In Situ fracture observation and fracture toughness analysis of pearlitic graphite cast irons with different nodularity

Effects of microstructural modification and microfracture mechanisms on fracture toughness of pearlitic graphite cast irons with different nodularity were investigated by in situ observation of microfracture process. Six pearlitic graphite cast irons were fabricated by adding a small amount of Mg as a nodularizing element for graphite, and their microstructures including pearlite, ferrite, graphite, and eutectic carbide were analyzed. Most of ferrites were observed in a layer shape around graphites because of carbon-depleted zones formed near graphites. As the nodularity and nodule count increased, fracture toughness linearly increased in the cast irons except the iron containing many fine graphites. According to in situ observation of microfracture process, cracks initiated at nodular graphites and carbides even at a small load, and then propagated readily through the adjacent graphites or carbides, thereby resulting in the lowest fracture toughness. The cast iron having widely spaced graphites and ferrite layers thickly formed around graphites showed the highest fracture toughness because of the blocking of crack propagation by ductile ferrite layers and the crack blunting and deflection by graphites, which was also confirmed by the R-curve analysis.     

Mechanical properties of compacted graphite cast iron with different microstructures

Tensile strength, fracture toughness and impact properties were evaluated in compacted graphite (CG) cast iron with ferritic, pearlitic and ausferritic microstructures. Ultimate tensile strengths for the ferritic and pearlitic samples were 337 and 632 MPa respectively. The austempered samples showed a significant increment in the strength and recording values between 675 and 943 MPa. The fracture toughness test revealed that the stress–intensity factor K_IC was 34·0 MPa m^1/2 for the ferritic CG iron, 39·7 MPa m^1/2 for the pearlitic and between 51·0 and 58·0 MPa m^1/2 for the austempered irons. On the other hand, CG iron with ferritic matrix exhibited the best impact properties with absorbed energy of 33·3 J. The absorbed energy of the pearlitic CG iron was the lowest, 14·3 J, while the austempered samples showed values between 17·2 and 28·4 J. Complementing these results, the critical crack size was also analysed.     

Ti处理改善船体钢焊接粗晶区的低温韧性研究

采用Gleeble 1500D热模拟试验机对Ti和Al处理船体钢进行不同热输入焊接热模拟实验, 并利用OM和SEM研究了母材和热模拟粗晶区氧化物夹杂及显微组织. 结果表明: Ti处理钢中弥散分布的Ti氧化物具有良好的高温稳定性, 75 kJ/cm的焊接热输入对其形貌、成分及尺寸无影响, 能有效促进晶内针状铁素体(AF)形核长大. Al处理钢中以Al₂O₃为核心的复合夹杂高温易分解, 不能促进晶内AF形核. 线能量大于50 kJ/cm的大热输入条件下, Ti处理钢模拟粗晶区的低温韧性明显高于Al处理钢. t_8/5>40 s时, Ti处理钢中较多的晶内AF组织抑制了M-A岛形成, 细化了基体铁素体组织, Al处理钢中的TiN和Nb(C, N)第二相粒子粗化, 粗晶区晶粒异常长大, 大于Ti处理钢中的奥氏体晶粒尺寸.     

Cracking resistance of spheroidal graphite bainitic carbide-free irons

The paper presents the results of studying the structure and cracking resistance of so-called carbide-free nonalloyed and alloyed bainitic spheroidal graphite irons. The effect of retained austenite on the static cracking resistance of the irons is shown. Transmission electron microscopy (TEM) is used to elucidate the structural affinity of bainitic ferrite (??_B) and dislocation or lath martensite. A high dislocation density (1.1 × 10¹¹ cm^?2) and a small amount of extremely dispersed carbides in bainitic ferrite are the main cause of the increased strength of the irons. The presence of hardened ferrite, along with the possibility of the plastic deformation of retained austenite, yield a high cracking resistance of the bainitic spheroidal graphite irons. The parameters of isothermal hardening are determined at which the maximum resistance to crack propagation is reached.     

