Ivan Vorel

Metallographic Observation for Evaluating Microstructural Evolution on Various Cross-Sections of Forged Part upon Air-Cooling from Finishing Temperature

Utility properties of forgings, particularly the mechanical ones, are among the primary aspects of interest to the customers of forge shops. These properties arise from internal structure whose evolution depends predominantly on the combination of parameters of deformation processes applied during forging, on the temperature profile during cooling and on the shape of the forged part. As microstructural evolution depends on the shape of the particular cross section of the forged part, an appreciable inhomogeneity of mechanical properties occurs in forgings. This article deals with observation of microstructural evolution in a chosen forged part, depending on cooling profiles of its various cross sections. The experimental programme of mechanical working and treatment of the forged part was based on the material-technological modelling approach. Microstructural evolution was studied using light and electron microscopic methods. Results of this analysis provided a basis for outlining optimization steps for mechanical working and treatment of the forged part.

Effect of silicon content on microstructure of low-alloy Q&P-Processed steels

Todays requirements for the design of functional parts and components demand low weight and, at the same time, high strength. There are several heat treatment methods which can satisfy such requirements. These include TRIP heat treatment, long-time low-temperature austempering, and Q&P processing. It is the Q&P processing which delivers excellent results in terms of mechanical properties and light weight. It relies on stabilising retained austenite through partitioning of carbon between martensite and austenite. The carbon-enriched austenite then becomes a ductile constituent in the otherwise brittle martensitic matrix. A precondition for successful Q&P processing consists in sufficient silicon content in the steel, which precludes precipitation of undesirable cementite. Cementite would otherwise form as a result of enrichment of retained austenite with carbon. To ascertain the usefulness of higher silicon level in steel for Q&P processing, one can compare Q&P processes in steels with various levels of this element

New heat treatment process for advanced high-strength steels

Todays advanced steels are required to possess high strength and ductility. It can be achieved by choosing an appropriate steel chemistry which has a substantial effect on the properties obtained by heat treatment. Mechanical properties influenced the presence of retained austenite in the final structure. Steels of this group typically require complicated heat treatment which places great demands on the equipment used. The present paper introduces new procedures aimed at simplifying the heat treatment of high-strength steels with the use of material-technological modelling. Four experimental steels were made and cast, whose main alloying additions were manganese, silicon, chromium, molybdenum and nickel. The steels were treated using the Q-P process with subsequent interrupted quenching. The resulting structure was a mixture of martensite and retained austenite. Strength levels of more than 2000 MPa combined with 10-15 % elongation were obtained. These properties thus offer potential for the manufacture of intricate closed-die forgings with a reduced weight. Intercritical annealing was obtained structure not only on the basis of martensite, but also with certain proportion of bainitic ferrite and retained austenite.

Impact of quenching temperature and isothermal holding time during austempering on bainite content in high-silicon steel

Silicon plays an important role in the manufacture and processing of steel. It is involved in metallurgical processes in the melt, improves castability, and, together with aluminium, belongs to the elements which suppress cementite formation during heat treatment of steels. The last-named aspect offers a great potential for developing high-strength steels with excellent ductility. The amount of bainite in the microstructure has a substantial impact in this respect. Bainite fraction depends mainly on temperature and on the isothermal holding time in the bainitic transformation region. In steels with low silicon levels, isothermal bainitic transformation continues until all austenite has transformed to bainite. At higher silicon levels, approximately 2 weight percent, the bainitic transformation is expected to be incomplete. The resulting bainite fraction would then be dictated by the thermodynamic equilibrium in the austenite-bainite system. As a result, one could control microstructural evolution, and thus the mechanical properties in high-strength bainitic-martensitic steels

Using of material-technological modelling for designing production of closed die forgings

Production of forgings is a complex and demanding process which consists of a number of forging operations and, in many cases, includes post-forge heat treatment. An optimized manufacturing line is a prerequisite for obtaining prime-quality products which in turn are essential to profitable operation of a forging company. Problems may, however, arise from modifications to the manufacturing route due to changing customer needs. As a result, the production may have to be suspended temporarily to enable changeover and optimization. Using material-technological modelling, the required modifications can be tested and optimized under laboratory conditions outside the plant without disrupting the production. Thanks to material-technological modelling, the process parameters can be varied rapidly in response to changes in market requirements. Outcomes of the modelling runs include optimum parameters for the forging parts manufacturing route, values of mechanical properties, and results of microstructure analysis. This article describes the use of material-technological modelling for exploring the impact of the amount of deformation and the rate of cooling of a particular forged part from the finish-forging temperature on its microstructure and related mechanical properties.

Effect of silicon on stability of austenite during isothermal annealing of low-alloy steel with medium carbon content in the transition region between pearlitic and bainitic transformation

In a vast majority of steels, a prerequisite to successful heat treatment is the phase transformation of initial austenite to the desired type of microstructure which may consist of ferrite, pearlite, bainite, martensite or their combinations. Diffusion plays an important role in this phase transformation. Together with enthalpy and entropy, two thermodynamic quantities, diffusion represents the decisive mechanism for the formation of the particular phase. The basis of diffusion is the thermally-activated movement of ions of alloying and residual elements. It is generally known that austenite becomes more stable during isothermal treatment in the transitional region between pearlitic and bainitic transformation. This is due to thermodynamic processes which arise from the chemical composition of the steel. The transformation of austenite to pearlite or bainite is generally accompanied by formation of cementite. The latter can be suppressed by adding silicon to the steel because this element does not dissolve in cementite, and therefore prevents its formation. The strength of this effect of silicon depends mainly on the temperature of isothermal treatment. If a steel with a sufficient silicon content is annealed at a temperature, at which silicon cannot migrate by diffusion, cementite cannot form and austenite becomes stable for hours.

Evolution of microstructure and mechanical properties during Q&P processing of medium-carbon steels with different silicon levels

Evolution of microstructure during heat treatment plays a fundamental role in the resulting mechanical properties of steel. Today, mechanical properties in conjunction with technological properties, such as weldability, formability, and machinability, and their optimum combinations, are widely discussed in a number of mechanical engineering disciplines. In this manner, requirements arise for developing steels which could offer high strength and good formability, and which could be used for making parts with high resistance to failure and with a long life. One present-day example of such steels involves Q&P-processed martensitic steels. Their properties are dictated by their treatment, as well as their alloying, particularly by the silicon content. Silicon fundamentally affects microstructure evolution during Q&P processing and, as a result, mechanical properties. With this way it is possible to receive microstructures consinsting of martensite and retained austenite with an ultimate tensile stress of more than 1600 MPa and a uniform elongation of more than 12 %.