Large Amount Of Martensite Research Articles

Research on microstructure evolution of laser surface quenching of ductile cast iron

Abstract Ductile cast iron is widely used in the manufacturing of components and dies due to its excellent performance and low cost. Laser surface quenching can further improve its performance and extend its service life. This article uses scanning electron microscopy, electron backscattered scattering detection and microhardness tester to systematically analyse the microstructure evolution of the melted zone, hardened zone, and matrix of ductile cast iron with laser surface quenching. The hardening zone can be divided into two layers: one layer contains a large amount of martensite, graphite, and bits of residual austenite, and the other layer contains a composite structure of graphite, ferrite, layered martensite+austenite+Fe3C. On the basis of microstructure research, the determination and optimization methods of the depth of the laser surface hardening zone and the process are formed, providing a basis for the selection and optimization of the laser surface hardening process for ductile cast iron.

On the likely origin of the Gold Dust Defect in the production line of industrial ferritic stainless steel

The “Gold Dust Defect” that sometimes appears on the surface of the AISI 430 ferritic stainless steel has been related to an improper recrystallization of the material. Hot-rolling, a decisive step in the production process, seems the step where this defect is induced and temperature during this treatment plays a key role. To trace back the origin of this defect, the microstructural evolution of a sample of an AISI 430 was monitored at the different stages in the production process of the steel. In particular, a detailed study combining electron backscatter diffraction with other techniques, such as optical microscopy and scanning electron microscopy has been performed. The whole set of results evidences that after hot-rolling and subsequent annealing, the material contains a large amount of martensite in the center of the cross-section due to a fast cooling of the austenite originated during rolling. In addition, the surface shows a top-layer made up by unrecrystallized subgrains separated from the matrix by an accumulation of chromium carbides. After pickling, the top-layer of subgrains semi-detaches from the surface forming flakes which cannot recrystallize. Most of these flakes do not peel off during the subsequent stages but survive until the end of the production process, forming Gold Dust Defect flakes.

Double-sided friction stir spot welding of ultra-high strength C-Mn-Si martensitic steel by adjustable probes

High-quality joints of ultra-high strength C-Si-Mn quenched and tempered martensitic steel were successfully fabricated by double-sided friction stir spot welding (FSSW) with adjustable probes. We found that the mechanical properties of the welded joints increased with the rotation speed, due to the increase of hard phase and strengthened welding interface. In the surroundings of the shoulder/probe interface, a large amount of martensite was formed resulting from the higher austenitizing temperature and the higher cooling rate. In addition, severe material flow vertical to the welding interface and strong relative slip along the interface conspicuously favored the fragmentation and dispersion of the oxides, and the drive force for grain boundary migration introduced by severe material flow further promoted the dispersion of the oxides. Consequently, a high-quality welding interface was fabricated, and a high-strength welded joint with a stable plug failure was obtained. Furthermore, we pointed out that soundly welded high strength interface of ultra-high strength steel can be obtained by higher welding temperature and introducing strong material flow vertical to the interface.

Effect of Reversed Austenite on Mechanical Properties of ZG06Cr13Ni4Mo Repair Welded Joint

In this work, gas tungsten arc welding (GTAW) was used to repair ZG06Cr13Ni4Mo martensitic stainless steel. Repair welding occurred either once or twice. The changes in the microstructure and properties of the repair welded joints were characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), tensile and impact tests. The effects of reversed austenite in repair welded joints on microstructure and mechanical properties were studied. The results show that the microstructure of the welded joint after repair welding consists of a large amount of martensite (M) and a small amount of reversed austenite (A), and the reversed austenite is distributed at the boundary of martensite lath in fine strips. With the increase in the number of welding repairs, the content of reversed austenite in the welded joint increases. The microstructure in the repair welded joints is gradually refined, the microstructure in the once and twice repaired joints is 45.2% and 65.1% finer than that in the casting base metal, respectively. The reversed austenite presented in the repair welded joints decreases the tensile strength by 4.8% and 6.7%, increases the yield strength by 21.3% and 26.4%, and increases the elongation by 25% and 56%, respectively, compared with the casting base metal. In addition, the reversed austenite mainly nucleates and grows at the boundary of lath martensite. The refinement of the martensite structure was due to the generation of reversed austenite and the refinement of original austenite grain by the welding thermal cycle. After repair welding, the reverse austenite appeared in the repair welded joints and the tensile strength decreased slightly, but the plastic toughness was significantly improved, which was conducive to the subsequent service process.

