Austempering of an Mn–Mo–Cu alloyed ductile iron Part 2 – Structure–mechanical property relationships

Abstract

Ultimate tensile strength, 0·2% proof strength, elongation, and impact energy measurements are reported for an alloyed ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·25Cu–0·23Mo–0·035Mg for austempering temperatures of 400, 375, and 350°C and a range of austempering times after austenitising at 920°C for 120 min. The ADI ASTM A897M:1990 standard is satisfied for an austempering temperature of 350°C but not at 375 or 400°C. This behaviour is discussed in terms of the influence of the unreacted austenite volume from the stage I austenitising reaction and the carbide product of the stage II austenitising reaction on the ductility. The present findings are predicted by the processing windows determined from the austempering kinetics.

MST/3393     

Influence of molybdenum on austempering behaviour of ductile iron Part 4 – Austempering behaviour of ductile iron containing 0·45%Mo

Abstract

Austempering kinetic measurements and mechanical property measurements are presented for a ductile iron of composition Fe–3·56C–2·77Si–0·25Mn–0·45Mo–0·43Cu–0·04Mg (wt-%) after austenitising at 870°C and austempering at 400, 375, 320, and 285°C. The austempering kinetic measurements show that increasing the Mo content of the iron, for example, to increase hardenability, does not delay the austempering reaction significantly and the processing window is open for all the austempering temperatures studied. The mechanical properties determined for austempering temperatures of 400 and 375°C show that the higher ductility grades of the austempered ductile iron standards can be satisfied as predicted by the open processing windows. The ductility of the 0·45%Mo austempered iron is reduced compared with that measured in 0·13%Mo and 0·25%Mo irons austempered under the same conditions. This is attributed to an increased amount and continuity of intercellular carbide as the Mo content increases.     

Influence of molybdenum on austempering behaviour of ductile iron Part 1 – Austempering kinetics and mechanical properties of ductile iron containing 0·13%Mo

Abstract

Measurements of the austempering kinetics and mechanical properties are presented for a ductile iron of composition Fe–3·51C– 2·81Si–0·25Mn–0·39Cu–0·13Mo–0·04Mg (wt-%) for austempering temperatures of 285, 320, 375, and 400°C after austenitising at 870°C for 120 min. The kinetic studies show that the alloying level is insufficient to cause a significant delay in ausferrite formation in the intercellular boundaries. This implies that the heat treatment processing window is open for all austempering conditions studied. The mechanical property measurements show that with the correct selection of austempering temperature all the grades of the ASTM Standard 897M : 1990 and BS EN 1564 : 1997 can be satisfied. The hardenability of the present iron is limited and it is therefore unlikely that these standards will be achieved in thicker section components.     

Influence of austenitising temperature on austempering of an Mn-Mo-Cu alloyed ductile iron Part 1—Austempering kinetics and the processing window

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austempering kinetics of an alloyed ductile iron of composition (wt-%) Fe-3·49C-2·33Si-0·42Mn-0·25Cu-0·23Mo-0·035Mg at austempering temperatures of 300, 350, 375, and 400°C and austenitising temperatures of 870 and 920°C. The stage I reaction during austempering occurs in two steps, the first in the eutectic cell and the second in the intercellular area. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have a lesser effect on the stage II reaction. Decreasing the austenitising temperature produces a more uniform austempered microstructure and reduces the amount of martensite in this structure. These changes move the processing window to shorter austempering times and increase the temperature at which the processing window closes.

MST/3390     

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     

Influence of heat treatment parameters on stepped austempering of 0.37%Mn-Mo-Cu ductile iron

Abstract

Measurements of the average austenite carbon content, retained austenite content, unreacted austenite content, and mechanical properties are reportedfor various austempering heat treatments of an alloyed ductile iron of composition (wt-%) Fe-3.39C-2.56Si-0.37Mn-0.25Mo-0.29Cu-0.04Mg. It is shown that a stepped austempering treatment can be used to considerably increase the ductility of the alloy compared with a single heat treatment and to extend the austempering time interval over which the ASTM standard can be satisfied. Decreasing the second step austempering temperature accelerates the stage I reaction during the second step treatment but produces little change in the mechanical properties of the alloy used in the present work. Decreasing the first step austempering time to 375°C accelerates the stage I reaction in the second step treatment but slightly decreases the maximum impact energy and increases the austempering time at which it is achieved.     

