Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500 °C, 600 °C and 700 °C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500 °C, where at 700 °C all deteriorated drastically. During fatigue testing, the samples endured at 500 °C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700 °C.     

Application of grey relational analysis based on Taguchi method for optimizing machining parameters in hard turning of high chrome cast iron

High chrome white cast iron is particularly preferred in the production of machine parts requiring high wear resistance. Although the amount of chrome in these materials provides high wear and corrosion resistances, it makes their machinability difficult. This study presents an application of the grey relational analysis based on the Taguchi method in order to optimize chrome ratio, cutting speed, feed rate, and cutting depth for the resultant cutting force (F_R) and surface roughness (R_a) when hard turning high chrome cast iron with a cubic boron nitride (CBN) insert. The effect levels of machining parameters on F_R and R_a were examined by an analysis of variance (ANOVA). A grey relational grade (GRG) was calculated to simultaneously minimize F_R and R_a. The ANOVA results based on GRG indicated that the feed rate, followed by the cutting depth, was the main parameter and contributed to responses. Optimal levels of parameters were found when the chrome ratio, cutting speed, feed rate, and cutting depth were 12%, 100 m/min, 0.05 mm/r, and 0.1 mm, respectively, based on the multiresponse optimization results obtained by considering the maximum signal to noise (S/N) ratio of GRG. Confirmation results were verified by calculating the confidence level within the interval width.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-018-0231-z     

Microstructural evolution and thermal fatigue resistance of grey cast iron

In the present work, thermal fatigue in grey cast iron has been investigated by means of a numerical and an experimental approach. Temperature gradients were generated within the material by means of a testing rig specifically designed for the experiments. The temperature gradients were responsible for the formation of severe stress fields that led to the failure of the specimens after a fairly low number of cycles. Crack growth was monitored during the tests, and the microstructure and hardness of samples were analysed after failure and compared with those of untested alloy. The repeated thermal cycles at peak temperatures of 600, 700, and 800°C led to important microstructural alterations of cast iron and to a drop in material hardness. The pearlite lamellae lost their original shape and became more fragmented. Oxygen‐rich regions surrounding the graphite flakes were produced by microgalvanic corrosion mechanism.     

Determination of presence of undesirable carbides at surface of cast iron parts using differential eddy current technique

Abstract

Machinability of grey cast iron parts can greatly suffer from the formation of carbide at the surface, resulting in a decrease in cutting tool life and higher production costs. Therefore, detection of the hardened layer and its hardness are the key factors in quality control and inspection processes. In the present paper, a number of metallurgical parameters (surface carbide, surface hardness and hardened depth) have been investigated using the non-destructive differential eddy current technique. The results show the high potential of the proposed method as a fast and accurate technique in inspecting and in consequent separation of undesirable parts.     

Stress/strain simulation model for grey cast iron

A simple model for estimating the stress/strain response of grey cast iron subjected to variable amplitude cyclic loading is presented. Material properties for the model are readily determined from the tensile and compressive stress/strain curve of the material.     

Fatigue behaviour and mean effects in grey cast iron

Strain cycle fatigue concepts are well integrated into fatigue life prediction methodologt for wrought components. Concepts developed for wrought materials cannot be directly extended to cast materials because of differences in the fatigue mechanisms, but the framework of a life prediction method for cast iron components will be similar. Observations and results of constant-amplitude completely reversed fatigue tests performed in strain and load control are reported for a pearlitic grey cast iron. Mean amplitude tests in both control modes have been conducted to evaluate the effects of mean stresses and strains. A parameter of the form (σ_maxΔ∈) gives good correlation of all the fatigue tests performed and provides a simple relationship to fatigue life.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

Effect of tandem submerged arc welding process and parameters of Gleeble simulator thermal cycles on properties of the intercritically reheated heat affected zone

The effects of real and Gleeble simulated double pass thermal cycles on the properties of the intercritically reheated coarse grained heat affected zones in X80 microalloyed pipeline steel has been investigated. The Gleeble simulated process involved heating the X80 steel specimens to the first peak temperature of 1400 °C and then reheating to the second peak temperature of 800 °C, with different cooling rates. The size and area fraction of martensite/austenite (M/A) constituents were obtained by a combination of field emission scanning electron microscopes and image analysis software. In addition, misorientation was characterized by electron back-scatter diffraction analysis. It is clear that the intercritically thermal cycles have a significant effect on morphology of M/A constituents. The M/A constituent’s size, such as mean diameter and length, are important factors influencing Charpy impact properties of thermally simulated intercritically reheated heat affected zones. The simulated thermal cycles of the intercritically reheated region in the high heat input tandem submerged arc welding processes, showed extremely poor Charpy impact absorbed energy. The intercritical reheated thermal cycles with lower heat input value showed higher Charpy impact absorbed energy due to a decrease in the prior-austenite grain and M/A particle size.     

