpus article; I recall it can be made from sodium silicate and an acid, but cannot source a method. The silica flour is quartz sand ground and sieved very fine; the corpus has no article on milling silica.
What You Need
- Wax (the corpus recipe is 70% beeswax, 20% pine resin, 10% tallow) and carving tools.
- Silica flour and a binder, or a clay-based refractory slurry.
- A drying space that stays warm and dry for a week.
- A kiln or furnace to burn out the wax and heat the shell.
- A furnace that melts steel (EAF Operation for Stainless), a crucible, and a ladle.
Method: Build, Burn, Pour
Step 1: Carve the pattern with allowance for shrinkage and for a sprue cross-section at least twice the largest section of the casting, as the corpus rule says.
Step 2: Coat in layers. Dip, drain, stucco and dry. At 24 hours per layer, 6 layers take 6 × 24 = 144 hours (6 days) and 8 layers take 8 × 24 = 192 hours (8 days).
Step 3: Burn out the wax at the carrier’s 700 °C. The shell must be bone dry first; the corpus warns a shell that feels cool still holds moisture.
Step 4: Preheat and pour the stainless melt at 1,550-1,600 °C into the hot shell in one continuous stream.
Step 5: Cool, break out, clean and finish (Passivation Treatment).
Worked Example
A wax pattern weighs 50 g. At a wax density near 0.95 g/cm³ (a recalled value) its volume is 50 / 0.95 = 52.6 cm³. At a stainless density near 7.9 g/cm³ (a recalled value) the casting weighs 52.6 × 7.9 = 416 g, about 8 times the wax. The sprue and runners add to the metal you need to melt; a 0.5 kg casting needs more than 0.5 kg in the crucible.
Common Mistakes
- Pouring into a damp shell. Water in the shell flashes to steam and throws metal.
- Using the bronze shell recipe. Test it at steel temperature first.
- Pouring at a casting-grade composition you have not checked. Carbon in the melt (Carbon Control) counts in a casting as much as in a forging.
Safety
Molten steel and silica dust
Pouring at 1,550-1,600 °C burns through leather and skin; a damp shell or cold mould flashes to steam and ejects metal. Dry and preheat everything, stand clear of the pour, and keep a dry-sand pit for spills. Grinding silica flour and breaking out the shell creates fine silica dust that damages lungs; wet it and wear a respirator (Dust and Respiratory Protection).
What’s Next
- Learn next: Solution Annealing - restoring corrosion resistance of a cast part
- Combine with: Welding Stainless - joining castings to fabricated parts