Fire-resistant Steel Research Articles

EXPERIMENTAL STUDY ON POST-FIRE MECHANICAL PROPERTIES OF HIGH STRENGTH FIRE-RESISTANT STEEL

A series of static tensile tests on high-strength fire-resistant steel such as grades Q460FR, Q420FR, Q345FR after fire were carried out in this study. The stress-strain curves and mechanical parameters of high-strength fire-resistant steel with post-fire temperatures from 200℃ to 800℃ were obtained. After comparing their mechanical properties with those of ordinary structural steels after high-temperature cooling, the coefficient of variation fitting equationthe of yield strength and tensile strength were proposed. It is shown that: the yield strength and tensile strength of high-strength fire-resistant steel with natural cooling after fire have been improved, while the modulus of elasticity is not influenced by exposing to fire. The results of this study can be applied to the assessment of load-bearing performance of fire-resistant steel structures after fire.

Strengthening mechanisms of solid solution and precipitation at elevated temperature in fire-resistant steels and the effects of Mo and Nb addition

Fire–resistant properties of Mo-/Nb-added structural steels were evaluated using hot tension tests and constant-load tests. Hot tension tests showed that an increase in the Mo and Nb contents led to a slow decrease in the strength as the holding time increased at a high temperature (600 °C), enhancing the fire resistance. Transmission electron microscopy (TEM) and atom probe tomography analyses revealed that this improved fire resistance stemmed from the annihilation of dislocations at high temperatures, which was suppressed by the solid solution of Mo atoms and fine precipitation of Ti/Nb-enriched MC carbides. Meanwhile, an increase in Ti content decreased fire resistance via precipitation of coarse TiC particles, i.e., in-situ TEM observations showed that dislocations moved easily along the surface of coarse particles by climb, indicating that coarse particles did not play a role in disturbing dislocation movement during deformation at high temperatures. Next, constant-load tests were carried out with a constant load of 50% of the yield strength, while the temperature was increased linearly until failure. The results were in good agreement with the results of the hot tension tests; i.e., the failure temperature increased with an increase in the Mo and Nb contents, indicating improvement in fire resistance.

The Influence of Temperature on the Microstructure and Properties of Nb-V-Ti-Mo Complex Microalloyed High-Strength Fire-Resistant Steel

Nb-V-Ti-Mo complex microalloyed high-strength fire-resistant steel was obtained through two-stage hot rolling and laminar cooling. The results showed that the microstructure of the steel included bainite ferrite and martensite-austenite constituent (i.e., MA) islands. The experimental steel displayed high strength at room temperature, with a yield strength (YS) of 617 MPa and tensile strength of 813 MPa (elongation = 18.5%). As the temperature increased to 700 °C, the high-temperature yield strength gradually decreased. Electron backscatter diffraction (EBSD) was used to analyze the experimental steels at different temperatures. The grain sizes did not grow significantly. A small number of nanoprecipitates with an average diameter of 29.2 nm were distributed in the matrix of the as-rolled specimen. Upon increasing the temperature, the number of fine nanoprecipitates gradually increased, resulting in a gradual decrease in their average diameter, reaching a minimum of 19.4 nm at 600 °C. The Orowan equation explained well the precipitation strengthening effect of the nanoprecipitates that formed at a high temperature. At both room temperature and 300 °C, the Ashby work hardening theoretical curves were consistent with the experimental true stress-strain curves. Dynamic recovery and recrystallization occurred at 600 °C, which caused the experimental true stress-strain curve to deviate from the calculated curve.

