Carbon potential control in MIM sintering furnace atmospheres

technical trends

Carbon potential control in MIM sintering furnace atmospheres Tony Palermo of Linde LLC and Akin Malas of Linde Gas Global Application Development describe a means to control the chemical carbon potential of the sintering atmosphere in order to produce higher quality products while in effect lowering the cost of production, enhancing customer satisfaction, and expanding the current and future market penetration of MIM- and PIM-produced parts.

M

ethods to make near-netshape parts and components hold the promise of reduced cost and inventory due to minimised yield loss of raw materials and fewer process steps. However, competition remains fierce among competitive processing routes to make metal parts and components for industries such as automotive, medical, agricultural/earth moving, and military and defence. Metal injection moulding (MIM) and powder injection moulding (PIM) methods for the production of parts and components are in competition with other processing methods on the basis of cost, quality, consistency and other factors. There are a number of materials and processing areas which may contribute to the generation of surface or internal defects and insufficient sintered part properties. These contributors include issues relating to the powder itself, binder material, powder mixing, packing pressure, heating rate, insufficient binder removal, temperature control, atmosphere composition, and part design among other factors. These,

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in turn, can result in increased costs, dissatisfied customers, and ultimately a loss of market share to alternative processes and materials. The root causes of these defects are sometimes obvious and other times not. One root cause resides in the surface decarburisation or carburisation of the sintered part. High quality and repeatable sintered part production is virtually impossible without a furnace atmosphere that is appropriate in each phase of the progressive sintering process. These progressive phases require that the atmosphere enables lubricant volatilisation and burn off, reduction of powder surface oxides, retention or addition of surface carbon (C), and prevention of oxidation. Regarding surface carbon control, it has not been a common practice to monitor and control the chemical carbon potential of the sintering atmosphere in a continuous, closed loop manner. While the optimal composition of the furnace atmosphere will vary throughout the furnace, depending on the process stage at any given location,

it is the primary focus of this paper to describe a means to control the chemical carbon potential of the sintering atmosphere in the highest temperature furnace zones. The objective is to produce higher quality product while in effect lowering the cost of production, enhancing customer satisfaction, expanding the current and future market penetration, and opening new market applications of MIM- and PIM-produced parts.

Atmospheres and furnaces in the MIM sintering process In a continuous sintering furnace, different atmosphere characteristics (for example, oxidising or reducing) are expected in different zones of the furnace, making sintering furnace atmospheres challenging in terms of control and optimisation. MIM furnaces employ nitrogen (N2) and hydrogen (H2) gases as inputs to create a sintering atmosphere.

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Figure 1: Sintering furnace atmosphere constituents.

Requirements for the atmosphere to influence oxidation/reduction and decarburisation/carburisation reactions in the sintering process are the same in principle with other metallurgical heat treatment operations. However, the functions in the progressive continuous sintering process that are facilitated by the atmosphere composition are somewhat more complex than most other heat treatment processes. Figure 1 shows most of the gaseous species in the sintering furnace atmosphere. Sintering process functions include: 1. Delubrication (or dewaxing or debinding) during preheating Since there is a separate pre-debinding process (that transforms the ‘green’ part to a ‘brown’ part) before the sintering operation in the MIM processing sequence, this is not considered a primary issue for the sintering process. However, the furnace still needs to be capable of oxidising the binder that remains to be removed from the final sintered part. Some furnaces employ humidifiers to facilitate the delubrication process, and the counter flow in the furnace of the hydrogen-containing atmosphere also assists in removal and combustion of lubricant vapours. 2. Oxide reduction at the sintering temperature By reducing the surface oxides, solid to solid mass transport and diffusion is facilitated. Reduction of oxides can take place in the presence of hydrogen or via reactions with carbon according to the reactions [1], [2] and [3] where Me denotes the metal.

