Elongated MnS Inclusions Research Articles

Formation mechanism of inspection defects in hundred-ton class 55NiCrMoV7 die steel forging

Using an ultrasonic flaw detector, this study precisely located the dimensions and distribution characteristics of defects during the manufacturing process of 55NiCrMoV7 die steel forgings weighing hundreds of tons. The probing defects of the forging are predominantly concentrated in the central region, spanning approximately 4200 mm in length, 400 mm height and 1500 mm width. Detailed research on the characteristics of these probing defects was conducted through dissection sampling and scanning electron microscope analysis. The research results reveal that the maximum diameter of an individual defect can reach Φ10 mm, and localised cracking phenomena exist in specific areas within the forging. The flaws exhibit characteristics such as millimetre-sized, large, elongated MnS inclusions, with spacing between inclusions ranging from 200μm to 500μm. Furthermore, based on elemental analysis, the sulphur content from the surface to the core of the forging increased from 0.001% to 0.002% to a maximum of 0.0085%. Through analysis using thermodynamic and growth kinetics models, it was determined that in regions with higher sulphur content (S = 0.0085) at a certain distance from the steel ingot surface, the radius of MnS inclusions can reach up to 380μm under low cooling rates (1 K/min). After forging deformation, these can evolve into millimetre-sized MnS inclusions, leading to internal inspection defects. Therefore, we propose that reducing the sulphur content in the molten steel and increasing the degree of undercooling can effectively inhibit the precipitation and growth of MnS inclusions. Practical results indicate that this improvement strategy effectively enhances the inspection quality of the 55NiCrMoV7 die steel forgings weighing hundreds of tons.

Modification of Sulfide in a 303 Ton Carbon Manganese Steel Heavy Ingot with Cerium Treatment

Herein, the cerium treatment is used to modify large MnS inclusions in a 303‐ton carbon manganese steel ingot. When the cerium content in steel in the tundish is 90 ppm, the inclusions are CeAlO3 with a small amount of Ce2O2S. The precipitation sequence of inclusions during the solidification and cooling process in the ingot is MgO·Al2O3 + CeAlO3→Ce2S3→MnS→MgS. Due to the inevitable macrosegregation, a small amount of elongated MnS inclusions is precipitated at the radial center. The maximum diameter is 257.80 μm, and the average aspect ratio is 3.28. For heterogeneously nucleated sulfide, the average diameter is less than 13.39 μm, and the average aspect ratio is less than 1.88. The morphology of MnS inclusion is well controlled when the mass ratio of Ce/S in steel is in the range of 0.7–2.8. The average aspect ratio of homogeneously nucleated MnS is less than 1.9. In addition, the formation of CeAlO3 clusters is avoided. According to the atomic mismatch obtained by the edge‐to‐edge matching model, the possibility of heterogeneous nucleation of MnS on the oxide core is CeAlO3 > Ce2O2S > MgO·Al2O3. The Ce2S3 inclusions are mainly precipitated with the core of MgO·Al2O3.

Morphological transformation of elongated MnS inclusions in non-quenched and tempered steel during isothermal heating

Morphological transformation of elongated MnS inclusions in non-quenched and tempered steel during isothermal heating

Influence of electrochemical hydrogenation parameters on microstructures prone to hydrogen-induced cracking

When pipeline steels are exposed to hydrogen-rich environments during service, this can lead to a degradation in mechanical properties and premature failure. These steels are often susceptible to hydrogen embrittlement and hydrogen-induced cracking (HIC) and contain very sensitive microstructures for HIC formation. This work aims to find a suitable hydrogenation methodology that allows to charge pipeline steel specimens without inducing any hydrogen related damage during the charging process. Therefore, different electrochemical hydrogen charging methodologies are evaluated. The susceptibility to HIC and blisters of two pipeline steels (API 5L grades X70 and X56), is first investigated in a sulfuric acid electrolyte. Both grades are prone to HIC in the mid-thickness section of the pipeline wall. For X56, significant blistering occurs as well, and hydrogen-induced damage could be linked to elongated MnS inclusions. For the X70 steel, hard bands are found to be vulnerable to HIC. The electrochemical charging conditions are modified for the X56 steel, where the extent of the damage is greatest due to its prone microstructure, as revealed by scanning electron microscopy. Lowering the sulfuric acid concentration and applied charging current density does lead to some reduction of blistering. However, changing to a different, sodium hydroxide-based electrolyte is more effective in mitigating the extent of the hydrogen-induced damage and the time at which it appears.

