Characterization of carbides in Q&amp;P steels using a combination of high-resolution methods

Klemm-Toole

et al. 2020

Metall Mater Trans A

Steels are ubiquitous due to their affordability and the landscape of useful properties that can be generated for engineering applications. But to further expand the performance envelope, one must be able to understand and control microstructure development by alloying and processing. Here we use multiscale, advanced characterization to better understand the structural and chemical evolution of AISI 4340 steel after quenching and tempering (Q&T), including the role of quench rate and short-time, isothermal tempering below 573 K (300°C), with an emphasis on carbide formation. We compare the microstructure and/or property changes produced by conventional tempering to those produced by higher temperature, short-time ''rapid'' tempering. We underscore that no single characterization technique can fully capture the subtle microstructure changes like carbon redistribution, transition carbide and/or cementite formation, and retained austenite decomposition that occur during Q&T. Only the use of multiple techniques begins to unravel these complexities. After controlled fast or slow quenching, g transition carbides clearly exist in the microstructure, likely associated with autotempering of this high martensite start temperature (M s ) steel. Isothermal tempering below 598 K (325°C) results in the relief of carbon supersaturation in the martensite, primarily by the formation of g transition carbides that exhibit a range of carbon levels, seemingly without substitutional element partitioning between the carbide and matrix phases. Ha¨gg transition carbide is present between 300°C and 325°C. After conventional tempering at or above 598 K (325°C) for 2 h, cementite is predominant, but small amounts of cementite are also present in other conditions, even after quenching. Previous work has indicated that silicon (Si) and substitutional elements partition between the cementite, which initially forms under paraequilibrium conditions, and the matrix. Phosphorous (P) may also be preferentially located at cementite/matrix interfaces after high temperature tempering. Slower quench rates result in greater amounts of retained austenite compared to those after fast quenching, which we attribute to increased austenite stability resulting from ''autopartitioning''. Rapid, high temperature tempering is also found to diminish tempered martensite embrittlement (TME) believed to be associated with the extent of austenite decomposition, resulting in mechanical properties not attainable by conventional tempering, which may have important implications with respect to industrial heat treatment processes like induction tempering. Controlling the amount and stability of retained austenite is not only relevant to the properties of Q&T steels, but also next-generation advanced high strength steels (AHSS) with austenite/martensite mixtures.

Section: Introductionmentioning

confidence: 99%

Perspectives on Quenching and Tempering 4340 Steel

Klemm-Toole

et al. 2020

Metall Mater Trans A

“…The martensite tempering has led to the formation of θ-carbide, and χ-carbide has existed in austenite. The APT measurements revealed that carbide precipitation resulted from austenite decomposition [25]. The effect of partitioning conditions during processing on the microstructure of 0. been studied by using Mössbauer spectroscopy and transmission electron microscopy.…”

Section: /2mentioning

confidence: 94%

“…S. Ebner et al 2020 also investigated carbide formation during the processing of Q&P steel with two different chemical compositions by in-situ high-energy X-ray diffraction. The author pointed that partial decomposition of austenite and martensite tempering leads to carbide precipitation [25].…”

Section: /2mentioning

confidence: 99%

Design and Optimization of Processing Conditions for a Recent Quenched and Partitioned Steel

El-Shenawy¹,

Hedia²,

Kama-El-Din³

et al. 2021

Esaform 2021

Q&P steels as a "Third Generation" of (AHSS) exhibit excellent tensile properties, which enable producing lightweight sections for the automotive industry and at the same time keep safety requirements. This research aims to predict the proper processing conditions for developing ultra-high-strength Q&P steel with a novel chemical composition of 0.37 C-3.65 Mn- 0.65Si- 0.87 Al- 1.5 Ni- 0.05P, wt. %. To design and optimize proper heat treatment conditions, the phase diagram, CCT curve, and critical temperatures of these alloys were first implemented using THERMO-CALC and JMATE PRO software and Gleeble 3500 machine. The heat treatment process included full austenitization, then quenching at 120°C followed by partitioning at 450°C for different times. The tensile properties, microstructure, and retained austenite volume fraction of heat-treated steel was studied at room temperature by tensile testing machine, optical microscope, and XRD. The finding summarized that partitioning of this steel for 100 s during processing had developed Q&P steel with ultra-high-strength of 1104 MPa with maximum total elongation and strength elongation balance 8.1 % and 8932 MPa %, respectively. The optical micrograph showed that heat-treated specimens at different partitioning times have had a microstructure of tempered martensite, carbide free bainite, and retained austenite. Besides, the retained austenite volume fraction has decreased with increasing partitioning time, which may be due to carbide precipitation during partitioning.

“…The main factors that may influence aγ are-carbon content, stress state, temperature, and carbides formation. As demonstrated in Figure 3a, carbides peaks cannot be distinguished from the background in the diffraction patterns during the deformation cycle, otherwise carbon atoms would be consumed by the carbides leading to a significant decrease in the lattice parameter of austenite [16,34]. Although no clear evidence of carbides was identified, cluster formation or transition carbides with fractions below 1 Mass.% cannot be excluded.…”

Section: Lattice Parametermentioning

confidence: 97%

“…Since Figure 2b shows a continuous increase in f α b during DT (γ→αb) , carbon partitioning from α b to γ is expected. Considering that carbon causes a lattice expansion [34], such contribution may act as an opposite effect to the lattice strain imposed by macroscopic deformation, implying that the observed non-linearity of aγ is due to carbon enrichment of austenite. The latter has an isotropic effect in the γ lattice parameter (e.g., carbon expands equally the lattice in the axial and radial directions), whereas the lattice strain induced by the macroscopic deformation is essentially anisotropic.…”

Section: Lattice Parametermentioning

confidence: 99%

Revealing the Dynamic Transformation of Austenite to Bainite during Uniaxial Warm Compression through In-Situ Synchrotron X-ray Diffraction

Bevilaqua

Épp

Meyer

et al. 2021

Metals

In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing.