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Reactions between ferrous powder compacts andatmospheres during sintering – an overview

Christian Gierl-Mayer

To cite this article: Christian Gierl-Mayer (2020) Reactions between ferrous powder compactsand atmospheres during sintering – an overview, Powder Metallurgy, 63:4, 237-253, DOI:10.1080/00325899.2020.1810427

To link to this article: https://doi.org/10.1080/00325899.2020.1810427

© 2020 The Author(s). Published by InformaUK Limited, trading as Taylor & FrancisGroup

Published online: 24 Aug 2020.

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INVITED REVIEW

Reactions between ferrous powder compacts and atmospheres duringsintering – an overviewChristian Gierl-Mayer

Institute for Chemical Technologies and Analytics, TU Wien, Vienna, Austria

ABSTRACTThis overview paper describes the interaction of powder metallurgical iron-base alloys with theatmosphere during sintering. The methods of thermal analysis serve to clarify the processes thattake place especially during the heating stage of the sintering cycle. After a discussion of thephysical and chemical fundamentals of the sintering process, the methods of thermalanalysis are explained. The differences between plain iron and alloyed systems are discussedin detail. Classical PM low alloy steels with alloying elements, such as Cu, Ni and Mo, react ina similar way as unalloyed carbon steels. The situation changes dramatically, when oxygensensitive elements as chromium, manganese and even more silicon come into play. Theremoval of the surface oxygen is much more crucial, and there are several competingreactions, which have to be considered when these systems should be sintered in industrialscale to reach the desired mechanical and dimensional properties.

ARTICLE HISTORYReceived 7 April 2020Revised 7 August 2020Accepted 11 August 2020

KEYWORDSSintered steels; atmospheres;thermal analysis; degassing;alloy elements

Introduction

The production of structural ferrous parts is a successstory of powder metallurgy. Although the principlesof sintering iron and steel parts are known to a widepublic within the PM community, the interactionbetween the atmosphere and the parts was not ofgreat importance over the years as productions ransmoothly as long as protective atmospheres wereused that were reducing for iron oxides and thus alsofor the oxides of the common alloy elements copper,nickel and molybdenum. Removal of the oxidesintroduced through the starting powders thus wasnot a problem and usually ran virtually unnoticed.Decarburisation was definitely recognised as a problembut was usually coped with by adding carburisingagents, as e.g. in endogas. The tendency towardsusing alloy steels containing chromium, manganeseor even silicon, which are widely used in ingot metal-lurgy due to their interesting relationship cost/benefitin terms of mechanical properties, hardenability andmicrostructure evolution, however, confronted thestructural parts industry with problems not knownfrom classically alloyed powder metallurgical steels.This article is intended to give an overview about theinvestigations done to understand the reactions takingplace between the pressed compacts and the atmos-phere, like the considerations given by Boccini [1]and Beiss [2], to ensure proper sintering also in thepresence of alloy element with high oxygen affinity.

The analysis of the surface of metal powders was pub-lished by Nyborg et al. in numerous papers, where thecomposition and the thickness of powder surfaces ofdifferent steel powders were reported. This startedwith the much easier analysis of gas atomised steelpowder [3–7] and was furthermore applied on irregu-lar shaped water atomised powders [8–10]. Thisbecame of special interest when chromium low-alloyedpowders appeared on the market.

Physical fundamentals

The main goal for the sintering process in precisionparts production is significantly different from otherpowder metallurgical production processes in whichthe main goal is to reach full dense products, such assintering of hardmetals or injection moulded parts.The sintering of precision parts is usually performedwith hardly any shrinkage, which implies that the start-ing material (the pressed green compact) should havethe maximum density already after compaction inorder to reach acceptable density values – and thusmechanical properties. Especially impact energy andelongation to fracture show a strong increase afterreaching a density level of about 7.6 g cm−3, whichcan be related to closed porosity [11–14]

This means that if the material should be sinteredwithout shrinkage, the compactibility of the powderwill determine the final properties, but the compactionpressures that can be applied by technical presses are

© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, orbuilt upon in any way.

CONTACT Christian Gierl-Mayer [email protected] Institute for Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9/164, CT, A-1060 Vienna, Wien, Austria

POWDER METALLURGY2020, VOL. 63, NO. 4, 237–253https://doi.org/10.1080/00325899.2020.1810427

limited to 600–800 MPa maximum according to tech-nical and economical reasons. Therefore, most of theprecision parts are produced within a density rangeof 6.8–7.1 g cm−3. To attain the desired microstructureduring sintering, the decisive process is the formationof stable metallic contacts between the compressed par-ticles. The grain size and the phase distribution can becontrolled by the alloying elements and the alloyingtechnique chosen and by sintering temperature andtime.

The principal driving force for sintering withoutexternal forces (free sintering) is defined by thermo-dynamics [15]: The trend for a system to reach a mini-mum of energy. For a single component system, suchas plain iron, the surface is the relevant feature for sin-tering. The surface of a solid (or a liquid) is connectedwith additional energy: specific surface free energy γ(J m−2) or surface tension γ (N m−1)

The energy of powder compacts is increased due tothe large surface area of the material, the more the finerthe powder is.

To reach a minimum of surface area, the drivingforce is defined by:

DE = g · DA (1)

where E is the energy of the System and A the sur-face area. This means that the driving force for sinter-ing is the minimisation of the surface area of thedispersed system. Therefore, fine powders will sintermore intensely than course ones. The material trans-port in the powder compact during sintering is diffu-sion at the surface, along the grain boundaries andthrough the volume of the powder grains, i.e. surface,grain boundary and bulk diffusion, respectively. Butprecondition for this material transport is the presenceof really metallic surfaces. Here the thermodynamics ofoxide reduction on metal powders comes into play,which means the interaction of powder compact andthe atmosphere.

Thermodynamic considerations

Most metallic elements are thermodynamically notstable in air (or oxidising environment in general) atroom temperature, which means that the Gibbs freeenergy ΔG of the reaction 2x/yM + O2 = 2/yMxOy isnegative. According to the base equation

DG = DH–TDS (2)

with ΔH as reaction enthalpy, T as absolute temp-erature and ΔS as reaction entropy, the oxides can bereduced either by heating until ΔG becomes positive– thermal decomposition of the oxide – which is realis-tic only for a few metals (e.g. silver) or by combinationwith a reducing agent for which ΔG of its oxide

becomes more negative. The most common reducingagents in this sense are hydrogen and carbon.

The consequence for sintering of powder compactsis that it is decisive to remove the surface oxides cover-ing every powder that has ever been exposed to air,since the material transport processes describedabove can only take place if a real metal–metal contactis given. As long as the contact region consists of metaloxides, the diffusion processes are hindered or at leastslowed down such that there will be no contact for-mation in realistic times. Furthermore, the drivingforce for sintering is also lower because of the signifi-cantly lower surface energy of oxides compared tometals, i.e. an oxide-covered metal powder will sintermuch less readily than a fully metallic one.

If a powder compact is introduced into a furnace, wehave to keep in mind that in principle the system canbe divided into four main components, which is sche-matically shown in Figure 1.

(1) The pressed powder itself: The compact consists ofpowder particles, pressed together with high forcesto ensure high green density and green strength.Although high pressures (400–800 MPa) are usedto consolidate the green body, the part still con-tains 10–15 vol.-% of pores when the sintered den-sity is taken as a measure. Furthermore, thealloying techniques in powder metallurgy mustbe considered. In contrast to ingot metallurgy,powder metallurgy has several ways to introducethe alloying elements, as shown schematically inFigure 2. The chemical activity α (the effective con-centration of a species), which is responsible forthe chemical interaction with the atmosphere,differs significantly between prealloyed andadmixed variants, for the latter, the chemicalactivity of the alloying element being 1. While pre-alloying reduces the activity most pronouncedly,the master alloy variant shows medium chemicalactivities of the alloying elements.

(2) The internal atmosphere: The atmosphere withinthe pores is the atmosphere the part carries whensubjected to a sintering cycle. For sure, this atmos-phere is strongly influenced by the interaction ofthe material with the atmosphere during thistemperature cycle and can significantly differfrom the atmosphere outside the body. It is poss-ible that the internal atmosphere is more oxidisingthan the free atmosphere in the furnace, but it isalso possible that it is the other way round.Owing to ‘internal gettering’-effects, the oxidisingspecies can be trapped within the compact and‘clean’ the atmosphere so that from thermodyn-amic point of view, the conditions turn to morereducing [17].

(3) External atmosphere: The atmosphere of the fur-nace, i.e. the gases introduced into the furnace

238 C. GIERL-MAYER

together with the evaporated (or trapped) gases ofthe material-gas reactions, forms the externalatmosphere. This is the only atmosphere we canmeasure and influence directly by analyticalmethods and gas introduction. In principal, ithas to fulfil four tasks:Task 1: The atmosphere protects the powder com-pacts against undesired reactions like oxidation,decarburisation, carburisationTask 2: The atmosphere removes the reaction pro-ducts of desired reactions as e.g. the water vapourproduced by the reduction of oxides with hydro-gen or the vapour of the lubricant in the burn-offzone of the furnace.Task 3: The atmosphere is able to remove undesir-able elements/contaminants like oxides byreductionTask 4: The atmosphere is able to introduce inter-stitial elements (carbon, nitrogen or boron) byspecial reactions

The possible atmospheres therefore can be groupedinto:

. Inert atmospheres: These atmospheres fulfil tasksT1 and T2. The most common ones are vacuumand noble gases (argon, helium). Nitrogen is fre-quently regarded as a member of this group, becausethere is hardly any reaction with steels, but theremight be a reaction with chromium in stainlesssteels, vanadium [18] and even stronger reaction

with titanium or the formation of aluminium nitrideduring the sintering of aluminium [19,20]. Also, theeffect of boron activated sintering is strongly depen-dent on the presence of nitrogen. Whereas in hydro-gen or vacuum, even stable h-BN decomposes toform a eutectic melt between boron and iron, onthe other hand, the reaction of boron, e.g. from bor-ides, is completely inhibited by the nitrogen atmos-phere through the formation of inert h-BN [21–23](see Figure 3).

. Reactive atmospheres:oOxidising atmospheres: Air and inert atmospheres

that contain amounts of oxidising species like watervapour, carbon dioxide and oxygen. The oxidisingeffect of these species is depending on their chemicalactivity, which in the gaseous phase usually is a

Figure 1. Interaction between sintering atmosphere and powder compact or sintered body.

Figure 2. Alloy variants for sintered steels (a) prealloyed; (b) elemental mixture; (c) master alloy, (d) diffusion bonded; (e) coated;(modified after [16]).

Figure 3. Dilatometry of Fe–1.5Mo–0.6C–0.3B (h-BN) in N2 andH2, 60 min 1250°C isothermal, 10 K min−1 heating and coolingrate [21].

POWDER METALLURGY 239

function of the partial pressure. For carbon monoxidecontaining atmospheres this means that endogasatmospheres with 20 vol.-% CO (+40 vol.-% H2 and40 vol.-%N2) may be oxidising for some systems andreducing for less sensitive ones. Usually these oxidisingconstituents of the sintering atmospheres mentionedabove are undesirable for sintering of PM parts exceptfor delubrication or binder burnout (‘rapid burnoff’techniques [24])

o Reducing atmospheres: hydrogen and to someextent carbon monoxide, depending on the partialpressure. These and their mixtures with inert gases arethe most widely used protective gases in PM [25,26]

o Carburising atmospheres: Carbon monoxide, pro-pane, acetylene or endogas; these gases are typicallyused during secondary treatment operations like gascarburising or low pressure carburising, but to someextent also in the sintering atmosphere, e.g. to compen-sate for the presence of decarburising agents.

o Decarburising atmospheres: All sintering atmos-pheres that contain too high amounts of water vapour,which is the most decarburising agent, but also carbondioxide, oxygen and exogas. Typically, these constitu-ents of the sintering atmosphere are extremely undesir-able to appear within a sintering furnace, but mayoccur due to leakages or other malfunctions. Theylead to unwanted decarburisation of the parts.

o Nitriding atmospheres: Ammonia, which formsnascent nitrogen, or atomic nitrogen in plasma areable to introduce nitrogen to a number of materials.For strong nitride formers as e.g. Ti or Zr, but alsofor stainless steels, also N2 is nitriding.

(4) Boundary layer: The interaction between externaland internal atmosphere is usually performed bydiffusion until a certain boundary layer is over-come and then by diffusion and convection inthe free atmosphere. Convection in the body canonly happen by ‘blowing’ of the pores during theheating process when the gases in the poresexpand, by massive formation of gases within thepores by chemical reactions there or by ‘sucking’during the cooling process.

The problem for atmosphere control is that all deci-sive processes like deoxidation of the surface oxidesoccur within the pore network – in the internal atmos-phere – and therefore cannot be directly measured byany known technology. The only atmosphere thatcan be measured and controlled is the externalatmosphere.

As already mentioned, it is decisive to remove theoxides from the particles in the early stages of sintering– during the heating stage – either by a reducingatmosphere or by addition of a reducing agent to thepowder mix, which usually means addition of carbon.

The possible reactions to reduce the surface oxides are:

MeO+H2 � Me+H2Owhich means reduction

with an atmospheric constituent (hydrogen)

(3)

MeO+ CO � Me+ CO2 which means indirect

carbothermic reduction favourable at low temperatures.

(4)

MeO+ C � Me+ CO which means direct

carbothermic reduction, which is favourable at

high temperatures

.

(5)

MeO +Mf�Me+MfO which means metallothermic

reduction by a metal forming more stable

oxides compared to the base material

(6)

The direct and indirect carbothermic reaction areconnected by the Boudouard equilibrium

12CO2 + 1

2C ↔ CO (7)

At high temperatures (and low pressures), the equi-librium is shifted towards carbon monoxide, whichmeans that the partial pressure of the specific gas com-ponents determines the equilibrium for industrial sin-tering. The driving force for the reactions therefore isthe partial pressure of the oxides.

Methods to measure the interactionbetween atmosphere and PM compact

There are several techniques to measure the inter-action, most of them belong to thermoanalyticalmethods, like dilatometry (DIL), differential thermalanalysis (DTA), thermogravimetry (TG) or evolvedgas analysis (EGA).

Dilatometry

Dilatometry (DIL) is usually employed to measurethermal expansion coefficients [27], but for powdermetallurgy, dilatometry is an instrument for processcontrol during sintering, and the dilatometer is moreor less a very precise sintering furnace [28]. Since thedimensional stability, shrinkage and expansion effectsduring sintering are extremely important to describethe sintering process, in particular, for PM precisionparts for which dimensional stability is crucial, dilato-metry is very helpful.

240 C. GIERL-MAYER

Simultaneous thermal analysis

The method of simultaneous thermal analysis (STA)combines DTA and thermogravimetry (TG) in oneoperating unit, by placing the DTA sensor on a bal-ance. The positive effect of one operating unit com-pared to separate DTA and TG systems is that thetemperature signal is without any doubt the same forboth methods. In STA systems, it is easier to correlateTG and DTA. One can be sure that the measurementhappened simultaneously and is not influencedby differences in the operating units such as furnaceshifts, etc.

