Influence of nickel and cobalt on microstructure of silicon solution strengthened ductile iron

‘Second Generation’ ductile iron with a silicon content of up to 4.3 wt-% exhibits a fully ferritic matrix, which is solution strengthened by silicon. Outstanding advantages of these ductile iron grades result in their strongly increasing demand. However, due to a presumed formation of a silicon long range order, the maximum strength is limited to 600 MPa at 4.3 wt-% silicon. At higher silicon content, the mechanical properties dramatically decrease. In order to increase the maximum achievable strength, the potential of additional solution strengthening elements is subject of present research. Initially, the effects of cobalt and nickel on matrix, graphite shape and nodule count are investigated. Cobalt and nickel are identified as promising candidates for further solid solution hardening.     

Effects of cobalt on mechanical properties of high silicon ductile irons

High silicon ductile irons are being developed due to their advantages relating to pearlitic-ferritic grades (high ductility, fully ferritic structures, good machinability, etc.). Recent studies reported that silicon contents higher than 5.2?wt-% originates drastic embrittlement due to chemical ordering. For improving the mechanical properties, the addition of other alloying elements becomes an interesting way of work. This study focuses on the cobalt effect on as-cast microstructures and mechanical properties of ductile irons with silicon contents that maximise ultimate tensile strength. The results obtained show that addition of 4?wt-% cobalt increases the ultimate tensile strength by about 10% and decreases the silicon content at the maximum in this property respecting the unalloyed alloys because cobalt enhances ordering as does silicon.     

Influence of aluminium on silicon microsegregation in solution strengthened ductile iron

The excellent static mechanical properties of solid solution strengthened grades of ferritic ductile iron (SSFDI) are adjusted by elevated silicon contents and undergo a rapid loss above a critical content of about 4.3?wt-% silicon. This phenomenon is attributed to the formation of iron-silicon superstructures that is intensified in particular by the formation of silicon microsegregation. In order to affect silicon microsegregation, the alloying concept of SSFDI has been modified. In the present investigations, an inversion of the silicon microsegregation profile could be achieved by alloying a nodular cast iron melt with 1.2?wt-% aluminium. The results provide a metallurgical tool to shift the silicon embrittlement to higher silicon contents and thus to further enhance the mechanical properties of SSFDI.     

Simplification of heat treatment process in a tool steel by aluminium addition

A novel concept for simplification of heat treatment process in a tool steel by the addition of aluminium has been proposed in this research. The addition of 1.08?wt-% aluminium leads to an approximately fully pearlitic state, of which the cementite lamellae are largely spheroidised. Excessive addition of 1.58?wt-% aluminium would result in the formation of a large amount of δ-ferrite. These results are mainly attributed to the synthetic effect of aluminium on the driving force of pearlite transformation and the inter-spacing distance between the proeutectoid carbides. The mechanical properties’ analyses show that aluminium has promising potential to substantially simplify the processing method for developing a relative low-cost mould steel without the concomitant mechanical properties’ reductions.     

Effect of silicon and aluminium in ferrite on tensile and impact properties

Two ferritic interstitial-free steels with approximately the same amount of solid solution strengthening by addition of 2?wt-% silicon and 4?wt-% aluminium are investigated using quasi-static tensile and dynamic impact tests. The addition of 2?wt-% silicon (2Si) results in brittle fracture in V-notched Charpy impact tests at ambient temperature, whilst the 4?wt-% aluminium-containing (4Al) steel has high absorbed energy of 320?±?12?J?cm^?2. In addition, the 4Al steel has a ductile-to-brittle-transition temperature (DBTT) ～60°C lower than the 2Si steel. It is proposed that the addition of silicon suppresses dislocation cross-slip at high strain rate and is responsible for the observed cleavage fracture and high DBTT in the 2Si steel. The ease of dislocation slip in the 4Al steel increases the impact toughness.     

Control of mechanical and wear properties of a commercial Al-Si eutectic alloy

The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.     

