Martensitic Matrices Research Articles

An in situ investigation of neighborhood effects in a ferrite-containing quenching and partitioning steel: Mechanical stability, strain partitioning, and damage

Understanding the micromechanical consequences of transformation-induced plasticity (TRIP)-assisted steels is challenging, due to the presence of multiple phase constituents and difficulties in spatially and temporally resolving the transformation of sub-micron scale retained austenite. Local stress or strain partitioning among constituents is considered important in determining the stability of retained austenite, which often competes with other effects such as composition or crystal orientation. In this work, we study the neighborhood effects on micromechanical behaviors of multi-phase TRIP steel using a ferrite-containing quenching and partitioning steel as a model system by employing in situ SEM/EBSD and in situ synchrotron X-ray diffraction tensile tests, phase-specific nanoindentation, and atom probe tomography. Our results reveal that retained austenite, bainite, and tempered martensite clusters behave similarly due to underlying strengthening mechanisms, resulting in the early transformation of retained austenite surrounded by bainite and tempered martensite matrices. Conversely, in large ferrite grains, the intragranular strain distribution develops heterogeneously, which retards the transformation of some embedded retained austenite grains. The in situ microstructure-based strain mapping also demonstrates that the local strain increment of the retained austenite inside ferrite grains decreases as the deformation level increases, leading to a non-linear strain partitioning behavior at high strain levels. Another neighborhood-induced effect is observed in ferrite grains. When ferrite grains are neighbored by hard fresh martensite, their lateral contraction is inhibited, causing a plane strain tension path locally in ferrite, even when the globally applied strain path is uniaxial tension.

On the effect of carbon content and tempering on mechanical properties and stiffness of martensitic Fe–18.8Cr–1.8B–xC high modulus steels

We studied the effects of C concentration (0.1, 0.3 and 0.5 wt%) and tempering temperature (100–700 °C) on the dynamic Young's modulus, mass density, hardness and impact toughness of quenched and tempered martensitic Fe–18.8Cr–1.8B (wt.%) based high modulus steels. Increasing C concentration lead to consistent hardness levels above 600 HV5 for tempering temperatures below 500 °C. While the tensile strength could be adjusted tailored from about 1000 MPa to more than 2000 MPa depending on the tempering temperature and respective C concentration, dynamic Young's modulus and mass density were left virtually unaffected. The E/ρ ratio shows little variability between approximately 31.2–33 GPa g−1 cm3, which is superior to conventional high strength steels and comparable to ferritic Fe–Cr–B steels without carbon additions. Particles identified within the martensitic matrices are mainly (Fe,Cr)2B type borides stemming from solidification, as well as an increasing fraction of (Fe,Cr)23(C,B)6 carbo-borides with increasing C content and tempering temperatures.

Formation and impact of functionally graded buffer layers between martensitic stainless steel and wrought steel substrate by laser metal deposition

In this study, the martensitic stainless-steel powder had been successfully deposited on the surfaces of 1045 wrought steel substrates using five types of laser metal deposition strategies. The results showed that a thin buffer layer can be naturally generated in-between the deposition layer and substrate owning to the dilution effect, producing functionally graded structures consisted of martensite dendrite matrices and intermetallic phases (e.g., M23(C, B)6). Unfortunately, the poor deformation characteristic of natural buffer layers can exacerbate the risk of cracking, leading to an incompatibility between martensitic stainless steel and forming steel. A thick artificial buffer layer was then designed by addition of Inconel 625 powder. Because of the formation of graded austenite phase in the artificial buffer layer, the yield strength of specimen was enhanced significantly. Hence, our study can be used for manufacture of reliable moulds with high surface hardness and structural strength and may be helpful in further developing hybrid forming strategy in the field of mould manufacture. When subjected to stress impact during mould filling, the artificial buffer layer with functionally graded properties would have a perfect capability to bear the deformation as the load increased, dramatically improving the reliability and functionality of moulds.

Research and analysis of processes occurring during abrasive wear of materials with heterogeneous cementitious structures

The article considers the impact of hardness and the number of solid-phase inclusions, as well as the properties of steels with ferritic, austenitic and martensitic matrices with heterophase cementitious structures on wear resistance. It is demonstrated that to ensure high wear resistance and impact toughness, solid phase particles in the structure of a heterophase metal material must be isolated from each other by sections of a relatively viscous matrix. Such structures can be obtained on the surface of a part or tool by surfacing, chemical-thermal treatment, and other methods. The results of calculations of the hardness of steel with different content of the carbide phase in its structure and the specific strain energy absorbed by the cementitious layer from the relative content of carbides in the ferritic, austenitic and martensitic matrices are presented. The contribution of carbide particles to determining the level of wear resistance of a two-phase material with different amounts of them in steels with different matrices is evaluated.

Dry Sliding Wear of Laser-Clad High-Vanadium Crucible Powder Metallurgy (CPM) 3V, 9V, and 15V Tool Steel Coatings

Dry sliding wear of laser-clad high-vanadium Crucible Powder Metallurgy (CPM) 3V, 9V, and 15V tool steel coatings was investigated in “as-clad” and “heat-treated” conditions against cemented WC-6%Co ball, where a hardened wrought AISI H13 hot-work tool steel substrate was used as a baseline for comparison. The 15V coating demonstrated a superior sliding wear resistance; the 9V coating possessed a better sliding wear resistance than the 3V coating, but was inferior to the 15V coating, and all the laser-clad CPM coatings showed a better sliding wear resistance than the hardened H13 substrate. The sliding wear resistances of the 3V and 15V coatings were further improved after double tempering at 540 and 550 °C (corresponding to their respective “secondary hardening”), respectively. By contrast, the sliding wear resistance of the 9V coating after double tempering at 550 °C (beyond its “secondary hardening” temperature) was slightly dropped instead, ascribed to the reduced hardness. The microstructures of the coatings, the morphologies, and chemical compositions of the worn surfaces were studied, revealing that the adhesion and oxidation played important roles in the dry sliding wear, depending upon the hardness of the martensitic matrices of the coatings and the morphologies and amount of (V, Cr)8C7 vanadium carbides dispersed on the matrices.

Microstructure and Tempering Behaviour of 28Cr-2.5C-1W Cast Irons

In this work, the effects of 1 wt.% tungsten addition and variation in tempering times on the microstructure and hardness of nominal 28 wt.%Cr high chromium irons were investigated. As-cast samples were destabilised at 1050 °C for 4 hours and then hardened by air cooling. Tempering after destabilisation was carried out at 450 °C for 2, 4 and 6 hours followed by air cooling. X-ray diffractometry, light microscopy and scanning electron microscopy were used to characterize the microstructures of the irons. The results show that the as-cast microstructure of the iron without W addition consisted of primary austenite dendrites with eutectic M7C3 and eutectic austenite partially transformed to martensite. The iron with 1 wt.%W addition contained primary M7C3 and eutectic M7C3 in an austenite matrix. Destabilisation treatment of the austenite matrix in both irons allowed precipitation of secondary carbides and transformation to martensite during air cooling. Phase transformation of eutectic M7C3 was also found in the iron with W addition. The formation of primary M7C3 in the 1 wt.%W iron increased the as-cast macro-hardness from 500 (no W) to 576 HV30. Destabilisation increased the macro-hardness up to 736 (no W) and 780 HV30 (1 wt.%W) since secondary carbide precipitation allowed austenite to transform to essentially martensitic matrices. At longer tempering times, the macro-hardness further increased up to about 820 HV30.

Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels

We studied the effects of Mn additions from 0 to 30wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB2 concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB2 based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~900MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217GPa and reduced density of 7460kgm−3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications.

Metallurgical characterization of coupled carbon diffusion and precipitation in dissimilar steel welds

The complex microstructures developed during post-welding heat-treatment in the vicinity of the fusion line between a ferritic and austenitic steel were examined in the case of submerged arc welded 18MND5/309L dissimilar joints. Quantitative measurements of the carbon distribution in the as-welded and post-weld heat-treated conditions were performed by both wavelength dispersive spectrometry and secondary ion mass spectrometry. The extent of carbon diffusion was confirmed by hardness profiles performed by nanoindentation. On the low-alloy ferritic side, decarburization resulted in cementite dissolution allowing the evolution of the bainitic structure toward a large-grained ferritic region. In the weld metal, the carbon content reached unusually high levels and an intense precipitation of chromium-rich carbides was observed in both the interfacial martensitic layer and the austenitic weld metal. The evolution of the precipitation as a function of the distance from the interface was analyzed in terms of crystallography, chemistry, volume fractions, and size distributions. Automated crystal orientation mapping in a transmission electron microscope allowed identification of the precipitates extracted on carbon replicas from both the martensitic and austenitic matrices. A 3D reconstruction of the carbides population in the martensitic layer was performed by serial cutting with a focused ion beam: M 7 C 3 and M 23 C 6 were found to coexist in the two carburized regions, but displayed different sizes, compositions, and morphologies, depending on their location with respect to the fusion line. This evolution in terms of precipitation was analyzed taking into account the local microstructure and composition.