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MeO + H2 = Me + H2O [1] MeO + C = Me + CO [2] MeO +CO = Me + CO2 [3] 3. Prevention of oxidation during cooling The atmosphere will require sufficient ratio of reducing to oxidising agents to protect the metal surfaces from oxidising during cooling. 4. Control of atmosphere carbon potential at sintering temperature as well as during cooling Traditionally and typically today, the carbon potential of the atmosphere in the furnace is not monitored and controlled in a continuous, closed-loop fashion. Some furnaces employ a continuous flow of a hydrocarbon, usually natural gas, in an attempt to maintain a carbon potential that will control surface carbon levels in the sintered parts. However, most MIM sintering furnaces (typically high-temperature pusher furnaces) do not use this method. Normally, only pure nitrogen and hydrogen comprise the gases introduced to the furnace. So carbon control, and specifically decarburisation, is a common problem in the MIM sintering process. In lower temperature PIM component sintering, when a hydrocarbon is injected into the sintering zone to increase the carbon potential of the atmosphere, a negative collateral effect can be the generation of high carbon potential at lower temperature zones in the furnace and cooling section. These areas of elevated carbon potential may lead to sooting and excessive carbide formation. The equilibrium approach

for sintering atmospheres is more challenging due to the factors: a) Temperature differences throughout the furnace profile will lead to different carbon and oxygen (O2) activities throughout the furnace; b) Graphite in the powder mixture may react with the atmosphere to cause decarburisation; c) Sintering furnaces usually have no circulation fans to assist with convection, as in carburising furnaces, therefore the atmosphere will not mix completely leading to local carbon and oxygen potential variations; d) Vapourisation of lubricants/binders will create gas mixtures that might affect the carbon potential of the overall furnace atmosphere; and e) There are differences in carbon potential among local pores and the free atmosphere within the furnace. Therefore, it is quite common for parts to be within a ±0.3%C compositional window after sintering. For carburising processes in batch furnaces this variation is much tighter, typically only ±0.03%C. Improved carbon control is a major challenge for improving the consistency of sintered parts. Compared to traditional lower temperature continuous sintering furnaces, MIM sintering furnaces are smaller in scale and capacity but better in terms of furnace atmosphere integrity.

Development of a sintering process carbon control system A new carbon-potential control technology developed by Linde Gas and Höganäs AB for sintering furnaces uses continuous atmosphere sampling, a proprietary oxygen probe situated in an external heated chamber, and an infrared carbon monoxide analyzer to continuously calculate atmosphere carbon potential and provide the addition, when needed, of trim gas from a flow train to maintain carbon potential within a desired, pre-set range. The objective of this technology is to maintain a furnace atmosphere carbon potential that contributes to the processing of sintered parts that meet required or desired carbon composition and consequent properties.

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The oxygen probe generates a millivolt signal that, combined with a temperature reading, calculates oxygen partial pressure of the sampled atmosphere. An earlier version of the oxygen probe is shown in Figure 2.

The complete atmosphere monitoring, analytical, and atmosphere adjustment system is shown schematically in Figure 3. It is mainly comprised of a sampling system, oxygen and CO analyzers, a programmable controller,

Figure 2: Early version of oxygen probe.

Figure 3: Schematic of atmosphere carbon potential control system.

Figure 4: Initial MIM test parts at Megamet.

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and a trim gas flow train which enables the control of carbon potential in the furnace zone from which the atmosphere was sampled.

Case study: Carbon control in a MIM high temperature pusher sintering furnace Megamet Solid Metals (Earth City, Missouri, USA) approached Linde Gas about the possibility of applying a continuous atmosphere carbon potential control system for sintering of its non-stainless steel MIM parts in its high-temperature, pusher-type furnace. Carbon control difficulties in sintering furnace atmospheres are amplified in the high-temperature MIM sintering process. Megamet had been experiencing decarburisation of parts with final part specification of 0.4 to 0.6% carbon. Each ceramic boat was loaded with at least 12 parts and they had been experiencing variation in part carbon content among parts on a given boat from 0.1% to 0.4%C. These outof-specification carbon levels would add costs due to additional post-sintering heat treatment and yield loss. BASF Catamold® FN0205 feedstock was used as the powder mixture with 0.4to 0.6%C, 1.9-2.2%Ni and the balance iron. Three different components were involved in the initial testing (see Figure 4). The furnace employs an atmosphere that in total is about 20% hydrogen and the balance nitrogen. Total batch process time is typically 7.5 hours with 14 minutes between each boat push. In order to establish the baseline for the carbon control trials, the sampling and analytical functions of the Linde atmosphere control system were employed to monitor the existing conditions. Then, for four days, the furnace atmosphere was continuously monitored, and carbon potential was influenced and controlled within programmed set point ranges. Propane and CO gases were used to influence carbon potential during these initial trials. Over 200 sintered parts were tested for carbon content in Megamet’s analytical laboratory and these results were correlated to the set

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points and realised carbon potential recorded by Linde’s carbon potential control system. The furnace and the setup of the Linde carbon control system are shown in Figure 5. The baseline data had indicated that the sintered parts were typically decarburised to varying degrees in virtually every batch (boat) for the subject parts without the use of a carbon control system. For the Megamet practice, when a given boat is discharged from the exit end vestibule, a hydrogen purge is used to minimise air ingress. Due to the pressure change caused by this purging, the Linde sampling and analytical system identified a change in the atmosphere’s carbon potential upon each of these door opening/purging events. As mentioned previously, baseline data showed erratic carbon content results ranging from 0.2 to 0.7%C both within and between boats. The graphs in Figure 6 show the carbon content on parts 1 and 2 (shown in Figure 4) in the baseline period prior to application of the Linde carbon control system. Figure 7 shows the microstructure of part 1 under the baseline atmosphere. Decarburisation on the surface and in the core can be clearly seen as white structures. Tests were then run with the furnace atmosphere then controlled and modified with the Linde carbon control system. The first test started with a forced CO addition of 2% and this practice increased the carbon content of the parts up to 1 to 1.4%C on the surface. The surface of these parts with high carbon content was melting due to the reduced melting temperature at these carbon levels. Figure 8 shows the phase diagram for a mixture of Fe - 2%Ni at different carbon contents. The graph shows that there will be partial melting at 1280 ºC when the carbon level is around 1.5%. The tests continued with reduced CO levels but with atmosphere carbon potentials higher than the baseline levels at which significant decarburisation had been the norm. The carbon potential set points in the control system were significantly higher than were seen during the monitoring of the baseline condition. The carbon potential of the furnace atmosphere is calculated knowing the amount of oxygen and

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Figure 5: Megamet test setup and furnace schematic.

Figure 6: %C for parts under baseline atmosphere for parts #1 and 2.

Figure 7: Microstructure of Part #1 sintered under the baseline atmosphere.

CO in the furnace as well as temperature. The effective carbon content (i.e., the required carbon potential of the atmosphere in order to be carbon neutral with the part material) can be calculated by using Gunnarsson’s formula [4].

Log Cp/C = 0.055 (%Si) – 0.013 (%Mn) – 0.040 (%Cr) + 0.014 (%Ni) – 0.013 (%Mo) + %C [4] Also, the factors in [5] can be used in a formula to calculate carbon potential. Cp = f (T, V, CO, q) [5]

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1550

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1: *FCC_A1 2: *LIQUID 4: *BCC_A2

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1350 2

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Figure 8: Phase diagram for Fe-Ni-C material.

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Conclusions

Figure 9: Phase diagram for Fe-Ni-C material.

Where Cp is the carbon potential (in %); T is the temperature (in Kelvin); V is the oxygen probe voltage (in millivolts); CO is the carbon monoxide (in %); and q is a steel alloy factor. Alpha iron undergoes a phase transition from body-centred cubic (BCC) to the face-centred cubic (FCC) crystal structure configuration of

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the decarburisation/carburisation behaviour of MIM sintered components. The furnace has several zones where the temperature is around this temperature and it is apparent from the microstructures that decarburisation also took place in the core of the part after the backbone binder was removed from the brown body. After the part is delubricated the porosity is quite high and the gas-metal interaction will cause carbon transfer in or out of the material. When the sintering starts (at a high temperature) the decarburisation reactions will slow down due to the low atmosphere carbon potential for the same atmosphere composition at high temperature. It was found that adjusting the atmosphere carbon potential to approximately 0.05%C at 1280 ºC will provide approximately a 0.6%C content in the part. The tests continued by setting the Linde carbon control system to this carbon potential largely for the rest of the trials. The result was that uniform carbon contents of around 0.5%C (±0.05%C) in the parts were achieved. Figure 10 shows the carbon content of parts 1 and 2 with the use of the Linde carbon control system on a continuous basis. Figure 11 shows the microstructures of product sintered with the Linde carbon control system in use. Compared to Figure 4, the decarburisation has been significantly reduced at the surface and almost eliminated in the core of the part. Figure 12 shows a general overview of the comparison of baseline atmosphere results with those after using the Linde carbon control system.

gamma iron, also called austenite. Carburising and decarburising start when the steel microstructure begins to transform to austenite. The temperature when this transformation starts differs depending on the alloy composition. This transformation temperature is approximately 700ºC for the alloy used in these tests (see Figure 9). Therefore, the carbon potential in the furnace at this temperature also plays an important role in determining

1. The Linde carbon control system has been demonstrated to control the carbon potential of the sintering atmosphere in the sampled area successfully. 2. With use of the Linde carbon control system it was possible to establish and sustain the furnace atmosphere conditions for three different sintered MIM parts so that the part carbon contents could be maintained in the 0.4 to 0.6%C specification window. This control was not repeatable

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without the use of the Linde carbon control system. 3. Carbon potential measured in the highest temperature zone can be used to direct the introduction of trim gases in order to optimise the process and improve part quality and consistency.

6. Impact of Decarburization on the Fatigue Life of Powder MetalForged Connecting Rods, Document Number: 2001-01-0403, Date Published: March 2001, Author(s): Edmond Ilia - Mascotech Sintered

Components, Russell A. Chernenkoff - Ford Research Laboratory.

Reprinted with permission from the Metal Powder Industry Federation.

References 1. Powder Injection Moulding International, Vol. 6, No. 1, March 2012, ‘Understanding defects in Powder Injection Moulding: Causes and corrective actions’ Randall M. German. 2. A new approach to sintering furnace atmosphere control and sinter hardening by gas impingement cooling, Akin Malas*, Soren Wiberg*, Daniel Nilsson**, and Sigurd Berg**, *Linde Gas Application Development, **Höganäs AB, PM2008 World Congress - Washington D.C. 3. Sintering of Steel, White Paper, published in 2011, Linde Gas Publications. 4. Advanced Carbon Control in Sintering Atmospheres, Christoph Laumen*, Akin Malas*, Sören Wiberg** and Sigurd Berg***,*Linde Gas Application Development, **AGA Gas AB, ***Höganäs AB, Euro PM 2009 Copenhagen. 5. Influence of decarburized layers and surface defects on the stress state of a specimen strained in bending. A. G. Rakhshtadt, V. M. Semenov, S. M. Serebrin, I. E. Stepanov and N. N. Shaposhnikov. Scientific-Research Institute for Automobile and Tractor Materials, Moscow. Translated from Problemy Prochnosti, No. 5, pp. 68–72, May, 1974.

Figure 10: %C in parts #1 and 2 using the Linde carbon control system.

Figure 11: Microstructure of part 1 sintered with carbon control system.

0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Base Atmosphere

Sinterflex Atmosphere Setting 1

Sinterflex Atmosphere Setting 2

Sinterflex Atmosphere Setting 3

Figure 12: Part carbon content variation between the highest and the lowest %C by part # (baseline and with Linde carbon control system).

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