High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106     

Austempering of spheroidal graphite cast iron

We study gray spherulitic cast iron, its microstructure, hardness ultimate strength in tension, elongation, and impact toughness as functions of the duration of isothermal austenitizing in the bainite region at 350 and 400°C after austenitizing at 900°C. As the temperature of quenching increases from 350 to 400°C, the microstructure of the analyzed cast iron changes from lower to higher bainite and the amount of retained austenite increases (its maximum is attained after 1 h). At the same time, the ultimate strength in tension and hardness decrease, whereas the elongation and fracture toughness increase.Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 79–83, July–August, 2004.     

High-temperature properties of ferritic spheroidal graphite cast iron

The behaviour of fracture mode and intermediate temperature embrittlement of ferritic spheroidal graphite cast iron is influenced by many factors. From the experimental results, intermediate temperature embrittlement can be considered to be dominated by dynamic strain ageing and the triaxial stress field developed in the ferrite matrix amongst the graphite particles. In order to understand the effect of dynamic strain ageing on high-temperature properties, tensile properties, push-pull low-cycle fatigue properties, rotary bending fatigue properties and creep-rupture properties were investigated from room temperature to 500° C. It was found that all the properties investigated were influenced by dynamic strain ageing. The intermediate temperature embrittlement of ferritic spheroidal graphite cast iron found in different load conditions is reported.     

A realistic computer-based fatigue comparison of spheroidal graphite cast iron and cast steel

Loads measured in a wheeled loader during service are used to compare two casts of SG iron with a cast steel to BS 592 grade A. Fatigue lives are predicted using computer based analytical methods. The conclusions reached are contrasted to those inficated by the more conventional static and fatigue tests. The suitability of SG iron as a material for cast axle housings is demonstrated.     

The fatigue strength of centrifugally cast spheroidal graphite iron pipe

The fatigue strength of centrifugally cast spheroidal graphite (SG) iron pipe was investigated. A parallel series of tests were carried out both on plain plate specimens which were extracted from an iron pipe, and on notched iron specimens. These results were compared with results for rolled steel beams, which were made from a steel with a tensile strength similar to the SG iron [1]. It was found that the strengths at a life of $\text{2 × 10}{}^6$ cycles differed only by 20% between the SG iron pipe and the rolled steel beam, whereas those of plain plate specimens of the two materials differed by 38%. The fatigue failure in the SG iron pipe initiated from the inherent gas pores existing in the inner surface of the pipe, while the fracture in the rolled steel beams originated from external notch defects. Thus, the steel beam appeared more sensitive to the external notches than the SG iron pipe, when the notch size was smaller than 1 mm. However, it was revealed from the fatigue tests on notched plates that, as the notch became severer, the fatigue strength of SG iron became more affected by the notches than did that of the steel. A fracture mechanics analysis indicated that this was because the fatigue crack growth rate for SG iron was three times as high as that for steel.     

多次多向等温变形对球墨铸铁石墨形态的影响

研究了球墨铸铁在900±10℃下,经不同方向多次压缩变形后石墨的形态变化.结果表明:单向压缩后石墨球变成近似平行分布的石墨片,经多向压缩后石墨片分裂;沿不同方向多次变形,石墨球在不同方向应力作用下被剪为小的石墨颗粒,起到了细化石墨球的作用.     

Caustic stress corrosion cracking of a spheroidal graphite cast iron: laboratory investigation

Abstract

This paper presents the results of an experimental investigation into the stress corrosion cracking (SCC) of an engineering cast iron (namely a spheroidal graphite (SG) cast iron), in a highly caustic solution (namely synthetic Bayer liquor (SBL)) at high temperature. In order to ascertain experimental conditions under which plain iron - carbon materials may fracture predominantly by SCC in a caustic environment, slow strain rate testing (SSRT) was performed on carbon steel specimens, employing various combinations of strain rates and temperatures, in SBL and an inert environment of liquid paraffin. Under the conditions identified to be most conducive for caustic SCC of mild steel, specimens of the SG cast iron were subjected to SSRT in SBL and liquid paraffin, and the fracture behaviour was investigated by detailed fractography and microstructural characterisation.     

