Martensite-retained Austenite Research Articles

Modification of electro gas weld metal microstructure reflecting mechanical property specifications in cruise vessel shipbuilding

AbstractTo identify the microstructural factors effecting the electro gas welding (EGW) weld metal properties, this study investigated the influence of different prototype welding consumables and shielding gases on the microstructural composition and mechanical-technological properties. The aim was to adjust the weld metal properties as a trade-off between strength, ductility, and impact toughness to fulfill typical weld metal material specifications in cruise vessel shipbuilding under consideration of the manufacturing conditions at European shipyards. The microstructure is analyzed by quantitative metallography of the ferritic matrix, martensite-retained austenite (M/A) constituents, and non-metallic inclusions (NMI). The influence of the Ni content, the deoxidation concept by variation of Si and Ti contents, and different shielding gas activity by variation of the Ar proportions is discussed. The interaction of ferritic matrix with high acicular ferrite content of about 70 ± 10%, the existence of larger grain boundary ferrite formations, and the M/A morphology plus distribution are considered as the determining factors for the material properties.

Improving the strength-ductility balance of medium-Mn Q&P steel by controlling cold-worked ferrite microstructure

The phase transformations, microstructure and properties of two Medium-Mn processed via quenching and partitioning steels were compared in this contribution and a new strategy for controlling mechanical properties by introducing and controlling cold-worked ferrite prior to heat treatment is proposed. It was found that during heating, the recovery and recrystallization of cold-worked ferrite compete with austenitization, thereby inhibiting the coarsening of austenite. The cold-worked ferrite interface will significantly delay the austenitization kinetics during the partitioning local equilibrium stage compared to martensite. These results lead to a diverse parent austenite, as well as a refined martensite substructure. As a result, the randomly distributed variants increase the number of effective grain boundaries, thus enhancing yield strength. The intercritical annealing process at a temperature of 860 °C resulted in the formation of fresh martensite-retained austenite (M/RA) constituents exhibiting a remarkably fine (<2 μm) and uniform grain morphology. Such microstructure yielded substantial improvement in both the strength and ductility of the steel. The proposed treatment led to excellent elongation (24%) at fracture, combined with very high ultimate tensile strength and yield strength of 1345 MPa and 1163 MPa, respectively, of the steel, resulting in a product of strength and elongation that exceed 32 GPa%.

Effect of Austempering Processes on the Tensile Properties and the Work-Hardening Behavior of Austempered Bainitic Steels Below the Martensite Start Temperature.

The tensile properties and work-hardening behavior of austempered bainitic steels below martensite start temperature (Ms) were investigated and compared with those of bainitic steel austempered above Ms. The results show that the tensile strength and yield strength increased from 1096 MPa and 734 MPa to 1203 MPa and 951 MPa, respectively, when the austempering temperature was decreased from 400 °C to 300 °C. However, the total elongation decreased from 23% to 16%. The martensite-retained austenite blocks and bainitic ferrite laths are significantly refined. With a decrease in the austempering temperature, the volume fraction of retained austenite decreased from 15.4 vol% to 6.2 vol%. The carbon content in retained austenite increased from 1.12 wt% to 1.69 wt%. All tensile specimens exhibited three stages of deformation in the differential Crussard-Jaoul (C-J) models. The difference in ductility is mainly attributed to the transformation of the retained austenite blocks into strain-induced martensite during deformation. The initial content of retained austenite is the main factor affecting the ductility of bainitic steels. Therefore, the work-hardening ability of austempered bainitic steel above Ms is higher than that of bainitic steel below Ms.

