Fine-grained Martensite Research Articles

Formability of the ultra high strength automotive steel docol 1500 bor by using hot processing maps and austenite reconstruction

In this study, the hot processing map was drawn to study the hot formability of high strength automotive steel Docol 1500 Bor. The quenching microstructure of the test steel was observed under optical microscope and EBSD and the deformed PA (parent austenite) was reconstructed under different conditions according to KS (Kurdjumov–Sachs) OR (orientation relationship). The results indicate that the optimal deformation conditions in the stable region are strain rates less than 1s−1 and temperatures higher than 900 °C. Overall, the fine-grained martensite structure with a larger Schmid factor has a better deformation ability for subsequent service. The texture component {011} <100> appears in the PA at the strain rate of 5 s−1 and the temperature of 1000 °C, while the dynamic recrystallization is significantly inhibited. The quenched martensite is found coarse. In the stable region, the recrystallized austenite grains with certain deformation transforms into the martensite grains with low cumulative misorientation values and higher Schmid factors. In the unstable region, the austenite grains are smaller in size, in which severe cumulative misorientation caused by higher strain rates leads to poor plasticity.

Heterogenous structure and formation mechanism of white and brown etching layers in bainitic rail steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle white and brown etching layers (WEL and BEL, respectively) on the rail raceway during service. Bainitic rail steels with an excellent strength–toughness balance are promising candidates for next generation rail steels. It is generally difficult to generate hard and brittle WEL/BEL in bainitic rails because of the low carbon content. This study reports, for the first time, the formation of a unique multilayer heterostructured WEL/BEL in a field-tested bainitic rail. The heterogenous structures of WEL/BEL were characterized using transmission electron microcopy, focused ion beam, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, electron probe microanalysis, atom probe tomography, and nano-indentation techniques. Results demonstrate that WEL is mainly composed of fine-grained martensite or ferrite and retained austenite, whereas BEL is a composite structure containing nanocrystalline martensite or ferrite, retained austenite, cementite, and oxide or O-rich particles. The oxide or O-rich particles are primarily formed within BEL through local oxidation, resulting in secondary partitioning of the alloying elements between the oxidation product and its surrounding matrix. BEL is hard but brittle due to the formation of brittle twinned martensite, cementite, and oxide particles, whereas the adjacent WEL is relatively soft and ductile. Based on these findings, the heterostructured WEL/BEL formation mechanism is preliminarily discussed in terms of inhomogeneous grain refinement, localized oxidation, chemical alterations (including both chemical segregation in the initial microstructure and secondary partitioning of the alloying elements during local oxidation), and phase transformation.

Enhancement of the strength-toughness balance in a quenched laser-additively-manufactured low alloy mild steel: Effect of grain refinement and nanotwin bundle formation

A laser additively manufactured (LAM) ferrite-austenite low-alloy mild 12CrNi2 has a low ultimate tensile strength but a larger elongation. To improve the strength but without compromising the toughness, a concept to martensitize the low alloy mild steel with introducing nanotwin bundles into the martensitic laths is proposed, through deliberately-designed cyclic quenching (CQ). It is found that the prior austenitic grain refinement helps to form a fine-grained martensite (α′) constituted of the laths with the formation of {112}<111> type nanotwin bundles. The steel with such a nanotwinned martensite structure, in comparison to the steel counterpart with a dislocated lath martensitic structure obtained through conventional direct quenching (DQ), is enhanced not only in ultimate tensile strength, but also in elongation. The mechanism for the nanotwin bundles formation is interpreted in connection with the grain-refinement-induced decrease in martensitic transformation starting temperature (Ms).

Grinding Temperature and Surface Integrity of Quenched Automotive Transmission Gear during the Form Grinding Process.