Evolved gas analysis

In most cases, EGA is not operated as stand-alone sys-tem. EGA is usually combined with DTA, TG or bothof them (STA) analysing the exhaust gases that hadpassed the systems. During thermal analyses, couplingtechniques are typically used to identify and in partquantify the exhaust gases. The common techniquesare:

. Infrared spectrometry: This method is often used forthe identification of decomposition products fromorganic compounds (feedstocks for metal injectionmoulding, polymer decomposition, etc.) [29]

. Mass spectrometry (MS): MS is the method ofchoice when inorganic gases should be detected.The quantification of mass spectrometry is knownto be tricky since not all evolved gases are trans-ferred into the measurement system. In principle,two methods of coupling a mass spectrometer to athermoanalytical device are reported, the main

task being the lowering of gas pressure from typi-cally 1 bar (ambient) to the vacuum of 10−5 mbarrequired in the spectrometer (see Figure 4): Theskimmer coupling, in which the vacuum is reachedin two steps. The first step from 103 mbar to 10–1

mbar is performed by a rotary pump and anorifice system, and the second step is performedby a high vacuum pump. The so-called skimmer isplaced within the furnace to prevent cooling of theevolved gases and condensation of material in thetiny orifice. The system is an expensive high-endsystem and only available for STA, but with highresolution, short transport distances and thereforealmost perfect temperature-time correlation. Thecapillary coupling, in contrast, is the more flexibledevice since the capillary entrance can be placede.g. also in the exhaust gas stream of a dilatometer.The necessary pressure drop is performed by thefused silica capillary only, which is usually heatedto 250–300°C to prevent condensation problems.The most important disadvantage is that duringthe flight, which is performed at supersonic speed,the gas species may react with each other and thefinal analysis result therefore might differ from theoriginal gases evolved from the specimen. For obser-vation of sintering, both systems are used, but themost severe limitation for MS is the most widelyused industrial sintering atmosphere which consistsof nitrogen–hydrogen mixtures. The most abundantmass m28 can be related both to molecular nitrogen(N2) and carbon monoxide (CO), i.e. the signalsinterfere and cannot be separated except by high-resolution MS. In principle, the distinction betweenboth gases often is realised by analysis of the break-down species: masses m12 for carbon and m14 for

Figure 4. EGA coupling systems (schematic), left: skimmer coupling, right: capillary coupling (courtesy of Netzsch GerätebauGmbH).

POWDER METALLURGY 241

atomic nitrogen, which are quite characteristic, butin excess of nitrogen the signals disappear due totoo high base lines of mass m28. Therefore, for sin-tering studies often neutral atmospheres are chosenlike argon or helium (or Ar–H2 in place of N2–H2),which do not interfere with masses of interest.

The first studies about the application of DTA/TG coupled with mass spectrometry in powdermetallurgy were published by Gille and Leitnerwho focused on the system WC-Co (hardmetals/cemented carbides). They showed that sintering ofthe system only started after deoxidation of theWC powder particles. Sintering and deoxidationwere further strongly influenced by alloyingelements that form more stable oxides comparedto WC, such as TaC, NbC, VC [30–32]. This is par-ticularly important for ultrafine hardmetal grades(in which esp. VC and Cr3C2 are necessary asgrain growth inhibitors) since the presence ofthese alloy elements shifts the deoxidation to highertemperatures while the ultrafine grades tend to formclosed porosity at lower and lower temperatures.Intersection of these two temperature ‘windows’has to be avoided at any rate, otherwise pore for-mation or even blistering would occur.

. Photoacoustic Spectroscopy (PAS): The principle isto detect an acoustic signal generated by lightabsorption. The method can be quantified from0.1 to 2000 ppm. An example of a sinteringsimulation is shown by Chasoglou et al. [33] inFigure 5. It is obvious that sintering atmospherescontaining nitrogen are not a limiting factor fordetecting carbon monoxide in PAS. The main limit-ation of this technique is that for PAS, gas has to besampled, and this limits the time–temperature cor-relation. For the usual number of gases (CO, CO2,CH4, H2O), the sampling time is approximately 60s [34], which means, a temperature resolution of10 K for the usual heating rates (10 K min−1).

Other gas analysing systems

In industrial practice, but also in laboratory scale, thereare numerous systems that allow the analysis of the sin-tering atmosphere. Some of them are operating on-lineand can directly be used as a control system for theatmosphere.

The most common sensors are infrared sensors forquantification of CO or CO2, which usually are com-bined with lambda sensors for the measurement ofoxygen content to control the atmosphere in industrialsintering furnaces. Also dewpoint meters are usefulinstruments to prevent decarburisation.

For sintering furnaces, there has been a systemintroduced by Linde and Höganäs AB which allowsthe determination and control of the sinteringatmosphere (Sinterflex). The system needs some COin the atmosphere (about 1%) and is operated notcompletely on-line, as it extracts some gas from the sin-tering atmosphere to analyse it in this extra unit (see[35–37]).

Reduction process during sintering

The following chapter describes in examples the reac-tions of the powder compacts with the atmosphere.The chapter is divided into three main parts.

Reactions with systems of low oxygen affinityReactions with systems of high oxygen affinitySpecial reactions in oxygen-sensitive systems

Systems with low oxygen affinity

The systems with low oxygen affinity do not containelements with higher oxygen affinity than iron inhigher amounts. All alloying elements described inthis chapter are less sensitive to oxygen, namely nickel,copper and molybdenum. In principle, also tungstenalso belongs to this group but usually is not used inPM due to its high price.

Plain ironTo remove surface oxides from plain iron compacts,hydrogen-containing atmospheres are necessary. If neu-tral atmospheres like vacuum of argon are used, the tracesof carbon still present in the powder are able to reducesome surface oxides to enable sintering, but the reactionis far away from being complete, which is, however, thecase in hydrogen atmosphere (Figure 6).

For sintering of plain iron, an extremely unsymme-trical phase transformation behavior has been reported[38,39] as shown in Figure 7. This coincides with unex-pected longitudinal growth of the sample due to slighttemperature gradients during the austenite–ferritephase transformation that causes excessive and direc-tional grain growth [40]. During the formation of the

Figure 5. PAS curve from sintering simulation of Fe–3Cr–0.5Mo–0.5C, 10 K min−1, Tmax 1120°C, 30 min in N2/3%H2 [32].

242 C. GIERL-MAYER

α-iron crystals, the oxygen is trapped and enriched atthe grain boundaries, and this causes severe embrittle-ment when sintering in argon and vacuum. Thesamples sintered in hydrogen also show enlarged grains[41, p. 119], but the impact energy is much higher (71.1J cm−2 compared to 2.3 J cm−2). It is important tomention that interstitials like carbon or nitrogen (see[42]) completely inhibit this behavior by extendingthe phase transformation over a wider temperaturerange. In addition, alloyed systems do not show thiseffect either because of the same reason (Figure 8).

The system iron-carbonIn contrast to plain iron compacts, the system Fe–Ccontains a strongly reducing agent within the compactin the form of carbon, which is intentionally admixedto the iron-base powder as alloying element, typicallyas fine graphite powder. Here it has to be consideredthat carbon consumed for reduction purposes leavesthe compact as gaseous CO/CO2 and is therefore nomore available for alloying, which means that car-bothermal reduction comes with the penalty of carbonloss, the more, the more effective the deoxidation is.However, this carbon loss is homogeneous within theentire specimen and usually well controllable, in con-trast to e.g. surface decarburisation through impureatmospheres. It can be compensated for by addingslightly more graphite.