Effect of austenitic state before ferrite transformation on the mechanical behavior at an elevated temperature for seismic-resistant and fire-resistant constructional steel

Abstract In this study, steels with different volume fraction of bainite and ferrite were obtained by tuning the rolling process to change the austenitic state before ferrite transformation. The volume fraction of bainite in the steel when the austenite was undeformed was ~83%, and in the steel when the austenite was deformed was ~36%. The yield strength at room temperature of the experimental steel with undeformed austenite was ~63.5 MPa higher than the experimental steel with deformed austenite, while the yield strength at 600 °C of the former was ~104.8 MPa higher than the latter. The main strengthening methods that led to difference in yield strength at elevated temperature are precipitation strengthening, dislocation strengthening and grain boundary strengthening. The experimental steel with higher volume fraction of bainite was characterized by higher Young's modulus and dislocation density at 600 °C, while the volume fraction of precipitates was relatively lower. The higher Young's modulus was mainly because of higher volume fraction of alloyed cementite precipitated from steel with higher volume fraction of bainite at 600 °C. Furthermore, the boundary densities of packet boundaries, block boundaries and sub-block boundaries had an important influence on the yield strength of steel at 600 °C.

Editorial: Special Issue on Materials for Extreme Environments, Part 1

Abstract Limits on materials behavior are among the greatest technical obstacles to improving the safety and reliability of engineering systems. When subjected to extreme environments, materials can behave in unexpected and unpredictable ways. The need to develop materials that can perform well in severe operating environments is a major challenge for materials scientists, and it requires a fundamental understanding of material response under extreme conditions of heat flux, stresses, strain rates, and corrosive environments. Chemical and physical processes must be probed, understood, and controlled at different length scales, over varying time periods for successful development of materials for engineering applications. Further, understanding and exploiting extreme environments is critical for meeting the demanding applications in diverse areas of engineering research for creating new materials, that not only survive but also function under the challenging conditions. It has been a real pleasure to be guest editors for the Materials Performance and Characterization two-part special issue on Materials for Extreme Environments. The call for papers attracted a large number of abstracts from leading scientists and engineers for these special issues. After scrutiny and their acceptance, a total of 48 full-length manuscripts have been submitted for consideration. The first part of this two-part special issue covers 16 papers that were submitted and subjected to the peer review process, performed by experts from the respective fields. Importantly, the issue consists of three review articles, authored by eminent researchers that are of significant addition to this volume. The first article, by Prof. Osamu Umezawa, is an authoritative review on the mechanical properties of high strength alloys at cryogenic temperatures. In his illustrious career, he has immensely contributed to the cryogenic testing procedures and to the understanding of fatigue and fracture of several structural materials operating at cryogenic temperatures. The second article is a review of steels for cryogenic applications from the materials researchers of the Indian Space Research Organization. This article presents an exhaustive study of different classes of steels used for cryogenic applications including recent advances in materials such as high entropy alloys and those processed through additive manufacturing for applications down to 4K. The third article is a review on processing of ZrB2 and HfB2 based ultra-high temperature ceramic materials from the research group of Homi Bhabha National Institute, India. This article discussed the challenges involved in the fabrication of dense di-borides for leading edge applications in hypersonic vehicles. This is followed by 13 manuscripts describing the latest advances in materials for extreme environments. A very interesting article by Perepezko et al. on Mo-Silicide alloys for high temperature structural applications brings out the latest developments in these classes of materials. Advanced materials and manufacturing technologies for gas turbines, creep, creep-fatigue, high strain rate behavior of different types of materials, fibers for high temperature applications, coatings for functional applications, C/SiC fasteners for high temperature applications, fire resistant steels, welding, and tribological properties of superalloys are presented in subsequent manuscripts. These articles describe the latest advances in the materials for extreme environments. We sincerely thank both the authors and reviewers for their hard work and dedication. We wholeheartedly thank the ASTM staff dealing with the issues related to the process of publishing an outstanding journal. We sincerely hope that you enjoy these papers and look forward to the second part of this special issue that is currently under processing.

