Question:
We manufacture automotive gaskets from 1008 low-carbon steel stampings that are 0.55-0.71 inch (14-18 mm) in diameter and up to up to 0.020 inch (0.508 mm) thick. We want to achieve no greater than 100 Knoop but are having trouble finding a commercial heat treater with a continuous mesh-belt furnace who can give us consistent and repeatable hardness results. Can you help?

Answer:
1008 material has virtually nothing to work with in terms of alloy or carbon content so its response to heat treatment is very likely to be erratic. Typical annealed values for this material fall in the range of 85-125 HRB range (about 110-155 Knoop). So, achieving values less than 100 Knoop may prove impossible. That being said, achieving the low end of the range will require the slowest cooling time; hence the slowest belt speeds.

The key to any good annealing operation is to control the process parameters such as temperature, time at temperature and cooling rate (°F/minute). Even if you do a superior job in this regard, you may still fall out of range. You haven’t specified if you are doing a straight anneal or a process anneal, which is an anneal designed to recrystallize cold-worked structures and as an intermediate step prior to further cold working.

Here are some things to consider when running your product in a continuous mesh-belt furnace (which may not be the best for this application):
  • Straight annealing requires an austenitizing 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) range.
  • Time at temperature is dependent on final hardness range, so adequate soak time is important.
  • You must also be sure that you are at the proper temperature throughout the load, so temperature uniformity is important as well.
  • Atmosphere for either annealing process can be exothermic gas (lean).
  • For clean (not necessarily bright) surfaces, parts must be cooled to below 200°F in the protective atmosphere, and you want to check or monitor the oxygen content in the cooling chamber (it should be less than 20 ppm).
  • Loading can be random (provided distortion is not an issue)
Finally, checking parts on the Knoop scale can lead to greater range of hardness values and may be contributing to the inconsistency you are observing. The use of HRB as an alternative should be considered.