Transformation Hardening Research Articles

Thermal fields induced by oscillating laser beams versus non-oscillating equivalent laser beams — A numerical study

In high power laser material processing, like laser transformation hardening, laser welding or laser cladding, tailoring the laser intensity profile is a method to improve stability of the laser material process and/or improve the processing results. A method to tailor the laser intensity profile in real-time is using a Galvanometer Scanner to quickly and iteratively oscillate the focal spot of the laser beam over a predefined path. Due to the oscillating nature of this beam shaping method, the temporal heat input will differ from the temporal heat input by an equivalent non-oscillated laser intensity distribution. Mathematically this equivalent non-oscillated beam profile is the convolution of the laser intensity distribution of the oscillated focal spot and the predefined oscillation path. This work presents simulation results of laser induced thermal fields. The thermal fields induced by oscillated laser beams at different frequencies are compared with the thermal field induced by the equivalent non-oscillated beam. These comparisons show that even at the highest oscillation frequencies the oscillated and equivalent non-oscillated laser beams induce significantly different in temperature fields (e.g. up to 325 K peak temperature difference) and different cooling rates (105 K/s and 5500 K/s respectively).

Low-calcium CO2 sequestration materials prepared with full-component solid waste based on lithium slag as crystal regulator

High performance low calcium CO2 sequestration materials (LCCSM) were prepared by using carbide slag (CS), copper tailings (CT) as main raw materials and lithium slag (LS) as crystal regulator at 1260 °C for 1 h. The effects of lithium slag content on the crystal transformation and carbonation hardening behavior of low calcium CO2 sequestration material (LCCSM) were studied. The results show that with the increase of LS content (i.e. from 0 % to 3 %), the C3S2 content was increased continuously, and the sum of C2S (i.e., β-C2S and γ-C2S) content was decreased continuously. Due to the activity energy of β-C2S was lowest in first stage carbonation, which accelerates carbonation reaction of the overall LCCSM clinkers and releasing a large amount of heat. Thereafter, the hydration reaction of β-C2S was accelerated significantly and the hydration products will be further carbonated. The minor LS (i.e., 1 %) significantly improved the carbonation activity of LCCSM clinker. Thus, the compressive strength development and degree of carbonation of carbonated compact made by LS-1% was obviously enhanced than that of other LCCSM clinker during the first 24 h. This finding could provide guidance for the obtaining high performance low-calcium CO2 sequestration material based on multiple solid wastes. And also proposed a new perspective for the mechanism among the multi-phase coexistence, crystal transformation and their synergistic carbonation sequestration of calcium silicate clinkers.

A comparative study of laser fluence effect on surface modification and hardness profile of austempered ductile iron

The impact of laser transformation hardening (LSH) and Laser surface melting (LSM) on an austempered ductile iron (ADI) alloy was investigated. Various laser fluences (J mm−2) were irradiated on the ADI specimens, obtained by different beam powers and scanning speeds to reach the optimum parameters to attain a surface with a good combination of high hardness and deep hardened depth, together with obtaining minimal distortion and crack-free surface. ADI surfaces were treated by means of high-power Nd:YAG laser in continuous wave mode. A detailed investigation of the obtained microstructure was characterized via optical and scanning electron microscopy attached to an EDX analyzer as well as X-ray diffraction analysis. Such a wide laser fluence range has a considerable effect on the achieved microstructure. A process parameter map was established which indicated that three different laser treatments were induced under such values of laser fluences: a laser transformation hardening, a partial laser melting, and a complete laser melting. The results revealed that to produce a hardened microstructure, specimens have to be operated within a very small range of laser fluence from 9.6 up to 29 J mm−2, achieving surface hardness values around 900 HV0.1 and depths of approximately 184–700 μm, respectively. Meanwhile, partial melting of the treated layer took place at a medium energy density range from 37 to 112 J mm−2, by a further increase in laser fluence (120–360 J mm−2), complete melting occurred which resulted in a deeper treated layer that reached 3.5 mm with an increase in microhardness value to almost 1100 HV0.1 at 360 J mm−2. The obtained microstructure in case of low laser fluence consisted of an austenite dendritic structure with undissolved graphite nodules, however, at medium and high laser fluence, the microstructure contains large cementite plates, iron-silicon carbides, a small area of ledeburite structure, and some retained austenite. The quantitative amounts of such phases directly depend on the applied laser fluence. Such phases were identified by means of EDX analysis.

