Decomposition Of Martensite Research Articles

Tailoring the microstructure and mechanical property of laser powder bed fusion fabricated Ti–6Al–2Sn–4Zr–2Mo via heat treatment

The heat treatment effect on the microstructure evolution and mechanical properties of the laser powder bed fusion fabricated Ti–6Al–2Sn–4Zr–2Mo (Ti6242) alloy was investigated in this study. The highly dense Ti6242 samples comprising acicular α΄ martensite and deformation twinning could be fabricated. The as-built alloy exhibited a high yield strength (σy, 1296 ± 44 MPa) yet a limited tensile elongation (ɛf, 5.7 ± 1.1%). Four different heat treatment profiles were conducted to tailor the microstructure and tensile properties. The direct annealing at 630 °C and 850 °C caused martensite decomposition. The annealing at 630 °C/8 h/air cooling led to the formation of β nano-particles and layers, leading to a slight increase of σy and a reduction of ɛf. Annealing at 850 °C/3 h/air cooling led to coarsening of α lamellae and growth of β phase, resulting in a decreased σy and enhanced ɛf compared with those at 630 °C. The two-stage heat treatment and thermal cycling triggered the formation of partially globularized primary α and Widmanstatten structure (secondary αs + β). These two heat treatments gave rise to a balanced combination of strength and ductility that could be comparable to those of wrought counterparts. This work demonstrates that a combination of laser powder bed fusion and post heat treatment could allow for the fabrication of complex-shaped Ti6242 components with little compromise of mechanical property for industrial applications.

Effect of alloying elements on the microstructure and mechanical properties of high chromium white cast iron and Ni-Hard iron

The present work deals with the effect of B, V, Ti on the microstructural evolution and mechanical behavior of high chromium white cast iron. The comparison is also made with Ni-hard iron in as-cast condition. The destabilization treatment provided after casting resulted in decomposition of austenite into martensite and secondary carbides and further tempering treatment resulted in decomposition of martensite into ferrite and secondary carbide phases. Phase analysis by x-ray diffraction revealed two main phases, i.e., a BCC-Fe matrix and a primary carbide phase. In addition to this, high chromium white cast iron and Ni-hard alloys contain austenitic phase (FCC-Fe) (Fig. 2(a)). The primary carbides are M7C3 for V additions whereas for all other cases, they are M3C2. Ti and B additions led to the formation of secondary carbides of M7C3 and M23C6, respectively. Effect of alloying elements on phases formed and further modification of the microstructure are discussed in detail. The hardness of unalloyed high chromium white cast iron is highest amongst all the alloys tested, however, Ti and V additions to it retained this hardness to almost the same extent. V addition to the alloy showed not only highest tensile strength but also longer elongation to fracture, thereby, resulting as toughest alloy in the lot. On the other hand, Ni-Hard iron showed the least tensile strength and elongation before fracture. This study, thereby concludes that high chromium white cast iron with V additions is suitable for manufacture of coal pulveriser grinding media owing to its higher hardness and toughness.

Structure and properties of near-a titanium products obtained by direct laser deposition and heat treatment

Advanced techniques of obtaining products require careful selection of materials for various industries. Titanium alloys are widely used in the aerospace, shipbuilding and mechanical engineering industries. The development of near-a titanium alloys should be considered a significant achievement in the field of metallurgy and heat treatment (HT) of titanium alloys. This article presents a study carried out with the aim of optimizing heat treatment modes for high-temperature titanium alloys obtained by direct laser deposition (DLD). Heat treatment was carried out in the temperature range (700-1000°C), covering three typical temperature ranges, i.e. the temperature range for the partial decomposition of martensite, the temperature range for the complete decomposition of martensite, and the phase transformation temperature were subsequently selected as the heat treatment temperatures. Based on metallographic analysis, the influence of heat treatment modes on the structure, as well as the tensile properties at room temperature, of TA15 titanium DLD-samples.

Reduction in anisotropy of mechanical properties of Ti-6Al-4V through martensite decomposition

Orientation induced anisotropic hardness and modulus of Ti-6Al-4V alloy has been minimized through microstructural engineering. This involves the formation of martensite upon quenching and further its decomposition facilitated by cold deformation and low temperature heat treatment. The martensite (α′) formed from the β phase has nearly equal distribution of multiple variants in the form of fine laths. After decomposition, this transforms to fine equiaxed β along the lath boundaries of α, instead of a thin lamellar β. The variations in the nano-indentation derived hardness and modulus are much smaller compared to the reported literature and is attributed to the fine nature and morphology of the constituent phases.

