Assessing Heat Treatment Carburization via the Combustion Method

Transcription

1 October 2011 Assessing Heat Treatment Carburization via the Combustion Method By Eric S. Oxley, Ph.D., Product Manager Gas Analysis, Bruker AXS Inc., Billerica, MA, USA Introduction We rely on them every time we jump into our vehicle to grab groceries, visit relatives, or simply cruise the open roads. Power production facilities use them to help reliably generate the energy we consume in our daily lives. They were implemented in the heavy machinery used to construct your favorite shopping mall. They are heat treated steel manufactured components (Figure 1) with reinforced surfaces that must withstand the friction and wear due to rotating, sliding, grinding and other compromising motions. Just a few applications for these hardened components include: camshafts and gears found in the engines of our vehicles; gear wheels used in wind turbines and hydroelectric and steamgenerated power stations; numerous components comprising heavy duty equipment used in construction. Figure 1: Examples of case hardened steel materials. Source: Internet Techniques of surface hardening include carburizing, nitriding, carbonitriding as well as induction and flame hardening. This article will focus on the process of carburization, or absorbing carbon atoms into a metal at elevated temperatures, to improve the mechanical properties of the part. The extent to which a work piece is strengthened by the carburization process must be assessed. Fortunately this evaluation process is made routine with the new Bruker G4 ICARUS C HF combustion analyzer. Carburization Overview Unlike hardening by traditional tempering, quenching and annealing processes, where the bulk work piece is hardened, carburization only reinforces the surface. Low carbon steel is typically chosen due to its excellent machinability properties. Figure 2. Carburization furnace used to harden the surface of materials. Source: Eickhoff Maschinenfabrik, Bochum, Germany During carburization the steel work pieces are immersed in a carbon rich gaseous atmosphere at elevated temperatures (typically C) to promote the absorption of carbon into the steel surface. By diffusion from the carbon enriched surface, carbon penetrates deeper into the material forming a depth profile whereby the carbon concentration decreases with distance from the surface. This absorption process is carried out in a carburization furnace (Figure 2) which generates the necessary carbon rich environment from a hydrocarbon source (e.g., methane, ethane or propane). After the surface has saturated with

2 carbon by reaching equilibrium and the desired case depth is established, the steel is then hardened by a quenching and annealing process. The result is a product with unmatched surface strength while retaining its strong inner core. The entire carburization process can take hours or even up to days to complete. The carburization level and its depth, which corresponds to the case depth or amount of carbon adsorbed into the material s surface, typically depends on the furnace temperature, atmospheric concentration and treatment time. Shim stocks, which are mm thick low carbon annealed steel sheets or gauze, are placed inside the carburization furnace and like the steel work pieces will absorb carbon. These test foils (Figure 3) are crucial to the carburizing process by providing a way of accurately estimating the carbon potential of the furnace atmosphere. The carbon content of the test foils can be quantified by a handful of techniques, 1 most notably via gravimetric means or the combustion method. The preferred technique 2 for accurately quantifying the carbon content in the test foils is with the combustion method. In this method the carburized foil is analyzed and quantified with a high frequency induction furnace and infrared detector, respectively, as featured on the G4 ICARUS HF analyzer described below. This method has been established as the reference method, does not suffer from the aforementioned limitations of the weight gain method and is more accurate than measuring mass gains. Figure 4. The G4 ICARUS HF carbon and sulfur analyzer. Figure 3. Shim stock test gauze used to evaluate the carbon potential of a carburization furnace. Source: Eickhoff Maschinenfabrik, Bochum, Germany The gravimetric, or weight gain, method of estimating the carbon content relies on comparing the test foil mass before and after being placed into the carburization furnace. Interferences, however, can arise from the presence of contaminant species in the furnace atmosphere (e.g., oxygen) that will react with the metal surface and artificially increase the foil mass. Additionally, the original carbon content of the test foils must be exactly known, adding another burden to this technique. G4 ICARUS CS HF The G4 ICARUS CS HF (Figure 4) is a new analyzer introduced by Bruker in early 2011 that utilizes the combustion method for rapidly and precisely measuring the carbon and sulfur content in metals, soils, and many other sample types. Like many combustion analyzers the G4 ICARUS converts the solid sample of interest into gaseous components which are measured by infrared detectors and processed into tangible carbon and sulfur concentrations. The ICARUS, however, has beneficial features not found on other combustion analyzers. Combustion Process The G4 ICARUS utilizes a high frequency (HF) induction furnace to rapidly combust solid samples. The sample, typically up to 1g depending on the

