Low-temperature Transformation Research Articles

Effect of Quenching Strategy and Nb-Mo Additions on Phase Transformations and Quenchability of High-Strength Boron Steels

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates.

Effect of Microstructure on Low-Temperature Impact Toughness of Multi-Pass Weld Metal of 1 GPa Class High Strength Steel

A 1 GPa class weld metal was prepared by multi-pass welding using a prototype flux-cored wire, and the correlation between microstructure and low-temperature impact toughness of the weld metal was investigated. The microstructure of the as-deposited columnar region and the equiaxed region reheated by subsequent welding passes was observed, and it was found that low-temperature transformation products such as bainite and martensite mainly developed in both regions. Since the prior austenite grains were coarse in the columnar region, the effective grain size of the final microstructure could be large; in this case, it was confirmed that the low-temperature impact toughness of the weld metal was very low. However, in the weld metal used in this study, acicular ferrite was formed inside the grains of the columnar region; the effective grain size and grain boundary misorientation distribution of the final microstructure were similar to those of the equiaxed region, and a good low-temperature impact toughness of the weld metal was obtained. Key words: High strength steel, Multi-pass weld metal, Acicular ferrite, Impact toughness

Crystal structure and magnetic properties of Al2NdNi3 studied by experimental and first-principles

The Al2NdNi3 single-phase alloy was prepared by melting stoichiometric Al, Nd, and Ni in the nonconsumable arc furnace filled with argon and then homogenizing annealing at a high temperature. The crystal structure of Al2NdNi3 was analyzed by an X-ray powder diffraction technique, Rietveld refinement analysis, and the first-principles simulation calculation method. The results show that Al2NdNi3 crystallizes in ordered variant of CaCu5-type structure (space group P6/mmm, No. 191, hP6), and the lattice parameters of a ​= ​5.1247 (1) Å, c ​= ​4.0941 (1) Å, Z ​= ​1 and Dcalc ​= ​6.68 ​g/cm3. The temperature-dependent susceptibility data shows field sensitive behaviour of Al2NdNi3 with low temperature transformation of magnetic ordering below ~100 ​K Al2NdNi3 exhibits ferromagnetic properties at low temperatures and has a Curie temperature (Tc) of 253 ​K in field of 50 ​Oe with the saturation magnetic moment (Ms) of 0.25 μB/f.u. in field of 8 ​T and coercivity (Hc) of 0.16 ​T at 2 ​K. The simulation results also show that Al2NdNi3 is a ferromagnet with a spontaneous magnetic moment of 0.28 μB/f.u., and it is believed that the spontaneous magnetization of this alloy originate from the electron spin in the Nd(1a)-4f band.

Взаємодія в системах Y2O3—Ln2O3 (Ln = Tb—Lu)

Based on the analysis of literature data from experimentally constructed phase diagrams of Y2O3 − Ln2O3 systems (Ln = Tb − Lu), as well as temperatures of polymorphic transformations of rare earth oxides (REE), tentative phase diagrams of Y2O3 − Ln2O3 systems (Ln = Tb − Lu) were constructed in wide intervals of temperatures and concentrations. Prediction of the binary phase diagrams structure of yttria − yttrium subgroup lanthanides systems was carried out on the basis of three principles: 1. Since double systems are formed by lanthanide oxides of one (yttrium) subgroup, it is very likely that in such systems continuous solid solutions will be formed between the components. 2. Intermediate binary phases are not formed in these systems. 3. The formation of continuous solid solutions occurs with a decrease in the temperatures of phase transformations in the solid state to a minimum shifted towards a lower transformation temperature of the system component. The forecast of the Y2O3 – Ln2O3 systems phase diagrams structure, where Ln = Tb – Lu, indicates the complete solubility of the components in the liquid and solid states. Binary compounds in the considered systems are not predicted. Phase transformations in the solid solutions on the basis of polymorphic modifications X, H, A, B and C of lanthanide oxides cascade at high temperatures by the peritectoid mechanism. Below 1850 °C regions of solid solutions with cubic C-structure of REE oxides are formed in the whole range of concentrations in the systems. Key words: REE oxides, yttria, polymorphs of REE oxides, phase diagram.