A machine learning model for multi-class classification of quenched and partitioned steel microstructure type by the k-nearest neighbor algorithm

The paper proposed a machine learning model for multiclass classification of quenched and partitioned (Q&P) steel microstructure type. In this work, we implemented the k-nearest neighbor (k-NN) algorithm to train the classifier. A Q&P steel microstructure-type database has been compiled from the previous research data comprising the information of 348 steel samples. The feature space was described by the steel composition, lower critical temperature (Ac1), upper critical temperature (Ac3), martensitic-start temperature (Ms), etc., and the Q&P heat treatment parameters. At the same time, the target or dependent variable was recorded as the microstructure type, for example, martensite-retained austenite {M, RA}, martensite-bainite-retained austenite {M, B, RA} etc. The proposed classifier could achieve an overall performance of 97.7% and 77.7%, measured as f1-Score in the training and testing dataset, respectively. The martensite-retained austenite {M, RA} type was found to be the most confusing class. The model explored the effect of compositional parameters and heat treatment variables on the evolution of microstructure. The re-engineering through model study for targeted martensite-retained austenite microstructure type has depicted a steel composition and heat treatment window, which has been validated by experimental development of steel microstructure. The optical and SEM micrographs, along with hardness, strongly corroborated the model analysis from a re-engineering perspective.

Effect of microstructure transformation below MS temperature in bainitic steels on the impact-abrasive wear behavior

The bainitic steel austempered below MS (martensite start temperature) with good strength and toughness has a great potential in the field of wear resistance. In this paper, the impact abrasive wear resistance of bainitic steel austempered below MS temperature was studied and compared with those of bainitic steel austempered above MS temperature. The results show that the wear resistance of bainitic steel austempered below MS is better than that of bainitic steel austempered above MS. The wear mechanism of bainitic steel austempered below MS is dominated by fatigue wear. This is attributed to the refinement of bainitic ferrite laths and martensite-retained austenite blocks by austempering processes below MS. The bainite steel austempered below MS with higher initial hardness can effectively resist the impact load and the damage of abrasive particles to the wear surface, thereby reducing the wear weight loss.

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.

Corrosion resistance of 13wt.% Cr martensitic stainless steels: Additively manufactured CX versus wrought Ni-containing AISI 420

The corrosion resistance of heat-treated additively manufactured (AM) precipitation hardening martensitic stainless steel (SS) CX and wrought components of AISI 420-SS with lower Ni content, were compared. The microstructure of heat-treated AM-CX comprised of a martensitic matrix and a few nano-scale particles, while the wrought AISI 420-SS contained approximately 8% of austenite and Cr-rich carbides in a martensitic matrix. Superior corrosion resistance was detected for the AM-CX part as compared to the AISI 420-SS ascribed to nearly absence of Cr-rich carbides and martensite-retained austenite interfaces in the AM-CX SS, while their presence in the AISI 420-SS destabilized the passive film.

Nanolath martensite-austenite structures engineered through DQ&P processing for developing tough, ultrahigh strength steels

A novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (DQ&P) was designed for developing tough, ductile, ultrahigh strength steels using 0.4 wt% carbon steels. A preliminary characterization of a laboratory-rolled, high-silicon DQ&P steel revealed an excellent combination of mechanical properties comprising high yield and tensile strengths (~1025 and ~2137 MPa, respectively), besides reasonable elongation (≈ 12%) and good impact toughness (T28J temperature ≈ −10 °C). The high mechanical properties are attributed to the formation of desired nanolath martensite-retained austenite (≈ 15%) structures. Features such as nano-twinned martensite, interlath austenite etc. were comprehensively characterized through high-resolution transmission electron microscopy. Attempts have been made to detect possible existence of metastable hexagonal omega phase within the boundaries of nano-twinned martensite and understand its nature of formation.