Effects of Alloying Elements on the Microstructures and Mechanical Properties of Heavy Section Ductile Cast Iron

The effects of alloying elements on the as-cast microstructures and mechanical properties of heavy section ductile cast iron were investigated to develop press die material having high strength and high ductility. Measurements of ultimate tensile strength, 0.2% proof strength, elongation and unnotched Charpy impact energy are presented as a function of alloy amounts within 0.25 to 0.75 wt pct range. Hardness is measured on the broken tensile specimens. The small additions of Mo, Cu, Ni and Cr changed the-as-cast mechanical properties owing to the different as-cast matrix microstructures. The ferrite matrix of Mo and Ni alloyed cast iron exhibits low strength and hardness as well as high elongation and impact energy. The increase in Mo and Ni contents developed some fractions of pearlite structures near the austenite eutectic cell boundaries, which caused the elongation and impact energy to drop in a small range. Adding Cu and Cr elements rapidly changed the ferrite matrix into pearlite matrix, so strength and hardness were significantly increased. As more Mo and Cr were added, the size and fraction of primary carbides in the eutectic cell boundaries increased through the segregation of these elements into the intercellular boundaries.     

Effect of heating temperature and magnesium content on the thermal cyclic failure behaviour of ductile irons

Abstract

This study elucidates the effect of residual magnesium content and heating temperature on the thermal cyclic failure behaviour of ductile irons by applying repeated heating and cooling cycles. Five irons with different residual magnesium contents ranging from 0.038 to 0.066 wt-% were obtained by controlling the amount of nodulariser additions. The thermal fatigue cracking behaviour was investigated during thermal cycling from 25°C to 650, 700, 750, and 800°C, respectively. Experimental results indicate that the thermal fatigue cracking resistance of ductile iron decreases with increasing residual magnesium content. The maximum heating temperatures of 700°C and 750°C led to the most severe thermal fatigue cracking in the specimens containing 0.054 wt-% and 0.060 wt-% residual magnesium content. Recrystallisation of ferrite grain occurred when the thermal cycles exceeded a certain number after testing at 800°C, which deferred the initiation of thermal fatigue cracking.     

Investigation of WC–Co alloy properties based on thermodynamic calculation and Weibull distribution

The influence of alloy composition and sintering temperature on the mechanical properties and reliability of WC–Co cemented carbides was studied theoretically and experimentally. For the first time, through a hybrid approach of thermodynamic calculations and Weibull distribution, the comprehensive performance of ultrafine WC–Co cemented carbides with different C contents and inhibitor type was investigated in detail. The carbon content of WC–10?wt-% Co–0.5?wt-% Cr cemented carbides was carefully controlled within the range of 5.38?5.52?wt-%. The contents of Cr and V are chosen to be in the range of 0–1?wt-%. It is found that WC–10?wt-% Co–0.5?wt-% Cr alloys with 5.46?wt-% C or 5.5?wt-% C show excellent mechanical properties and high reliability. WC–10?wt-% Co alloys with 0.5?wt-% Cr and 0.4?wt-% Cr–0.2?wt-% V demonstrate high mechanical property and reliability. The results of this study can be used to design process parameters during the manufacture of WC–Co cemented carbides.     

Analysis of mechanical properties and its associated fracture surfaces in dual‐phase austempered ductile iron

This work aims at evaluating the fracture surfaces of tensile samples taken from a new kind of ductile iron referred to as ‘dual‐phase Austempered Ductile Iron (ADI)’, a material composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. The tensile fracture surface characteristics and tensile properties of eight dual‐phase ADI microstructures, containing different relative quantities of ferrite and ausferrite, were studied in an alloyed ductile cast iron. Additionally, samples with fully ferritic and fully ausferritic (ADI) matrices were produced to be used as reference. Ferritic–pearlitic ductile irons (DI) were evaluated as well. For dual‐phase ADI microstructures, when the amount of ausferrite increases, tensile strength, yield stress and hardness do so too. Interesting combinations of strength and elongation until failure were found. The mechanisms of fracture that characterise DI under static uniaxial loading at room temperature are nucleation, growth and coalescence of microvoids. The fracture surface of fully ferritic DI exhibited an irregular topography with dimples and large deformation of the nodular cavities, characteristic of ductile fracture. Microstructures with small percentages of ausferrite (less than 20%) yielded better mechanical properties in relation to fully ferritic matrices. These microstructures presented regions of quasi‐cleavage fracture around last‐to‐freeze zones, related to the presence of ausferrite in those areas. As the amount of ausferrite increased, a decrease in nodular cavities deformation and a flatter fracture surface topography were noticed, which were ascribed to a higher amount of quasi‐cleavage zones. By means of a special thermal cycle, microstructures with pearlitic matrices containing a continuous and well‐defined net of allotriomorphic ferrite, located at the grain boundaries of recrystallised austenite, were obtained. The results of the mechanical tests leading to these microstructures revealed a significant enhancement of mechanical properties with respect to completely pearlitic matrices. The topographies of the fracture surfaces revealed a flat aspect and slightly or undeformed nodular cavities, as a result of high amount of pearlite. Still isolated dimple patterns associated to ferritic regions were observed.     

