Formation Of Lath Martensite Research Articles

The formation control and microstructure evolution of macrosegregation in laser offset welding on steel-nickel dissimilar alloy

To precisely control the melting of two base metals, laser offset welding is an effective method for managing the interfacial microstructure in dissimilar metal welds. Macrosegregation exhibits a markedly different morphology near the steel-nickel fusion line, which plays an important role in premature creep failure. In this study, it was found that an offset of the laser beam towards the nickel correlated with an increase in macrosegregation. When the laser beam was biased towards the steel, the partially mixed zone (PMZ) and transition zone (TZ) of 9Cr steel were identifiable within the macrosegregation, displaying a distinct solidification mode. Lath martensite and fine austenite formed in the PMZ, with alternating distributions of martensite and austenite observed in the TZ. Conversely, when the laser beam was biased towards the nickel, the unmixed zone (UMZ) of 9Cr steel was observed within the macrosegregation. Nanoindentation tests revealed that the maximum nanohardness in macrosegregation reached 5.90 GPa in the PMZ due to the formation of lath martensite, while the minimum nanohardness was 1.30 GPa in the TZ due to reduced martensite formation. Additionally, the TZ exhibited a significant reduction in resistance to plastic deformation. The various microstructures of macrosegregation were attributed to the solidification process and the dilution rate of 9Cr steel. When the liquidus temperature of WM exceeded 1455℃, a wide TZ formed in the macrosegregation due to adequate mixing. In contrast, when the WM had a significantly lower liquidus temperature than the 9Cr steel, only UMZ and PMZ formed in the macrosegregation due to preferential solidification and insufficient mixing. To minimize macrosegregation and eliminate the softening TZ, a laser beam offset towards the steel between 0.1 mm and 0.2 mm is recommended.

Evaluation and Modeling of the Rate of Formation of Lath Martensite in Fe-C Alloys, Extracted from Ultra-Rapid Quenching Experiments

Isothermal information is rarely available for the formation of martensite in Fe or Fe alloys due to a very high rate of transformation compared to the rate of heat conduction. Such information has now been extracted for lath martensite in some sets of Fe alloys from available information on ultra-rapid quenching but only at a single temperature for each alloy, related to its two MS temperatures. The temperature dependence could, thus, be studied only on binary sets of alloys. Those results have been applied to mathematical models based on the Arrhenius equation and illustrated with Arrhenius plots. For three sets of binary Fe alloys, a large group of rates came close to the rate of an almost pure and carbon-free Fe-C alloy. It illustrated that Cr, Ni, and Ru in low contents have relatively small effects on the rate of formation of lath martensite in Fe. It also demonstrated that the present measurements have considerable reproducibility. In contrast, a set of Fe-C alloys did not give a straight line in the Arrhenius plot. Using a new mathematical model based on the concept of the Arrhenius equation to express the effect of carbon, it was possible to predict the rate of formation of lath martensite for Fe-C alloys with fixed C content and their temperature dependencies which are not available experimentally due to the very high rate of formation.

Effect of welding parameters on weld surface roughness, hardness, and microstructure in laser-welded 340BH-steel and pure vanadium

This study delves into the influence of laser welding parameters on the microstructural evolution, surface roughness, and hardness of dissimilar lap joints formed by welding 340BH steel (upper) to cold-rolled pure vanadium (bottom) in an attempt to successfully develop metal membranes by adopting a novel technological route. Specifically, we examined power levels of 0.3 kW and 0.4 kW combined with varying welding speeds (40 mm/s, 50 mm/s, and 60 mm/s). The results indicate that defect-free joints were achievable at the higher welding speed of 60 mm/s. This conclusion is supported by the observed surface roughness profile. Elemental interdiffusion and temperature play pivotal roles in the formation of these dissimilar joints. These factors determine the dimensions of the heat-affected zone (HAZ) and the fusion zone (FZ) under all welding conditions. Notably, the high cooling rates during laser welding hindered the formation of intermetallic phases both in the FZ and at the interface between the metals. Microstructural examination of the HAZ/FZ on the Fe side showed a coarser microstructure when using a power of 0.4 kW, in contrast to the lath martensite observed at 0.3 kW. These observations align with the cooling curve derived from finite element analysis (FEA). FEA was also used to estimate temperature profiles throughout the welded area, and the results were consistent with experimental data. Further microstructural analysis using electron backscattered diffraction (EBSD) revealed a significant phenomenon: the formation of recrystallized grains of vanadium in the HAZ near the FZ on the vanadium side. Microhardness tests, conducted to gauge joint quality, displayed varying hardness levels across the base metal (BM), HAZ, and FZ interfaces. At a power of 0.3 kW, the HAZ displayed greater hardness than the FZ and BM due to the formation of lath martensite. Conversely, at a power setting of 0.4 kW, a reduction in hardness was observed within the HAZ, attributable to coarser ferrite formation.

