The Annealing Process Revealed (Part Two: Furnace Atmosphere Considerations)
Annealing is a relatively simple heat treatment to perform, but there are a number of factors that must be carefully considered and controlled. One of the most important is the furnace atmosphere that surrounds the parts as they heat and cool. Let’s learn more.
The purpose of a furnace atmosphere varies with the desired end result of the heat-treating process. In general, the atmospheres used in the heat-treating industry have one of two common purposes:
- To protect the material being processed from chemical reactions that could occur on their surfaces (e.g., oxidation), that is, to be passive (chemically inert) to the metal surface.
- To allow the surface of the material being processed to change (e.g., adding carbon, nitrogen or both), that is, to be reactive (chemically active) to the metal surface.
When annealing wire, we most often want the atmosphere to protect the product in question rather than be reactive with it. Air atmospheres or those using products of combustion from direct-fired burners are not capable of preventing steel parts from oxidation or decarburization. It is for this reason that various protective atmospheres (generated, pure or mixed) are used (Table 1). The type of gas or gas mixtures used depends on the metal being treated, the treatment temperature, part contamination and the surface requirements of the product being annealed. Atmosphere control is often an important consideration as well.
Purging is another critical first step in the annealing process and should take place when the workload is cold (prior to heating). Nitrogen or lean (non-combustible) exothermic gas are common choices, and purging is considered complete when less than 1% oxygen is present as measured by use of an oxygen analyzer. The “rule of thumb” of five volume changes in an hour is often used to establish the required flowrate.
|Fig. 1. Typical annealing cycle for steel wire (nitrogen-hydrocarbon mixture)|
Types of Annealing Atmospheres
Characteristics of the most common annealing furnace atmospheres can be summarized as follows:
Nitrogen and Nitrogen-Hydrocarbon Blends
Pure (100%) nitrogen atmospheres are seldom used in annealing because they are not air (oxygen) excluding. Leaks or other sources of air infiltration cannot be compensated for. However, nitrogen is often blended with small percentages of hydrocarbon gas (methane, propane, propylene) or combined with methanol (CH3OH) to create a suitable atmosphere provided it is properly controlled (Table 1). The nitrogen-hydrocarbon flows are often dictated by a predetermined recipe (Fig. 1).
Hydrogen and Hydrogen-Nitrogen Blends
Annealing under a controlled atmosphere of 100% hydrogen prevents oxidation and product discoloration. Hydrogen is a highly reducing gas, so hydrogen annealing promotes “surface cleaning” of oxidized parts by reducing the oxides present on the wire. It is commonly referred to as “bright annealing.” Hydrogen may decarburize steel parts, so appropriate cautions must be taken. From an economic perspective, an atmosphere of 100% hydrogen is the most expensive. Lower cost nitrogen-hydrogen blends can also be used effectively.
Dissociated ammonia is produced by the cracking (dissociation) of ammonia into a gas having a composition of 75% hydrogen, 25% nitrogen. It has many of the same benefits as hydrogen but can be supplied at lower cost.
Rich exothermic gas is generated by partially combusting an air-gas mixture of (approximately) 6.5 parts of air to 1 part natural gas. Other hydrocarbon fuels can be used as well. Exothermic gas is the most widely used protective atmosphere for annealing, especially for annealing of low-carbon steel. However, it will decarburize medium-carbon and high-carbon steels because of the carbon dioxide (CO2) and water vapor. When decarburization concerns exist, coolers are used to reduce the dew point from typically +38°C (+100°F) to +4.5°C (+40°F). In some cases, refrigerant dryers are used to further reduce the dew point from typically +4.5°C (+40°F) to -40°C (-40°F).