Grinding burn is an undesired defect in gear machining, and a white layer is an indication of severe burn that is detrimental to gear surface performance. In this work, the influence of grinding parameters on the thickness of the white layer during form grinding of quenched transmission gear was investigated, and the microstructure evolution and mechanism of severe burn formation were analyzed. The grinding temperature increased with the grinding depth and grinding speed, with the highest level of ~290 °C. The thickness of the white layer exceeded 100 μm when the grinding depth was 0.03 mm, and the top layer was a plastic deformation layer followed by a fine-grained martensite layer. Coarse-grained acicular martensite was found at the interface between the white layer and softened dark layer. The mechanical effect and thermal softening mainly contributed to the formation of white layer stratification. The ground surface topography showed several scratches and typical grooves; when grinding depth increased to 0.03 mm, the grinding surface roughness Sa was relatively high and reached up to ~0.60 μm, mainly owing to severe plastic deformation under grinding wheel extrusion and the thermal effect.

Microstructural characterization of 15-5PH stainless steel processed by laser powder-bed fusion

The 15-5PH (UNS S 15500) stainless steel combines high mechanical strength, ductility, and good corrosion resistance for aircraft and aerospace applications. This set of properties and its high added-value applications make it an excellent choice for additive manufacturing processes such as laser powder-bed fusion (LPBF). However, there is a need to understand the complex microstructure developed in LPBF-processed parts, which may show particularities such as columnar solidification, preferential orientation, different kinds of porosities, chemical segregation along the melt pools, metastable phases, and oxide nanoinclusions. We report the microstructural characterization of LPBF-processed 15-5PH stainless steel in the as-built condition and after aging. Low porosity (<1%), high hardness (420 HV0.1), and a large amount of retained austenite (15%) were found in the as-built samples. The microstructure shows the usual “fish scale”-like morphology. Grain size and hardness vary depending on the location within the melt pools. Reconstruction of the parent grains from EBSD maps indicates a microstructural refinement due to in situ reaustenitization of parts of the previous consolidated layers, where these heat-affected zones are harder and present larger amounts of retained austenite. After aging, as-built samples were harder and more resistant to overaging than annealed or wrought counterparts. Transmission electron microscopy reveals a large amount of nanometric crystalline silicon-oxide inclusions, indexed as cristobalite. The combination of fine-grained martensite, coherent Cu-rich clusters, nanometric oxide particles, and retained austenite makes LPBF-processed 15-5PH stainless steel a very promising material for high-end structural applications.

Neutron Bragg-edge transmission imaging for microstructure and residual strain in induction hardened gears

A time-of-flight Bragg-edge neutron transmission imaging was used to investigate the microstructure and strain distributions in a gear hardened by a newly developed two-step induction-heating method: precursor (Sample 1) and final product (Sample 2). The edge-position and edge-broadening were determined and mapped with high spatial resolution, which enabled us to confirm the two-dimensional distributions of the microstructure and residual strain. A deep hardened layer was made for Sample 1 in which martensite was formed on the entire teeth and the outer peripheral portion of the gear body. Sample 2 was subjected to double induction-hardening, where a tempered martensite was formed as the thermal refined microstructure between a fine-grained martensite at the tooth surface and a ferrite-pearlite microstructure at the core. The relationship between edge-broadening and the Vickers hardness described by a linear equation was employed to derive the elastic residual strain. The residual strain map for Sample 2 revealed that a steep compressive strain was introduced into the fine-grained martensite at the tooth surface by the super rapid induction-heating and quenching process. The reversal of tension was speculated to occur below 2 mm from the tooth tip, and the strain was almost zero in the core region.

Effect of Composition System on Microstructure and Properties of 1000-MPa Grade Cold Rolled DPSteel

1000 MPa grade cold-rolled duplex steel with 5 component systems was trial-produced in the laboratory. After hot rolling, pickling and cold rolling, the test steel was simulated continuous annealing on Gleeble-3800 thermal simulator. The microstructure and properties of 5 kinds of experimental steels after simulated annealing were analyzed. The results show that C and Mn elements are the basic alloy systems of all duplex steels. In the case of low Si element, the C element content of the 1000 MPa grade dual phase steel should be 0.15 % or higher, and the Mn element content should be higher than 1.8 %. Si element increased to the A3 point, while the amount of austenite was less in the two-phase region during annealing, and the volume fractionof martensiteafter the rapid cooling is insufficient; Nb element is a high-efficiency fine-grain strengthening element, which greatly improvedthe uniformity of the steel species. The fine-grained martensite islands have enhance the tempering stability of martensite and thereby to reduce the adverse strength effect from over-aging temperature.