Microstructure evolution and fire-resistant properties of 690 MPa anti-seismic fire-resistant steel plate

In this precent study, microstructure evolution of a newly developed 690 MPa composite microalloyed anti-seismic fire-resistant steel plate after quenching at 740 °C , quenching at 740 °C + tempering at 400 °C were studied and the fire resistance mechanism was analyzed. Experimental results show that the microstructure of hot rolled state of the tested steel is lath martensite, acicular ferrite, bainite and M/A island. After quenching at 740 °C, the microstructure of steel was composed of lath martensite, polygonal ferrite and retained austenite. The mixed microstructure of tempered troostite + bainite + a small amount of M/A island was obtained after quenching at 740 °C + tempering at 400 °C , it transforms into tempered sorbite + bainite after holding at 600 °C for 3h. A great number of fine carbides containing Mo, Nb, V and Ti precipitate at 600 °C of the quenched + tempered state of tested steel. The mixed microstructure and fine composite carbides make the quenched +tempered steel have excellent fire resistance.

Welding procedure qualification of Q345 grade fire-resistant steel based on Jmat-pro calculation

The welding process of two Q345 grade low Mo (about 0.25 wt.%) fire-resistant steels will be calculated and evaluated in this paper. The continuous cooling transformation (CCT) curve and isothermal transformation (TTT) curve of test steel were calculated by Jmat-pro software. The welding processes of steels were evaluated by measuring the continuous cooling transition (SH-CCT) curves of welding heat affected zone. The test results show that when the actual cooling time of Q1 steel is t 8/5>43s, cracking does not occur in the HAZ of welding and near the fusion zone, while when t 8/5<43s, cracking may occur in the HAZ or fusion zone; When the actual cooling time of Q2 steel is t 8/5>44s, cracking does not occur in the welding heat affected zone and the vicinity of the fusion zone, while when t 8/5<44s, cracking may occur in the heat affected zone or the fusion zone. In addition, Jmat-pro software has a certain accuracy for the calculation of Ac 3, but has a large error for the continuous cooling transformation (CCT) curve, especially the difference between the calculated value and the measured value of Q2 steel is larger, which only has reference value.

Microstructure and High-Temperature Strength in the Weld Coarse-Grained Heat-Affected Zone of Fire-Resistant Steels and the Effects of Mo and Nb Additions

The microstructural evolution and fire-resistant properties in the weld heat-affected zone (HAZ) of Mo and Mo + Nb-added fire-resistant steels were investigated. For this purpose, three Fe-0.1 wt%C-1.5 wt%Mn-0.1 wt%Si steels containing various Mo and Nb contents were prepared. HAZ samples were experimentally simulated using a Gleeble simulator at a welding heat input of 30 and 300 kJ/cm. The yield strength of the HAZ samples was higher than those of base steels at both room temperature and 600 ℃, whereas a greater decrease in the yield strength at 600 ℃ compared to that at room temperature occurred in the HAZ samples than in the base steels, indicating that the fire-resistance deteriorated in the HAZs as compared to the base steels. This is due to the formation of hard phases such as bainite and martensite in the HAZs, i.e., bainite and martensite phase have very high yield strength with high dislocation density at room temperature, while their strengths decrease rapidly at high temperature due to a great annihilation and recovery of dislocations at high temperature. In addition, the fire-resistance of the HAZ improved as the heat input was increased. The alloying of Mo and Nb improved the fire-resistance of both the base steels and the HAZs. Finally, the changes in the microstructures of the base steels and the HAZs upon alloying and the heat input and corresponding effects on the fire-resistance were carefully explored and discussed through transmission electron microscopy analyses, atom probe tomography analyses, and calculations of continuous cooling transformation diagrams.

Mechanical properties of conventional structural steel and fire-resistant steel at elevated temperatures

There has been a growing shift towards the use of fire-resistant steel (FRS) in the construction industry as a replacement for conventional structural steel (CSS). However, only limited research has been undertaken in the past on the structural properties of cold-formed FRS, especially at elevated temperatures. This paper presents the results of an experimental investigation on key mechanical properties of FRS after subjecting to elevated temperatures. The steady-state tensile tests were performed on two different types of FRS and a CSS at different elevated temperatures ranging from 100° to 800 °C with a holding time of 60 min. A total number of 99 tensile coupon test specimens were tested in this study, both at room and elevated temperatures. Important observations have been made in the paper about the performance of FRS at elevated temperatures in terms of stress-strain relationships, mechanical properties, and deformability parameters. The test results indicate the better performance of FRS compared to CSS at elevated temperature greater than 400 °C suggesting a possible application of FRS in steel structures liable to be exposed to fire conditions. Constitutive modeling for steels is also proposed for strength and stiffness properties.

