Cr Mo Low Alloy Steel Research Articles

Influence of temperature and exploitation time on hardness and micro-structure of a welded joint in a reactor mantle

This paper presents the analysis of the influence of temperature and exploitation time on the cross-sectional hardness and micro-structure changes of characteristic zones of a welded joint made of low-alloyed Cr-Mo steel A-387 Gr. B. Exploited parent metal is a part of a reactor mantle which was working for over 40 years and is in the damage repair stage, i.e. part of its mantle is being replaced with new material. Cross-sectional hardness of a butt-welded joint was measured and macroscopic investigation of the welded joint and microstructural analysis of the parent material, weld metal and the heat affected zone were performed. The comparison of parameters obtained for characteristic zones of a welded joint provides a way to meausre the justifiability of the selected welding technology.

Bainitic reaction and microstructure evolution in two normalized and tempered steels designed for service at elevated temperatures

Abstract In the present work conventional heat treatment like normalizing (bainitic microstructure) and tempering of the alloys has been performed. The materials used in this study were two steels, one the laboratory prepared experimental low alloy Cr-Mo steel in comparison to typical commercial 10CrMo9-10 steel. The determined carbon concentrations of the residual austenite at the different temperatures of bainite transformation supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. It was found that bainitic reaction has stopped when average carbon concentration of the untransformed austenite is close to the T0 line and supports formation of bainitic ferrite by a shear mechanism, since diffusionless transformation is not possible beyond the T0 curve. Normalized samples were air cooled down to room temperature before tempering at various temperatures in the range of 500-750°C. Samples have been austenitized at 980°C for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750°C for 1 hour. After heat treatment, the assessment in the microstructure and phase precipitation was made using the samples prepared for metallographic and transmission electron microscope (TEM) on thin foils analysis. Quantitative X-ray analysis was used to determine the retained austenite content after heat treatment like normalizing and tempering and the total volume fraction of the retained austenite was measured from the integral intensity of the (111)γ and (011)α peaks. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C precipitates. The magnitude of these changes depended on the temperature of tempering.

Comprehensive Understanding of Ductility Loss Mechanisms in Various Steels with External and Internal Hydrogen

Hydrogen-induced ductility loss and related fracture morphologies are comprehensively discussed in consideration of the hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316, 316L), high-strength austenitic stainless steels (HP160, XM-19), precipitation-hardened iron-based super alloy (A286), low-alloy Cr-Mo steel (JIS-SCM435), and low-carbon steel (JIS-SM490B). External hydrogen is realized by a non-charged specimen tested in high-pressure gaseous hydrogen, and internal hydrogen is realized by a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile tests (SSRT) of the materials with external or internal hydrogen could be comprehensively categorized into five types: hydrogen-induced successive crack growth, ordinary void formation, small-sized void formation related to the void sheet, large-sized void formation, and facet formation. The mechanisms of hydrogen embrittlement are broadly classified into hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). In the HEDE model, hydrogen weakens interatomic bonds, whereas in the HELP model, hydrogen enhances localized slip deformations. Although various fracture morphologies are produced by external or internal hydrogen, these morphologies can be explained by the HELP model rather than by the HEDE model.

Effect of Elemental Sulfur and Sulfide on the Corrosion Behavior of Cr-Mo Low Alloy Steel for Tubing and Tubular Components in Oil and Gas Industry.

The chemical degradation of alloy components in sulfur-containing environments is a major concern in oil and gas production. This paper discusses the effect of elemental sulfur and its simplest anion, sulfide, on the corrosion of Cr-Mo alloy steel at pH 2 and 5 during 10, 20 and 30 h immersion in two different solutions. 4130 Cr-Mo alloy steel is widely used as tubing and tubular components in sour services. According to the previous research in aqueous conditions, contact of solid sulfur with alloy steel can initiate catastrophic corrosion problems. The corrosion behavior was monitored by the potentiodynamic polarization technique during the experiments. Energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) have been applied to characterize the corrosion product layers after each experiment. The results show that under the same experimental conditions, the corrosion resistance of Cr-Mo alloy in the presence of elemental sulfur is significantly lower than its resistance in the presence of sulfide ions.

