The development of the compound layer during gaseous nitriding and nitrocarburizing of Fe-based m... more The development of the compound layer during gaseous nitriding and nitrocarburizing of Fe-based material is described. The first nucleation of the compound layer at the surface depends on the competition between the dissociation of ammonia and the removal nitrogen from the surface by solid state diffusion and desorption or the competition with a carburizing reaction. During compound layer growth the nitriding kinetics are determined by a combination of solid state diffusion of interstitial elements and the kinetics of the surface reactions. Moreover, phase transformations within the compound layer play a role in determining the overall layer growth kinetics during nitrocarburizing.
Edited by leading authorities in the field, Thermochemical Surface Engineering of Steels provides... more Edited by leading authorities in the field, Thermochemical Surface Engineering of Steels provides a comprehensive scientific overview of the principles and different techniques involved in thermochemical surface engineering, including thermodynamics, kinetics principles, process technologies and techniques for enhanced performance of steels A Volume in the Woodhead Publishing Series in Metals and Surface Engineering . KEY FEATURES Reviews the fundamentals of surface treatments and current performance of improved materials Covers nitriding, nitrocarburizing and carburizing of iron and iron carbon alloys Examines how different thermochemical surface engineering methods can help against corrosion DESCRIPTION Thermochemical surface engineering significantly improves the properties of steels. Edited by two of the world's leading authorities, this important book summarises the range of techniques and their applications. It covers nitriding, nitrocarburizing and carburizing. There are also chapters on low temperature techniques as well as boriding, sheradizing, aluminizing, chromizing, thermoreactive deposition and diffusion. 10 Classical nitriding of heat treatable steel L. Barrallier, Arts et Métiers ParisTech, France 11 Plasma-assisted nitriding and nitrocarburizing of steel and other ferrous alloys E. Rolinski, Advanced Heat Treat Corp., USA 12 ZeroFlow gas nitriding of steels 16 Plasma-assisted processes for surface hardening of stainless steel J. P. Lebrun, Lebrun-Consulting, France 17 Low temperature surface hardening of stainless steels J. P. Lebrun, Lebrun-Consulting, France Part V Dedicated thermochemical surface engineering methods 18 Boriding to improve the mechanical properties and corrosion resistance of steels
ABSTRACT Laboratory testing on selected alumina and silica-forming alloys was performed to evalua... more ABSTRACT Laboratory testing on selected alumina and silica-forming alloys was performed to evaluate their performance against high temperature corrosion induced by potassium chloride (KCl). The alloys studied were FeCrAlY, Kanthal APM, Nimonic 80A, 214, 153MA and HR160. Exposure was conducted at 600 °C for 168 h in flowing N2(g)+5%O2(g)+15%H2O(g) (vol.%) with samples covered under KCl powder. A KCl-free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was observed that alloying with aluminum did not lead to the formation of protective alumina for the studied alloys. The silicon containing stainless steel 153MA showed an analogous performance to low-silicon austenitic stainless steels of similar chromium and nickel contents. For alloy HR160, a potassium-chromium-silicon-oxygen containing layer forms as the innermost corrosion product. The layer was uniformly distributed over the surface and appears to render some protection as this alloy exhibited the best performance among the investigated alloys. To reveal further aspects of the corrosion mechanism, Nimonic 80A was exposed in static laboratory air for the same duration and temperature with either KCl or K2CO3 deposits. Comparison of results obtained with these experiments showed that both potassium and chlorine can play a role in material degradation by KCl.
Ni-based alloys with high Cr contents are not only known for their excellent high temperature and... more Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650°C or 700°C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular a-Cr and d (Ni 3 Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable c¢¢ (Ni 3 Nb). A co-dependent growth morphology was found, where the preferred growth direction of c¢¢, the {001} planes of c-Ni, caused precipitates of both a-Cr and d to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100°C) followed by aging at 973 K (700°C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb.
Complementary microscopic and diffraction based methods revealed a peculiar microstructure of ele... more Complementary microscopic and diffraction based methods revealed a peculiar microstructure of electrodeposited nickel. For the as-deposited layer, thus, without any additional treatment, multiple twinning yields a high population of R3 n boundaries, which interrupts the network of normal high angle grain boundaries. A peculiar arrangement of R3 boundaries forming five-fold junctions is observed. The resulting microstructure meets the requirements for grain boundary engineering. Twinning induced effects on the crystallographic orientation of grains result in one major texture component being a h210i fiber axis and additional minor orientations originating from first and second generation twins of h210i, i.e., h542i and h20 2 1i. V C 2013 AIP Publishing LLC.
