Chromium Carbide Precipitation Research Articles

Magnetization study of the sensitization in SS304LN

Sensitization is precipitation of chromium carbide in stainless steels during exposure to high temperatures service that leads to premature corrosion failure of components. It is well known that during sensitization process, chromium carbide forms at the grain boundary along with chromium depletion near the grain boundary resulting in the change of the magnetic property of the steel from paramagnetic to ferromagnetic. From this study, it is clear that the magnetic characterisation is sensitive enough to detect the sensitisation. However, the magnetic behavior has contributions from both ferromagnetic and super paramagnetic nature of the phase.

Influence of the welding current intensity and nitrogen content on the corrosion resistance of austenitic stainless steels

The resistance to pitting and intergranular corrosion of the welded joint of X5CrNi18‐10 austenitic stainless steel is analyzed in this paper. The resistance of the welded joint to intergranular corrosion does not depend on the nitrogen content in the shielding gas. It depends on the heat input into the welded joint, that is, on the welding current intensity. The resistance to pitting corrosion depends on the content of nitrogen in the shielding gas and on the welding current intensity. With the increase in the nitrogen content in the shielding gas, the resistance of the heat affected zone (HAZ) to pitting corrosion increases. The welding current intensity (heat input into the welded joint) shows two opposite effects. On the one hand, the increase of the heat input into the welded joint causes a more intensive precipitation of chromium carbides along the grain boundaries, which then leads to depletion in chromium of the grain boundary areas. The sensitization degree of the HAZ is thus increased and the formation of pits easier. On the other hand, with the increase in the welding current intensity, diffusion of nitrogen from the weld metal into the HAZ is facilitated, which contributes to the increased HAZ resistance to pitting corrosion. Possible mechanisms for increasing the HAZ resistance to pitting corrosion in the presence of nitrogen are also considered.

Martensite coarsening in low-temperature plasma carburizing

This work shows microstructural evidences of morphological transformation from as-hardened martensite to C-expanded martensite as a result of the low-temperature plasma carburizing treatment carried out in AISI 420 martensitic stainless steel. Treatments were carried out at different treatment temperatures (350, 400, 450, and 500 °C, for a 12 h fixed time) and times (12, 24, 36, and 48 h, at 400 °C fixed temperature). Confocal laser scanning microscopy and SEM results show martensite coarsening in the treated surfaces regarding the respective substrate bulks, which is related to the surface C-alloying, thus being dependent on the treatment time and temperature. Hardness measurement and XRD results strongly suggest that the martensite coarsening is related to the transformation from lath- to plate-type martensite due to the enhancement of the martensite C content, in accordance with the Zener ordering theory. For samples carburized at relatively low temperatures (namely 350, and 400 °C), as C diffuses in paraequilibrium, strain-induced transformation mechanism would govern the changes in the initial martensite morphology as a result of its C content increase. On the other hand, as the chromium carbide precipitation occurs for samples treated at relatively high temperatures (higher than 450 °C for 12 h), and long times (longer than 36 h at 400 °C), C atoms would leave the supersaturated (expanded) martensite phase solid solution, leading the plate martensite to transform back to lath martensite.

Age-hardening behaviour, microstructure and corrosion resistance of the copper alloyed stainless steel 1.4542

The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ϵ–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.

Individual and synergistic influences of microstructural features on intergranular corrosion behavior in extra-low carbon type 304L austenitic stainless steel

The individual and synergistic effects of the microstructural features viz. grain size, residual strain, grain boundary character distribution (GBCD) and its connectivity on intergranular corrosion (IGC) behavior is evaluated in type 304 L stainless steel. Straining accelerated sensitization by promoting chromium carbide precipitation via diffusion through dislocations. Coarse-grained microstructure suppresses IGC by delaying both the onset and progression of sensitization. Microstructure comprising of optimized GBCD exhibits remarkable resistance to IGC owing to the substitution of random high-angle boundaries by Σ3 and its variants. Impressively, evidences suggested that GBCD has higher implication than grain size or residual strain in controlling IGC behavior.