Process parameter optimization and EBSD analysis of Ni60A-25% WC laser cladding

In this research work, the parameters of the preset powder feeding laser cladding process of Ni60A-25% tungsten carbide (WC) powder were optimized by using the response surface methodology of the central composite design. The dilution rate η and unit effective area ϕ of the laser cladding layer were considered as the response. Based on the factors and levels in the design matrix, the Ni60A-25% WC powder was fused on the 42CrMo alloy structural steel substrate with the laser cladding process to form a high-microhardness cladding layer. In this work, the microstructural characterization and crystal orientation analysis of the laser cladding layer obtained from the optimal process parameters were performed by using a Field Emission Scanning Electron Microscope (FE-SEM) and the Electron Backscatter Diffraction (EBSD) technique. The microhardness of the specimen processed with the optimal process parameters was evaluated using a Vickers microhardness tester. The results show the order of influence of four laser cladding process parameters on the dilution rate η and unit effective area ϕ. The developed mathematical models for the dilution and the unit effective area were validated. The errors between the experimentally obtained dilution rate η and unit effective area ϕ and predicted values were within acceptable limits. The EBSD analysis revealed that there was no meritocratic orientation of the substrate, while the laser cladding layer showed a pronounced meritocratic orientation. A large amount of martensite was produced within the coating after the laser cladding, resulting in a significant increase in the microhardness compared with the substrate. The microhardness test results were consistent with the microstructure morphology.

Unraveling the significant role of retained austenite on the dry sliding wear behavior of medium manganese steel

For wear resistant steels, there is generally a linear relationship between the initial hardness and wear resistance, yet a high initial hardness limits the formability of components. The intrinsic ductility of austenite and strain hardening produced from the transformation-induced plasticity (TRIP) effect during service opens a new window for this contradiction. Herein, the wear resistance and the corresponding failure mechanisms of 5Mn steels after different heat treatment processes were evaluated using a ball-on-disk sliding test under dry conditions. The wear behaviors of the counterbodies were also studied to elucidate the wear mechanism of the tribo-system. The results show that the microstructures of the hot-rolled and air-cooled steel were mainly martensite. When the intercritical annealing temperature was relatively low (<695 °C), the microstructure primarily consisted of reversed austenite and ferrite. The samples annealed at 630 °C, 645 °C, 660 °C, and 675 °C had a similar initial hardness, while the sample annealed at 675 °C possessed a wear resistance that was 3.25 times that of the steel annealed at 630 °C. The large difference in the wear resistance was predominately due to the gradient of strain hardening provided by the varied amount and mechanical stability of the retained austenite. The transformation from austenite to martensite, which has an increased hardness, improves the work hardening. A strengthening layer formed with a certain thickness that provided stress support during the wear process and enhanced the wear resistance. When the annealing temperature reached 695 °C, a large amount of secondary martensite was generated from austenite that had a low thermal stability, which improved the initial hardness and wear resistance. The steels with a large amount of martensite had an enhanced wear resistance, but the elongation severely deteriorated, leading to unsatisfactory formability. The present work provides guidance for designing novel steels with an excellent balance of good formability and decent wear resistance by precisely adjusting the characteristics of the retained austenite.

Effects of Peening Direction on Reverse Transformation Induced by Shot-Peening for Fe-33%Ni Alloy

Effects of peening direction on the reverse transformation induced by the shot-peening for the Fe-33 mass%Ni alloy with large amount of martensite (α’) are investigated. When the angle between the peened surface and the peening direction (Hereafter, peening angle) is 90 o, the reverse transformation occurs and subsequently martensitic transformation is induced by the shot-peening. On the other hand, in case of the peening angle of 30 o, only reverse transformation occurs. Furthermore, the volume fraction of austenite (γ) in the specimen after the shot-peening increases as the peening angle decreases. This means that the reverse transformation induced by the shot-peening is enhanced by decreasing the peening angle. Moreover, residual compressive stress around the peened surface increases as the peening angle decreases. Since the hydrostatic compressive stress decreases phase transformation temperature, the phase transformation temperature around the peened surface would be decreased by the shot-peening. Therefore, the reverse transformation behavior depending on the peening angle can be explained by the residual compressive stress due to the shot-peening.

Laser power modulated microstructure evolution, phase transformation and mechanical properties in NiTi fabricated by laser powder bed fusion

In this study, two sets of optimized laser powder bed fusion (LPBF) additive manufacturing parameters with similar energy density but different laser powers (HP: high laser power with high scanning speed, LP: low laser power with low scanning speed) were used to produce fully dense and crack-free NiTi samples. The microstructure, phase transformation and mechanical properties of the LPBF fabricated HP and LP NiTi were investigated using scanning electron microscopy (SEM), differential scanning calorimeter (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). The results showed that the HP and LP NiTi samples had different microstructures, phase transformation temperatures and mechanical properties. It was found that the HP NiTi samples predominantly contained austenite at room temperature and exhibited lower phase transformation temperatures. In contrast, the LP NiTi samples contained a large amount of martensite and had larger thermal memory recovery and better damping capacity. Additionally, the microstructure, phase transformation temperatures and mechanical properties were found to vary at different locations along the building direction in both HP and LP NiTi. This study implies that by manipulating the LPBF processing parameters, in particular the laser power, the phase transformation, microstructure and dynamic mechanical properties of the LPBF fabricated NiTi can be controlled to target different applications.

