NiCrMoV Steel Research Articles

SCC fracture location shifting affected by stress-controlled fatigue damage of NiCrMoV steel welded joints

The influence of stress-controlled fatigue (SCF) damage on the fracture location shifting of welded joints for NiCrMoV steels in the corrosive solution of 3.5wt.% NaCl is systematically investigated by electrochemical measurements, immersion tests, slow strain rate tensile (SSRT) tests, and fractural morphology observations in the present work. The results show that both the yield strength (YS) and the ultimate tensile strength (UTS) of welded joints are increased after the SCF tests. The susceptibility of stress corrosion cracking (SCC) and galvanic corrosion of the SCF damaged welded joints are higher than that of the as-received specimens. In addition, the fracture locations of welded joints after SSRT tests in the corrosive solution are shifted with the degree of SCF damage. Different degrees of fatigue damage are accumulated in the three zones of welded joints during the cyclic deformation, which could change the competitive relation of the most susceptive SCC zone between the fusion zone and the middle of WM.

Crack branching behavior and amorphous film formation mechanism during SCC expanding test for multi-layers weld metal of NiCrMoV steels

Multi-layer weld metal of NiCrMoV steels used in nuclear rotor was adopted to perform stress corrosion cracking (SCC) test at 180 ℃, which indicated the crack growth rate in 3.5% NaCl solution was lower than that in pure water. The results obtained from high resolution transmission electron microscopy (HRTEM) analysis revealed that the combined effect of crack branching behavior and amorphous film made contributions to the slower crack growth rate. The aggressive Cl− ions invaded grain in multi-directions that was promoted by dislocation motion, facilitating the main crack to bifurcate. Besides, nanocrystals also enhanced branching behavior by deflecting the crack growth path. An amorphous film forming in crack evolved from the disorder of atom arrangement produced by dislocation accumulation and grew stably with the supplement of carbon atom generated in selective corrosion. The complete amorphous film was absent connected path for migration of aggressive anions, thus weakened the driving force for crack propagation and improved resistance to localized corrosion attack.

Microstructure and Mechanical Properties of NiCrMoV Steel Fabricated by Double-Electrode Gas Metal Arc Additive Manufacturing

Microstructure and Mechanical Properties of NiCrMoV Steel Fabricated by Double-Electrode Gas Metal Arc Additive Manufacturing

A Comparison Study on the Strengthening and Toughening Mechanism between Cu-Bearing Age-Hardening Steel and NiCrMoV Steel.

Cu-bearing age-hardening steel has significant potential in shipbuilding applications due to its excellent weldability as compared to conventional NiCrMoV steel. Not much research has been carried out to analyze the differences in the mechanisms of strength and toughness between Cu-bearing age-hardening and NiCrMoV steel. Both steels were heat treated under the same conditions: they were austenized at 900 °C and then quenched to room temperature, followed by tempering at 630 °C for 2 h. The uniaxial tensile test reveals that the Cu-bearing age-hardening steel exhibits relatively lower strength but larger plasticity than NiCrMoV steel. The lower contents of Carbon and other alloying elements is one of possible reasons for these differences in mechanical properties. Transmission Electron Microscope observations show that two types of precipitates, Cr carbides and Cu-rich particles, exist in tempered Cu-bearing age-hardening steel. Cu-rich particles with sizes of 20–40 nm can inhibit the dislocation motion during deformation, which then results in dislocation pile ups and multiplication; this makes up the strength loss of Cu-bearing age-hardening steel and simultaneously improves its plasticity.

Microstructural Evolution along the NiCrMoV Steel Welded Joints Induced by Low-Cycle Fatigue Damage

The degradation of mechanical properties of materials is essentially related to microstructural changes under service loadings, while the inhomogeneous degradation behaviors along welded joints are not well understood. In the present work, microstructural evolution under low-cycle fatigue in base metal (BM) and weld metal (WM) of NiCrMoV steel welded joints were investigated by miniature tensile tests and microstructural observations. Results showed that both the yield strength and ultimate tensile strength of the BM and WM decreased after low-cycle fatigue tests, which were attributed to the reduction of dislocation density and formation of low-energy structures. However, the microstructural evolution mechanisms in BM and WM under the same cyclic loadings were different, i.e., the decrease of dislocation density in BM was attributed to the dislocation pile-ups along the grain boundaries, dislocation tangles around the carbides at the lower strain amplitudes (±0.3% or ±0.5%). Additionally, when the strain amplitude was ±8%, the dislocation density was further decreased by the formation of subgrains in BM. For WM, the dislocation density decreased with the increase of strain amplitude, which was mainly caused by the dislocation pile-ups along the grain boundaries and the formation of subgrains.

