Relationship between mechanical properties and structure in austempered alloyed compacted graphite cast iron

Abstract

Austempering kinetic measurements and mechanical property measurements are presented for a compacted graphite iron of composition Fe-3.4C-3.5Si-0.25Mn-0.50Mo-0.50Ni (wt-%) austempered at 375°C after austenitising at 890 and 940°C. Analysis of the austempering kinetics shows that alloying elements have a similar effect on the processing window as in ductile irons. The mechanical properties show optimum values at austempering times within the processing window. However, the graphite morphology limits the mechanical property enhancement achieved by austempering. Nevertheless, it is possible to double the strength of the as cast compacted graphite iron without loss in ductility.     

Stepped heat treatment for austempering of high manganese alloyed ductile iron

Abstract

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m^?2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.

MST/3119     

Influence of austenitising temperature on austempering kinetics of high manganese alloyed ductile cast iron

Abstract

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics of an alloyed ductile iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, during austempering at 285 and 375°C after austenitising at 870, 900, and 920°C. The austenitising kinetics show that 120 min is sufficient time to produce afully austenitic matrix. The stage I reaction during austempering occurs in two distinct steps: first in the eutectic cell and then in the intercellular areas. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have only a small effect on the stage II kinetics. Decreasing the austenitising temperature results in a more uniform austempered microstructure and reduces the amount of martensite in the structure. These changes shift the heat treatment processing window for high Mn irons to shorter timesfor austempering at 285°C and come close to, but do not open the processing window at 375°C.

MST/3117     

Microsegregation of manganese and silicon in high manganese ductile iron

Abstract

Manganese is known as an inexpensive element and a potent promoter of hardenabilty in ductile iron but it also segregates severely and encourages the formation of carbides in the matrix. Thus, it is suggested that when the Mn content is high, attempts should be made to minimise the Mn segregation. In the present work the effect of solidification rate and homogenisation treatment on the severity of segregation of Mn and Si in several types of wear resistant high Mn ductile iron was investigated. It is demonstrated that increasing the solidification rate, leading to a high nodule count, decreases not only the heterogeneity of these elements but also the carbide content.     

Influence of matrix structure on the fatigue properties of an alloyed ductile iron

Rotary bending fatigue tests were conducted on ductile iron containing 1.25 wt% nickel, 1.03 wt% copper and 0.18 wt% molybdenum with various matrix structures. Several heat treatments were applied to obtain ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic structures in the matrix of a pearlitic as-cast alloyed ductile iron. The tensile properties (ultimate tensile strength, 0.2% yield strength and percent elongation), the hardness and the microstructures of the matrixes were also investigated in addition to fatigue properties. Fractured surfaces of the fatigue specimens were examined by the scanning electron microscope. The results showed that the lowest hardness, tensile and fatigue properties were obtained for the ferritic structure and the values of these properties seemed to increase with rising pearlite content in the matrix. While the lower ausferritic structure had the highest fatigue strength, the upper ausferritic one showed low fatigue and tensile properties due to the formation of the second reaction during the austempering process.     

Influence of matrix structure on physical properties of an alloyed ductile cast iron

Abstract

Measurements of thermal diffusivity, specific heat capacity, and density are reported for seven different matrix structures of a ductile iron of composition Fe–3·5C–2·64Si–0·67Mn–0·007P–0·013S–0·25Mo–0·25Cu–0·04Mg (wt-%). These measurements are used to calculate the thermal conductivity over the temperature range 200–580°C. The matrix structures examined were ferritic–pearlitic, martensitic, and austempered. Matrix structure is shown to play a significant role in determining the thermal conductivity of the ductile iron. The ferritic–pearlitic matrix structures display a higher thermal conductivity than the austempered matrix structures. Matrix structure changes during heating were identified from the thermal diffusivity and specific heat measurements and from microstructural observations. Transformations which occur during heating and promote ferrite formation (such as tempering of martensite and the stage II reaction in the austempered matrix structure) increase the thermal conductivity.     

Effect of matrix structure on the impact properties of an alloyed ductile iron

An investigation was performed to examine the influence of the matrix structure on the impact properties of a 1.03% Cu, 1.25% Ni and 0.18% Mo pearlitic ductile iron. Specimens were first homogenized at 925 °C for 7 h and a fully ferritic structure was obtained in all ductile iron samples. Then, various heat treatments were applied to the homogenized specimens in order to obtain pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic matrix structures. The unnotched charpy impact specimens were tested at temperatures between − 80 °C and + 100 °C; the tensile properties (ultimate tensile strength, 0.2% yield strength and elongation) and the hardnesses of the matrix structures were investigated at room temperature. The microstructures and the fracture surfaces of the impact specimens tested at room temperature were also investigated by optical and scanning electron microscope. The results showed that the best impact properties were obtained for the ferritic matrix structure that had the lowest hardness, yield and tensile strength. Ductile iron with a lower ausferritic matrix had the best combination of ultimate tensile strength, percent elongation and impact energies of all structures.     

