Austempering of low manganese ductile irons

Abstract

X-ray diffraction and optical microscopy were used to follow the stage I and stage II kinetics of a low Mn, Cu iron and a low Mn, Ni–Cu iron during austempering at 300, 370, and 440°C. Nickel and Cu are shown to delay the bainitic transformation without the undesirable features displayed by Mn and Mo and to influence the stage I kinetics through their effect on the C content of the austenite. The rate of transformation varies with the driving force for the transformation. In contrast, the rate at which the stage II reaction proceeds depends on the total alloying content through its effect on the nucleation and/or growth of the ferrite and carbide phases. The concept of the processing window is discussed and definitions using mechanical property measurements and microstructural and X-ray observations are described. It is shown that the processing window defined from the microstructural observations is not coincident with the optimum mechanical properties. Modifications to the microstructural criteria are discussed.

MST/1742     

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.     

Surface laser treatment of ductile irons

The effect of the laser surface treatment of three types of ductile iron (Grades 60-40-18, 80-55-06 and 100-70-03) was studied. Using a continuous CO₂ laser with a square 10 mm × 10 mm beam and uniform power density, the effect of beam scan rate at 2.5 and 5 kW power output was investigated. At each power rate, a range of scan rates was used to produce treatments with and without surface melting. The microstructure and hardness of the different zones of the treated material were analysed. It was found that layers of white iron of the same depth were produced in the three test irons when the operating conditions melted the surface material. Surface porosity can be eliminated by melting, although transverse cracks appeared on the surface with this treatment. Surface martensitic hardening produced a layer of uniform hardness only in the case of the grade 100-70-03 ductile iron with a perlitic matrix.     

The effects of heat treatment on the mechanical properties of multicomponent white cast irons

Multicomponent white cast irons contain many kinds of strong carbide-forming elements in order to obtain a very hard microstructure characterized by the presence of different carbides that are well dispersed in a martensitic matrix. The heat treatment of these products consists of high temperature austenization followed by quenching and two temperings, as required in order to increase their overall hardness and to completely eliminate residual austenite. The influence of tempering temperatures on the mechanical properties of these products, determined using tensile, hot compression and fracture toughness tests, was studied in this research work. Their corresponding failure micromechanisms were defined by means of the analysis of fracture surfaces.     

Austempering heat treatments of ductile iron using molten metal baths

This work aims to evaluate the use of two different zinc–tin and zinc–aluminum molten metal baths on austempering heat treatments performed in ductile cast iron. Samples were extracted from as-cast standard Y-blocks for austempering heat treatments. The samples were heated for austenitization at 910°C for 90?min and further cooled in two different molten metal baths for austempering: zinc–tin and zinc–aluminum alloys at 370 and 400°C, respectively, for 30, 60 and 90?min. The Zn–50?wt% Sn hypoeutectic alloy and the Zn–5?wt% Al eutectic alloy were chosen for the molten metal baths. After heat treatments, the samples were analyzed by optical and scanning electron microscopy, Brinell hardness, Vickers microhardness, Charpy impact and tensile tests, and fracture mode analysis. The results indicated the viability of using Zn–Al and Zn–Sn molten metal baths as a substitute of molten salts. When the austempering temperature was increased from 370 to 400°C, the hardness, tensile strength, and elongation decreased, while impact energy increased. The ideal processing parameters were obtained for austempering at 370°C for 60?min, where the austempered ductile cast iron presented a microstructure completely formed by finer ausferrite.     

Stepped heat treatment for austempering of high manganese alloyed ductile iron

Abstract

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m^?2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.

MST/3119     

Influence of 1 wt-% addition of Ni on structural and mechanical properties of ferritic ductile irons

Abstract

Two sets of ductile irons with and without Ni additions containing various low Si contents have been prepared in order to study the effect of Ni on structural and mechanical properties of thermal analysis cups and standard keel blocks. Because contradictory results appearing in literature, this work has been focused on the influence of this element on matrix structure and on impact properties at room temperature as well as at low temperatures. The structures of Ni free and Ni bearing alloys have been related to the features of cooling curves recorded on both casting types and to the tensile and impact properties of the materials.     