Effect of Tin on the Phase Transformations of Cast Irons

Copper, manganese and essentially tin are used as alloying elements for obtaining cast irons with a fully pearlitic matrix in the as-cast state. Addition of tin, at a level of about 0.10-0.15 mass%, seems to be the only practical way for avoiding growth of ferrite in the stable eutectoid reaction and to fully transform the matrix of the material to pearlite in the metastable eutectoid system. While the role of copper and manganese has been previously rationalized, the way tin affects the eutectoid transformation in cast irons is still a matter of debate. The present work makes use of an assessment of the Fe-Sn system and of experimental data in the Fe-C-Sn system to evaluate the effect of tin on phase equilibria in this latter system. One ternary parameter is estimated and the resulting modification is applied to literature data on Fe-C-Si-Sn equilibria. Finally, solid-state phase transformation temperatures are calculated and used to discuss experimental information dealing with pearlitic cast irons. It is proposed that pearlite formation in Sn-bearing cast irons is associated to the transient formation of a Fe₃SnC compound which has an ordered FCC structure.     

Effects of carbon content and solidification rate on thermal conductivity of grey cast iron

The thermal conductivity or diffusivity of pearlitic grey irons with various carbon contents is investigated by the laser flash method. The materials are cast in controlled thermal environments and produced in three dissimilar cooling rates. The cooling rate together with the carbon content largely influence the thermal conductivity of grey iron. Linear relationships exist between the thermal conductivity and the carbon content, the carbon equivalent and the fraction of former primary solidified austenite transformed into pearlite. The work shows that optimal thermal transport properties are obtained at medium cooling rates. Equations describing the thermal conductivity of pearlite,solidified as pre-eutectic austenite, and the eutectic of grey iron are derived. The thermal conductivity of pearlitic grey iron is modeled at both room temperature and elevated temperature with good accuracy.     

In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C–Mn steel

Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C–Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C–Mn steel.     

Microstructure and properties of laser cladding FeCrBSi composite powder coatings with higher Cr content

FeCrBSi alloy powder with higher Cr content was used for laser cladding by employing a 3 kW solid-state laser. Ni- and Fe-based alloy powders, which were more resistant to cracking, were added into FeCrBSi alloy powder with higher Cr content to increase the ductile phases, lower thermal expansion coefficient, and reduce the crack sensitivity of the cladding layer. FeCrBSi alloy powder with higher Cr content combined Ni- and Fe-based alloy powder were cladded on the substrates, which yield two different phases. The hard phases of the cladding layer were mainly composed of carbide phase M₂₃C₆, and the ductile phases which played a lubrication function in the cladding layer were mainly composed of austenite γ-Fe and γ-Ni. The ductile phases increased by adding Ni- and Fe-based alloy powder into FeCrBSi alloy powder with higher Cr content, and the hard phases became sparser relatively. Smooth cladding layers, which were free of macroscopic pores, cracks and void between the adjacent tracks, were achieved. Therefore, the toughness of the cladding layer was improved, and the crack tendency was reduced. Three kinds of composite powder were obtained. The composition and morphology of the cladding layer were analyzed, and the microhardness between the hard phases and the ductile phases was compared. The average microhardnesses of the three cladding layers varied from HV_0.2 760 to HV_0.2 950.     

Effect of low temperatures on charpy impact toughness of austempered ductile irons

Impact properties of standard American Society for Testing Materials (ASTM) grades of austempered ductile iron (ADI) were evaluated at subzero temperatures in unnotched and V-notched conditions and compared with ferritic and pearlitic grades of ductile irons (DIs). It was determined that there is a decrease in impact toughness for all ADI grades when there is a decrease in content of retained austenite and a decrease in test temperature, from room temperature (RT) to −60 °C. However, the difference in impact toughness values was not so noticeable for low retained austenite containing grade 5 ADI at both room and subzero temperatures as it was for ADI grade 1. Furthermore, the difference in impact toughness values of V-notched specimens of ADI grades 1 and 5 tested at −40 °C was minimal. The impact behaviors of ADI grade 5 and ferritic DI were found to be more stable than those of ADI grades 1, 2, 3, and 4 and pearlitic DI when the testing temperature was decreased. The impact toughness of ferritic DI was higher than that of ADI grades 1 and 2 at both −40 °C and −60 °C. The impact properties of ADI grades 4 and 5 were found to be higher than that of pearlitic DI at both −40 °C and −60 °C. The scanning electron microscopy (SEM) study of fracture surfaces revealed mixed ductile and quasicleavage rupture morphology types in all ADI samples tested at both −40 °C and −60 °C. With decreasing content of retained austenite and ductility, the number of quasicleavage facets increased from ADI grade 1–5. It was also found that fracture morphology of ADI did not experience significant changes when the testing temperature decreased. Evaluation of the bending angle was used to support impact-testing data. Designers and users of ADI castings may use the data developed in this research as a reference.     

