High Carbon Martensite Research Articles

The Effect of Addition Amount of Chromium Iron on the Bonding Strength between Alloy Steel Surfacing Layer and Steel Base Metal

Fe-C-Cr-Nb alloy steel surfacing layers with different contents of C and Cr were prepared on 45 steel base metal by selfshielded flux-cored wires with distinct amounts of high carbon chromium iron addition and melt arc surfacing. The composition and microstructure changes of the surfacing layer were tested and analyzed. The surfacing test plate was processed into a pulling specimen, and the bonding strength between the surfacing layer and the 45 steel base metal was tested with a self-designed pulling test method. The fracture location of the pulling specimen and fracture characteristics were observed by a metallurgical microscope and a scanning electron microscope. The result shows that with the increase of the amount of high carbon chromium iron added to flux-cored welding wire, the content of C and Cr in the surfacing layer increases, and the NbC hard phase disperses. The microstructure of the steel matrix changes from mixed martensite + residual austenite to high carbon martensite + residual austenite, and then independent austenite appears. The hardness of the surfacing layer first increases and then decreases. The bonding strength between the surfacing alloy and the 45 steel base metal first decreases and then increases, and the fracture location is at the bottom of the surfacing layer or the fusion zone with mostly quasi-cleavage characteristics. When the additional amount of high carbon chromium iron reaches 13%, thee pulling specimen exhibits significant deformation with the highest bonding strength, and the fracture is close to the fusion line, where there are numerous tearing edges and shallow dimples.

Multi-aspect Characterization of Low-temperature Tempering Behaviors in High-carbon Martensite

Multi-aspect Characterization of Low-temperature Tempering Behaviors in High-carbon Martensite

Achieving High Plasticity and High Toughness of Low-Carbon Low-Alloy Steel through Intercritical Heat Treatment

Medium manganese steel has excellent comprehensive properties due to the TRIP effect of retained austenite, but its welding performance is unsatisfactory for its high alloy content. This study obtained retained austenite in low-carbon low-alloy steel with low contents of silicon and manganese elements through intercritical heat treatment. The influence of intercritical quenching temperature on the content and characteristics of the retained austenite, as well as the functional mechanism of the retained austenite during low-temperature impact, was studied. The results showed that the content of the retained austenite increased from 12% to 17%, and its distribution extended from grain boundaries to martensite lath boundaries, with increasing intercritical quenching temperature. The retained austenite on the grain boundaries was in blocks, and that on the martensitic lath boundaries formed slender domains. The stability of the retained austenite was achieved through the enrichment of C and Mn during intercritical heat treatment. The contribution of retained austenite to low-temperature mechanical properties was closely related to its stability. The retained austenite with poor stability underwent martensite transformation at low temperatures, and the high-carbon martensite was a brittle phase that became the nucleation site of cracks or the path of crack growth during impact. Stable retained austenite passivated crack tips and hindered crack propagation during impacts, which improved the impact performance of the steel.

Mapping of lattice distortion in martensitic steel—Comparison of different evaluation methods of EBSD patterns

To visualize the varying tetragonal distortions in high carbon martensitic steels by EBSD, two different approaches have been applied on backscattered Kikuchi diffraction (BKD) patterns. A band-edge refinement technique called Refined Accuracy (RA) (Oxford Instruments) is compared with a technique called Pattern Matching (PM), which optimizes the fit to a simulated BKD signal. RA distinguishes between hypothetical phases of different fixed c/a, while PM determines a best fitting continuous c/a by projective transformation of a master pattern. Both techniques require stored BKD patterns.The sensitivity of the c/a-determination was tested by investigating the microstructure of a ferritic steel with an expected c/a=1. The influence of the Kikuchi pattern noise on c/a was compared for a single or 40 averaged frames per measuring point, and turned out to be not significant. The application of RA and PM on the martensitic microstructure delivered qualitatively similar maps of c/a. The comparison of RA and PM shows that RA is suitably fast and precise during mapping the martensite c/a ratio in analyses of high carbon martensite, especially for fast initial surveys. As RA leads quantitatively to higher noise in c/a, the PM analysis can be used for higher precision results.

