Microstructures and mechanical properties of creep resistant steel for application at elevated temperatures

The subject of this paper is the microstructural and mechanical characterisation of regions of the heat-affected zone (HAZ) in steels containing 9–12% Cr that are used for operation at elevated temperatures. Tests were performed on regions in the HAZ, which was created by physical simulation using a thermal welding simulator. Half of the simulated samples (SSs) were tested at room temperature (RT) and at an operating temperature (OT) of 600 °C immediately after simulation/welding, while the rest of the simulated samples were tested at RT and at the OT after heat treatment following the welding, i.e., post-weld heat treatment (PWHT). In addition to the results from mechanical testing, the results from microstructural analysis using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are also presented. The manner in which PWHT contributes to the creep resistance of the HAZ in P91 steel is demonstrated.     

Thermal and mechanical properties of high-strength structural steel HSA800 at elevated temperatures

The temperature dependent material properties of structural steel are important for fire resistant (FR) design and fire simulation of steel structures. In this study, the material tests were conducted for a high-strength steel developed in Korea, namely HSA800, to determine thermal and mechanical properties at elevated temperatures up to 1000 °C. Then test results were compared with current design models – ASCE, Eurocode 3 and AISC to verify which design models are fit well to the HSA800 at the elevated temperature. The thermal properties test results show that the specific heat and thermal strain at elevated temperatures agreed well with those predicted by current design models but thermal conductivity was slightly less than the design models. In the mechanical properties test results, the ASCE model was adequate for predicting yield strength of HSA800. Eurocode 3 and AISC models were unconservative for predicting yield strength but conservative for elastic moduli. The stress–strain relationships of HSA800 at elevated temperatures were developed using Ramberg–Osgood model and they agreed well with the test results up to 2% strain.     

Influence of elevated temperatures on the mechanical properties of foamed geopolymer materials

Current research focuses heavily on geopolymer concrete as possible applications for insulation materials. The aim of the research is to test the strength properties of lightweight geopolymer concrete after exposure to high temperatures. Waste material from the Wieczorek mine (Poland) was used to produce the foamed geopolymers. Alkaline activation took place by mixing the mine powder with an aqueous solution of sodium hydroxide combined with an aqueous sodium silicate with a concentration of 10 M. Prepared geopolymer samples after temperature curing at 75 °C for 24 hours in a laboratory dryer, they were seasoned for 28 days, after which the strength properties were determined. Mechanical tests: compressive strength and bending strength were carried out at temperatures: 20 °C, 200 °C, 600 °C, 800 °C, 1100 °C. Research has shown the precursor activation with the presence of hydrogen peroxide enabled the manufacturing of foamed geopolymers. Heating in the temperature range up to 1100 °C influenced, to some extent, the total porosity of the tested foams. The geopolymer foams based on coal gangue present stable mechanical properties in the range up to 800 °C. No sharp mechanical performances decrease or material chipping was observed. Only colour change of heated samples occurred.     

Mechanical and electrical properties of medium carbon steel with eutectic coatings at elevated temperatures and tensile stresses

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 1, pp. 84–87, January–February, 1990.     

Creep of mild steel at low temperatures

Creep data for mild steel at low temperatures (360–400°C) have been analysed and it has been found that the creep behaviour is similar to that at higher temperatures. The variation of secondary creep rate with stress and temperature, in the temperature range (360–538°C) may be described by two equations with a transition stress. This transition stress is 410 MPa at 360°C. For low stresses the creep equation is given by $\text{ε}{}_{\mathrm{<mtext>s</mtext>}}\text{= 250 σ}{}^{3.7}\text{exp}\text{{?(2.17 × 10}{}^5\text{/}\text{RT}\text{)}}$. Material response to stress is highly dependent on the amount of cold work; secondary creep rate decreases with increasing prior deformation.     

Phase stability and mechanical properties of wire+arc additively manufactured H13 tool steel at elevated temperatures

Wire+arc additive manufacturing(WAAM)is considered an innovative technology that can change the manufacturing landscape in the near future.WAAM offers the benefits of inexpensive initial system setup and a high deposition rate for fabricating medium-and large-sized parts such as die-casting tools.In this study,AISI H13 tool steel,a popular die-casting tool metal,is manufactured by cold metal transfer(CMT)-based WAAM and is then comprehensively analyzed for its microstructural and mechanical properties.Location-dependent phase combinations are observed,which could be explained by nonequilibrium thermal cycles that resulted from the layer-by-layer stacking mechanism used in WAAM.In addition,remelting and reheating of the layers reduces welding anomalies(e.g.,pores and voids).The metallurgical characteristics of the H13 strongly correlate with the mechanical properties.The combinations of phases at different locations of the additively manufactured part exhibit a periodic microhardness profile.Martensite,Retained Austenite,Ferrite,and Carbide phases are found in combination at different locations of the part based on the part’s temperature distribution during additive deposition.Moreover,the tensile properties at elevated temperatures(23℃,300℃,and 600℃)are comparable to those from other WAAM and additive manufacturing(AM)processes.The X-ray diffraction results verify that the microstructural stability of the fabricated parts at high temperatures would allow them to be used in high temperatures.     

