Carburized Zone Research Articles

Manufacturing of high strength bimetallic section steel with hot-rolling process

This study aimed to develop a 316L/35# stainless steel–bimetallic section steel to satisfy plasticity and strength standards. The stainless steel bimetallic section was made of 316L stainless steel, whereas the base metal was made of 35# carbon steel, rolled using 13 rolling mills at 1150 ​°C by applying interface vacuum technology. Moreover, the mechanical properties and microstructure of the composite interface of hot-rolled stainless steel–bimetallic section steel were analyzed at multiple scales. The finished, rolled-section steel was selected as the research object. Various experiments were conducted from multiple scales: transmission electron microscopy (TEM), electron probe micro-analyses (EPMA), and scanning electron microscopy (SEM) were utilized, among other techniques for testing the mechanical properties. The results revealed that distinct carburized and decarburized zones were formed on both sides of the composite interface due to the diffusion of elements. The width of the carburized zone was 30–50 ​μm, and the hardness was 184.3HV, whereas the width of the decarburized zone was 80–100 ​μm, and the hardness was 146.4HV. In the carburized zone, short, rod-like martensite, second-phase precipitates, and chromium carbide and oxides were observed. The shear strength of the stainless clad section steel was 384.62 ​MPa and the ultimate strength was 599.76 ​MPa, far exceeding the 210 ​MPa required in the standard, which revealed that the bimetal achieved good metallurgical bonding. This study serves as an important theoretical basis for conducting structural integrity evaluations of bimetallic materials.

Effect of heat input on the microstructure and performance of dissimilar welded joint between Q345B low-alloy steel and 2205 duplex stainless steel

Q345B/2205 dissimilar steel multi-layer multi-pass welded joints were prepared using gas metal arc welding (GMAW). The effects of heat inputs on the microstructures and properties of welded joints were studied. The morphology and composition distribution of Q345B base metal near the fusion line were investigated. The results showed that the welded joints under different heat inputs were all present “carbon depleted zone” and “carburized zones” on both sides of the Q345B base metal and transition layer interface. The microstructure of the “carburized zones” was composed of martensite phase and granular bainite phase. It was found that only the welded joints with heat inputs of 1.765 kJ/mm and 2.860 kJ/mm met the usage requirements. The corrosion behavior and characteristics of the welded joint with a heat input of 1.765 kJ/mm in a 3.5 wt % NaCl solution were investigated intensively. During the soaked process of 14 days, the total impedance value of the welded joint showed first increasing and then decreasing. The maximum value of 8.7 kΩ cm2 was obtained on the 5th day. As the soaked time increased, the corrosion products of the welded joint underwent the formation, thickening, cracking and detachment, which was the main reason of the change in the corrosion resistance. The research results of this article have significant implications for the application of low-alloy steel and stainless steel welded joints in engineering.

Eddy current analysis for predicting deterioration stages in alumina former radiant coils

The alumina former radiant coil is typically used as a high-temperature material. It is inevitable that the material undergoes deterioration after prolonged use. This deterioration is primarily caused by damage from the service environment and initiates carburization, resulting in a loss of its mechanical properties. Currently, the damage to the materials is only discovered when they are already broken and a precise non-destructive testing method to determine the extent of material deterioration before the damage occurs has not successfully been developed. Therefore, this research aims to study and analyze the deterioration and the causes of damage to alumina former radiant coil materials. Additionally, the study was conducted to establish a relationship between the extent of deterioration and the impedance phase angle using the eddy current test method. The experimental analysis included both as-received and in-service materials of alumina former radiant coil, which consisted of 45 % nickel by weight, 30 % chromium by weight and 4 % alumina by weight. The additional testing methods included microstructure and elemental analysis, deterioration distance measurement and impedance phase angle measurement. The results indicated that the deterioration was primarily caused by damage to the oxidation layer, facilitating the diffusion of carbon and initiating a carburization reaction on the surface of the coil. These factors unavoidably reduced the corrosion resistance of alumina former radiant coil materials. Furthermore, the reduction in chromium content as materials entered the deterioration stage led to increased hardness in the carburization zone and reduced hardness in the oxidation zone. This deterioration caused the material to exhibit ferromagnetic behavior. It was observed that as the area of deterioration increased, the size of the ferromagnetic regions also increased. Resulting in, the eddy current testing results show a variation in the impedance phase angle. Therefore, the relationship can be concluded that the impedance phase angle increases in the counterclockwise direction, corresponding to a greater extent of deterioration. The benefit of this paper is to forecast the level of the severity of deterioration quantitatively when using eddy current testing.

