Effect of bath and specimen temperature on the thermal stress resistance of brittle ceramics subjected to thermal quenching

The effect of specimen and bath temperature on the failure of brittle ceramics in a thermal quench experiment was studied by quenching glass and alumina rods in water and silicon oil baths at different temperatures. The results were discussed in terms of the variation of heat transfer coefficient of the quenching media and the change in material properties as a function of temperature. It was found that the usual assumption of constant heat transfer coefficient and material properties may lead to considerable errors in the quantitative interpretation of the results of thermal quench experiments. Effective values for the film coefficient of heat transfer for water and oil baths were estimated as a function of film temperature from thermal quench data. Recommendations were made for the selection of quenching media and for the procedure to be followed in reporting the results.     

W18Cr4V合金工具钢在不同介质淬火后性能比较

通过对W18Cr4V合金工具钢在清水、锭子油和高压气体等淬火介质中淬火对比实验,研究W18Cr4V合金工具钢在不同介质淬火工艺处理后的硬度和微观结构.研究结果表明,水淬火时试件表面与中心的温差较大,锭子油次之,高压气体较小;试件高压气体淬火时,温度剃度小,内部冷却比较均匀,可以预计,相应的热应力和热变形也比较小;应用较小压力的氮气能够实现淬透性比较好的W18Cv4V工具合金钢的高压氮气淬火处理.     

Thermal shock behavior of ZrB2--SiC ceramics with different quenching media

The thermal shock behavior of ZrB₂--SiC ceramics was studied with water, air and methyl silicone oil as quenching media, respectively. The temperature of all coolants was room temperature (25°C) and the residual strength of the ceramics after quenching was tested. The strength of the ceramics after water quenching had an obvious drop when the temperature difference, ΔT, was about 275°C, while the residual strength of the specimens quenched by air and silicone oil only varied a little and even increased slightly when the temperature difference was higher than 800°C. The different thermal conductive coefficient of the coolants and surface heat transfer coefficient resulted in the differences in the thermal shock behavior. The formation of oxidation layer was beneficial for improving the residual strength of the ceramics after quenching.     

Analysis of cracking of lithium tantalate (LiTaO3) single crystals due to thermal stress

Quantitative estimation of failure of a LiTaO₃ single crystal due to thermal stress was investigated. Cylindrical test slabs were heated in a silicone oil bath, then subjected to large thermal stress by pouring silicone oil with room temperature. Cracking occurred during cooling. A transient heat conduction analysis was performed to obtain a temperature distribution in a test slab at the time of cracking, using the surface temperatures measured in the test. Then thermal stress was calculated using a temperature profile of the test slab obtained from the heat conduction analysis. It is found from the results of thermal stress analyses and the observation of the cracking in the test slabs that the cracking induced by thermal stress occurs mainly in the cleavage planes due to the stress component normal to the plane. As for a size effect of failure stress, large-sized cylindrical test slabs show lower failure stress than small-sized ones. Four-point bending tests were also performed to examine the relationship between the critical stress for cracking induced by thermal stress and the four-point bending strength. A useful relation was derived for predicting the critical stress for cracking induced by thermal stress from the four-point bending strength.     

基于第一抗热震因子的BN纳米管/Si_3N_4复合材料抗热震性能评价

利用Kingery抗热震断裂理论构建了BN纳米管(BNNTs)强韧化陶瓷复合材料的第一抗热震因子模型,通过真空热压烧结法制备了四组BNNTs含量分别为0.5wt%、1.0wt%、1.5wt%和2.0wt%的BNNTs/Si_3N_4复合材料,并采用水浴淬冷法和三点弯曲法测试了复合材料的抗热震性能(震后弯曲强度和临界热震断裂温差)。测试结果验证了在急剧加热和急剧冷却条件下第一抗热震因子模型的正确性。结果表明:添加BNNTs使BNNTs/Si_3N_4复合材料第一抗热震因子增大,抗热震性能提升。分布在晶界上的BNNTs起到裂纹钉扎、桥联和裂纹偏转作用,增加了裂纹扩展的阻力;纳米管孔隙的存在改变了裂纹扩展路径,提高了BNNTs/Si_3N_4的断裂韧度,从而有效提高了其抗热震断裂能力。     

Finite element calculation and measurement of thermal stresses in quenched plates of high–strength 7075 aluminium alloy

