bdGas carburizing of steel with furnace atmospheres formedin situ from methane and air and from butane and air

Carburizing experiments were conducted at 927°C (1700°F) and 843°C (1550°F) using furnace atmospheres formed from methane and air and from butane and air introduced directly into the carburizing furnace. Gas flow rates were low to promote equilibration of the reaction products within the furnace. The air flow rate was held constant while the methane or butane flow was automatically regulated to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. In comparing the results of these experiments with earlier results obtained using propane and air, several differences were noted: (a) The methane content of the furnace atmosphere, measured by infrared analysis, was about twice as great when methane was the feed gas rather than propane or butane. This was true despite the fact that the mean residence time of the gas within the furnace was greater in the methane experiments. Methane appears to be less effective than propane or butane in reducing the CO₂ and H₂O contents to the levels required for carburizing. (b) There was a greater tendency for the CO content of the furnace atmosphere to decrease at high carbon potentials when methane is used instead of propane or butane. The decrease in CO content is due to hydrogen dilution caused by sooting in the furnace vestibule. These differences in behavior make propane or butane better suited than methane forin situ generation of carburizing atmospheres. However, there is no difference in the amount of carburizing occurring at a specified carbon potential when methane, propane, or butane are used as the feed gas in this process. J.A.Pieprzak, formerly a member of the Engineering and Research Staff     

Hydrogen attack of carbon steel

A model is developed for the kinetics of nucleation and growth of methane bubbles in the hydrogen attack of carbon steel. It is concluded that at high temperatures the time to incubate fissuring along grain boundaries is determined by the rate of iron diffusion away from microscopic growing bubbles. At lower temperatures and/or higher hydrogen pressures carbon supply is limiting. The equations fit the observed incubation times if the bubble density is high (~10⁷/cm²) and essentially independent of temperature (T) and hydrogen pressure (P) over a wide range. It is postulated that the number of growing bubbles is limited at high nucleation rates (lowT and highP) by carbon starvation. At hiT and lowP chemisorption to lower the solid-vapor surface energy or fine inclusions are required to aid nucleation. A quantitative analysis of these processes leads to several predictions which can be checked experimentally. Formerly Director, Division Material Research, National Science Foundation, Washington, DC     

Kinetics of methane bubble growth in a 1020 steel

A 1020 carbon steel and a sensitive dilatometer have been used to study the changes in the kinetics, and morphology, of methane bubbles with temperature, methane pressure, and bubble size. The transition from roughly spherical bubbles at low methane pressure to lenticular ones at high pressure, as predicted by theory, has been demonstrated along with the predicted change in pressure exponent and activation energy. That is, at high pressures the rate of bubble growth increases as (P _{CH 4} ³ and exhibits an activation energy characteristic of surface diffusion, while at quite low pressures the pressure exponent is about 3/2 with an activation energy between that for grain boundary and lattice diffusion. When the lenticular bubbles grow to a diameter roughly equal to that of the grains, their growth kinetics change to be limited by the power law creep of the matrix.     

A Model for the Growth of Hydrogen Attack Cavities in Carbon Steels

A detailed analysis of precision dilatometry data of McKimpson on plain carbon steel during the incubation stage of hydrogen attack has been carried out in the light of both the surface and grain boundary diffusion limited cavity growth models. The results are consistent with cavity growth by grain boundary vacancy diffusion provided the methane pressure in the cavity exceeds a threshold pressure. The threshold pressure observed by the analysis decreases with an increase in temperature from ~400 MPa at 658 K to ~200 MPa at 723 K. The magnitude of the threshold pressure is consistent with a constrained cavity growth model in which the strain due to atom plating is accommodated by local plastic deformation around the precipitates in the boundary. Quantitative agreement between model predictions and experimental data on expansion rates is good.     

Gas-Carburizing Kinetics of a Low-Alloy Steel

Gas-carburizing kinetics of a low-alloy steel (Pyrowear 53) was investigated by thermogravimetric experiments. Kinetic curves were modeled by adapting the approximate integral method, and the diffusion coefficient of carbon as well as the rate constant of the surface reaction were estimated. These parameters were evaluated after several carburizing procedures, which differ from each other in the surface treatments performed before the carburizing step. It is known that the carbon enrichment is low when this steel is carburized without any pretreatment, and this behavior was found to be related to a low value of carbon diffusivity. The interaction between the selective oxidation of alloying elements by the carburizing atmosphere and carbon diffusion is discussed. The pretreatment procedures investigated in this work consist of different combinations of oxidation, reduction, and grit-blasting processes. The most effective procedures involve oxidation in dry air or oxidation in wet air followed by grit blasting.     

碳钢表面冶金W-Mo强化层的研究

利用等离子表面冶金技术在碳钢表面进行W-Mo共渗,在表面形成数十～数百微米的合金扩散层,随后将试样进行超饱和离子渗碳,结果表明:试样表面钨当量在100μm之内大于10%,含碳量平均达到1.03%,淬火和回火后合金层表面硬度达到1000HV以上;合金层成分呈梯度分布,形成的合金碳化物细小、均匀、弥散,碳化物类型主要为M6C、M2C、MC。     

高铬合金表面渗碳研究

针对合金篦条材料表面进行等离子渗碳技术的试验研究和渗层的耐磨性能实验分析。利用等离子技术成功地在舍金篦条试样表面渗碳，使表面w（C）达到1．5％以上，渗碳层深1mm左右，表面硬度大大提高。合金篦条试样经渗碳后，在合金表面形成了碳化铬化合物，表面耐磨性能发生了质的变化。经磨损试验。耐磨性能较未经表面处理试样有较大的提高。     

Simultaneous plasma treatment for carburizing and carbonitriding using hollow cathode discharge

Ion carburizing and nitriding are effective processes for saving energy and providing polutionless surface treatment but have the disadvantage of using much electric energy. A cylindric subsidiary cathode was set up around a rod-shaped workpiece with a gap, and hollow cathode discharge for ion carburizing was studied. Thus, simultaneous plasma treatments for ion carburizing and ion car-bonitriding in one workpiece were researched using Cr-Mo steel to save electric treatment power. First, the effects of the gap between the test piece and subsidiary cathode and the pressure of electric discharge gas, including methane gas, on fundamental plasma treatment conditions were experimen-tally researched. It was found that the temperature for ion carburizing in a H₂-N₂-Ar-CH₄ gas mixture was 1123 to 1193 K with a gap of 3 to 5 mm under a gas pressure of 133 to 532 Pa. Next, the test piece was ion carburized with hollow cathode discharge and carbonitrided with normal glow dis-charge simultaneously. The ion-carburized layer was formed in the area covered by the subsidiary cathode. The surface hardness was 800 Hv, the effective case depth was 0.6 mm, and the surface carbon content was 0.75 wt pct. An ion carbonitriding layer was formed in the area without the subsidiary cathode. The surface hardness was 700 Hv and the case depth was 0.1 mm. It is useful to form the different layers of ion carburizing and ion carbonitriding in one treatment process and to give different mechanical and tribological properties on one workpiece simultaneously.     

Simulation of Case Depth of Cementation Steels According to Fick＇s Laws

 The carburizing process is the enrichment of the depth of low carbon steels with carbon. It leads to samples with a combination of high surface hardness and high core toughness and to an impact strength that is required for many engineering parts. The material studied is a low carbon steel. The carbon content is little in this type of steel (wC=0.2%). The calculation of case depth is very important for cementation steels that are hardened in the carburizing process. The effective case depth is defined as the perpendicular distance from the surface to a place at which the hardness is HV 550. Nowadays, a great number of studies have been carried out on the simulation of effective case depth, but no studies have been conducted to determine the numerical relation between the total case depth on one hand and the carburizing time and the effective case depth on the other hand. The steel specimens were subjected to graphite powder. Then, they were heat treated at 925 ℃ for about 3, 5, 8 and 12 h, respectively. Then, these parts were quenched in oil. To determine the effective case depth, the microhardness test was performed on the cross-section of specimens. Plotting the case depth vs carburizing time, the required conditions for obtaining the specified case depth were determined. Also, the comparison between the case depths in numerical solution and the actual position in pack carburizing was performed.     

