Atmosphere / Material Interaction

Powder Metallurgy Summer School 2016, EPMA, Valencia

Christian Gierl-Mayer

TU Wien Institute of Chemical Technology and Analytics What‘s the connection between sintering of PM-steels of composition Fe-C-xM-yM2 and the atmosphere?

(Miba Sinter Austria GmbH) (Fe-4%Ni-2.5%Cu-0,5%Mo-0,5%C)

Provocative statement: “OK, protecting gas is necessary, but we usually don‘t care much” Alloying Elements Pressing Aid Base Powder

Mixing

Shaping Soft Annealing

Sintering Double-Press

Post Treatment: Thermal Treatment, Calibration, joining, Impregnation, machining,…

Finished Production scheme for PM process Part (after E.Mosca) Sintering: One possible definition: Thermal treatment where loose powder becomes densified and the desired composition.

Driving force: Energy generated by the minimization of surfaces Heating period Isothermal sintering 1. Forming of contacts by diffusion processes

Forming 2. Shrinkage: accelerated by higher of Densification contacts stage Final stage starting energy of the system (fine powder) e.g. MIM Liquid phases: e.g. HM Density Densification,

3. Final stage: desired microstructure is Temperature Time generate

Schematic illustration of pressure less sintering solid state sintering (after W.Schatt) Metallic core

Oxide layer

Sintering of spheres Metallic (after W.Schatt) core

Necessity to remove oxide layer  diffusion between metallic structures  forming of contacts Most important factor for Generation of Contacts: Degassing and deoxidation processes

Principle: Metallic powders are thermodynamically unstable  covered by Oxide layer on the surface

Behaviour is described by Richardson Ellingham-diagram:

Free Gibbs energy of the reaction n*M + y*1/2O2  MnOy* Metal + Oxygen  metal oxide towards temperature Richardson-Ellingham-diagram for reduction with C (after A.R.Glassner) 6 Possible statement: Oxides of the usual alloying elements Ni, Cu, Mo are easier to reduce than the oxides of Fe This means: Describes the stability of oxide (= how difficult it is to reduce), shows if a metal can be oxidized

It does not say if the reaction is really happening, for a given case also kinetic effects must be considered, e.g. passivation (see Al vs. Fe)

Holds for wrought as well as for sintered metals

Richardson-Ellingham-diagram for the reduction with C 7 (after A.R.Glassner) -10 TG und MS in Argon TG /% Ionenstrom *10 /A 4.0 m28 100.00 TG 3.5

99.95 3.0

2.5 Fe-0,5C 99.90 2.0

99.85 1.5

[1.1 1.0

[8] 99.80 200 400 600 800 1000 1200 Temperatur /°C

TG /% Ionenstrom *10-9 /A

100.05 m28 1.2 TG 100.00 1.0 Fe-4Ni-1,5Cu- 99.95

0,5Mo-0,5C 99.90 0.8

99.85 0.6 99.80

[1.1 0.4 99.75

0.2 99.70 [8]

200 400 600 800 1000 1200 Temperatur /°C 8 TG /% Ionenstrom *10-9 /A

TG und MS in H2 TG 100.00 1.000 m18 m28 0.900 99.95 0.800

0.700 99.90 Fe-0,5C 0.600 99.85 [7] 0.500 0.400

99.80 0.300 [2.1] [8] 0.200 99.75 200 400 600 800 1000 1200 Temperatur /°C

TG /% Ionenstrom *10-9 /A

100.00 TG 3.0 m28 99.95 Fe-4Ni-1,5Cu- 2.5 0,5Mo-0,5C 99.90 [8] 99.85 m18 2.0 99.80

99.75 1.5

99.70 [1.1] 1.0 99.65 [7] 200 400 600 800 1000 1200 Temperatur /°C 9 Conclusion:

• Thermodynamics of the reduction is important factor in the success story of these alloying elements

• Desoxidation processes  formation of sintering contacts are depending on the behaviour of iron, only little influence by the alloying elements

• Low stability of their oxides  Sintering is possible in almost all atmospheres, that are useful for sintering of carbon steels

• Neutral atmospheres (N2, vacuum) or reducing atmospheres (N2-H2, endogas) with rather modest quality (dew point, Oxygen-content) can be used

• No problems with open furnace concepts (belt furnaces)

10 Tasks of sintering atmospheres:

1. Protection of powder compacts against undesirable reactions (oxidation, decarburization, carburization, …)

2. Removing the products of desirable reactions (e.g. reduction of oxides, delubrication)

3. Controlled removal of undesirable elements / contaminants (e.g. reduction of oxides)

4. Controlled introduction of interstitials (C, N, B)

Different degrees of reactivity : • Inert atmospheres (fulfil tasks 1 and 2) • Reactive atmospheres (fulfil tasks 1, 2, 3 and/or 4) Groups of sintering atmospheres (depending on reactivity)