Effect of repeated-tempering induced martensite on the microstructural evolution in 17–7 PH stainless steel

Single long-time tempering and triple tempering processes have been applied to 17–7 PH stainless steel (chemical composition: Fe-0.09C-16.4Cr-7.7Ni-0.9Al, wt%) to investigate the effects of the stability of austenite on subsequent martensitic transformation and aging hardening. The as-received steel was composed of 0.55 austenite and 0.45 martensite (volume fraction). Scanning electron microscopy images showed that the volume fraction of martensite (up to 0.98) in triple-tempered specimens ((0.5 h @ 760 °C) × 3) was about 0.15 higher than that (0.83) in single-tempered ones ((1.5 h @ 760 °C) × 1). The results clearly indicate the effectiveness of the triple-tempering process in progressively improving martensitic transformation. Direct observation of the austenite/martensite interface has provided evidence that the preceding martensitic transformation induces accommodation strain into the adjacent austenite, which brings about the mechanical stabilization of austenite but will facilitate the succeeding martensitic transformation after three consecutive tempering operations. The relevant microstructural evolution has been explored and discussed. Furthermore, subsequent aging at 565 °C led to precipitation hardening associated with the formation of NiAl nanoparticles. The triple-tempered specimen yielded the higher peak hardness value (424 HV) in 20 min, significantly faster than the single-tempered specimen, which took 90 min to reach the lower one (357 HV). The corresponding number density of NiAl nano-precipitates and dislocation densities of martensite at various aging times have been examined by transmission electron microscopy to elucidate the benefits of the triple tempering process for precipitation strengthening.

Laser Surface Transformation Hardening for Automotive Metals: Recent Progress

This article discusses recent advancements in the Laser Surface Transformation Hardening (LSTH) process applied to industrial metals. It focuses on examining the microstructure of the metal surface layer and explores different methods of performing LSTH to evaluate mechanical and surface properties. The study also investigates the utilization of various industrial lasers and simulation software for the LSTH process. The careful analysis of heat transfer and temperature control during LSTH aims to prevent the generation of surface defects like micro-cracks and surface melting. Finite element method (FEM) software effectively simulates the LSTH process. The research provides a comprehensive overview of recent developments in LSTH, categorized based on different metals and subsequent testing, highlighting its applications in the automotive industry. Electrochemical, wear, and microhardness tests are investigated to assess the potential applications of automotive metals.

Effect of Rolling Temperature on Martensitic Transformation and Strain Hardening of Fe–21.2Mn–0.68C Steel

Effect of Rolling Temperature on Martensitic Transformation and Strain Hardening of Fe–21.2Mn–0.68C Steel

The Effects of Ultrasonic Impact Modification on the Surface Quality of 20CrNiMo Carburized Steel

Ultra-high residual compressive stress can be introduced via the ultrasonic impact on the basis of transformation hardening, and further enhance the overall performance of 20CrNiMo carburized steel. In order to achieve the best surface quality of 20CrNiMo carburized steel, ultrasonic impact modification testing with varying static loads (900 N, 1200 N, and 1500 N) and rounds (1, 3, and 6) was conducted. By characterizing microhardness, microstructure, the surface roughness and residual compressive stress, the influence of ultrasonic impact modification parameters on its surface quality were analyzed. The experimental results indicated that the static loads and rounds of ultrasonic impact modification had a significant impact on the surface quality. The best surface quality could be obtained after six rounds of ultrasonic impact modification under a static load of 1200 N. In addition, the surface roughness decreased from 0.40 μm to 0.04 μm, the surface microhardness increased from 679 HV0.1 to 1086 HV0.1, and the maximum residual compressive stress of 1195.36 MPa was formed. Furthermore, the surface quality would deteriorate when the static load and ultrasonic impact rounds were increased.

