Abstract: A method is disclosed for making compacted graphite cast iron of improved strength and hardness while retaining excellent thermal conductivity, low shrinkage, and excellent damping characteristics. A ferrous alloy is melted consisting essentially of, by weight, 3-4% C, 2-3% Si, 0.2-0.7% Mn, 0.25-0.4 Mo, 0.5-3.0% Ni, up to 0.002% sulfur, up to 0.02% phosphorus, and impurities or contaminants up to 1.0%, with the remainder being essentially iron. The melt is subjected to a graphite modifying agent to form compacted graphite upon solidification. The solidified casting is heat treated by austempering and quenching to produce an iron having a matrix of bainite and austenite.

Abstract: Using a dilatometer the isothermal transformation of austenite to bainite has been studied in ductile cast iron with 0.05 % Mn and a silicon content varying from 2.4 to 3.8 %. The alloys were austenitized to a carbon content in the matrix of 0.65 %. It appears that silicon retards the formation of carbides in the upper bainite region (400 °C), resulting in an amount of retained austenite up to 40 % present in the final structure at room temperature. Silicon improves the strength; in the lower bainite region the yield strength in particular. An elongation up to 10 % or more is obtained after austempering at 400 °C independent of the silicon content.

Abstract: Interest in austempered bainitic ductile irons has increased dramatically in the past few years as more and more successful applications are being reported. The remarkable combination of properties attainable has caused this material to emerge as a new class of ductile iron. Austempered ductile irons achieve strength levels as high as twice those of standard ductile iron grades at the same level of toughness and ductility, see Figure 1. In addition they respond to work-hardening treatments at the surface and thus have exceptionally high bending fatigue strength and wear resistance. These good properties are directly related to the unique bainite-austenite microstructure produced by austempering.

Abstract: PURPOSE:To convert only the necessary part of cast iron parts to a bainite structure excellent in mechanical properties such as strength, toughness or the like, by a method wherein only the necessary surface part of cast iron parts heated to and held at a bainite transformation temp is heated up to an austenitizing temp by high frequency induction heating and the heated part is subjected to rapid self-cooling to the aforementioned transformation temp CONSTITUTION:A crank shaft for a gasoline engine is prepared from a spherical graphite cast iron of which main components consist of, for example, 36%C, 24%C, 025%Mn, 0012%P, 0009%S, 004%Mg and the remainder Fe In this case, the objective hardness value of a surface layer part subjected to austempering treatment is set to Hv 300 and the roughly pre-processed crank shaft is heated to and held at 375 degC while a pin, the parallel part of a journal or the like where high load is applied during the operation of a practical machine are subjected to high frequency heating to austenitize the same over a range reaching a depth 5mm from the surface thereof In the next step, the heated parts are self-cooled to 375 degC being the temp of the crank shaft main body and, after the whole is held at 375 degC for 2hr, allowed to cool in the air The hardness of each austempering treatment part is Hv 290-300 and a predetermined bainite structure is shown

Abstract: Experiments were carried out on unalloyed ductile cast iron to evaluate the % retained austenite (%RY) and its lattice parameter as a function of austenitizing time and temperature for austempering temperatures ranging from 270 to 420°C. Results are related to expected carbon levels in the gamma iron matrix at the austenitizing temperature. It is shown that the rate of austenitization can be described as a two step process and experiments demonstrate that 900°C austenitizatlon is complete after 8m. An 1100°C homogenization has been shown to have a small effect upon %RY and rate of austenitization.

Abstract: PURPOSE: To manufacture spheroidal graphite cast iron having superior strength and toughness by properly selecting the composition of spheroidal graphite cast iron or spheroidal graphite alloy cast iron and conditions during the austempering of the cast iron so as to adjust the amounts of retained austenite and bainite in the structure. CONSTITUTION: Ordinary spheroidal graphite cast iron or spheroidal graphite alloy cast iron contg. at least one among <1.0% Mo, <1.0% Ni, <1.5% Cu and <1.0% Cr is austempered by austenitizing at 850W950°C for 0.5W3hr and isothermal heat treatment at 200W500°C for 0.5W10hr to adjust the amount of retained austenite in the matrix structure of the cast iron to 20W70vol% and the amount of bainite to 30W80vol%. At the same time, the retained austenite structure in the matrix structure is distributed in the form of a chain. Spheroidal graphite cast iron having superior strength and toughness is obtd. COPYRIGHT: (C)1985,JPO&Japio