• Inert Atmospheres: Vacuum; • Oxidizing atmospheres: air, noble gases (Ar, He). H2O, CO2, occasionally CO; Caution: Never completely usually undesirable except for inert, traces of O2, N2, .. delubrication/binder burnout (Richard Kieffer: „There is (debinding of PIM Al in O2) reducing and oxidizing • Reducing atmospheres: H2, to vacuum“) some extent CO • Partly inert atmospheres: • Carburizing atmospheres: CO, Nitrogen (hardly any reaction C3H8, C2H2, endogas.. with steels, reaction with Cr in • Decarburizing atm.: H O, CO , stainless steels; strong 2 2 O , exogas reaction with Ti; formation of 2 AlN during sintering of Al alloys • Nitriding atmospheres: NH3 (forms nascent nitrogen), N2 in plasma Nitrogen: inert or reactive atmosphere?

ASC 600MPa Ar TG /mg ASC Ar DTA /µV ASC 600MPa N 2 Inflexion point: 1180 .4 °C [6.1][6.1] ASC N2 22 Exo 0.2 20 Inflexion point: 1172.7 °C [3.1] 0.1 [1.1]18 [4.1]

[2.1] 0.0 16

[8.1] [7.1] -0.1 14 12 -0.2

1000 1050 1100 1150 1200 1250 1300 1350 Temperature /°C

TG/heating section of plain iron in different atmospheres: mass gain in the temperature range 1100 … 1200°C when sintering in N2 ASC 600MPa Ar

DTA /µV ASC 600MPa N2 Peak: 861.0 °C ASC 600MPa vacuum -15 Exo Onset: 881.3 °C -20 -25 -30 -35 Peak: 882.9 °C -40 Peak: 872.9 °C Onset: 895.1 °C -45 Onset: 884°C -50 -55 800 820 840 860 880 900 920 940 Temperature /°C

DTA/cooling section of plain iron in different atmospheres: shifting of austenite-ferrite transformation to lower T when sintering in N2 Gas volumes in the sintering atmosphere External (free) atmosphere: composition adjustable by the furnace operator

(also with regard to O2, H2O content); affected also by the furnace type, design and gas tightness Transport through convection and diffusion Diffusion and Convection External atmosphere Diffusion Internal atmosphere

Internal atmosphere: composition in part defined by reactions between atmosphere and solid body = local equilibria, in particular regarding content of H2O, CO, CO2 Transport through diffusion; convection through nonisothermal effects

Boundary layer at the compact surface: transport through diffusion; convection only by „blowing“ of the pores, e.g. during heating, or „sucking“ during cooling Crucial period for reactions between atmosphere and compact: Heating section

• Continuous change of temperature and thus shifting of equilibria: high temperature favours reduction and decarburization

• Change of gas density in the pores  gas flow from the pores into the free atmosphere

• Most pronounced change of specific surface, which lowers the reactivity; on the other hand faster diffusion processes with increasing temperature

• Dissolution reactions of alloy elements with matrix  lowering of their chemical activity Most important interaction between powder compact and atmosphere: removal of oxides Virtually all metal powders bear oxygen; oxide layers on the surfaces essential in some case for safety reasons (Al powder; Carbonyl Fe)

Oxide layers inhibit formation of stable sintering bridges, the more, the higher the stability of the oxides is

Remedy: Reduction of the oxide layers in the early stages of sintering (heating stage); reduction either by components of the atmosphere

(mostly H2) or by carbon present in the powder compact (sintered steels, hardmetals, ….)

Reduction reactions for metal oxides:

• MeO + H2  Me + H2O reduction with atmospheric constituent • MeO + CO  Me + CO2 indirect carbothermic reduction (at low T) • MeO + C  Me + CO direct carbothermic reduction (at high T) • MeO + Mf  Me + MfO metallothermic reduction Conclusion (recapitulation):

• Thermodynamics of the reduction is important factor in the success story of these alloying elements

• Desoxidation processes  formation of sintering contacts are depending on the behaviour of iron, only little influence by the alloying elements

• Low stability of their oxides  Sintering is possible in almost all atmospheres, that are useful for sintering of carbon steels

• Neutral atmospheres (N2, vacuum) or reducing atmospheres (N2-H2, endogas) with rather modest quality (dew point, Oxygen-content) can be used Disadvantages of Ni, Cu, Mo:

Expensive, massive price push in the last decade (Mo up to 30x) Ni – fine powder hazardous

Alloying elements in wrought steels: Cr, Mn, V, Si

Oxides much more stable than oxides of Iron

Desoxidation of surface oxides not only dependant on the desoxidation of iron, but from alloying elements

Richardson-Ellingham-diagram for reduction with C 19 (nach A.R.Glassner) Demands on furnace atmospheres: Alloying elements Fe-C; Mo, Cu, Ni: can be sintered in any atmospheres (no high demands) Alloying elements Cr, Mn, V: CO is oxidising at the usual temperatures (1120°C, belt furnaces)

Criteria: pCO < pCOeq.