Precipitation behavior of Ti–Nb–V–Mo quaternary microalloyed high strength fire-resistant steel and its influence on mechanical properties

Abstract The microstructures and mechanical properties of Ti–Nb–V–Mo quaternary microalloyed high strength fire-resistant steel are investigated under different experimental conditions. The results show that the fire-resistant steel with high strength (ultimate tensile strength (UTS), 746 MPa; yield strength (YS), 597 MPa) and a low yield ratio (YS/UTS, 0.80) is successfully developed by rapid cooling combined with a low final cooling temperature. The microalloyed high strength fire-resistant steel tempered at 600 °C × 15 min and 600 °C × 180 min exhibit yield strength of 440 and 419 MPa, which values significantly exceed two-thirds of the specified yield strength at room temperature (i.e., 307 MPa) for the Q460 (YS ≥ 460 MPa) fire-resistant steel. The mechanical properties at room temperature and 600 °C even meet the requirements for the Q550 fire-resistant steel. The ferritic grain boundaries in the experimental steel tempered for 180 min at 600 °C show excellent stability. The analysis indicates that solid-solution strengthening and precipitation strengthening are important in improving the strength of the studied fire-resistant steel at room temperature and 600 °C, respectively. M(C, N) (M = Nb, V, Ti, and Mo) and M3C (M = Fe, Cr, Mn, and Mo) particles in small quantities are found in the matrix of the as-rolled sample. However, after reheating and tempering at 600 °C, large amounts of M(C, N) (mainly MC) and M3C particles precipitate. With an increasing in tempering time, the amount of nanoscale MC precipitate increases; especially, the amount of the MC particles size smaller than 10 nm, and the mass fraction of Nb, V, and Mo in the MC precipitate also increases, which is consistent with the equilibrium thermodynamic calculation results. This enhancement of the precipitation strengthening of secondary phase particles compensates for strength loss attributing to a decrease in shear elastic modulus and solid solution strengthening at 600 °C.

Constitutive model of high-performance bolts at elevated temperatures

Mechanical properties of steel at elevated temperatures are critical to the fire-resistant analysis and the fire safety design of steel structures. Thus, one solution for improving the fire resistance is use of high-performance (HP) steel, such as fire-resistant (FR) steel. However, practical use of the FR steel in steel structures implies requirements of sufficient fire resistance for the high-strength (HS) bolts as well, so that the loading capacities of bolted connections commonly used in steel structures can be guaranteed in case of fire. In this paper, material properties of grade 10.9 HP bolts recently developed in China, possessing both fire resistance and corrosion resistance, are tested at various elevated temperatures. Their stress-strain curves, modulus of elasticity, yield strength, ultimate tensile strength (UTS), and elongation percentage after fracture are obtained. These results are compared with that of conventional HS bolts and with reduction factors given in national standards. Constitutive models and prediction equations of the HP bolts at elevated temperatures are proposed. Besides, the relationship between degradation in macro mechanical properties and change in microstructure is clarified. The research outcomes may provide essential bases for the fire response analysis of steel structures applying the FR steel and the HP bolts.