Purified rich exothermic gas, in which CO2 levels are lowered to less than 0.1% CO2 (typically 800 ppm or less), is used for short-cycle annealing and process annealing of medium- and high-carbon steels of the straight-carbon and certain alloy types. For long-cycle annealing, however, the high carbon monoxide (CO) content results in soot deposits on the work and other surface effects as a result of the relatively long soak times in which the work is in the critical low-temperature range of 480-700°C (900-1300°F), where these adverse gas reactions can occur. In short-cycle annealing, these effects are minimal, and in some instances, high CO gas is desirable because of its high carbon potential. Purified lean exothermic gas is sometimes used for long-cycle annealing of medium- and high-carbon steels of the straight-carbon and alloy types.
Endothermic gas (also called Endo or Rx™ gas) is produced when a mixture of air and fuel is introduced into an externally heated retort at 2.5:1 to 3.5:1 air-to-gas ratios. The retort contains an active catalyst, which is needed for cracking the mixture. Leaving the retort, the gas is cooled rapidly to avoid carbon reformation (in the form of soot) before it is sent into the furnace. An endothermic gas atmosphere requires carbon control for precise and repeatable carbon levels.
Simply stated, vacuum is the absence of an atmosphere and represents the most ideal condition under which annealing can take place. Vacuum annealing, which can also be done in a partial-pressure atmosphere of an inert (e.g., nitrogen or argon) or reactive gas (e.g., hydrogen), is sometimes referred to as “bright annealing” due to the surface finishes produced. Because vacuum annealing is also the most costly, time consuming and least conducive to mass production, it is usually restricted to only certain materials such as titanium or tantalum.
|Fig. 2. Typical bell annealing furnace sampling system (courtesy of Super Systems Inc.)|
The composition of the furnace atmosphere is constantly changing, so we must use measurement and control devices (Fig. 2) to ensure good metallurgical quality control. This is accomplished by making sure that several of the following control methods are monitoring and/or controlling the process:
- Dew-point analysis
- Infrared analyzer (single or multiple gas analyzers)
- Oxygen probes
The trend today is to use multiple measurement tools to obtain the most accurate snapshot of the atmosphere in real time.
The measurement of oxygen is important to judge both the effectiveness of the purging cycle (<1% O2) at the beginning of the run as well as the effectiveness of the atmosphere during cooling (ppm range). Oxygen analyzers and oxygen probes are used for this purpose.
Gas Analysis Methods
Dew point is defined as the temperature that water vapor starts to condense. In simplest terms then, a dew-point analyzer measures the amount of water vapor present in the furnace atmosphere (Table 2). This information can then be used to determine the carbon potential of the atmosphere. It is considered an indirect measurement technique since it involves pulling a gas sample from the furnace into the instrument.
Dew point, infrared (3-gas analyzers) and oxygen probes provide measurements commonly used to monitor the furnace atmosphere. The latter is an in-situ measurement device (it does not rely on a sample being drawn from the furnace). It is important to recognize that contaminants present on the wire or rod will tend to volatilize off during the initial heating phase. These contaminants must be removed by flushing (i.e. volume changeover) of the atmosphere, and the atmosphere must be stable prior to the onset of further heating or soaking at temperature.
For example, a properly controlled nitrogen/propylene atmosphere can achieve repeatable results on subcritical annealing of ferrous wire and wire rod. The balance of process parameters and cost include:
- Accurate and repeatable measurement/inference of decarburizing agents that occur at trace levels (O2, H2O and CO2)
- Integration of the temperature control with the atmosphere control
- Control of the ramp from 1100°F (590°C) to the final soak temperature, typically 1300°F (705°C), in order to prevent decarburization
- Avoidance of carbon deposition (soot) by shutting off the propylene during soak after all major reactions are complete
- Lowering nitrogen flows to absolute minimum levels
- Using additional higher flows of propylene to speed up cycle time (leading to cost savings in fuel, electricity and nitrogen)
- Controlling depth of partial decarburization to meet specific manufacturing requirements of wire users
- Reducing/eliminating the air-burnoff segment to introduce less oxidation and speed up the ramp time on subsequent cycles