Numerical and experimental investigations of the anisotropic transformation strains during martensitic transformation in a low alloy Cr-Mo steel 42CrMo4

Abstract Hot forming as a coupled thermo-mechanical process comprises of numerous material phenomena with a corresponding impact on the material behavior during and after the forming process. Within the subsequent heat treatment, possible rapid cooling of the hot formed parts leads to the diffusionless decomposition of austenite into martensite. In this context, in addition to the elastic, plastic and linear thermal strain components, complex isotropic as well as anisotropic transformation strains can occur. Irreversible anisotropic transformation strains account for the plastic deformation at the phase boundary between the emerging and the parent phase and are related to the transformation induced plasticity (TRIP or TP) phenomena. Moreover, TRIP strains can be reduced or amplified by varying the current stress state. These phenomena significantly contribute to the final residual stress state and may be responsible for the cost-intensive component defects arising due to thermal shrinkage. This study aims at developing an FE-based material model in order to describe and quantitatively visualize stress dependence of the transformation induced anisotropic strains for a typical forging steel 42CrMo4. The developed material model as well as the aspects of its implementation in a commercial FE-system (Simufact.forming) is presented. Consequently, the discussed material model is tested by comparison of experimental and numerical results with respect to resulting dilatation under various stress states.

Effect of rare earth cerium and impurity tin on the hot ductility of a Cr-Mo low alloy steel

The specimens of 1Cr-0.5Mo low alloy steel, undoped, Sn-doped and Ce+Sn-doped, were austenitized at 1300 °C and then cooled down to different temperatures in the range of 700–1050 °C, followed by tensile tests with the aid of a Gleeble machine. The reduction of area (RA) obtained from the test was employed to evaluate the hot ductility of the steel. The tested specimens were characterized using different techniques. Minor Sn could considerably reduce the RA values of the steel in the whole temperature range, and the hot ductility curve could be widened and deepened. Nevertheless, minor Ce could improve the hot ductility of the Ce+Sn-doped steel by fully suppressing the Sn-induced hot ductility deterioration. FEGSTEM microanalysis showed that the Sn or Ce and Sn atoms segregated to austenite grain boundaries in the Sn-doped or Ce+Sn doped specimens. The detrimental effect of Sn on the hot ductility could be attributed mainly to the segregation of Sn as it could decrease the grain boundary cohesion and in turn enhanced the grain boundary sliding and cracking. However, this detrimental effect of Sn could be counteracted by the segregation of Ce which could increase the grain boundary cohesion and in turn restrained the grain boundary sliding and cracking. Accordingly, a minor addition of rare earth Ce could be an effective method of suppressing the detrimental effect of impurity elements on the hot ductility of a Cr-Mo low alloy steel.

Sensitivity analysis of a 2D cohesive model for hydrogen embrittlement of AISI 4130

A finite element cohesive zone model able to reproduce the effect of hydrogen exposition on the fracture toughness properties of a low alloy Cr–Mo steel, AISI 4130, is here presented. The model simulates the embrittlement effect of hydrogen by decreasing the constitutive law of cohesive elements that reproduces the crack propagation behaviour of the material according to the calculated total hydrogen concentration.The model is validated by comparison with experimental data. Finally, a sensitivity analysis of the model that mimics both different material parameters and environment conditions proves its efficiency as a predictive tool for the mechanical response of the material.

Influence of temperature and exploitation period on fatigue crack growth parameters in different regions of welded joints

-The influence of exploitation period and temperature on the fatigue crack growth parameters indifferent regions of a welded joint is analysed for new and exploited low-alloyed Cr-Mo steel A-387 Gr. B. Theparent metal is a part of a reactor mantle which was exploited for over 40 years, and recently replaced with newmaterial. Fatigue crack growth parameters, threshold value Kth, coefficient C and exponent m, have beendetermined, both at room and exploitation temperature. Based on testing results, fatigue crack growth resistancein different regions of welded joint is analysed in order to justify the selected welding procedure specification

Very high cycle fatigue properties of Cr–Mo low alloy steel containing V-rich MC type carbides