ОЦІНЮВАННЯ ВОГНЕСТІЙКОСТІ ВОГНЕЗАХИЩЕНИХ СТАЛЕВИХ КОНСТРУКЦІЙ

Purpose. Evaluation of fire resistance of fire-resistant steel structures using the developed calculation and experimental method. Methods. Finite difference method, landfill fire test method, mathematical and computer modeling of non-stationary heat exchange processes, determination of thermophysical characteristics of fire-retardant coatings based on solving direct and inverse thermal conductivity problems. Results. Geometric, physical, computer models have been developed, with the help of which the fire resistance of fire-resistant steel structures has been evaluated by the calculation-experimental method. The adequacy of the developed method for assessing the fire resistance of fire-resistant steel structures in assessing the fire resistance of fire-resistant I-beam steel column has been checked. The analysis of tests on fire resistance of fire-resistant steel columns exposed to fire at the standard temperature of the fire without the load applied to them has been carried out. A computer model of the “steel column – reactive flame retardant coating” system has been built for numerical simulation of non-stationary heating of such a system. The fire resistance of fire-resistant steel columns of I-beam section without load applied to them has been evaluated using the calculation-experimental method. Verification of results of experimental research with results of numerical modeling has been carried out. Scientific novelty. The convergence of the results of experimental data on the duration of fire exposure at the standard temperature of the fire to reach the critical temperature of steel with the results of numerical simulations has been determined. Based on the comparison of the experimental results and numerical modeling, the adequacy of the developed model to the real processes that occur when heating fire-retardant steel columns without applying a load under fire conditions at a standard fire temperature has been confirmed. The efficiency of the proposed calculation and experimental method for assessing the fire resistance of fire-resistant steel structures has been confirmed. Practical significance. It consists in the implementation of the results on objects of different purposes in assessing the fire resistance of fire-resistant steel structures by evaluating the effectiveness of fire-retardant coatings of steel building structures.

The fire-risks of cost-optimized steel structures: Fire-resistant and hot-rolled carbon steel

This work studies the behaviour of a steel portal frame selection under fire exposure, considering both span lengths and fire exposure times as variables. Such structures combine carbon steel (S275), fireproof micro-alloyed steel (FR), and coatings of intumescent paint with variable thicknesses, improving thereby the flame retardant behaviour of the steel structure. Thus, the main contribution of this study is the optimization of the portal frames by combining both steels, analysing the resulting costs influence on the final dimensions. Besides, the topological optimization of each steel component within the structure is also defined, in accordance with the following variables: weather conditions, span, paint thickness, and cost of steel. The results mainly confirmed that using both FR and S275 grades with intumescent painting is the Pareto optimum when considering performance, feasibility and costs of such portal frames widely used for industrial facilities.

Розрахунково-експериментальний метод оцінювання вогнестійкості вогнезахищених сталевих конструкцій

Physical and mathematical models for assessing the fire resistance of fire-resistant steel structures have been developed. An algorithm is used, which includes experimental and computational procedures in determining the fire resistance of fire-resistant steel structures. The initial and boundary conditions for the construction of these models are formulated, which allow to predict the fire resistance of the fire-resistant steel structure with sufficient accuracy for engineering calculations. The peculiarity of the developed models is taking into account the thermophysical characteristics of steel structures and fire-retardant coatings, the peculiarities of the formation of fire regimes. Based on the proposed physical and mathematical models, a computational and experimental method for estimating the fire resistance of fire-resistant steel structures has been developed. The adequacy of the developed method was checked when assessing the fire resistance of a fire-retardant steel column. A computer model of a fire-retardant steel column was built to simulate non-stationary heating of such a system in the FRIEND software package. The results of determining the convergence of experimental data on the duration of fire exposure at the standard temperature to reach the critical temperature of steel with the results of numerical simulations in the software package FRIEND. Based on the comparison of the experimental results and numerical simulations, a conclusion is made about the adequacy of the developed model to the real processes that occur when heating fire-retardant steel columns without applying a load under fire conditions at standard fire temperature.