Abstract: PURPOSE: To obtain a material suitable for manufacturing thick-wall machine parts which is to sustain a high repeated stress, by providing specific amounts of C, Si, Mn, Mo, and 1 kind among Cu, Ni, and Co, making a matrix form into bainite and reducing graphite grains into fine particles to be dispersed uniformly. CONSTITUTION: The spheroidal graphite molten cast pig consists of, by weight, 3W4% C, 1.8W3% Si, <0.7% Mn, 0.1W1% Mo, 0.1W3% of 1 kind among Cu, Ni and Co, and the balance Fe. This molten pig is subjected to metal mold casting and rapid cooling, and the heat-treated at 800W1,000°C to be austenized which is subjected to austempering at a constant temp. of 200W450°C to make the matrix form into bainite. In this way, the graphite grains are reduced to fine particles of 15W20μ to be dispersed uniformly. COPYRIGHT: (C)1986,JPO&Japio

Abstract: The influence of microstructure on the properties of austempered ductile cast irons have been studied along with an analysis of the necessary process control to insure consistent properties. Both austempered ductile iron transformed at high temperatures with a matrix structure of carbide-free ferrite and substantial amounts of austenite, and austempered/bainitic ductile iron transformed at low temperatures with a more characteristic bainitic structure also containing austenite have been studied. The good toughness and ductility of these high strength ductile irons is due to the microplasticity associated with both the ferrite and austenite phases; however carbide precipitation during the later stages of austempering reduces room temperature toughness considerably and results in a change to a quasi-cleavage fracture mode. The influence of casting quality control and heat treatment control on the microstructures and properties obtained are discussed.

Abstract: PURPOSE:To obtain abrasion resistance spheroidal graphite cast iron which secures high toughness and is provided with high abrasion resistance and fatigure strength by heating and holding only the surface part which requires abrasion resistance, of parts cast by ferrite type spheroidal graphite cast iron, and thereafter, air-cooling it, and subsequently, hardening the whole parts. CONSTITUTION:Only the surface part of parts cast by ferrite type spheroidal graphite cast iron whose pearlite area rate is <=30% is heated and held at 900- 1,000 deg.C for five seconds - five minutes. As a result, the base ground of said surface is converted to austenite. Subsequently, said surface which is heated and held is air-cooled, and the austenite is converted to pearlite. Thereafter, said whole parts are brought to hardening or austempering treatment, and the pearlite is transformed to a single structure of martensite (at the time of hardening) or bainite (at the time of austempering treatment) which does not contain ferrite. In this way, abrasion resistance and fatigue strength of the surface of said parts are improved.

Abstract: Ion implantation (N+ and C+) effect on fatigue strength of structural chromo-nickel-manganese steel is investigated after austempering. A noticable increase of fatigue strength is shown under the influence of ion implantation. It is found, that the depth of hardening is far larger than the length of ion path.

Abstract: Recent interest in the development of austempered ductile cast irons has resulted in considerable study of the physical metallurgy and mechanical properties of these high strength, high toughness cast irons. Equally important is the identification of process control and quality assurance factors to achieve the desired properties successfully and consistently. In this study, aspects of austempered ductile iron quality control are reviewed including the production of quality ductile iron that will respond to austempering heat treatments, heat treatment process control variables to achieve the desired properties, and non-destructive techniques for quality assurance. A distinction is made between austempered ductile irons transformed at high temperatures and austempered/bainitic ductile irons transformed at low temperatures. Austempering response is a complex interaction between all heat treatment variables and chemical composition on a macroscopic and microscopic level. Alloying elements are essential to provide sufficient hardenability (or austemperability) for heavy section heat treatment. Austenitizing temperature as well as austempering time and temperature affect the transformation response for a given alloy. In addition, segregation causes a non-uniform transformation response within the material on a microscopic level. In many cases the final properties of austempered ductile iron can be directly related to the amount of stabilized (retained) austenite present in the final structure. Non-destructive techniques to measure stabilized austenite are discussed and evaluated. Problems associated with dimensional control for critical tolerance components are highlighted.

Abstract: The mechanical properties obtained in austempered ductile iron castings are dependent on metallurgical process variables, including metal composition and heat-treatment conditions. The former determines also, to a large extent, the size of section which can be fully transformed. The effects of these metallurgical variables on both the structure and mechanical properties of austempered ductile irons having predominantly bainitic structures are described, on the basis of work carried out at BCIRA and elsewhere. Areas where further work is required have been identified and include the need for more data concerning impact, fatigue, and fracture toughness properties. Some practical aspects of the heat-treatment of these materials are reviewed, and the potential of near-net shape casting processes is briefly examined. Further extensive exploitation of these materials, particularly as gears, will require close cooperation with design engineers and the development of foundry control and inspection methods to guarantee the integrity and properties of castings supplied to the user.