Investigation of Microstructure and Magnetic Properties of HP40-Nb Reformer Tubes

In this paper, the results of nondestructive evaluation, structure characteristics, and magnetic properties of HP-Nb steam reformer tubes of a direct reduction unit are presented. An eddy current testing technique based on a special normal probe was performed to estimate the service life of the outer surface of the reformer tubes at a frequency of 60 kHz. The magnetic properties were investigated by vibrating sample magnetometer testing. Microstructural transformations of reference samples were performed by an optical microscope equipped with image analysis software and scanning electron microscopes. With exposure to severe operational conditions in oxidizing atmospheres and high temperature, microstructural changes such as oxidation and transformation of M23C6 and M7C3 carbide in the carburization zone, and chromium depletion in the austenite matrix, can be intensified. The tubes were inspected on a point basis and the eddy current data were presented as an impedance map. The results of the eddy current testing and vibrating sample magnetometer showed that microstructural changes due to operational conditions affect the electromagnetic response of the tubes.

Mechanical Properties of a Stainless Steel after Annealing in Uranium Carbide

The aim of this study was to investigate the interaction of carbide nuclear fuels with steel that is being used as cladding material for nuclear reactors. The specimens prepared from steel EN 1.4988 were consecutively annealed in three uranium carbide (UC) powders, having different carbon contents, at 600 °C for 1000 h. Both Ar and Na were used as bonding elements. The increase in the carbon content of the carburized specimens was determined and evaluated according to the bound and free carbon contents in the UC powders. The migration of free and bound carbon atoms into steel via self-diffusion and over Fe3C formation is interpreted as carburizing. Microhardness measurements and stress-strain tests were used to determine the mechanical properties of crude and carburized steel specimens. Maximum hardness at the contact surface and depth of the carburized zone were determined from the microhardness profiles and discussed depending on the bonding elements and carbon content in the specimens. These variables have a significant impact on the elongation percent, 0.2% yield stress, and tensile stress.

An innovative process of clad teeming and rolling for preparing stainless steel/carbon steel clad plate

An innovative process of clad teeming and rolling (CTR) is proposed to prepare stainless steel/carbon steel clad plate. A 20 kg clad ingot was prepared by pouring carbon steel melt around stainless steel core. It was rolled into a clad plate with total reduction ratio of 62.1%. Interfacial morphologies and mechanical properties were studied before and after hot rolling. Interfacial element diffusion was analyzed and the carbide phases were identified by XRD. The result shows that an excellent bond interface is obtained by clad teeming. The interfacial shear strength reaches 242 MPa in as-cast clad ingot. A chromized zone containing large amount of chromium-rich M7C3 carbides and a decarburized zone form in carbon steel substrate. A carburized zone containing small amount of M23C6 carbides occurs in stainless steel layer. The interface defects are quickly eliminated by the first rolling pass. The interfacial shear strength further increases to 353 MPa after hot rolling of four passes.

Microstructure Evolution and Mechanical Properties of AISI 430 Ferritic Stainless Steel Strengthened Through Laser Carburization

Abstract Carburization assisted by laser processing is a promising method to strengthen metallic materials. Direct laser beam carburization is implemented for the first time on thin AISI 430 ferritic stainless steel (FSS) sheets with graphite coating under different conditions. Microstructural morphology, phase constitution, carbon content, microhardness, and tensile behavior are investigated to evaluate the laser carburization effect. The carburized zone presents different morphologies according to the linear energy density of the laser beam. The least carbon content is around 0.4 wt% in the carburized zone where austenite becomes the leading phase. Delta ferrite is found in a cellular carburized area, which resembles a duplex microstructure. The hardness of carburized zone has been at least increased by 130%, the yield strength and ultimate tensile strength of a fully carburized sample can be increased by respectively 90% and 85%. This hardening effect is driven by the precipitation of carbides formed during solidification offering pinning points for dislocations and grain boundaries. These improvements could be useful to modify locally ferritic stainless steel to meet industrial needs such as wear-resistant surfaces.