The system Fe–C was systematically studied in [43]where the respective deoxidation behavior of compactsprepared from different base powders such as atomisedand sponge grades (Höganäs ASC 100.29; HöganäsABC 100.30 and Höganäs SC 100.26) admixed withcarbon was studied in neutral (argon) and hydrogen(reducing) atmosphere (both plain hydrogen and N2/H2 mixture 90/10) by dilatometry (sample size: 10 ×10 × 8 mm³). The results showed that – if desorptionof humidity at low temperatures is neglected – thedeoxidation takes place in two stages for all atmos-pheres. In neutral atmospheres the first, very pro-nounced peak of CO appears at 640–730°C,depending on the purity of the base powder, but alwaysbefore the α−γ-transition. This peak marks thereduction of the surface oxides and is followed athigher temperature by reduction of the internal oxides(including those trapped in the pressing contacts) [44].Since the oxygen from the internal oxides must diffuseto the surface before being able to react with the redu-cing agent to form gaseous products, the deoxidationpeaks are much broader since the diffusion distanceis not the same for each oxide. It is also obvious thatthe particularly pure grade ABC 100.30, with extralow manganese content of 0.074 mass% Mn comparedto ASC 100.29 with 0.113 mass% Mn [41], showsmarkedly less reduction of internal oxides by theadmixed carbon (Figure 9). Furthermore, it has also

Figure 6. Dilatometry +MS of Fe, heating ramp of plain iron compacts (600 MPa) in (a) argon and (b) hydrogen, 10 K min−1, Tmax

1300°C (please note the different ion current scales for the MS graphs), sample size 55 × 10 × 8 mm³ [27].

Figure 7. Dilatometry of plain iron compacts (600 MPa) in (a) argon and (b) hydrogen, Tmax 1300°C, 60 min, 10 K min−1, sample size55 × 10 × 8 mm³ [27].

POWDER METALLURGY 243

been shown that the ratio of the two peaks depends onthe particle size of the base powder, with finer powdersthe first peak being more pronounced – because of thelarger specific surface – while the second one is smallerand is shifted to lower temperatures [43].

In reducing atmospheres, the reduction of thesurface oxides is performed at much lower tempera-tures, but now by the hydrogen in the atmosphere.The CO peak at about 700°C completely disappearswhile a water peak (mass m18) appears at around400°C, depending on the hydrogen content of theatmosphere. Reduction of the internal oxides, incontrast, is once more performed by the added

carbon, as indicated by the pronounced CO peakin Figure 10(a). This shows that at high tempera-tures, above about 900°C, carbon is the preferredreducing agent even in presence of H2, because ofthe increased thermodynamic stability of CO withhigher temperatures compared to H2O. In the nitro-gen–hydrogen mixture, this carbothermal reductioncan only be detected by the breakdown peak ofmass 12, since, as already mentioned before; massm28 for carbon monoxide will be masked by nitro-gen from the atmosphere (Figure 10(b)).

The production process of the metal powders alsoimplies differences in oxygen content and allocation

Figure 8. Dilatometry of (a) Fe–1.5Mo (600 MPa) and (b) Fe–0.5C, in Argon, Tmax 10 K min−1, sample size 55 × 10 × 8 mm³.

Figure 9. Dilatometric and MS graphs of Fe–0.8C in argon, heating stage, Tmax 1300°C, 10 K min−1 (a) ABC 100.3 (b) ASC 100.29,sample size 55 × 10 × 8 mm³ [43].

Figure 10. Dilatometric and MS graphs of Fe (ASC 100.29)–0.8C in (a) hydrogen and (b) nitrogen–hydrogen mixture 90/10, heatingstage, Tmax 1300°C, 10 K min−1, sample size 10 × 10 × 8 mm³; please note the different ion current scales for the MS signals [43].

244 C. GIERL-MAYER

of the oxides. Especially sponge iron powders with itsinternal porosity, which is to a large extent closedduring pressing, show a significant shift of ratiobetween surface and internal oxides towards the latterones (see Figure 11).

When full-size Charpy bars (ISO 5754, 55 × 10 ×8 mm³) are used for the dilatometric measurement,the resolution of the mass peaks is not that sharp anymore – the gaseous reaction products being releasedmore slowly and gradually – but in principle, theresults are the same [28]. In argon, there are oncemore two m28 deoxidation peaks, and in hydrogen,the surface deoxidation is performed by hydrogen, acorresponding water (m18) peak being formed atlower temperatures, <500°C. Since the mass is muchhigher with these large specimens, the first peak ofmass m28 does not disappear completely even inplain H2, indicating some carbothermal reduction ofthe surface oxides. It stands out rather clearly thatthe dissolution of graphite in the austenite matrixtakes place slightly faster in hydrogen, although thephase transformation starts at higher temperatures.The former effect can be seen by the more pronouncedexpansion in the austenite range after the ferrite–auste-nite transition (Figure 12).

Chromium and manganese alloyed systems

Chromium alloyed steels have been thoroughly investi-gated as described in [28,44,45]. All these studies showthat chromium as an alloying element is responsible fordecisive changes in the deoxidation behaviour. Inargon atmosphere, the first intense peak that indicatesthe deoxidation of the surface oxides is shifted to muchhigher temperatures and somehow overlaps with thepeak for the internal oxides, only for small samples,for which better resolution is attained, the separationis possible (see Figure 13).

This means that deoxidation does not start before900–950°C and is not even finished at 1300°C, the criti-cal temperatures depending on the chromium content,as also shown in [46]. The consequence for the mech-anical properties is a pronounced effect of the sinteringtemperature: below the first degassing peak, there isvirtually no formation of metallic sintering contacts[47], and even at temperatures above this threshold,higher temperatures have a strongly beneficial effecton both monotonic and cyclic mechanical properties[47, 48]. Molybdenum as an alloying element (alsocontained in these steel grades) hardly affects deoxida-tion, Mo alloyed steels being comparable to Fe–C [49].In hydrogen atmosphere, there is somewhat less differ-ence between the Fe–C system and Fe–Cr(–Mo)–C.There is a rather pronounced water peak in the low-temperature region, just as it is in Fe–C, whichmeans that at least part of the deoxidation is performedaround 400°C by hydrogen, and only the more stableoxides have to be reduced at higher temperatures bythe added carbon, which is clearly confirmed by [50].Nevertheless, when comparing the peak areas form18 and m28, it stands out clearly that the vastmajority of the initial oxygen content is removed car-bothermally at temperatures >900°C (see Figure 14).It has been shown that prealloyed manganese steelsreact in a very similar way when the manganese con-tent is in a similar range (>1.5 mass %) [51]. In [52],a comprehensive study on the efficiency of reducingatmospheres was performed on Cr-alloyed steels. The

Figure 11. TG/DTG graphs for Fe–0.8% C prepared from wateratomised (black) and sponge iron powders (green), respect-ively. 10 K min−1, flowing He [44].

Figure 12. Dilatometric and MS graphs of Fe (ASC 100.29)–0.5C in (a) argon and (b) hydrogen, heating stage, Tmax 1300°C,10 K min−1, sample size 55 × 10 × 8 mm³ [28].

POWDER METALLURGY 245

results show clearly that it needs hydrogen-containingatmospheres for effective reduction. CO addition isonly helpful, when lean H2/CO (concentration of activegas ≤ 5 vol.-%) is used, otherwise, oxidation and/orcarburisation is observed.

The main reason for this behavior was found byapplying high-resolution electron microscopy andphotoelectron spectroscopy (XPS) on the surfaces ofthe prealloyed powders. The analysis showed thatalloyed steel powders are mainly covered by iron oxides(90% of the area) which form a homogeneous thinlayer of about 6–7 nm thickness. The rest of the surfaceis covered by islands formed of mixed oxides [Fe, Mn,Cr]xOy and oxides of spinel type like FeCr2O4, MnFe2-O4, MnCr2O4, etc. (Figure 15). The size of these par-ticulate features rises up to 300 nm with higher alloyelement content [53]. According to the calculationsin [54], these oxide islands contain about 50% of thesurface oxygen in as delivered powder.

