Feb_March_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 0 4 7 atmosphere composition inside the furnace changes contin- uously with time due to leaks and drafts in the furnace, de- sorption of impurities such as moisture from the surface of components or decomposition of lubricant. With the thermodynamic background described before and years of experience on annealing atmosphere practice, Air Products developed an atmosphere control system for annealing atmospheres which measures the oxygen and hydrogen in the atmosphere and calculates the dew-point. To adjust the atmosphere inside the furnace to the desired parameters, the system can either control flow rates or gas composition by opening solenoid valves or mass flow con- troller. Control set-points can be put in to avoid surface oxi- dation and decarburization for: a) Oxygen probe signal b) K H = H 2 O/H 2 ratio c) Dew point Figure 5 shows an example of optimized N 2 /H 2 atmo- sphere in a roller hearth furnacewith the atmosphere control system, for annealing of steel plates. Top part of the figure shows sensor readings and furnace temperature. Lower part of the figure shows the H 2 O/H 2 ratio, H 2 % in furnace and H 2 % in feeding gas blend during normal operation. The H 2 O/H 2 ra- tio of furnace atmosphere goes up (light blue line) because air andmoisture enter the furnacewhenopening the door. H 2 flow is controlledby turning on/off the solenoid valve to keep the H 2 O/H 2 ratio (reducing condition of the atmosphere) at the desired level. In most charges it was possible to signifi- cantly reduce the introduced H 2 -level over the annealing cy- cle, as shown by the blue line (H 2 inside the furnace) and light blue line (H 2 in the gas blend to the furnace). Because the H 2 flow is not a fixed setting in the atmosphere control system, the overall H 2 consumption is reduced by approximately 35% in the optimized operation compared to the furnace opera- tion with fixed N 2 and H 2 flowrate. The atmosphere control system also can be integrated into the cloud-based Air Products’ Process Intelligence Sys- tem for process data management and process optimization by analytical evaluations of the process data based on cus- tomized algorithms. This allows further process optimiza- tions to reduce operating costs. 16 SUMMARY The present study focused on the better understand- ing of annealing furnace atmosphere and its control and optimization. Oxidation of metal parts from free oxygen and moisture is different and should be treated differently. Con- trol and optimization of N 2 /H 2 annealing furnace atmosphere need both thermodynamic knowledge of reactions inside the furnace and accurate atmosphere measurement solu- tion with a robust atmosphere sensor. With better control of furnace atmosphere, consistent product quality can be guar- anteed, and production cost can be reduced. ~HTPro For more information: Liang He, application engineer, Air Products and Chemicals Inc., 7201 Hamilton Blvd., Allen- town, PA 18195, 610.481.8181, HEL5@airproducts.com . References 1.  ASM Handbook Volume 4: Heat Treating , ASM Inter- national, 1991. 2. Guido Plicht and Diwakar Garg, “Advanced Atmos- phere Control System, for Improving Annealing of Steel Components,” www.airproducts.com , 2019. 3. Daniel H. Herring, “The Annealing Process Revealed (Part Two: Furnace Atmosphere Considerations),” In- dustrial Treating, September 2010. 4. The Ellingham Diagram, http://www.doitpoms.ac.uk/ tlplib/ellingham_diagrams/ellingham.php, 2019. 5. Liang He and Zbigniew Zurecki, “Optimization of Protective Atmospheres for Annealing and Hardening Operations in Continuous Furnaces,” 28th Heat Treating Society Conference, Detroit, 2015. 6. C.W. Bale, P. Chartrand, S.A. Degterov, G. Eriksson, K. Hack, R.B. Mahfoud, J. Melançon, A.D. Pelton, and S. Petersen, “FactSage Thermochemical Software and Databases,” Calphad, Vol 26, p 189-228, 2002. 7. A.S. Reeves and W.W. Smeltzer, Decarburization of an Iron 0.8 w/o Carbon Alloy in the Presence of a Wustite Scale, Journal of the Electrochemical Society, Vol 117(1) p 117-121, 1970. 15