Fracture location transition in constant load tests of a NiCrMoV steel welded joint

The fracture location transition of the NiCrMoV steel welded joints induced by different tensile loads has been investigated through the constant load tests in 3.5 wt% NaCl at 180 °C. The mechanical properties of different zones in the welded joints were measured by the hardness measurement and localized tensile tests in air at room temperature. The corrosion properties were characterized by potentiodynamic polarization curve tests and scanning vibrating electrode technique (SVET) tests. The results reveal that the most susceptive corrosion zone and the weakest strength zone do not locate in same zone of the welded joint. This mismatch leads to the rupture locations of the welded joints change with the applied load in the constant load tests. For the applied load larger than or equal to 765 MPa, the rupture section locates in the middle of WM due to its weakest strength. But the fracture position shifts to the fusion zone for the applied load less than or equal to 760 MPa, which is attributed to stress corrosion cracking (SCC) originating from the pits under the combined effect of galvanic and stress-assisted corrosion.

Effect of Heat Treatment on the Microstructure and Wear Resistance of NiCrMoV Steel

The effect of an applied load within a 50-200 N range, speed of 10-45 mm/s, and sliding of 10-120 min on the wear behavior of a NiCrMoV low alloy steel quenched in air at 1123-1323 K, followed by tempering at 873 K, was investigated. The material’s microstructure was further studied. The wear was measured using a ball-on-disc type wear machine at room temperature under dry sliding conditions. The micromorphology, worn surface and debris were investigated using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results indicate that all the tempered specimens’ microstructure of the NiCrMoV low alloy steel consists of ferrite and carbide and that the grain size increases with an increase in quenching temperature. The friction coefficient and mass loss increased with an increase in the applied load, the sliding speed and time, as well an increase in quenching temperature. The friction coefficient and mass loss significantly increased at a load of 200 N. The worn surface of all the specimens revealed a discontinuous adherent layer. Furthermore, the microstrucure revealed that the specimens underwent a combination of delamination, plastic deformation, fatigue, and oxidation. The wear mechanism involves predominantly adhesion with a mixture mode of abrasive, adhesion, and oxidation.

Enhanced Galvanic Corrosion Phenomenon in the Welded Joint of NiCrMoV Steel by Low-Cycle Fatigue Behavior

The effect of microstructural changes on the corrosion of a NiCrMoV steel welded joint induced by low-cycle fatigue behavior in chloride solution has been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), potentiodynamic polarization curve tests and scanning vibrating electrode technique (SVET) tests. During the low-cycle fatigue tests, cyclic softening takes place in the three zones (base metal, BM/weld metal, WM/heat affected zone, HAZ) of the welded joint due to the decrease of dislocation density or the formation of low-energy structures. Compared to the as-received specimens, the corrosion potential of each zone in the low-cycle fatigue damage specimens moves to the positive direction and the corrosion current density decreases. The corrosion resistance of the individual zone increases, which is related to the reduction of dislocation density or the formation of low-energy structures. However, the difference of corrosion potential between base metal and weld metal in the low-cycle fatigue welded joints becomes lager, which means that the galvanic corrosion susceptibility of the low-cycle fatigue welded joint increases. This may be caused by the difference of microstructural changes in the various regions of welded joints during cyclic plastic deformation.

Effect of Long-term Service on Microstructure and Mechanical Properties of NiCrMoV Steel Welded Joint

The influences of prolonged service on microstructure evolution and mechanical properties of NiCrMoV steel welded joint in an ex-service welded steam turbine rotor were investigated. The welded rotor had been operated for 22 years since 1991. The specimens for the present study were taken from the location where the temperature was as high as 230°C. The optical microscope (OM) showed that even after long-term service, there were no obvious defects such as creep cavities, cracks found in the microstructure of the whole welded joint after such a long term service. The microstructure was uniform and no obvious grain coarsening was observed. However some black strip-shaped zones were found in base metal and heat affected zone (HAZ). The distribution of hardness across the welded joints showed no anomalies. The results of tensile strength and fracture toughness tests demonstrated that the welded joint still exhibited excellent. Mechanical performance after long-term service, indicating that the welding process of Shanghai Turbine Plant was reliable and stable. With the improvement of forging and welding qualities and improved heat treatment furnaces with more accurately controlled temperature, it is reasonable to assume that the current large low-pressure (LP) welded rotors are definitely safe to operate under similar service conditions for designed life.