Development of high impact toughness offshore engineering steel by yttrium-based rare earth addition

ABSTRACT The effect of yttrium-based rare earth on the inclusions and impact toughness of EH36 offshore engineering steel were investigated by scanning electron microscopy, electron backscattered diffraction and electron probe x-ray microanalysis techniques. It was found that impact toughness and isotropy of EH36 were enhanced significantly by rare earth addition. The longitudinal and transverse impact toughness at −60°C were increased by 33.5% and 113.7%, respectively. Moreover, the differences between the longitudinal and transverse textures decrease significantly with the addition of rare earth, the low-temperature impact property isotropy increased by 61.1%. Meanwhile, rare earth can modify elongated irregular MnS inclusions into small spherical regular RE-inclusions. Most of the RE-inclusions are within 3μm in diameter, which account for 97.84% of total inclusions. With the addition of rare earth in EH36 offshore engineering steel, the average effective grain size was reduced by 9.7%.

Experimental Study on Rare Earth and Magnesium Composite Treatment of 49MnVS3 Non‐quenched and Tempered Steel

Nowadays, under the context of severe resource shortage, it is urgent to develop non‐quenched and tempered steel. Herein, a new method of combining rare‐earth cerium (Ce) and magnesium (Mg) to treat the 49MnVS3 steel with low oxygen and rather high sulfur is proposed. The inclusions modification, microstructure evolution, and mechanical properties change of 49MnVS3 steel with different Ce and Mg content composite treatment are investigated by optical microscope (OM), scanning electron microscope (SEM)–energy‐dispersive spectrometer (EDS), electron probe microanalyzer (EPMA), and tensile and impact tests. The results indicate that the small spherical Ce–Mn–Mg–O–S composite inclusions replace the Al2O3wrapped MnS composite inclusions and elongated MnS inclusions after Ce and Mg joint treatment. After Ce and Mg composite treatment, the proportion of ferrite is significantly improved. Meanwhile, the interlamellar spacing of pearlite is refined. Attributed to modified inclusions and optimized microstructure, the strength and impact toughness are gradually enhanced. In contrast to Ce–Mg free steel, the impact energy of 4#steel with 0.0122 wt% Ce and 0.0012 wt% Mg addition is increased by 33%. In the current study, the optimal mass fractions of Ce and Mg are 0.0122 and 0.0012 wt%, respectively.

Behavior of MnS inclusions during homogenization process in low-alloyed steel FAS3420H

Abstract Sulfur is added to low-alloyed steel FAS3420H to improve the free-cutting property of the steel. Elongated MnS inclusions usually deteriorate the mechanical and process performance of steels, so the control of shape and size of MnS is essential. The statistical analysis of MnS inclusions in as-cast and rolled steels at different positions shows that the distribution and aspect ratio of MnS in rolled steel are closely related to the quantity and size of MnS in as-cast one. The in situ observation and experimental results reveal that homogenization treatment effectively reduced MnS inclusions size and transformed their shape to globule or ellipsoid. Significant change of quantity and size of MnS inclusions was identified only when the soaking time exceeds 5 h at 1,250℃. Additionally, a kinetic model is presented to quantitatively characterize the behavior of MnS inclusions during homogenization process.

Research on the Effect of Rare Earth on the Structure and Impact Performance of Armor Steel

In order to study the impact performance of 685 armored steel with different mischmetal content, the impact performance of the steel at room temperature and -40°C was tested, and the impact fracture morphology and microstructure were observed and analyzed. The results show that the addition of mixed rare earth La-Ce to the test steel improves the tempered martensite structure and refines the austenite grains. At the same time, the inclusions in the steel are modified, and the elongated MnS inclusions are modified. It is a spherical composite rare earth inclusion, and the size of the inclusion is reduced from 6 μ m to 2 μ m. Compared with the test steel without rare earth, the room temperature impact performance of the mixed rare earth test steel and the -40 °C low temperature impact performance are improved. When the La-Ce content is 102 × 10−6, the room temperature impact energy of the protective steel increases by 24.5%, and the low temperature impact energy at -40 °C increases by 17. 4%. The results of the shooting test showed that the protection level of rare earth-containing armored steel reached NATO Level 2.