Study on High-Temperature Mechanical Properties of Fe–Mn–C–Al TWIP/TRIP Steel

In this study, high-temperature tensile tests were carried out on a Gleeble-3500 thermal simulator under a strain rate of ε = 1 × 10−3 s−1 in the temperature range of 600–1310 °C. The hot deformation process of Fe–15.3Mn–0.58C–2.3Al TWIP/TRIP at different temperatures was studied. In the whole tested temperature range, the reduction of area ranged from 47.3 to 89.4% and reached the maximum value of 89.4% at 1275 °C. Assuming that 60% reduction of area is relative ductility trough, the high-temperature ductility trough was from 1275 °C to the melting point temperature, the medium-temperature ductility trough was 1000–1250 °C, and the low-temperature ductility trough was around 600 °C. The phase transformation process of the steel was analyzed by Thermo-Calc thermodynamics software. It was found that ferrite transformation occurred at 646 °C, and the austenite was softened by a small amount of ferrite, resulting in the reduction of thermoplastic and formation of the low-temperature ductility trough. However, the small difference in thermoplasticity in the low-temperature ductility trough was attributed to the small amount of ferrite and the low transformation temperature of ferrite. The tensile fracture at different temperatures was characterized by means of optical microscopy and scanning electron microscopy. It was found that there were Al2O3, AlN, MnO, and MnS(Se) impurities in the fracture. The abnormal points of thermoplasticity showed that the inclusions had a significant effect on the high-temperature mechanical properties. The results of EBSD local orientation difference analysis showed that the temperature range with good plasticity was around 1275 °C. Under large deformation extent, the phase difference in the internal position of the grain was larger than that in the grain boundary. The defect density in the grain was large, and the high dislocation density was the main deformation mechanism in the high-temperature tensile process.

Relationship between Residual Stress and Net Strain in Low-Temperature Transformation Weldments Considering Microstructure

Weldments inevitably shrink during cooling from the melt pool. Residual stresses then occur owing to surrounding constraints. Tensile residual stresses in weldments cause various problems, such as deformation and reduction of fatigue strength. Low-temperature transformation (LTT) welding consumables can reduce the tensile residual stress through volume expansion, which accompanies a phase transformation from austenite to martensite. In this study, the relationship between residual stress and net strain was examined, mainly by controlling the martensite start (Ms) temperature, and the result was related to the weld’s microstructure. The Ms temperature and the expansion accompanying the phase transformation were analyzed by the dilatometric method. A hole drilling test was carried out to measure the residual stress in the weldments. The highest compressive stress was observed in the most expanded weldment at room temperature, and a linear relationship between the net strain and residual stress was derived. This linear relationship was analyzed with a microstructural approach.

Control of residual stress in metal additive manufacturing by low-temperature solid-state phase transformation: An experimental and numerical study

Directed energy deposition (DED) technology commonly known as additive manufacturing (AM), is a rapid prototyping method for fabricating metallic functional components with minimum finishing operations. However, excessive tensile residual stress (RS) is often a potential source of structural failure due to the fact that it can cause surface/subsurface cracking of DED parts. In this work, the RS properties in DED specimens made of the low-temperature transformation (LTT) pre-alloyed powders was investigated. A DED model coupling the finite element (FE) and thermo-mechanical-metallurgical (TMM) analysis was constructed, and was experimentally validated by thermographic system and X-ray diffraction (XRD) stress measurements. An additional set of RS prediction for the DED specimen, using increased inter-track idle time was developed to reveal the effects of thermal history on TMM evolution behavior and RS distribution. The combination of experiment and predictive simulation provided an insight into the non-linear thermo-metallurgical evolution and the formation mechanism of compressive RS during DED of LTT alloy, and showed the great potentiality of LTT powder in minimizing the RS in DED parts by the austenite-to-martensite transformation on cooling.

Effect of Microstructures on the Mechanical Behavior of Fire‐Resistant Dual‐Phase Steels at Elevated Temperature

Herein, the start laminar cooling temperature to obtain different volume fractions of ferrite and bainite/martensite is optimized. Lower start laminar cooling temperature yields a higher volume fraction of ferrite and also decreases the transformation temperature of bainite/martensite. The yield strength of the experimental dual‐phase steel at 600 °C increases from ≈484.8 to ≈526.2 MPa by decreasing the start laminar cooling temperature from 650 to 600 °C. To understand the effect of microstructure, the dislocation density, volume fraction of precipitates, and Young's modulus at elevated temperature are statistically analyzed. A machine learning method, known as gradient boosting decision tree, is used to classify the grain boundaries. The statistical results suggest that the steel subject to lower start laminar cooling temperature shows higher Young's modulus and larger volume fraction of precipitates at elevated temperature. Furthermore, at elevated temperature, the boundary is stable and the proportion of Σ3 boundary is greater, when lower start laminar cooling temperature is adopted. Theoretical calculations results suggest that the lower transformation temperature of bainite/martensite improves the fire resistance of dual‐phase steels.

