The Influence and Control of Gases and Their Blends During Sintering Of

Home , Sintering

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. Figure 3 decarburisation. This results in carbon dioxide, which results in a and 4 show the microstructures of parts with lower surface hardness carbon pick-up out of the steel parts, sintered in endothermic than desired and dimensional surface. The atmosphere generated atmospheres. The bulk changes outside the desired range. composition, produced by an ideal carbon content of the sintered bars As described in section 2.1., the endo generated atmosphere, was 0,74%. The microstructure of reason for the surface contains less moisture and carbon • Reduces Variations and Spread in Physical Properties (Dimensional Changes and surface hardness, transfers rupture strength) • Improved microstructural homogeneity and less surface decarburisation • Producing Components with Stringent Specifications

Another improvement can be achieved by Sintering in Nitrogen- Figure 5: Microstructure of sintered parts in nitrogen diluted endo atmosphere Hydrogen Atmosphere, whereas the H2-level in sintering of carbon steel can be reduced below 5%. Parts treated in this atmosphere have shown • A tight spread inside the desired physical properties range • a uniform Microstructure

3. Effect of the Atmosphere Composition on the Service Life of Conveyor Belts

3.1. Material Degradation of a Figure 6: Microstructure of dry nitrogen- 5%hydrogen atmosphere Stainless Steel Belt in a dioxide, which results in less carbon dioxide and a dew point of Sintering Furnace pronounced partial surface <-65°C. Figure 6 shows the decarburisation (Figure 4). microstructure of a sintered part Wire mesh belts, used to convey in nitrogen/5% hydrogen- powder metal parts through This partial decarburisation can be atmosphere. It can be seen that continuous sintering furnaces, are minimized by further reducing the surface decarburisation was totally commonly made of austenitic moisture and carbon dioxide level avoided. stainless steel (AISI type 314 or in the atmosphere. Figure 5 shows 1.0314). Mechanical properties of a microstructure of a part sintered 2.4. Summary of the atmo- the belt deteriorate during service in endothermic atmosphere sphere comparison because of high temperature diluted with dry nitrogen and material degradation, related to In principle all used atmospheres hence having a significantly creep, oxidation, carburization and can deliver acceptable sintering reduced moisture level. The nitridation [4-6]. Conveyor belts qualities. However, using purely decarburisation problem is nearly are usually removed from service endothermic produced avoided. There is only a slight due to excessive deformation or atmospheres require a very surface decarburisation visible, cracking of the wire spirals or sensitive atmosphere setup in the which can be explained by the cross-rods. relatively high carbon dioxide generator and control in the level. However, the parts have furnace. Dilution with N2 can Carburization and nitridation of shown improvements in hardness make endothermic generated austenitic stainless steel belts and fulfilled the customer atmospheres more reliable. occurs in all sintering requirements. A further increase of Significant improvements have atmospheres. Even though quality can be realized using a dry been achieved in part quality by nitrogen-hydrogen atmospheres nitrogen/5% hydrogen atmosphere dilution of Endogas with Nitrogen do not consist of carbon-bearing having no carbon monoxide and resulting in: compounds, contrary to endothermic atmospheres, the belt (a) (b) Figure 7: Microstructures of the belt spiral wire, backscattered electron images (BEC): (a) from the endo atmosphere; (b) from nitrogen-hydrogen atmosphere