Morphological changes of elongated MnS inclusions during heat treatment process

ABSTRACT The element sulphur is usually added to form MnS inclusions and improve the machinability of micro-alloyed steel. However, the MnS inclusions tend to elongate in rolling direction during hot working process and deteriorate the mechanical properties of steels. In this work, the effect of heat treatment on the morphological changes of elongated MnS inclusions was investigated. The research results show that the as-cast type II MnS inclusions tend to flattened and elongated morphology after rolled. The shape changes of the MnS inclusions during the heat treatment process is flat →cylinderization→ovulation→spheroidization. The morphological changes leading to cylinderization, ovulation and spheroidization of the MnS inclusions are controlled by the potential gradient induced by the inherent curvature difference. The transformation modes of ovulation and spheroidization depend on the initial aspect ratio (Length to width, L/W). The transport mechanisms for cylinderization, ovulation and spheroidization of the MnS inclusions are all controlled by surface diffusion.

Effects of Zr addition on microstructure and toughness of simulated CGHAZ in high-strength low-alloy steels

The effect of Zr addition (0.005, 0.013, and 0.054 wt.%) on the microstructure and toughness of simulated coarse-grained heat-affected zone in high-strength low-alloy steels was revealed using a Gleeble 2000 thermal simulator. It was observed that elongated MnS inclusions were formed in the lowest Zr-containing steel, while only pure equiaxed ZrO2 existed in the 0.054Zr steel (Zr content of 0.054 wt.%). Complex oxide–sulfide inclusions (ZrO2 + MnS) with size of (1.40 ± 0.25) μm were formed in 0.013Zr steel (Zr content of 0.013 wt.%). The complex inclusions refined the prior austenite grain, and the nucleation of acicular ferrite was promoted compared to those of 0.005Zr steel (Zr content of 0.005 wt.%) and 0.054Zr steel. Consequently, the 0.013Zr steel possessed superior low-temperature impact toughness in relation to 0.005Zr and 0.054Zr steels. Thus, moderate Zr addition can be considered as an effective method to refine the structure and improve the mechanical properties of the coarse-grained heat-affected zone.

Effects of vanadium precipitates on hydrogen trapping efficiency and hydrogen induced cracking resistance in X80 pipeline steel

In this study, the number and size distribution of vanadium precipitates and their effects on hydrogen trapping efficiency and hydrogen-induced cracking (HIC) susceptibility were investigated in X80 pipeline steel. The results showed that as the vanadium content increased, the number of nanoscale vanadium precipitates clearly increased. Furthermore, the amount of hydrogen atoms trapped by vanadium precipitates gradually increased and the hydrogen diffusion coefficient decreased from 4.74 × 10−6 cm2 s−1 in the vanadium-free V0 steel to 8.48 × 10−7 cm2 s−1 in the V4 steel with 0.16% V, according to hydrogen permeation results. It also reduced the possibility of hydrogen atoms diffusing into the sites of harmful defects such as large-size oxides and elongated MnS inclusions, where cracks were caused more easily. In addition, the V3 steel with 0.12% V, containing the largest number of vanadium carbide particles of less than 60 nm, had the lowest HIC susceptibility.

Study on nonmetallic inclusions in clean Cu-P-RE weathering steels

The compositions, morphologies and sizes of nonmetallic inclusions in rare earth (RE) weathering steels with different oxygen and sulfur contents were analyzed by using electron probe micro-analyzer (EPMA), quantitative image analyzer and scanning electron microscope (SEM-EDS). The formation of inclusions was studied by the thermodynamics theory. The RE solid-soluble content in steel was analyzed by non-aqua electroanalysis and inductively coupled plasma (ICP). To RE weathering steels with different contents of oxygen and sulfur, the optimal RE content was obtained from the perspective of RE modifying inclusions. The results showed the cleanliness of steel was an important factor which influenced the modifying effect of RE on the inclusions. RE changed the character and shape of inclusions in the clean Cu-P weathering steels. The elongated MnS inclusions and brittle oxide inclusions were modified as the small globular RE oxysulfide and RE compound inclusions in the RE weathering steels. The RE inclusions distributed uniformly in matrix and the sizes of more than 90% of the inclusions were less than 2 μm. It was obtained the optimum content of RE was 0.0065 wt% ∼ 0.016 wt% to the Cu-P-RE weathering steels with 0.002 wt% oxygen and 0.004 wt% sulfur, and the optimum content of RE was 0.0092 wt% ∼ 0.023 wt% to the RE weathering steels ( O: ∼ 0.007 wt%, S: ∼ 0.008 wt%). The RE content was the most favorable to modify the harmful inclusions and improve the properties of Cu-P weathering steels.