Characterization and formation mechanism of periodic solidification defects in deep-penetration laser welding of NiCrMoV steel with heavy section

With the thickness increasing of steel, application of laser beam welding on heavy section has been arousing broadly interests. Deep-penetration laser welding was performed on thick plate of NiCrMoV steel, and the characteristics of welding defects were analyzed in terms of their formation and distribution. The periodic defects were found to occur frequently at the central longitudinal section of the bead-on-plate welds that have a certain distance from fusion line. The formation and distribution of these defects were deemed as unique for deep-penetration laser welding on heavy plate due to mismatch of welding parameters with weld profile. Defect susceptibility was raised firstly and reduced subsequently as the heat input decreased, and a large amount of periodic solidification defects should occur when penetration-to-width ratio (aspect ratio) is higher than 4.0. By means of detail characterization on defect by optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), the periodic defects were proved to be hot cracking defects induced by contact and rapid shrinkage of transverse columnar crystals in mid part of the welds before feeding in sufficient liquid. The formation of periodic solidification defects in welds was greatly related to weld profile and cooling rate limited by the heat dissipation conditions. The optimized parameters to avoid periodic solidification defects were determined for deep-penetration laser welding of this steel.

Microstructural Characteristics and Mechanical Properties of Low-Alloy, Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV- and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature, regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.

A comparison of microstructure and mechanical properties of low-alloy-medium-carbon steels after quench-hardening

The microstructural characteristics of three medium carbon steels, namely MnCrB, NiCrSi and NiCrMoV containing steels, have been investigated when the steels were hardened by quenching in water or oil from different austenitisation temperatures (i.e. 850, 900 and 950 °C). The microstructure was characterised using optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, and X-Ray diffraction technique, whereas the mechanical properties were measured by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The microscopy observations suggested a fully martensitic microstructure, whereas martensite was considerably finer in NiCrSi and NiCrMoV steels compared to MnCrB steel. Moreover, the NiCrSi and NiCrMoV steels showed significantly higher strengths and lower ductility than MnCrB steel. The results suggested that the small additions of alloying elements and different prior austenite grain sizes were mainly responsible for the observed microstructural and mechanical properties variations.

Investigation of local brittle zone in multipass welded joint of NiCrMoV steel with heavy section

Abstract

Effect of formic acid on corrosion behavior of 3.5NiCrMoV steel in the boiler water contained chloride ions

Effect of formic acid on corrosion behavior of 3.5NiCrMoV steel in the boiler water contained chloride ions

Pit evolution around the fusion line of a NiCrMoV steel welded joint caused by galvanic and stress-assisted coupling corrosion.

The corrosion of NiCrMoV steel welded joints is performed in an aqueous solution of 3.5 wt% NaCl at 180 °C in a container at a pressure of 0.8 MPa. The specimens in the shape of cylindrical tensile rods are immersed in the aqueous solution under the action of tensile stress in a range of 0 to 0.9 of the yield stress of the base metal. The experimental results suggest that there is macro-galvanic corrosion in the welded joint with the coarse-grained heat affected zone (CGHAZ) as anode due to the highest corrosion susceptibility of the CGHAZ. The CGHAZ has the highest positive current density in the welded joints as measured by the scanning vibrating electrode technique. The two-parameter Weibull distribution function, which is represented by the Weibull modulus and characteristic strength, is used to analyze the distribution of the depth of pits at different immersion times. Both the Weibull modulus and characteristic strength are calculated, and found to be dependent on the applied tensile stress. The values of the characteristic pit depth and the average pit depth reveal that there are two mechanisms controlling the corrosion of the NiCrMoV steel welded joints; one is galvanic corrosion, and the other is stress-assisted corrosion.