Kinetic Study of the Aging and Overaging of Alloy Pb0.058%Ca0.12%Sr1.09%Sn for Battery Grids

In this paper, we examine the effect of a small amount of Sn, Sr and Ca on the hardening and the kinetic properties of Pb. Experimentally, we analyze the specific process from the structural hardening evolution to equilibrium of PbCaSrSn alloy occurring in two steps. In the first one, a discontinuous transformation hardening and a continuous precipitation take place. A lamellar discontinuous precipitation that occurs after aging, characterizes the second step. Theoretically, we study the aging and overaging kinetics of the alloy by using the two complimentary methods needed to calculate the apparent activation energies of the Pb0.058%Ca0.12%Sr1.09%Sn associated with the various transformations, characterizing its hardening at 20 and 80 °C. We show that activation energies of aging and overaging of Pb0.058%Ca0.12%Sr1.09%Sn alloy are 25 and 28 kJ/mole, respectively, which are four times lower than the self-diffusion energy of Pb. Moreover, the addition of Ca, Sr and Sn leads to an amplification of the maximum hardness from 5 ± 5% HV to 21 ± 5% HV and an acceleration of the transformation hardening process responsible for aging.

Anomalous effect of nitrogen flow ratio on mechanical properties of NbMoTaW:(N,O) medium entropy alloy films

In this work, nanocrystalline NbMoTaW:(N,O) thin films were prepared by magnetron sputtering. The effect of nitrogen flow ratio [RNN2/(N2 + Ar)] ranging from 0 to 0.5 on the microstructure and mechanical properties of as-deposited films is investigated. It is found that the microstructure and the mechanical properties are both dependent on RN. The structures of NbMoTaW:O films and NbMoTaW:(N,O) films are body-centered cubic and face-centered cubic, respectively. With increasing RN, the hardness of the film reduces first and then increases, showing an anomalous effect of RN on the mechanical properties compared with previous results. The anomalous effect origins from the phase transformation, dislocation density and grain boundary hardening. The surface roughness and residual stress havelittle influence on the mechanical property.

State of art: Review on laser surface hardening of alloy metals

Surface hardening treatments are performed to increase the hardness, wear resistance, durability and their micro structural properties. After the development of High-Powered Diode Lasers (HPDL), the laser surface hardening process is widely applied due to their non-contact treatment process, rapid cooling without any medium, controlled method of heat treatment and depth with automation and precision with relatively minimum energy input. Laser surface hardening of steels, carbon alloys, iron and other metals in manufacturing of components, stamping and forging dies, tools and other components in the field of mechanical, automotive and metallurgical have shown a positive progression in hardness and wear resistance with controlled phase changes. The factors of laser power density and travel speed plays key roles in phase changes and the surface transformation hardening. The surface hardening of heavily loaded components under motion should possess good surface roughness, tribological characteristics of wear resistance, hardness and temperature coefficients. Thus, laser surface hardening is optimum, controlled and economic way of surface treatment processes in manufacturing of mechanical components.

Laser transformation hardening of metal sheets under air and water environment

Laser transformation hardening of metal sheets under air and water environment

Study of hardening and structure of maraging powder steel grade PS-H18K9M5TR (18%Ni+9%Co+5%Mo+1%Ti+1%Re+66%Fe)