A 1.5%Cr, 1%C bearing steel was sub-zero Celsius treated after quenching. Transmission and reflec... more A 1.5%Cr, 1%C bearing steel was sub-zero Celsius treated after quenching. Transmission and reflection (synchrotron) X-Ray Diffraction were applied ex-situ at the HZB-BESSY II synchrotron facility to quantify the phase fractions of martensite and austenite and determine the stress state in austenite.
The austenite-to-martensite transformation at temperatures below room temperature was investigate... more The austenite-to-martensite transformation at temperatures below room temperature was investigated in situ by magnetometry in Fe-N, Fe-Cr-C and Fe-Cr-Ni based alloys. After quenching to room temperature, samples were immersed in boiling nitrogen and martensite formation was followed during subsequent heating to room temperature. Different tests were performed with heating rates ranging from 0.5 K/min to 10 K/min. An additional test consisted in fast heating the samples in water to verify whether martensite formation can be suppressed. Thermally activated formation of martensite during heating was convincingly demonstrated for all investigated materials by a heating rate dependent transformation kinetics. Moreover, magnetometry showed that the heating rate has an influence on the fraction of martensite formed during the thermal treatment. The activation energy for thermally activated martensite formation as quantified by a Kissinger-like method lies in the range 11‒18 kJ/mol and incr...
The present paper addresses the influence of chemical induced stresses on diffusion in interstiti... more The present paper addresses the influence of chemical induced stresses on diffusion in interstitial systems. This is exemplified by simulations of carbon diffusion in austenite at high temperatures and it is shown that old well established literature data is flawed by the occurrence of composition induced stress. For the technological relevant system of expanded austenite the diffusion can be dramatically affected by composition induced stress.
International Heat Treatment and Surface Engineering, 2012
The development of the compound layer during gaseous nitriding and nitrocarburising of Fe based m... more The development of the compound layer during gaseous nitriding and nitrocarburising of Fe based material is described. The first nucleation of the compound layer at the surface depends on the competition between the dissociation of ammonia and the removal of nitrogen from the surface by solid state diffusion and desorption or the competition with a carburising reaction. During compound layer growth, the nitriding kinetics is determined by a combination of solid state diffusion of interstitial elements and the kinetics of the surface reactions. Moreover, phase transformations within the compound layer play a role in determining the overall layer growth kinetics during nitrocarburising.
ABSTRACT Surface layers of expanded austenite resulting from nitriding typically exhibit large gr... more ABSTRACT Surface layers of expanded austenite resulting from nitriding typically exhibit large gradients in residual stress and composition. Evaluation of residual-stress profiles is explored by means of grazing incidence X-ray diffraction (GI-XRD), probing shallow depths, combined with successive layer removal. Several factors complicating the stress determination are analysed and discussed: (1) ghost stresses arising from a small variation in the shallow information depths probed with GI-XRD, (2) selection of the grain interaction model used to calculate the X-ray elastic constants for conversion of lattice strains into residual stress and (3) the composition dependence of these elastic constants.
According to classical theories precipitate interfaces are described by their degree of coherency... more According to classical theories precipitate interfaces are described by their degree of coherency with the matrix, which affects their strengthening contribution. Investigations of nitride precipitate interfaces in 12% Cr steels with transmission electron microscopy have shown the nitrides to be enveloped in an amorphous shell a few nm thick, thus leaving them without any coherency with the matrix. The amorphous nature of the shells could be ascertained with high resolution microscopy and dark field techniques. When extracted from the ferrite matrix the amorphous shells were observed to crystallize during electron beam exposure. The amorphous shells were observed around Ta-and Nb-based nitrides, which are considered to have a high interfacial energy with the ferrite matrix. They were not observed around V-based nitrides which have a Baker-Nutting relationship with low-misfit to the matrix.
ABSTRACT This article addresses an investigation of the influence of plastic deformation on low-t... more ABSTRACT This article addresses an investigation of the influence of plastic deformation on low-temperature surface hardening by gaseous nitriding of two commercial stainless steels: EN 1.4369 and AISI 304. The materials were plastically deformed to several levels of equivalent strain by conventional tensile straining, plane strain compression, and shear. Gaseous nitriding of the strained material was performed in ammonia gas at atmospheric pressure at various temperatures. Microstructural characterization of the as-deformed state and the nitrided case produced included X-ray diffraction analysis, reflected-light microscopy, and microhardness testing. The results demonstrate that a case of expanded austenite develops and that the presence of plastic deformation has a significant influence on the morphology of the nitrided case. The presence of strain-induced martensite favors the formation of CrN, while a high dislocation density in a fully austenitic structure does not lead to such premature nucleation of CrN.