Effect of nodule count on austenitising and austempering kinetics of ductile iron castings and mechanical properties of thin walled iron castings

Abstract

In the present work, the potential for producing thin walled ductile iron castings with an ausferritic matrix is presented. Experimentally, thin walled iron castings of 2 mm in thickness were obtained and characterised by a nodule count of 1992 mm^?2. In addition, a reference casting was produced with a 25 mm thick wall and a nodule count of 330 mm^?2. Austenitising was carried out at 920°C, whereas austempering was implemented in the 300–400°C temperature range. The austenitising and austempering transformation rates were determined by dilatometry, and the results were confirmed by microstructural analyses. It was found that in thin walled castings, the austenitising and austempering times were reduced by either one-half or one-third of the ones corresponding to the reference casting. The exhibited mechanical properties of the thin walled castings were also determined as a function of austempering time and temperature. It was found that austempering at 300°C for 1200 s leads to thin walled castings with a tensile strength of 1500 MPa. Accordingly, from this work, it is plausible to produce high strength thin walled castings that satisfy all the ASTM 897M grades of ausferritic ductile iron through proper heat treating.     

Effect of telmperature and strain rate on tensile behaviour of M250 maraging steel

Abstract

The effect of temperature and strain rate on the 0·2% yield strength, ultimate tensile strength, and percentage elongation of M250 maraging steel was investigated under uniaxial tensile conditions in the temperature range from 25 (room temperature) to 550°C and strain rate range 10^?4–10^?1 S^?l. Up to 400°C the steel shows essentially strain rate insensitive behaviour with a gradual decrease in the 0·2% yield strength and ultimate tensile strength. The elongation remains constant at all strain rates up to 300°C. Fractographic analysis indicates that the increasing strain rate induces strain constraint resulting in an increased dimple size. An elongated structure was observed at temperatures above 400°C. X-ray diffraction reveals the presence of reverted austenite in the specimens tested at 550°C.

MST/3263     

Relationship between structure and mechanical properties in high manganese alloyed ductile iron

Abstract

Measurements of ultimate tensile strength, 0·2% proof stress, elongation, and impact energy are reported for an alloyed ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg,for a range of austempering temperatures and times after austenitising at 920°C for 120 min. It is shown that the mechanical properties satisfy the high strength grades of the standard AST MA897 M:1990, but fail to satisfy the higher ductility grades because of poor ductility. This is attributed to overlapping of the stage I and II reactions and the occurrence of the transformation induced plasticity mechanism during deformation, particularly in irons austempered at higher temperatures.

MST/3054     

Effects of pearlite formation on mechanical properties of austempered ductile iron

Abstract

Mechanical property measurements are described for a ductile iron alloyed with Mn, Mo, and Cu in the fully austempered condition, andfor irons with various amounts of pearlite introduced by isothermal transformation at 550°C after austenitising at 920°C for 120 min and before austempering at 370°C for 60 min. The ultimate tensile strength, 0·2% proof strength, elongation, impact energy, and hardness all decrease as the amount of pearlite in the structure increases. A smaller amount of pearlite can be tolerated in the alloyed iron compared with an unalloyed iron before itfails to satisfy the standard.