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

High-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.     

Characterization of expanded austenite developed on AISI 316L stainless steel by plasma carburization

Expanded austenite generation through ion carburizing of AISI 316L using two different reactive gas mixtures (Ar 50%, H₂ 45%, CH₄ 5% and Ar 80%, H₂ 15%, CH₄ 5%) has been studied. It was found that an ∼ 14 µm surface layer of expanded austenite was developed with 30 min processing for both gas mixtures. Nevertheless, AES analyses have shown that on the ∼ 150 nm surface layer carbon in a concentration of ∼ 12% was diffused and located as carbide. For longer periods of processing, while for the gas mixture with 50% of Ar no significant modifications within those 150 nm surface layer were produced, for the gas mixture with 80% of Ar a gradual increase in the carbon concentration with time was found, with the extra carbon remaining as free carbon. The difference between both situations can be attributed to the different resulting current densities that have been of 7.0 mA cm^− 2 and 8.1 mA cm^− 2 for 50% and 80% of Ar respectively. Higher current densities result in higher carbon and Ar ions fluxes inducing, from one side surface element concentration modification through sputtering, and from the other the enhancement of carbon diffusion on the first hundred nanometers of the surface layers. This free carbon on top of the surface layers can act as solid lubricant reducing wear rate. Nevertheless, and in spite of the fact that expanded austenite was proved to be corrosion resistant, a reduction against NaCl solution corrosion in relation to the base material was observed. This lost to corrosion resistance can be attributed to carbide development on the layers closer to the surface that can work as a trigger for localized corrosion.     

Microindentation fracture behavior of surface modified alumina ceramic using glass infiltration

This study investigates the effect of a low expansion glass (Mg₃Al₂Si₆O₁₈) treatment on the surface fracture toughness of sintered alumina. The surface fracture toughness was determined by direct indentation method (Vickers indentations), carried out at different loads ranging from 9.8 to 196 N. The crack lengths on the surface at each load were found to be decreased (8–12%) by glass treatment and the corresponding crack resistance values increased by about 17–20%. Both sintered and glass treated specimens showed rising trend in crack resistance values as the indentation load was increased. There was also a significant increase in the Weibull modulus value of crack resistance. Improved properties of glass treated sample were attributed to the formation of a relatively larger process zone surrounding the crack, crack arrest behavior due to the compressive stresses and the crack bridging phenomena. The compressive stresses were generated from the thermo-elastic properties mismatch: (a) between the glass and the ceramic in the glass infiltrated zone, and (b) the glass–ceramic composite layer and the ceramic substrate.     

超低碳贝氏体钢埋弧焊焊接接头韧化机理分析

对超低碳贝氏体钢埋弧焊焊接接头焊缝区显微组织进行分析,发现焊缝区韧性与针状铁素体的含量存在一定的关系。对焊接接头组织进行扫描电子显微镜观察和能谱分析。结果表明,焊缝区针状铁素体以Al₂O₃夹杂物为核心多维形核呈放射状生长;利用EBSD对焊接接头组织进行晶体学取向研究,结果发现,焊缝针状铁素体晶粒取向并不完全随机分布,在某些晶体学方向上存在取向择优;各针状铁素体之间呈大角度晶界;从同一夹杂物上长出的针状铁素体,沿同一方向背向生长的针状铁素体具有相同的取向,可能因为它与奥氏体具有一定的晶体学取向关系。     

含Ti微合金钢中的第二相粒子对焊接粗晶热影响区组织及韧性的影响

利用碳萃取复型技术研究了含Ti微合金钢及其模拟粗晶区 (CGHAZ)中的第二相粒子 ,并利用OM(光镜 )、TEM(透射电镜 )及系列冲击试验对含Ti微合金钢及一种成分相近的不含Ti低合金高强钢焊接粗晶区的组织及韧性进行了研究。研究结果表明 ,含Ti微合金钢中含有大量的、尺寸细小的TiN粒子 ,这些粒子非常稳定 ,在焊接热循环过程中能有效地阻止奥氏体晶粒长大 ,抑制粗大贝氏体的形成 ,促进针状铁素体析出及M -A组元的分解 ,从而显著改善低合金高强钢焊接粗晶热影响区的韧性 ,t8/5(80 0～5 0 0℃冷却时间 )越大 ,这种改善作用越明显     

Thermodynamic study of inclusion formation in low alloy steel weld metals

Abstract

The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO₂.3Al₂O₃) is found to be formed in addition to Ti₃O₅ in the steel melt at 1800 K when the melt has a wt-%Al/wt-%O ratio of ～0·6 optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al₂O₃), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with pre-existing mullite. Titanium additions are found to be beneficial to acicular ferrite production by decreasing the formation of ineffective glassy oxide.     

