Dew Point in Metal Heat Treatment

The water content of the atmosphere takes part in the chemical equilibrium inside the furnace: it determines whether the metal surface tends to oxidize or stay reduced during the thermal cycle. An atmosphere wetter than specified translates into surface oxidation, sintered parts out of specification or defective brazed joints — defects usually discovered only at quality control, with the batch already processed.

That is why the process specification is expressed as a target dew point. Depending on the process and the metal, control values range from −60 °C up to +35 °C or even higher — a range no ambient relative humidity measurement can meaningfully cover.

The practical difficulty is temperature: furnace gas runs far above the operating range of any humidity sensor. Measurement is normally done on a sampling line that draws a small gas flow and lets it cool before it reaches the dew point probe. Since the absolute water content of the gas does not change as it cools — as long as it does not condense along the way —, the dew point measured in the line still represents the furnace atmosphere.

The instrument has to be rugged, leak-tight, easy to install and maintain, and compatible with different furnace types and protective gases.

Vaisala's DRYCAP® technology is specified for this duty: it stays stable at low dew points, withstands condensation and recovers undamaged if the sensor gets wet, and its sensor purge — a controlled heating cycle that removes residual chemicals — sustains accuracy in contaminant-laden atmospheres between calibrations.

For blanket gases in heat treatment, a probe like the DMP7 — with a short probe head for tight spaces and sensor purge — integrates into the sampling line or the equipment; for your furnace's specific configuration, consult a specialist.