Neutron Diffraction Study of Kinetics of Low Temperature Decomposition of Martensite in Medium Carbon Steel

Neutron Diffraction Study of Kinetics of Low Temperature Decomposition of Martensite in Medium Carbon Steel

Martensite decomposition during post-heat treatments and the aging response of near-α Ti–6Al–2Sn–4Zr–2Mo (Ti-6242) titanium alloy processed by selective laser melting (SLM)

Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near-[Formula: see text] titanium alloy explicitly designed for high-temperature applications, consists of a martensitic structure after selective laser melting (SLM). However, martensite is thermally unstable and thus adverse to the long-term service at high temperatures. Hence, understanding martensite decomposition is a high priority for seeking post-heat treatment for SLMed Ti-6242. Besides, compared to the room-temperature titanium alloys like Ti–6Al–4V, aging treatment is indispensable to high-temperature near-[Formula: see text] titanium alloys so that their microstructures and mechanical properties are pre-stabilized before working at elevated temperatures. Therefore, the aging response of the material is another concern of this study. To elaborate the two concerns, SLMed Ti-6242 was first isothermally annealed at 650[Formula: see text]C and then water-quenched to room temperature, followed by standard aging at 595[Formula: see text]C. The microstructure analysis revealed a temperature-dependent martensite decomposition, which proceeded sluggishly at [Formula: see text]C despite a long duration but rapidly transformed into lamellar [Formula: see text] above the martensite transition zone (770[Formula: see text]C). As heating to [Formula: see text]C), it produced a coarse microstructure containing new martensites formed in water quenching. The subsequent mechanical testing indicated that SLM-built Ti-6242 is excellent in terms of both room- and high-temperature tensile properties, with around 1400 MPa (UTS)[Formula: see text]5% elongation and 1150 MPa (UTS)[Formula: see text]10% elongation, respectively. However, the combination of water quenching and aging embrittled the as-built material severely.

In-situ XRD study on the effects of stress relaxation and phase transformation heat treatments on mechanical and microstructural behaviour of additively manufactured Ti-6Al-4V

Additively Manufactured (AM) titanium (Ti) components are routinely post-thermal heat treated (HT), to reduce internal stresses, as well as to obtain more desirable microstructural features, yielding improved mechanical performance. Currently, there is no consensus on the optimum HT method for AM Ti-6Al-4V, as the mechanism for the main phase transformation (α′ (martensite) → α + β (equilibrium)) is still ambiguous. In this study, stress relaxation and phase transformation in the alloy are investigated in detail, via isothermal heat treatments and in situ high temperature X-ray Diffraction (XRD). The latter was carried out at heating rates of 5 and 200 °C/min. The relationship between crystallographic evolution during isothermal treatments and mechanical behaviour was determined. Isothermal holding at 400 °C resulted in an increase in ultimate tensile strength (UTS) and yield strength (YS) by 3.4% and 2.1%, respectively, due to the relief of tensile microstrain. It was found that isothermal treatment conducted between 550 and 700 °C promotes martensitic decomposition, resulting in the formation of a transitional - αtr phase, which has an asymmetrical hexagonal crystal lattice. The formation of this αtr phase was determined to be the main factor contributing to a major decrease in ductility.

Investigation into Competitive Mechanism of Carbon Partitioning and Pseudospinodal Decomposition of Supersaturated Martensite in Q&P Steel Containing High Silicon

In this study, the competing mechanisms of carbon partitioning and concurrent pseudospinodal decomposition of supersaturated martensite, forming superlattice-ordered α″-Fe16C2, are elucidated in a quenching and partitioning (Q&P) steel containing high silicon based on various microstructural characterizations. Our results demonstrate that the fluctuation of carbon content caused by high-density dislocations and transformation residual stress in martensite may stimulate the pseudospinodal decomposition. Furthermore, the sluggish diffusion kinetics of silicon and nickel inhibits further transformation from α″-Fe16C2 to carbide precipitation. The experimental results provide new insights into the pseudospinodal decomposition and carbon redistribution mechanism during the carbon partitioning process.