Further insights on compositional control and heat treatment for improving the impact behaviour of hot-rolled steels containing Nb and Al

The influence of Al and Nb on the strength and impact behaviour of hot rolled steels containing 0.06 wt% of C, 1.4 wt% of Mn has been determined after hot rolling to 15 mm and 30 mm thick plate. When 0.16 wt% of Al was added to the plain C-Mn steel, the impact behaviour significantly improved even though Widmanstätten ferrite (WF) was present. This improvement was due to refinement of the grain boundary carbides and removing the N from solution as AlN. The hot rolled steels all contained WF but when Nb was added more WF formed as well as martensite/retained austenite (MA) giving poor impact behaviour. Reducing the hardenability from that shown in previous work by decreasing C from 0.1 to 0.06 wt%, Nb from 0.03 to 0.02 wt%, and cooling rate from 33 to 17 K/min had no effect in improving the impact performance of hot rolled Nb steels. To ensure optimum properties not only is it necessary to reduce the hardenability, but WF formation must be discouraged by having a high Ar3. This can only be presently achieved by refining the austenite grain size via control rolling the Nb containing steels; the benefit of adding Al can then, readily be seen. Suggestions are made as to how this might be achieved for hot rolling.

Relation of martensite-retained austenite and its effect on microstructure and mechanical properties of the quenched and partitioned steels

A two-step quenching and partitioning (Q&P) treatment was applied to low-carbon alloy steels. The relation of initial martensite - retained austenite - fresh martensite and its effect on microstructure and mechanical properties were investigated by experiments. The results reveal that the volume fraction of retained austenite can reach the peak value of 17%, and the corresponding volume fractions of initial martensite and fresh martensite are 40% and 43%, respectively, when the tested steel is treated by initial quenching at 330°C, partitioning at 500°C for 60s and final quenching to room temperature. Moreover, the micromorphologies of austenite and martensite become finer with the increasing of initial martensite fraction. The elongation is the highest when the volume fractions of initial martensite and retained austenite are 70% and 11%, respectively, meanwhile, the yield strength increases and tensile strength decreases gradually with the increase of initial martensite fraction, which proves that the mechanical properties including elongation, yield strength and tensile strength are based on the comprehensive effect of the retained austenite fraction, the finer microstructure and austenite stability.

Corrosion of simulated weld HAZ of API X-80 pipeline steel

Four peak temperatures and two cooling rates were used in a Gleeble thermomechanical processing machine to simulate weld HAZ of API X-80 pipeline steel. Bainitic microstructures were observed at both 30 and 100°C/s cooling rates. Bainite favoured higher passive current densities in alkaline solutions. The lowest peak temperature, 770°C, produced the highest martensite-retained austenite (M–A) phase in the microstructure. Presence of M–A phases in the microstructure extended the passive region to higher potentials in alkaline solutions. Corrosion current density of the as-received sample was higher in acidic solutions and lower in alkaline solutions compared to the heat-treated ones.

Characterization Microstructural and Mechanical of X-60 Steel Heat-Treated

ABSTRACTIn this paper was study the effect of heat treatment on mechanical properties of an API X-60 steel used for storage and transportation of hydrocarbons.In the first stage evaluation are mechanical properties of steel heat treated by the technique of the three-point test according to ASTM 399-90 was carried out. In the second stage, characterization of the type of failure and microstructure through optical microscopy (OM) was determined; also heat treated samples were then mechanically tested for hardness (HRC) and nano-indentation. The presence of alloying elements by scanning electron microscopy (SEM) and the fracture surfaces generated in the steel with varying times, temperatures and cooling medium generated by different modes of solicitation (Bending), likewise with loading rates were determined. The results revealed a ductile fracture and microstructures (PF) ferrite-pearlite (DP), bainite -ferrite (BF) and martensite-retained and martensite/retained austenite (MA). Finally, this article discusses the effect of heat treatment followed by precipitation hardenable of steel API X-60 on the mechanical properties

Manganese partitioning in low carbon manganese steel during annealing

For 6Mn16 steel experimental soft annealing at 625 °C for periods from 1 h to 60 h and modeling with Thermo-Calc were performed to estimate the partitioning of alloying elements, in particular Mn, between ferrite, cementite and austenite. Using transmission electron microscopy and X-ray analysis it was established that the increase of Mn concentration in carbides to a level 7%–11.2% caused a local decrease of the Ac 1 temperature and led to the presence of austenite around the carbides. Thus, after cooling, small bainite–martensite or bainite–martensite-retained austenite (BM-A) islands were observed. A dispersion of carbides and a coarsening process were observed. The measured amount of Mn in the carbides was in good agreement with theoretical predictions.