The influence of the green density on deoxidation ismarginal for classical PM steels until the parts reach 7.4g cm−³, as shown in [55]. For the chromium alloyedvariants, the deoxidation is shifted to higher tempera-tures as the density increases. As green densitiesreach 7.5 g cm−³, which can be performed, e.g. byhigh-velocity compaction, the deoxidation virtuallystops at temperatures above 1150°C indicating a fastclosing of the pore channels with resulting trappingof the remaining oxygen. However, in [56], it has

been shown that the effect of this trapped oxygen onthe mechanical properties is apparently insignificant,even in the gigacycle fatigue regime.

Special reactions in oxygen-sensitive systems

In the above mentioned oxygen-sensitive systems,there is a certain tendency to special reactions thatare described in the following sub-chapters.

Methane formationAs can be seen in Figure 14(b), there is somemethane formation, indicated by the mass16. Thismethane formation has always been a topic of discus-sions since it influences carbon control, which isessential for carbon containing steels. The commonreason for inhomogeneous decarburisation observedin sintering of PM steels (in contrast to the ‘natural’,homogeneous carbon loss described above) is thepresence of water vapour in the atmosphere, carbonremoval taking place according to

C+H2O ↔ CO+H2 (8)

but in the literature [29], also the reaction

2H2 + C ↔ CH4 (9)

is described. However, the deoxidation experimentswith the system Fe–C have never shown significant

Figure 13. Dilatometric and MS graphs of Fe–3Cr–0.5Mo–0.6C in (a) argon and (b) hydrogen, heating stage, Tmax 1300°C,10 K min−1, sample size 10 ×10 ×8 mm³ (please consider different y axis scalings) [45].

Figure 14. Dilatometric and MS graphs of Fe–1.5Cr–0.2Mo–0.5C (solid lines) and Fe–3Cr–0.5Mo–0.6C (dashes lines) in (a) argon and(b) hydrogen, heating stage, Tmax 1300°C, 10 K min−1, sample size 55 × 10 × 8 mm³ [28].

246 C. GIERL-MAYER

amounts of methane; neither in the old literature [57,58] nor in the more recent one [26, 59], significant car-bon loss via methane formation is reported. However,when the advanced chromium and manganese preal-loyed steels were introduced, the appearance ofmethane in the temperature range of about 700–950°C was observed. This was first reported in [60, 61]and also in [62]. The calculation of the Gibbs free ener-gies of possible reactions [equations (10–13)] showedthat the probability of CH4 formation in the systemFe–C is rather low because the ΔG values for the directreactions of carbon containing species with hydrogenbecome positive at rather low temperatures (Figure 16),i.e. the reactions are thermodynamically inhibited,while at lower temperatures at which ΔG would benegative, the kinetics of the reactions are not favour-able.

Methane reaction:C+ 2H2 ↔ CH4 (10)

Sabatier reaction:CO2 + 4H2 ↔ CH4 + 2H2O (11)

Methanisation: CO+ 3H2 ↔ CH4 +H2O (12)

2CO+ 2H2 ↔ CH4 + CO2 (13)

The analysis of the chromium and manganese pre-alloyed steels sintered in hydrogen showed distinctmethane peaks in the range of 600–1100°C, which dis-agrees with the performed calculations. The introduc-tion of oxides of silicon, chromium and manganeseopened new ways of probable reactions. The tempera-ture thresholds when ΔG = 0 of some of these reactionsare at much higher temperature levels compared to thereactions cited above. Especially the findings in [63]showed that silicon is that particular element that pro-motes methane formation very strongly. By the for-mation of methane, steels containing 1 wt-% ofsilicon lost about 0.5 wt-% of carbon during a soakingperiod of 2 h in the mentioned temperature range. ForCr prealloyed materials, it is striking that it is not thematerial with the highest Cr content that shows thestrongest methane formation (see Figure 14(b)), butthe 1.8% chromium material. The proposed reactionmechanisms all include the presence of CO, whichfor sure will be higher when there is a reduction ofthe surface oxides by carbon at the given temperaturerange. In [64], it is shown that different steel compo-sitions result in the formation of methane at differenttemperature ranges, and the intensity of the methaneformation depends on the alloying elements used.There seems to be a correlation between the oxygensensitivity of the elements, i.e. the Gibbs free energyof the oxides, and CH4 formation, following thesequence Cr < Mn < Si. Manganese-containing mixesshow more intense methane formation than chromiumand even more does silicon. However, binary mixes Si+ 0.5C mixes do not show any methane formation atall, which indicates that the presence of iron is necess-ary (see Figure 17).

The current working hypothesis is that the methaneformed and measured is a final product of an ‘internalgetter’ effect. Systematic experiments have shown thatboth iron as base material and oxygen-sensitiveelements have to be present to produce significantamounts of methane. Carbon monoxide seems to be

Figure 15. Qualitative SEM + EDX analysis of particulate features on the surface of manganese alloyed steel powder [54].

Figure 16. Gibbs free energies of reactions possibly formingCH4 [60].

POWDER METALLURGY 247

a necessary intermediate product which finally oxidisesthe sensitive alloying elements (Si, Mn, Cr) by the reac-tion Me + 2H2 + CO → MeO + CH4. The metallo-graphic section shown in Figure 18 prove that thereis much more homogeneous carbon loss in hydrogencompared to sintering in argon atmosphere, whichwas also confirmed by combustion analysis [65]. Thissupports the hypothesis that the homogeneous decar-burisation through CH4 formation is preceded byhomogeneous carbothermal reduction. If a direct reac-tion of C with H2 would be responsible for CH4 for-mation, heterogeneous decarburisation from thesurface would be expected, similar to the decarburisa-tion caused by H2O from impure atmospheres.

‘Internal gettering’From a thermodynamic point of view, an elemental ormasteralloy powder mix is in extreme inequilibrium,which has pronounced consequences if elements withwidely differing oxygen affinity are present. The pres-ence of agents that become reducing for the ironoxide surfaces above a given temperature during the

heating stage generates reaction products (e.g. H2Oand/or CO) which are, however, heavily oxidising forthe more oxygen-sensitive elements at the same temp-erature. The largest source of oxygen is usually not theelemental or master alloy powder (unless treatedimproperly), but the base iron powder itself. Therefore,the gaseous reaction products, as oxidising agents, willoxidise silicon, manganese and chromium, present aselemental powders or combined in a masteralloy, bysimply transferring the oxygen from the iron via thegas phase to the elemental or master alloy particles(see schematic in Figure 19).

This phenomenon usually does not occur at lowtemperatures (300–450°) at which only reduction byH2 is possible and when water is the reaction product,since the kinetics of the oxidising reactions is usuallystill too slow at these temperatures. However, in inertatmospheres when the reduction is performed by theadded graphite, the temperatures of first CO/CO2 for-mation are significantly higher, and the effect can beobserved indirectly, by the disappearance of the firstCO peak, as given in Figure 20(b) for admixed

Figure 17. Thermogravimetric signals and MS graphs of different powder mixes sintered in H2, (a) Fe–0.5C–4.0X (X = Cr, Mn or Si)(b) Si–0.5C [64].

Figure 18. Metallographic sections of masteralloy-containing steels Fe–4MA–0.5Cadmixed (MA: Fe–40Mn–17Si) sintered in thedilatometer at 1300 °C for 60 min in Ar or H2 (a) Argon: O: 0.025%, combined: 0.43% (b) Hydrogen: O: 0.013%, combined:0.31% [65].

248 C. GIERL-MAYER

elemental Mn. As the CO does not leave the com-pound, the absence or at least the diminution of theexpected CO peak at about 700°C is a clear hint forthis internal oxygen transfer. In fact, the net reactionmechanism is a metallothermic reduction of the ironoxide surfaces by the alloy element(s) via gaseous inter-mediate products (see Figure 20 [66]).