Enhanced mechanical properties of a high-carbon martensite steel processed by heavy warm rolling and tempering

The strength-ductility trade-off in steel, especially high-carbon steel, has been a long-standing and troubling issue; steel with increased mechanical properties has attracted much attention in recent years. In this study, an ultrafine-grained ultrastrong high-carbon steel with good ductility was produced by stabilizing retained austenite (RA) and accelerating carbide precipitation by warm rolling, quenching and tempering. The ultimate tensile strength reaches approximately 2550 MPa, with a total elongation of 5.1% after tempering at 220 °C. The corresponding microstructure was composed of stable high-carbon RA and ultrafine-grained tempered martensite with carbide precipitates. The thermodynamic stability of RA was increased by carbon partitioning during quenching. The volume fraction was optimized to 6%. As the grain size of the primary austenite was refined to approximately 3 μm or smaller by warm rolling along the normal direction, the substructure of the high-carbon martensite was characterized as ultrafine twins and high-density dislocations. Carbide precipitations from the martensite matrix are accelerated at lower tempering temperatures. The main strengthening mechanism was still the strengthening of the solid solution and dislocations. Good tensile ductility was mainly derived from the effective weakening of the solid solution of martensite, whose carbon content decreased from 0.86 wt% to 0.61 wt%. Additionally, the greater ductility was attributed to the transformation-induced plasticity effect of larger amounts of high-carbon RA. These results may help in the design and production of ultrahigh strength and ductile high-carbon steel.

Double Diffraction or ω-Fe Diffraction in Body-Centered Cubic {112}<111>-Type Twinned Martensite

The martensitic substructure in steels plays a major role in understanding the nature of martensite transformation and improving mechanical properties. However, the substructure in the quenched martensite of carbon steel, particularly in high carbon martensite, has not been clarified yet. Local crystal structure in martensite has been characterized by means of transmission electron microscope (TEM). A specimen tilting experiment in TEM is a practical method to simply confirm that the electron diffraction spots are caused by either double diffraction effect or a second crystalline phase. By tilting the TEM specimen containing Fe–C twinned martensite structure, the so-called double diffraction spots of {112}<111>-type twin are still visible even after the diffraction spots from the twinned crystals become invisible. This experimental result is the direct evidence that the so-called twinning double diffraction spots come from a second crystalline phase (ω-phase), which is distributed in the body-centered cubic (BCC){112}<111>-type twinning boundary region. An ideal BCC {112}<111>-type twin is just a model, which has not been reported yet in any experiment.

Failure analysis of the piston used in a pneumatic down the hole impactor

In this paper, the failure mechanism of the piston in the pneumatic down the hole (DTH) impactor has been studied. Failed piston microstructure has been analyzed using the light optical microscopy (OM) and scanning electron microscope (SEM). Impact toughness test and the Rockwell hardness test were conducted. Results showed the following: a) The carburizing layer of IQCT failed piston is mainly composed of acicular martensite and lower bainite after the carburizing-tempering-isothermal quenching-cryogenic treatment-tempering (IQCT), and there are collapses on the impact side and outer surface of the piston, mainly fatigue failure. b) IQT failed piston samples are subject to carburizing-tempering-isothermal quenching-tempering treatment (IQT). The carburized layer is mainly composed of high-carbon martensite, carbide and retained austenite. The transition layer is still composed of lower bainite and acicular martensite, but the volume fraction of lower bainite increases significantly and the core is mainly composed of lower bainite. It has the best impact toughness, up to 61.5 J/cm2, which is mainly fatigue failure. c) OQT failed piston sample is subject to carburizing-tempering-oil quenching-tempering (OQT). The carburized layer is mainly composed of acicular martensite, the transition layer is composed of acicular martensite (AM) and lath martensite (LM), and the core is mainly composed of LM. The impact toughness of the sample is the worst. Therefore, fatigue fracture failure is caused.

The effect of Nb on the hydrogen embrittlement susceptibility of Q&P steel under static and dynamic loading

