Development and characterization of a wear resistant bainite/martensite ductile iron by combination of alloying and a controlled cooling heat-treatment

In the present work, mechanical and microstructural properties of Austempered Ductile Iron (ADI) alloyed with Cu-Ni-Mn-Mo submitted to different hot air austempering cycles in a two-step mechanism were evaluated. In the first step, cooling and austempering were performed in an experimental hot air blower device. In the second step, the austempering was performed in a muffle furnace. Bars with O16.5 mm and 90 mm of length were austenitized in an electric resistive furnace for 90 min at 860°C. Austempering first step was performed for 10 min at 290°C, 320°C and 350°C corresponding to cycles I, II and III, respectively. Austempering second step was performed for 60 min at 380°C for all cycles. The measurements of Brinell hardness, elongation, yield strength, and ultimate tensile strength for cycle I were 288 HB, 5.0%, 840 MPa, and 988 MPa. For cycle II, the measurements were: 264 HB, 5.9%, 717 MPa, and 876 MPa. And for cycle III: 257 HB, 6.6%, 711 MPa, and 888 MPa. Microstructural analysis indicated the presence of pearlite-free ausferrite.

Microstructural and mechanical properties of Cu-Ni-Mn-Mo austempered ductile iron obtained from two-step hot air austempering

The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360 °C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24 vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270 MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6 × 10−7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described.

Structure–property correlation in austempered alloyed hypereutectic gray cast irons

Low variability in mechanical properties is required for pipeline project designs to meet a strain-based design, which is used in regions of large ground movements. The objective of this study is to elucidate the influence of centerline segregation in continuously cast slab on variability in the mechanical property of pipeline steel, and controlling centerline segregation can meet the requirements of a strain-based design. Mannesmann rating method was used to evaluate the degree of segregation of two slabs and its effect on variability in mechanical properties of corresponding plates. Microstructural characterization indicated that bainite/martensite was formed in a segregated area where the content of C and Mn enriched. The mechanical property results indicated that controlling the degree of centerline segregation can reduce tensile strength variability and improve ductile-brittle transition temperature (DBTT).

https://www.mdpi.com/2075-4701/9/7/749/pdf

Controlling Variability in Mechanical Properties of Plates by Reducing Centerline Segregation to Meet Strain-Based Design of Pipeline Steel

6th Congress and Exhibition on International Advances in Applied Physics and Materials Science (APMAS) -- JUN 01-03, 2016 -- Istanbul, TURKEY

/pdf/the-effects-of-austempering-and-induction-hardening-on-the-33hffm4ovn.pdf

The Effects of Austempering and Induction Hardening on the Wear Properties of Camshaft Made of Ductile Cast Iron

The effect of segregation of alloying elements on the phase transformation of ductile iron during austempering was investigated. Four heats, each containing 0.4%Mn, 1% Cu, 1.5% Ni, or 0.4% Mo (wt%) separately, were melted; then three different sizes of casting bars (3,15, and 75 mm diameter) were poured from each heat. The distribution and the degree of segregation of certain elements were quantitatively analyzed using an electron microprobe. A personal computer (PC)-controlled heat treating system was used to measure electrical resistivity, and the information on resistivity variations was used to analyze the effect of segregation on phase transformations during austempering. Also, Charpy impact and Rockwell hardness tests were performed to determine the effect of segregation on properties.

The effect of segregation on the austemper transformation and toughness of ductile irons

A computer-controlled system for measuring electrical resistance has been developed and used to study the isothermal transformation of austenite in a ductile iron (3.31 % C, 3.12 % Si, 0.22 % Mn, 0.55 % Cu). The ability of the technique to follow the isothermal decomposition of austenite was established by measurements on an AISI4340 steel. The times at which the austenite decomposed to primary ferrite, pearlite, and bainite were accurately detected. In the ductile iron, the formation of pearlite and of bainite was easily detected, and an isothermal transformation diagram was constructed from the results. The temperature range for the formation of bainite is especially important in producing austempered ductile iron (ADI) and was mapped. An initial stage of decomposition of austenite to ferrite and high-carbon austenite is followed by a time delay; then the high-carbon austenite decomposes to bainite. The formation of ADI requires austempering to a structure of ferrite and high-carbon austenite, then quenching to retain this structure, thus avoiding the formation of bainite. This is achieved by isothermal transformation into the time-delay region. For the ductile iron studied here, this time region was about 2.6 h at 400 °C and increased to 277 h at 300 °C.

Study of the isothermal transformation of ductile iron with 0.5% Cu by electrical resistance measurement

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The effect of segregation on the austemper transformation and toughness of ductile irons

Study of the isothermal transformation of ductile iron with 0.5% Cu by electrical resistance measurement