Interplay between Diffusive and Displacive Phase Transformations: Time-Temperature-Transformation Diagrams and Microstructures

Materials which can undergo extremely fast displacive transformations as well as very slow diffusive transformations are studied using a Ginzburg-Landau framework. This simple model captures the essential physics behind microstructure formation and time-temperature-transformation diagrams in alloys such as steels. It also predicts the formation of mixed microstructures by an interplay between diffusive and displacive mechanisms. The intrinsic volume changes associated with the transformations stabilize mixed microstructures such as martensite-retained austenite (responsible for the existence of a martensite finish temperature) and martensite-pearlite.

Use of aqueous polymer quenchants for hardening of carbonitrided parts

Water, 10% aqueous polymer solutions, and conventional quench oil were evaluated to harden carbonitrided parts. The resulting as-quenched and quenched and tempered microstructures were compared. Typically, the gas carbonitriding process produced (carbonitrides) – martensite-retained austenite in the case and martensite-retained austenite or bainite-retained austenite in the core. The effect of the amount of retained austenite and carbides (carbonitrides) on hardness, abrasive wear resistance, and impact strength showed that the best results were obtained when a 10% aqueous solution of a polyalkylene-glycol quenchant was used. However, most surprisingly, the impact strength for the aqueous polymer quenched specimens was dramatically greater than either oil or water quenched specimens. A rationale for this important observation is presented, which suggests that thermal stresses may be more important than transformational stresses as it relates to these quenching conditions and the test pieces quenched.

Influence of martensite-austenite constituents on heat affected zone toughness of C-Mn steel welds

"Martensite-retained austenite" constituents play an important role on Heat Affected Zone (HAZ) toughness in the welding of structural steels. The influence of thermal cycles - especially in the case of multipass welding - and of steel composition on the presence of such constituents is discussed here. Fracture initiation mechanisms related to these constituents are also examined.

Martensite crystalline structure of the high-nickel alloys at cryogenic temperatures

Using the X-ray method on single crystal specimens an investigation ismade of crystal structure of newly-formed martensite of Fe-30,8wt.%Ni alloy and Fe-24wt.%Ni-0,5wt.%C alloy and the change during heating from liquid helium and nitrogen temperature. The decrease in tetragonality of martensite lattice of both alloys at low temperature is associated with relaxation of elastic stresses on the martensite-retained austenite interface. The further increase in tetragonality of martensite lattice of second alloy is explained by the process of ordering of the carbon atoms on martensite lattice sites.

Elastic Limit and Microplastic Response of Hardened Steels

Tempered martensite-retained austenite microstructures were produced by direct quenching a series of 41XX medium carbon steels, direct quenching and reheating a series of five 0.8C-Cr- Ni-Mo steels and intercritically austenitizing at various temperatures, and quenching a SAE 52100 steel. All specimens were tempered either at 150 °C or at 200 °C. Specimens were subjected to compression and tension testing in the microstrain regime to determine the elastic limits and microplastic response of the microstructures. The retained austenite and matrix carbon content of the intercritically austenized specimens were measured by X-ray diffraction and Mossbauer spectroscopy. The elastic limit of the microstructures decreases with increasing amounts of retained austenite. Refining of the austenite distribution increases the elastic limit. Low elastic limits are mainly due to low flow stresses in the austenite and not internal stresses. The elastic limit correlates with the largest austenite free-mean path by a Hall-Petch type equation. The elastic limit increases with decreasing intercritical austenitizing temperature in the SAE 52100 due to (1) a lower carbon content in the matrix reducing the retained austenite levels and (2) retained carbides that refine grain size and, therefore, the austenite distribution in quenched specimens. The microplastic response of stable austenite-martensite composites may be modeled by a rule of mixtures. In the microplastic region, the strain is accommodated by successively smaller austenite regions until the flow strength matches that of the martensite. Reheating and quenching refines the microstructure and renders the austenite unstable in the microplastic regime, causing transformation of the austenite to martensite by a strain-induced mechanism. The transformation of austenite to martensite occurs by a stress-assisted mechanism in medium carbon steels. The low elastic limits in medium carbon steels were due to the inability of the strain from the stress-assisted transformation of austenite to martensite to balance the plastic strain accumulated in the austenite.