In the present study, the effect of Niobium (Nb) on the hydrogen embrittlement resistance of Quenched and Partitioning (Q&P) steel is investigated. For this purpose, the hydrogen uptake level and its impact on the mechanical properties of a Nb-free and a 0.024 wt% Nb Q&P steel are thoroughly analysed. The hydrogen trapping capacity is evaluated via thermal desorption spectroscopy (TDS). In-depth analysis of the desorption kinetics at different heating rates allows identification and quantification of the available trapping sites. The hydrogen embrittlement sensitivity of both steels is characterized using static and dynamic tensile tests. The addition of Nb results in an increase of the hydrogen concentration by more than 25%. The larger hydrogen content in the Nb steel, as a result of the higher fraction of grain boundaries/interphases, gives rise to a more severe embrittlement of the Nb steel compared to the Nb-free one. In addition to the larger hydrogen fraction in the Nb Q&P steel, the larger retained austenite fraction of low stability is detrimental due to the larger fraction of high carbon martensite formed when straining. This results in higher susceptibility to hydrogen embrittlement of the Nb microalloyed steel due to the brittle character of the high carbon martensite that forms easily during straining. Under dynamic loading conditions, the hydrogen embrittlement of both steels is minimal, which is attributed to a reduced hydrogen diffusion and the suppression of the transformation induced plasticity (TRIP) effect due to adiabatic heating.

Study on mechanism of improving wear and corrosion properties of 20CrMnTi ring gear surface by laser carburizing

Laser carburizing is an effective method to improve the surface properties of materials. In order to improve the corrosion resistance and wear resistance of 20CrMnTi ring gear surface, firstly, the material surface was carburized by laser. Secondly, the hardness, wear resistance and corrosion resistance of the surface before and after carburization were tested. Finally, the electronic structure of 20CrMnTi before and after carburization was calculated based on the first principle method of density functional theory. The results show that the surface microstructure after laser carburizing is mainly high carbon martensite and cementite, and the hardness is increased by 109.5%. There are adhesive wear and abrasive wear before and after carburizing. Interstitial carbon increases the density of dislocation and lattice distortion, and significantly improves the wear resistance of the surface. At the same time, the addition of interstitial carbon reduces the corrosion current density, increases the self-corrosion potential, and improves the corrosion resistance of the material. The first principle calculation results show that the interstitial carbon increases the strength of covalent bond and ionic bond, which is the main reason for the improvement of hardness and wear resistance, and the interstitial carbon reduces the density of states at Fermi level and inhibits electron transfer, which is the essential reason for improving corrosion resistance.

Effect of Alloying Elements on the Sharpness Retention of Knife Blades Made of High Carbon Martensitic Stainless Steels

Blades usually become blunt as the blade tip suffers wear during cutting, and improving the sharpness retention of steel blades has become an attractive prospect in various industries. In this study, blades were fabricated from commercial high carbon martensitic stainless steels (154CM, 440C and N690) with different contents of alloying elements. 154CM with higher Mo content demonstrated superior capability in sharpness retention to 440C and N690, although these steels exhibit similar chemical composition, carbide phases, microstructure and HRC hardness. Further investigations via SEM and nanoindentation indicated that the faster deterioration of sharpness in 440C and N690 may result from the fatigue peeling of the carbides, which was aggravated by the modulus mismatch between carbide particles and the martensitic steel matrix.

Study on the hydrogen-induced delayed fracture behavior of Q-P980 and MS980

In this paper, the hydrogen diffusion behavior and hydrogen induced delayed fracture (HIDF) of Q-P980 (Q-P: Quenching and Partitioning) and MS980 (MS: Martensitic steel) steels were investigated using hydrogen penetration, slow strain rate tensile (SSRT) tests, thermal desorption spectroscopy (TDS) tests, fracture analysis, and microstructural examination in this paper. The austenite in Q-P980 is massive retained-austenite (RA) with low stability. The TRIP (Transformation Induced Plasticity) effect will occur in the process of strain and change into high carbon martensite. HIDF is caused by a substantial amount of surplus hydrogen being enriched at the border and flaws. The fracture has a broad cleavage surface and is a typical quasi-cleavage fracture. MS980 has been sufficiently tempered, resulting in a substantial quantity of distributed spherical cementite (150nm) precipitating around the lath martensite. This size and form of cementite may successfully trap hydrogen while maintaining the material’s mechanical characteristics. And tempering can effectively reduce the local stress level of steel, so MS980 has a very low HE susceptibility. HIDF is related to local stress and hydrogen accumulation. We suppose that Z is a constant and Z C is a critical value which associated to σ and C H (the local stress and local hydrogen concentration), rising as σ and C H rises. The atomic bonds at the crack tip, lattice position and the phase interface will fracture when Z C reaches a particular value Z. Tempering to minimize local stress and carbide precipitation to capture hydrogen are two strategies for reducing hydrogen embrittlement (HE) susceptibility, particularly for dislocation strengthened steel. Microalloying elements can generate precipitates that function as hydrogen traps and obstruct the HELP (Hydrogen Enhanced Localized Plasticity) process, lowering local stress and hydrogen accumulation.

