Low-temperature Carburization Research Articles

Increased hardness value due to the diffusion of low-temperature carburizing process

This research to determine the effect of the tensile load on the carburizing process and the heating temperature lower than the normal carburizing temperature on the hardness of carbon steel with a percentage of 80% buffalo bone charcoal and 20% BaCO3, 20 mesh grain size, 4 hours holding time, and slow cooling with a tensile load of 1/4σp 1/2σp 3/4σp, and σp (proportional stress). The heating temperatures below normal carburizing temperatures, namely 6000C, 6500C, 7000C, and 7500C while being pulled by 1/4σp, 1/2σp, 3/4σp, and σp. After reaching the heating temperature, the material is held in the furnace for 4 hours and cooled slowly. After the material is cold, mechanical testing is carried out with Vickers microhardness. Hardness value at a temperature of 6000C is 103.93 HRB, at a temperature of 6500C is 104.33 HRB, at a temperature of 7000C is 104.80 HRB, and at a temperature of 7500C is 106.60 HRB, while the process of pack carburizing without tensile at a heating temperature of 8000C is 105.2 HRB. This proves that the application of loads at lower heating temperatures during the heating process can exceed the hardness value without tensile loads at higher temperatures of 8000 C.

Surface characteristics and tribological behavior of 3D-printed 316 L steel after plasma assisted low temperature carburizing

Conventional carburizing treatments of steels are usually carried out above 550 °C thus they are not suitable for austenitic stainless steels because surface hardening is achieved to detriment of corrosion resistance. To overcome this drawback different carburizing treatments at lower temperature have been developed and among them one of the most efficient is a plasma assisted process at 475 °C. With respect to austenitic stainless steels produced through traditional processes additive manufacturing allows to obtain mechanical parts of complex shape with enhanced strength and without loss of ductility. However, for certain applications the components still suffer of limited hardness and wear resistance. In this work the plasma assisted process at 475 °C has been employed to treat the 316 L steel manufactured by Laser Powder Bed Fusion (L-PBF) with the aim of improving its tribological behavior. The treatment time was 7 h and gas mixtures (CH4 + H2) with different amounts of CH4 have been used. The samples were then submitted to wear tests in pin-on-flat (POF) mode at room temperature with applied loads of 10 N and 20 N. The surface characteristics were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy (RS), glow discharge optical emission spectroscopy (Rf-GDOES), and X-ray photoelectron spectroscopy (XPS). The results show that plasma treatments induce the formation of a ~ 25 μm thick layer of expanded austenite (S-phase) that is covered by a diamond like carbon (DLC) over-layer of ~2 μm. Such over-layer exhibits a complex sub-structure: the inner part consists of amorphous C (sp2 bonds) with a degree of topological disorder depending on the gas mixture whereas in the external part both sp2 (graphite-like) and sp3 (diamond-like) bonds are present with relative amounts changing with the distance from the surface. Independently on the gas mixture used in plasma treatment, all the samples exhibit a wear resistance much better than that of the untreated material. Moreover, wear resistance also depends on plasma treatment conditions: the greater the CH4 content, the lower the wear resistance. The results have been discussed by considering the intrinsic DLC brittleness that fractures and detaches from the metal surface during the tests thus hard debris take part to the wear process acting as abrasive particles.

Enhanced carbon diffusion efficiency and work hardening effect of AISI 9310 steel via pre-laser shock peening

By using pre-laser shock peening (pre-LSP), low temperature carburizing (LC) is successfully applied in AISI 9310 steel to obtain a carburized layer that is approximately 34 μm thickness, which is thicker than the carburized layer achieved by LC alone of about 10 μm. Pre-LSP enhances diffusion mechanism including two factors: structural and kinetic. Structurally, LSP induces density dislocation such as dislocation walls (DWs) that evolve into grain boundaries (GBs), thereby providing paths for accelerating carbon atom diffusion. This transformation results in an increase in Kernel average misorientation (KAM), high angle GBs (HAGBs), and deformed grains. Kinetically, LSP lowers activation energy, making it easier for carbon to diffuse in BCC ferrite. Furthermore, the LSP-LCed sample exhibits a work hardening depth of 400 μm, which is significantly higher than the 60 μm work hardening depth of LCed sample. The main composition of carburized layer is hard Fe3C phase, which exists mostly along α-Fe GBs. The compound work hardening effect observed in the LSP-LCed sample is attributed to plastic deformation induced by pre-LSP and the formation of hard Fe3C phase via LC. This study presents a new method for strengthening contact component with interfacial damage requirements, such as wear and contact fatigue resistance.