Effect of cooling rate on intercritically reheated microstructure and toughness in high strength low alloy steel

High strength low alloy steels have been shown to be adversely affected by the existence of regions of poor impact toughness within the heat affected zone (HAZ) produced during multipass welding. One of these regions is the intercritically reheated coarse grained HAZ or intercritical zone. Since this region is generally narrow and discontinuous, of the order of 0.5 mm in width, weld simulators are often employed to produce a larger volume of uniform microstructure suitable for toughness assessment. The steel usedfor this study was a commercial quenched and tempered steel of 450 MN m -2 yield strength. Specimen blanks were subjected to a simulated welding cycle to produce a coarse grained structure of upper bainite during the first thermal cycle, followed by a second thermal cycle where the peak temperature T p2 was controlled. Charpy tests carried out for T p2 values in the range 650-850°C showed low toughness for T p2 values between 760 and 790°C, in the intercritical regime. Microstructural investigation of the development of grain boundary martensite-retained austenite (MA) phase has been coupled with image analysis to measure the volume fraction of MAformed. Most of the MA constituent appears at the prior austenite grain boundaries during intercritical heating, resulting in a 'necklace' appearance. For values of T p2 greater than 790°C the necklace appearance is lost and the second phase areas are observed throughout the structure. Concurrent with this is the development of the fine grained, predominantly ferritic structure that is associated with the improvement in toughness. At this stage the microstructure is transforming from the intercritical regime structure to the supercritically reheated coarse grained HAZ structure. The toughness improvement occurs even though the MA phase is still present, suggesting that the embrittlement is associated with the presence of a connected grain boundary network of the MA phase. The nature of the second phase particles can be controlled by the cooling rate during the second cycle and variesfrom MA phase at high cooling rates to a pearlitic structure at low cooling rates. The lowest toughness of the intercritical zone is observed only when MA phase is present. The reason suggested for this is that only the MA particles debond readily, a number of debonded particles in close proximity providing sufficient stress concentration to initiate local cleavage. © 1993 The Institute of Materials.

The intermediate transformation of Mn-Mo-Nb steel during continuous cooling

The continuous cooling transformation diagram for low-carbon low-alloy steel containing 0.05% C, 1.99% Mn, 0.31% Mo and 0.06% Nb was constructed by dilatometry and metallography. The intermediate transformation between martensite and polygonal ferrite involves two typical stages: the formation of ferrite matrix and the formation of microphases. Four intermediate transformation products obtained from various cooling rates and designated B1, B2, A1 and A2, were studied. The B1 and B2 structures are composed of pockets of parallel ferrite laths and interlath microphases, which are films of retained austenite in B, and are fragments of retained austenite or martensite or martensite-retained austenite (M-A) constituents in B2. The B1 structure is further characterized by the appearance of martensite particles inside the ferrite laths. The A1 structure is comprised of the randomly arranged ferrite groups. Each group contains several short ferrite laths in the same crystallographic orientation and granular M-A constituents or martensite located at the rim of ferrite laths or groups. The A2 structure is morphologically analogous to Widmanstatten ferrite. The formation mechanisms of these products are also discussed.