Grain boundary discontinuity and performance improvement mechanism of wire arc additive manufactured Ti–6Al–4V

Most recent studies for improving tensile properties of wire arc additive manufactured Ti–6Al–4V focused on layer-by-layer plastic deformation but with complicated operation. Previous study demonstrated that a sub-transus solution increased the strength and decrease the anisotropic ductility by generating a new micro-lamellae recrystallization to discontinue α grain boundaries (α GB ). The current study investigated the effects of near β transus solution and aging treatment on the microstructure and tensile properties. Both α lamellae recrystallization and α' martensite decomposition were observed, which induced a fine lamellar (α + β) structure and a discontinuous α GB. These changes increased the yield strength (YS) up to 904 MPa while maintaining a high elongation above 15.4%. According to the data, the Hall-petch relationship between the YS and α lamellae width was established, where the quadratic relationship between the elongation and α lamellae width was found. The anisotropic ductility disappeared when the discontinuous ratio of α GB was higher than 0.6 because the discontinuous α GB changed the failure mode of horizontal sample from opening to sliding modes similarly with that of vertical sample. The results would provide a pathway to improve the tensile properties of additive manufactured Ti–6Al–4V without complicated layer-by-layer plastic deformation. • The effect of the grain boundary discontinuity on tensile properties in wire arc additive manufactured titanium alloys was investigated. • The improvement mechanism of tensile properties was elucidated. • A quantitative model has been successfully applied to investigate the anisotropic ductility.

Semi in-situ local structure observation during PWHT of Cr–Mo weldments extent of the PWHT on local structure changes in heat affected zone microstructure

Soft zone formation in dissimilar Cr–Mo welding occurs during post weld heat treatment that could compromise the integrity of the weldment. The decomposition of martensite could cause this formation at the weld heat affected zone adjacent to the fusion boundary. There could not yet determine the actual width of this zone. The extent of martensite decomposition in soft zone Cr–Mo steel dissimilar weld was observed in semi in-situ stages using the x-ray absorption spectroscopy (XAS) at Synchrotron facility. 2.25Cr–1Mo was welded using ER90S–B9 using GTAW process. PWHT was performed at 730 °C for 0.5 h and 2.0 h in a chamber with opening for Synchrotron light analysis at various locations away from the fusion boundary. This was complemented by microstructure and grain size analysis. The results showed the decomposition of martensite started at the fusion boundary and could extend beyond 1000 microns covering both CGHAZ and FGHAZ. Since the experiment was performed in normal atmosphere, the oxide film could form during PWHT on the surface of the specimen. More oxide formed was observed more in the location with high carbide contents. There was more oxide formation at the locations further distance from fusion boundary due to less martensite decomposition. Therefore, the extent of the phenomena could be stated by using information from oxide formation and martensite decomposition which could help to determine the risk on both creep life and oxidation.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C system alloys. Part 2. Metallographic studies

Original metallographic studies of annealed medium carbon steel (steel 45, steel 40X and steel 35XGA) are carried out in order to reveal leakages in pearlite of these steels of low-temperature carbide transformation of peritectoid type, in which solid solutions of ferrite and cement form a solid solution of wide area of homogeneity based on ε-carbide Fe2C. Most of the inclusions of pearlite cement 45 steel are almost entirely covered with a light grey “foam” shell of nanoglobular crystals ε-сarbide Fe2C. In the process of crystallization of ε-сarbide Fe2C on the cement plates of pearlite steel 45 three morphological types of structure of ε-сarbide Fe2C can be distinguished: “foam” globular shell, granular “outgrowths” and wrapping of particles of decomposed and partially dissolved cement plates. Chromium, which is a part of steel 40X, abruptly inhibits peritectoid transformation. On many cement plates pearlite surface is smooth. In areas where the concentration of chromium is low, the process of formation of ε-сarbide Fe2C is active, with the formation of individual sites with light gray “foam” shell of nanoglobular crystals ε-сarbide Fe2C. It can be expected that in high-alloy chrome steels, the peritectic transformation can be completely blocked through the stabilization of chrome cement or its transformation into thermodynamically stable high chrome carbides. In 30ХГСA steel a sharp intensification of the process of peritectoid transformation of solid solutions of ferrite and cement which are part of pearlite is fixed. The reason for acceleration of the disintegration process of pearlite cement into individual fragments and intensification of release of ε-сarbide Fe2C in the form of column-shaped crystals between the plates of pearlite cement is silicon and manganese, which are part of steel 30ХГСА. As a result of the acceleration of martensite decomposition, the morphology of the released crystals of ε-сarbide Fe2C has changed from “foam” nanocrystals of ε-сarbide Fe2C, typical for steel 45 and steel 40X, to granular. There was also intensive fragmentation or disintegration of cement plates with the appearance of plane-parallel boundaries between the fragments and the formation of large longitudinal flat inclusions of ε-сarbide Fe2C above 100 nm, whose axis is mainly perpendicular to the axis of the cement plate. On the basis of the performed experiments it is possible to consider as proved the presence of low-temperature carbide peritectoid phase transformation in the Fe-C alloy system as a result of interaction of solid solutions of ferrite and cement at 3820C with formation of solid solution on the basis of ε-сarbide Fe2C with wide area of homogeneity. The influence of the chemical composition of steel on the peritectoid transformation between ferrite and cement slurries opens up additional possibilities for regulating the microstructure of pearlite, such as the degree of dispersion of pearlite, which has a determining influence on a number of performance characteristics of steel, such as wear resistance, yield strength and others.