Some Features of the Influence of Titanium and Nitrogen Addition to NiCrMoV Steel

This paper reports a study of the addition effects of either titanium or titanium and nitrogen of steel grade DIN 56NiCrMoV7 on mechanical properties. Three steel grades were produced in 30 kg-induction furnace, one conforms the chemical composition of conventional 56NiCrMoV7 while the other two produced steels were microalloyed by either titanium or titanium and nitrogen. The produced cast steel grades were reheated to 1150°C and hold for 2 hours, followed by forging process. The forging process was carried out in temperature range 950°C - 1100°C. Solution treatment of hot forged steels was conducted at 880°C, 850°C followed by air and oil quenching, respectively. Quenched steel samples of different steel grades were tempered at different temperatures in the range of 300°C to 650°C for 45 min. The hardness variations after tempering of the two modified steels comparing with the conventional 56NiCrMoV7 steel were studied. Microadditions of titanium or titanium and nitrogen were found to produce secondary hardening at 550°C to 575°C (45 min) with a hardness peak higher than that attained in the conventional 56NiCrMoV7 steel. The effect of titanium and nitrogen additions on phases formation was investigated by Thermo-Calc. SEM was used to confirm Thermo-Calc analysis. Interpretation between hardness and formed phases has been illustrated.

Determination of the equivalent hydrogen fugacity during electrochemical charging of 3.5NiCrMoV steel

A new thermal desorption spectroscopy (TDS) apparatus was used to identify the hydrogen trapping peaks, and to measure the hydrogen concentrations in 3.5NiCrMoV steel after hydrogen charging electrochemically, and in gaseous hydrogen. The hydrogen concentration increased with (i) increasingly negative charging potential and (ii) increasing hydrogen gas pressure. The equivalent hydrogen fugacity versus charging overpotential was derived. There was a difference in the diffusible hydrogen traps activated during electrochemical and gas phase charging, attributed to the difference in the hydrogen fugacity. A two-site model for Sieverts’ Law explained the positive Y-intercept, as the density of already filled hydrogen traps.

Effect of chloride and sulfate ions on crevice corrosion behavior of low-pressure steam turbine materials

Crevice corrosion behavior between homogeneous and dissimilar materials of 13Cr and 3.5NiCrMoV steels was investigated in simulated boiler waters containing chloride ions (Cl−) and sulfate ions (SO42−), using open circuit potential (OCP) measurements in addition to morphology analysis. Pitting occurred inside the crevice not only on 13Cr steel but also on 3.5NiCrMoV steel in the test water containing 100 ppm Cl−, while it was suppressed by the coexistence of 50 ppm SO42− in the water with the 100 ppm Cl−. However, the presence of 50 ppm SO42− in the test waters promoted uniform corrosion on 3.5NiCrMoV steel inside the crevice.

The Influence of Hydrogen on the Low Cycle Fatigue Behavior of Medium Strength 3.5NiCrMoV Steel Studied Using Notched Specimens

The influence of hydrogen on low cycle fatigue (LCF) of 3.5NiCrMoV steel electrochemically hydrogen charged in the acidified pH 2 0.1 M Na2SO4 solution is studied. In the presence of hydrogen, the fatigue life decreases significantly by ≈70 to ≈80% by: (i) the crack initiation period is decreased; and (ii) the crack growth rate is accelerated. SEM observation indicates that in the presence of hydrogen, the fracture surface shows flat transgranular fracture with vague striations and some intergranular fracture at lower stresses. The fatigue crack growth rate increases with increasing cyclic stress amplitude and with hydrogen fugacity. Once the fatigue crack reaches a critical length, the specimen becomes mechanical unstable and fracture due to ductile overload occurs. The hydrogen contribution to the final fracture process is not significant.

Equivalent Hydrogen Fugacity during Electrochemical Charging of 980DP Steel Determined by Thermal Desorption Spectroscopy

Thermal desorption spectroscopy (TDS) is used to analyze hydrogen in 980DP after (i) electrochemical charging, and (ii) gaseous charging. The hydrogen concentration increases with (i) a more negative charging potential and (ii) an increasing hydrogen gas pressure. For charging in 0.1 M NaOH, the hydrogen fugacity for 980DP is similar to that for (i) low interstitial steel, and (ii) MS1500, and is greater than that for the 3.5NiCrMoV steel. This indicates an influence of steel chemistry on the hydrogen evolution reaction. The de‐trapping activation energies are 40.5 and 50.2 kJ mol−1, indicating hydrogen traps at boundary defects.