A probabilistic approach to predict the very high-cycle fatigue behaviour of spheroidal graphite cast iron structures

A new probabilistic approach is developed to study structures made of spheroidal graphite cast iron and subjected to very high-cycle fatigue. Until now, the probabilistic approach was based on S–N curves obtained from experiments carried out only until 10⁷ cycles. To validate this approach, failure predictions relating to the safety of components are computed and compared to experimental results. In addition to this development, an extension is proposed in order to improve the very long life assessment of complex structures. An extrapolation of the previous fatigue results to 10⁹–10¹¹ cycles illustrates the error made on cumulative failure probabilities. Finally, the respective influence of the casting flaw distribution, volume and stress field heterogeneity within specimens and industrial components is studied.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

Caustic stress corrosion cracking of a spheroidal graphite cast iron: characterisation of ex-service component

Abstract

This paper discusses observations of features suggesting grain boundary embrittlement ahead of stress corrosion crack tips in ex-service cast iron components exposed to strong caustic environments during Bayers process for alumina processing. The cracks and the neighbouring areas in the ex-service specimens were examined using conventional metallography, SEM, the extraction replica technique in the TEM and Auger electron spectroscopy. In all cases, the cracking was initiated at the surface of the steel exposed to the corrosive environment and it appeared that crack nucleation may have been aided by local stress concentrations and/or zones of local residual stress concentration. The fracture mode was almost exclusively intergranular and the crack path followed ferrite grain boundaries. There was clear evidence of a local zone of grain boundary embrittlement extending ahead of the tip of the major cracks examined. The phenomenon was established by investigating the fresh fracture surfaces produced by extending pre-existing cracks under impact loading at liquid nitrogen temperature. Auger electron spectroscopy of the fresh intergranular fracture surface failed to reveal any evidence of local elemental grain boundary segregation that might account for the observed embrittlement. In the absence of evidence of any other embrittling species on the exposed intergranular facets, there arises the possibility of hydrogen being involved in the embrittlement. The paper discusses hydrogen assisted intergranular cracking, as observed in the case of similar materials, to be the possible mechanism.     

Heterogeneous nuclei of graphite in grey cast iron

Abstract

In the present paper, the initial nuclei of flake graphite in grey cast iron are investigated. Using liquid quench, deep electrolytic erosion, and electron microanalysis, it is shown that the initial graphite in grey cast iron is nodular and involves a heterogeneous nucleus which can be one of several types: complex oxides, MnS, sulphides of rare earth, etc. The elements Si, Ca, Mn, Ce, La, Al, etc. can promote heterogeneous nucleation of flake graphite in grey cast iron.

MST/709     

Modeling of austenite to ferrite transformation

In this research, an algorithm based on the $\boldsymbol{Q}$ -state Potts model is presented for modeling the austenite to ferrite transformation. In the algorithm, it is possible to exactly track boundary migration of the phase formed during transformation. In the algorithm, effects of changes in chemical free energy, strain free energy and interfacial energies of austenite?Caustenite, ferrite?Cferrite and austenite?Cferrite during transformation are considered. From the algorithm, the kinetics of transformation and mean ferrite grain size for different cooling rates are calculated. It is found that there is a good agreement between the calculated and experimental results.     

Intragranular formation of austenite during delta ferrite decomposition in a duplex stainless steel

A Fe–22.5%Cr–4.53%Ni–3.0%Mo duplex stainless steel was solution treated at 1,325 °C for 1 h, quenched in water and isothermally treated at 900 °C for 5,000 s. The crystallography of austenite was studied using EBSD technique. Intragranular austenite particles formed from delta ferrite are shown to nucleate on inclusions, and to be subdivided in twin-related sub-particles. Intragranular austenite appears to have planar-only orientation relationships with the ferrite matrix, close to Kurdjumov–Sachs and Nishyiama–Wassermann, but not related to a conjugate direction. Samples treated at 900 °C underwent sparse formation of sigma phase and pronounced growth of elongated austenite particles, very similar to acicular ferrite.     

A useful technique for studying graphite in cast iron

A new experimental procedure for studying eutectic graphite in cast iron is described. It consists of slow dissolution of the matrix, which is simulteneously replaced by a transparent one in which the graphite remains entrapped. This provides advantages for the study of nucleation using light microscopy. Afterward, the complete graphite lattice can be released through dissolution of the amorphous matrix, retaining unaltered the structure of the eutectic grains and the spatial distribution produced during the solidification process. In this way, the crystallography of the phase can be followed by different methods in zones with a well-defined morphology and also in zones where there are transitions between different morphologies. In the last case, the close bonding with the surroundings in which they were produced is maintained. Finally, the study of the particles included exclusively in the graphite phase can be optimized because of the elimination of matrix interference in microanalysis.