Control of Secondary Phases by Solution Treatment in a N-Alloyed High-Mn Cryogenic Steel

The secondary phases of the steels have significant effects on the microstructure and mechanical properties, making controlling these secondary phases important. The control of MnS inclusions and AlN precipitates in a N-alloyed high-Mn twin-induced plastic cryogenic steel via solution treatment was investigated with several different techniques including microstructural characterization, 298 K tensile testing, and 77 K impact testing. The solutionizing temperature (ST) increased from 1323 to 1573 K, where the elongated MnS inclusions and large-sized AlN precipitates became spheroidized and dissolved. The aspect ratio of the MnS inclusions decreased as the ST increased and the number density increased. The impact toughness of the steels showed anisotropy and low impact energy values, due to the elongated MnS inclusions and large-sized AIN precipitates. The anisotropy was eliminated by spheroidizing the MnS inclusions. The impact energy was improved by dissolving the large-sized AlN precipitates during the solution treatment. The austenite grain size increased when the dissolution of the AlN precipitate increased, but the effect of the grain size on the yield strength, toughness, and the strength–ductility balance was weak.

BİR LEVHA HADDEHANESİNDEKİ ÜST EĞME SİLİNDİRİ KOVANINDA OLUŞAN HASARIN ANALİZİ

Premature failure of a bending cylinder bushing in a plate mill was investigated in this study. Visual, chemical and microscopic analyses were performed to detect the possible reasons of this failure and possible measures to be taken were mentioned not to be encountered for this type of failures in the future. No traces of welding were observed in the performed studies. Intensive elongated MnS inclusions, fatigue striations and splits from the grain boundaries were detected in the microstructural investigations. It was also proved that the base of the bushing was worked without chamfering. The importance of the removing hydrogen from the structure by annealing to decrease the hydrogen brittleness risk which would occur after the chromium coating process, and the chamfering of the bushing base to decrease the crack susceptibility was stressed. Keywords: Failure analysis, bending cylinder bushing, hot rolling, fatigue

Mechanical anisotropy in Ca-treated and ultra-low sulphur HSLA-100 steel

A few elongated MnS inclusions were observed in ultra-low sulphur HSLA-100 steel which significantly decreased the yield strength (YS), ultimate tensile strength and notch toughness of short transverse direction. The slight anisotropies of YS (≈5 MPa) and notch toughness (≈15 J) between longitudinal and transverse directions are correlated to the presence of texture components in the vicinity of {113}<110> and {554}<225>. In the case of anisotropy between longitudinal and short transverse directions, it is found that recognising the contribution of each effective factor in the creation of strength anisotropy is not possible. However, the most primary effective factor in the creation of remarkable notch toughness anisotropy (≈70 Pct), especially at −85°C, is related to the existence of elongated MnS inclusions.

Crack initiation at high loading rates applying the four-point bending split Hopkinson pressure bar technique

Dynamic crack initiation with crack-tip loading rates of $$\dot{K} \approx 2 \cdot 10^6\,\mathrm {MPa\,m^{0.5}\,s^{-1}}$$ in a high strength G42CrMoS4 steel was investigated. To this end, a previously developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and pre-cracked Charpy specimens were tested. The detection of dynamic crack initiation was performed by analyzing the dynamic force equilibrium between the incident and the transmission bar. High-speed photography of the tests and analysis of the dynamic stress intensity factor revealed that the vibration of the specimen had to be considered. The dynamic and static analyses of the tests lead to nearly the same results when a force equilibrium was achieved. Fracture-surface analysis revealed that elongated MnS inclusions strongly affected both the dynamic crack initiation and growth. Blunting of the precrack did not take place when a group of MnS inclusions was located directly at the precrack tip. Due to the direction of the elongated MnS inclusions perpendicular to the direction of crack growth, the crack could be deflected. The comparison with a 42CrMo4 steel without elongated MnS inclusions revealed the detrimental effect in terms of resistance to crack initiation.