Influence of long term Sodium Exposure on the Corrosion and Tensile Properties of AISI Type 316LN stainless steel and modified 9Cr-1Mo steel

AISI Type 316LN stainless steel (SS) and modified 9Cr-1Mo (P91) steel are used in the intermediate heat exchanger and steam generator (SG), respectively of prototype fast breeder reactor (PFBR). This paper presents the effect of long-term sodium exposure (50000 h) on the corrosion and tensile properties of 316LN SS and P91 steel. The results showed that the prolonged sodium exposure of 316LN SS can lead to selective leaching of alloying elements (depletion of Cr, Ni, and Mo), enrichment of Fe, formation of ferrite layer and carburized zones with increased hardness at the surface (272 HV), and a reduction in ductility by nearly 40 %. Double-loop electrochemical potentiokinetic reactivation (DLEPR) results of sodium exposed 316LN SS showed a 25% higher degree of sensitization (DOS) as compared to thermally aged one. The microstructural examination of sodium exposed 316LN SS revealed a ditch structure near the surface and carbide precipitates along the grain boundaries indicating the diffusion of carbon through grain boundaries and surface carburization. XRD studies of sodium exposed 316LN SS revealed predominant phases of ferrite and Cr23C6 phases, which further corroborated the results of DLEPR, microstructure, and hardness data. The sodium exposed P91 steel showed no significant microstructural, microchemical changes and tensile properties after sodium exposure. The carbon depth profiles carried out using SIMS in sodium exposed 316LN SS and P91 steel showed a surface carbon concentration of 0.85 and 0.32 wt.%, respectively. Fractography of sodium exposed 316LN SS showed intergranular fractures whereas the fractographs of P91steel showed fibrous dimple and cleavage type of fractures indicating the growth of carbides. The present results confirm that P91 steel can sustain the reactor life in flowing sodium containing 25 ppm carbon and less than 2 ppm oxygen.

Determination of residual stress gradient in a Ti-stabilized austenitic stainless steel cladding candidate after carburization in liquid sodium at 500 °C and 600 °C

Non-destructive residual stress profile measurement with a micrometric depth resolution within the depth of carburized Ti-stabilized stainless steel cladding candidate was carried out by high-energy X-ray diffraction. The samples were carburized in carburizing nuclear liquid sodium at 500 °C and 600 °C for 1000 h. The full residual stress tensor profile was determined thanks to the estimation of the strain-free lattice parameter evolution using the carbon concentration profile measured by electron probe microanalysis and thermodynamic simulation. For the sample carburized at 500 °C, the residual stress genesis was governed by the carbon concentration within the steel and the formation of expanded austenite. For the sample carburized at 600 °C, the residual stress profile in the austenitic matrix depended on the precipitation of M23C6 carbide. Stress relaxation was observed in the intragranular carburization zone. For the two temperatures, compressive residual stresses developed in the carburized zone and tensile stresses developed in the rest of the sample.

Effect of carbon content of substrate on the microstructure changes and tensile behavior of clad layer of stainless steel composites

This work aims to estimate the effect of interfacial carbon diffusion on the diffusion behavior of interfacial Cr element, interfacial microstructure, and tensile behavior of stainless steel (SS) composites. Here, 316L SS was selected as the clad material, and two low-alloyed steels (LASs) with different carbon contents (0.06 and 0.20 wt%) were used as the substrate. Two SS composites were prepared via vacuum hot compression in a Gleeble-3500 simulator, followed by normalization treatment. The results revealed that as the carbon content increases, the zone of the decarburized and the carburized of the composite sample become wider. The M23C6 carbides precipitated at the grain boundaries of the carburized zone of SS clad increase, and their morphology transformed from discontinuously granular to continuously lamellar. Therefore, the ultimate tensile strength (UTS) and yield strength (YS) were also increased. Simultaneously, the uniform elongation decreased from 24% to 17% due to the formation of local microcracks at the intergranular lamellar carbides.