The present study was attempted to investigate the very high cycle fatigue (VHCF) properties of Cr–Mo low alloy steel containing V-rich MC carbides by using ultrasonic fatigue testing machine. All the specimens were quenched and tempered at 500°C and 600°C and part of them were electrochemical hydrogen charged. Mainly owing to the precipitation of fine V-rich MC particles at 600°C, the introduced hydrogen content of the 600°C tempered specimen is much higher than that of the 500°C tempered specimen. Both the hydrogen-charged and uncharged specimens revealed internal fractures in the long-life region and no notable fracture surface difference were observed. Both fatigue strength and fatigue strength ratio increase with increasing tempering temperature from 500°C to 600°C for both the hydrogen-charged and uncharged specimens, though the 600°C tempered specimen possessed higher hydrogen content. Increasing hydrogen content was definitely detrimental to the fatigue properties. However, compared to other high strength steels investigated previously, the detrimental effect of hydrogen on fatigue properties of the tested steel was comparatively small, which was ascribed mainly to the presence of fine V-rich MC carbides acting as hydrogen trapping sites. In is concluded that appropriate controlling of fine V-rich MC carbides is a promising way to alleviate the detrimental effect of hydrogen on VHCF properties of high strength steels.

Fatigue behavior of hydrogen pre-charged low alloy Cr–Mo steel

Aim of the present work is to study the mechanical behavior in presence of hydrogen of a quenched&tempered low alloy Cr–Mo steel, AISI 4130, used for hydrogen storage and transportation. The sensitivity of steels to hydrogen embrittlement is generally determined by means of two kinds of experimental procedures to hydrogen charge the specimens: gaseous hydrogen in environmental chambers or autoclaves or H2S solution (recommended by NACE standard). In this work experimental tests are carried out on specimens pre-charged by means of an electrochemical method. A campaign of toughness and fatigue crack growth tests is performed on specimens without and with pre-charged hydrogen. The results are compared and discussed with other, from literature, obtained by following more traditional testing procedures.By this comparison the adopted experimental procedure, that is safer and easier, seems to provide useful information.

Nanostructured Y2O3 plasma coating for high temperature oxidation protection

This paper investigates the effect of nano-size and micron-size Y2O3 powder-coated specimen on high-temperature oxidation resistance of low-alloy Cr–Mo steel (2.25 Cr-1 Mo steel). Both are powder coated by plasma spray coating method. The oxidation rate and reaction kinetics are studied and the post-corroded scales are characterized in Field Emission Scanning Electron Microscope and X-ray Diffraction. The results clearly indicate that nanosize Y2O3-coated (nanostructured) specimen improves the high-temperature oxidation resistance in comparison to the micron-size Y2O3-coated specimen. The detail mechanism of the oxidation of both the coated specimen is discussed in detail in this paper.

Existence form of boron in dissimilar weld metals of low-alloy steel with boron-bearing high-chromium steel during post-weld heat treatment†

Recently, higher steam inlet temperature and pressure has been required for fossil power plants in order to increase the power generation efficiency. Therefore, boron-bearing high-chromium (high-Cr) steels with higher long-term creep strength are applied to structure materials in these plants. Dissimilar (metal) weld joints between a boron-bearing high-Cr steel and a low-alloy steel are applied for various parts of boilers and other equipment in the plants. Caution has to be paid to maintain the quality of these weld joints because boron is diffused to the low-alloy steel weld deposits by dilution from the boron-bearing high-Cr base metal. However, the existing form of boron and the effect of boron on reheat cracking susceptibility during post-weld heat treatment (PWHT) have not been surveyed in the previous literature. Therefore, using Cr–Mo low-alloy steels with varied contents of B and Cr, reheat cracking susceptibility of the weld metal was evaluated, and the precipitation behaviour of the carbides during PWHT was observed. From these results, the existing form of boron in boron-bearing Cr–Mo steel weld metals is discussed in this study. Reheat cracking susceptibility clearly increased with boron addition, whereas it decreased with increasing chromium content. It was shown by thermo-dynamic calculation that possible existing forms of boron were of four types, namely BN, M2B, M23(C, B)6 and dissolved boron. The amount of M23C6-type carbides was increased with increasing chromium content. On the basis of these results, it was presumed that the amount of dissolved boron in boron-bearing low-chromium steel weld metals was larger than in those with high-Cr. It was suggested that the larger amount of dissolved boron enhanced the strength difference between the matrix and the grain boundary in low-chromium steels, hence it led to higher susceptibility to reheat cracking.