Thermo-mechanically processed dual matrix ductile iron produced by continuous cooling transformation

Ductile irons with dual matrix structure were attained by controlled cooling in the austenite+ferrite region, after austenitization. Afterwards, the ductile irons were quenched to either 375°C, so as to transform the austenite into ausferrite or to room temperature to transform austenite to martensite. Fully ausferritic and fully martensitic matrices were also produced by direct quenching from the austenite region. Furthermore, three different deformations with true-strain values of 0, 0.3 and 0.5 have been applied in the austenite region. The structures were produced in three ductile irons with aluminum contents of 0.31wt.%, 0.96wt.% and 1.7wt.%. The different microstructures were produced in a thermo-mechanical simulator equipped with a dilatometry system. Dilatometry was used to monitor the structure development throughout the thermo-mechanical processes. The microstructural changes and hardness property exhibited by the produced structures were investigated. Increasing the aluminum widened the intercritical region making the control of the pro-eutectoid ferrite volume fraction easier. The quenching temperature in the intercritical region was shifted to higher value by both increasing the aluminum content and increasing the deformation. This shift resulted in a higher carbon level saturating the intercritical austenite which resulted in fundamental effects on the subsequent transformation kinetics.

Impact Toughness of Medium‐Mn Transformation‐Induced Plasticity‐Aided Steels

The impact toughness of 0.2% C–1.5% Si–(1.5–5.0)% Mn (mass%) transformation‐induced plasticity (TRIP)‐aided steels with bainitic ferrite and/or martensite structure matrices produced by isothermal transformation process is investigated for automotive body applications. The highest impact toughness, highest Charpy impact absorbed value (CIAV) at 298 K (130 J cm−2), and lowest ductile–brittle transformation temperature (DBTT, 203 K) is achieved in 1.5% Mn steel subjected to an isothermal transformation process at a temperature below the martensite start temperature MS − (50–100) K. An increase in the Mn content results in a small decrease in the CIAV (5–40 J cm−2) and an increase in the DBTT (20 K). The highest CIAV of 1.5% Mn steel compared with steels containing higher Mn content is mainly caused by (i) a more softened matrix structure consisting of wide lath‐martensite and bainitic ferrite, (ii) a smaller amount of the narrow‐lath martensite‐retained austenite‐like phase, and (iii) more stable retained austenite. The lowest DBTT of 1.5% Mn steel compared with the other steel compositions is mainly associated with the low Mn concentration of the matrix in this steel.

Dry sliding wear behaviour of cast roller materials

In this study, wear behaviour of roller materials produced by different casting techniques was investigated. Static and centrifugal casting techniques were used to produce roller materials having pearlitic and martensitic matrices due to the processing method and alloy composition. The roller materials were examined using light and scanning electron microscopes to determine the matrix phase. The samples were tested using 'ball–on–disc' type tribometer to study the wear characteristics of cast morphologies. DIN 100Cr6 steel ball with a 5 mm diameter was used as counterface, load force was selected as 50 N and total sliding distance was 500 m. After wear tests, the surfaces of cast parts were examined by light and scanning electron microscopes to evaluate the wear mechanisms under dry sliding condition. The friction coefficient, weight loss and type of wear mechanisms were determined. It was concluded that the material having finer martensitic matrix produced by centrifugal casting had a higher wear resistance than the one produced by static casting.

Friction Evolution in Running-In of Sliding Wear of Cast Iron Processed in Gleeble

As limited results were reported in terms of the evolution of sliding friction with growth of oxide layer in thickness during running-in, a pin-on-disc wear test was carried out in this study. 4.8Ni-1.5Cr cast iron as core layer and low carbon steel as outer layers, were thermo-mechanically processed via three different routes. For samples with lower hardness due to their predominantly austenitic or martensite retarded matrices, we found that initially rapid increase of thickness of oxide layer continually lowers the sliding friction. However, after the oxide layer was beyond a certain thickness, the sliding friction began to increase consecutively. After a fluctuation of friction caused by the break-down of oxide, a mild equilibrium wear with roughly constant friction followed.

Wear Behaviour of Heat Treated 100Cr6 Steels

Abstract In this study, several heat treatments were applied to DIN 100Cr6 steel to obtain different matrices. In the first stage of the study, solution annealing treatment was applied to the steel and cooling was carried out in various media (furnace, oil, and salt bath). In order to eliminate the stresses after transformation from austenization, a low temperature tempering treatment was applied to the quenched samples. All heat treated samples were examined using light microscopy after metallographic preparations. In the second step, ‘ball-on-disc’ type tribometer was used to determine the friction coefficient of the steels depending on the matrix phase. Weight loss was recorded and the friction coefficient versus distance was plotted for each steel. Worn surfaces of the steels were examined using scanning electron microscopy to characterize the wear mechanisms. It turned out that (i) pearlitic, bainitic and martensitic matrices could be obtained depending on the cooling medium, (ii) martensitic matrix had higher wear resistance based on its weight loss, (iii) abrasive and adhesive wear tracks were present on the worn surfaces of the steels.

Wear Behaviour of Heat Treated Hot Work Tool Steels under Dry Sliding Conditions

Abstract In this study, commercial hot work tool steels were selected and their wear behaviour under dry sliding condition was investigated after heat treatment applications. In the first stage of the study, solution annealing, quenching, and finally tempering treatments were applied to the experimental steels. All samples were examined by light microscopy after metallographic preparations. In the second stage, hardness values (HRC) of the steels were determined to evaluate the surface resistance to wear. Finally, wear tests were carried out using ‘ball-on-disc’ type tribometer under dry sliding condition and all worn surfaces were characterized using both light and scanning electron microscopy. It is concluded that (i) the steels had typical tempered martensite matrices, (ii) alloy content affected the final microstructure and a finer matrix was obtained due to vanadium addition, (iii) hardness levels directly affected the coefficient of wear of steels, and (iv) worn surfaces exhibited abrasive and adhesive wear tracks.

Effect of particle hardness on mild–severe wear transition of hard second phase materials

This study analyzes the effect of particle hardness on the transition from mild to severe abrasive wear. White cast iron with 1.9wt% Cr with austenitic and martensitic matrices, white cast iron with 13.8wt% Cr and 24.4wt% Cr and mottled cast iron, and four types of abrasive with different hardness were used in pin-on-abrasive paper tests at 4.6N of load. The microstructure of the alloys was characterized by optical microscopy and the micromechanisms of abrasive wear were characterized using scanning electron microscopy. The hardnesses of both the abrasive and the alloys were selected to obtain a HA/H range ratio (in which HA is the hardness of the abrasive and H is the alloys' hardness) much wider than the one reported in the literature until now. The relationship between wear rate and friction coefficient versus HA/H ratio was analyzed. The results show that when the HA/H ratio is less than 1.9, an increase in abrasive hardness produces significant increase in wear rate. However, when the ratio HA/H is more than 1.9, increasing the abrasive hardness has little effect on the wear rate. Comparing these results with the ones from the literature for homogeneous materials and hard second phase materials, the regions found in this study correspond to the severe wear region, the mild–severe wear transition, and a small mild wear region. A correlation between the friction coefficient and the HA/H ratio was found: the friction coefficient increases proportionally with the increase of HA/H to achieve a HA/H ratio of about 3.3. For HA/H values above 3.3 the friction coefficient is relatively constant.

Effect of abrasive size on wear of metallic materials and its relationship with microchips morphology and wear micromechanisms: Part 1

In this paper, the effect of abrasive particle size on the wear of white cast iron with M3C carbide and with austenitic and martensitic matrices was investigated. Abrasive wear tests using a pin on alumina paper were carried out using abrasive sizes between 16 μm and 192 μm. The wear surface of the specimens was examined by SEM for identifying the wear micromechanism and the type of microchips (wear debris) formed on the abrasive paper. The results show that the mass loss for the white cast iron with both austenitic and martensitic matrices increases linearly with the increase of particle size until the critical particle size is reached. After the critical particle size is reached, the rate of mass loss of the iron with austenitic matrix increases at a lower linear rate, and for the iron with martensitic matrix the curve of mass loss is non-linear and flattens when the critical particle size is reached. The abrasive paper in contact with the iron of both austenitic and martensitic matrices presents continuous microchips and the main wear mechanism was microcutting before reaching critical particle size, and after that, it presents deformed discontinuous microchips and the main wear mechanism was microploughing. The results show that the critical abrasive size on wear is related to the wear micromechanisms and the microchips morphology.