Surface Roughness Effect in the Case of Welded Stainless Steel Corrosion Resistance

Abstract It is known that fusion welding can cause a decrease in the corrosion resistance of the heat affected zone of unstabilized stainless steels. The reason for this problem is that the welding heat (in the heat affected zone (HAZ)) can cause chromium-carbide (Cr23C6) precipitation with the simultaneous reduction of chromium content at the local grain boundaries. The chromium content dictates the corrosion resistance level. The relationship between surface roughness and corrosion behaviour is well known. We sought to find the difference between the corrosion resistance and surface roughness relationship in the case of cold rolled stainless steel and in the case of heat treated (welding heat effect simulated) stainless steel [1-3].

Research on the Diffusion of Ti-Based Stabilizer through Flux Coated GTAW to Reduce Carbide Precipitation in SS304 Weldment

Sensitization is a major problem, which leads to intergranular corrosion in stainless steel weldments due to precipitation of Chromium (Cr) carbides. Thus, Cr depleted from heat affected zone (HAZ) results in intergranular corrosion causes failure of structure. In the present research work an attempt has been made by the diffusion of titanium based stabilizer into HAZ during welding of SS 304 by gas tungsten arc welding (GTAW). The Ti powder (< 20µm size) has been used and coated on v-joint before welding in optimized coating thickness. This process is termed as flux coated gas tungsten arc welding (FC-GTAW). The surface microstructure, surface morphology and surface chemistry have been analyzed by Microscope, Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS). The EDS results along with SEM revealed that Ti has been diffused up to 2mm from the fusion boundary. The EDS line scan has confirmed the depth of Ti diffusion. Intergranular corrosion resistance is evaluated by DLEPR test, as a result of this test the sample coated with flux shows better corrosion resistance. The result indicates that FCGTAW is novel technique for reducing the formation of Cr carbides thus enhancing the corrosion resistance of stainless steel.

Effects of Electrode Types on the Microstructure, Tensile and Hardness Properties of 304 L Austenitic Stainless Steel Heat-Affected Zone (HAZ)

Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by improper choice of electrode. Therefore, in this study, effects of different types of electrode on the microstructure and tensile property of type 304 L austenitic stainless steel heat-affected zone (HAZ) were studied. Chemical composition of the as-received sample was determined. A number of samples were cut from the as-received sample. Afterwards, two half were joined together with 308 L, 312 L and 316 electrodes at a controlled welding speed, current and voltage of 4.6 mm/s, 160 A and 30 V to produce a constant heat input of 626.09 J/mm. An automatic SAW machine with Model Type: DX3-301, and Frequency: 50 Hz was used. And based on ASTM standard, tensile and hardness samples were prepared from the as-received and HAZs. Tensile and hardness measurements were made. Also, specimens for microscopy studies were prepared from the HAZ and as-received samples. From the results, microstructures of the HAZs revealed varied volume fraction of austenite and ferrite phases and grain sizes, and at austenite and ferrite grain boundaries, chromium carbide formation and precipitation were observed. The weld joint produced with 308 L electrode revealed optimum UTS value and YS value of 475 and 325 respectively. While weld joint produced with 316 L electrode has superior ductility of value 41%. Irrespective of the types of electrode used, the as-received sample revealed superior tensile properties over the weld joints. Also, optimum hardness value of 45.7 HRA was obtained with 308 L. Hardness value of the as-received sample was higher than that of HAZ samples.

Quantitative Evaluation of Secondary Phases in a Weld Joint Made of COST F and FB2 Creep Resistant Steels

Conventional (CCT) and accelerated (ACT) creep tests of a weld joint made of COST F and COST FB2 steels were carried out over a temperature range from 550 °C to 650 °C. Fracturing of the crept specimens was located in the heat affected zone (HAZ) of the F steel. Two specimens were selected after CCT and ACT for quantitative evaluation of the precipitates and compared to the weld joint in as-received conditions. Scanning and transmission electron micrographs were used to measure the precipitate size. Both methods were compared and the accuracy of the results was discussed. It was concluded that ACT can simulate the precipitation of chromium carbides and structure recovery during long term creep exposures. However, precipitation of Laves phase during CCT was not recorded after ACT. Therefore, it is difficult to use ACT in this experiment for estimating the long term creep strength.