(a) (b) (c) Figure 8: Distribution of carbon (yellow, a), nitrogen (red, b) and chromium (cyan, c) on the cross-section of the belt spiral wire after 9 months of service in nitrogen-hydrogen atmosphere. is always exposed to carbon- Figure 8. 3.2. Protective Effects of the bearing compounds that form Dew Point Controlled A surface layer of chromium oxide during delubrication of powder Hydrogen-nitrogen can considerably protect stainless metal parts. Carbide and nitride Atmosphere precipitation, resulting from steels from carburization and nitridation and carburization, leads nitridation [6, 4]. However, the 3.2.1. Belt Trials to material embrittlement [4-6]. oxides scales are destroyed in the reducing environment of the high Two long-term belt trials were An example of an extensively conducted in an industrial furnace brittle belt after service in the heat zone of a sintering furnace operating with a standard to compare the effects of two endothermic atmosphere is shown nitrogen-hydrogen atmospheres in Figure 7a. Examinations nitrogen-hydrogen atmosphere. The belt material experiences on service-related belt degradation. conducted by scanning electron The high heat zone temperature of microscopy (SEM), combined with cyclic oxidation and reduction within the furnace, as it is reduced the furnace was 1129 °C. The first energy dispersive x-ray trial was carried out using microanalysis (EDX), indicate that in the high heat zone and re-oxidized in the lower standard nitrogen-6 % hydrogen the precipitated particles are atmosphere. The dew point of this chromium-rich carbides (light temperature, more oxidizing cooling zone. However, the oxide atmosphere in the high heat zone gray) and chromium-rich of the furnace was -51 °C. The carbonitrides (dark gray). reduction in high heat zone of the furnace can be eliminated by second trial was conducted using a An example of a belt after about 9 controlled increase of moisture newly developed humidification months of service in the nitrogen- content of a nitrogen-hydrogen system. In this test, the dew point 6%hydrogen atmosphere is shown sintering atmosphere, which will of the nitrogen-6 % hydrogen in Figure 7b. SEM/EDX result in increased service life of a atmosphere in the high heat zone examinations indicate that the conveyor belt [3]. of the furnace was maintained in particles that precipitated on grain the range of -40 to -37 °C. This dew boundaries and within grains are point range was selected based on chromium-rich carbonitrides, the oxidation-reduction diagram for the Fe-23Cr-19Ni system and The material examinations particles as well as length of pure iron, which was calculated included microstructure analysis, continuous chains of particles using FactSage™ software [7]. The tensile tests, microhardness tests increased with time. The diagram in Figure 9 shows that the and analysis of nitrogen and concentration of particles and the dew point of -40 to -37 °C, which carbon concentrations. length of continuous chains of was maintained throughout the particles are higher in the Routine quality control procedures second trial, was oxidizing to the specimens from the standard for sintered compacts did not stainless steel belt at the sintering atmosphere after the same time of reveal any nonconformance temperature of 1129 °C and at the service, Figure 10. So, the related to the increased moisture same time was reducing to the deterioration of the belt from the content of the sintering sintered metal powder materials standard atmosphere is more atmosphere. Statistical analysis of (point B in Figure 9). The position advanced than the deterioration of the data for two part types made of point A in Figure 9 corresponds the belt from the humidified of nickel steel containing 0.5 to the dew point and temperature atmosphere. weight % carbon did not detect of the standard furnace any significant difference in the The phenomena causing atmosphere in the high heat zone. apparent hardness and degradation of both belts included The conditions indicated by point dimensions after sintering in the internal oxidation of chromium, A are reducing to the austenitic modified atmosphere, as compared manganese and silicon. An stainless steel belt. to the standard nitrogen-hydrogen example of surface and subsurface The two belts that were used in atmosphere. zone of the wire, containing the trials were made of the same internal oxidation of chromium, type steel (AISI type 314). Both 3.2.2. Material Examination manganese and silicon is shown in belts had the same designation Results Figure 11. Overall, the depth of BEF-36-10-8-10, which stands for The material examinations of internal oxidation for the belt from balanced extra flat weave with 36 samples taken from the two belts the modified atmosphere was spiral loops per foot of width and that were tested in the standard about half that for the belt from 10 cross-rods per foot of length, 8 and humidified atmospheres the standard atmosphere, Figure gauge rod and 10 gauge spiral. revealed the following. Chromium 12. or chromium-rich carbonitrides Service-related belt degradation Surface oxide scales of both belts have been detected in the was evaluated based on material consisted of chromium and microstructures of the both belts. examinations of the samples taken manganese oxides. However, a The concentration and size of at 4, 7, 8 and 11 months of service. less protective iron-rich outer oxide layer was observed on the belt from the standard atmosphere. No iron-rich outer oxide layer was observed on the belt from the humidified atmosphere. This is an indication of a more protective oxide scale on the belt from the modified atmosphere [8].

The depth of chromium-depleted zones in the subsurface was deeper in the belt from the standard atmosphere. The average combined carbon and nitrogen concentrations and the rate of concentration increase with time were higher for the belt from the Figure 9: Oxidation-reduction diagram for Fe-23Cr-19Ni system and pure iron standard atmosphere [8]. under partial pressure of hydrogen equal to 6079.5 Pa (0.06 atm). (a) (b) Figure 10: Central areas of belt wires after 8 months of service, axial cross-sections, BEC: (a) belt from standard atmosphere; (b) belt from humidified atmosphere.

(a) (b) (c) (d) (e) Figure 11: Surface and subsurface region of the wire from the belt after 11 months of service in the humidified atmosphere: (a) BEC; (b) overlay of chromium on BEC; (c) overlay of manganese on BEC; (d) overlay of silicon on BEC; (e) overlay of oxygen on BEC.