A similar phenomenon was reported by Hryha [67],when using elemental manganese as alloying elementduring sintering at 1120°C. Intergranular decohesionfacets were found around Fe–Mn residues with ahuge amount of point inclusions consisting of oxides,forming networks. This observation also is an indicatorfor internal gettering since the manganese that diffusesalong the grain boundaries reduces the surface ironoxides of the particles, forming a network of point-like Mn oxides that locally weaken the grain bound-aries and finally lead to intergranular fracture in thisarea. The mechanical behavior is strongly improvedat a higher sintering temperature, because then alsothese Mn oxides can be reduced.

The practical consequence is that effective deoxida-tion of such powder compacts requires temperaturestypical for carbothermal reduction of the alloy elementoxides, although initially the oxygen contained hasbeen present mainly as – easily reducible – iron oxides.To some extent, the temperature thresholds for

reduction are lowered by the dissolution of the alloymetal formed in the iron matrix, with a resultingdecrease of the chemical activity. In any case, there isnot much difference in the deoxidation behaviorbetween mixed and prealloyed systems, respectively,in both cases, high sintering temperatures, >1200°C,are required for reasonably complete deoxidation.

‘Internal gettering by diffusion’As stated above, the introduction of alloy elements intosintered steels can be done by admixing these elements,as elemental powders or through masteralloys, but alsoby prealloying the entire powder. The deoxidation ofchromium or manganese prealloyed powders in inertatmospheres shows a macroscopically similar effect asthe ‘internal gettering’ observed with mixes, althoughhere the transport of the reaction partners does notoccur via gaseous products but by diffusion. As alreadyshown, the surface of commercially available preal-loyed powders consists primarily of iron oxide, andthe oxides of the more sensitive elements are locatedin particulates on the surface and within the powderparticle [46,68]. This is confirmed by the fact that sin-tering in H2 results in a pronounced H2O peak at about400°C, as observed for Fe–C. When this powder is,however, heat treated (or heated) in inert atmosphere,the chromium (or the manganese) dissolved in the iron

Figure 19. Schematic of ‘internal getter effect’ (‘AE’: alloy element).

Figure 20. Thermogravimetric signals and MS graphs of Fe–0.5C and Fe–0.5C–4.0Mn (elemental) powder mixes heated in Ar, Tmax

1300°C, 20 K min−1, (a) Fe + 0.5C, (b) Fe + 0.5C + 4Mn [66].

POWDER METALLURGY 249

matrix starts to diffuse towards the surface where oxy-gen is present bonded to iron, the driving force beingthe much more negative Gibbs free energy for the for-mation of, e.g. MnO compared to iron oxides (seeFigure 21). There, the transfer from iron oxide tomore stable oxides – which is in fact also a metallother-mic reduction of the iron oxide – can happen if the kin-etics of the reaction is fast enough. As can be seen inFigure 22, heat treatment at 400°C in argon does notchange the deoxidation behaviour in hydrogen at low

temperatures compared to as delivered powder (seeFigure 13(b) and Figure 22(a)), the typical m18 peakat 400°C still being present. This confirms that the sur-face oxides still have been iron oxides in this case. Thesituation changes dramatically when the material isheated to 650°C (even without holding time). For theFe3Cr0.5Mo–C system, the water peak disappearsalmost completely, which is a clear indicator for thepresence of more stable oxides on the surface thatrequire much higher temperatures for (carbothermal)reduction. The kinetics aspects of this oxide transform-ation have also been discussed in [53], where it isshown, that the reduction of the surface oxides byhydrogen is beneficial, especially when it is performedbefore a part the stable oxides are entrapped within thegrowing sintering necks. The critical temperatureidentified by this study for oxygen transfer is 800–1000°C, where an unfavourable balance between ther-modynamics and kinetics of oxide reduction existsduring the heating stage. This means increased masstransfer of oxygen-sensitive elements but still poorconditions for oxygen reduction.

The transformation of oxides towards more stableones has been shown through XRF and SEM studiesand microanalysis, e.g. by Karamchedu et al. [37],but at much higher temperature levels (900°C), at450°C delubrication temperature, no effects havingbeen measured. Chasoglou et al. [69] demonstratethe change in morphology of stable oxides and theincorporation of these in the sintering necks. Thisfor sure influences the mechanical properties, notso much the static ones but it can be expectedand was already demonstrated in [47] that dynamicproperties suffer from these inclusions. The presentstudy, however, shows that such ‘internal getter’phenomena occur at quite moderate temperaturelevels, below 700°C.

One possible solution might be a reduction/presin-tering treatment in H2 at temperatures at which theinternal getter effect does not yet appear, but for PMprecision parts, this collides with the necessity to doeffective lubricant burnout, which in industrial practicerequires temperatures well above 400°C. Furthermore,as indicated by the graphics in Figure 22(a,b), even atoptimal conditions presintering in H2 removes onlypart of the oxygen – that one bonded to iron – whilethe remaining oxygen must be removed by

Figure 21. Schematic of ‘internal getter‘ effect in Fe–Cr–Mo prealloyed powders.

Figure 22. Dilatometry and mass spectra of Fe–3wt-%Cr–0.5wt-%Mo–0.5wt-%C in H2. Tmax 1300°C, heating rate 10 Kmin−1. In part, pre-annealed in Ar. (a) Reference run, no pre-anneal. (b) Pre-annealed in Ar at 400°C. (c) Pre-annealed inAr at 650°C [66].

250 C. GIERL-MAYER

carbothermal reduction at much higher temperatures.Therefore, it is as necessary to apply high sinteringtemperatures for Cr/Mn/Si prealloyed powders as it isfor the mixed systems described above, at least if thepotential of these advanced alloy elements is to befully exploited.

Conclusions

It has been shown that thermoanalytical techniques,such as dilatometry or DTA/TG combined with EGAby mass spectrometry, are powerful tools to understandthe interaction of powder components with the sinter-ing atmosphere, in particular, the deoxidation pro-cesses in the early stages of sintering that are essentialfor obtaining excellent mechanical properties. Themain findings are:

1) The classical materials for ferrous structural partsare more or less easy to sinter, since the deoxida-tion processes are dominated by the base ironpowder. The classical alloying elements, such asnickel, copper and molybdenum form oxides,that are less stable than the iron oxides and aretherefore easier to reduce, and the system is fairlyrobust towards the quality of the sintering atmos-phere, the limiting factor being rather surfacedecarburisation.

2) Advanced high-performance sintered steels con-tain alloying elements, such as chromium, manga-nese (and in the future possibly silicon), and thepresence of these oxygen-sensitive elementschanges the chemical behaviour dramatically.

3) The reducing of the oxides of these elements in theearly sintering stage becomes decisive for a suc-cessful sintering.

4) One of the most critical points is the transfer ofoxygen from the surface of the starting powder,which consists mainly of iron oxides, to the alloy-ing elements.

5) In inert atmospheres, a transformation of the orig-inal iron oxides to more stable complex oxidesoccurs by diffusion of the alloying elements tothe particle surface during the heating stage ofthe sintering process in case prealloyed powdersare used.

6) When powder mixes – or more modern – masteralloys are used, this transfer takes place via the gasphase, in a process designated ‘internal gettering’,the transporting agent being CO within the porestructure of the compact.

7) In reducing atmospheres, it is possible that thisCO is reduced by hydrogen to form methane indifferent temperature ranges depending on thealloying system.

8) All these reactions influence the final carbon andoxygen content of the material and therefore the

mechanical properties. Therefore, it is importantto be aware of these reactions also in industrialpractice.

9) The high-performance steels based on advancedalloying systems are much more sensitive to thequality of the furnace atmosphere than are tra-ditional grades. Therefore, more attention has tobe paid to keep the processing conditions as stableas possible for a successful sintering of thesematerials. On-line measurement systems to ensureproper atmospheres are available for industrialpractice

10) Furthermore, reduction of the ‘natural’ oxides pre-sent on the powders requires significantly highertemperatures than applied for standard PM steels,and therefore high-temperature sintering is a mustif the full benefit of the new alloy systems is to beexploited.