Low temperature mullitization of oxime-modified aluminosiloxane by sol-gel process

ABSTRACT In this study, mullitisation at low temperature was observed in an oxime-modified aluminosiloxane precursor [(CH3)2CNO}2Al{OSi(Ph)2O}Al{ONC(CH3)2}2]. Oxime-modified aluminosiloxane was synthesised by carrying out a reaction between oxime-modified aluminium(III) isopropoxide [{(CH3)2CNO}2Al(O i Pr)] and diphenylsilanediol in a 2:1 molar ratio. 29Si and 27Al NMR spectroscopic data suggest the presence of tetra- and penta-coordinated environment around Si and Al elements. The presence of Al-O-Si linkages was observed in the FTIR spectra of the precursor as well as in the formed mullite. Thermal study of precursor suggests clean decomposition of precursor molecules to amorphous aluminosilicate 3Al2O.2SiO2at ~500°C. Low temperature (900°C) transformation of precursor to mullite was observed by the sol-gel process. XRD (X-ray diffraction) and FTIR (Fourier Transform Infrared) studies suggest the formation of mullite in the orthorhombic phase without phase segregation. Scanning electron microscopy suggests agglomeration of nanocrystallite, while HRTEM (High Resolution Transmission Electron Microscopy) results corroborate with the XRD data.

Metabasic rocks from the Variscan Schwarzwald (SW Germany): metamorphic evolution and igneous protoliths

In the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.

Size effect on residual stress in low transformation temperature welded joints

It has been reported that low transformation temperature (LTT) weld metals are beneficial to generation of compressive residual stress around weld zone. In this study, the relationship among residual stress, size effect of LTT welded joints with different plate width and thickness as well as martensite start (Ms) temperatures was investigated by experimental and finite-element analysis. It was found that heat dissipation and thermal expansion coefficient of LTT weld metal had a significant impact on residual stress. Welded joint with a small plate width led to greater compressive residual stresses in the LTT weld, which was due to the lower heat dissipation and smaller thermal expansion coefficient of the LTT weld metal in due course of cooling process. Additionally, the finite-element analysis revealed that increasing plate width mainly affected the longitudinal residual stress, while increasing the plate thickness influenced all the residual stress components in the LTT weld. Furthermore, the LTT weld with a lower Ms temperature of 191 °C resulted in greater compressive residual stresses, and was less sensitive to the LTT joint size, as against the Ms temperature of 398 °C.

Martensite phase structure of Mg-Sc lightweight shape memory alloy and the effect of rare earth elements doping

With the continual increase of application requirement of lightweight shape memory alloys (SMAs), MgSc alloys have drawn great attention as a new SMA. However, the ambiguous martensitic structure, unclear transformation path, and low martensitic transformation temperature (Tm) of Mg-Sc alloys are still issues. Herein, the martensitic structure and transformation path of Mg-Sc alloys were investigated systematically. The martensite phase was identified finally to be an orthorhombic structure by formation energy. Moreover, to improve the Tm, the effect of rare earth elements doping on Tm was studied, and Tm was increased by 28.2 K with 1.25 at.% Gd doping. The total density of states indicated that the phase stability determined the increase of Tm. Furthermore, the partial density of states indicated that the d orbital electrons coupling between Sc and Gd atom led to the changing of Tm. Our results provide a firm base for designing the new Mg-Sc-based lightweight SMAs.