INFLUENCE OF MICROSTRUCTURE ON IMPACT TOUGHNESS OF G18CrMo2-6 STEEL DURING TEMPERING

Low alloy CrMo steels are widely used for high temperature applications in the power and petrochemical industries as structural materials. The most crucial mechanical properties for these steels are sufficient strength to withstand internal pressure and high impact toughness to assure safety from momentary shock owing to unexpected accidents. Particularly, impact toughness deteriorates because of continuous high temperature during the operation of a high pressure vessel and embrittlement can occur. Thus, the use of steels with high impact toughness is extremely important to guarantee sufficiently the safe operation of the nuclear reactor. Usually low alloy CrMo steels enter service after the normalized and tempered treatment or annealed treatment with a mixed ferritebainite or full bainite microstructure. The G18CrMo2-6 steel is one of the most popular materials with the mixed microstructures of ferrite and bainite for the pressure vessel in nuclear industry due to its good impact toughness,high strength and good creep resistance. In this work, the influence of microstructures, including the parent phases and precipitates, on the impact toughness is investigated in detail. The experimental results show that the constituent of the parent phases, namely the ferrite, pearlite or bainite, is not the reason resulting in the ultra-low impact energy. The microstructure characterization implies that the morphology and the distribution of precipitates play the key role in controlling the impact toughness of the G18CrMo2-6 steel. The lower tempering temperatures result in the blocky martensite/austenite(M/A) island and lathy M3C carbides with the large particle size. The finely granular M3C carbides with the uniform distribution on the bainite matrix can be found at the higher tempering temperatures. As the tempering temperature increased, the Charpy absorbed energy at room temperature increased. After the tempering below 600 ℃, Charpy absorbed energy has the ultra-low value of 17 and 29 J. Generally speaking,the weak softening of matrix during the lower tempering temperature increases the accumulative residual stress at particle-ferrite interface. The other important factor should be attributed that the blocky M/A island and lathy M3C carbides result in the lower critical fracture stress of a particle-ferrite interface.

Enhanced hot ductility of a Cr–Mo low alloy steel by rare earth cerium

The hot ductility of a 1Cr–0.5Mo low alloy steel is investigated over a temperature range of 700–1050°C using a Gleeble thermomechanical simulator in conjunction with various characterization techniques. The steel samples undoped and doped with cerium are heated at 1300°C for 3min and then cooled with a rate of 5Ks−1 down to different test temperatures, followed by tensile deformation until fracture. The results show that the hot ductility of the steel, evaluated by the reduction in area, can be substantially enhanced by a minor addition of cerium, especially in the range 800–1000°C. In the austenite–ferrite dual-phase region, cerium may delay the formation of proeutectoid ferrite layers along austenite grain boundaries, thereby increasing the hot ductility of the steel. In the single austenite region, grain boundary segregation of cerium may increase the grain boundary cohesion, toughening the steel and thus raising the resistance to grain boundary sliding as well as promoting dynamic recrystallization. Consequently, the hot ductility of the steel is enhanced.

A quantitative characterisation of porosity in a Cr–Mo sintered steel using image analysis

A systematic characterisation of the porosity in the bulk and surface regions of a sintered Cr–Mo low alloy steel was carried out using image analysis. Fractional porosity in the different regions varies, mainly due to the scatter of the maximum pore size. A higher porosity is found in the bulk region and lower porosity in the regions that contact the die surface during compaction. The maximum pore size is larger in the bulk region than in the surface layers. The large pores are more irregular. With increasing green density, both the fractional porosity and maximum pore size decrease. The fraction of load bearing section in the bulk and surface regions was calculated from fractional porosity and the shape factor of the pores and compared in the different regions. The load bearing section fraction and the maximum pore size were used to predict tensile and fatigue resistance for different densities. The data from the characterisation of the bulk images can predict tensile strength. For fatigue, where the crack nucleates in the surface regions, the use of bulk data underestimates the fatigue resistance.

Nitriding Effect on Corrosion Fatigue Strength of Low Alloy Steel in 1% HCl Aqueous Solution

In this paper the emphasis is focused upon nitriding effect on corrosion fatigue strength of Cr-Mo low alloy steel in 1% HCl aqueous solution.Corrosion fatigue strength enhancement of Cr-Mo low alloy steel by nitriding is discussed on the basis of the corrosion fatigue testing results on gas and ion nitrided Cr-Mo low alloy steel plate specimen with 3.5mm thickness in 1%HCl aqueous solution. It can be concluded that residual compressive stress distributed on the nitrided specimen surface caused improvement of corrosion fatigue strength of Cr-Mo low alloy steel.