It can be expected that the alloy development willcontinue and alloying elements that are today in usefor wrought steels like silicon or vanadium becomemore and more interesting for powder metallurgy.Therefore, investigations in this field will be stillnecessary to further improve the performance of PMsteels. In any case, however, tools for these studiesare available.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes on contributors

Christian Gierl-Mayer studied Technical Chemistry at TUWien, Vienna Austria. He made his Master Degree in Inor-ganic Chemistry in 1996. He got his PhD in 2000 from TUWien for ‘Production of PM structural parts by optimisedsintering‘. After three years as scientist in private researchinstitute (ofi-Austrian Institute for Chemistry and Technol-ogy) he became senior scientist at TU Wien, Institute forChemical Technologies and Analytics (2003). 2017 he waspromoted Associate professor and 2019 after his habilitationin powder metallurgy he became Associate Professor formetallic sintered materials. He is currently leading theresearch group ‘Powder metallurgy’ at the above mentionedInstitute. He is currently leading the research group ‘Powdermetallurgy’ at the above mentioned Institute.

References

[1] Boccini GF. Influence of controlled atmosphere on theproper sintering of carbon steels. Powder Metall Prog.2004;4(1):1–33.

[2] Beiss P. Control of protective atmospheres during sin-tering. Proceedings Euro PM 2000 – Sintering andEquipment. Munich; 2000. p. 147–157.

[3] Bracconi P, Nyborg L. Quantitative phase analysis andthickness measurement of surface oxide layers in

POWDER METALLURGY 251

metals and alloy powders by the chemical granularmethod. Appl Surf Sci. 1998;133:129–147.

[4] Oleford I, Nyborg L. Surface analysis of gas atomizedferritic steel powder. Powder Metall. 1985;28:237–243.

[5] Nyborg L, Nylund A, Olefjord I. Thickness determi-nation of oxide layers on spherical shaped metal pow-ders by ESCA. Proc. of ESCASA 1987. Stuttgart; 1987.

[6] Nyborg L, Norell M, Olefjord I. Surface studies of pow-der metallurgical stainless steel. Surf Interface Anal.1992;19:607–614.

[7] Norell M, Nyborg L, Tunberg T, et al. Thickness deter-mination of surface oxides on metal powder by AESdepth profiling. Surf Interface Anal. 1992;19:71–76.

[8] Tunberg T, Nyborg L. Surface reactions during wateratomisation and sintering of austenitic stainless steelpowder. Powder Metall. 1995;38:120–139. doi:10.1179/pom.1995.38.2.120.

[9] Karlsson H, Nyborg L, Berg S. Surface chemical analy-sis of pre-alloyed water-atomized steel powder. PowderMetall. 2005;48(1):51–58.

[10] Karlsson H, Nyborg L, Berg S, et al. Surface productformation of chromium alloyed steel powder particles.Proc. EuroPM 2001 Vol. 1. Shrewsbury: Nice: EPMA;2001. p. 22–27.

[11] Bocchini G. The influence of porosity on the character-istics of sintered materials. Int J Powder Metall.1986;22(3):185–202.

[12] Dlapka M, Danninger H, Gierl C, et al. Defining thepores in PM components. Met Powder Rep. 2010;65(1):30–33.

[13] Murphy T, Lindsley B. Metallographic analysis of PMfracture surfaces. In: Murphy CB, editor. Advances inpowder metallurgy and particulate materials.Princeton, NJ: Metal Powder Industries Federation;2007. p. 11-1–11-15.

[14] Danninger H, Spoljaric D, Jangg G, et al.Characterization of pressed and sintered ferrousmaterials by quantitative fractography. PractMetallogr. 1994;31(2):56–59.

[15] Schatt W. Sintervorgänge – Grundlagen. Düsseldorf:VDI; 1992.

[16] Zapf G. Handbuch der Fertigungstechnik, Bd.1.Urformen. (G. Spur, Hrsg.). München: HanserVerlag; 1981.

[17] Gierl-Mayer C, de Oro Calderon R, Danninger H. Therole of oxygen transfer in sintering of low alloy steelpowder compacts: a review of the “internal getter”effect. JOM. 2016;68(3):920–927.

[18] Danninger H, Gierl C. New alloying systems for fer-rous powder metallurgy precision parts. Sci Sinter.2008;40(1):33–46.

[19] Sercombe T, Schaffer G. The effect of trace elements inthe sintering of Al-Cu alloys. Acta Mater. 1999;2:689–697.

[20] Schaffer G, Huo S, Drennan J, et al. The effect of traceelements on the sintering of an Al-Zn-Mg-Cu alloy.Acta Mater. 2001;49(14):2671–2678.

[21] Gierl C, Ul Mohsin I, Danninger H. Boron activatedsintering of PM steels – alterntive boron sources.Powder Metall Prog. 2008;8(2):135–141.

[22] Momeni M, Gierl C, Danninger H, et al.Thermoanalytical sintering studies of Fe–C admixedwith ferroboron performed in different atmospheres.Powder Metall. 2012;55(1):54–64.

[23] Vassileva V, Danninger H, Strobl S, et al. The role ofthe atmosphere on boron-activated sintering of ferrous

powder compacts. Powder Metall Prog. 2018;18(1):6–20.

[24] Crease, Jr, A. Rapid burn-off system for removinglubricant from P/M parts. J Powder Metall. 1977;13(1):21–24.

[25] anonymous. Funace atmospheres No.6 sintering ofsteelsLinde AG; 2011. https://www.linde-gas.com/en/images/Furnace.

[26] Nayar H. Sintering atmospheres. In: Metal handbookVol. 7 powder metal technologies and applications.Materials Park, OH: ASM International; 1998.p. 1066–1094.

[27] ASM International. Thermal expansion. In: Cverna F,Editor. Thermal properties of metals. Materials Park,OH: ASM International; 2002. p. 9–12.

[28] Gierl-Mayer C, Danninger H. Dilatometry coupledwith MS as instrument for process control in sinteringof powder metallurgy steels. Powder Metall Prog.2015;15(1):3–12.

[29] Quadbeck P, Schreyer B, Strauß A, et al. In-Situ moni-toring of gas atmospheres during debinding and sinter-ing of PM steel components. (EPMA, Edt.)Proceedings of PM21010 World Congress, Vol. 2Sintering; 2010.

[30] Gille G, Leitner G, Roebuck B. Sintering behaviour andproperties of WC-Co hardmetals in relation to the WCpowder properties. Proceedings of EuropeanConference of Advanced Hard Metals Production,Stockholm (S. 195–201). Shrewsbury: EPMA (Edt.);1996.

[31] Gestrich T. In situ Charakterisierung der Vorgängebeim Entbindern, Ausgasen und Sintern von WC-Co-Hartmetallmischungen mittlels komplexer ther-mischer Analyse [PhD Thesis]. Dresden, TU; 2005.

[32] Gestrich T, Kaiser A, Pötschke J, et al. Thermal behav-ior of cermets and hardmetals during debinding andsintering. Int J Refract Met Hard Mater. 2018;73:210–214.

[33] Chasoglou D, Hryha E, Bergman O. Efficient sinteringof Cr-prealloyed powders by careful adjustment of pro-cess parameters. Adv Powder Metall Part Mater.2014:142–153.

[34] Frisk K, Söderberg H, Caddeo S. Studies of surfaceoxides on steel powders using photo acoustic spec-troscopy coupled with thermodynamic calculations.Proc. Euro PM2009, Copenhagen. Shrewsbury,EPMA (Edt.); 2009.