The present work deals with the evaluation of the residual-stress profile in expandedaustenite by... more The present work deals with the evaluation of the residual-stress profile in expandedaustenite by successive removal steps using GI-XRD. Preliminary results indicate stresses of several GPa's from 111 and 200 diffraction lines. These stresses appear largest for the 200 reflection. The strain-free lattice parameter decayed smoothly with depth, while for the compressive stress a maximum value is observed at some depth below the surface. Additionally a good agreement was found between the nitrogen profile determined with GDOES analysis and the strain-free lattice parameter from XRD.
A steel containing 16 wt.% Cr, 5 wt.% Ni and 3 wt.% Cu was transformed into martensite by applyin... more A steel containing 16 wt.% Cr, 5 wt.% Ni and 3 wt.% Cu was transformed into martensite by applying isochronal, i.e. constant rate, cooling followed by isothermal holding. The formation of martensite was monitored with dilatometry. A series of retardations and accelerations of the transformation was observed during isochronal cooling for cooling rates ranging from 1.5 to 50 K min À1 . The cooling rate in the isochronal stage was observed to influence the transformation rate in the isothermal stage. Electron backscatter diffraction was applied to determine the morphology of the martensite, which was of lath type, and to investigate the microstructure of the material. No influence of the cooling rate on the scale of the microstructure was observed. The series of retardations and accelerations of the transformation is interpreted in terms of the combined effect of the strain and interfacial energy introduced in the system during martensite formation, which stabilizes austenite, and autocatalytic nucleation of martensite.
The transformation of austenite to martensite in an Al-Mn-Si transformation-induced plasticity st... more The transformation of austenite to martensite in an Al-Mn-Si transformation-induced plasticity steel was investigated with in situ electron backscatter diffraction (EBSD) measurements under tensile straining. The visualisation of the microstructure upon straining allows for an investigation of the stability of austenite grains against strain-induced transformation, with particular focus on the grain size and the location of the austenite grains. The findings confirm that size and location of austenite grains are significant parameters for their stability. Small austenite grains were observed to be more stable than large grains, while austenite grains located beside bainitic ferrite are the most stable. Moreover, it is demonstrated that austenite grains transform gradually.
The development of the compound layer during gaseous nitriding and nitrocarburizing of Fe-based m... more The development of the compound layer during gaseous nitriding and nitrocarburizing of Fe-based material is described. The first nucleation of the compound layer at the surface depends on the competition between the dissociation of ammonia and the removal nitrogen from the surface by solid state diffusion and desorption or the competition with a carburizing reaction. During compound layer growth the nitriding kinetics are determined by a combination of solid state diffusion of interstitial elements and the kinetics of the surface reactions. Moreover, phase transformations within the compound layer play a role in determining the overall layer growth kinetics during nitrocarburizing.
Edited by leading authorities in the field, Thermochemical Surface Engineering of Steels provides... more Edited by leading authorities in the field, Thermochemical Surface Engineering of Steels provides a comprehensive scientific overview of the principles and different techniques involved in thermochemical surface engineering, including thermodynamics, kinetics principles, process technologies and techniques for enhanced performance of steels A Volume in the Woodhead Publishing Series in Metals and Surface Engineering . KEY FEATURES Reviews the fundamentals of surface treatments and current performance of improved materials Covers nitriding, nitrocarburizing and carburizing of iron and iron carbon alloys Examines how different thermochemical surface engineering methods can help against corrosion DESCRIPTION Thermochemical surface engineering significantly improves the properties of steels. Edited by two of the world's leading authorities, this important book summarises the range of techniques and their applications. It covers nitriding, nitrocarburizing and carburizing. There are also chapters on low temperature techniques as well as boriding, sheradizing, aluminizing, chromizing, thermoreactive deposition and diffusion. 10 Classical nitriding of heat treatable steel L. Barrallier, Arts et Métiers ParisTech, France 11 Plasma-assisted nitriding and nitrocarburizing of steel and other ferrous alloys E. Rolinski, Advanced Heat Treat Corp., USA 12 ZeroFlow gas nitriding of steels 16 Plasma-assisted processes for surface hardening of stainless steel J. P. Lebrun, Lebrun-Consulting, France 17 Low temperature surface hardening of stainless steels J. P. Lebrun, Lebrun-Consulting, France Part V Dedicated thermochemical surface engineering methods 18 Boriding to improve the mechanical properties and corrosion resistance of steels
ABSTRACT Laboratory testing on selected alumina and silica-forming alloys was performed to evalua... more ABSTRACT Laboratory testing on selected alumina and silica-forming alloys was performed to evaluate their performance against high temperature corrosion induced by potassium chloride (KCl). The alloys studied were FeCrAlY, Kanthal APM, Nimonic 80A, 214, 153MA and HR160. Exposure was conducted at 600 °C for 168 h in flowing N2(g)+5%O2(g)+15%H2O(g) (vol.%) with samples covered under KCl powder. A KCl-free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was observed that alloying with aluminum did not lead to the formation of protective alumina for the studied alloys. The silicon containing stainless steel 153MA showed an analogous performance to low-silicon austenitic stainless steels of similar chromium and nickel contents. For alloy HR160, a potassium-chromium-silicon-oxygen containing layer forms as the innermost corrosion product. The layer was uniformly distributed over the surface and appears to render some protection as this alloy exhibited the best performance among the investigated alloys. To reveal further aspects of the corrosion mechanism, Nimonic 80A was exposed in static laboratory air for the same duration and temperature with either KCl or K2CO3 deposits. Comparison of results obtained with these experiments showed that both potassium and chlorine can play a role in material degradation by KCl.