Нейтронографическое исследование кинетики низкотемпературного распада мартенсита среднеуглеродистой стали

It is found that the low-temperature decomposition of martensite in quenched medium-carbon steel occurs in two stages. In the first stage, the rate of decomposition is higher than that in the subsequent stage. Application of the neutron diffraction method allows the identification of two stages of transformation in the first stage of martensite decomposition. It is shown that the first stage is associated predominantly with carbon segregation at dislocations, and the second, with the outdiffusion of carbon from the supersaturated solid solution with the formation of dispersed particles of metastable carbides. It is shown that the change in the concentration of carbon and, accordingly, the degree of tetragonal lattice of martensite at aging and low tempering occurs to a certain limit, independent of the cooling rate during quenching and tempering temperature. This is due to the establishment of a relative equilibrium between a supersaturated solid solution and fine particles of metastable iron carbide. It is found that the determining process, which leads to a change in the microhardness the low-temperature decomposition, is the out diffusion of carbon from the supersaturated solid solution.

Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C-system alloys. Part 1. Basics of theory

In the present work low-temperature carbide phase transformations in the system of Fe-C alloys based on ε-сarbide Fe2C with consideration of identification of θ-Fe3C cement as a solid solution were studied. It has been proved that the θ-Fe3C cement slurry is colourfastonide, and the ε-Fe2C carbide slurry is bertollide. When tempering hardened steels, ε-сarbide Fe2C is emitted in the structure of hardened martensite, which is absent in the phase diagram of iron-carbon system alloys. It is believed that ε-сarbide Fe2C is not a stable phase, and since it is metastable, it is formed only in quenched steels under non-equilibrium conditions. The isolation and dissolution of ε-сarbide Fe2C is a classic phase transformation and the absence of this transformation on the diagram is not caused by the metastable phase of ε-сarbide Fe2C, but by the incomplete iron-carbon diagram. The martensite decomposition phase transformation is based on the formation of carbon enriched zones. The processes of carbon segregation on dislocation structures and grid planes of martensite create zones with excess energy. Beginning approximately with temperature 100 °С in structure of martensite begins to allocate ε-сarbide Fe2C, finishing a stage of two-phase segregational disintegration of martensite. At rather small concentrations of carbon in cluster zones the fastest and most effective way of relaxation of redundant energy in these zones, as well as in the tetragonal lattice of martensite is the formation of phases with low value of work of nucleation, first of all ε-сarbide Fe2C and α-Fe(C) or ferrite. The main stages of phase transformations in the peritectoid reaction of martensite decomposition are considered. It is proposed to introduce the peritectoid transformation horizontal at 382 °C and the peritectic transformation horizontal of cement at 1650 °C into the Fe-C alloy state diagram.

Gradient microstructure evolution and hardening mechanism of carburized steel under novel heat treatment

The microstructure evolution of 15MnNi4Mo steel after carburizing was studied, and the hardening mechanism of carburized layer under the influence of sub-temperature quenching was discussed. The results show that due to the high Ni content, the high-temperature tempering process of 15MnNi4Mo is slow, and the carburizing layer evolution behavior is mainly the decomposition of martensite and retained austenite, while the heart region is mainly the decomposition of M/A island. The sub-temperature quenching retains some of the carbides after tempering which increases the hardness (60HV) of the carburized layer by reducing the retained austenite, refining the substructures of martensite and accumulating dislocations around carbides.