Laser Oscillating Welding of TC31 High-Temperature Titanium Alloy

The joining of high-temperature titanium alloy is attracting much attention in aerospace applications. However, the defects are easily formed during laser welding of titanium alloys, which weakens the joint mechanical properties. In this work, laser oscillating welding was applied to join TC31 high-temperature titanium alloy. The weld appearance, microstructure and mechanical properties of the laser welds were investigated. The results show that sound joints were formed by using laser oscillating welding method, and a large amount of martensite was presented in the welds. High mechanical properties were achieved, which was approaching to (or even equaled) the strength of the base material. The joints exhibited a tensile strength of up to 1200 ± 10 MPa at room temperature and 638 ± 6 MPa at 923 K. Laser oscillating welding is beneficial to the repression of porosity for welding high-temperature titanium alloy.

The sensitivity analysis of microstructure and mechanical properties to welding parameters for linear friction welded rail steel joints

In this study, high carbon rail steel (U71Mn, Chinese grade) was welded by linear friction welding (LFW) under different welding parameters. The microstructure and mechanical properties of the joints were characterized to investigate the connection mechanism of the joint. In order to fully yield the metal at the welding interface, sufficient heat input must be provided with friction to fully offset the heat loss. Therefore, welding parameters have a decisive influence on joint quality. The joint can be divided into the weld center zone (WCZ), the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the base material (BM). The BM have a mixed microstructure of the pearlite and upper bainite, and the TMAZ consists of pearlite and a small amount of martensite. In WCZ, the martensite content increases further due to the higher cooling rate. During LFW, dynamic recrystallization occurs in the weld, which results in a significant change in the ratio of the deformed grain, sub-structure grain and recrystallized grain compared with that in BM. In the tensile test, all samples have fractured in WCZ. The highest tensile strength reached of the joint is 78.5% of the BM strength. In addition, there is a large amount of martensite in WCZ, which increases the microhardness while reduces the toughness of the weld.

Effects of impact energy on the wear resistance and work hardening mechanism of medium manganese austenitic steel

Medium manganese austenitic steel (MMAS) fabricated through the hot rolling process has been used in the mining, military, and mechanical industries. In this paper, the abrasion performance and hardening mechanism were measured under a series of impact energies. The impact wear was tested at different impact energies from 0.5 J to 6 J using a dynamic load abrasive wear tester (MLD-10). Microstructure and surface morphologies were analyzed using scanning electron microscopy, X-Ray diffraction, and transmission electron microscopy. The results suggest that MMSA has the best wear resistance at 3.5 J and the worst wear resistance at 1.5 J. Furthermore, the wear mechanism and worn surface microstructure change with different impact energies. There are small differences between a large amount of martensite on the worn surfaces under different impact energies and the shapes of dislocation and twins change with different impact energies.

In Situ Neutron Diffraction Studies of Increasing Tension Strains of Superelastic Nitinol

A micromechanical study of the effect of varying amounts of tensile strains on the microstructures and subsequent mechanical behaviors of superelastic Nitinol rods is presented. It is found that strains up to ~8–9 % develop microstructures that assist both forward and reverse transformation relative to un-strained material. This superelastic phenomenon is explained to be analogous to two-way shape memory effect in Nitinol actuation materials. These results provide understanding as to why such “pre-strains” may lead to improvements in subsequent superelastic fatigue life. Beyond 9 %, a drastic change is observed, as large amounts of martensite (75 % and more) are retained in unloaded samples. Thus, a competition between transformation, plasticity, and reorientation is found to give rise to microstructures that inhibit complete transformation. Furthermore, even though similar inelastic strain magnitudes are observed in loading and unloading plateaus, micromechanical mechanisms differ substantially from samples with less pre-strain. For example, in highly pre-strained samples at least half of the plateau strains are due to martensite reorientation, whereas, in low and moderately pre-strained samples nearly the entirety of the plateau strain is due to transformation. We also find that latent heat of plastic flow is larger than latent heat of transformation.

Effect of martensite on cold cracking in 600-MPa grade flux-cored arc weld metals using the Y-groove test

This study observed the cold crack susceptibility of three types of low-hydrogen flux-cored arc (FCA)-weld metals developed for a yield strength of 600 MPa. Cold cracks in the weld metal were measured using the Y-groove test, and four different preheating temperatures were employed from room temperature to 150 °C. The microstructure of the Y-groove weldment consisted of acicular ferrite, bainite, and martensite. All weld metals showed cold cracking at room temperature, and a preheating temperature of 50 °C produced cold cracking only for weld metal C, having the largest carbon equivalent and the largest volume fraction of bainite and martensite. The volume expansion ratio of the weld metals was measured when the austenite was transformed to martensite during simulated weld cooling. Weld metal C showed the largest volume expansion ratio (2.6 %), resulting in the largest probability of cold crack production. Moreover, the martensite affected the cold crack fractography in the weld metal. Intergranular fracture occurred in the region occupied by large volume fractions of martensite, and quasi-cleavage fracture was observed in regions of small-volume fractions of martensite. Large amounts of martensite were related to the regions of high Mn content and corresponding to intergranular fracture.