Experimental and Numerical Investigations of Wear Resistance Characteristics of XCr13 during Advanced High-Strength Steel Stamping

Owing to the high strength and hardness of advanced high-strength steels (AHSS), the die materials for AHSS sheet stamping must have superior mechanical properties. A novel carburization technology on high-chromium, low-carbon, low-cost stainless steel XCr13 was used to achieve a hardness of approximately 60 HRC, with a carburized layer of approximately 0.5 mm; these specifications meet the requirements of AHSS sheet stamping. To evaluate the wear characteristics of the treated XCr13, regular pin-on-disk wear tests were performed under dry sliding conditions at room temperature. JIS SKD11, a popular cold work die material, was also tested and compared with XCr13. A numerical simulation model for AHSS sheet flanging with a W-shaped specimen was set up in ABAQUS, and the user subroutine UMESHMOTION was used to evaluate the wear resistance of the two die materials, including the wear surface profile, wear depth, and wear severity index value. For JIS SKD11, the agreement between the simulation and experiment results was good, which verifies the reliability and accuracy of the proposed wear simulation model and method. It was found that the wear resistance of carburized XCr13 was at the same level as that of JIS SKD11 because of the carburized zone. Further, carburized XCr13 can work well as a die material for AHSS stamping.

Investigation of hardness behavior after carburizing and hardening of 15CrNi6 steel

DIN 15CrNi6 is the most representative grade of the case-hardened steels. The present work analyses the influence of carburizing time on hardness of the specific steel. Specimens with similar chemical composition were heated at 900°C in liquid carbonaceous media for one, two, three and four hours, correspondingly. Then samples were oil quenched and tempered at 180°C for two hours. Microhardness was measured across the carburized zone and case profiles were acquired. The effective case depth was determined as function of carburizing holding time. Core macro hardness was carried out and the impact of holding time on the substrate hardness was discussed. The optimum case depth was defined and the carburizing parameters determined. The hardness control is critical in case hardening practice and results provide practical information to heat treaters, useful both to control the treatment parameters and to minimize the risk of failure.

Carburization effect of Austenitic alloys with various Cr and Al additions under the methane/hydrogen atmosphere on the corrosion behaviors of steels

The corrosion behaviors of six Fe-19Ni-13/21Cr-xAl (x = 0, 2, 6 at. %) alloys in 10% CH4/H2 at 800oC were investigated. 2 at. % Al did not affect the corrosion resistance obviously, while 6 at. % Al reduced the carbon attack completely for Fe-19Ni-13Cr-6Al but was still insufficient to form protective alumina scales for alloys with 21 at. % Cr. An increase of Cr content changed the appearance of the internal carburization zone under the optical microscope. Stability diagrams of M-C-O system（M= Cr, Fe）were established to estimate the diffusion paths of carbon in the alloys.

Importance of trimethyl borate temperature used during gas boriding for microstructure, nanomechanical properties and residual stresses distribution on the cross-section of the produced layer