The effect of elevated temperatures on the mechanical properties of B-Al composites

The mechanical properties of an aluminium alloy reinforced with 20 and 50 vol % boron fibres have been obtained from specimens in the as-received condition and after heating to 600° C for various periods of time. Heating the specimens caused a reduction in the load-bearing capacity of each specimen and the eventual growth of a reacted layer at the fibre-matrix interface. As-received specimens fractured in a sudden brittle manner; however, those specimens heated for 4 h at 600° C slowly pulled apart. The variation in the static strength of the as-received and the heated specimens is explained in terms of upper and lower bounds calculated from a knowledge of the statistical strength variation exhibited by individual fibres.     

纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO2的钢纤维混凝土(NSFC)、钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、劈裂和抗折强度等力学性能,对不同温度热处理后的微观结构进行了SEM分析,对钢纤维与过渡区界面的相结构进行了XRD分析.结果表明:在测试温度范围内,NSFC的抗压、劈裂和抗折强度均高于SFRC和NC的强度,且在400℃时达到最大值.在常温下,NSFC的抗压、劈裂和抗折强度较NC分别提高27.01％、63.28％和54.12％,400℃高温热处理后比NC分别高35.09％、84.62％和87.23％; SEM分析表明,在钢纤维与过渡区的界面处,致密度提高,显微硬度提高.由于固相反应,使界面区结构发生变化,在钢纤维表层形成扩散渗透层(白亮层),即化合物层,呈锯齿状,XRD分析证明,白亮层主要由FeSi2和复杂的水化硅酸钙组成,从而增强了钢纤维与基体的粘结力,提高了混凝土的高温力学性能.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Protection of steel against hydrogen embrittlement at elevated temperatures

This article reports data on the effect of inhibitors, electrolyte (6 N and 14 N H₂SO₄ solutions), temperature and time of preliminary holding in the electrolyte on hydrogen embrittlement of steel. It was established that inhibitor additions offer an effective protection against hydrogen embrittlement of steel strained in an electrolyte at room temperature, but are less effective at elevated temperatures.     

新型耐火耐候钢材高温力学性能与本构模型研究

火灾下钢构件断裂破坏是导致整体结构连续性倒塌的主要原因之一,掌握钢材高温断裂性能是研究钢结构抗火承载性能和评估高温后结构安全性的基础。以Q355钢为研究对象,设计光滑圆棒和缺口圆棒试件,进行火灾全过程(升温段、降温段、高温后)拉伸断裂试验,研究复杂应力状态(应力三轴度)和温升历程(峰值温度和拉伸温度)对钢材工程和真实应力-应变曲线、断裂应变的影响,并通过电镜扫描研究其微观断裂机理,结合数值模拟对高温断裂模型进行参数标定。研究表明：火灾全过程下Q355钢材呈现韧性断裂特征,拉伸温度和应力三轴度对其断裂性能影响较大;拉伸温度和峰值温度越高,断裂应变越大,延性越好,高温后断裂性能与常温相似;应力三轴度影响材料断裂性能对温度的敏感性,温升历程影响真实应力-应变曲线塑性段斜率;SMCS断裂模型适用于预测Q355钢材火灾全过程断裂行为,需采用不同参数表征钢材升温段和降温段断裂性能。

     

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10?nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.     

Influence of elevated temperatures on the mechanical properties of blended cement concretes prepared with limestone and siliceous aggregates

The investigation performed was aimed at showing the influence of high temperatures on the mechanical properties and properties that affect the measurement by non-destructive methods (rebound hammer and pulse velocity) of concrete containing various levels (10% and 30%) of pozzolanic materials. Three types of Pozzolans, one natural pozzolan and two lignite fly ashes (one of low and the other of high lime content) were used for cement replacement. Two series of mixtures were prepared using limestone and siliceous aggregates. The W/b and the cementitius material content were maintained constant for all the mixtures. Concrete specimens were tested at 100, 300, 600 and 750 °C for 2 h without any imposed load, and under the same heating regime. At the age of 3 years, tests of compressive strength, modulus of elasticity, rebound hummer and pulse velocity were come out. Results indicate that the residual properties of concrete strongly depend on the aggregates' and the binder type. Relationships between strength of concrete as well as rebound and pulse velocity versus heating temperatures are established. The above results are evaluated to establish a direct relationship between non-destructive measurements and compressive strength of concrete exposed to fire.