Dew Point vs. Oxygen Content in Vacuum Processing
It is well known that accurate measurement of any heat-treating atmosphere can have a significant effect on the quality and process yield of heat-treated components. Traditionally, dew-point analysis has always been the bellwether in determining our heat-treating atmospheric conditions. This is because it was discovered very early on that moisture parameters can have a tremendous impact on carbon potential and thus on final properties.
Since the main objective of any neutral furnace atmosphere is to prevent detrimental effects such as carburization, decarburization, hydrogen embrittlement, oxidation and soot formation, one must analyze much more than moisture content of a furnace atmosphere. Contending with such constituents as CO, CO2, H2, H2O, N2 and hydrocarbons (e.g., CH4), a better, more-robust instrument was needed to analyze endothermic or exothermic atmospheres. Today, either oxygen probes or three-gas analyzers are the industry’s preferred analytical instruments to determine carbon potential within an atmospheric furnace.
As the analysis and controls of atmospheric furnaces have evolved over the years, this author has questioned the antiquated method of measurement of purity of specialty gases within the vacuum processing arena. Why is dew-point measurement still the bellwether of specialty-gas providers and, more importantly, within the vacuum heat-treating industry?
With the advent of such innovative manufacturing processes as additive-manufactured (AM) components, the new metallurgy that comes with AM parts requires ultraclean atmospheres (Fig. 1). Is the dew-point analyzer really the best instrument that we have in our toolbox?
Analyzing Dew Point
Examine this picture of a rose (Fig. 2). The appearance of liquid water occurs only because of one simple fact: The temperature on the surfaces of the rose petals collecting the dew is below the dew point of the surrounding air. Likewise, the amount of water within specialty-gas sampling lines in terms of the number of molecules or mass of water determines at what temperature the water vapor starts to go into liquid phase.
Therefore, an “exact” dew-point measurement is most dependent on the surrounding temperature. Note how the dew-point values within the environs of a state-of-the-art heat-treatment plant’s process gas lines (i.e. N2, AR, etc.) vary from summer to winter (Fig. 3). The fact is today’s heat-treatment plants are probably the worst environment to test for dew point. The majority of heat-treating facilities in the world experience tremendous swings of ambient temperatures and relative humidity even within a 24-hour period of time.
A trace-oxygen analyzer is a versatile microprocessor-based instrument used for detecting parts per million (ppm) levels of oxygen. Oxygen-sensing instruments are typically sealed units and require reasonably regulated sample pressures (0.2-2.4 slpm). The response time is dependent on the flow rate (e.g., a low flow rate will result in a slower response to O2). More importantly, a trace-oxygen analyzer results in a signal that is independent of temperature.
Solar Atmospheres’ engineers realized the advantages of trace-oxygen instruments versus dew-point instruments and decided to build a combination instrument that would employ both methods (Fig. 4). Solenoid controls automatically sample each of the four specialty gases (nitrogen, argon, helium and hydrogen), utilizing both instruments every six hours, 24 hours a day. All dew-point and trace-oxygen results are recordable and traceable along with the plant’s ambient temperature and relative humidity. Alarm features are set for any values of dew point above -60°F and /or values of oxygen greater than 5 ppm in the process-gas feed lines.
After one full year of side-by-side operation, it was very clear to see the trace-oxygen analyzer is the instrument of consistency (Fig. 5).
So, why has the vacuum heat-treating community been slow to react when it comes to determining purity of their specialty inert gases as compared to the atmospheric heat-treating community? It is this author’s opinion we are all being driven blindly by the specifications that govern us. Many specifications require only dew-point measurements for gas purity.
Some specifications, such as Boeing, acknowledge the Compressed Gas Association’s designations, which include dew point AND ppm of oxygen for various gases (Fig. 6). However, AMS 2769B paragraph 18.104.22.168 counteracts this allowance by addressing only dew-point issues, more specifically the installation of sampling lines, the location of the dew-point cell and recording frequencies (Fig. 7). This paragraph is often fertile ground for “findings” by any auditors who perform outside accreditation audits on heat-treating facilities.
When vacuum heat treating metal alloys that oxidize readily in the presence of small concentrations of water vapor or oxygen, data suggests that dew point should not be the stand-alone gas purity analyzer. Dew point only measures the water vapor, not oxygen in the gas line. Including an oxygen analyzer as an additional quality tool provides the heat-treat shop greater assurance that the process gas entering the furnace is of the highest purity and meets the specifications of the customer.
For more information:Contact Robert Hill, FASM, president, Solar Atmospheres of Western PA, 30 Industrial Road, Hermitage, PA 16148; tel: 1-866-982-0660; fax: 1-724-982-0593; e-mail: firstname.lastname@example.org; web: www.solaratm.com
- Solar Atmospheres’ Publication No. 3 in its Vacuum Furnace Reference Series entitled “Operating a Vacuum Furnace Under Humid Conditions,” available on www.solaratm.com
The world is beginning to realize that additive manufacturing (AM) is not just another manufacturing tool. It is actually a new way to manufacture exotic metal components. As the benefits of AM – such as reduced costs, reduced lead times, increased flexibility and closer near-net shaping – are fully understood, increased production of metal-powder feedstock is necessary. It is currently estimated that the world will need more than 30 metric tons of spherical metal powder to support the AM revolution.
It is no secret that vacuum thermal processing will be a critical path for all AM parts. Light and complex geometries that are virtually printed to size will have to have robust assurances that the final post-heat-treated parts are void of any surface contamination.
Within the AM value stream, leak-tight vacuum chambers along with prescribed high-temperature bake-outs will be mandatory for AM parts. Additionally, specialty inert gases must be continually sampled in such a way to provide the heat treater 100% confidence that the gases, which are primarily used to cool the parts, are also pure and not contaminated.
As the AM revolution continues to grow, new standards and specifications will need to be developed. This author believes the way one analyzes specialty-gas purities must be improved within the heat-treating industry in order to provide the world with quality 3D-printed parts.