Molybdenum alloying in cast iron and steel

Metal casting is an important manufacturing technology for efficiently producing massive components with complex shape. A large share of industrial castings is made from iron and steel alloys, combining attractive properties and low production cost. Upgrading of properties in cast iron and steel is mainly achieved by alloying and in fewer cases by heat treatment. Molybdenum is an important alloying element in that respect, increasing strength, hardness and toughness. It also facilitates particular heat treatments such as austempering. The paper describes the metallurgical functionality of molybdenum alloying in iron-based castings and demonstrates its effectiveness for applications in the automotive and mining industry.The full text can be downloaded at https://link.springer.com/content/pdf/10.1007/s40436-019-00282-1.pdf     

Influence of fusion zone size and failure mode on mechanical performance of dissimilar resistance spot welds of AISI 1008 low carbon steel and DP600 advanced high strength steel

The work here addresses the investigation of the effect of the welding parameters (welding time, welding current and electrode force) on the overload failure mode and mechanical performance of dissimilar resistance spot welds between drawing quality special killed AISI 1008 low carbon steel and DP600 dual phase steel. Mechanical properties of spot welds are described in terms of failure mode, peak load and energy absorption during the quasi-static tensile-shear test. Three distinct failure modes were observed during the tensile-shear test: interfacial, pullout and partial thickness–partial pullout failure modes. Correlations among failure mode, welding parameters, weld physical attributes and weld mechanical performance are analyzed. Effect of expulsion on mechanical performance of welds is also investigated.     

Effect of brazing temperature on tensile strength and microstructure for a stainless steel plate-fin structure

This paper presented a vacuum brazing technology for 304 stainless steel plate-fin structures with BNi2 filler metal. The effect of brazing temperature on tensile strength and microstructure has been investigated. The tensile strength is increased along with the increasing of brazing temperature. The microstructure is very complex and some Boride compounds are generated in the brazed joint. Full solid solution can be generated in the middle zone of joint when the brazing temperature is increased to 1100 °C. The brittle phases always exist in the fillet no matter how the brazing temperature changes, but the microstructure in fillet becomes more uniform and the tensile strength is increased with the brazing temperature increasing. In total, the brittle Boride compounds are decreased with the brazing temperature increase. Brazing with a filler metal thickness 105 μm and 25 min holding time, 1100 °C is the best suitable brazing temperature and a tensile strength of 82.1 MPa has been achieved for 304 stainless steel plate-fin structure.     

Effect of tool pin profile on microstructure and mechanical properties of friction stir welded AZ31B magnesium alloy

In this investigation the effect of friction stir welding pin geometry on the microstructure and mechanical properties of AZ31B magnesium alloy joints is studied. The considered pin geometries are simple cylindrical, screw threaded cylindrical and taper. The joints are friction stir welded at different traverse and rotational speeds. Microstructures of the joints are examined using the optical and scanning electron microscopes. Also, the tensile properties and hardness of the joints are measured. The results show that taper and screw threaded cylindrical pins produce defect free joints. In addition, the taper pin results in finest microstructure and highest mechanical properties. Furthermore, it is found that rotational speed has a more significant role on the final microstructure and mechanical properties of the joints, compared to the traverse speed.     

The study on defects in aluminum 2219-T6 thick butt friction stir welds with the application of multiple non-destructive testing methods

The present study focused on the relationship between primary friction stir welding process parameters and varied types of weld-defect discovered in aluminum 2219-T6 friction stir butt-welds of thick plates, meanwhile, the weld-defect forming mechanisms were investigated. Besides a series of optical metallographic examinations for friction stir butt welds, multiple non-destructive testing methods including X-ray detection, ultrasonic C-scan testing, ultrasonic phased array inspection and fluorescent penetrating fluid inspection were successfully used aiming to examine the shapes and existence locations of different weld-defects. In addition, precipitated Al₂Cu phase coarsening particles were found around a ‘kissing-bond’ defect within the weld stirred nugget zone by means of scanning electron microscope and energy dispersive X-ray analysis. On the basis of volume conservation law in material plastic deformation, a simple empirical criterion for estimating the existence of inner material-loss defects was proposed. Defect-free butt joints were obtained after process optimization of friction stir welding for aluminum 2219-T6 plates in 17–20 mm thickness. Process experiments proved that besides of tool rotation speed and travel speed, more other appropriate process parameter variables played important roles at the formation of high-quality friction stir welds, such as tool-shoulder target depth, spindle tilt angle, and fixture clamping conditions on the work-pieces. Furthermore, the nonlinear correlation between weld tensile strengths and weld crack-like root-flaws of different lengths was briefly investigated.     