Ni合金化球墨铸铁组织和性能研究

为获得兼具较高强度和良好低温冲击韧性的球墨铸铁铸件,向球墨铸铁中加入质量分数约0.5%的Ni进行合金化,并对其进行中温奥氏体化(880℃+3 h)和低温退火(720℃+4 h)处理.采用光学显微镜(OM)、扫描电子显微镜(SEM)对铸态和热处理态试样的显微组织和冲击断口形貌进行分析;利用万能试验机、布氏硬度计和摆锤式冲击试验机等对铸态和热处理态试样进行了室温拉伸、硬度检测、低温冲击等力学性能测试.结果表明:铸态球墨铸铁的微观组织由珠光体、铁素体和球状石墨及少量的渗碳体组成,其强度、硬度偏高,塑性、韧性较差;热处理态试样中的珠光体向铁素体转变后为铁素体和球状石墨,试样强度、硬度有所降低,塑性、韧性得到明显的改善;铸态试样呈现典型的脆性断裂特征,热处理态试样冲击断口处存在少量韧窝,断裂模式以解理断裂为主,伴有少量塑性变形的韧脆混合断裂,且在-40℃冲击功达到12.4 J;比较铸态与热处理态的冲击断口形貌可知,试样断裂方式由脆性断裂转变为韧脆混合断裂.     

Effect of microstructure on properties of ADI and low alloyed ductile iron

Microstructure, tensile, impact, hardness, fractography and wear characteristics were investigated for: (1) Austempered ductile iron (ADI); and (2) low alloyed ductile iron. Comparison has been made between the properties of these two types and that of conventional ductile iron. Detailed analysis, of the fracture mode for the 3 types of ductile iron, which failed under tensile and impact testing, were presented using the SEM. The wear properties were determined using pin-on-ring machine, under dry sliding conditions. The variation of mass loss and coefficient of friction with sliding distance, at different loads and speeds were presented and discussed. The wear mechanisms were investigated by means of subsurface observations. Microhardness test was used to study the change in the matrix strength with distance from the worn surface due to plastic deformation.     

Austenitisation kinetics of unalloyed and alloyed ductile iron

Abstract

Specimens with different microstructures of an unalloyed iron and a 0.75 wt-%Mn ductile iron were austenitised at 870°C to study the kinetics of austenitisation. The specimens were austenitised in a salt bath furnace for different periods. The wafer specimens were ground, polished and etched before metallography and hardness tests. The thermodynamic procedure was used to calculate the Ae ₃ phase boundary as a function of composition. The results were used to explain the effect of segregation on the kinetics of austenitisation. The results showed that the transformation rate in both the alloyed and the unalloyed pearlitic ductile iron is higher than that of the ferritic iron. As far as the ferritic structure is concerned, at the early stage of transformation the alloyed iron showed a higher austenitisation rate than the unalloyed iron, while after longer transformation the unalloyed iron showed the higher rate. This change in transformation rate is supposedly because of the effect of Mn segregation in the intercellular region. In the case of specimens with a pearlitic microstructure, unalloyed iron has a higher austenitisation rate than alloyed iron because of the effect of Mn on the stability of carbides in the pearlite. It was also shown that in all the specimens the cell boundaries are the most potent sites for nucleation of austenite. The Avrami equation was used to calculate k and n parameters as a function of microstructure. The results obtained indicate that n for specimens with different microstructures varies from 1.5 to 3.5. The results also show that the effect of the initial microstructure on the n and k values is greater than that of the chemical composition of the iron.     

The austempering kinetics and mechanical properties of an austempered Cu–Ni–Mo–Mn alloyed ductile iron

Measurements of austempering kinetics and mechanical properties are presented as a function of austempering time over the range 1–4320 min for different combinations of austempering temperature (275, 315, 370 and 400 °C) and austenitizing temperature (870, 900 and 950 °C) for a ductile iron of composition 3.5% C, 2.6% Si, 0.48% Cu, 0.96% Ni, 0.27% Mo and 0.25% Mn. The austempering kinetics are used to calculate processing windows for the three austenitizing temperatures. The mechanical properties are analysed to show that the processing windows accurately predict the austempering times over which the mechanical properties satisfy the ASTM standard. The analysis shows the role of austenitizing temperature, austempering temperature and time in optimizing the mechanical properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of mechanical properties of boro-tempered ductile iron

In this study, the effects of boro-tempering heat treatment on mechanical properties of ductile iron were investigated. Standard tensile test samples and unnotched Charpy specimens were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Micro-hardness measurements were performed on cross-section of the metallographically prepared samples, where cut from fractured impact test samples. The hardness of boride layers was measured in the range of 1654–1867 HV_0.05. It was observed that tempering temperature was more effective on the mechanical properties of the material than boronizing time. Optimum mechanical properties were obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h.     

Study of microstructural evolution and mechanical properties exhibited by non alloyed ductile iron during conventional and stepped austempering heat treatment

It was evaluated the microstructural and mechanical response that a non alloyed ductile iron (DI) presented when was subjected to Conventional Austempering (CA) and Stepped Austempering (SA) heat treatments. X-ray Diffraction (XRD) quantification techniques demonstrated to be the more reliable method for monitoring phase transformations taking place during both CA and SA. When CA was applied some intercellular areas remain untransformed even for long time, however when samples were subjected to SA those untransformed areas disappeared and instead finer ausferrite was found. Additionally mechanical properties values obtained from tensile and impact tests confirmed that for all times used, SA was superior to the CA.     