Effect of frictional treatment and low temperature plasma carburizing on the microhardness and electromagnetic characteristics of metastable austenitic steel

Microhardness and electromagnetic characteristics of corrosion-resistant chromium-nickel (wt. %: 16.80 Cr; 8.44 Ni) austenitic steel subjected to electron beam plasma carburizing at temperatures of 350 and 500°C, frictional treatment with a sliding indenter and combined treatments, including frictional treatment and plasma carburizing have been investigated. It has been found that plasma carburizing increases the microhardness of the steel surface from 200 to 1100 HV0.025. The total hardening depth was 25 microns after carburizing at T = 350°C and 300 microns after carburizing at T = 500°C. Frictional treatment increases the microhardness of the steel to 600 HV0.025 with a total hardening depth of 500 microns. It has been shown that the diffusion-active layer with a dispersed structure formed during preliminary frictional treatment contributes to additional hardening of the steel (up to 1275 HV0.025) during subsequent low-temperature (350°C) carburizing. Combined treatment with carburizing at a temperature of 500 °C increases the microhardness of the steel to 820 HV0.025, and the total hardening depth is 500 microns for both combined treatments. It has also been found that plasma carburizing of the steel leads to a decrease in the eddy-current readings compared to the quenched steel and their growth compared to the steel subjected to frictional treatment, which can be used to develop quality control techniques for such treatments.

Low-Temperature Carburization: Ex Situ Activation of Austenitic Stainless Steel

Surface engineering of chromium-oxide-passivated alloys (e.g., stainless steels) by low-temperature infusion of interstitial solutes (carbon, nitrogen) from a gas phase requires “surface activation” by removing or perforating the passivating oxide film. We demonstrate a new approach for surface activation based on pyrolysis of a reagent powder, introduce advanced methodology to study its microstructure, and compare it to an established activation method. Rather than a bare alloy surface, stripped of its oxide, we find that an “activated” surface involves a reaction layer containing high concentrations of Cl, carbon, or nitrogen. We propose a model for the microscopic mechanism of surface activation that will enable future systematic development toward more effective process schemes.

OPTIMASI PARAMETER KARBURISASI TEMPERATUR RENDAH PADA BAJA TAHAN KARAT AUSTENITIK MENGGUNAKAN METODE TAGUCHI

Austenitic stainless steel is a popular material for its corrosion resistant properties, however it has low hardness which limits its application. Low-temperature carburizing can be used to improve the mechanical properties of the austenitic stainless steel by producing expanded austenite layer. In order to get a high-quality layer and an efficient processing operation, the carburizing process must be optimized. In this research, a Taguchi method was utilized to investigate the effect of processing parameters related to the formation of the expanded austenite layer depth in austenitic stainless steel. Four factors were selected to be optimized namely temperature, gas flow rate, time, and gas composition with three levels each. L9(34) orthogonal array was applied with nine experimental tests to get the diffusion depth value of carbon in the expanded austenite layer. S/N ratio was used to determine the optimum factor combination with nominal-the-better quality characteristic and the most significant factor was obtained by applying the Analysis of Variance. Temperature was found to be the most significant factor with 54.91% contribution. The optimum combination was also successfully defined with temperature at 450°C (level 2), gas flow rate at 15 liter per minute (level 2), time at 12 hours (level 3), and gas composition at 15% CH4 – 5% H2 – 80% N2 (level 3). Clearly, in this study the Taguchi method was proven to be appropriately used as one of robust tools in optimizing the thermochemical treatment process parameters.

Kinetic triplet from low-temperature carburization and carbon deposition reactions

Kinetic triplet from low-temperature carburization and carbon deposition reactions

Low Temperature Carburizing of Stainless Steels and the Development of Carbon Expanded Austenite*

Abstract Low-temperature carburizing dramatically enhances the inherently low wear resistance of austenitic stainless steels due to the formation of a carbon-supersaturated solid solution, i.e. expanded austenite. The formation of expanded austenite from low-temperature carburizing has been intensively investigated. However, the influence of chemical composition of the stainless steel on the carburizing response has not received the same interest. This contribution addresses the effect of the chemical composition on low-temperature carburizing in terms of carbon solubility, decomposition of expanded austenite upon exceeding the solubility limit and the elasto-plastic accommodation of the carbon-induced lattice expansion. The results demonstrate that the carbon solubility increases with an increasing Cr-equivalent and that higher Cr- and Ni-equivalents favor the formation of Cr-based M7C3 over Fe-based Hägg (M5C2) carbide.

Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Understanding the efficacy of concentrated interstitial carbon in enhancing the pitting corrosion resistance of stainless steel

By introducing a high fraction of interstitial carbon through low temperature carburization, the pitting corrosion resistance of austenitic stainless steel can be significantly improved. Previous work attributed this to strengthening of the passive film by carbon. Here, however, we present a paradigm shift by showing that carbon actually weakens the passive film but strengthens the bonding within the alloy by forming localized (covalent) bonds with the metal atoms (Fe, Cr, Ni). Accordingly, the enhancement in pitting resistance is a result of carbon reducing the metal dissolution rate in a local pit environment by many orders of magnitude, which extremely decreases the growth stability of a pit and prevents it from transitioning into stable growth. Electronic structure calculations confirm that carbon bonds to the metal atoms and that the metal–carbon bonds are 1.4 to 2.0 times stronger than the metal–metal bonds. These results suggest a new strategy for the design of corrosion resistant alloys, namely surface engineering by infusion of concentrated interstitials that form strong bonds with the matrix atoms.

Notch fatigue behaviour of low-temperature gaseous carburised 316L austenitic stainless steel

ABSTRACT The influence of low-temperature gaseous carburisation on notch fatigue behaviour of 316L steel under cyclic axial loading was investigated. After carburisation, the carburised case was well distributed at the surface region and was not influenced by the notch geometry. Low-temperature carburisation considerably enhanced the notch fatigue performance, which led to 32% and 44% increase in the endurance limits for the specimens with stress concentration factors K t = 1.91 and 3.91, respectively. The notch sensitivity of 316L steel reduced after carburisation. Irrespective of the applied stress amplitude, the fatigue crack nucleation sites were always at the notch root surface for the untreated specimens. For the carburised specimens, fatigue cracks nucleation changed from surface at high-level stress to subsurface at low-level stress.

On surface carbides in low-temperature carburized austenitic stainless steels

Surface carbides are occasionally found at the surface of austenitic stainless steels subjected to low-temperature carburizing surface treatments which are intended to induce an interstitially supersaturated surface layer. The crystallographic nature and chemical composition of the carbides can have a significant influence on the surface properties, especially on the corrosion resistance of the steel. However, their phase identification and chemical characterization via conventional methods (e.g. X-ray diffraction) is difficult due to their complex structure and metastable nature. In this paper, a combined metallographic and electron microscopic approach is applied to characterize the morphology and chemical composition of surface carbides found on AISI 304L and SS2343 after the same Kolsterising® treatment. Exclusively M5C2, also known as Hägg or χ-carbide was found at the surface of 304L, preferentially oriented along (111) deformation bands. In SS2343, however, χ-carbide coexisted with M7C3 (or ω-carbides) and M23C6. The carbides had preferential orientation relationships with the matrix, suggesting a carbide evolution during carburizing. All the carbides show elemental partitioning, with Cr enrichment and Ni depletion. A microstructure development sequence regarding precipitation and growth of surface carbides is proposed in this paper.

Microstructure and mechanical properties of low temperature carburizing layer on AISI 440C martensitic stainless steel

In this study, the microstructure and mechanical properties of AISI 440C martensitic stainless steel with low temperature carburizing (LTC) layers were investigated by microscopes, X-ray diffraction (XRD), Vickers indenter and pin-on-disc test device, etc. LTC treatment was conducted at 420 °C for 10–40 h on the quenched-tempered steel samples by pack method. Results indicated that the LTC layers formed on base metal exhibited compact and regular morphology, and carbon-expanded martensite (α′c), Fe3C and Cr23C6 compounds were examined in the LTC layers by XRD analysis. Besides, the layer hardness from 1430 to 1720 HV0.025 with processing time increased, which was approximately three times higher than that of the specimen without LTC treatment. However, as the processing time increased, the adhesion strength of LTC layers deteriorated and the indentation grade decreased from HF1 to HF3. In addition, the wear resistance was significantly improved after the LTC treatment, and the wear rate of the LTC layer decreased by 93.7% in comparison with the untreated condition.