(a) (b) Figure 12: Internal oxidation in the subsurface region of belt spiral wire after 11 months of service, transverse cross-section, BEC: (a) standard atmosphere; (b) humidified atmosphere. The strength of the belt material 3.2.3. Benefits of Modified • More protective surface oxide decreased with time of service and Nitrogen-Hydrogen Atmosphere scales was significantly higher for the versus Standard Nitrogen- • No reduction of the protective belt from the modified Hydrogen Atmosphere surface oxide scales atmosphere. Assuming that the • Higher tensile strength and • Lower concentration of particles; tensile strength is the main factor elongation; lower rate of tensile shorter chains on grain in determining service life, the strength reduction modified atmosphere increased boundaries • The effects listed above result in the service life by about 30 % [8]. • Lower nitrogen pickup extended service life of • Lower depth of internal conveyor belts used in oxidation humidified nitrogen-hydrogen • Lower depth of chromium- atmospheres depleted zone 4. Conclusions Acknowledgement

The evaluation of sintered parts We would like to acknowledge Mr. under several types of standard Anthony M. Zaffuto, President of sintering atmospheres has shown Metaltech, Inc. Dubois, PA and Mr. that the highest quality results Jeremy Gabler from Metaltech for were produced in a dry nitrogen/ their support of the humidification <5%H2- atmosphere. This is related technology field testing, for to a very low dew point, resulting providing samples of the belts for in a limited driving force for material examinations, and for decarburization as well as the quality control results in relation elimination of carbon species like to parts sintering. carbon monoxide and hydrocarbons, resulting in sooting. However, the strongly reducing atmosphere destroys the protective surface oxide scales on the transport belt of the furnace. Although high surface quality parts are produced in this atmosphere, the service life of the transport belt is not optimal.

The results of belt tests in an industrial furnace using nitrogen- hydrogen atmospheres showed that the service life of a belt could be extended by increasing the moisture content of the sintering atmosphere. The belt used in the humidified atmosphere was better protected from high temperature oxidation, nitridation and carburization, which delayed the material degradation leading to the belt failure. A 30 % increase in belt life was attributed to the protective oxide scales that were maintained on the belt surface in the nitrogen-hydrogen atmosphere modified using our newly developed humidification system.

The described work shows that the set-up and control of the sintering furnace atmosphere is the key to producing parts with stringent high quality properties and can also be used to lower operating costs by reducing the maintenance efforts as achieved by extending the service life of a conveyor belt. References

[1] Beguin, C.: Einführung in die [7] FactSage™ thermochemical Technik der Schutz- und software and database package Reaktionsgase, Schweizerischer developed jointly between Verband für die Thermfact/CRCT (Montreal, Wärmebehandlung der Canada) and GTT-Technologies Werkstoffe, 1995 (Aachen, Germany), version 6.2, 2010. [2] Neumann, F. : Methoden zum Messen und Regeln von [8] A. Wehr-Aukland, D. J. Bowe, A. Aufkohlungsatmosphären M. Zaffuto and J. Gabler, unter besonderer “Service Life Extension of Berücksichtigung des Stainless Steel Wire Mesh Belts Sauerstoffpotentials, for Sintering Furnaces”, Härtereitechnische Advances in Powder Metallurgy Mitteilungen 44 (1989) 5 & Particulate Materials, to be published in 2011. [3] J.G. Marsden, D.J. Bowe, K.R. Berger and D. Garg, D.L. Mitchell, “Atmospheres for Extending Life of Belts within Sintering Furnaces”, U.S. Patent No. 5,613,185, March 18, 1997.

[4] H. J. Grabke, Carburization – A High Temperature Corrosion Phenomenon, MTI publication no. 52, 1998, Material Technology Institute of the Chemical Process Industries, St. Louis, MO.

[5] G. Y. Lai, High Temperature Corrosion and Materials Applications, 2007, ASM International, Materials Park, OH.

[6] S. R. Pillai, “High Temperature Corrosion of Austenitic Stainless Steels”, Corrosion of Austenitic Stainless Steels: Mechanism, Mitigation and Monitoring”, edited by H.S. Khatak and B. Raj, Woodhead Pub., Cambridge, UK, Alpha Science, Pangbourne, UK, 2002, pp. 265-286. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labcuntbore et dolore magna aliquyam erat.

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod temuniiat por invidunt ut labore et dolore magna aliquyam erat. tell me more

For more information, please contact us at:

Corporate Headquarters South America Legal name Air Products and Chemicals, Inc. Building, Road Name 7201 Hamilton Boulevard Town, Province, Country Allentown, PA 18195-1501 Postal Code T 610-481-4911 T 11-4791-8833 F 610-481-5900 F 11-4791-8830 [email protected] [email protected] or visit www.airproducts.com or visit www.airproducts.co.br

Europe Legal name Sales Region Legal name Building, Road Name Building, Road Name Town, Province, Country Town, Province, Country Postal Code Postal Code T 44-1234-567899 T 123-098-7654 F 44-1234-567890 F 123-456-7890 [email protected] [email protected] or visit www.airproducts.co.uk or visit www.airproducts.com

Asia Legal name Building, Road Name Town, Province, Country Postal Code T 65-1234-5678 F 65-1234-5670 [email protected] or visit www.airproducts.com.cn