The aim of this work was to indicate the possibility of applying organic compounds as a boron source for gas boriding. In the present work the trimethyl borate was used as an organic boron source for gas boriding process. The process was carried out at 950 °C for 2 h in gaseous atmosphere composed of N2–H2–B(CH3O)3. The temperature of trimethyl borate influenced on its concentration in gas atmosphere. As a result, depending on B(CH3O)3 temperature of 20 °C or 50 °C, it was possible to arranging the two types of process: boriding and borocarburizing, respectively. In the case of gas boriding the single-phase Fe2B layer was produced. The high temperature of B(CH3O)3 caused release of free atoms of carbon, therefore there existed favorable conditions for carburizing. The produced borocarburized layer consisted of two zones: an outer Fe2B borided layer and an inner carburized zone. The thickness of boride layer was higher after boriding process than simultaneous borocarburizing process, 10.8 μm and 7.8 μm, respectively. Whereas, the depth of zone of carbon diffusion was equal ca. 400 μm. For nanomechanical properties, as well as, the residual stress distribution the nanoindentation tester Anton Paar NHT3 equipped with the Berkovich diamond tip under a maximum load of 10 mN was used. In both layers, the highest hardness HIT (7.8–17.9 GPa) and highest Young's modulus (222–368 GPa) were measured in Fe2B layer. However, the presence of thick zone of carbon diffusion was the reason for gradually decrease in hardness in the cross section of borocarburized layer. Moreover, the presence of carburized zone advantageous influenced on residual stresses distribution across the layer. The gradually changes of residual stresses from compressive to tensile were observed in the case of simultaneous gas borocarburized layer. Such a situation was more advantage than those obtained for gas borided layer.

Effect of carbon migration on mechanical properties of dissimilar weld joint

After welding or servicing at high temperature, carbon migration will occur in dissimilar welded joint. Correspondingly, the carburized layer with high carbon content and decarburized layer with low carbon content are formed near weld fusion line. Carbon migration has a great influence on the properties of the joint and leads to premature failure. However, due to the small size of carburized layer and decarburized layer, it is difficult to use the traditional uniaxial tensile test method to study their mechanical properties. In this work, the elastoplastic properties of base metal (BM), carburized zone (CZ), decarburized zone (DZ) and weld metal (WM) in A302/ Cr5Mo dissimilar weld joint were investigated by nanoindentation tests. nanoindentation tests results indicate that the decarburized zone is the weakest position in the weld joint. Metallographic and carbon content distribution analysis were also carried out and found that the width of CZ is related to the width of fusion zone (FZ) and the CZ does not exceed the boundary of FZ.

Effect of carburization (pyrolysis furnace tube main failure factor) on the microstructure and properties of HPNb alloy tube

The structure characteristics and mechanical properties of as-cast and carburized HPNb alloy tubes were investigated in this paper. Through magnetism test by the vibrating sample magnetometer, we find that the paramagnetic structure of as-cast HPNb alloy tubes has changed into ferromagnetic structure after carburization. Electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) analyses show that the M23C6 type carbide transformed into M7C3 type carbide in carburization zone. Due to the coarser primary carbides and blocky-shaped secondary carbides after carburization, the ductility and toughness of the ethylene cracking furnace tubes are very low and become brittle. At high temperature, the tensile strength and yield strength of carburized tube are higher than those of as-cast tube, but the tensile elongation is lower. Owing to the microstructure degradation, the creep strength of HPNb alloy decreased after carburization. Hence, the carburization also could lead to the decrease of the remnant life of the pyrolysis furnace tubes.

Behaviour of Metallic Materials in Simulated Service Environments of CO2/H2O Co-electrolysis Systems for Power-to-X Application

In the present study, the ferritic steel Crofer 22 H as potentially suitable interconnect material for SOEC stacks as well as joints between the steel and Ni- and CuNi contact materials was investigated with respect to the behaviour in simulated service environments of an SOEC system for CO2/H2O co-electrolysis. Exposures up to 1000 h at temperatures between 600 and 800 °C were carried out in CO2/H2O- and CO/H2-rich gases, thus simulating conditions at the stack inlet and outlet, respectively. It was found that the steel formed protective surface oxide scales consisting of chromia and/or Cr/Mn spinel in all studied test conditions. No indication of carbon transfer from the gas atmosphere into the steel was found even in the high carbon activity CO/H2-rich gas simulating stack outlet conditions. However, in the latter gas substantial carbon transfer from the gas to the steel via the Ni- or CuNi-wires resulted in the formation of a carburized zone with substantial M23C6 and/or M7C3 precipitate formation. This effect was more pronounced for the joints of the steel with the Ni-wire than with the CuNi-wire. In the gas simulating the service environment at the stack inlet, only minor carbon transfer was found in case of the Ni/steel joint at 600 °C but not at 800 °C. In case of the CuNi-wires, partial loss of contact between wire and interconnect steel and formation of Kirkendall voids as a consequence of interdiffusion between wire and steel were observed. The experimental results are discussed using thermodynamic considerations involving gas equilibria and stability of possible external and/or internal formation of oxide and carbide phases.