The Effects of Process Temperatures on the Microstructure and Tensile Properties of Warm-Stamped Nb-Bearing Medium-Mn Steel

The effects of process temperatures for warm stamping, such as annealing (Tann), austenitizing (Taus), and stamping temperatures (Ts), on the tensile properties of Nb-bearing medium-Mn (5.9 wt pct) steel were investigated. The cold-rolled tensile specimens were first annealed at 650 °C to 750 °C, austenitized at 650 °C to 900 °C, held at Ts (500 °C to 700 °C), and then air-cooled to room temperature. Of the three process temperatures, only Taus significantly influenced the tensile properties of medium-Mn steel; in the Taus range of 650 °C to 900 °C, the amount of variation in yield strength (YS), ultimate tensile strength (UTS), and total elongation (TE) were 555 MPa, 570 MPa, and 20 pct, respectively. Particularly, when the specimens were austenitized at 730 °C to 790 °C, high YS (1060 to 1100 MPa), UTS (1760 to 1795 MPa), and TE (10.0 to 11.7 pct) were obtained due to fine-grained martensite embedded with nano-sized Nb carbides. The Nb-bearing medium-Mn specimen was successfully warm-stamped into a T-part of a B-pillar prototype at 600 °C without any cracks after austenitized at 750 °C for 5 minutes, and then air-cooled to room temperature. The warm-stamped Nb-bearing medium-Mn steel revealed the higher TE (~ 9.3 pct) than the hot-stamped 30MnB5 steel (5.3 pct) at the similar level of UTS (1855 to 1900 MPa).

Influence of electrolyte-plasma surface hardening on the structure and properties of steel 40KhN

The optimal electrolyte composition for electrolyte-plasma surface hardening of 40XH steel, which does not lead to the surface layer to erosion, oxidation and decarburization are determined in this work. It is shown that after electrolytic-plasma surface hardening a modified layer with a thickness of 1–1.2 mm is formed with high hardness and wear resistance which consisting of a hardened layer of fine-grained martensite, an intermediate layer of perlite and martensite.

Hardness Evolution and High Temperature Mechanical Properties of Laser Welded DP980 Steel Joints

AbstractDP980 steels were joined using fiber laser welding. The welded joint was characterized in terms of hardness distribution and tensile behavior at room temperature, 150 ℃, and 300 ℃, respectively. The fine-grained martensite in supercritical heat affected zone (HAZ) resulted in the highest hardness (428 Hv), while the tempered martensite contributed to the hardness decreasing (‒31 Hv). Both the ultimate tensile strength and yield strength of the base metal and welded joint decreased at 150 ℃, and then increased at 300 ℃ due to dynamic strain aging (DSA). The welded joint exhibited slightly higher yield strength and lower elongation at all the test temperatures compared to base metal due to the hardened fusion zone. The energy absorption reduced slightly with increasing temperature both for base metal and welded joint, and the weld posed a minor effect on the energy absorption. Deformation was one of the requirements for DSA effect. DSA enhanced the hardness of base metal (+78 Hv) and softened zone (+53 Hv). HAZ was not softened enough to become the weakest position during tensile test.