Abstract

Thick parts of high–strength aluminium alloys usually undergo a drastic quench which gives rise to thermal stresses and may cause distortion of products. The control of complex phenomena involved during quenching is achieved by determining the temperature distribution, thermal strains, and residual stresses using the MARC finite element program. In this approach, the thermo mechanical problem is assumed to be uncoupled, i.e. the thermal and mechanical calculations are solved consecutively. First a non–linear heat transfer analysis is performed taking the temperature dependence of the thermophysical properties and transient conditions of convection at the surface of the plate into account. This is followed by a thermo–elastoplastic stress analysis using the predicted temperature distributions, assuming an isotropic behaviour of the material and taking the temperature dependence of its mechanical properties into account. The calculation of thermal stresses occurring during the cold and hot water quenching of a 70 mm thick plate of 7075 alloy using this method shows a good agreement between theoretical predictions and experimental values of residual stresses, as measured by the layer removal method.

MST/2     

Nondestructive Testing of Thermal Shock Resistance of Cordierite/Silicon Carbide Composite Materials after Cyclic Thermal Shock

Two different types cordierite/silicon carbide composite ceramic materials (KS 50 and KZ 50) were used for this investigation. Both materials were exposed to the water quench test from 950°C, applying various numbers of thermal cycles. When refractory samples are subjected to the rapid temperature changes crack nucleation and propagation occurs resulting in loss of strength and materials degradation. The formation of cracks decreases the density and elastic properties of material. Therefore, measuring these properties can directly monitor the development of thermal shock damage level. Dynamic Young modulus of elasticity and strength degradation were calculated using measured values of ultrasonic velocities obtained by ultrasonic measurements. Level of degradation of the samples was monitored before and during testing using Image Pro Plus program for image analysis. The capability of ultrasonic velocity technique and image analysis for simple and reliable nondestructive methods of characterization was presented in this investigation. It was found that both composite materials exhibit good thermal shock resistance.     

Effects of viscous flow on generation of thermal stress and strain during quenching of steel plates

Abstract

Substantial improvements have been made recently in the quality of the mathematical models used to represent the generation of thermal stress and strain during the quenching of steel components. However, viscous processes have received little attention in these models, even though there is frequently sufficient time for creep processes to occur during a quench. The inclusion of stress–relaxation and creep effects in one such model has led to a marked improvement in the degree of agreement between the experimental and predicted residual–stress distributions obtained after an oil quench, at the expense of a modest reduction in the corresponding level of agreement after a water quench. The best correlation is obtained by the representation of stress–relaxation and creep effects by means of a standard linear solid model at temperatures above 230°C, with stress–relaxation rates a function of temperature only. Since the isothermal stress–relaxation tests indicated significant viscous processes at temperatures above 130°C, it is possible that the model could be further refined by consideration of the interaction of viscous processes that occur at different stages in the quench. Results are also presented of the predicted relationship between stress and strain at the surface and centre of a plate during quenching in both water and oil.

MST/3     

The ultrasonic and image analysis method for non-destructive quantification of the thermal shock damage in refractory specimens

In the present work, Mg-exchanged zeolit and silicon carbide were used as starting materials for obtaining cordierite/SiC composite ceramics with weight ratio 50:50.Samples were exposed to the water quench test from 950 °C, applying various number of thermal cycles (shocks). Level of surface deterioration before and during quenching was monitored by image analysis. Ultrasonic measurements were used as non-destructive quantification of thermal shock damage in refractory specimens. When refractory samples are subjected to the rapid temperature changes crack nucleation and propagation occurs resulting in loss of strength and materials degradation. The formation of cracks decreases the density and elastic properties of material. Therefore measuring these properties can directly monitor the development of thermal shock damage level. Dynamic Young modulus of elasticity and strength degradation were calculated using measured values of ultrasonic velocities obtained by ultrasonic measurements. Level of degradation of the samples was monitored using Image Pro Plus program for image analysis. The capability of non-destructive test methods such are: ultrasonic velocity technique and image analysis for simple, and reliable non-destructive methods of characterization were presented in this paper.     

Effect of environmental factors on thermal shock behaviour of polycrystalline alumina ceramics

The effect of the environmental factors on thermal shock behaviour of polycrystalline alumina ceramics was studied by quenching the alumina specimens into various quenching media. The environment factors of quenching media were controlled by changing the temperature of water and changing the concentration of the propylene glycol/water solution. The convection heat transfer coefficient and thermal stress increased as the temperature of cooling water increased and decreased as the concentration of the propylene glycol in water increased. The critical thermal stress which makes the cracks grow catastrophically was found to be generated by the critical cooling rate, and the critical cooling rate of alumina ceramics was found to be a certain value (550 °C/s) and same for all cooling liquids. Therefore, cooling rate was found to be the most influential of the environmental factors in thermal shock.     