On the fatigue behavior of low-temperature gaseous carburized 316L austenitic stainless steel: Experimental analysis and predictive approach

Low-temperature gaseous carburization is a surface modification method for austenitic stainless steels. In order to investigate the effects of low-temperature gaseous carburization on the fatigue behavior of AISI 316L, fully reversed axial fatigue tests were performed at room temperature on specimens with various remaining case depths. The fatigue performance of AISI 316L could be significantly improved; a 15% higher endurance limit is achieved after low-temperature gaseous carburization. After removal of the outer, brittle part of carburized case by electropolishing, the improvement of the fatigue performance is reduced. Fractography showed that for untreated specimens, fatigue cracks always initiated on the surface. For the carburized specimens, however, the locations of crack initiation sites depend on the applied stress levels. Compressive residual stresses in the case move the crack initiation site to the sub-surface; the lower the applied stress, the deeper the initiation site. A quantitative analysis of the effect on fatigue behavior forms the basis for a life prediction model, which can accurately predict the fatigue life of AISI316L steel after low temperature carburization.

Improving tribological properties of P91 steels through carburizing process

The wear conduct of P91 steel has been researched under a dry sliding contact at consistent load. It was discovered that Carburizing was successful in improving the wear conduct of P91 steel material. A low temperature carburizing was completed for three distinct examples at an hour, an hour and a half and 120 min. Unnitrided austenitic treated steel was utilized as a source of perspective material and experienced wear test for comparision with nitride examples. Wear tests were conducted at 1000 rpm with an applied load of 50 N to examine the wear loss and volume wear loss of the material. The nitrided samples uncovers that hard nitride layers are formed which improves the surface hardness. Miniaturized scale hardness estimations uncovered a huge increment in hardness after treatment. The layers were further by checking electron magnifying instrument investigation.

Low-Temperature Nitridation of 2205 Duplex Stainless Steel

Low-temperature gas-phase nitridation has been studied in $$\updelta $$-ferrite in 2205 duplex stainless steel. High-resolution spatially-resolved compositional and structural analysis revealed two competitive responses to nitridation. Some regions revealed a nitrogen atom fraction approaching 25 at. pct—greater than $$10^6$$ times the equilibrium solubility limit at room temperature. Remarkably, there is no expansion or distortion of the body-centered cubic lattice. This is similar to the response of $$\updelta $$-ferrite in this alloy to low-temperature carburization. In conventional transmission electron microscopy bright-field images, the supersaturated ferrite grains show no diffraction contrast—resembling the appearance of amorphous structures—suggesting an unusually high defect density. These grains exhibit spinodal decomposition of the ferrite to nanometer-scale Fe-rich and Cr-rich ferrite domains. High-resolution imaging reveals pristine Fe-rich nanocrystals, whereas the Cr-rich domains are apparently amorphous. Elsewhere in the nitrogen-rich case, an isothermal ferrite-to-austenite phase transformation occurred. The austenite transformation product formed martensitically with a high-aspect-ratio plate-like morphology in the Nishiyama–Wassermann orientation relationship to the ferrite matrix.

High-temperature phase transformations in AISI 316 stainless steel infused with concentrated interstitial carbon

For the austenitic stainless steel AISI 316 treated by low-temperature carburization, we report a first study on high-temperature phase transformations (up to the melting point). This study was carried out on uniformly carburized foils of AISI 316. Upon heating, M23C6 was found to precipitate in grain boundaries between 900 and 1100 K. At around 1325 K, these precipitates dissolve. Upon heating to 1473 K and subsequent cooling, the carbide M7C3 forms in grain boundaries and then also within the austenite grains. The Cr-depletion caused by precipitation of Cr-rich carbides converts austenite to ferro-nickel, causing the material to become ferromagnetic. Unless oxygen is available at the alloy surface, carbon is not lost from the alloy. Cooling down already after reaching 1173 K retains a substantial fraction of the initial interstitial carbon in solid solution, meaning that AISI 316 foils surface-engineered by low-temperature carburization have significant “accident tolerance.” Even after exposure to temperatures close to the melting point (1473 K), the mechanical properties will be improved over those of non-carburized AISI 316, owing to precipitation hardening by an exceptionally high volume fraction of intra-granular carbide.