Effect of welding current on the microstructural evolution and lap-shear performance of resistance spot-welded 340BH steel

In this study, we used experimental characterizations and finite element analysis (FEA) techniques to investigate the effect that welding current exerts on the microstructural features and mechanical properties of bake-hardened (BH) steels. Resistance spot welding (RSW) experiments were conducted using 5.0 and 7.0 kA of welding current. The microstructure across the weld regions was analyzed using optical microscopy (OM) and electron backscatter diffraction (EBSD). FEA was employed to simulate the RSW process, predict nugget dimensions, and determine cooling rates. The results of the OM analysis showed that nugget size increased with an increase in the welding current, and EBSD characterization revealed similar microstructures under both levels of current, but significant variations were observed within weld zones. Lath martensite formed as a result of rapid cooling in the fusion zone (FZ). Whereas, fine and coarse grains were found in the heat-affected zone (HAZ). In addition, the formation of lath martensite in the FZ resulted in high hardness on a 2D map of microhardness. The welding current had no substantial effect on microhardness, but it did impact the mechanical properties under lap-shear test conditions, which resulted in significant changes in the fracture mode. The deformation and fracture behavior of spot-welded specimens was studied using a lap-shear test coupled with 3D digital image correlation (DIC). The experimental results indicated a fracture in the 5.0 kA current samples near the FZ/HAZ boundary, while 7.0 kA welding current resulted in base metal (BM) necking and tensile fracture. FEA was also employed to elucidate the local deformation properties and fracture modes during lap-shear testing. The results of this study show that nugget size has no effect on a failure site during lap-shear loading, but size does affect the nugget load-carrying capabilities. The failure behavior was influenced by the microstructural properties of the distinct weld areas.

Microstructure and Mechanical Properties of Laser-Welded Joints between DP590 Dual-Phase Steel and 304 Stainless Steel with Preset Nickel Coating

Dissimilarities in metal laser welding lead to brittleness in welded joints due to differences in the thermophysical and chemical properties between dissimilar base materials. To overcome such brittleness, the addition of a preset coating onto the base materials as an interlayer is a method for attaining reliable welded joints. Nd:YAG laser butt welding of DP590 dual-phase steel and 304 stainless, both with a thickness of 1 mm, was performed with a preset nickel coating as an interlayer using an electroplating process. The relationship between the microstructure and the mechanical properties of the welded joints was researched, the microstructure and composition of the weldment were analyzed, and the microhardness, tensile strength and corrosion resistance were tested. The results showed that the preset nickel coating increased the content of Ni element in the welded joints, which is beneficial to the formation of lath martensite. The average hardness of the welded joints increased by 12%, and the tensile strength was higher than 370 MPa. The corrosion rate of the welded joints can be slowed down, and the corrosion resistance can be improved by increasing the nickel coating.