Finite element prediction of crack formation induced by quenching in a forged valve

Quenching treatment is commonly used to improve the mechanical properties of steel components. However, in these components, quench cracks are often observed as a result of improper material choice and thermal treatment process. Prediction of quench cracks is important to reduce production cost and to prevent in-service component failure; however, a generally accepted criterion for their formation has to be still established. In this study, finite element prediction of crack formation induced by quenching in a forged valve used in the offshore oil drilling field, is performed by means of the commercial finite element software Abaqus®. Microstructures (austenite, martensite, bainite, pearlite and ferrite) which can be formed during the thermal treatment are taken into account. The simulation results, compared with the effects of the quenching on the actual component, indicate that this type of simulation can effectively predict the quench cracks formation in 3D components.     

Effect of NiCrAlY platelets inclusion on the mechanical and thermal shock properties of glass matrix composites

NiCrAlY platelets modified glass matrix composites were prepared. Their microstructures were characterized, their Young's modulus, fracture strength in bending, Vickers hardness, and indentation toughness were measured, and their thermal shock resistance was studied using quenching-strength and indentation-quench methods. With increasing NiCrAlY content, evident enhancements of the Young's modulus and indentation toughness were obtained. The NiCrAlY alloy inclusion could exert significant influences on the retained bending strength of the samples after quench tests, from 9.6 MPa for NiCrAlY-free glass to 32.0 MPa for 30 wt.% NiCrAlY-containing composites. The indentation-quench tests showed that NiCrAlY alloy inclusion elevated the critical quenching temperatures for propagation of pre-crack, from 150 °C for NiCrAlY-free glass to 225 °C for 30 wt.% NiCrAlY-containing composites. Inclusion debonding and intersection, crack deflection and bridging were observed, and are likely the micromechanisms accounted for the improvement of fracture resistance.     

感应淬火热处理工艺对10CrNi3MoV对称球扁钢力学性能的影响

研究了感应淬火热处理工艺对10CrNi3MoV对称球扁钢力学性能的影响。结果表明,10CrNi3MoV对称球扁钢的淬火温度随着感应淬火轨道频率的提高而显著降低,控制轨道频率7～11 Hz,即可控制球扁钢加热温度840～890℃。感应淬火温度和淬火水量对球扁钢的力学性能存在显著影响,随着感应淬火温度的提高以及淬火水量的降低,腹板部位的屈服强度显著降低,低温韧性变化不明显。二次调质热处理时钢的屈服强度显著降低,应提高钢的奥氏体化温度及奥氏体化均匀程度。感应加热淬火时,高温停留时间短,奥氏体化程度低,淬火后获得了细小的马氏体和贝氏体混合组织,钢的屈服强度较加热炉加热显著提高。     

Investigation of quench sensitivity of high strength Al–Zn–Mg–Cu alloys by time–temperature-properties diagrams

The quench sensitivity of some typical high strength Al–Zn–Mg–Cu alloys, including 7075, 7175, 7050, 7010, 7055, 7085 and 1933, was investigated by time–temperature-properties (TTP) diagrams which were from the present paper and literature. The drop in the mechanical properties due to decreased quenching rate was predicted by quench factor analysis method. The nose temperature of TTP diagrams was the highest for 7055 alloy and the lowest for 7085 alloy. The critical time at the nose temperature was the shortest for 7055 alloy and the longest for 1933 alloy. Decreased quenching rate to 10 k/s led to drop in the properties less than 2% for 7085 and 1933 alloys, but more than 20% for 7075, 7175 and 7055 alloy. Thus, 7075, 7175 and 7055 alloys were the most quench sensitive alloys, while 7085 and 1933 alloys were the least quench sensitive ones. The differences in the quench sensitivity of these alloys were explained mainly based on its chemical compositions.     