Povećanje otpornosti na trošenje elemenata brodskih dizelskih motora koji se izrađuju od nodularnog lijeva

This paper examines the influence of austempering and shot peening on the wear resistance of ductile iron. Samples for further testing were made from mechanically processed casts. The hardness and microstructure of ductile iron were examined on the prepared samples. Metallographic analysis of ductile cast iron samples in the cast state determined the pearlitic-ferritic structure of the matrix characteristic of this type of cast iron. Pearlite predominates in the structure, and the rest consists of ferrite and graphite nodules. The samples were then austempered. The isothermal conversion was 240 °C. After austempering, the hardness was measured and the microstructure was characterized, and the changes that occurred in relation to the heat-treated ductile iron were studied. Austempering created a new austempered structure, harder than that in the cast state, which led to improved mechanical properties. A needle-like structure of lower bainite (ADI) with high-carbon austenite, untransformed austenite, martensite and graphite nodules was achieved. The selected parameters of the shot peening process deformed the surface of the ADI and achieved a mostly martensitic structure without the presence of a soft phase, which increased resistance to abrasion. Additional hardening of the substrate by the shot peening process resulted in a composite material with the properties of comparative steel parts, which are lighter, cheaper and easier to manufacture, more resistant to wear and quieter in operation. Such material should have an even wider application in mechanical engineering, and thus in the construction of marine machinery and plants.

Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&amp;B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.

Influence of Mo–Nb–Ti additions and peak annealing temperature on the microstructure and mechanical properties of low alloy steels after ultrafast heating process

Abstract The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.

Effect of Silicon Content on the Decomposition of Austenite in 0.4C Steel during Quenching and Partitioning Treatment

Although quenched and partitioned (Q&amp;P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&amp;P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the Ms -Mf range, retaining about 20% austenite at the quench-stop temperature (TQ), and held for 1000 seconds above TQ in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&amp;P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&amp;P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

The effect of Nb on the high strain rate hydrogen embrittlement of Q&amp;P steel

Quenching and Partitioning (Q&amp;P) steels are, due to their excellent combination of strength and ductility, seen as good candidates for the third generation advanced high strength steels (AHSS). Although the TRIP effect is beneficial for the overall mechanical behaviour of these steels it potentially can have detrimental effects when strained in a hydrogenenriched environment. The solubility of hydrogen is high in austenite but low in high carbon martensite. Martensite is even in the absence of hydrogen already a possible damage initiation spot. The effect of hydrogen under static and dynamic tensile loading was evaluated in a Q&amp;P and a Nb micro-alloyed Q&amp;P steel. Experiments were carried out under a strain rate ranging from 0.03 s-1 till 500 s-1 and correlated with the hydrogen uptake characterised via thermal desorption spectroscopy (TDS). The presence of Nb resulted in a 25% increase in the hydrogen uptake capacity. A higher susceptibility to hydrogen was observed in the Nb steel partially due to the high hydrogen fraction, but also because of the larger fraction of low stability austenite. However, when tested under dynamic conditions the hydrogen susceptibility is minor and even improved in the micro-alloyed Q&amp;P steel compared to the standard Q&amp;P steel.

Evaluation of the Corrosion Resistance of Tubing of Various Strength Groups

The corrosion resistance of tubing pipes of four strength groups is evaluated: E, Λ, K and D by measuring the saturation current density of steel anodic dissolution. It is shown that tubing of increased strength (groups E and Λ) has lower corrosion resistance compared with pipes of strength groups D and K. The low corrosion resistance of tubing of strength groups E and Λ is associated with the formation of adverse structural components, in particular high-carbon martensite having a high dislocation density. Formation of this structure is due to the fast cooling rate of the pipes in sprayer equipment. In order to increase the corrosion resistance of tubing of strength groups E and Λ it is recommended that additional heat treatment consisting in particular of quenching and tempering is performed in order to relieve internal stresses within the steel.