Effects of Postprocess Hot Isostatic Pressing Treatments on the Mechanical Performance of EBM Fabricated TI-6Al-2Sn-4Zr-2Mo.

An essentially fully acicular alpha-prime martensite within an equiaxed grain structure was produced in an Electron Beam Melting (EBM)-fabricated Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy using two different Arcam EBM machines: An A2X system employing tungsten filament thermionic electron emission, and a Q20 system employing LaB6 thermionic electron emission. Post-process Hot Isostatic Pressing (HIP) treatment for 2 h at 850, 950, and 1050 °C resulted in grain refinement and equiaxed grain growth along with alpha-prime martensite decomposition to form an intragranular mixture of acicular martensite and alpha at 850 °C, and acicular alpha phase at 950 and 150 °C, often exhibiting a Widmanstätten (basketweave) structure. The corresponding tensile yield stress and ultimate tensile strength (UTS) associated with the grain growth and acicular alpha evolution decreased from ~1 and ~1.1 GPa, respectively, for the as-fabricated Ti6242 alloy to ~0.8 and 0.9 GPa, respectively, for HIP at 1050 °C. The optimum elongation of ~15–16% occurred for HIP at 850 °C; for both EBM systems. Because of the interactive role played by equiaxed grain growth and the intragrain, acicular alpha microstructures, the hardness varied only by ~7% between 41 and 38 HRC.

Influence of aging on transformation characteristic and phase structure of a Cu–12.4Al–1.2Ti–1.2Ta shape memory alloy

The effect of aging on the properties of a Cu–12.4Al–1.2Ti–1.2Ta (wt.%) high-temperature SMA was investigated throughout SEM/EDS, XRD and DSC experiments. It was found that the transformation behavior, the thermal properties and the phase structure of the alloy remarkably changed with homogenization process performed at 900 °C for 20 min and then aging at 400 and 500 °C for 1 h. Structural and morphological investigations revealed that the homogenization process caused martensitic decomposition in the alloy and austenite phase domains remained. Afterward, with the effect of thermal aging, the amount of the austenite phase increased and the α-phase domains nucleated and grew in the alloy. At elevated aging temperature, the amounts of the austenite and the α-phases in the alloy considerably increased and martensitic morphology disappeared. As a result of these, the phase structure of the alloy affected its thermal behavior and led to exhibit irreversible one-stage martensitic transformation behavior.

Thermodynamics of Phase Transformation and Microstructure Evolution of 18Cr2Ni4WA Carburized Steel during High‐Temperature Tempering

The microstructural evolution and phase transformation mechanism in 18Cr2Ni4WA carburized steel during high‐temperature tempering is investigated using a thermodynamic model. The experimental results demonstrate that the retained austenite in the matrix decreases from the surface to the core, and martensite transform from acicular to lath‐type. Furthermore, the hardness distribution of the carburized layer increase initially, and then decrease. Based on thermodynamic calculations, the driving forces for the decomposition of martensite decomposition and nucleation of cementite in martensite are larger than those for the decomposition of austenite during tempering. Furthermore, cementite preferentially precipitate in martensite. The driving force for the overall phase transformation of the decomposition of the retained austenite is negative, and the driving force for the nucleation of cementite in the low‐carbon retained austenite is positive, indicating that cementite can nucleate only in high‐carbon retained austenite. The low‐carbon retained austenite transform into secondary martensite on cooling after the tempering process while the driving force gradually increase. The decomposition of the initial quenched martensite during the tempering process leads to a decrease in the overall hardness of the carburized layer. The secondary martensite obtained from the retained austenite contribute to the increase in the surface hardness.

