We manufacture gaskets for an automotive application made of both 1008 low-carbon steel and 305 stainless steel and want to anneal them. The gaskets are typically stampings around 0.50-0.75 inches (13-19 mm) in outside diameter and up to 0.020 inches (0.51 mm) thick. The anneal specification calls for a hardness less than 100 HK. Can this be done and what style unit would you recommend?
1. The application involving the 1008 low-carbon steel is ideal for a continuous belt-type furnace. Either a conventional anneal or a process annealing can be considered. Process annealing is typically used to recrystallize a cold-worked structure and as an intermediate anneal prior to further cold working.
Loading can be random (provided distortion is not an issue). A standard annealing process requires a temperature of (approximately) 1670°F (910°C). Process annealing temperature depends on the degree of prior cold working and the desired final microstructure, but for your purposes anticipate it would take place at 1100-1300°F (595-705°C).
The atmosphere for either annealing process can be exothermic gas (lean), but if you are thinking of running both 1008 and 305 SS in the same furnace, the 305 SS atmosphere requirements (see below) must apply. For clean (not necessarily bright) surfaces (which I assume you are interested in) parts must be cooled to below 200°F (95°C) in the protective atmosphere. Time at temperature is dependent on final hardness range.
The literature tells us that a hardness range of 85-125 HRB is achievable (about 100-150 Knoop), so limiting the hardness to not more than 100 Knoop is unrealistic. This is the lowest range, so it will require the longest time and slowest belt speed. My guess (and it is only a guess) is a minimum of one hour at temperature.
2. The application involving the 305 stainless steel gaskets is a bit more involved. Annealing is normally done in the range of 1850-2050°F (1010-1120°C) with 1900°F (1040°C) being typical. Parts should be spaced out to leave room for expansion because distortion is a concern. Remember as well that heat-up time will be about twice as long as the 1008 material (stainless steels do not have the same thermal conductivity as steel). Choice of atmosphere is finish-dependent. For bright annealing you are talking hydrogen, dissociated ammonia with a dew point of -80°F to -100°F (-62° to -73°C) or vacuum are the best choices – argon, helium or nitrogen/hydrogen are acceptable. Nitrogen alone might cause problems if you do not have a certain percentage of hydrogen in the atmosphere, however, since you will see some degree of discoloration. Exothermic gas is a poor choice (due to the potential of lowered corrosion resistance due to carbon pickup).
You must also be able to cool the parts from annealing temperature to black heat, that is, approximately 900°F (485°C) in three minutes or less (normally in under 90 seconds). This may necessitate a fan-cooled section on the back end of the belt furnace. If these were aerospace parts in the thickness range you are talking about, they would be water quenched. A soak time of 20 minutes should be adequate. Hardness should be in the 78-80 HRB (<100 Knoop) range.