20%CO

Equilibrium partial pressure of CO as function of the temperature for the carbothermal reduction of the oxides in Fe-C and Fe-3%Cr-C (after Danninger) Chromium as alloying element:

Successfully introduced into the market 1998: Astaloy CrM with 3% Cr (+0,5%Mo), prealloyed; subsequently „Astaloy CrL“ (1.5%Cr-0.2%Mo); „Astaloy CrA“ (1.8%Cr) Why prealloyed? Cr can be sintered only at high temperatures and low dew points (if a = 1)

Alloy Fe-3%Cr  aCr = 0.025  purity requirements towards the atmosphere are somewhat lower (Lit. Arvidsson et al.)

Astaloy CrM (SEM) 21 SE of surface of Cr-containing powders (by Hryha et. al)

Surface analysis (XPS): Surface is covered by thin layer of Fe- oxide (app. 6-7 nm) with particles of Cr-oxides  Decisive: Removal of oxides from the surface and from the Oxide layer thickness by XPS of AstCrM (nach Chasoglou et al. Applied Surface Science 268 pressing contacts! (2013) 496– 506) Desoxidation behaviour of Cr-alloyed steels: In neutral Atmospheres:

Redistribution of oxides: Fe-Cr-Spinells are formed by diffusion of Cr to the surface

Fe-Cr-Spinells: easier to reduce that Cr, but still more stable than FexOy

Ionenstrom *10-10 /A

Increasing Cr-content 4.0 Fe-0,5C 3.5

Fe-Oxid: massive reduction 3.0 of the peak area of surface 2.5 Fe-1,5Cr-0,2Mo-0,5C reduction 2.0 Fe-3Cr-0,5Mo-0,5C 1.5 Maximum for the surface 1.0 [12] [14] reduction is shifted to [8] 0.5 200 400 600 800 1000 1200 higher temperatures Temperatur /°C

mass 28 (here CO) 23 sintered in Dilatometer, Hydrogen sintered in Dilatometer, appears at 750°C O and MS for Fe-1.5Cr-0.2Mo-0.5C and Fe-3Cr-0.5Mo-0.5C ; and Fe-3Cr-0.5Mo-0.5C and MS for Fe-1.5Cr-0.2Mo-0.5C 2 4 0 l/l

 : 2 Fe + H  2 Reduction of surface oxides still occurs at low temperatures FeO + H Broad peak of mass 28 (CO) at high temperatures (internal oxides) Small peak of CH Fe-xCr-0.5C in H • • • • Mangan as alloying element: Motivation: strong effect on the hardenability and low prize Introduction into PM was not successful although intense research (e.g. Šalak)

Major Problem: Mn-oxide is even more stable than Cr-oxide  reduction is more difficult

1st attempt: Mixing of Mn oder Mn-master- alloys: Strong hardening effect of Mn on the ferrite  fear of low compressibility Special feature of Mn: High vapour pressure  strong mobilisation via gas phase • Self-getter effect: makes sintering possible • Demanganizing of surfaces; swelling effects • Risk: furnace lining is attacked The self cleaning effect of Mn (after Šalak) 25 New attempt: prealloyed powder Compressibility = OK Demanganizing of the surfaces = OK  Reduction of Mn-activity is very effective

Degassing behaviour: Similar to Cr-steels

Increasing Mn-content:

Reduction: Lower mass loss between 600 und 700°C

Mass change on Mn-alloys ion powders with increasing Mn-content during heating 26 Shift to lower temperatures of the first CO-Peak is dependent on the Mn- content

Peak temperature as function of the Mn-content

Mass signal (28, here CO) of Mn-containing iron 27 powders Slight oxidation of the material at the higher alloyed variants  importance of the heating rate for sensitive systems against oxidation

Mass change of Mn-containing iron powders with increasing Mn-content 28 Dissolution of Carbon allows reduction of internal Oxides Peaks become broader by Mn Complete reduction is shifted to much higher temperatures

Mass signal 28 (here CO) of Mn-containing powders

29 Master alloys as alloy carrier: + compressibility of base powder - Allloying elements with high oxygen affinity in direct contact with Fe-powder

Alloys made from powder mixtures with oxygen sensitive elements: Oxygen will be transferred during the heating stage from Fe-oxides to the more sensitive elements Chromium and Manganese.

dL/Lo *10-3 Ionenstrom *10-9 /A ARGON:

[1.1] 1.000 High Mn-alloyed masteralloy 10 0.900 [7] 0.800 powder (MA) acts like internal 8 0.700 getter.