Relevance. High-strength steels are increasingly in demand in modern industry for various applications. Maraging steels are the primary material in the manufacture of most aircraft parts as well as machine-building components. This type is low-carbon and is rich in nickel, which forms martensite when cooled as well as demonstrates properties such as high hardness, wear resistance, etc. The hardening process is the main factor affecting the functional properties of maraging steel. At certain temperatures, austenite has the ability to transform into various kinds of phases. However, the shortcoming that lies in the presence of some impurities limits the established types of improvement technologies, leading to the search for innovative methods to improve the characteristics of steel without losing any of the desired properties. Good qualities appear in maraging steels mainly after treatment with a solution at a temperature of about 1000℃ and during aging at a temperature of about 490℃. Purpose. Thus, the purpose of this research paper is to analyze the structure of maraging steel powders and study the thermal effect on its properties. Methodology. In this paper, powder steel was pressed by spark plasma sintering technology at a pressure of 60 MPa to a powder compact and heated at a temperature of 1100℃ for 180 s at a rate of 20℃/s, after which the samples underwent phase and elemental analysis, their hardness was measured, the value of which amounted to about 60 HRC. Results. The results of this scientific research demonstrate the presence of a variety of precipitates. The presence of impurities such as Co, Ti, and Re led to an improvement in strength due to martensitic phase transformation and precipitation hardening, as well as slowed down the diffusion process. Conclusions. In addition, tasks for further research on the issue of manufacturing maraging steels by the additive manufacturing method were identified. This technology enables obtaining strong maraging steels based on a powder mixture with the required characteristics

Microstructural characteristics and tribological properties of the localized laser surface treatment of AISI 420 stainless steel

In this study, a localized laser surface treatment (LLST) of AISI 420 including transformation hardening and surface melting processes was utilized using a high-power diode laser (HPDL) under different beam travel speeds in order to obtain transformed hardened and surface melted samples. Microstructural characteristics of the laser processing areas were evaluated using field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS). Moreover, the tribological properties of the as-received, transformed hardened and surface melted samples were investigated. Obtained tribological results indicated that the surface melted sample has a higher coefficient of friction (COF) and greater sliding wear rate compared with the transformed hardened sample due to the increment of surface roughness and formation of δ-ferrite precipitates respectively.

A feasible route to produce 1.1 GPa ferritic-based low-Mn lightweight steels with ductility of 47%

High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility. However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facile strategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embedded in a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain, which act through dislocation engineering. We ascribe the simultaneous improvement in strength and total elongation to both the size-dependent dislocation movement in austenite grains and the controlled deformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation and high dislocation density-induced hardening and by interface strengthening. Additionally, high-carbon-partitioned small austenite grains enhance the strength and ductility by planar dislocation glide (in the low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strain regime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP may be desirable for broad industrial applications at an economic cost.

A multi-featured shape memory alloy constitutive model incorporating tension–compression asymmetric interpolation

The mechanical response of shape memory alloys is heavily governed by phase transformation and plasticity. These inelastic mechanisms present added complexities in the form of asymmetries with respect to the stress state (tension–compression asymmetry) and the direction (forward or reverse) of transformation, leading to the formation of internal loops. With this in mind, a constitutive model is developed incorporating phase transformation and yield plasticity. The model represents the tension–compression asymmetry through a novel approach by interpolating the maximum transformation strain, the Young’s modulus of martensite and a plasticity factor in the deviatoric plane of the transformation strain direction. Internal loops are also considered by scaling the transformation hardening and the hysteresis width based on memory points. The interaction between the processes of reorientation and phase transformation is also taken into account. The model is validated with several proportional and non-proportional loading tests with considerable success. The capability of the model to simulate complex loading scenarios is demonstrated by simulating the manufacturing and loading of a stent structure.

Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy

In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200 ℃, 250 ℃ and 300 ℃ are β″(DO19) → β′(BCO) → β(FCC), β″(DO19) → β′(BCO) → β1(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200 ℃, 250 ℃ and 300 ℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200 ℃ and 250 ℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β′′ to β′ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β′ precipitate, the hardness of the alloy aged at 200 ℃ is higher than that aged at 250 ℃. For the alloy aged at 300 ℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.

