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Independently Controlled Carbon and Nitrogen Potential: A New Approach to Carbonitriding Process

  • Karl-Michael Winter
Article

Abstract

Recent research projects show that retained austenite, if stabilized by nitrogen, has a positive influence on the fatigue strength of work pieces. The combined diffusion profile of carbon and nitrogen applied in a carbonitriding process plays a major role, besides the process temperature. Yet today, only the carbon potential is somehow controlled and even this is not easy to achieve. This paper will present a new system able to measure and control both the carbon potential and the nitrogen potential independently. The knowledge of the activities of nitrogen and carbon in iron and the effect of alloying elements on such activities as well as the solubilities offers a way to apply the potentials on real steels.

Keywords

aerospace automotive carbon/alloy steels heat treating modeling processes surface engineering 

Nomenclature

[C], [N]

Carbon atom and nitrogen atom adsorbed on the metal surface

lg(x)

Logarithm to the base of 10 of x

pCO, pH2, …

Partial pressures in bar

pYXX

Partial pressure of XX in bar powered by Y

aCatm

Carbon activity of process atmosphere, based on graphite

aCch4aCatm

Out of methane reaction

aCBoudaCatm

Out of Boudouard reaction

aChetaCatm

Out of heterogeneous water gas reaction

aCFe

Carbon activity in iron

aNatm

Nitrogen activity of process atmosphere, based on 1 bar nitrogen

aNFe

Nitrogen activity in iron

T

Temperature in Kelvin

JC

Carbon flux between process gas and part’s surface, typically in g/(cm2 s)

JN

Nitrogen flux between process gas and part’s surface, typically in g/(cm2 s)

kch4

Carbon transfer coefficient of methane reaction in cm/s

kBoud

Carbon transfer coefficient of Boudouard reaction in cm/s

khet

Carbon transfer coefficient of heterogeneous water gas reaction in cm/s

βC

Carbon transfer coefficient based on equilibrium conditions of Boudouard and het. water gas reactions in cm/s

βN

Nitrogen transfer coefficient of ammonia dissociation in cm/s

CPatm

Atmosphere carbon potential in mass% C, as if derived by the absence of a nitriding effect, based on gamma iron

NPatm

Atmosphere nitrogen potential in mass% N, as if derived by the absence of a carburizing effect, based on gamma iron

CP = cCequ

Equilibrium carbon content in mass% C in gamma iron, if exposed to a carbonitriding atmosphere

NP = cNequ

Equilibrium nitrogen content in mass% N in gamma iron, if exposed to a carbonitriding atmosphere

KN

Nitriding potential in bar−0.5 as known from nitriding and nitrocarburizing processes

cCgraphite

Maximum solubility of carbon in gamma iron toward cementite formation, also representing the maximum solubility of carbon in the process gas toward sooting. Parameter given in mass% C

cCmax

Maximum solubility of carbon in gamma iron toward cementite formation in the presence of interstitially solved nitrogen, given in mass% C

Fe2-3[N+C]

Epsilon iron carbonitride

cNeps

Maximum solubility of nitrogen in gamma iron toward epsilon formation, given in mass% N

cNmax

Maximum solubility of nitrogen in gamma iron toward epsilon formation in the presence of carbon, given in mass% N

kj

Alloying factor for interstitial j

kC

Alloying factor for carbon

kN

Alloying factor for nitrogen

cj-Fe

Equilibrium solubility of interstitial j in gamma iron

cC-Fe

Equilibrium solubility of interstitial j in gamma iron in mass% C

cN-Fe

Equilibrium solubility of interstitial j in gamma iron in mass% N

cj-Steel

Equilibrium solubility of interstitial j in steel

cC-Steel

Equilibrium solubility of carbon in steel in mass% C

cN-Steel

Equilibrium solubility of nitrogen in steel in mass% N

ci

Mass% of element i in steel composition

eji

Parameter of element i in austenitic iron on Henry’s activity coefficient for interstitial j, element concentration given in mass%

eCi

Parameter of element i in austenitic iron on Henry’s activity coefficient for carbon, element concentration given in mass%

eNi

Parameter of element i in austenitic iron on Henry’s activity coefficient for nitrogen, element concentration given in mass%

cCmax-Steel

Maximum solubility of carbon in steel toward carbide formation in mass% C

cNmax-Steel

Maximum solubility of nitrogen in steel toward epsilon formation in mass% N

pCOatm, pH2-atm

Partial pressures of carbon monoxide and hydrogen in the process atmosphere in bar

vol.% COcarrier

Carbon monoxide percentage in carburizing carrier gas, typically 20% if using endogas made from natural gas and air

vol.% H2-carrier

Hydrogen percentage in carburizing carrier gas, typically 40% if using endogas made from natural gas and air

NH3-add

Ammonia added to carrier gas flow in vol.%; note that the total flow will be 100% plus NH3-add

ΔRC

Delta in electrical resistivity of iron caused by interstitially placed carbon

ΔRN

Delta in electrical resistivity of iron caused by interstitially placed nitrogen

DC

Carbon diffusion coefficient in austenite in cm2/s

DN

Nitrogen diffusion coefficient in austenite in cm2/s

Notes

Acknowledgments

The author would like to thank the IWT team in Bremen for giving his company the opportunity of testing and modifying the control equipment to their needs and letting him participate in their research.

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Copyright information

© ASM International 2013

Authors and Affiliations

  1. 1.PROCESS-ELECTRONIC GmbH, a Member of United Process ControlsHeiningenGermany

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