Phase instability of the topmost surface layer of Inconel-718 superalloy in low-temperature gas carburization atmosphere, revealed by means of (HR)(S)TEM

The surface microstructure modifications induced by the industrial treatments with the effect of enhancement of the surface engineering parameters in metallic alloys (hardness, wear, corrosion resistance) are utterly important. The recently invented, industrially successful surface hardening by carburization of 316L stainless steel (SS) in gas atmosphere at low-temperature (LT) [1] was extended with similar results to the Ni-Fe base Inconel-718 superalloy (IN-718) [3] and to several other alloys. The accepted model of surface microstructure modification due to processing is not based on full data about the modified topmost layer microstructure. It is missing the visualization of the new microstructure by means of (HR)(S)TEM and analysis. The effect of LT gas carburization is currently explained by a model of surface grains lattice dilatation due to the so called colossal carbon supersaturation (LTCSS) [2] that is not conform to the classical phase diagrams and is claimed to be valid in all alloys including IN-718 [4]. Our work reports new findings that reveal the up to now unknown nanostructure of the topmost carburized (at 570ºC) layer ~5 µm thick of IN-718 and cast doubts on the general validity of the LTCSS. The (HR)(S)TEM and nanoscale analysis were used to reveal the nanostructure of samples prepared in a special way by ion beam milling. We got no evidence of a classical crystalline microstructure of the carburized IN-718 upper-most layer that can match the requests of the LTCSS model. Instead, we put into evidence the nano-structure of the topmost ~ 5 µm thick layer of the carburized surface showing that it consists both (i) in nanocrystalline austenite and carbides and (ii) in an amorphous phase that contains the main alloy elements, with or without carbon (Fig-1). Segregated Ni nanoclusters (Fig-2) are occurring at large areas, apparently not including other main alloy elements. A fragmentation mechanism of the topmost surface layer (consisting initially of µm-sized grains) can be figured out by starting from the observed segregation in the initial alloy grains of the main alloy elements, agglomerated both inside grains close to grain boundaries (GBs) and at the GBs (Fig-3) and by the segregation of carbon both inside grains along GBs (Fig-4) and outside grains at GBs. Those chemical segregates can be considered as precursors in the process of fragmentation and are proving the phase instability of the surface alloy grains in the LT carburizing gas atmosphere, thereby revealing the chemical reactivity of the IN-718 surface grains with the LT carburization gas mixture. The surface reactivity can be valorized industrially through developing new research for finding novel compositions of LT gaseous mixtures able to modify the engineering properties of the alloys surfaces.

Characteristics of carbon-expanded α phase layer on AISI 431 stainless steel using experimental and fist-principles calculation methods

In the present investigation, carbon-expanded α (αC) phase layers were prepared by low-temperature carburizing of AISI 431 martensitic stainless steel. The microstructures of the layers were characterized by optical microscopy and X-ray diffractometry, the mechanical properties of the carburized layers were characterized by nanoindentation tester. Then, first-principles calculation method was used to characterize αC phase from atomistic level. Experimental results show that there are the uniform layers formed on stainless steels after plasma carburized at 703 K. The layer thicknesses for the steel carburized for 14,400 s, 28,800 s and 57,600 s are about 25 μm, 38 μm and 46 μm, respectively. The surface phases are mainly of αC phase with a small amount of Fe3C phase. The surface hardness of the carburized specimen can be higher than 15 GPa by nanoindentation tests, the carburized layers possess a good wear resistance based on the higher H/E and H3/E2 ratio evaluation criterions. Crystal cell of αC phase were established and describing as the nominal molecular formula of Fe13Cr3Cx (x = 0, 2, 4, 6). The calculation results indicate that with increasing of carbon content, the structural and mechanical stabilities of Fe13Cr3Cx (x = 0, 2, 4, 6) system decrease gradually, the hardness increases first then decrease, and can reach up to about 25 GPa. The elastic moduli decrease from about 210 GPa to 66 GPa with increasing of carbon content, while the toughness increase gradually based on B/G values evaluation criterion. The calculated and the experimental results are consistent within a reasonable range, which shows the validity of the established model for αC phase.

Влияние низкотемпературной цементации в плазме электронного пучка на упрочнение и шероховатость поверхности метастабильной аустенитной стали

Влияние низкотемпературной цементации в плазме электронного пучка на упрочнение и шероховатость поверхности метастабильной аустенитной стали