New insight into the hardenability of high strength low alloy steel from the perspective of crystallography

A Jominy specimen of low carbon low alloy steel was investigated to elucidate the relationship between crystallographic features and hardness. As the Jominy distance increased from 2 to 20 mm and then to 50 mm, the morphological structure changed from lath martensite to lath bainite and then to granular bainite. The cooling rate was lowered with increasing Jominy distance, and the density of high angle grain boundaries (HAGBs) and the hardness were decreased, showing a near-linear relationship between the HAGBs density and hardness. The high cooling rate led to a large driving force and a low transformation temperature, which contributed to producing high density of HAGBs and hardness.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C system alloys. Part 2. Metallographic studies

Original metallographic studies of annealed medium carbon steel (steel 45, steel 40X and steel 35XGA) are carried out in order to reveal leakages in pearlite of these steels of low-temperature carbide transformation of peritectoid type, in which solid solutions of ferrite and cement form a solid solution of wide area of homogeneity based on ε-carbide Fe2C. Most of the inclusions of pearlite cement 45 steel are almost entirely covered with a light grey “foam” shell of nanoglobular crystals ε-сarbide Fe2C. In the process of crystallization of ε-сarbide Fe2C on the cement plates of pearlite steel 45 three morphological types of structure of ε-сarbide Fe2C can be distinguished: “foam” globular shell, granular “outgrowths” and wrapping of particles of decomposed and partially dissolved cement plates. Chromium, which is a part of steel 40X, abruptly inhibits peritectoid transformation. On many cement plates pearlite surface is smooth. In areas where the concentration of chromium is low, the process of formation of ε-сarbide Fe2C is active, with the formation of individual sites with light gray “foam” shell of nanoglobular crystals ε-сarbide Fe2C. It can be expected that in high-alloy chrome steels, the peritectic transformation can be completely blocked through the stabilization of chrome cement or its transformation into thermodynamically stable high chrome carbides. In 30ХГСA steel a sharp intensification of the process of peritectoid transformation of solid solutions of ferrite and cement which are part of pearlite is fixed. The reason for acceleration of the disintegration process of pearlite cement into individual fragments and intensification of release of ε-сarbide Fe2C in the form of column-shaped crystals between the plates of pearlite cement is silicon and manganese, which are part of steel 30ХГСА. As a result of the acceleration of martensite decomposition, the morphology of the released crystals of ε-сarbide Fe2C has changed from “foam” nanocrystals of ε-сarbide Fe2C, typical for steel 45 and steel 40X, to granular. There was also intensive fragmentation or disintegration of cement plates with the appearance of plane-parallel boundaries between the fragments and the formation of large longitudinal flat inclusions of ε-сarbide Fe2C above 100 nm, whose axis is mainly perpendicular to the axis of the cement plate. On the basis of the performed experiments it is possible to consider as proved the presence of low-temperature carbide peritectoid phase transformation in the Fe-C alloy system as a result of interaction of solid solutions of ferrite and cement at 3820C with formation of solid solution on the basis of ε-сarbide Fe2C with wide area of homogeneity. The influence of the chemical composition of steel on the peritectoid transformation between ferrite and cement slurries opens up additional possibilities for regulating the microstructure of pearlite, such as the degree of dispersion of pearlite, which has a determining influence on a number of performance characteristics of steel, such as wear resistance, yield strength and others.

Assessment of the chemical composition of LTT fillers on residual stresses, microstructure, and mechanical properties of 410 AISI welded joints

The present paper aims to characterize the welding residual stresses, microstructure, and mechanical properties of some AISI 410 plates welded with low-transformation temperature (LTT) fillers containing different Ni wt.%. The LTT fillers with 11 wt.% Cr and Ni content varying between 3 and 11 wt.% were produced using an arc melting furnace. Effect of Ni wt.% on Ms of LTT fillers was studied using a dilatometer. Five-millimeter-thick AISI 410 plates were welded by GTAW using LTT fillers mentioned above and ER410 conventional filler as a reference. Welding residual stresses were measured by using the hole drilling strain gage method (HDSG). Analyzing the effect of welding dilution on the chemical composition of weld metal was carried out using energy-dispersive X-ray spectroscopy (EDS) followed by a dilatometric test for the evaluation of weld metals Ms after welding. Microstructural examinations show that conventional filler leads to ferritic–martensitic weld metal while in the cases welded with LTT fillers, delta ferrite (δ-ferrite) is decreased when Ni increases. Also, by increasing Ni more than 9 wt.%, the retained austenite is detected in the weld metal. Furthermore, weld metal hardness rises with increasing Ni. Performing tensile and impact tests on welded joints endorse the successful use of LTT fillers for welding the AISI 410 plates.

Investigation of the Residual Stress in a Multi-Pass T-Welded Joint Using Low Transformation Temperature Welding Wire.

