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.     

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.     

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.     

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.     

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     

Initial stage of isothermal decomposition of austenite to ferrite and graphite in spheroidal graphite cast iron

Abstract

The isothermal decomposition of austenite to ferrite and graphite in a spheroidal graphite cast iron was investigated by dilatometry, metallography, and image analysis. The results gave quantitative, although indirect, information on the kinetics of carbon transfer from austenite to graphite, as well as on the kinetics of the austenite to ferrite and graphite transformation. The relationship between the kinetics and the graphite nodule count was highlighted. The incubation period before ferrite growth was found to be associated with a global contraction of the material, in spite of the related increase in the graphite volume fraction. Calculations showed good agreement with experimental results, except for the rate of carbon transfer, which was found to be much slower experimentally than predicted by calculations based on the volume diffusion of carbon. Possible reasons for this discrepancy are discussed.     

Round‐robin prediction of the fatigue limit of a ring of spheroidal graphite cast iron

A round‐robin investigation has been performed, where stress analysts from eight different organisations carried out a total of 11 predictions of the expected fatigue limit of a diametrically loaded cast ring subjected to fluctuating tensile or compressive loading. Whereas geometry, load parameters, and type and quality of material (spheroidal graphite cast iron EN‐GJS‐600‐3) had been prescribed, the participants were free to use computational tools and models, and fatigue assessment models and data of their own choice. The objectives of the investigation were to compare the 11 predictions (i) among themselves, and (ii) with a posteriori experimental fatigue limits determined by means of stair‐case testing. The fatigue limit predictions showed coefficients of variation of as large as 25%. Even for a group of analysts from a single organisation, the coefficients variation were around 15%. Fatigue tests gave mean fatigue limits 60% (tensile loading) and 30% (compressive loading) above the a priori predictions. Possible reasons for the large deviations between single predictions and for their conservatism have been proposed. It seems that design engineers (i) make use of the available room for interpretation of models and data, and (ii) have an unconscious tendency to make conservative assumptions. Only if models and data for fatigue assessment are prescribed in great detail, can the ‘scatter’ among fatigue limit predictions be expected to decrease below 15–25%. Improved ‘absolute’ predictions would require more accurate fatigue data.     

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.     

Relation between microstructural heterogeneities and damage mechanisms of a ferritic spheroidal graphite cast iron during tensile loading

In this work, the influence of the metallic matrix heterogeneities and the spheroidal graphite nodules distribution on both crack initiation and propagation and damage evolution during tensile loading of ferritic spheroidal graphite cast iron is examined. The experimental methodology involves specialized metallographic techniques, step by step tensile loading, microscopic observation by using optical and scanning electron microscopy and three‐dimensional (3D) reconstruction of the graphite nodules distribution. The results show that the graphite nodules are the major heterogeneities responsible for inducing the development of cracks in the metallic matrix. Crack initiation is preferentially located at the irregular contour of graphite nodule cavities, ferritic grain boundaries and internodular areas highly strained. The final fracture involves cracks mainly propagating through the internodular ligaments of matrix‐nodule debonded areas belonging to the first‐to‐freeze zones resulting from the solidification process.     

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.     

Static cracking resistance of cast iron

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 6, pp. 30–36, November–December, 1988.     

Identification of Cu-rich precipitates in pearlitic spheroidal graphite cast irons

This investigation studies the partitioning of Mn and Cu – and Si– between ferrite and cementite in the pearlite of two alloys containing similar Mn and Si concentrations, but different Cu contents, keeping the Cu concentration within the usual industrial limits. The results confirm the partitioning of all the elements between ferrite and cementite during pearlite growth. Furthermore, the presence of nanometric Cu-rich precipitates was detected for an alloy with typical Cu contents for achieving pearlitic structures in spheroidal graphite cast irons. This might be linked to the microsegregation of Cu developed during the solidification step. These precipitates have not been reported before and cannot be ignored as they can affect the transformation kinetics and the mechanical properties of the alloy.     

On the fatigue damage micromechanisms in Si‐solution–strengthened spheroidal graphite cast iron

Graphite nodules in spheroidal graphite cast iron (SGI) play a vital role in fatigue crack initiation and propagation. Graphite nodules growth morphology can go through transitions to form degenerated graphite elements other than spheroidal graphite nodules in SGI microstructure. These graphite particles significantly influence damage micromechanisms in SGI and could act differently than spheroidal graphite nodules. Most of the damage mechanism studies on SGI focused on the role of spheroidal graphite nodules on the stable crack propagation region. The role of degenerated graphite elements on SGI damage mechanisms has not been frequently studied. In this work, fatigue crack initiation and propagation tests were conducted on EN‐GJS‐500‐14 and observed under scanning electron microscope to understand the damage mechanisms for different graphite shapes. Crack initiation tests showed a dominant influence of degenerated graphite elements where early cracks initiated in the microstructure. Most of the spheroidal graphite nodules were unaffected at the early crack initiation stage, but few of them showed decohesion from the ferrite matrix and internal cracking. In the crack propagation region, graphite/ferrite matrix decohesion was the frequent damage mechanism observed with noticeable crack branching around graphite nodules and the crack passing through degenerated graphite elements. Finally, graphite nodules after decohesion acted like voids which grew and coalesced to form microcracks eventually causing rapid fracture of the remaining section.     

Thermal cracking of multicomponent white cast iron

Abstract

The effect of the volume fraction of eutectic carbides on the thermal fatigue resistance of multicomponent white cast iron has been investigated. Thermal fatigue tests were carried out for 100 and 500 cycles. Nucleation of thermal fatigue cracks took place mostly at the specimen surface, induced by mechanical and metallurgical stress raisers. The crack nucleated in the matrix as well as at the carbide/matrix interface or at the carbide itself. The surface crack density increased slightly for increasing volume fraction of eutectic carbides from 9 to 14%, approximately. Crack propagation took place mostly at the carbide/matrix interface or through the carbide. The propagation rate was affected by the carbide distribution: the higher was the 'carbide continuity/mean free path between carbides' ratio, the higher was the propagation rate. The propagation rate decreased with increasing test time, regardless of the volume fraction of eutectic carbides.