Ni-based alloys with high Cr contents are not only known for their excellent high temperature and... more Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650°C or 700°C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular a-Cr and d (Ni 3 Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable c¢¢ (Ni 3 Nb). A co-dependent growth morphology was found, where the preferred growth direction of c¢¢, the {001} planes of c-Ni, caused precipitates of both a-Cr and d to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100°C) followed by aging at 973 K (700°C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb.
Complementary microscopic and diffraction based methods revealed a peculiar microstructure of ele... more Complementary microscopic and diffraction based methods revealed a peculiar microstructure of electrodeposited nickel. For the as-deposited layer, thus, without any additional treatment, multiple twinning yields a high population of R3 n boundaries, which interrupts the network of normal high angle grain boundaries. A peculiar arrangement of R3 boundaries forming five-fold junctions is observed. The resulting microstructure meets the requirements for grain boundary engineering. Twinning induced effects on the crystallographic orientation of grains result in one major texture component being a h210i fiber axis and additional minor orientations originating from first and second generation twins of h210i, i.e., h542i and h20 2 1i. V C 2013 AIP Publishing LLC.
A 1.5%Cr, 1%C bearing steel was sub-zero Celsius treated after quenching. Transmission and reflec... more A 1.5%Cr, 1%C bearing steel was sub-zero Celsius treated after quenching. Transmission and reflection (synchrotron) X-Ray Diffraction were applied ex-situ at the HZB-BESSY II synchrotron facility to quantify the phase fractions of martensite and austenite and determine the stress state in austenite.
The austenite-to-martensite transformation at temperatures below room temperature was investigate... more The austenite-to-martensite transformation at temperatures below room temperature was investigated in situ by magnetometry in Fe-N, Fe-Cr-C and Fe-Cr-Ni based alloys. After quenching to room temperature, samples were immersed in boiling nitrogen and martensite formation was followed during subsequent heating to room temperature. Different tests were performed with heating rates ranging from 0.5 K/min to 10 K/min. An additional test consisted in fast heating the samples in water to verify whether martensite formation can be suppressed. Thermally activated formation of martensite during heating was convincingly demonstrated for all investigated materials by a heating rate dependent transformation kinetics. Moreover, magnetometry showed that the heating rate has an influence on the fraction of martensite formed during the thermal treatment. The activation energy for thermally activated martensite formation as quantified by a Kissinger-like method lies in the range 11‒18 kJ/mol and incr...
The present paper addresses the influence of chemical induced stresses on diffusion in interstiti... more The present paper addresses the influence of chemical induced stresses on diffusion in interstitial systems. This is exemplified by simulations of carbon diffusion in austenite at high temperatures and it is shown that old well established literature data is flawed by the occurrence of composition induced stress. For the technological relevant system of expanded austenite the diffusion can be dramatically affected by composition induced stress.
International Heat Treatment and Surface Engineering, 2012
The development of the compound layer during gaseous nitriding and nitrocarburising of Fe based m... more The development of the compound layer during gaseous nitriding and nitrocarburising of Fe based material is described. The first nucleation of the compound layer at the surface depends on the competition between the dissociation of ammonia and the removal of nitrogen from the surface by solid state diffusion and desorption or the competition with a carburising reaction. During compound layer growth, the nitriding kinetics is determined by a combination of solid state diffusion of interstitial elements and the kinetics of the surface reactions. Moreover, phase transformations within the compound layer play a role in determining the overall layer growth kinetics during nitrocarburising.