Effect of low-temperature isothermal holding on microstructure and mechanical properties of hot rolled high carbon Nb microalloyed steel plate

Two high carbon steels, one plain carbon (HC0) and the other, Nb microalloyed (HC1) are selected in the present research. Both experimental steels were initially hot rolled after complete homogenisation and then air cooled to room temperature. These hot rolled plates were then subjected to low temperature isothermal holding followed by air-cooling to room temperature. Isothermal holding above the martensite start temperature (Ms) results in the formation of bainite with some amount of high carbon martensite whereas, isothermal holding below Ms leads to the formation of predominantly martensite. The increase in isothermal holding temperature from 200°C to 300°C results in lowering the hardness, estimated yield strength and ultimate tensile strength values marginally for HC0 whereas, it decreases the hardness, estimated yield strength and ultimate tensile strength values significantly for HC1 steel which may be attributed to the expected formation of some amount of coarse niobium carbide (NbC).

High-speed thermo-cyclic nitriding carburized steels

The task of developing new methods of chemical and heat treatment (CHT), aimed at increasing wear resistance, mechanical properties and reducing length of process operations seems to be quite vital. The objective of the work is development of a method of complex surface strengthening of structural steels with application of the mechanism of additional self-strengthening at wear. Suggested is application of high-speed thermo-cyclic nitriding of carburized structural steels for formation of gradient meta-stable phase-structural modifications in saturated layers. Specimens of 18Cr2Ni4Мo steel underwent preliminary carburization in solid carburetor at 930 °С temperature for 12 hours it leading to obtaining a carbonized layer 1.2…1.3 mm in thickness. Then high-speed thermocyclic nitriding (HSTCN) was carried out in carbamide medium with CHT heating, according to heating-cooling mode 1000↔20 °С, the number of thermo cycles being 2 to 12. The following investigation methods were applied: metallographic, scanning electronic microscopy, durometric, wear testing, according to dry friction-sliding metal against metal mode. As a result of combined CHT with thermo-cycling a thin nitrited layer 0.07…0.14 mm thick is formed on the surface (following 2 HSTCN cycles) and up to 0. thick (after 12 HSTCN cycles) with increased nitrogen and carbon content. Further structure along the depth of the carbonized layer consists of high-carbon martensite, carbonitrides and increased amount of residual austenite (Аres). Along the depth of the saturated layer the microstructure is changed in gradient: the amount of Аres and carbonitrides is reduced, while the amount of quenching martensite is increased. Depending upon the number of HSTCN cycles relative wear resistance is extremely maximally changed with maximum (ε=4,1) at 7 cycles. Аres is meta-stable and undergoes deformation induced martensite transformation γres→αʹ in the process of wear testing (DIMTWT) in the thin surface layer, it bringing some additional contribution into deformational self-strengthening and improvement of relative wear resistance. Thus, improvement of wear resistance ensures additional amount on nitrides and carbonitrides in the preliminarily carbonized and then nitrited steel layer, as well as high-carbon martensite and increased amount of meta-stable Аres (45…70 %), that undergoes γres→αʹ DIMTWT. Thermocyclic character of additional nitriding ensures grain refinement, it promoting improvement in steel impact strength.

Hydrogen Desorption Behavior Trapped in Various Microstructures of High-Strength Steels Using Thermal Desorption Analysis

The relationship between different microstructures and hydrogen trapping states of cold-drawn pearlitic (CD-P) steel, medium-carbon martensitic (MC-M) steel, and high-carbon martensitic (HC-M) steel was studied based on the hydrogen desorption profile, activation energy of hydrogen desorption (Ea), and hydrogen release behavior at room temperature by means of thermal desorption analysis (TDA). The TDA results revealed that hydrogen in CD-P and HC-M was present as both Peak 1H and Peak 2H, but only as Peak 1H in MC-M. Hydrogen release behavior indicated that Peak 1H for all three types of steel was diffusible (Ea ≤ 34 kJ/mol). For CD-P, Peak 2H was non-diffusible (Ea = 62 kJ/mol), but diffusible for HC-M (Ea = 43 kJ/mol). TDA results combined with SEM and/or TEM observation and XRD analysis clarified that the former hydrogen was desorbed from strained incoherent α/θ interfaces in a detrapping-controlled process. The latter hydrogen was desorbed via trapping sites involved in retained austenite in a diffusion-controlled process. It is essential to judge hydrogen trapping states as either diffusible or non-diffusible by analyzing not only hydrogen desorption profiles but also hydrogen release behaviors and Ea values, taking into account microstructure characteristics related to hydrogen trapping and diffusion processes.