Optimization of the Double Loop Electrochemical Potentiokinetic Reactivation Method for Detecting Sensitization of Nickel Alloy 690

Nickel alloy 690 (UNS N06690) is one of the current choices for nuclear power plant steam generator tubing. The severity of certain stress corrosion cracking submodes in Ni-Cr-Fe alloys, such as Alloy 690, may be a function of the alloy sensitization condition. Sensitization results from chromium carbide precipitation at or near grain boundaries when the alloy is exposed to temperatures from 500°C to 800°C, which causes local chromium depletion in the adjacent matrix. The objective of this work was to study the parameters that control the double loop electrochemical potentiokinetic reactivation (DL-EPR) method and to develop an optimized test routine for Alloy 690. Alloy 690 specimens were tested under different thermal treatments, tailored to obtain a variety of concentrations of chromium in solid solution near the grain boundary (GB) region. The DL-EPR method was applied to Alloy 690 in solutions with different concentrations of sulfuric acid and potassium thiocyanate, at 30°C and 50°C. The optimal condition for detecting sensitization of Alloy 690 was 0.5 M H2SO4 + 0.001 M KSCN, at 30°C. Optical micrographs confirmed an intergranular type of attack under thermally aged (sensitized) conditions. The ratio of the reactivation to the activation peak intensities (Ir/Ia) obtained in the DL-EPR test agreed well with mass loss obtained in boiling nitric acid solution with Cr(VI) additions (modified Huey test). Chromium depletion profiles near the GB of thermally treated specimens under different conditions were measured with the energy dispersive x-ray spectroscopy technique, performed on a transmission electron microscope. A model available in the literature was fitted to those results. The Ir/Ia ratio for Alloy 690, obtained when using the optimal DL-EPR testing conditions, correlated well with the Cr depletion zone width and depth near the GB.

Dependence of corrosion properties of AISI 304L stainless steel on the austenite grain size

AbstractThe corrosion resistance of austenitic stainless steels is known to be hampered by the loss of chromium available for passive surface layer formation as a result of chromium carbide precipitation at austenite grain boundaries during annealing treatments. Although high-temperature annealing can promote carbide dissolution leading to better corrosion resistance, grain coarsening also results, which would lead to poorer mechanical properties. Processing methods to achieve both good corrosion resistance and mechanical properties are thus highly desirable for austenitic stainless steels. In the present study, we show that the corrosion resistance of AISI 304L stainless steel can be improved by grain refinement into the ultrafine-grained regime. Specifically, samples with different austenite grain sizes in the range of 0.65–12 μm were studied by potentiodynamic polarization and electrochemical impedance spectroscopy tests in a 3.5 wt.% NaCl solution. All samples showed a typical passive behavior with similar corrosion potential, but the corrosion current density decreased significantly with decreasing grain size. The results show that the sample with the finest grain size had the best corrosion resistance due to a higher resistance of the passive layer to pitting attacks. This study indicates that grain refinement which improves mechanical properties can also significantly improve the corrosion resistance of AISI 304L stainless steel.

Understanding the microstructure-properties relationship of low-temperature carburized austenitic stainless steels through EBSD analysis

The present article is dedicated to the microstructural characterization of the surface layer of two different austenitic stainless steels, 304L and 904L, subjected to a low-temperature carburizing process (Kolsterising®, Bodycote) and a subsequent annealing at high-temperature. The carburizing treatment forms a hard expanded austenite in both materials. However, thermal decomposition occurs at high temperatures through precipitation of chromium-carbides, hence compromising the surface hardness of the treated materials.The purpose of this paper is to explore the potential applicability of electron backscatter diffraction (EBSD) technique to reveal the correlation between phase transformation and hardness. First of all, EBSD was used to create kernel average misorientation (KAM) maps of the modified surface layers to identify the internal strains. Moreover, the preferential sites for precipitation of chromium-compound during annealing were identified. We prove here that EBSD can provide useful information to distinguish the main hardening mechanisms within modified surface layers at different annealing conditions. When combined with nano-indentation, X-ray diffraction (XRD) and glow discharge optical emission spectrometry (GDOES), an effective bridge between macro and microanalysis can be obtained. Solid solution hardening was found to be the dominant mechanism in as-carburized materials, with pre-existing strain promoting a higher supersaturation. In the annealed materials, the alloy composition and surface finish can also dictate the preferential sites of precipitation and can therefore affect the residual hardening.