Microstructure and mechanical properties of laminated ZrB2-SiC ceramics with ZrO2 interface layers

Laminated ZrB₂-SiC ceramics with ZrO₂ interface layers were successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and 14.4 ± 0.3 MPa m^1/2 for parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m^1/2 for perpendicular direction. The fracture toughness for parallel direction is improved significantly compared to monolithic ZrB₂-SiC ceramics. The toughening mechanism was attributed to the deflection and branch of the crack and the new microcracks, which would increase the propagation path and fracture work.     

Effect of austenitising conditions on the impact properties of an alloyed austempered ductile iron of initially pearlitic matrix structure

This study sets out to investigate the effect of austenitising conditions on the microstructure and impact properties of an austempered ductile iron containing Copper and Nickel and having an initially fully pearlitic structure. Un-notched Izod impact test specimens were solution treated in the range 850–1000 °C for durations between 15 and 360 min. and then austempered at 360 °C for 180 min.

It was shown that increasing the austenitising temperature increases the amount of carbon taken in solution by the original austenite. This reduces the driving force controlling the transformation of the austenite to the ausferrite product, ferrite and austenite. As a result, the retained austenite volume in the final microstructure increases but simultaneously its stability falls. This places an upper limit on the austenitising temperature and the amount of retained austenite permissible. On the other hand, for optimum properties, the austenitising temperature and time must be high and long enough respectively to ensure complete austenitisation.

It was also shown that generally, in irons with an initially pearlitic structure, the impact properties increase steadily to a maximum value as the austenitising time increases to about 180 min. and remain constant as the soaking period extends further. Optimum properties are obtained following austenitising between 850 and 900 °C for durations of 120 to 180 min. and correspond to heat treatment cycles which saturate the initial austenite with carbon.     

外场处理对低合金钢焊接接头组织和性能的影响

研究了电磁搅拌、氩弧表层处理、超声喷丸处理对低合金钢焊接接头组织和性能的影响。结果表明 :电磁搅拌促进奥氏体晶粒内针状铁素体生成和细化 ,使针状铁素体的含量由 85%提高到 92 % ,从而提高了焊缝金属的韧度。氩弧加热处理使焊缝表层发生熔化和重结晶 ,可消除焊缝表层的柱状晶 ,处理后的组织主要由针状和细晶铁素体组成。对接头焊趾部位进行氩弧快速重熔处理可消除焊趾部位的几何截面突变 ,使焊缝与母材圆滑过渡 ,有利于消除焊趾部位的应力集中 ,对抗应力腐蚀性能十分有益 ;焊趾部位热影响区中粗大的晶粒细化。表面高能喷丸处理使焊接接头表面层形成尺寸均匀的等轴状纳米晶 ,不仅提高了表层的硬度 ,而且可使焊接接头的疲劳强度提高 50 %。     

Sinter and hot isostatic pressing (HIP) of multi-wall carbon nanotubes (MWCNTs) reinforced ZTA nanocomposite: Microstructure and fracture toughness

This work describes the microstructure and fracture toughness of zirconia toughened alumina (ZTA) nanocomposite in which multi-wall carbon nanotubes (MWCNTs) and nanosized ZrO₂ particles were used as reinforcement. The ZTA nanocomposites with additions of 0, 0.005, and 0.01 wt.% MWCNTs and 2 wt.% nanosized ZrO₂ particles were pressureless sintered in an anti-oxidant sagger with graphite powder bed at 1520 °C during 1 h in air and then HIPed at 1475 °C in argon atmosphere 1 h at a pressure of 150 MPa. Relative densities ranging 94–98% were reached. In HIPed composites the hardness and fracture toughness values were increased up to ∼17% and ∼37%, respectively, compared to the “as sintered” composites free of carbon nanotubes. A combined fracture mode, crack deflection, pull-outs of a small amount of carbon nanotubes, and bridging effect were the mechanisms leading to the improvement in fracture toughness.