乙酸乙酯作潜伏性酸对铁黄表面改性处理的研究

为了改善铁黄颜料的耐热性,运用乙酸乙酯作为潜伏性酸试剂,研究了一种新的铁黄表面均匀包覆二氧化硅的方法,即通过控制乙酸乙酯的水解速率来实现在铁黄表面均匀包覆二氧化硅。对表面均匀包覆二氧化硅后的铁黄颜料的颜色变化、分散稳定性、吸油量进行了表征。结果表明,均匀包覆二氧化硅可有效改善铁黄颜料的综合性能。     

Ca0.5Sr0.5Zr4（PO4）6低膨胀陶瓷的抗热震性

Ｃａ０．５Ｓｒ０．５Ｚｒ４（ＰＯ４）６陶瓷热震后的强度行为和裂纹扩展依赖于材料的膨胀系数和轴膨胀特性，与Ａｌ２Ｏ３陶瓷的热震行为不同，ＣＳ陶瓷热震后的残留强度大于室温强度。作者提出裂纹部分闭合模式解释了这种现象。利用计算的有效裂纹长度分析了常温强度对临界淬冷温差Δｔｃ的影响。     

Effect of water quench process on mechanical properties of cold rolled dual phase steel microalloyed with niobium

Effect of water quench process on tensile properties of microalloyed dual phase (DP) steel was investigated. The results showed that the tensile strength and yield strength decrease while total elongation increases with decreasing quenching temperature. Different quenching temperatures lead to not only different amounts of martensite, but also different volume fractions of new ferrite. Due to exemption of precipitation strengthening, new ferrite will contribute to not only the increase of ductility, but also the improvement of microalloyed DP steel strength.     

双组元掺杂钛硅再结晶石墨传导性能的研究

用煅烧石油焦作填料,煤沥青作粘结剂,钛粉和硅粉作添加剂,采用热压工艺制备了一系列双组元掺杂再结晶石墨.考察了不同质量配比的添加剂对再结晶石墨的热导率、电阻率和抗弯强度的影响以及微观结构的变化.实验结果表明,与相同工艺条件下制备的纯石墨材料相比较,掺杂15wt％钛粉再结晶石墨的传导以及力学性能有较大幅度的提高.在掺杂钛粉15wt％、硅粉<2wt％时,双组元再结晶石墨的常温热导率随着硅粉的掺杂量的增加有所提高.当掺杂钛粉及硅粉分别为15wt％和2wt％时,再结晶石墨RG-TiSi-152的常温热导率可达494W/m·K.但是当掺杂钛粉15wt％、硅粉>2wt％时,随着硅粉的继续增加,再结晶石墨的常温热导率反而降低.而双组元掺杂钛硅再结晶石墨的导电以及力学性能却随着硅粉的掺杂量的增加而降低.XRD分析表明,对于双组元掺杂钛硅再结晶石墨而言,钛元素最终在材料中以碳化钛形式存在,而硅元素则大都以气态形式被逸出,XRD物相图谱中未发现硅及其碳化物的存在.材料RG-TiSi-152的微晶尺寸La以及晶面层间距d₀₀₂分别为864和0.3355nm.     

Ordering and stabilization in quenched CuAlNiMnB alloys

The ordering and stabilization behaviour of a set of CuAlNi alloys with manganese and boron additions have been studied. Specimens were subjected to different thermal treatments, mostly consisting of a direct quench into martensite from different temperatures. The transformation was studied by differential scanning calorimetry, and X-ray diffraction was also used to check the martensite and parent phase structures. According to the results, the first reverse transformation after quenching into martensite stems from two competitive processes, namely the stabilization of the martensite and a decrease of the transformation temperatures due to quenched disorder. The temperature from which the quench is performed is the critical parameter determining whether stabilization or disorder suppression predominates. The degree of stabilization increases with the manganese content.     

Na2O-B2O3-SiO2玻璃焊料连接碳化硅陶瓷接口抗热震性能

采用水淬法对以Na₂O-B₂O₃-SiO₂体系玻璃焊料连接的常压烧结碳化硅试条的抗热震性能进行了研究. 对不同温度下淬火后接口以及断面处微观结构进行了比较与分析, 并对比了不同淬火温度以及固定淬火温度为150℃时多次热循环后连接试条的残余弯曲强度. 结果表明, 对于单次淬火, 当淬火温度为150℃时, 由于热应力引起中间层内部微裂纹扩展, 导致残余弯曲强度迅速降低到(152±28) MPa. 淬火温度在150℃~320℃时, 中间层内部裂纹保持在亚临界状态, 相应弯曲强度基本保持在140 MPa. 继续升高淬火温度至420℃时, 裂纹进一步扩展, 试条残余弯曲强度迅速降低至(32±8) MPa. 对于多次热循环, 当淬火温度固定在150℃时, 经多次热循环, 残余弯曲强度与热循环次数变化不明显, 基本保持在120 MPa左右, 这说明在150℃以下淬火, 连接试条具有较好的抗热循环冲击性能.