Vacuum carburization of 12Cr2Ni4A low carbon alloy steel with lanthanum and cerium ion implantation

As bearing parts, 12 Cr2 Ni4 A is expected to have high hardness and excellent fatigue strength, so carburizing is employed to improve the inherit properties of 12 Cr2 Ni4 A. However, the traditional carburizing is limited by poor microstructure distribution and low rate of carburizing. The rare earth ion implantation is known to help improving the properties of tribology, corrosion resistance and oxidation resistance of metal. In this article, the RE implantation is employed to assist the carburizing. Lanthanum and cerium ion implantations are initially used to assist 12 Cr2 Ni4 A low pressure vacuum carburization.The microstructure, content of retained austenite, hardness, thickness of layer and carbon diffusion were analyzed by optical microscopy(OM), scanning electron microscopy(SEM), X-ray diffraction(XRD) and Rockwell/Vickers hardness tester, respectively. It was shown that lanthanum and cerium implantations can improve structure of the vacuum carburizing layer, and enhance the uniformity of carbon element distribution on the carburized surface. Meanwhile the RE implantation plays a positive role in promoting the surface hardness and carburized rate. The lanthanum element has more significant effect on surface hardness and content of retained austenite than cerium element. The surface hardness of lanthanum element implanted layer was 62.9 HRC with 9.6% content of retained austenite, while the carburizing rate of cerium implanted layer increased by 12.4%.     

模铸低碳复合保护渣的开发与应用

本文针对钢锭浇铸时使用中高碳保护渣对钢锭表面增碳的影响,对钢锭进行解剖,研究钢锭中C偏析的情况,证实保护渣增碳是造成钢锭大幅度增碳的主要原因,进而开发低碳复合保护渣。结果表明,使用中高碳保护渣钢锭表面增碳明显,采用开发的低碳复合保护渣能达到降低钢锭表面增碳的效果,从而提升钢锭的轧板质量。     

Carbon diffusion and phase transformations during gas carburizing of high-alloyed stainless steels: Experimental study and theoretical modeling

Gas carburizing of high-alloyed stainless steels increases surface hardness, as well as the overall mechanical characteristics of the surface. The growth of chromium-rich carbides during carbon transfer into the steel causes precipitation hardening in the surface, but decreases the chromium content in solid solution. In order to maintain a good corrosion resistance in the carburized layer, the stainless steel composition and the carburizing process need to be optimized. To limit the experimental work, a methodology using software for modeling the thermodynamic and kinetic properties in order to simulate carbon diffusion and phase transformations during gas carburizing is presented. Thermodynamic calculations are initially used to find the optimum parameters (T, carbon wt pct, etc.) in order to maintain the highest Cr and Mo contents in the austenitic solid solution. In a second step, kinetic calculations using the diffusion-controlled transformations (DICTRA) software are used to predict how the amount of the different phases varies and how the carbon profile in the steel changes as a function of time during the process. Experimental carbon profiles were determined using a wavelength-dispersive spectrometer for electron-probe microanalysis (WDS-EPMA), while carbide compositions were measured by energy-dispersive spectroscopy_X (EDS_X) analyses. A good agreement between calculated and experimental values was observed for the Fe-13Cr-5Co-3Ni-2Mo-0.07C and the Fe-12Cr-2Ni-2Mo-0.12C (wt pct) martensitic stainless steels at 955 °C and 980 °C.     

无莱氏体超硬高韧高速钢M2Si

与Ｍ２钢对比，新型高速钢Ｍ２Ｓｉ的硅含量提高到１．０％，碳含量降至０．５５％。该钢渗碳、淬、回火后的表面硬度高于６７￣６８ＨＲＣ，心部硬度为５５ＨＲＣ，无碳化物偏析及粗大碳化物，是一种价廉的超硬高韧高速钢。     

真空渗碳制备双相梯度硬质合金的研究

介绍了采用真空渗碳来制备试样外层为无η相正常组织而芯部为η相、且粘结相钴呈成分梯度分布的双相硬质合金的方法,试验发现,在真空烧结后期引入甲烷与丙酮蒸气的混合气体进行摆式渗碳,是制备该类梯度硬质合金非常有效的方法。     

The kinetics of hydrogen attack of steels

The hydrogen attack of steel involves the formation of methane bubbles along the grain boundaries and their subsequent link-up to form fissures. This paper presents a detailed model for the kinetics of growth of such methane bubbles. The model considers two parallel processes which can control the growth—one involving the bubble growth by direct power-law creep and the other involving combinations of surface diffusion, grain boundary diffusion and matrix accommodation processes. The proposed model is more general and complete than the earlier ones and considers for the first time the possibility of bubble growth being controlled by surface diffusion and accommodation processes. The predictions of the model are shown to compare well with the experimental results obtained in our lab and with the literature data. The model also indicates the relative importance of lower carbon activity and increased creep stength of steel to its hydrogen attack resistance.     

