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The Influence and Control of Gases and Their Blends During Sintering Of The influence and control of gases and their blends during sintering of carbon steel parts Guido Plicht, Air Products GmbH, Anna Wehr- Aukland, Rana Ghosh, Diwakar Garg, Don Bowe, Air Products and Chemicals Inc., Allentown/USA This paper is published to encourage the sharing and transfer of technical data. Abstract: Producers of sintered parts face 1. Introduction higher challenges in their production due to higher demands Heat treatment atmospheres were on variation in mechanical discussed in several papers in the properties of the parts going along past and are treated as “technical with the pressure on reduced standards” in the sintering process operating costs due to a of carbon steel parts. However, the competitive market. The operator of sintering furnaces composition and quality of the always faces challenges in sintering atmosphere as well as material properties in the day-to- the full understanding of the best day business due to atmosphere way to use atmosphere issues. Typically these challenges compositions thereby plays an are related to carburizing or important role in both, the decarburizing of the steel parts, influence on the final properties of resulting in undesired variations the sintered parts as well as on the of mechanical parameters of the specific operating and sintered parts as well as to maintenance costs, such as the maintenance problems of the belt life. This paper provides an furnace due to surface reactions of overview about the influence of the transport belt with the the atmosphere on the quality of sintering atmosphere or sintered the sintered carbon steel parts and parts. also provides ideas to improve the This paper will describe on a performance of the furnace practical level the impact of equipment such as the belt life to atmosphere components like reduce maintenance costs. Carbon monoxide, Carbon dioxide, Methane, Hydrogen and Moisture and their gradients inside the furnace on the part quality as well as on the lifetime of the transport belt. As a protective atmosphere for the sintering process of carbon steel, some companies use an endothermic generated atmosphere. This atmosphere, produced by an under stoichiometric partial burning process of natural gas or propane with air, can be generated by 1) external endothermic generators supplying several furnaces, 2) in-situ generators installed inside the furnace or 3) by an endothermic reactor installed on top or inside of the furnace. Compared with atmospheres composed of technical gases, such as nitrogen (N2) and hydrogen (H2), these endo generated gases have variations in produced gas composition. Operating the endogenerator to hard on the hydrocarbon side to avoid decarburization of the parts will result in high maintenance efforts and sooting problems with an impact on the reliability on the atmosphere supply. Using a higher concentration of air (oxygen) to reduce the danger of sooting creates a lower carbon potential and will decarburize the part surfaces. For this reason, most sintering companies now use atmospheres based on a nitrogen blend, mostly with hydrogen-levels up to 5%. However, Nitrogen/Endo- blends also deliver significant advantages on the product quality compared to pure Endo. 2. Impact of the Atmosphere Composition on the Sinterparts 2.1. Thermodynamic Figure 1: The Ellingham-Richardson- Diagram [1] Background In general, sintering atmospheres 2 Me + O2 ó 2 MeO (3) ó ó are used to prevent oxidation on 2 H2 + O2 2 H2O (1) Me + H2O MeO + H2 K = pH2/pH2O (4) ó ó the compact surface and to 2 CO + O2 2 CO2 (2) Me + CO2 MeO + CO K = pCO/ pCO2 (5) provide a bright surface finish. Air, therefore, has to be purged from Reactions (4) and (5) show that the sintering than Iron. the furnace and the remaining oxidising or reducing potential Especially in the sintering process traces of oxygen have to be always depends on the of carbon steel, surface reduced by a reactive component equilibrium of pH /pH O or pCO/ 2 2 decarburisation has to be avoided. in the atmosphere blend. However, pCO and not only on the amount 2 By producing water vapour or the reducing reaction should not of oxidising components in carbon dioxide in the furnace produce too many oxidising and general. In atmospheres without atmosphere, the operator has to decarburising components such as reducing components or used at ensure that the amount of these moisture (H O) (1) or carbon low furnace temperatures, the 2 decarburising components are not dioxide (CO ) (2), so it is important amount of residual oxygen is 2 leading to decarburisation of the to provide a sufficient flow rate of critical. The Ellingham-Richardson steel surface according to the a protective atmosphere to prevent Diagram [1] (Figure 1) therefore following reactions (CS: carbon in air ingress. It can be seen that heat shows the reaction equilibria for steel surface): treatment atmospheres always different types of metals. It can be have residual