We investigated whether low transformation temperature (LTT) welding materials are beneficial to the generation of compressive residual stress around a weld zone, thus enhancing the fatigue performance of the welded joint. An experimental and numerical study were conducted in order to analyze the residual stress in multi-pass T-welded joints using LTT welding wire. It was found that, compared to the conventional welded joint, greater tensile residual stress was induced in the flange plate of the LTT welded joints. This was attributed to the reheat temperature of the LTT weld pass during the multi-pass welding. The formerly-formed LTT weld pass with a reheat temperature lower than the austenite finish temperature converted the compressive residual stress into tensile stress. The compressive residual stress was generated in the regions with a reheat temperature higher than the austenite finish temperature, indicating that LTT welding materials are more suitable for single-pass welding.

Toughness Property Control by Nb and Mo Additions in High-Strength Quenched and Tempered Boron Steels

The synergetic effect on hardenability by combining boron with other microalloying elements (such as Nb, Mo and Nb + Mo) is widely known for high-strength medium carbon steels produced by direct quenching and subsequent tempering treatment. The improvement of mechanical properties could be reached through optimization of different mechanisms, such as solid solution hardening, unit size refinement, strain hardening, fine precipitation hardening and the effect of carbon in solid solution. The current study proposes a procedure for evaluating the contribution of different microstructural aspects on Charpy impact toughness. First, the effect that austenite conditioning has on low-temperature transformation unit sizes and microstructural homogeneity was analysed for the different microalloying element combinations. A detailed crystallographic characterization of the tempered martensite was carried out using electron backscattered diffraction (EBSD) in order to quantify the effect of unit size refinement and dislocation density. The impact of heterogeneity and presence of carbides was also evaluated. The existing equations for impact transition temperature (ITT50%) predictions were extended from ferrite-pearlite and bainitic microstructures to tempered martensite microstructures. The results show that microstructural refinement is most beneficial to strength and toughness while unit size heterogeneity has a particularly negative effect on ductile-to-brittle transition behaviour. By properly balancing alloy concept and processing, steel having a yield strength above 900 MPa and low impact transition temperature could be obtained by direct quenching and tempering.

Cold Cracks in Fillet Weldments of 600 MPa Tensile Strength Low Carbon Steel and Microstructural Effects on Hydrogen Embrittlement Sensitivity and Hydrogen Diffusion

Hydrogen flowing into a weld causes hydrogen embrittlement (HE), so to prevent cold cracks in the weld, HE must be suppressed. In this study, single-pass fillet weldments were fabricated using two different welding fillers with the same strength level as the American Welding Society standard. The specimen F-HNi, which had a high, reversible hydrogen content, exhibited cold cracking in the fillet weld. The HE sensitivity index (HE index) was calculated using the in-situ slow strain rate test (SSRT). The reversible hydrogen trap concentration (<i>Crev</i>) and hydrogen diffusion coefficient (<i>Deff</i>) were calculated using the permeation test. The formation of low-temperature transformation phases such as bainite and martensite increased the <i>Crev</i> and decreased the <i>Deff</i>, thereby increasing the HE index. In addition, it was determined that reversible hydrogen was most effectively trapped in the low-temperature transformation phase, as confirmed by silver decoration. We concluded that the cold cracks in the F-HNi specimen were associated with the large <i>Crev</i>, small <i>Deff</i>, and large HE index. HE sensitivity can be controlled by optimizing the microstructure, even when welding fillers with the same level of strength are used. To reduce HE sensitivity, it is important to reduce the number of reversible hydrogen trap sites, by reducing the fraction of the low-temperature transformation phase where the reversible hydrogen trapping most occurs.

Paper Review of “Fatigue strength of transverse attachment steel joints with single-sided arc weld using low transformation temperature welding consumable”

Paper Review of “Fatigue strength of transverse attachment steel joints with single-sided arc weld using low transformation temperature welding consumable”

新低変態温度溶接材料と全姿勢伸長ビード補修法による溶接接手の高疲労寿命化 -第2報：全姿勢伸長ビード補修法による角回し溶接継手の疲労寿命延伸-

新低変態温度溶接材料と全姿勢伸長ビード補修法による溶接接手の高疲労寿命化 -第2報：全姿勢伸長ビード補修法による角回し溶接継手の疲労寿命延伸-