Microstructure and heat treatment effect on transformation strain in steels: part1 experiment

Anisotropic strain is observed during phase transformation even though no external stress is applied when band structure is formed. The anisotropic strain can result in a change in shape and dimension or residual stress after heat treatment process and thus it achieved much attention. In this paper, three steel grades with different carbon content were prepared to explore the effect of carbon amount and resulting pearlite band thickness as well as heat treatment condition on anisotropic dilatation behaviour during phase transformation. Steel grades containing 0.05 and 0.15?wt-% carbon exhibit a pronounced anisotropy during reverse transformation, whereas 0.44?wt-% carbon shows more isotropic transformation strain. During cooling, transformation strain is anisotropic but less pronounced than that during heating.     

Influence of silver on structure and properties of low-carbon steel

Abstract

Silver has only slight solubility in low–carbon steel at elevated temperatures (~ 0·08 wt-% at the solidus) and precipitates on cooling as fine particles (< 10 nm) in the interphase mode. When present in as-cast steel at an estimated volume fraction of ~ 0·0006, silver can cause precipitation strengthening of up to 50 MN m^?2. In as–rolled and in normalized steels, silver (0·02 wt-%) improves both strength and toughness mainly through grain refinement, with only a small influence from precipitation hardening. An important observation is the improvement to the heat affected zone toughness of high heat input welds which can result from small silver additions (0·02 wt-%) to C–Mn–AI–V steels. However, because of its relatively high cost, it remains to be demonstrated whether silver will impart sufficient benefit to steel properties to be commercially acceptable as an alloying addition.

MST/44     

Combined effects of high-temperature rolling and ultrafast cooling on the mechanical properties of low-carbon steel

The role of ultrafast cooling (UFC) on the grain refinement of ferrite, the precipitation behavior of cementite particles and the mechanical properties of a mild steel (Q235 grade) was evaluated by applying laminar cooling and UFC and varying the finish cooling temperature ranges during UFC after hot rolling. While UFC refined the ferrite grains, it accumulated the degeneration of pearlite, resulting in complete disappearance of the laminar pearlite at relatively low finish cooling temperatures. The minimum mean size of spheroidized cementite particles reached ~110?nm. Meanwhile, the enhancement of UFC on tensile strengths of mild steels mainly resulted from the grain refinement of ferrite and the precipitation strengthening of cementite particles; however, the contribution varied with the finish cooling temperature of UFC. A modified Ashby–Orowan model was also used for evaluating the yield strength increment of medium plates. This work will provide a theoretical basis for the diversity control of microstructure and for developing stronger and tougher mild steels by introducing UFC technology after high-temperature rolling.     

Effect of Cooling Rate and Vanadium Content on the Microstructure and Hardness of Medium Carbon Forging Steel

This paper reports the effect of cooling rate on the microstructure and hardness of a kind of medium carbon steel microalloyed with two levels of V content(0.15%and 0.28%)after hot deformation by using single compression tests on a Gleeble-3800 thermal simulator.The results show that cooling rate has a significant effect on the microstructure and hardness of the tested steels.Both the fraction of pearlite and hardness increase with increasing cooling rate,whereas a further increase of the cooling rate above a critical value promotes the formation of acicular ferrite(AF),and thus leads to a decrease of hardness mainly owing to the decrease of pearlite fraction and replacing it by AF and the less effective precipitation strengthening.Increasing V content results in a significant increase of hardness,and this tendency enhances with increasing cooling rate until the formation of AF.Furthermore,increasing V content also significantly enhances the formation of AF structure at a lower cooling rate.The results also suggest that by controlling microstructure,especially the precipitation of fine V(C,N)particles through adjusting postforging cooling,the strengthening and gradient function in one hot-forging part could be obtained.     