Mechanical properties and nugget evolution in resistance spot welding of Zn–Al–Mg galvanized DC51D steel

Abstract Zn–Al–Mg coating galvanized steel in resistance spot welded (RSW) in different configurations of DC51D was investigated to illustrate the nugget evolution process and mechanical properties of the joints. Results show that the microstructure of welded joints can be divided into nugget zone (FZ), heat-affected zone (HAZ), and base metal zone (BM). FZ was composed of lath martensite. The average hardness value of the weld joint was 110 HV0.2 while the FZ was up to 300 HV0.2 due to the formation of lath martensite. The failure modes can be divided into interface fracture (IF) and pull-out fracture occurred (PF) under different welding parameters, in which shear dimples showed had a typical plastic fracture morphology. The best range for welding parameters was found to be 12–18 cycles in which the nugget diameter reached 5.5 mm. The process of nugget evolution in HAZ and FZ was discussed.

Shunting effects on the resistance spot welding parameters of DP600

Abstract In this study, the effects of spot welding parameters on the weld quality of DP600 steel were experimentally investigated. The welding parameters such as welding current and time were determined based on microstructural characterization, microhardness tests and tensile-shear tests. The joints were welded with two parallel welds perpendicular to the tensile-shear test direction. The shunting effect was evaluated based on microhardness and nugget size. A microstructural examination was performed using optical microscopy. The results show that parallel welding increased current diversion along the shunt path, reducing hardness and nugget size in the second weld. Additionally, higher welding current and welding time caused increased hardness and a higher tensile load due to the formation of lath martensite.

Comparison between experimental data and a cellular automata simulation of martensite formation during cooling

Computer simulations of steel microstructural development provide a powerful tool, which can form the basis of mechanical property predictions. However, in order to create detailed understanding of the factors affecting the properties, the model should predict microstructural evolution during cooling. The present study compares the results of cellular automata simulations with experimental data for two distinct austenite conditions, recrystallized and deformed. Detailed microstructural features were studied using a laser scanning confocal microscope, FESEM and FESEM-EBSD. The two-dimensional cellular automata (CA) model for simulating the formation of lath martensite was parameterized using fitted Johnson-Mehl-Avrami-Kolmogorov and Koistinen-Marburger equations. The parent austenite microstructure for the CA model was determined from the final martensitic microstructure using austenite grain reconstructions based on the use of MATLAB software and the MTEX toolbox. The results of this cellular automata simulation can be used to estimate the shapes and sizes of martensite blocks, which offers new possibilities for the qualitative estimation of the mechanical properties of high-strength steels formed from recrystallized or deformed austenite.

Special aspects of thermal treatment of steel for hot forming dies production

Abstract It was found the influence of heat treatment parameters on the structure and properties of perspective die steel for hot working 70Cr3Mn2WTiB. It was specified the optimal mode of heat hardening for hot working hammer die from offered steel including spheroidizing annealing by the temperature of 780 °C with furnace cool up to 550 °C and then in a smooth air to the normal temperature; hardening by 1000 °C with cooling in oil; high-temperature tempering by 550–600 °C with cooling in a smooth air. During the hardening the formation of lath martensite with the high density of dislocation and the hardness HRC 57–62 occurs however it stays 0, 5–0,6% insoluble carbide phase. It’s shown that at the stage of high-temperature tempering by 550–600 °C in steel 70Cr3Mn2WTiB can be seen the process of hardness stabilization through the formation of special complex carbide intermetallic inclusions. The analysis of carbide phase by the temper showed that the combinations of type Me7C3 and Me3C are carbides the composition of which is changing during thermal influence. Thus, some atoms Fe and Cr are changed by titanium and vanadium atoms at different ratio. The simulation of temperature stressed condition of hammer die by thermal treatment was made. It’s shown that the offered mode of thermal treatment of steel 70Cr3Mn2WTiB leads to dispersed ferrite cementite structure with dispersed in it special carbides, high level of mechanical characteristics, provides homogeneous spreading of heat isotherms in extent of the hot deformation stamp and as a consequence insignificant inner residual strength. At the stage of high-temperature soak the temperature difference reduces to 25–30 °C.

Simulation of bainite and martensite formation using a novel cellular automata method

A novel two-dimensional cellular automata model for simulating the formation of lath martensite and bainite in steels during cooling is presented. The model is parameterized using fitted Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Koistinen-Marburger equations and by comparing predictions to scanning electron microscopy images of the actual microstructures after cooling. The results of this simulation can be used to estimate the fractions, shapes and sizes of bainite and martensite for different cooling rates, which should offer new possibilities for the qualitative estimation of the mechanical properties of high-strength steels with bainitic – martensitic microstructures formed from recrystallized or thermomechanically deformed austenite.

