These needles are mixed with Refractory Mixes to provide crack resistance and structural reinforcement. These 3/4" long needles are made of 406 Stainless Steel, slightly more heat tolerant but fractionally less rust resistant than our 304 Stainless Steel 1" Refractory Needles.
Each has a gently wavy shape for optimal castable refractory reinforcement.
For most applications, the choice between 406 and 304 and 1" vs 3/4" is arbitrary so don't get too hung up on selecting the 'best one' unless building a very technical forge or furnace.
Each 406 "needle" is 1 mm x 19 mm (.020 x 3/4") and crimped-shaped (newer photos coming soon).
1) These needles minimize breakout even if hairline cracks occur, giving you the option to repair them rather than having to overhaul and replace the entire structure.
2) AMOUNT to add (in lbs) General guidelines (adjust to suit):
a) 50 or 55 lbs = 1 lb of Needles
b) 2 gallons refractory = 1/2 lb of Needles
c) 1 gallon of refractory = 1/4 lb of Needles
d) 32 oz jar or less of refractory = 1/8 lb of Needles
3) A Thermal Expansion Key: Differences in thermal expansion are important considerations. Different expansion rates generate internal forces risking stress cracks and breakouts.
These fine short suspended needles avoid the stresses that concrete rebar anchors would cause in refractory casting. Being short, fine, and suspended without external anchors, these needles only expand microscopically within the modulus of elasticities of their environments. The result - simple yet profound elevations in structural integrity.
4) Don't use more water than you need, excess water dilutes strength.
5) Early & quick vibration helps eliminate voids but excess risks 'layering'.
6) Power mix quickly. Most insulating castables should only be mixed 2 minutes. More time increases density and weakness. Medium and dense castables' average mix time is 4-7 minutes.
7) Keep castables warm (70-90°F) before adding water and until fully set and dry-out cured.
8) Dry-Out cure slowly to avoid steam - slow heat dry/cure between days 2 & 30. Proceed slow enough remove water by gentle evaporation, avoiding steam which risks microscopic cracks & weakened castings.
Excerpt from 107-page Research Thesis by Peterson in 2014:
"After prolonged heat and thermal shock exposure, needles help maintain the integrity and mechanical properties of samples. The study also found that corrosion due to oxidation was a major contributing factor to the way needles performed, and concluded that a concentration of 3% 406 stainless steel reinforcing needles added to the working lining of a taconite furnace is recommended."