Effect of MnS inclusion and crystallographic texture on anisotropy in Charpy impact toughness of low carbon ferritic steel

The combined effect of microstructure and crystallographic texture on the anisotropy in Charpy impact toughness has been studied in two hot-rolled low-carbon steel plates having different sulphur contents: 0.03wt% S in high-sulphur, HS, steel and 0.01wt% S in low-sulphur, LS, steel. Sub-size Charpy impact specimens were prepared from the rolled plates along 0°, 30°, 60° and 90° to the rolling direction and tested over the temperature range of +40°C to −196°C. Microstructure, inclusions and crystallographic texture have been characterized on the plane parallel to the fracture plane of each sample. The variation in upper shelf energy (USE) was more severe (∼69%) in HS steel due to the presence of coarse and elongated MnS inclusions. Crystallographic texture, especially, higher fraction of alpha- to gamma-fibre texture and stronger cube texture resulted in stronger variation in ductile to brittle transition temperature (DBTT) in LS steel (by ∼26°C). Increase in the fraction of {001} planes of the crystals on the fracture plane of the sample increased the DBTT by helping the cleavage crack propagation. On the other hand, increase in the fraction of {110} planes of the crystals on the shear planes (inclined at 45° to fracture plane) increased the USE by promoting the plastic deformation.

Permanent effect of a cryogenic spill on fracture properties of structural steels

Fracture analysis of a standard construction steel platform deck, which had been exposed to a liquid nitrogen spill, showed that the brittle fracture started at a flaw in the weld as a consequence of low-temperature embrittlement and thermal stresses experienced by the material. In the present study, the permanent effect of a cryogenic spill on the fracture properties of carbon steels has been investigated. Charpy V-notch impact testing was carried out at 0 °C using specimens, from the platform deck material. The average impact energy appeared to be below requirements only for transverse specimens. No pre-existing damage was found when examining the fracture surfaces and cross sections in the scanning electron microscope. Specimens of the platform deck material and a DOMEX S355 MCD carbon steel were tensile tested immersed in liquid nitrogen. Both steels showed a considerable increase in yield- and fracture strength and a large increase in the Lüders strain compared to the room temperature behavior. A cryogenic spill was simulated by applying a constant tensile force to the specimens for 10 min, at -196 C. Subsequent tensile tests at room temperature showed no significant influence on the stress-strain curve of the specimens. A small amount of microcracks were found after holding a DOMEX S355 MCD specimen at a constant force below the yield point. In a platform deck material tensile tested to fracture in liquid nitrogen, cracks associated with elongated MnS inclusions were found through the whole test region. These cracks probably formed as a result of the inclusions having a higher thermal contraction rate than the steel, causing decohesion at the inclusion-matrix interface on cooling. Simultaneous deformation may have caused formation of cracks. Both the microcracks and sulphide related damage may give permanently reduced impact energy after a cryogenic exposure.

Improvement of Anisotropic Mechanical Behavior by Sulfide Control in Quenched and Tempered 4340 Steel

The anisotropic mechanical behavior of quenched and tempered 4340 steel with different Ca contents was investigated by means of a macro/micrograph analysis, Charpy impact test, and rotating bending fatigue test. The 4340 steel with Ca added formed small spherical (Ca,Mn)S inclusions and effectively decreased both the inclusion size and the aspect ratio (length to width) of the MnS inclusions as compared to the Ca-free 4340 steel. The anisotropic impact value and fatigue strength were effectively improved due to the Ca addition that prevented the growth of MnS inclusions, which provided increased resistance against deformation to maintain a spherical shape because the elongated MnS inclusions acted as a crack propagation path and promoted the crack propagation due to higher stress concentrations.

Crack initiation at high loading rates applying the four-point bending split Hopkinson pressure bar technique

Dynamic crack initiation with crack-tip loading rates of K ≈ 2. 106 MPa√ms-1 in a high strength G42CrMoS4 steel was investigated. To this end, a previously developed split Hopkinson pressure bar with four-point bending was utilised. V-notched and pre-cracked Charpy specimens were tested. The detection of dynamic crack initiation was performed by analysing the dynamic force equilibrium between the incident and the transmission bar. Additionally, the signal of a near-field strain gauge and high-speed photography were used to determine the instant of crack initiation. To account for vibrations of the sample, a dynamic analysis of the stress intensity factor was performed. The dynamic and static analyses of the tests produced nearly the same results when a force equilibrium was achieved. Fracture-surface analysis revealed that elongated MnS inclusions strongly affected both the dynamic crack initiation and growth. Blunting of the precrack did not take place when a group of MnS inclusions was located directly at the precrack tip. Due to the direction of the elongated MnS inclusions perpendicular to the direction of crack growth, the crack could be deflected. The comparison with a 42CrMo4 steel without elongated MnS inclusions revealed the detrimental effect in terms of resistance to crack initiation. Taking the loading-rate dependency into consideration, it was shown that there was no pronounced embrittlement due to the high loading rates.