Elemental distribution in the martensite–austenite constituent in intercritically reheated coarse-grained heat-affected zone of a high-strength pipeline steel

We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49wt%) and Mn (2.32wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent.

Mechanical and Metallographic Effects of Laser Hardening of Two AHSS Steels

Abstract The search for suitable materials for the transport industry, to meet more stringent regulations related to crashworthiness, emissions, and fuel economy led to the development of advanced high-strength steels (AHSS). Thermal treatments, for example, using lasers as a process tool, can locally improve some characteristics of these materials like plastic deformation capability. The high energetic efficiency of a laser process, and its capability to be automated, are some of the advantages of using such a process. This study aims to investigate the effects of local heat treatment (involving changes in the solid state), by laser radiation, in the mechanical properties and microstructures of two types of advanced high-strength steels, the dual-phase DP 600, and the transformation-induced plasticity TRIP 750. A method to evaluate the interaction between laser radiation and the materials is proposed. Previous studies in this area focused, basically, in welding or cutting technology, and for the modern TRIP steel studied here, there is a scarcity of published material regarding laser–material interaction. Hardness and tensile tests revealed, for the range of process parameters studied, an improvement (up to 30 % with relation to the base material) in yield strength and ultimate strength (up to 15 %). Revealed also is a dramatic reduction in the elongation (up to 80 %) for both materials. Optical metallography analysis revealed that the resulting microstructures presented grain refinement and formation of lath martensite according to the level of laser absorption, improving up to twice the original hardness.

Transformation-rate maxima during lath martensite formation: plastic vs. elastic shape strain accommodation

Recently, a modulated formation behaviour of lath martensite in Fe–Ni(-based) alloys was observed, exhibiting a series of transformation-rate maxima. This peculiar transformation behaviour was explained on the basis of the hierarchical microstructure of lath martensite, minimising the net shape strain associated with martensite formation, by a block-by-block formation of martensite packages occurring simultaneously in all packages. In the present work, the martensitic transformation upon slow cooling of two Fe–Ni alloys, containing 22 and 25 at.% of Ni, respectively, was investigated by high-resolution dilatometry with the aim of identifying the influence of alloy composition on the modulated transformation behaviour. The differences observed for the two alloys, a more rapid sequence of the transformation-rate maxima and a narrower temperature range in case of Fe-25 at.% Ni, can be explained consistently as a consequence of the lower transformation temperatures in Fe-25 at.% Ni, highlighting the role of temporary accommodation of the shape strain during formation of the lath martensite microstructure: the depression of the transformation toward lower temperatures leads to a higher strength of the austenite, hence resulting in a more elastic (less plastic) temporary accommodation of the shape strain upon block formation and thereby in a more effective mutual compensation of the shape strain by neighbouring blocks. A kinetic model on the basis of energy-change considerations is presented which is able to describe the observed modulated transformation behaviour.

Modulated formation of lath martensite: Influence of uniaxial compressive load and transformation-induced plasticity

The austenite → martensite transformation occurring upon cooling of an FeNiCoMo alloy, similar to commercial FeNiCoMo maraging steels, thereby forming lath martensite, was investigated by dilatometry with regard to the influence of an externally applied compressive load on the peculiar transformation behavior in this system, which exhibits a step-wise mode of transformation upon slow cooling, as shown recently. Temporary, external loading during isothermal interruption of the continuous cooling at different stages of the transformation revealed a decreasing degree of plastic deformation proportional with the decreasing amount of retained austenite, implying that the plastic deformation is predominantly accomplished within the austenite phase. Upon resuming the cooling, the step-wise transformation was found to be unaffected. Application of a constant, comparatively high external load during continuous cooling caused a step-wise length decrease in the temperature range of the transformation instead of the step-wise length increase observed for the martensitic transformation in absence of an external load. The plastic deformation happening during the transformation occurs also for loads below the yield limit of pure austenite. This genuine transformation-induced plasticity was ascribed to the generation of mobile dislocations within the austenite upon the formation of martensite, thereby enabling plastic deformation in addition to the intrinsic deformation induced by the load for a specimen in the absence of transformation. Because the transformation into martensite also during loading takes place in a step-wise manner, the generation of these mobile dislocations occurs step-wise too, thus leading to the observed step-wise plastic deformation and corresponding step-wise length decrease during the transformation.