AN EXPERIMENTAL INVESTIGATION OF LASER TRANSFORMATION HARDENING OF UNALLOYED TITANIUM USING ND:YAG LASER

This research paper presents the investigation on laser transformation hardening (LTH) of unalloyed titanium of 1.6mm thickness sheet, nearer to ASTM Grade 3 of chemical composition was investigated using 2KW CW Nd: YAG Laser. The effects of laser process parameters: laser power750-1250W), scanning speed (1000-3000 mm/min) and focal point position (-10mm to -30mm) on the hardened bead profile parameters such as hardened bead width (HBW) and hardened depth (HD) was investigated using Response Surface Methodology (RSM). The experimental design is based on an independent quadratic Box-Behnken Design (BBD) matrix. The reduced linear and quadratic polynomial equations for predicting hardened bead width and hardened depth of bead profile were developed. Adequacies of models were examined by Analysis of Variance (ANOVA) technique. The results designate that the mathematical models developed predict the responses adequately adequate within the limits of hardening parameters being employed. It is recommended that the regression equations can be used to find optimum laser hardening conditions for desired criterion.

Композиты системы Al2O3/Yb-TZP, модифицированные катионами кальция, стронция и бария

The paper discusses the possibility of implementing two strengthening mechanisms in a ceramic system of Al2O3 and ZrO2 simultaneously: namely, transformational, due to the tetragonal form of zirconium dioxide and dispersion, due to the in situ formation of hexaaluminates of alkaline earth elements. The concentrations of modifying additives of calcium, strontium and barium, providing the formation of the corresponding hexaaluminates, have been established. The influence of the formed phases on the microstructure and strength characteristics of the obtained composites is considered. On composites including strontium cations, high strength parameters were obtained, flexural strength — 700 MPa, the critical stress intensity coefficient K1c reaches 10 MPa·m1/2, which is due to the optimal effect of a combination of transformation and dispersion hardening effects.

Swift heavy ion induced phase transformations in partially stabilized ZrO2

The paper reports on the effect of irradiation by swift heavy Xe ions on partially stabilized zirconia ceramics. XRD analysis identified two tetragonal phases with different degrees of tetragonality (transformable t phase and non-transformable metastable t” phase). TEM analysis of the irradiated samples showed that the efficiency of high-energy ion track formation decreases when the fluence and, hence, the fraction of t” phase increase. Changes in nanohardness, elastic modulus, and microhardness of partially stabilized ZrO2 ceramics before and after irradiation were investigated by nanoindentation and microindentation techniques. Mechanisms of ceramic layer hardening associated with phase rearrangement, compressive stress accumulation, and transformation and ferroelastic hardening are discussed.

Laser transformation hardening of various steel grades using different laser types

Laser surface hardening has become a promising replacement technique for conventional heat treatment. The laser is a heat source, deployed with optics to modify the surface along with the desired geometry. Laser density and its interaction time can be controlled and varied depending upon the final requirement at the surface. Increased hardness, diffusion-less phase transformation, self-quenching, uniform case depth over the entire scanned area are the reasons for laser-assisted hardening to become very popular and successful. Another reason for its success is that there is no bulk transformation of the material; only selected regions are subjected to laser irradiation for better surface improvements. This paper is a precis of various investigations done with a gas type CO2 laser, a solid-state Nd: YAG laser, high-power diode laser and fiber laser (only Continuous Wave mode) over a variety of steel grades. Laser wavelength, power intensity, transverse/scanning speed, focal plane distance, surrounding atmosphere, pre-treated surface and surface absorptivity are parameters involved in laser surface hardening technique. By varying these independent variables, the desired surface property is achieved on the substrate subjected. A significant analysis of various experimental investigations done in this area is provided. Particular reference is made to surface hardening using different lasers as mentioned above. Although laser hardening is a potential technique for enhancing the surface properties, multi-track hardening/back tempering of the large surface area is still a concern and needs some attention. The main reason is a decrease in hardness in the overlap region. Various studies indicate that a single-track laser hardened region results in 3- or 4-times higher hardness than that of the original or base metal hardness. In contrast, in the overlapped region, the hardness value decreases, i.e., the difference between the hardness values at the single-track hardened region and an overlapped region is around 100–400 HV. This article reviews the chronological development of laser transformation hardening method explicitly by outlining the effect of process parameters on hardened depth and width, hardening influence in wear and corrosion resistance, numerical simulation and experimental validation techniques. Particular reference is made to CW (continuous wave) type laser hardening.