Improving mechanical properties for pure irons by local surface mechanical attrition treatment

After a short-time surface mechanical attrition treatment process, a planar uneven structure was obtained on the surface of irons. The yield strength of the sample subjected to surface mechanical attrition treatment for 60?s is almost twice than that of the annealed sample, but the ductility is still well retained. The good combination of strength and ductility is mainly due to the extra strain hardening and the coexistence of soft and hard phases. The extra strain hardening delays the annihilation of dislocations and reduces the plastic instability, while the hard phase in the planar uneven structure suppresses the crack initiation, and the soft phase delays the crack propagation.     

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     

Triboengineering properties of austenitic manganese steels and cast irons under the conditions of sliding friction

We analyze the influence of parameters of the graphite phase on the ultimate strength in bending, deflection, and the intensity of fracture of manganese cast irons and steels. The least intensity of wear is exhibited by alloys with graphite inclusions 30–50 μm in length and a hardness of 250 HB. The decrease in the hardness of alloys, improvement of the parameters of the graphite phase, and increase in its amount (occupied area) to 6–10% lead to the disintegration of the metal matrix, decrease its strength characteristics, and promote the intensification of the fracture processes under the conditions of friction. It is shown that, unlike 40, 45, 45G2, and 20KhN hardened steels, the austenitic cold-worked manganese steel is capable of preserving the coating formed by the adsorbed layers of oil and graphite on the surface for 3–4 h under the conditions of shortage of oil. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 55–60, September–October, 2005.     

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200 MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200 °C for 60 min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850 °C for 30 min and then quenched at 70 °C in oil and tempered at 210 °C for 60 min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122 MPa and 18–168 MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12 GPa and 0.47–3.47 GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.     

Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

Abstract

In the present study, an unalloyed ductile iron containing Fe–3·50C–2·63Si–0·318Mn–0.047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region α+γ at various temperatures of 795, 805, 815 and 830°C for 20 min and then quenched into salt bath held at austempering temperature of 365°C for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferrite+high carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in α+γ temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite was absent in these samples. The volume fraction of proeutectoid ferrite, new ferrite and ausferrite can be controlled to determine the strength and ductility. Austempered specimens in α+γ temperature range exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing AFVF. On the other hand, the ductility increased with increasing proeutectoid ferrite and new ferrite volume fractions at the expense of strength. The specimen with ～47·2%AFVF exhibited the best combination of high strength and ductility. The strength and ductility of this material is much higher than that of ferritic grades. Its strength is at the same level as while ductility almost more than four times higher than that of pearlitic grades. Meanwhile, the specimen with ～ 75%AFVF exhibited the best combination of high strength and ductility compared with those of pearlitic grades. The strength of this material is much higher and its ductility is almost more than two times higher than that of pearlitic grades yet slightly lower than that of ferritic grades. This material also meets the requirements for the strength of quenched and tempered grades and its ductility is higher than that of this grade.     

The effects of boro-tempering heat treatment on microstructural properties of ductile iron

In this study, the effects of boro-tempering heat treatment on microstructural properties of ductile iron were investigated. Test samples with dimensions of 10 × 10 × 55 mm were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Both optical microscopy and scanning electron microscopy were used to reveal the microstructural details of coating and matrix of boro-tempered ductile iron. X-ray diffraction was used to determine the constituents of the coating layer. The boride layer formed on the surface of boro-tempered ductile cast iron is tooth shape form and consisted of FeB and Fe₂B phases. The thickness of boride layer increases as the boronizing time increases and tempering temperature decreases. Tempering temperature is more effective than boronizing time on the matrix structure. Boro-tempering heat treatment reduces the formation of lower and upper ausferritic matrix temperature according to classical austempering. This causes formation of upper ausferritic matrix in the sample when tempered at 300 °C. This is in contrast to general case which is the formation of lower ausferritic matrix via austempering at this temperature.     