SURFACING OF DETAILS OF MINING EQUIPMENT BY POWDER WIRES OF C – Si – Mn – Mo – V – B AND C – Si – Mn – Cr – Mo – V SYSTEMS

A new composition of the powder wires based on the C – Si – Mn – Mo – V – B and C – Si – Mn – Cr – Mo – V doping systems represented by steels 40GMFR and 40Kh3G2MF was developed. The influence of alloying elements in the composition of C – Si – Mn – Mo – V – B and C – Si – Mn – Cr – Mo – V powder wires on the size of the martensite needles was studied, as well as the primary austenite grain size and the contamination of the weld layer with non-metallic inclusions. The infl uence of the structure on the strength and durability of the deposited metal was defi ned. It is shown that an increase in the content of carbon and alloying elements, in particular chromium, in the composition of the welded steel contributes to the formation of a structure with a medium-carbon fi ne-grained martensite and a small volume fraction of δ-ferrite. It provides reduction in size of the primary austenite grain and decrease in the contamination degree of the deposited layer with non-metallic inclusions. The established improvement in the structure aff ects the hardness increase up to 22 % and the decrease in the abrasion rate of the deposited layer up to 34 %. The chemical composition of samples welded with a fl ux-cored wire of C – Si – Mn – Mo – V – B system is characterized by a lower content of carbon and alloying elements compared to a 40Kh3G2MF steel, which causes the formation of low-carbon martensite in the structure after surfacing. It was established that a signifi cant increase in the content of carbon, manganese, chromium, vanadium and molybdenum in 40GMFR welded steel provides a structure with fine needle and medium needle martensite, reduces the size of the primary austenite grain and the contamination level of the deposited layer with non-metallic inclusions, in particular, non-deformed silicates. Changes in the micro structure are accompanied by an increase in hardness up to 16 % and a decrease in the attrition rate of the deposited layer up to 20 %. As a result of a comparative analysis of the two studied systems of powdered wires, it was found that it is more eff ective to use C – Si – Mn – Cr – Mo – V wire for surfacing mining equipment, since the content and ratio of alloying elements in steel of type 40Kh3G2MF contributes to obtaining a dispersed martensitic structure with an insignifi cant volume fraction of δ-ferrite, which provides high hardness and wear-resistance of the deposited layer.

HARDENING OF CRANE RAILS BY PLASMA DISCRETE-TIME SURFACE TREATMENT

Crane wheels and rails are subjected to intensive wear in the process of operation. Therefore, improvement of these components’ performance can be considered a task of high importance. A promising direction in this regard is surface treatment by highly concentrated energy flows such as laser beams or plasma jets. This thesis suggests that the use of gradient plasma surface treatment can improve the performance of crane rails. A research was conducted, according to which hardened zones were deposited on crane rails under different treatment modes. Microhardness was measured both at the surface and in depth using custom-made microsections. The article includes the results of study of plasma surface hardening effects on wear resistance of crane rails. Change of plasma surface treatment parameters (current, plasma torch movement speed, argon gas flow rate) allows for desired steel hardness and structure, while the choice of optimal location for hardened zones makes it possible to significantly improve wear resistance and crack resistance. As a result of plasma surface hardening, the fine-grained martensite structure is obtained with mainly lamellar morphology and higher hardness rate compared toinduction hardening or overlaying. Wear test of carbon steels revealed that plasma surfacing reduces abrasive wear rate compared to the irinitial state by 2 to 3 times. Enough sharp boundary between hardened and non-hardened portions has a positive effect on the performance of parts under dynamic loads, contributing to the inhibition of cracks during the transition from solid to a soft metal. For carbon and low alloy rail steels, the properties achieved by plasma surface hardening can effectively replace induction hardening or overlaying.The mode range for plasma surface treatment that allow sobtaining a surface layer with certain operating properties has been determined.