6 0.600 Oxygen content of the MA Dl/l 0.500 4 0.400 changes and implements a m28 2 0.300 part of the oxygen of the iron 0.200 powder 0 0.100 200 400 600 800 1000 1200 Temperatur /°C Lower peak at 630°C and bigger at 1100-1200°C Fe-4%MA in Argon 30 Avoiding the getter effect:

Hydrogen: dL/Lo *10-3 Ionenstrom *10-9 /A Transfer is less effective as [1.1]

8 4.00 reduction is performed at

7 3.50 Dl/l lower temperatures (Fe- 6 m18 3.00 oxides are reduced, MA is not m28 [7] 5 2.50 oxidised) 4 2.00

3 1.50 m16 2 1.00 Side reaction: m15

1 [6] 0.50 [4] [10] Mass 16 + Mass 15  0 0 200 400 600 800 1000 1200 Temperatur /°C Methane (CH4)

Fe-4%MA in H2

31 Formation of stable sintering bridges in the powder compact:

Oxides are present as islands  not particularly detrimental for contact formation  inclusions in the sintering contact  internal oxides = defects in the contact Reduction at high temperatures always with carbon, of the iron oxide layers at lower temperatures also with hydrogen

Complex oxides of oxygen sensitive Homogenous layer of Fe-oxides elements Fe-3Cr-0.5Mo; 30 min 1120°C (6-7 nm) (after Chasoglou et. al.)

32 Consequences: • Reduction of iron oxides (oxides of Ni, Cu, Mo) is performed in part by H2  minimization of carbon loss • Location of Oxides becomes important: oxide (surface) / oxide (internal) • Maximum performance of the alloy (esp. for dynamic applications) only possible after reduction of the internal oxides • Potential decarburizing side reactions must be considered. Methane formation! (usually not relevant during sintering; decarbuzing reactions usually caused by water vapour)

33 Possible reactions leading to Methane:

Methane-reaction: C + 2H2 =CH4 Sabatier-reaction: CO +4H =CH +2HO 2 2 4 2 until 620°C G >0 Methanisation: CO + 3H2 =CH4 +H2O 2CO + 2H2 =CH4 +CO2

Gibb free Enthalpie (G) for methane formation 34 Reactions to methane formation including metal oxides:

Mn7C3 + 20H2 +7CO=10CH4 +7MnO Mn + 2H2 +CO=CH4 +MnO SiC+4H2+CO = 2CH4 +SiO Si+CO+2H2 =SiO+CH4 SiC+6H2+2CO = 3CH4 +SiO2 Si+2CO+4H2 =SiO2 +2CH4

500 SiC + 4H2(g) + CO(g) = 2CH4(g) + SiO(g) Si + CO(g) + 2H2(g) = CH4(g) + SiO(g) SiC + 2CO(g) + 6H2(g) = 3CH4(g) + SiO2 Si + 2CO(g) + 4H2(g) = 2CH4(g) + SiO2

0 G(kJ) 

Manganese Silicon -500 400 600 800 1000 1200 T(°C)

Gibb free Enthalpie (G) for methane formation 35 Mechanical properties: Atmos- density Hardness sample phere g/cm³ HV30 H 7,15 146 Fe-4MA-0,8C 2 Ar 7,06 162

Decarburizing reaction through methane formation is confirmed by mechanical properties as well as my microstructure Hydrogen Argon

Fe-4MA-0,8C, sintered in Dilatometer 1300°C, 60 min, 10K/min 36 Consequences: If Manganese and Silicon is used further facts have to be considered Dilemma: • Neutral atmospheres: internal getter effect, as degassing is happening at rather high temperatures • Reducing atmospheres: undesired carbon losses through methane formation, combined with oxidation of alloying elements

Way out: fast heating trough the critic temperature range  minimizing of the reaction; using N2-H2 mixes which lowers the reaction rate of methane formation

Deduction: PM-steel, alloyed with effective and cheap elements can be produced in a commercial manner, if the chemistry is controlled during the sintering process!

37 Conclusion: • Reduction of the surface oxides is essential for formation of sintering bridges • Reduction can be performed by the atmosphere or by added components (C) • Reduction of surface oxides is strongly dependant on the thermodynamical behaviour of the alloying elements • FeO is rather easy to reduce • Oxides of Ni, Mo, Co are even easier to reduce than FeO  reduction of FeO is decisive • Cr, Mn, Si form more stable oxides  reduction of the surface oxides is possible only at higher temperatures • Reducing atmospheres during the heating stage are important to remove as much surface oxides as possible  if not stable oxides are formed and are introduced into sintering bridges • Reducing agent for Cr, Mn u. Si-Oxides is always C  C loss is inherent (compensation possible?) • Ratio surface oxides / internal oxides becomes important  control of the final C-content Thank you for listening