The relationship between fatigue strength and microstructure in an austempered Cu-Ni-Mn-Mo alloyed ductile iron

Rotating bending fatigue measurements are reported for an austempered ductile iron containing 3.5 wt% C, 2.6 wt% Si, 0.48 wt% Cu, 0.96 wt% Ni, 0.27 wt% Mo, and 0.25 wt% Mn. The iron was austenitized at 870, 900 and 950°C and then austempered at 370 and 400°C for times between 30 and 240 min to obtain various austempered microstructures. The correlation between fatigue strength and austempered microstructure represented by the parameter XγCγ, where Xγ is the amount of high C austenite and Cγ its C content is examined. It is shown that fatigue strength increases as XγCγ increases. The highest fatigue strength is obtained with an ausferrite structure; the presence of martensite and/or carbide in the structure reduces the fatigue strength. Lower austenitizing temperatures increase the fatigue strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Isothermal transformation diagrams for alloyed ductile cast iron

Abstract

Isothermal transformation (IT) diagrams which were determined metallographically are presented for a ductile cast iron alloyed with manganese, molybdenum, and copper, following austenitisation at 870 and 920°C. Isothermal transformations were conducted over the temperature range 275–650°C for times between 0·5 and 120 min. The IT diagrams displayed a nose at ~650 and 425°C for the pearlite and upper bainite reactions respectively. The diagrams could be separated into two overlapping diagrams corresponding to transformations in the eutectic cell and intercellular regions. When compared with previous data for an unalloyed iron, the alloying additions were found to delay the transformation, and to a greater extent above 450°C than below this temperature. Increasing the austenitising temperature was observed to delay the transformation. The effect of a 50 K increase was equivalent to increasing the total alloy content in the eutectic cell area by about 0·4 wt-% in this alloy.

MST/2040     

Modelling microstructural and mechanical properties of ferritic ductile cast iron

Abstract

It is well known that the mechanical properties of ductile cast iron (DCI) depend on its microstructure, and that the microstructure depends on the properties of the melt and the cooling conditions during casting. There have been many studies of the individual elements of the process of casting DCI, but as yet there have been very few examples of modelling the entire process to predict cooling rates, microstructure, and mechanical properties, particularly for large castings. The present paper describes a method of modelling the microstructural and mechanical properties of ferritic DCI, and applies the methods to the case of a large (13 t) thick walled (300 mm thickness) casting. The microstructure calculated includes nodule count, nodularity, ferrite grain size, and percentage ferrite. The mechanical properties calculated include yield stress, tensile strength, elongation, and static upper shelf fracture toughness (J _1C and K _JC). The calculated results compare well with those of a test casting.     

Modelling microstructural evolution and mechanical properties of austempered ductile iron

Abstract

Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminium alloys. The aim of the present research is to develop a generic model that will enable the producers of ADI to optimise their product in terms of microstructure and mechanical properties, hence minimising the need for expensive and exhaustive experimental trials and reducing alloy development lead times.     

Relationship between structure and mechanical properties for aramid fibres

The relationship between structure and mechanical properties for a series of twelve wellcharacterized aramid fibres has been determined. The fibres were produced under a variety of processing conditions and the fibre structure has been characterized using transmission electron microscopy. In particular, both the overall degree of molecular orientation in the fibres and the difference in structure between the fibre skin and core regions have been investigated in detail. The mechanical properties of the fibres have been evaluated using conventional mechanical testing and molecular deformation followed using Raman microscopy to monitor strain-induced band shifts. It has been shown that the mechanical properties of the fibres are controlled by the fibre structure. In particular, it is shown that the fibre modulus is governed by the overall degree of molecular orientation. It is also demonstrated that the fibre strength is controlled principally by the overall molecular orientation but may also be reduced by the presence of a highly-oriented skin region. It has been found that the rate of shift of the Raman bands per unit strain is proportional to the fibre modulus except for fibres with large differences in molecular orientation between fibre skin and core regions. For these fibres the rate of shift reflects the higher orientation of the skin.     

Dry sliding wear of low alloyed austempered ductile iron

Abstract

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s^-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472     

Metallurgical parameters,mechanical properties and machinability of ductile cast iron

Results on the effect of solidification cooling rate on the microstructure, mechanical properties and machinability of spheroidal graphite (SG) iron have been presented. The effect of ferritic heat treatment on the same properties has been also investigated. The microstructural observation, tensile properties and hardness values of the present SG iron has been developed. The tool life criterion was used as a measure of machinability. It was found that during turning of SG iron by using a single point cutting tool, its life increased with decreasing the solidification cooling rate for both sand and metal moulds. The tool life was found to be significantly affected by the variation of nodule characteristics. A decrease in tool life due to an increase of nodule count was observed. The tool life was found to be directly proportional to the ductility of SG iron whether for the as cast or ferritic heat-treated ingots.