On methane generation and decarburization in low-alloy Cr-Mo steels during hydrogen attack

Low-carbon, low-alloy Cr-Mo steels may fail by hydrogen attack when they are exposed to high hydrogen pressures at elevated temperatures. During this process, the dissolved hydrogen reacts with the carbides of the steel to form methane in grain boundary cavities. The methane pressure inside these cavities depends on the microstructure of the used steel, which consists of a ferritic matrix and alloy carbides such as M₇C₃, M₂₃C₆, M₆C, and M₂C. The different phases in the multicomponent system Fe-Cr-Mo-V-C are modeled with the sublattice model. Their Gibbs energies are then used to calculate the equilibrium methane pressure as a function of the microstructure. Driven by the methane pressure, the cavities grow due to grain boundary diffusion and dislocation creep, which is described by analytical relations. This leads to progressive development of damage inside the material but, at the same time, to a decrease of the carbon content in the steel. This reduction depends on, among other factors, the methane pressure and the damage state. As the carbon content also affects the creep parameters, this process of decarburization may accelerate the cavity growth. Model calculations are used to obtain insight into the influence of this decarburization process on damage evolution and the final lifetime.     

Effects of tempering on the carbon activity and hydrogen attack kinetics of 2.25 Cr-1 Mo steel

The variation with tempering of the carbon activity and the Hydrogen Attack rates of a Q&T 2.25 Cr-1 Mo steel (A542 C13) was studied at 550 °C. A highly sensitive capacitance dilatometer was used to measure the HA strain rates as a function of hydrogen pressure, and an equilibration technique was used for the carbon activity. Both the carbon activity and the HA strain rate decreased monotonically with the extent of tempering. A strong correlation existed between carbon activity and the HA strain rate of the samples. Excessive tempering beyond the commercial practice did not eliminate HA, and the carbon activity of a sample tempered for 500 hours at 700 °C was as high as 0.05. The high carbon activity of the excessively tempered samples is explained as due to the effects of low Cr/Fe ratio in M₂₃C₆ carbides and to less than the equilibrium Cr content next to the M₂₃C₆ resulting from the low diffusivity of Cr in α-iron at the tempering temperature of 700 °C. The methane pressure dependence of the HA strain rates suggests a grain boundary diffusion controlled growth of bubbles for hydrogen pressures up to 20 MPa at 550 °C.     

Mechanical Properties of a Ni–Cr–Mo Steel Subjected to Room Temperature Carburizing Using Surface Mechano-Chemical Carburizing Treatment (SMCT)

Surface mechano-chemical carburizing treatment (SMCT) is a modified version of surface mechanical attrition treatment and it is one of the cutting-edge technologies for producing hard nano-crystalline surface in metallic materials. In the present study, a case carburized surface layer is achieved in 1.75 Ni–Cr–Mo steel at room temperature using SMCT. Activated charcoal powder is continuously fed during the process so as to achieve the carbon diffusion into the surface layer. The SMCT process has been carried out for different periods say 15, 30, 45 and 60 min respectively. The microstructure and surface chemical composition is investigated by using TEM and XRF analysis. The mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), fracture toughness and surface hardness of SMCT samples have been investigated using universal testing machine, Plain strain fracture toughness test and Microvickers hardness test respectively. The surface carbon content has been found to increase linearly and grain size reduces continuously with processing time. A 60 min SMCT samples reveal 0.8% C and about 10 nm grains over the surface. The SMCT samples show significant improvement in mechanical properties. The surface hardness increases from 180 HV_0.1 to ~ 878 HV_0.1 by 60 min of treatment. About 55% increment in the YS and 30% increment in UTS is achieved by 60 min of SMCT. It is also interesting to note that the fracture toughness of the samples enhances from 24 to 47 MPa \( \sqrt m \) after 60 min of SMCT.     

Diffusion aluminide coatings for internal surface of rhenium- and rhenium-ruthenium-containing single-crystal superalloys turbine blades: Part I

Process of formation rhenium or refractory carbides based diffusion barrier layer (coating) on internal surface of rhenium- and rhenium-ruthenium-containing single-crystal high-temperature alloys (superalloy) turbine blades, prior to diffusion aluminide coating deposition, is studied. It is shown that diffusion barrier layer is preventing deleterious secondary reaction zone formation under aluminide coating during long-term high-temperature operation. The kinetics of powder carburizing process of rhenium- and rhenium-ruthenium-containing high-temperature alloys is investigated, and conditions for carburizing these alloys are determined. The phase composition of the surface layer after carburizing is studied, and the effect of the fractional composition of a carbon-based powder mixture on the carburizing rate is determined.