Effect of substrate preheating by induction heater on direct energy deposition of AISI M4 powder

This study discusses the effect of substrate preheating by an induction heater on the deposition characteristics of high-speed tool steel M4 powder. The powder is highly susceptible to cracking because of the very rapid heating and cooling rates induced by a high-energy laser during the deposition process and the residual stress evolution after deposition. Experimental investigations showed that a slower cooling rate can be induced by substrate preheating with the assistance of induction heating, which results in a coarser microstructure and a significantly longer spacing between cellular and dendrite arms. A high micro-hardness level may be obtained despite the microstructure evolutions occurring during the induction heating-assisted laser metal deposition because of the higher volume fraction of hard and stable carbides resulting from the increased holding time for precipitation. The tensile strength of the specimen prepared with substrate preheating increases. This effect may be attributed to the fine-grained martensite structure exhibited in the inter-diffusion and heat-affected zones in the sample that was not preheated. On the contrary, no distinct martensite structure was observed in the induction-heated sample. The fracture surface showed a typical brittle fracture, in which the crack propagated along the inter-dendritic boundaries. The results presented in this paper can be considered as guidelines for practical hardfacing applications that use high-alloy tool steel powders to achieve highly improved die and mold performances in terms of wear resistance and toughness.

Experimental Investigation of White Layer in Grinding of AISI 52100 Annealed Steel

Ground white layer is caused primarily by grinding temperature induced phase transformation. So, it may form when grinding temperature exceeds the nominal phase transformation temperature. However, no attempt is made to investigate mechanical effect on formation of white layer. In this study, grinding temperature is measured by using thermocouple technique in grinding of AISI 52100 annealed steel. The specimens are investigated by using scanning electron microscope (SEM), energy disperse spectroscopy (EDS), micro hardness tester and X-ray diffraction (XRD). The microstructure and formation mechanism of white layer are analyzed. Ground whiter layer is confirmed to be composed of fine-grained cryptocrystalline martensite and retained austenite. High hardness of white layer is caused by transformation hardening through fine grain and high dislocation density due to severe deformation. White layer can form at temperatures below the nominal austenitization temperature of the material. Plastic deformation is also important in white layer formation during grinding process. High pressure on grinding surface exists under severe deformation. Phase temperature can be reduced due to high pressure; while crystal grain can be refined by plastic deformation. Oxidation and carburizing phenomenon exist during formation of white layer.

Contact fracture of rolling bodies of open bearings of three-cone rock bits in aqueous environment

We have analyzed the formation of corrosion damages during the standby time of a rock bit in operation and the low crack resistance of high-strength steel under the action of aqueous environment, which lead to the loss of serviceability of the rolling friction pair of the rock bit. We have investigated the relation between the structure of 55SM5FA high-strength steel, used for rollers of the rock-bit bearings, and its short-term crack resistance with regard for the influence of water on it. With decrease in the fracture toughness of steel, the negative influence of environment on its crack growth resistance increases. The optimal structure of steel for rollers is fine-grained martensite with ~ 1% of troostite and hardness HRC 56 to 58. To retard the stress corrosion fracture of rock bits, we recommend conserving them during standby times in the boring process by lubrication not only of the nipple part, but also of the open bearings of bits.

Wear resistance of spark-deposited TiN, TiB2, and Mo coatings on 30KhGSA steel after exposure to focused solar radiation

The use of focused solar radiation to improve the mechanical properties of spark-deposited TiN, TiB 2 , and Mo coatings is studied. It is shown that the tribotechnical characteristics of sparkdeposited coatings improve after exposure to focused solar radiation over steel coatings after thermal treatment. Solar treatment of 30KhGSA steel leads to its strengthening—hardening with fine-grained annealed martensite being formed.

Structure and properties of laser alloyed surface layers on the hot-work tool steel

Investigations include alloying the X40CrMoV5-1 hot-work tool steel surface layer with the tungsten carbide, using the high-power diode laser (HPDL). The structural mechanism was determined of surface layers development, effect was studied of alloying parameters, gas protection method, and thickness of paste layer applied onto the steel surface on structure refinement and influence of these factors on the mechanical properties of surface layer, and especially, on its hardness, abrasive wear resistance, and roughness. The remelted zone structure is characterised by the significant martensite dispersion with its lathes length several times shorter than those of those developed during the conventional quenching. Martensite twining occurs in some locations, retained austenite, and very fine precipitations of the M 6C and M 7C 3 type carbides are observed as broken network on dendrite boundaries, as well as the high-dispersive ones inside of certain grains. The fine-grained martensite structure is responsible for hardness increase of the alloyed layer. It has the important cognitive significance and gives grounds to the practical employment of these technologies for forming the surfaces of new tools and regeneration of the used ones.