Dependence of the mechanical properties and microstructure of ultralight magnesium-lithium-aluminum alloy on heat treatment conditions

Although magnesium-lithium-based alloys demonstrate superior workability and lower densities than conventional magnesium alloys, their mechanical properties require improvement. In this study, the effect of heat treatment conditions on the mechanical properties and microstructure of magnesium-lithium-aluminum alloys was investigated. Tensile tests were conducted on the alloys, and the results showed that the yield stress, ultimate tensile strength, and total elongation were significantly dependent on the heat treatment conditions. The relationship between the yield stress and grain size was not governed by the Hall-Petch relationship. The activation volume of various heat-treated samples estimated from the strain rate jump test was smaller for higher yield stress. Wide-angle x-ray scattering indicated that the second phase with a Bragg spacing of 1.7 nm was generated after heat treatment. It is found that the state of the second phase periodic structure affects the mechanical properties of the magnesium-lithium-aluminum alloy.     

Study of microstructural evolution and mechanical properties exhibited by non alloyed ductile iron during conventional and stepped austempering heat treatment

It was evaluated the microstructural and mechanical response that a non alloyed ductile iron (DI) presented when was subjected to Conventional Austempering (CA) and Stepped Austempering (SA) heat treatments. X-ray Diffraction (XRD) quantification techniques demonstrated to be the more reliable method for monitoring phase transformations taking place during both CA and SA. When CA was applied some intercellular areas remain untransformed even for long time, however when samples were subjected to SA those untransformed areas disappeared and instead finer ausferrite was found. Additionally mechanical properties values obtained from tensile and impact tests confirmed that for all times used, SA was superior to the CA.     

Investigation of mechanical properties of boro-tempered ductile iron

In this study, the effects of boro-tempering heat treatment on mechanical properties of ductile iron were investigated. Standard tensile test samples and unnotched Charpy specimens were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Micro-hardness measurements were performed on cross-section of the metallographically prepared samples, where cut from fractured impact test samples. The hardness of boride layers was measured in the range of 1654–1867 HV_0.05. It was observed that tempering temperature was more effective on the mechanical properties of the material than boronizing time. Optimum mechanical properties were obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h.     

变形制度对低碳锰钢组织性能的影响

为了获得细晶铁素体/贝氏体的复相组织,通过控轧控冷工艺研究了低碳锰钢在奥氏体区变形时变形量、终轧温度和卷取温度对组织演变和力学性能的影响规律.研究表明,增加变形量(对应道次间隔时间缩短)可以细化铁素体晶粒,但当终轧温度降低到800℃时,变形量的增加以及开冷温度的降低不利于贝氏体组织的获得.通过调整变形量、终轧温度、可开冷温度并适当降低卷取温度,可使实验钢获得晶粒尺寸约为5μm的铁素体和10%~20%的贝氏体组织,低碳锰钢强塑性能良好.     

Effect of heat treatment on mechanical properties of alloy 909

Abstract

Alloy 909 is a Ni–Fe–Co alloy that exhibits a low coefficient of thermal expansion and high strength at elevated temperatures. These characteristics are suitable for demanding high temperature uses such as turbine components for aerospace and energy generation. The mechanical properties are achieved by heat treatment (solution and aging) that affects the microstructural characteristics such as grain size and precipitation of intergranular and transgranular particles. This work summarises the results of trials conducted to determine the conditions that assure the desired mechanical properties in high temperature ring rolled pieces. Results of tensile, hardness and stress rupture tests are complemented by microstructural analyses. These studies allowed for establishing a robust method for the manufacture of rings; no degradation of the product, either metallurgical or due to properties, has been reported after the method was implemented.     

The effect of heat treatment on the toughness, hardness and microstructure of low carbon white cast irons

The effect of destabilisation and subcritical heat treatment on the impact toughness, hardness, and the amount and mechanical stability of retained austenite in a low carbon white cast iron have been investigated. The experimental results show that the impact energy constantly increases when the destabilisation temperature is raised from 950°C to 1200°C. Although the hardness decreases, the heat-treated hardness is still greater than the as-cast state. After destabilisation treatment at 1130°C, tempering at 200 to 250°C for 3 hours leads to the highest impact toughness, and secondary hardening was observed when tempering over 400°C. The amount of retained austenite increased with the increase in the destabilisation temperature, and the treatment significantly improves the mechanical stability of the retained austenite compared with the as-cast state. Tempering below 400°C does not affect the amount of retained austenite and its mechanical stability. But the amount of retained austenite is dramatically reduced when tempered above 400°C. The relationship between the mechanical properties and the microstructure changes was discussed.