Anomalous kinetics of lath martensite formation in stainless steel

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.

Modulated martensite formation behavior in Fe–Ni-based alloys; athermal and thermally activated mechanisms

Abstract

Kinetics of anomalous multi-step formation of lath martensite in steel

A steel containing 16wt.% Cr, 5wt.% Ni and 3wt.% Cu was transformed into martensite by applying isochronal, i.e. constant rate, cooling followed by isothermal holding. The formation of martensite was monitored with dilatometry. A series of retardations and accelerations of the transformation was observed during isochronal cooling for cooling rates ranging from 1.5 to 50Kmin−1. The cooling rate in the isochronal stage was observed to influence the transformation rate in the isothermal stage. Electron backscatter diffraction was applied to determine the morphology of the martensite, which was of lath type, and to investigate the microstructure of the material. No influence of the cooling rate on the scale of the microstructure was observed. The series of retardations and accelerations of the transformation is interpreted in terms of the combined effect of the strain and interfacial energy introduced in the system during martensite formation, which stabilizes austenite, and autocatalytic nucleation of martensite.

Elasto-plastic phase-field simulation of martensitic transformation in lath martensite steels

Phase-field simulation of lath martensite formation in Fe–0.1C mass% steel was carried out based on the two types of slip deformation (TTSD) model, which is recently developed as a result of analytical solution for the martensitic transformation being composed of Bain deformation and plastic deformation but without lattice rotation matrix. The simulation results reveal that the plastic deformation along the two types of slip system is complementary. The simulation result of the relationship between the two types of slip deformation is consistent with the analytical result calculated by TTSD model, indicating the validity of TTSD model for explaining the formation of lath martensite.

Martensite in quenched Fe–C steels and Engel–Brewer electron theory of crystal structures

The Engel–Brewer (E–B) electron theory of crystal structures has been used to develop a model of lath martensite formation in quenched Fe–C steels. Lath martensite is described as a cluster of eight iron atoms surrounding each carbon atom in a matrix of fine subgrained body centred cubic iron. The E–B theory is used to describe the valence state of the cluster and the matrix of iron. Three sequential transformations take place during quenching, involving face centred cubic austenite with valence of 3, hexagonal close packed hexagonite with valence of 2 and body centred cubic ferrite with valence of 1. The E–B theory predicts the maximum solubility of carbon in hexagonal close packed iron, i.e. hexagonite, to be 2·625 at-%C, supporting the phenomenological value of 2·75 at-%C (0·6 wt-%C). The electron binding energies associated with the creation of hexagonite are consistent with experimental observations. The E–B theory also predicts the volume change from austenite to hexagonite and from hexagonite to ferrite that is in agreement with the values obtained from pressure–temperature studies. The exceptional hardness of lath martensite is attributed to the presence of iron–carbon clusters and subgrains and is proportional to its specific volume.

Prediction of the Maximum Dislocation Density in Lath Martensitic Steel by Elasto-Plastic Phase-Field Method

On the basis of the two types of slip deformation (TTSD) model of lath martensite, the martensitic transformation was simulated in Fe­ 0.1mass% C steel by an elasto-plastic phase-field method. The TTSD model allowed us to predict the total dislocations for the necessity of the formation of lath martensite, which is taken as the upper limit of dislocation density in lath martensite. The calculated dislocation density by the simulation was reasonable to be higher than the observed dislocations in value but to be the same in order. This consistence indicates that the calculation method based on the TTSD model is credible, together with the calculation of the habit plane predicted by the TTSD model. [doi:10.2320/matertrans.M2012067]