Monitoring Hydrogen and Oxygen in Steel Mills

Controlling heat treatment processes with gas analysers is essential to achieve consistent quality of the desired metal characteristic. Heat treating is a range of processes designed to alter or achieve certain characteristics in different materials (metals). Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching.

Binary Gas Analysers Measure Hydrogen in Continuous Type Furnace

The Michell XTC601 binary gas analyser can be used to measure the hydrogen concentration in either the heating or quenching zones of the furnace or in the hydrogen / nitrogen feed gas to the furnace.

In a continuous steel furnace there is usually a hot zone and a quench (cooling) zone. The hot section of the furnace is heated electrically or by natural gas fired burners. The quench zone has a series of cooling water pipes. At the inlet and outlet of the furnace there is generally a nitrogen “curtain” which prevents large amounts of air (containing oxygen and moisture) to enter into the heating or quenching zones. It also prevents hydrogen mixing with air outside the furnace and creating a potentially explosive atmosphere.

Oxygen Transmitters Monitor Bell Type Furnace

A bell type furnace is typically used for sheet steel, cable and wire spools or smaller parts. The parts are stacked on a base plate and then the bell furnace is lowered over them. Air is trapped inside the bell when it is first lowered, so it is necessary to purge the bell furnace with nitrogen until the oxygen concentration is below a certain limit (this is monitored with the Ntron Minox i Intrinsically save oxygen transmitter. The bell furnace is then filled with the hydrogen / nitrogen atmosphere and allowed to treat for a time interval. A Michell XTC601 binary gas analyzer monitors the gas mix with typical measurement ranges: 0 –10%, 0 –25% & 0 –100% H2 in N2.

Background

The presence of hydrogen serves two purposes in heat treating furnaces.

  • The high thermal conductivity of hydrogen allows for very fast cooling and/or heating of the metal in the furnace. The H2 concentration, the temperature and the rate at which the metal is fed to the furnace are all controlled and optimized. This allows the plant to produce the best product, for the lowest cost at the required quantities.
  • Any oxygen present in a steel furnace will cause corrosion. A simple way of reducing the O2 content is to consume the oxygen with hydrogen.
  • RECOMMENDED MEASUREMENT TECHNIQUES

  • The Michell XTC601 Binary gas analyzer is ideally suited to measure H2 in N2. It has been designed to measure binary gas concentrations using the thermoconductivity technique.
  • The Ntron Minox-I is an intrinsically safe, compact, low powered O2 transmitter that is suitable for measuring oxygen from 20.9% down to 0.01% (100ppm) O2. The transmitter is rated for use in hazardous areas with hydrogen environments.
  • The sensor will last in excess of 5 years.