Overview of Mechanisms Involved During the Quenching and Partitioning Process in Steels

Abstract

The application of the quenching and partitioning (Q&P) process in steels involves a microstructural evolution that is more complex than just the formation of martensite followed by carbon partitioning from martensite to austenite. Examples of this complexity are the formation of epitaxial ferrite during the first quenching step and the formation of bainite, carbides, and carbon gradients as well as migration of martensite/austenite interfaces during the partitioning step. In this work, recent investigations on the mechanisms controlling microstructural changes during the application of the Q&P process are evaluated, leading to phase-formation based concepts for the design of Q&P steels. NEW strategies for the creation of advanced high strength steels with improved mechanical properties of strength, toughness, and ductility are based on the development of microstructures consisting of ultrafine phases formed in nonequilibrium conditions such as martensite and bainite in combination with retained austenite. [1] The refined and highly dislocated martensite and bainite contribute to a simultaneous increase of strength and toughness. Retained austenite contributes to the improvement of the strength/ductility combination via the transformation induced plasticity (TRIP) effect and to the improvement of the toughness if the retained-austenite grains have a filmlike morphology. One of the most innovative procedures to create microstructures consisting of martensite and retained austenite is the so-called quenching and partitioning (Q&P) process. [2] This process starts with a total or partial austenitization, followed by a quench of the microstructure to a temperature (quenching temperature) below the martensite start (M s ) temperature but above the martensite finish (M f ) temperature to form a controlled fraction of martensite. This microstructure is then subjected to a treatment at the same or higher temperature (partitioning temperature) in order to accomplish the carbon diffusion from the carbon-supersaturated martensite to the neighboring austenite. Finally, the material is quenched to room temperature and the austenite that has been sufficiently carbon enriched remains metastable at room temperature, whereas the rest transforms into martensite.Investigations on the Q&P process were mainly focused on the application of this heat treatment to steels with chemistries very close to commercial TRIP steels, [3][4][5][6][7][8] designed to promote bainite formation, whereas studies on steels specially designed to be subjected to the Q&P process are fewer. [9][10][11] Depending on the steel composition and the particular heat treatments, formation of bainite, ferrite, and carbides during the Q&P process can overlap with carbon partitioning from martensite to austenite, reducing the effectiveness of this heat treatment leading to the desired microstructures. An adequate theoretical knowledge of the mechanisms occurring during the Q&P process would lead to a well control of these overlapping phenomena. In the present work, ...

Critical Assessment 7: Quenching and partitioning

“…83 It should be noted that alloy design in this work focused on lowering the calculated quench temperature to room temperature. While this alloy 35,[83][84][85] as the original intent of some of the studies involved retarding the bainite transformation and increasing austenite amounts at the quench temperature. Additions of Mo to a 0?2C-2?0Mn-1?5Si (wt-%) alloy resulted in greater austenite fractions, with less dependence of austenite fraction on partitioning time.…”

Study of The Precipitation Behavior and Strengthening of Carbides during the Partition Process

“…Although the Si and Al elements in the spring steel can inhibit the precipitation of cementite in the Q&P process,E.P.Da Siva, D.De knijf, D.V.Edmods and Moor [5][6] all indicate that carbides precipitation in the partition period cannot be completely suppressed even in low carbon steels with relatively high Si and Al content, especially transition carbides [7][8].While precipitation of carbides and the diffusion of carbon in martensite into austenite are a competition mechanism.Carbides precipitation not only lead to the less content of retained austenite at room temperature, but also contribute to poor carbon in martensite,which can reduce comprehensive mechanical properties.As the second phase particle precipitation, the size of carbides will affect the mechanical properties of steel [9][10].The large size carbide particles formed in partition time are easy to cause stress concentration, reduce the plasticity and toughness,and it will result in serious brittle fracture.So,in addition to martensite,precipitation of carbides is also an important factor determining the mechanical properties of spring steel treated by Q&P process [11][12].…”

“…It mainly work in bending periodic, alternating torsion stress conditions, subjected to pull, pressure, shock, torsional fatigue, corrosion and other effects, sometimes bear high short-term sudden load. So the comprehensive mechanical properties of spring steel high requirements.The microstructure of martensite and retained austenite can be obtained by heat treatment of spring steel by Q&P process, and it has the advantages of both high strength and high plasticity in performance [1][2][3][4].Although the Si and Al elements in the spring steel can inhibit the precipitation of cementite in the Q&P process,E.P.Da Siva, D.De knijf, D.V.Edmods and Moor [5][6] all indicate that carbides precipitation in the partition period cannot be completely suppressed even in low carbon steels with relatively high Si and Al content, especially transition carbides [7][8].While precipitation of carbides and the diffusion of carbon in martensite into austenite are a competition mechanism.Carbides precipitation not only lead to the less content of retained austenite at room temperature, but also contribute to poor carbon in martensite,which can reduce comprehensive mechanical properties.As the second phase particle precipitation, the size of carbides will affect the mechanical properties of steel [9][10].The large size carbide particles formed in partition time are easy to cause stress concentration, reduce the plasticity and toughness,and it will result in serious brittle fracture.So,in addition to martensite,precipitation of carbides is also an important factor determining the mechanical properties of spring steel treated by Q&P process [11][12].There were no systemic study of precipitation of spring steel in the partition of carbides and its effects on the mechanical properties in the domestic and foreign literatures.The purpose of the present work was to explore the influence ofcarbides on mechanical properties by the statistics of carbides precipitation types, quantity and size distribution of spring steel after different Q&P processes,and calculate the contribution to strength.It is determined that In addition to the changes in mechanical properties caused by different tissue structures, the contribution of carbides precipitation strengthening is confirmed ,which can provide guidance for optimizing mechanical properties. …”

Effect of Tempering on Microstructure and Mechanical Properties of Laser Welded and Post-Weld Treated AHSS Specimens

“…Depending on volume fraction of martensite and austenite and the partitioning time, different microstructures can develop, e.g. retained austenite, martensite and bainite [6]. In this work, the idea behind Q&P is used after laser welding of AHSS in order to control the microstructure and the mechanical properties of the weld and heat affected zone, by a very fast procedure.…”