Microstructure and Mechanical Properties of Dissimilar Friction Stir Spot Welding Between St37 Steel and 304 Stainless Steel

In the present study, St37 low-carbon steel and 304 stainless steel were welded successfully, with the thickness of 2 mm, by a friction stir spot welding process carried out at the tool dwell time of 6 s and two different tool rotational speeds of 630 and 1250 rpm. Metallographic examinations revealed four different zones including SZ and HAZ areas of St37 steel and SZ and TMAZ regions of 304 stainless steel in the weld nugget, except the base metals. X-ray diffraction and energy-dispersive x-ray spectroscopy experiments were used to investigate the possible formation of such phases as chromium carbide. Based on these experiments, no chromium carbide precipitation was found. The recrystallization of the weld nugget in the 304 steel and the phase transformations of the weld regions in the St37 steel enhanced the hardness of the weld joint. Hardness changes of joint were acceptable and approximately uniform, as compared to the resistance spot weld. In this research, it was also observed that the tensile/shear strength, as a crucial factor, was increased with the rise in the tool rotational speed. The bond length along the interface between metals, as an effective parameter to increase the tensile/shear strength, was also determined. At higher tool rotational speeds, the bond length was found to be improved, resulting in the tensile/shear strength of 6682 N. Finally, two fracture modes were specified through the fracture mode analysis of samples obtained from the tensile/shear test consisting of the shear fracture mode and the mixed shear/tensile fracture mode.

Damage associated with the interaction between hydrogen and microstructure in a high sulfur 17-4PH steel for studs

A great number of failure cases of precipitation hardenable 17-4PH steels have been described in the literature. Structural components working in harsh environments failed due excessive hardness. A maximum hardness of 33 HRC was specified for service in aggressive media. However, high impurity levels and chromium carbide precipitation may cause failure, even when the correct final heat treatment is applied and the hardness is below the maximum limit. This fact is investigated in this work, where a 17-4PH steel with 0.027%S was evaluated after hydrogen cathodic charging. Specimens with three different commercial heat treatments were evaluated. Specimens with treatments A (solution treated) and H900 (aged for the peak of hardness) showed inter and transgranular microcracks, pores and cracks associated with sulfide inclusions. Specimen with treatment H1150 (overaged), with low hardness, experienced intergranular attack and pores nucleation in the inclusions. The high sulfur steel was also more susceptible to pitting corrosion, which can also contribute to the hydrogen embrittlement process in high chloride solutions. It is recommended to limit the sulfur and other impurities contents to values lower than those specified by the usual standards.

Influence of deep cryogenic treatment on the microstructure and properties of AISI304 austenitic stainless steel A-TIG weld

Appropriate deep cryogenic treatment can improve comprehensive mechanical properties of the AISI304 austenitic stainless steel activating flux tungsten inert gas (A-TIG) welds. The microstructure of the welds before and after deep cryogenic treatment was all austenite with a small amount of δ-ferrite. The vermiform ferrite + austenite distributed in the whole weld, but the lath-shaped ferrite + austenite mixed components only distributed in the centre of the weld. The phases in the two welds were all Cr–Ni–Fe–C and Fe–Ni solid solutions, ferric carbide (i.e. Fe3C) and chromic carbides (i.e. Cr23C6 and Cr7C3). After deep cryogenic treatment, the grain size of the weld was decreased a certain of degree, and the carbide phase content was increased. The strength and micro-hardness of the weld joints were increased due to the grain refinement. The intergranular corrosion resistance of the weld was reduced because the precipitation of chromium carbides at the austenite grain boundary.

Effect of annealing conditions on in situ formation of Cr2O3 diffusion barrier

The previous investigation suggested the approach for an in situ formation of Cr2O3 diffusion barrier by annealing the cold-sprayed Ni coatings on 310SS. In this paper, the influences of annealing conditions on the growth kinetics of Cr2O3 and substrate microstructure were investigated. Results show that Cr2O3 formed at the selected annealing temperatures of 850, 900 and 950°C for different durations of 4, 8 and 20 h. Increasing temperature enhanced the growth kinetics of Cr2O3 and the Mn content in the oxide layer. The annealing process for the growth of Cr2O3 improves the coating adhesion compared to the as-deposited coating. However, annealing at 950°C resulted in the precipitation of chromium carbides and enhanced the element inter-diffusion across the substrate/coating interface.