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     

Effect of consolidating temperature on strengthening mechanism in Fe–Cu alloy from rapidly solidified powder

Abstract

The strengthening mechanism of Fe-Cu alloy manufactured from rapidly solidified powder was investigated. Powders of Fe-Cu with copper content ranging from 0.5 to 5 wt-% were prepared by high pressure water atomisation and consolidated by groove rolling at 973, 1073 or 1273 K. Analysis by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) were carried out to evaluate the resulting structures. The microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Yield stress under tensile loading and hardness after aging were measured. The copper states in consolidated specimens were determined based on the results, and the states were correlated to the mechanical properties of the specimens. At each of the consolidating temperatures, the yield stress increased with an increase in copper content. However, the strengthening mechanism differed according to the temperature. Specimens consolidated at 973 and 1073 K were strengthened by microstructure refinement,whereas precipitation hardening was the main strengthening mechanism in specimens consolidated at 1273 K.     

The Determination of Optimum Forging Conditions for the Production of High Strength-High Impact Toughness Automotive Parts

The influences of various reheating and forging temperatures as well as cooling rates on the microstructure and mechanical properties, particularly impact energy, during the forging of a Nb-V microalloyed steel to be used for automotive safety parts were investigated. Increasing the prior austenite grain size increased the volume percent of acicular ferrite and reduced pearlite content in the microstructure even for very low post-forging cooling rates, resulting in improved toughness and tensile strength values. Increasing the cooling rate enhanced the acicular ferrite content, thereby increasing the impact energy properties. At lower reheating temperatures the yield strength and impact energy levels are determined by the percentage of pearlite present in the microstructure; while as the cooling rate is increased the amount of acicular ferrite and retained austenite are increased, improving the toughness and tensile strength of the forged part. This effect is more pronounced for the parts solutionized at 1250°C and is related to the presence of very fine carbonitride precipitates under these conditions, which contributes to improved yield strength, particularly at higher cooling rates. An optimized forging process was determined and adapted to a 25 MN production forging press to validate the experimental results on semi-industrial production scale. By adequate control of the above parameters, high-strength, high-toughness parts (T.S. = 800 MPa, CVN = 35 J) were forged and optimum mechanical properties were achieved without the need for any additional heat treatment.     

含Zr多组元掺杂石墨材料的性能研究

以天然石墨为原料，通过热压工艺，制备了含Zr多组元掺杂石墨材料。研究了掺杂元素对材料性能的影响。实验结果表明：随着Zr含量增加，基体石墨的强度、导电和导热性成线性增加；但是过量的ZrO2会消耗基体炭原子，生成金属Zr蒸汽逸出基体，形成孔隙和缺陷，导致材料的性能下降，因此应控制ZrO2的加入量。另外，采用SEM、XRD等分析手段研究了材料微观结构，探讨了微观结构对其性能的影响。     

Characterization of Austempered Ductile Iron Through Barkhausen Noise Measurements

The outstanding mechanical properties of austempered ductile irons (ADI) are linked to the microstructure of the matrix obtained by subjecting a ductile iron with an appropriate composition to a heat treatment called austempering. Then the microstructure of the matrix consists of bainitic ferrite with different volume fractions of retained austenite. The aim of this work is to use the magnetic Barkhausen noise (MBN) as a nondestructive method for characterizing the microstructure of ADI. First, it is shown that the amplitude and position of the peak-shaped MBN response is quite sensitive to the microstructure of the matrix of ductile irons. Thus each type of constituent (equiaxial ferrite, pearlite, martensite or bainite) exhibits a typical response and, in turn, it can be identified from the MBN response. Furthermore, a good correlation is found between MBN signal parameters and ADI heat treatment parameters, indicating that MBN is also quite sensitive to fine evolutions of the microstructure of ADI. MBN peak position is especially sensitive to the type of bainite, whereas peak amplitude is linked to the progress of the bainite reaction. Hence MBN measurements appear to be a powerful tool to assess some important microstructural features of ADI castings.