Pulsed laser spot welding of intersection points for Zircaloy-4 spacer grid assembly

A pulsed laser spot welding technology has been developed for joining intersection points of 0.457 mm-thick Zircaloy-4 straps. Weld beads size, mechanical properties and microstructure characterization of the weld beads were investigated. The results indicate that peak power of the pulsed laser has significant influence on the penetration depth of weld beads. Pulse width and number of shots should be taken into consideration mainly to control the weld width. The average value of ultimate tensile loads of the intersection points are continuously increasing as the penetration depth and weld width of weld beads increase. Fracture was located at the fusion line and the fracture surface could be characterized as a mixture of ductile and cleavage feature. Column grains and equiaxed grains were observed in fusion zone and heat affected zone, respectively. The fusion zone consists of a mixture of α-Zr and β-Zr phases among which lamellar Widmanstatten structure α-Zr + Zr₃Fe was distributed. The Zr(Fe, Cr)₂ second phase particles were precipitated inter and intragranular of α-Zr grains.     

Microstructural features and mechanical properties of AM60 and AZ31 friction stir spot welds

The microstructural features and mechanical properties of AM60 and AZ31 friction stir spot welds are investigated in joints made using different tool designs (threaded and three-flat/threaded tools) and dwell time settings. Since the hook regions are curved inwards towards the keyhole periphery in AM60 friction stir spot welds made using threaded and three-flat/threaded tools and different dwell time settings, the distance from the tip of the hook region to the keyhole periphery mainly determines their failure load properties. In contrast, the hook regions are curved outwards from the axis of the rotating tool in AZ31 friction stir spot welds and their failure strength properties are determined by the bonded width, the distance from the tip of the hook region to the sheet intersection, the depth of tool shoulder penetration into the surface of the upper sheet and the distance from the tip of the hook region to the top of the welded joint.     

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 applied pressure on microstructure and mechanical properties of squeeze cast ductile iron

In this study, the effects of applied pressure during solidification on the microstructure and mechanical properties of cylindrical shaped ductile iron castings were investigated. Magnesium treated cast iron melts were solidified under atmospheric pressure as well as 25, 50 and 75 MPa external pressures. Microstructure features of the castings were characterized using image analysis, optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Tensile properties, toughness and hardness of the castings were also measured. The results showed that average graphite nodule size, free graphite content and ferrite content of the castings decreased and pearlite and eutectic cementite contents increased as the applied pressure was raised from 0 to 75 Mpa. Graphite nodule count was first increased by raising the applied pressure up to 50 MPa and then decreased. The highest graphite nodule count was obtained at 50 MPa applied pressure. The microstructural changes were associated with the improved cooling rate and the expected changes in the corresponding phase diagram of the alloy under pressure. The ultimate tensile strength (UTS), yield point strength (0.2% offset) and fracture toughness of the castings were improved when the applied pressure was raised from 0 to 50 MPa. Further increase of the applied pressure resulted in slight decrease of these properties due to the formation of more cementite phase in structures as well as reduced graphite nodule count. Hardness of the castings continuously increased with increasing the applied pressure.     

Microstructural and mechanical properties of friction stir welded Cu–30Zn brass alloy at various feed speeds: Influence of stir bands

In this study, the effect of various feed speeds on microstructure and mechanical properties of friction stir welded Cu–30Zn brass alloy is investigated. Rotation speed was fixed at 950 rpm and feed speed varied in the range of 190–375 mm/min. Examination of the microstructure showed very fine grains with some deformed grains in the stirred zone and some coarser grains in the thermo-mechanically affected zone and base metal. A unique deformation pattern, namely “stir band” in the stirred zone region was identified and its density increased by increase in feed speed. Results showed that the grain size profile was independent of feed speed and the hardness values decreased by increase in feed speed. Increase in feed speed led to a slight improvement of yield strength and ultimate tensile strength, associated to continuous spring-like morphology of stir bands acting as a strengthening structure. However, ductility reduces considerably from 57 to 27%. Moreover, it is observed that during tensile test, fracture cracks originate exactly adjacent to the stir bands.