Mass transfer and doping during electrolyte-plasma treatment of cast iron

The surface and subsurface layers of cast iron articles upon electrolyte-plasma treatment (EPT) were studied for the first time by a combination of methods, including Rutherford backscattering, elastic recoil detection analysis, X-ray diffraction, optical metallography, and microhardness measurements. It was found that tungsten and copper ions are incorporated into the surface layers of the metal. Total thickness of an EPT-hardened layer is about 3 mm, the maximum hardness (up to 7000 MPa) being observed at a depth from 1 to 2.5 mm. The X-ray diffraction analysis (i) shows that the EPT-hardened layer comprises a mixture of α and γ phases, (ii) indicates the formation of a fine-grained martensite structure with high dislocation density (above 1012 cm−2) and an oxide (FeO) phase, and (iii) reveals mass transfer involving the doping elements (W and Cu).

Wear and corrosion resistance of laser surface hardened structural steel

CO 2 laser surface treatment in solid state has been performed on a CK60 structural steel. The laser energy density was lower than the melting threshold and only caused surface hardening. The observed heat affected zone (HAZ) ranged in depths of 50–430 μm. Microstructure of the HAZ was composed of needle, fine-grained martensite and small amount of pearlite and retained austenite. Microhardness values changed in areas of overlapping successive laser passes. Wear tests revealed the improved wear resistance of the laser treated specimens in comparison with the untreated one. Specimens laser treated using 0 and 38% overlapping ratios exhibited improved wear resistance. The corrosion resistance exhibited a tendency to decrease with increasing overlapping ratios. Among all specimens studied the one, which was laser treated with 0% overlapping ratio presented the best wear and corrosion behaviour.

On the Microstructure-Polarization Behavior Correlation of a 9Cr-1Mo Steel Weld Joint

The use of 9Cr-1Mo ferritic steel necessitates its fabrication by the process of welding. The heat-affected zone (HAZ) of 9Cr-1Mo ferritic steel is a combination of many microstructures. In the present study, the corrosion properties of the base metal, weld metal, and the various regions of the HAZ are assessed with respect to their microstructures. The various microstructures in the HAZ were simulated by heat treatment of the normalized and tempered base metal at 1463, 1200, and 1138 K for 5 min followed by oil quenching. The microstructure of the base metal in the normalized and tempered condition revealed martensite laths with M23C6 carbides at lath boundaries, and uniform dispersion of fine, acicular M2C. The weld metal showed predominantly martensitic structure with dispersion of carbides. Simulation of the microstructures of the HAZ by heat treatment resulted in the following microstructures: coarse-grained martensite of 75 µm size at 1463 K, fine-grained martensite at 1200 K, and martensite + proeutectoid α-ferrite at 1138 K. In all cases, carbide precipitation was observed in the martensitic matrix. Microhardness measurements of HAZ-simulated base metal showed increasing hardness with increasing heat treatment temperature. The hardness values obtained corresponded very well with the regions of the actual HAZ in the weld joint. Electrochemical polarization studies were carried out on the base metal, weld metal, weldment (base metal + weld metal + HAZ), and the simulated HAZ structures in 0.5 M sulfuric acid solution. Critical current densities (icrit1 and icrit2), passive current densities (ipass and isec-pass), and transpassive potential (Etp) were the parameters considered for evaluating the corrosion resistance. The HAZ structures simulated at 1463 and 1200 K, corresponding to coarse- and fine-grained martensitic regions of an actual HAZ, had corrosion properties as good as the normalized and tempered base metal. Of the various simulated HAZ structures, the intercritical region, which was simulated at 1138 K, possessed the worst corrosion resistance. The weld metal possessed the worst corrosion resistance of the various microstructural regions in the weld joint. The weldment adopted the degraded corrosion properties of the weld metal.