ABSTRACT Surface layers of expanded austenite resulting from nitriding typically exhibit large gr... more ABSTRACT Surface layers of expanded austenite resulting from nitriding typically exhibit large gradients in residual stress and composition. Evaluation of residual-stress profiles is explored by means of grazing incidence X-ray diffraction (GI-XRD), probing shallow depths, combined with successive layer removal. Several factors complicating the stress determination are analysed and discussed: (1) ghost stresses arising from a small variation in the shallow information depths probed with GI-XRD, (2) selection of the grain interaction model used to calculate the X-ray elastic constants for conversion of lattice strains into residual stress and (3) the composition dependence of these elastic constants.
According to classical theories precipitate interfaces are described by their degree of coherency... more According to classical theories precipitate interfaces are described by their degree of coherency with the matrix, which affects their strengthening contribution. Investigations of nitride precipitate interfaces in 12% Cr steels with transmission electron microscopy have shown the nitrides to be enveloped in an amorphous shell a few nm thick, thus leaving them without any coherency with the matrix. The amorphous nature of the shells could be ascertained with high resolution microscopy and dark field techniques. When extracted from the ferrite matrix the amorphous shells were observed to crystallize during electron beam exposure. The amorphous shells were observed around Ta-and Nb-based nitrides, which are considered to have a high interfacial energy with the ferrite matrix. They were not observed around V-based nitrides which have a Baker-Nutting relationship with low-misfit to the matrix.
ABSTRACT This article addresses an investigation of the influence of plastic deformation on low-t... more ABSTRACT This article addresses an investigation of the influence of plastic deformation on low-temperature surface hardening by gaseous nitriding of two commercial stainless steels: EN 1.4369 and AISI 304. The materials were plastically deformed to several levels of equivalent strain by conventional tensile straining, plane strain compression, and shear. Gaseous nitriding of the strained material was performed in ammonia gas at atmospheric pressure at various temperatures. Microstructural characterization of the as-deformed state and the nitrided case produced included X-ray diffraction analysis, reflected-light microscopy, and microhardness testing. The results demonstrate that a case of expanded austenite develops and that the presence of plastic deformation has a significant influence on the morphology of the nitrided case. The presence of strain-induced martensite favors the formation of CrN, while a high dislocation density in a fully austenitic structure does not lead to such premature nucleation of CrN.
The present work deals with the evaluation of the residual-stress profile in expandedaustenite by... more The present work deals with the evaluation of the residual-stress profile in expandedaustenite by successive removal steps using GI-XRD. Preliminary results indicate stresses of several GPa's from 111 and 200 diffraction lines. These stresses appear largest for the 200 reflection. The strain-free lattice parameter decayed smoothly with depth, while for the compressive stress a maximum value is observed at some depth below the surface. Additionally a good agreement was found between the nitrogen profile determined with GDOES analysis and the strain-free lattice parameter from XRD.
A steel containing 16 wt.% Cr, 5 wt.% Ni and 3 wt.% Cu was transformed into martensite by applyin... more A steel containing 16 wt.% Cr, 5 wt.% Ni and 3 wt.% Cu was transformed into martensite by applying isochronal, i.e. constant rate, cooling followed by isothermal holding. The formation of martensite was monitored with dilatometry. A series of retardations and accelerations of the transformation was observed during isochronal cooling for cooling rates ranging from 1.5 to 50 K min À1 . The cooling rate in the isochronal stage was observed to influence the transformation rate in the isothermal stage. Electron backscatter diffraction was applied to determine the morphology of the martensite, which was of lath type, and to investigate the microstructure of the material. No influence of the cooling rate on the scale of the microstructure was observed. The series of retardations and accelerations of the transformation is interpreted in terms of the combined effect of the strain and interfacial energy introduced in the system during martensite formation, which stabilizes austenite, and autocatalytic nucleation of martensite.
The transformation of austenite to martensite in an Al-Mn-Si transformation-induced plasticity st... more The transformation of austenite to martensite in an Al-Mn-Si transformation-induced plasticity steel was investigated with in situ electron backscatter diffraction (EBSD) measurements under tensile straining. The visualisation of the microstructure upon straining allows for an investigation of the stability of austenite grains against strain-induced transformation, with particular focus on the grain size and the location of the austenite grains. The findings confirm that size and location of austenite grains are significant parameters for their stability. Small austenite grains were observed to be more stable than large grains, while austenite grains located beside bainitic ferrite are the most stable. Moreover, it is demonstrated that austenite grains transform gradually.
Uploads
Papers by Marcel Somers