Effect of microstructure evolution on strength and impact toughness of G18CrMo2-6 heat-resistant steel during tempering

G18CrMo2-6 steel is one kind of low alloy CrMo steel with ferrite/bainite phase constituent. It is widely adopted in the large casting parts of pressure vessels. This paper aims at investigating evolution of G18CrMo2-6 steel microstructure, tensile strength and impact toughness during the tempering (680°C). The tempered microstructure characterization shows that the tempering time mainly affects the precipitation of three kinds of carbides, naming MC, M3C and M23C6. Both tensile strength and impact toughness do not obey the monotonic evolution with the tempering time but the complex changes. The shift of mechanical properties results from the interaction among matrix phases and the evolution of three different kinds of carbides. The softening of matrix due to the dislocation recovery and the decrease of the amout of carbon and other alloy elements in matrix lattice due to the precipitation of M23C6 cause the decreasing tensile strength for the whole tempering except for that an increase during the short-term tempering controlled by the precipitation of MC and the spheroidization and refinement of M3C carbide. On the other hand, both the softening of matrix and the spheroidization and refinement of M3C carbide are responsible for the increasing impact toughness. However, the precipitation and coarsening of M23C6 at ferrite grain boundaries during long-term tempering results in the sharp deterioration of impact toughness. It attributes to the larger grain boundary carbides result in the lower critical fracture stress of a carbide-ferrite interface.

Tin-induced hot ductility degradation and its suppression by phosphorus for a Cr–Mo low-alloy steel

Using a Gleeble 1500D thermomechanical simulator, the samples of 1Cr–0.5Mo low-alloy steel, undoped, doped with Sn and doped with P+Sn, were heated at 1300°C for 3min and then cooled at a rate of 5°Cs−1 down to different temperatures between 700 and 1050°C, followed by tensile deformation until fracture. The results show that the hot ductility of the steel, evaluated by the reduction in area, can be substantially deteriorated by Sn, but it is considerably enhanced by P+Sn, especially in the range 850–1000°C. Therefore, the detrimental effect of Sn on the steel hot ductility is completely suppressed by the beneficial effect of P. Grain boundary chemistry measurements indicate that the hot ductility deterioration by Sn may mainly arise from its grain boundary segregation. Nanohardness measurements and microscopy demonstrate that, in the austenite–ferrite region, the hot ductility enhancement by P+Sn is mainly caused by the P-induced solid solution hardening of ferrite layers formed along austenite grain boundaries, and in the austenite region, by the P-promoted dynamic recrystallization of austenite.

Antimony-induced embrittlement in welding heat-affected zones in a Cr–Mo low-alloy steel

Heat-affected zones (HAZs) were simulated with a peak temperature of 1320 °C at different welding heat inputs for a Cr–Mo low-alloy steel. The ductile-to-brittle transition temperature (DBTT) of the HAZs increased with increasing heat input. When 0.05 wt.% Sb was added to the steel, the DBTTs of the HAZs became 1 and 10 °C higher than those for the undoped steel at heat inputs of 36 and 60 kJ/cm, respectively. Sb segregation to austenite grain boundaries during thermal cycling was found to be mainly responsible for the DBTT increase.

Grain boundary segregation of antimony in heat affected zones during welding thermal cycles for a Cr–Mo low alloysteel

Grain boundary segregation of Sb in heat affected zones during welding thermal cycles is examined using field emission gun scanning transmission electron microscopy (FEGSTEM) for an Sb-doped Cr–Mo low alloy steel. A Gleeble-1500 thermomechanical simulator is used to simulate the welding heat affected zone with a peak temperature of 1320°C for heat inputs of 36, 60 and 100kJ/cm. The results show that non-equilibrium segregation of Sb occurs during thermal cycles and critical welding heat input is found to be between 36kJ/cm and 100kJ/cm, at which the maximum segregation of Sb may be obtained during corresponding thermal cycling. If possible, this critical heat input should be avoided in engineering practice.