Comparison of Wear Performance of Austempered and Quench-Tempered Gray Cast Irons Enhanced by Laser Hardening Treatment

The current research studied the effects of laser surface hardening treatment on the phase transformation and wear properties of gray cast irons heat treated by austempering or quench-tempering, respectively. Three austempering temperatures of 232 °C, 288 °C, and 343 °C with a constant holding duration of 120 min and three tempering temperatures of 316 °C, 399 °C, and 482 °C with a constant holding duration of 60 min were utilized to prepare austempered and quench-tempered gray cast iron specimens with equivalent macro-hardness values. A ball-on-flat reciprocating wear test configuration was used to investigate the wear resistance of austempered and quench-tempered gray cast iron specimens before and after applying laser surface-hardening treatment. The phase transformation, hardness, mass loss, and worn surfaces were evaluated. There were four zones in the matrix of the laser-hardened austempered gray cast iron. Zone 1 contained ledeburite without the presence of graphite flakes. Zone 2 contained martensite and had a high hardness, which was greater than 67 HRC. Zone 4 was the substrate containing the acicular ferrite and carbon-saturated austenite with a hardness of 41–27 HRC. In Zone 3, the substrate was tempered by the low thermal radiation. For the laser-hardened quench-tempered gray cast iron specimens, three zones were observed beneath the laser-hardened surface. Zone 1 also contained ledeburite, and Zone 2 was full martensite. Zone 3 was the substrate containing the tempered martensite. The tempered martensite became coarse with increasing tempering temperature due to the decomposition of the as-quenched martensite and precipitation of cementite particles. In the wear tests, the gray cast iron specimens without heat treatment had the highest wear loss. The wear performance was improved by applying quench-tempering heat treatment and further enhanced by applying austempering heat treatment. Austempered gray cast iron specimens had lower mass loss than the quench-tempered gray cast iron specimens, which was attributed to the high fracture toughness of acicular ferrite and stable austenite. After utilizing the laser surface hardening treatment, both austempered and quench-tempered gray cast iron specimens had decreased wear loss due to the high surface protection provided by the ledeburitic and martensitic structures with high hardness. In the worn surfaces, it was found that cracks were the dominant wear mechanism. The results of this work have significant value in the future applications of gray cast iron engineering components and provide valuable references for future studies on laser-hardened gray cast iron.

Temperature sensitivity of mechanical properties and microstructure during moderate temperature deformation of selective laser melted Ti-6Al-4V alloy

In this paper, the temperature sensitivity of mechanical properties and microstructure of the additive manufactured Ti-6Al-4V alloy via selective laser melting was investigated in the deformation temperature range between room temperature (RT) and 550 °C. Mechanical properties and microstructure were characterized by the means of tensile testing, optical microscope, scanning electron microscope and transmission electron microscope. The results show that the ultimate tensile strength (UTS) of the experimental Ti-6Al-4V alloy exhibits two-stage temperature sensitivity, which is reflected in the fact that the temperature sensitivity of UTS below 500 °C is conventional, whereas, the UTS decreases drastically from 834 MPa to 562 MPa as the temperature increases from 500 °C to 550 °C. Compared with the Ti-6Al-4V alloys prepared by conventional processing methods and other additive manufacturing methods, this experimental Ti-6Al-4V alloy exhibits a competitive strength below 500 °C and possesses excellent comprehensive mechanical properties at RT. The analysis shows that the microstructure is composed of α′ martensite with plenty of twinning structures when the as-built samples are deformed below 500 °C. However, above 500 °C, martensite gradually transformed into equilibrium α phase, the microstructure changed from acicular to lamellar, and β and α2 (Ti3Al) appear due to the elemental distribution homogenization and martensite decomposition, which account for the dramatic decrease of the tensile strength of the experimental Ti-6Al-4V alloy above 500 °C.

Study on Transformation Mechanism and Kinetics of α’ Martensite in TC4 Alloy Isothermal Aging Process

The law of microstructure evolution and transformation mechanism of the α′ martensite decomposition during 400–600 °C were studied by the isothermal dilatometry. The transformation process of α′ martensite was quantitatively characterized based on Johnson–Mehl–Avrami (JMA) model, and the microstructure evolution under different aging processes was observed and compared on Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that α′ → α + β is the elemental diffusion transformation, the position and shape of the precipitate gradually change with the holding time and temperature. The decomposition rate of α′ martensite was positively correlated with the aging temperature. The whole transformation process was divided into two stages based on the value of the Avrami exponent n, and the corresponding average values of the transformation activation energies Q are 46.1 kJ/mol and 116.8 kJ/mol, respectively. The calculated model had good agreement with the experimental data, and the transformation curve of α′ martensite with time and temperature during the isothermal aging at 400–600 °C was drawn.