[35] Hryha E, Nyborg L, Malas A, et al. Carbon control inPM sintering: industrial applications and experience.Powder Metall. 2013;56(1):5–10.

[36] Hryha E, Dudrova E, Nyborg L. On-line control of pro-cessing atmospheres for proper sintering of oxidation-sensitive PM steels. J Mater Process Technol. 2012;212(4):977–987.

[37] Karamchedu S, Hryha E, Nyborg L. Changes insurface chemistry of chromium alloyed powdermetallurgical steel during delubrication and theirimpact on sintering. J Mater Process Technol.2015;223:171–185.

[38] Danninger, H. Asymmetrical ferrite-Austenite trans-formation during vacuum sintering of plain iron com-pacts. Powder Metall Prog 2003, 3(2), p. 75–85.

[39] Danninger H, Gierl C, Vassileva V. Asymmetry effectsin ferrite-austenite transformation during sintering ofcarbon-free ferrous alloys. Proceedings ofEuroPM2005, Vol. 1. Prague, Shrewsbury: EPMA(Edt); 2005. p. 15–20.

252 C. GIERL-MAYER

[40] Kuroki, H., Suzuki, H. Y. Coarse columnar structure oftransformation-grown ferrite in pure iron – onwrought iron and sintered iron. Mater Trans 2006,47(10), p. 2449–2456.

[41] Jaliliziyaeian M. Sintering of manganese prealloyedpowder metallurgy steels [PhD thesis], TUWien. 2008.

[42] Gierl C, Danninger H. Thermal analysis of plain iron –the effect of “inert” atmospheres. ProceedingsEUROPM2005, Vol. 3. Prague: EPMA (Edt.),Shrewsbury; 2005. S. 3–8.

[43] Gierl C, Jaliliziyaeian M, Danninger H, et al.Dilatometry of PM carbon steels in different atmos-pheres – deoxidation effects. Proceedings EuroPM2009. Copenhagen. Shrewsbury: EuropeanPowder Metallurgy Association; 2009. Paper-Nr. 29-1, on CD.

[44] Danninger H, Gierl C, Kremel S, et al. Degassing anddeoxidation processes during sintering of unalloyedand alloyed PM steels. Powder Metall Prog. 2002;2(3):125–140.

[45] Kent D, Drennan J, Schaffer G. A morphological studyof nitride formed on Al at low temperature in the pres-ence of Mg. Acta Mater. 2011;59(6):2469–2480.

[46] Ortiz P, Castro F. Thermodynamic and experimentalstudy of role of sintering atmospheres andgraphite additions on oxide reduction in AstaloyCrM powder compacts. Powder Metall. 2004;47(5):291–298.

[47] Kremel S, Danninger H, Yu Y. Effect of sintering con-ditions on particle contacts and mechanical propertiesof PM steels prepared from 3%Cr prealloyed powders.Powder Metall Prog. 2002;2(4):211–221.

[48] Dlapka M, Danninger H, Gierl C, et al. Fatigue behav-ior and wear resistance of sinter-hardening steels. Int JPowder Metall. 2012;48(5):49–60.

[49] Danninger H, Leitner, G. Gas formation in Fe-Mo-Cbelow the α−γ transition. In: German R, Messing G,Cornwall R. editors. Sintering technology (Proc.Conf. “Sintering’95”). University Park, PA; 1996;New York-Basel-Hong Kong: Marcel Dekker. p. 165–172.

[50] Hryha E, Wendel J. Effect of heating rate and processatmosphere on the thermodynamics and kinetics ofthe sintering of pre-alloyed water-atomized powdermetallurgy steels. J Am Cer Soc. 2019;102:748–756.

[51] Danninger H, Jaliliziyaeian M, Gierl C, et al. Chemicalreactions during sintering of Mn and Mn-Cr preal-loyed steels in inert versus reducing atmospheres.Mater Sci Forum. 2011;672:203–206.

[52] Hryha E, Nyborg L. Thermogravimetry study of theeffectiveness of different reducing agents during sinter-ing of Cr-prealloyed PM steels. J Therm Anal Calorim.2014;118(2):825–834.

[53] Hryha E, Nyborg L. Surface oxides on metal powders:an overview of the effect of alloy composition andmanufacturing method. Proc. World PM 2016;Hamburg. Shrewsbury: EPMA (Edt.); 2016.

[54] Hryha E, Gierl C, Nyborg L, et al. Surface compositionof the steel powders pre-alloyed with manganese. ApplSurf Sci. 2010;256(12):3946–3961.

[55] Danninger H, Xu C. Degassing and reduction duringsintering of high density PM steels. Proc. EuroPM2003, Vol. 1. Valencia; 2003. p. 269–274.

[56] Danninger H., Xu C., Khatibi G., Weiss B., LindqvistB., Gigacycle fatigue of ultra high density sinteredalloy steels. Powder Metall. 2012, 55(5), p. 378–387

[57] Kieffer R, Hotop W. Sintereisen und Sinterstahl.Springer-Verlag: Wien; 1948.

[58] Jones W. Fundamental principles of powder metal-lurgy. London: Edward Arnold; 1960.

[59] Lenel F. Powder metallurgy – principles and appli-cations. Princeton (NJ): MPIF; 1980.

[60] de Oro Calderon R, Campos M, Gierl C, et al.Atmosphere effects on liquid phase sintering of PMSteels modified with master alloy additions. Proc.PM2010 World Congress Florence, Vol. 2 Sintering.Shrewsbury: EPMA (Edt.); 2010.

[61] Gierl C, Danninger H. Charakterisierung vonSintervorgängen in Eisenbasis- Pulverpresslingen mit-tels Prozessanalytik durch thermischeAnalysenmethoden. In: Kolaska H. Hrsg.Pulvermetallurgie in Wissenschaft und Praxis Band27: Sintern – der zentrale Prozess derPulvermetallurgie. Hagen: FachverbandPulvermetallurgie; 2011. p. 215–236.

[62] Gierl-Mayer C, Danninger H, de Oro Calderon R, et al.“Internal gettering” – metallothermic reduction pro-cesses in the early stage of sintering. Int J PowderMetall. 2015;51:47–53.

[63] Danninger H, Avakemian A, Gierl-Mayer C, et al.Methane formation through substrate-atmosphereinteraction during sintering of Si containing steels.Proc. EuroPM2014 Salzburg. Shrewsbury, UK: EPMAEdt; 2014. p. 6.

[64] Gierl-Mayer C, de Oro Calderon R, Danninger H.Sintering of ferrous metallic compacts: chemical reac-tions that involve interstitial elements. J Am Cer Soc.2018;102:695–705.

[65] Gierl-Mayer C, de Oro Calderon R, Danninger H.Methane formation during sintering of PM steelsalloyed with chromium and/of manganese. Proc.World PM 2016 Hamburg. Shrewsbury, UK: EPMA(Edt.); 2016.

[66] Danninger H, de Oro Calderon R, Gierl-Mayer C.Chemical reactions during sintering of PM steel com-pacts as a function of the alloying route. PowderMetall. 2018;6(3):241–250.

[67] Hryha E, Nyborg L. Oxide transformation in Cr-Mn-prealloyed sintered steels: thermodynamic and kineticaspects. Metall Mater Trans A. 2014;45(4):1736–1747.

[68] Hryha E, Nyborg L, Dudrova E, et al. Brittleness ofstructural PM steels admixed with manganese studiedby advanced electron microscopy and spectroscopy.Powder Metall Prog. 2008;8(2):109–114.

[69] Chasoglou D, Hryha E, Nyborg L. Effect of atmospherecomposition on the surface interactions during sinter-ing of chromium-alloyed PM steels. Proceedings EuroPM2011 Congress and Exhibition on PowderMetallurgy, Barcelona, Vol. 3. Shrewsbury: EPMA;2011, p. 111–117.

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