amounts or traces of seen, that alloying elements like ó CS + H2O CO + H2 (6) ó oxidising components. The Chromium, Silicon, Vanadium, CS + CO2 2 CO (7) ó following reactions can represent Manganese require a much higher CO + H2O CO2 + H2 (8) the oxidation of metal (Me): ratio of H2/H2O for oxide free Furthermore, it is important to consider, that all gases stay in an equilibrium (water-gas equilibrium) (8) Due to the high temperature in the sintering zone, the amounts of CO2 and H2O in the endothermic generated atmosphere have to be very low to achieve the required carbon potential for a neutral atmosphere to the steel surface. This has to be realised by providing a sufficient amount of CO and by reducing the decarburising components with additional introduction of hydrocarbons into the furnace (Reaction 9 and 10). However, to make sure most of the moisture will be converted into CO and H2 Figure 2: Equilibrium relations between dew point and temperature for gas an overstoechiometric amount of carburising of γ-iron [2] hydrocarbons is used, resulting in unreacted CH4 and as well in components in the blend can be catalyst is already in worse sooting. reduced down to less than 5% condition, but the residual CH4 (which is below the explosion might help in the furnace CO + CH ó 2 CO + 2 H (9) 2 4 2 point). atmosphere to balance the carbon H O + CH ó CO + 3 H (10) 2 4 2 losses out of the material and to 2.2. Sintering Tests help reducing the moisture inside Because the same atmosphere the high temperature zone of the composition is also used in the Tests were done with different furnace. cooler sections of the sintering Endogas compositions and the furnace, this very often results in effect on the microstructure of Looking at the equilibrium carbon sooting inside the furnace, carbon steel containing 0.74 potential calculations of the depositing soot on the belt or the weight % carbon was evaluated: atmosphere composition of parts. Figure 2 provides an Table 1: Sintering atmospheres used in the tests overview about the variation of the carbon potential by using Generator 1 Generator 2 N2/ Endo N2/H2 endothermic generated CO/% 19 20 6 0 CO /% 1,2 0,3 0,3 0 atmospheres at different 2 CH /% 1,2 0,04 1,1 0 temperatures and compositions. 4 H2/% 40 40 13 5 Dp/°C 21 1 -16 <-65°C This thermodynamic process can be simplified by using a dry The produced gas composition in generator 1 and 2 (Figure 2) the nitrogen/hydrogen atmosphere. In the generator 2 is based on the expected Carbon Potential in the Nitrogen/Hydrogen-atmospheres optimal setting to avoid sooting, atmosphere composition in the the driving force for but also run on a reasonable low high temperature zone of the decarburisation is mainly related dew point and CO2 level, resulting furnace is far below the C-level of to moisture created by air ingress in an optimal Carbon potential in the steel powder (0,74%C). or vaporisation of water an equilibrium atmosphere containing residuals. Practically in condition in the retort of the To compensate the decarburising most cases for sintering of carbon generator. Generator 1 operates on effect in the high temperature steel, due to the low dew point of a high dew point, but has high zone, some operators introduce technical gases the flammable residual CH4. This means the additional hydrocarbons in the hot zone. This helps to balance the carbon losses of the steel in endothermic atmospheres at the high temperature level, but results in carburising and sooting in the low temperature areas. Apart from reactions with the parts, that might also effect furnace equipment like transport belts (Chapter 3). This means in endothermic generated atmospheres a well balanced atmosphere composition and distribution is key for homogeneous material properties. Figure 2: Calculated carbon potential for generator 1 and 2 in the furnace related In N2-Endo diluted atmospheres, to furnace temperature this effect does not appear so much, because of the far lower dew point and thereby lower decarburising tendency. The carbon potential in the atmosphere thereby is not such important anymore, since there is no carbon balance required to compensate the effect of decarburisation due to the higher dew point. In Nitrogen-Hydrogen Atmospheres the whole effect of Figure 3: Microstructure of sintered parts in endothermic generated atmosphere decarburisation is just based on residual moisture. In atmospheres below -30°C the decarburising effect inside the sintering zone is typically too slow to effect the surface composition during the sintering time. 2.3. Sintering of carbon steel, test results Sintering trials of carbon steel compacts were carried out with endothermic generated Figure 4: Microstructure of sintered parts in endothermic generated atmosphere atmosphere, endothermic generated atmosphere diluted the parts in Figure 4 shows a very decarburisation is an unacceptable with dry nitrogen and nitrogen/5% severe or total surface high presence of moisture and hydrogen atmospheres.
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