Microstructural Effects on the Mechanical Integrity of a TRIP-800 Steel Welded by Laser-CO2 Process

In this study, a TRIP-800 steel was welded using a Laser CO2 process, and the resultant microstructures were characterized by optical, scanning, and transmission electron microscopy (TEM) means. It was found that the microstructure of the steel in the as-received condition consisted of ferrite, bainite, and retained austenite (RA), including some martensite. In particular, TEM observations indicated that the developed martensites were high carbon twinned martensites. It was found that laser beam welding (LBW) promoted the development of up to 23% martensite in the fusion zone (FZ) and up to 30% in the heat-affected zone (HAZ). In addition, determinations of RA using x-ray diffraction indicated that the amount of RA developed in the FZ was relatively small (<6%). Confirmation for the relatively large amounts of martensite in both the FZ and HAZ was indirectly made by the shape of microhardness profiles, which resembled a “top hat.” Tensile testing in welded strips indicated a loss of strength and ductility. An examination of the fracture surfaces indicated that the steel fractured in a brittle fashion at the HAZ-BM interface. Apparently, the development of relatively large amounts of martensite in the HAZ reduced the steel toughness. In turn, this indicated that LBW leads to martensite embrittlement in the HAZ regions, but not at the parting line of the FZ.

Microstructural evolution of simulated heat-affected zone in modified 2.25Cr-1Mo steel during high temperature exposure

The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700°C for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed.

Correlation of microstructure and surface roughness of disc drums fabricated by hot forging of an AISI 430F stainless steel

The surface roughness of disc drums fabricated by hot forging of an AISI 430F stainless steel was investigated. Emphasis was placed on the role of microstructural changes, which depend on postforging annealing conditions. In the forged drum specimens, MnS inclusions were heavily elongated, and a large amount of martensite was found. Three different annealing conditions were used on the forged drum specimens to completely or partially decompose martensite. The surface roughness tests were conducted on these specimens, and then the test data were compared via hardness and microstructures. The top plane of disc drums, where MnS inclusions were elongated perpendicular to the machined surface, showed a good surface roughness because of the free-machining effect of MnS. However, the side plane, where MnS inclusions were aligned parallel to the machined surface, showed an increased surface roughness due to reduction of the MnS effect. These findings suggested that a proper amount of martensite formed during forging and annealing would be beneficial for cost efficiency, as well as machinability of the 430F stainless steel products.

ＳＵＳ３０４鋼単結晶における加工誘起α′マルテンサイトの生成挙動に及ぼす引張軸方位の影響

We have studied the effect of tensile axis on strain induced martensitic formation in SUS304 steel. Single crystal specimens having various tensile axes were deformed by the Instron type testing machine at 293K. After deformation, quantitative analysis of α' martensite formed in specimens were carried out by using a newly developed apparatus. This apparatus estimates an amount of α' martensite with measuring the magnetic force between a specimen and a permanent magnet, of which force was calibrated by standard samples.The formation of α' martensite appeared to be classified into 3 groups. (1) In case of ‹001› specimen, no martensite was found without necking down part. (2) In ‹111› specimen, relatively small amount of martensite formation was found. (3) In specimen with the other tensile axes, large amount of martensite was formed. This classification agrees with previously reported hydrogen induced fracture

Measurement of retained austenite using scanning electron metallography

The volume percentage and distribution of retained austenite in two steels has been studied by scanning electron microscopy. Metallographically polished and etched samples were examined in the as-etched and replica-cleaned conditions. SEM results provide more information compared to other microscopic techniques because they take into account the presence of holes as well as the very fine distribution of retained austenite in areas where large amounts of martensite are present. The value of this approach should further increase when the SEM is directly coupled to commercially available automatic data recording equipment for rapid determination of second phases.

Impact toughness of austenitic steels at low temperatures

1. If austenite undergoes no change in the impact test than the impact toughness decreases monotonically with decreasing temperatures. 2. The martensite forming during the test increases the work of formation of the crack but reduces the work of its propagation. The curves of the impact toughness have peaks. The less the samples are deformed during the impact tests, the less martensite is formed, and the less distinct the peaks. 3. When the samples contain martensite before the tests then the impact toughness decreases because of the values of Ap and, with large amounts of martensite, Af.