Numerical investigations of fire-resistant steel welded I-section columns under elevated temperatures

Based on the experimental investigations on compressive buckling behavior of fire-resistant steel columns subjected to elevated temperatures, the numerical analysis models were established and validated by the experimental results of 12 fire-resistant steel columns and 15 conventional steel columns. The parametric analysis was performed to examine the influence of cross-section types, axial load ratios, normalized slenderness ratios, initial imperfections, residual stress and temperature elevation procedure on critical temperatures of fire-resistant steel columns. The parametric analysis indicated that: (1) The critical temperatures increased with decreasing axial load ratios and slenderness ratios; (2) The fire resistance of column with higher axial load ratios and middle slenderness ratios were more sensitive to the residual stress and initial imperfections; (3) Cross-section size and temperature elevation curve variations had less influence on critical temperatures. Furthermore, a total of 1674 numerical simulations were compared with the results determined from the current standards. It was found that provisions in Chinese GB 51249 and European EC 3 tended to overestimate the compressive buckling coefficient while underestimate the critical temperature, when applied to fire-resistant steel columns. Based on Perry-Robertson formula and EC 3 critical temperature formula, the fire resistance determination methods were proposed to facilitate the engineering application of the fire-resistant steel. The proposed methods were proved to meet both safety and economic requirements of structural design.

Mechanical properties and modelling of superior high-performance steel at elevated temperatures

Steel structures have been increasingly applied in contemporary infrastructures, which always require high loading capacities and strong resistance to disasters such as earthquake actions, fire and environmental corrosion, etc. Recently a superior high-performance (SHP) steel is developed accordingly, possessing combined beneficial features of high-strength, high-ductility, corrosion-resistance and fire-resistance, which may well satisfy the structural requirements. The present study aims to investigate mechanical properties of the SHP steel at elevated temperatures and to propose its constitutive models for further usage in fire design and analyses of structural members. A comprehensive experimental programme including a series of standard tensile coupon tests at both ambient and elevated temperatures is described herein, and key mechanical properties including modulus of elasticity, yield strength, ultimate tensile stress as well as stress-strain curves of the SHP steels are obtained based on the test results. Reduction effects of elevated temperatures on their mechanical properties are quantitatively clarified, and further comparisons with prediction results in accordance with national standards as well as with independent test results reported in literature are made. It is indicated that the mechanical properties of the SHP steels differ significantly from that of conventional mild steel and fire-resistant steel. Specific prediction equations are proposed for evaluating the temperature-dependent mechanical properties of the SHP steel; furthermore, an adapted Ramberg-Osgood model is developed for describing its full range stress-strain relation. The research outcomes of the present research may serve as an essential basis for better understanding mechanical performance of the SHP steel at elevated temperatures as well as for further investigations on its structural members' behaviour; and it is helpful to promote its application in practice.

The Impact of Interphase Precipitation on the Mechanical Behavior of Fire-Resistant Steels at an Elevated Temperature.

In this study, we address the challenge of obtaining high strength at ambient and elevated temperatures in fire-resistant Ti–Mo–V steel with ferrite microstructures through thermo-mechanical controlled processing (TMCP). Thermally stable interphase precipitation of (Ti, Mo, V)C was an important criterion for retaining strength at elevated temperatures. Electron microscopy indicated that interphase precipitation occurred during continuous cooling after controlled rolling, where the volume fraction of interphase precipitation was controlled by the laminar cooling temperature. The interphase precipitation of MC carbides with an NaCl-type crystal structure indicated a Baker–Nutting (B–N) orientation relationship with ferrite. When the steel was isothermally held at 600 °C for up to 3 h, interphase precipitation occurred during TMCP with high thermal stability. At the same time, some random precipitation took place during isothermal holding. The interphase precipitation increased the elastic modulus of the experimental steels at an elevated temperature. It is proposed that fire-resistant steel with thermally stable interphase precipitation is preferred, which enhances precipitation strengthening and dislocation strengthening at elevated temperatures.