Fracture surfaces and the associated failure mechanisms in ductile iron with different matrices and load bearing

Ductile iron (DI) is a family of cast alloys that covers a wide range of mechanical properties, depending on its matrix microstructure. For instance, ferritic matrices used in parts, such as automotive suspension components, demand high impact properties and ductility among some of their main requirements. On the other hand, pearlitic and martensitic matrices are used when hardness, strength and wear resistance are of particular concern. When it comes to very high strength parts, ausferritic matrices, typically austempered ductile iron (ADI), are widely used. DI has been employed to replace cast and forged steels in a large number of applications and its production has shown a sustained rate of growth over the last decades. Knowing about failure modes and fracture mechanisms associated to materials with the properties mentioned above is crucial, since they can be of great value for designers of mechanical components. This paper deals with the analysis of fracture surfaces of ductile cast iron generated under different conditions of load application, temperature and environments. The studies include the examination of fracture surfaces obtained by means of tensile tests, impact tests and by samples used to determine fracture toughness properties, where the zones of fatigue pre-crack and monotonic load condition were evaluated. A special case of ductile iron fracture is also examined. The study of the different surfaces permitted to establish patterns that contributed to unveil the fracture mechanisms of ductile iron with different matrices, nodule count, etc.

Morphology and crystallography of Ti2Pd phase precipitated in B2 parent and B19 martensite matrices in Ti-rich Ti–Pd alloys

Morphology and crystallography of Ti2Pd phase of C11b structure precipitated in B2 parent and B19 martensite matrices in Ti-rich Ti–Pd shape memory alloy have been investigated by transmission electron microscopy observations. Three Ti2Pd variants were recognized in the specimen aged above Af temperature, i.e., in the B2 matrix. On the other hand, four Ti2Pd variants were confirmed in the specimen aged below As temperature, i.e., in the B19 matrix. The orientation relationship between the Ti2Pd precipitate and each matrix is consistent with the number of Ti2Pd variants. The Ti2Pd precipitates in the B2 and the B19 matrices have a disk and a polygonal plate shapes, respectively. The c-axis of Ti2Pd precipitate in the B2 matrix is perpendicular to the disk surface. On the other hand, that in the B19 matrix is parallel to the plate surface. These differences are well explained by the magnitude of coherent strain.

Effect of matrix microstructure on precipitation of Laves phase in Fe–10Cr–1.4W(–Co) alloys

Ferritic heat-resistant steels involving intermetallic Laves phase have drawn a growing interest for the enhancement of creep strength, while the brittleness of Laves phase may lower the toughness of the alloy. We believe it is possible to modify the morphology of Laves phase precipitates by controlling the α-Fe matrix microstructure. In order to make clear the influence of matrix microstructures on age-hardening, the precipitation behavior of Laves phase was investigated by transmission electron microscopy (TEM). The matrix of the Fe–10Cr–1.4W–4.5Co (at%) alloy is controlled by heat treatments so as to provide three types of microstructures; ferrite, ferrite+martensite, and martensite. Alloys with ferrite and ferrite+martensite matrices show age-hardening behavior comprised of two hardness peaks. At around the first hardness peak, it is revealed by TEM observation that fine particles precipitate coherently within the ferrite matrix. In the martensite matrix, most of R-phase and Laves phase precipitates exist on laths and dislocations.

Morphological and crystallographic aspects of Cl1b-type precipitates nucleated in martensitic and parent phase matrices in Ti-rich Ti-Pd shape memory alloys

Ti 2 Pd phase with C11 b -type structure which precipitates in Ti-rich Ti-Pd shape memory alloys may nucleate either in martensitic phase or parent phase depending upon thermal treatment conditions, since the transformation temperatures are around 800 K. In the alloy aged above A f , three C11 b -type precipitate variants are always observed by TEM. The orientation relationship between those three variants and the B19 structure, which was transformed from B2 structure after the precipitation, is as follows: Variant I (001) Ti2Pd //(101) B19 ; Variant II (001) Ti2Pd //(101) B19 ; Variant III (001) Ti2Pd //(010) B19 . The shape of the precipitates was deduced to be disk-like and/or elliptic. On the other hand, only the variants I and II are observed when the precipitates are nucleated in martensitic phase. Since the difference between the lattice spacing of (010) B19 and (001)/3 Ti2Pd is about 20%, the formation of a third variant in the martensitic matrix is energetically unfavorable.