Study on microstructure and properties of centrifugal casting 35Cr45NiNb+MA furnace tubes during service

ABSTRACTIn the present study, the microstructure evolution of 35Cr45NiNb+MA ethylene cracking furnace tubes during service and its effect on the properties were investigated. According to the results, in the early stage of service, the skeletal M7C3 and vermicular NbC were transformed into blocky M23C6 and G phase (Ni16Nb6Si7), respectively, accompanied with many dispersed M23C6 secondary carbides. With the extension of service time, M23C6 carbides on the grain boundaries were transformed into M7C3 with high stacking fault structure and coarsened, and the blocky G phase was transformed into granular NbC. The yield strength and ultimate tensile strength of the aging furnace tubes decreased by 9%-18%, yet the elongation after fracture decreased significantly from 10.0%-14.0% to 3.0%-5.0%. The hardness of the carburized zone of the carburized tubes increased by 10%-17%, and the rupture time decreased by 45%-75% under the test condition of 1100℃ and 16MPa. Finally, the evolution map was summarized.

Microstructure, corrosion and mechanical properties characterization of AISI type 316L(N) stainless steel and modified 9Cr-1Mo steel after 40,000 h of dynamic sodium exposure at 525 °C

This paper presents results of a comprehensive study on 316L(N) stainless steel (SS) and modified 9Cr-1Mo steel exposed to flowing sodium at 525 °C for 40,000 h in a bi-metallic (BIM) loop. A non-uniform ferrite layer of 8–14 μm thickness was formed on the surface of 316L(N) SS due to elemental leaching of Ni and Cr. Higher carbon activity (aC) in sodium resulted in carburization of 316L(N) SS with a surface carbon content of 0.5 wt.%. The effective carbon diffusion coefficient, DeffC was estimated to be 3.64 × 10 −19 m2/s. Consequent to carbon transport in 316L(N) SS, extensive sensitization was detected and the charge density, Pa, values for sodium exposed, thermally aged and carburized zone were found to be 23.3, 18.5 and 4.8 C/cm2, respectively. 42 and 60% decrease in percent total elongation (%TE) and percent reduction in area (%RA), respectively were observed in sodium-exposed 316L(N) SS, as compared to thermally aged specimen, due to surface carburization. Unlike 316L(N) SS, modified 9Cr-1Mo steel did not show carburization, however, it showed a decrease in the impact energy values due to the presence of Laves phase and M23C6 precipitates in sodium-exposed and thermally aged specimens.

The Relationship Between Magnetism and Microstructure of Ethylene Pyrolysis Furnace Tubes after a Long-term Service.

The magnetism and microstructure of Cr25Ni35Nb and Cr35Ni45Nb alloy tubes after 5 years of service were investigated in this paper. The saturation magnetization of the Cr25Ni35Nb alloy tube in the thickness direction is more than 20 emu/g, and the tube becomes ferromagnetic. The inner and outer walls of Cr35Ni45Nb alloy tubes also become ferromagnetic. But the saturation magnetization of the Cr35Ni45Nb alloy tubes approaches to zero in the center zone. The primary carbides M7C3 and NbC are changed into M23C6 and G phase at the outer region of the furnace tube. However, the M23C6-type carbides were replaced by carbon-rich carbides M7C3 at the carburization zone. Cr-depleted zones are formed at the inner and outer walls of the furnace tubes owing to oxidation. Carburization and oxidation reduce the Cr content of the matrix. Accordingly, the saturation magnetization is very high at the carburization zone and Cr-depleted zone. The magnetism of Cr25Ni35Nb and Cr35Ni45Nb alloy tubes has a high correlation with the Cr content of the matrix. Carburization and oxidation are the main reasons that make the paramagnetic ethylene pyrolysis furnace tube change to ferromagnetic.