3 application, can be analyzed directly with the G4 ICARUS regardless of configuration. Shapes such as powders, pins, chips, drillings, and many others are possible. The sample is placed in a ceramic crucible along with a material (i.e., accelerator) that will readily couple with the electromagnetic field produced by an RF coil that surrounds the crucible. Accelerators are chosen based on the sample application and include most commonly tungsten, copper, iron, and tin. By providing a high pressure, oxygen rich (O 2 ) environment in the furnace, the sample material and accelerator combust reaching temperatures above 1500 C while liberated carbon and sulfur compounds are oxidized to form carbon dioxide (CO 2 ) and sulfur dioxide (SO 2 ), respectively. The accelerator ensures a good coupling with the electromagnetic field and provides additional energy to the combustion process so that a fluid melt is achieved. With the G4 ICARUS HF, the efficiency of the combustion process can be monitored with a viewing port on the front of the analyzer, as shown in Figure 5. This viewing port also allows the integrity of the combustion tube to be monitored so that it can be cleaned or replaced only when necessary. Figure 5. Combustion viewing port found on the front of the G4 ICARUS HF analyzer. Extraction Nozzle The liberated sample gases must be extracted from the furnace and transported downstream for eventual detection. This transport mechanism inherent with the Bruker G4 ICARUS HF analyzer provides many tangible benefits for its users. By utilizing the high pressure in the furnace and an extraction nozzle located directly above the crucible, the gaseous components can be actively and efficiently extracted from the furnace with the oxygen carrier gas. The components comprising the novel furnace design are shown in Figure 6. Note specifically the extraction nozzle and its slight immersion in the top of the ceramic crucible. Destructive byproducts that can splatter against a surrounding quartz combustion tube (Figure 6) with traditional furnace designs are virtually eliminated with this extraction design. Users benefit from extended quartz tube lifetimes. The extraction nozzle also eliminates the need for a lance, which is a narrow oxygen supplying orifice located above the crucible (i.e., in lieu of a large diameter nozzle) that can become clogged with splatter from the combustion process. Further, the active extraction principle of the sample gases with the G4 ICARUS improves analysis times and measurement sensitivities compared to traditional, diffusiondominated furnace purge designs. Visible Dust Filter Once extracted from the furnace via the nozzle, the sample stream flows through a mesh based micron filter (Figure 7) where fine particulates are removed. Over time this filter must be cleaned as the accumulated dust can retard or trap the CO 2 and/or SO 2. On the G4 ICARUS HF this filter is secured with quick disconnect fittings and conveniently located on the front panel behind transparent housing. By integrating a dust filter that is clearly visible on the front panel, the G4 ICARUS eliminates preventative cleaning. No longer must the user disassemble an entire furnace assembly simply to access the concealed dust filter, only to discover it did not yet require maintenance. The dust filter assembly on the G4 ICARUS provides a visible indication when cleaning is required. When maintenance is necessary, no tools are required to access the filter.

4 Quartz Combustion Tube Extraction Nozzle Crucible RF Coil Crucible Holder Figure 6. Furnace of the G4 ICARUS HF analyzer featuring a patent pending extraction design. Quick connects Transparent housing Dust filter Figure 7. Dust filter located behind transparent housing on the front panel of the G4 ICARUS. Gas Purification and Flow Control Once particulates are removed from the gas stream additional purification is achieved by directing it through a drying reagent, magnesium perchlorate, to remove any moisture that may have been produced or released during sample combustion. Failure to remove this moisture could result in the formation of sulfuric acid through combination with the SO 2 that is present in the stream. This would be not only detrimental to the fittings and components within the analyzer, but would result in losses in sulfur recovery. Also located downstream of the furnace are pressure and flow regulating components. These ensure consistent combustion, transport and detection is achieved from one analysis to the next. The analyzer can also be checked for undesirable atmospheric leaks through incorporation of these components. Infrared Detection The purified gas stream, consisting of O 2, CO 2, SO 2 and possibly small amounts of CO, is now ready to be quantified. This quantification is achieved by using selective and stable non dispersive infrared (NDIR) detectors found in the G4 ICARUS. The gases are directed through an infrared detector that will respond exclusively to the amount of SO 2 in the stream. Upon exiting this detector the gas stream flows through a heated oxidation furnace filled with platinized silica (PtSiO 2 ). This furnace will catalytically oxidize CO to CO 2 and convert most of the previously detected SO 2 to sulfur trioxide (SO 3 ). Because carbon content in the sample is assessed with a selective CO 2 IR detector, this CO to CO 2 oxidation process ensures a representative quantification of total carbon content with no losses as a result of CO. Passing the sample stream through cellulose will trap SO 3 to protect downstream components. The gas stream, now comprising only O 2, CO 2, and possibly a small amount of SO 2, flows through a selective CO 2 cell to measure the carbon content before exiting through the exhaust.