Design analysis of a steel industrial building with wide openings exposed to fire

In order to design a fire-resistant steel structure, the change in the physical and mechanical properties of the steel at high temperatures must be known. As the temperature of steel structural elements increases during fire, their strength decreases considerably. After a certain temperature, these strength drops reach critical levels. Therefore, collapses and various deformations (buckling, arching, etc.) occur. To prevent these collapses during the fire, various fire protection materials must be applied to the structural members such as column and beam. Columns are the most critical structural elements in a steel bearing system. While the possible collapse of the columns may cause the collapse of the whole structure, the beams alone may not cause the collapse of the structure, and the column-beam junctions directly affect the spread of fire. Since there will be many openings and gaps in industrial buildings, the spread and growth of a possible fire becomes very serious. Special fire protection measures are therefore required. In this study, the behavior of a steel industrial structure designed and designed under the influence of Standard Fire (ISO 834) was investigated, the distribution of the temperatures in the structural elements was determined, the required fire protection material was selected, and both protected and unprotected steel temperatures were determined. This design against fire is designed to provide fire resistance for 1 hour (60 min) for this structure. During this period, the type and optimum thickness of the protection material to be applied before reaching the critical temperature values for which the strength of the steel material would lose and would be damaged and compared with the temperatures that would occur in the structural elements without applying fire protection. According to the findings of the study, it was concluded that 25 mm drywall box protection material should be applied on the inner columns and 20 mm on the edge columns and 15 mm on the corner columns. In addition to this, it was concluded that spray beams (intumescent coating) of different thicknesses between 15-20 mm were applied to the beams depending on the location and the load to be affected and the type of joint. After these applied passive fire protection materials, the temperatures obtained in the structural elements reached to 500-550 as a result of 1-hour fire design. These temperatures are acceptable temperature values given the strength drop in critical temperature ranges for steel under the 1-hour fire condition.

Mechanisms for improving tensile properties at elevated temperature in fire-resistant steel with Mo and Nb

Fire resistance of structural steels used in buildings is typically evaluated based on the ratio of the yield strength at 600 °C to that at room temperature. Fire-resistant steel shows a yield strength ratio of more than 2:3 between 600 °C and room temperature and achieves an excellent high-temperature strength. Alloying elements such as Mo and Nb are used to improve the strength at elevated temperatures. In this study, the effects of the addition of Mo and Nb on the fire resistance of steel were carefully investigated using SEM, TEM, in-situ TEM, EPMA, 3D-APT, positron annihilation lifetime spectroscopy, and first principles calculations. The fire resistance in Mo and Nb added steel (Mo + Nb) was shown to be drastically improved compared to plain carbon steel (CMn). The improvement in the high-temperature strength in Mo + Nb was attributed to the precipitation of fine Nb-rich MX particles and a solid solution of Mo and Nb hindering the dislocation movement, and thereby minimizing the dislocation annihilation at elevated temperature. The solid solution of Mo and Nb lowers the vacancy formation energy and allows a vacancy to form more easily, leading to a large lattice distortion which results in a slower dislocation mobility in Mo + Nb.

Longitudinal tensile test on weld metal in fusion zone at high temperature

To determine the mechanical properties of welds used in fire resistant steel structures at different temperatures, the mechanical properties of welds subjected to elevated temperatures are investigated in this study. This is achieved by carrying out steady-state tensile tests on welds composed of Q235 base metal with an E4303 electrode and Q345B base metal with an E5015 electrode. This paper presents the results of analyses on the failure mechanisms and metallurgical structures of welds at high temperatures. Based on the stress-strain curves of welds at a specific temperature, the mechanical parameters and reduction factors are listed (including the elastic modulus, yield strength, and ultimate strength), and comparisons with current design standards are presented. This paper reveals that the elastic modulus and yield strength of the welds gradually decreases with increasing temperature. By contrast, the ultimate strength of welds first increases and then decreases with increasing temperature, reaching a maximum value at 300 °C. The comparison shows that CECS 200:2006, AS 4100, Eurocode 3, and AISC 2010 are very conservative in predicting the degradation of the elastic modulus. The recommendations of current codes are not fully applicable to the reduction of yield strength and ultimate strength of E4303 and E5015 welds, especially at 500 °C and higher, and it is unsafe to use the recommendations of current codes to predict the mechanical properties of welds.