MST/2041     

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     

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     

Austempering of low manganese ductile irons Part 3 Variation of mechanical properties with heat treatment conditions

Abstract

Measurements of 0·2% proof stress, ultimate tensile strength, elongation, and impact energy are presented for low Mn, Cu iron and low Mn, Ni–Cu iron as a function of austempering time, austempering temperature, and austenitising temperature. The mechanical properties show optimum values at times corresponding to the processing window derived from kinetic measurements. Austempering temperature is shown to influence the properties significantly and by varying the austempering temperature most of the grades of the standard ASTM A897M: 1990 can be achieved with the irons studied. Austenitising temperature is shown to influence the elongation and impact energy significantly at higher austempering temperatures and it is possible by lowering the austenitising temperature to open the processing window at a particular austempering temperature and improve the mechanical properties.

MST/1899     

Solidification structures and mechanical properties of Zn–27Al alloy cast in metal moulds

Abstract

Rectangular and round cross-section test bars of Zn–Al alloy containing (wt-%) 27·67%Al; 2·30%Cu; 0·13%Mg; 0·04%Fe; <0·0025%Pb, Cd, and Sn; balance Zn were cast in metal moulds. The variables studied were: bar cross-section 3–25 mm thickness and 14·5–28 mm dia., pouring temperature 550–700°C, feeder location at bar shoulders or over full gauge length, and cast iron mould preheating temperature 100 or 200°C. The results show that the above variables can lead to variations of 0·14–1·72% bar porosity, 18–0·5% elongation, 428–305 MN m^?2 tensile strength, and 108–128 Brinell hardness. Metallographic examination leads to the conclusion that these properties are influenced by the volume, distribution, and morphology of shrinkage porosity and of eutectic constituent. The observed variations in the microstructure and their effects on the mechanical properties are related to the cooling and feeding conditions of the bars.

MST/549     

Dynamic recrystallisation and dynamic precipitation in AA6061 aluminium alloy during hot deformation

Abstract

Deformation behaviour of AA6061 alloy was investigated using uniaxial compression tests at temperatures from 400 to 500°C and strain rates from 0·01 to 1 s^?1. Stress increases to a peak value, then decreases monotonically until reaching a steady state. The dependence of stress on temperature and strain rate was fitted to a sinh-Arrhenius equation and characterised by the Zener–Hollomon parameter with apparent activation energy of 208·3 kJ mol^?1. Grain orientation spread analysis by electron backscattered diffraction indicated dynamic recovery and geometrical dynamic recrystallisation during hot compression. Deformation at a faster strain rate at a given temperature led to finer subgrains, resulting in higher strength. Dynamic precipitation took place concurrently and was strongly dependent on temperature. Precipitation of Q phase was found in the sample deformed at 400°C but none at 500°C. A larger volume fraction of precipitates was observed when samples were compressed at 400°C than at 500°C.     

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.     

Computer modelling of phase diagrams

Abstract

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d^?1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.

MST/2050     

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     

Improved low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel through modified heat treatments

Abstract

The low temperature mechanical properties of 0·4C–Cr–Mo–Ni steel can be improved significantly by thefollowing treatments. Modified oil quenching (MOQ): interrupt quenching at 573 K just below the martensitic transformation temperature followed by short time tempering at 673 K (up-quenching) before oil quenching and subsequently 473 K tempering (after conventional 1133 K austenitisation). Modified austempering (MA): the same up-quenching treatment followed by austempering at 673 K and subsequently water cooling. Each modified treatment was compared with its corresponding conventional treatment. The MOQ treatment significantly improved the notched tensile strength of the steel with slightly increased 0·2%PS and UTS, owing to an increase infracture ductility over the temperature range 123–203 K and also improved the Charpy impact energy of the steel over the temperature range 203–373 K. As a result of the MA treatment, the 0·2%PS and UTS and the notched tensile strength were developed remarkably with little change of fracture ductility over the temperature range 123–293 K. This treatment also improved the Charpy impact energy of the steel over the temperature range 203–293 K. The beneficial effect of the modified heat treatments on the mechanical properties is briefly discussed in terms of a modified law of mixtures, fibre loading theory, and fracture profiles.

MST/1157     

Formation of strain-induced martensite in austempered ductile iron

The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.