Microstructure and properties of 1Cr12Ni2WMoVNb (GX-8) steel bored barrels with and without QPQ treatment

Bored barrels are one of the primary parts of small arms. In order to enhance the service life of bored barrels, specimens of 1Cr12Ni2WMoVNb (GX-8) steel bored barrels were treated by QPQ salt bath at temperatures 530°C for 1–4h, followed by the same post-oxidation process with heating temperature of 440°C for 1h. QPQ nitrided layers were formed on the surface of GX-8 steel after QPQ salt bath and the optimized treatment time was determined to be 1h. Specimens with and without QPQ-1h were studied by micro-hardness test, SEM, wear resistance test, Tafel polarization curves, salt spray corrosion and ball firing. Layers formed during QPQ-1h contains a mixture of magnetite (Fe3O4), Fe1-XO, CrN, ε phase, γ′-Fe4N and α-Fe phases. This layer leads to significant improvement in wear resistance as a result of remained Fe3O4 film, while the fine distribution of nitride precipitates introduces high hardness and large residual microstresses. The formation of thick Fe3O4 film results in an improvement in the corrosion resistance of steel. After the breakdown of Fe3O4 film, the absence of chromium in solid solute by precipitation of chromium nitrides and carbides leads to significant decreases in corrosion resistant. The QPQ-1h process results almost one times increases in the lifetime of bored barrels, as a result of wear resistance improvement after QPQ treatment.

Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in high chromium steel

ABSTRACTIn an attempt to investigate Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in high chromium steel, Cs–Te corrosion out-pile tests of two 9Cr steels with different distributions of chromium carbide were carried out at 975 K for 100 h, and their corrosion depths were compared. The corrosion is obviously more advanced in a specimen which has grain boundary carbide than in the one that does not. A considerable reason of the result is that the carbide distributed at grain boundaries promoted the corrosion reaction and the corrosion extended along the grain boundary. This is the first case in which the Cs–Te corrosion depth dependence on distribution of chromium carbide precipitation in Fe–Cr steel is clarified experimentally.

Failure of Stainless Steel Welds Due to Microstructural Damage Prevented by In Situ Metallography

Abstract In stainless steels, microstructural damage is caused by precipitation of chromium carbides or sigma phase. These microconstituents are detrimental in stainless steel welds because they lead to weld decay. Nevertheless, they are prone to appear in the heat affected zone (HAZ) microstructure of stainless steel welds. This is particularly important for repairs of industrial components made of austenitic stainless steel. Non-destructive metallography can be applied in welding repairs of AISI 304 stainless steel components where it is difficult to ensure that no detrimental phase is present in the HAZ microstructure. The need of microstructural inspection in repairs of AISI 304 is caused because it is not possible to manufacture coupons for destructive metallography, with which the microstructure can be analyzed. In this work, it is proposed to apply in situ metallography as non-destructive testing in order to identify microstructural damage in the microstructure of AISI 304 stainless steel welds. The results of this study showed that the external surface micrographs of the weldment are representative of HAZ microstructure of the stainless steel component; because they show the presence of precipitated metallic carbides in the grain boundaries or sigma phase in the microstructure of the HAZ.

Improving the toughness of thermally sprayed Cr 3C 2-NiCr hardmetal coatings by laser post-treatment

Thermally sprayed hardmetal coatings typically exhibit a pronounced embrittlement of the metallic binder matrix due to carbide dissolution during the deposition process. This characteristic renders the coatings prone to wear by brittle fracture, which lays at the core of many relevant wear phenomena. The present work introduces laser post-treatments as a suitable means of “curing” the microstructure of high-velocity oxygen-fuel sprayed Cr3C2-NiCr coatings from this spray-process induced deterioration. While operating well below the remelting threshold, the essential impact of the laser-generated heat flux is precipitation of secondary chromium-carbides from the supersaturated binder matrix. The concomitant transition from a solid-solution to a precipitation-hardened phase significantly increases the fracture toughness of the binder matrix and renders the coatings more resistant against mechanical wear. In the present work, the microstructural modifications of the coatings upon laser post-treatments were investigated by means of scanning electron microscopy and microhardness probing, and the corresponding impact on the abrasive wear resistance was tested under both high- and low-stress conditions. Major improvements of the high-stress abrasive wear resistance by up to a factor of three were determined and discussed in the context of the microstructure of the wear scars.