5 The analysis time for a single analysis with the Bruker G4 ICARUS CS HF is nominally 40 seconds depending on the sample application, sample mass and carbon/sulfur concentration. Carburization Applications with the G4 ICARUS A special version of the G4 ICARUS HF analyzer has been developed exclusively for heat treatment applications. The principles and components are similar to the aforementioned G4 ICAURS HF, except this new derivative features a precise, single CO 2 NDIR detector that has been optimized for the target carbon concentration range of these carburization foils (i.e., 0.1% for base foil and up to 1.5% for the saturated foil). The combustion analyzer is even recommended as the referee in the event carburization measurements yield contradictory results. 2 One 4 All User Interface The analysis software of the G4 ICARUS HF (Figure 8) shares commonality with all other gas analyzer products offered by Bruker. The primary tasks are organized into four individual screens to maximize convenience and productivity: Analysis. This is the primary view and where samples are queued and analyzed, allowing the sample peak profiles to be viewed in real time during each analysis. Parameters. Configurations that control the sample combustion are defined and saved in this pane. Statistics. This tab provides the ability to statistically evaluate the analysis results and generate sample reports. Calibration. Screen which allows the instrument to be calibrated with results from pure substances or certified reference materials via single point, twopoint or multi point calibrations. Figure 8: Screen capture of the universal One 4 All user interface found on the G4 ICARUS HF and all other gas analyzers manufactured by Bruker. Representative Data Like other sample applications developed for the G4 ICARUS HF detailed reports are available for the analysis of shim stock test foils. These Application Notes describe an overview of the application and measuring principle, recommend analysis conditions and sample preparation techniques, and show the typical results one can expect with this application. Table 1 shows representative data for the analysis of carburization test foils and Figure 9 shows a typical peak profile for a single test foil analysis using the G4 ICARUS HF. Table 1. Typical carbon results from the analysis of carburization test foils using the G4 ICARUS C HF analyzer. Lot Sample Mass (g) Carburization Temp ( C) Expected Carbon (%) Measured Carbon (%)

6 Table 3. Instrument specifications of the special G4 ICARUS C HF designed for carburization applications. Measuring Range* Carbon % *Can be extended by adjusting the sample mass Nominal Sample Weight 0.5 1g Analysis Time 40s nominal** **Sample mass and concentration dependent Figure 9. Representative peak profile of a single test foil analysis using the G4 ICARUS HF. Instrument Specifications Table 2. General instrument specifications of the G4 ICARUS CS HF. Measuring Ranges* Carbon 1ppm 6% Sulfur 1ppm 0.5% *Can be extended by adjusting the sample mass Nominal Sample Weight 1g Analysis Time 40s nominal** **Sample mass and concentration dependent Resolution Precision/Reproducibility Carbon Carrier Gas Pneumatic Gas Dimensions and Weight Dimensions (w x d x h) Weight Electrical Supply 0.1ppm 0. 5% RSD O 2, 99.5%, ~50psi min Air, oil & water free, ~72.5psi min 22 x 24 x 19 in 88 lbs 230V ± 10%, 50/60Hz, 16A Resolution Precision/Reproducibility Carbon Sulfur Carrier Gas Pneumatic Gas Dimensions and Weight Dimensions (w x d x h) Weight Electrical Supply 0.1ppm 2ppm or 0.5% RSD (whichever is greater) 2ppm or 1.0% RSD (whichever is greater) O 2, %, ~50psi min Air, oil & water free, ~72.5psi min 22 x 24 x 19 in 88 lbs 230V ± 10%, 50/60Hz, 16A References 1. ASTM Standard G79 83 (Reapproved 1996), Standard Practice for Evaluation of Metals Exposed to Carburization Environments, ASTM, West Conshohocken, PA, Herring, Daniel H., September 2004, Furnace Atmosphere Analysis by the Shim Stock Method, IndustrialHeating.com, treatdoctor.com/documents/shimstockanalysis.pdf Bruker Corporation. All rights reserved. Bruker AXS Billerica, MA, USA +1 (978) Info@bruker axs.com axs.com