Hot-work Steel Research Articles

Impact of residual stresses on mechanical behaviour of hot work steels

Abstract This paper presents a multi-scale analysis of the mechanical behaviour of hot work steels taking into account the effect of residual stress generated by the initial microstructure and the external load and induced industrially by heat treatment and machining process. The methodology is based on: i) laboratory study to evaluate the macroscopic behaviour of the material, ii) numerical simulation based on finite element and homogenisation approaches to evaluate the material's microstructure complexity and characterize the residual stress distribution, iii) industrial tests during pretreatment and forging process of valve gas parts in order to complete the results obtained at laboratory scale. It was proved that residual stress induced by the initial microstructure of the hot steel material has significant effects on the stress behaviour of the hot forging steel under static tensile load. Besides, the residual stress generated during the manufacturing process affected more significantly the stress distribution on the die surface than the heat treatment, inducing residual stress close to plastic deformation.

Corrosion behavior of a spark plasma sintered Fe–20Mn–11Al–1.8C–5Cr alloy in molten aluminum

The corrosion behavior of an Fe–20Mn–11Al–1.8C–5Cr alloy prepared by spark plasma sintering was investigated via immersion tests in molten aluminum at 750 °C for 1 and 4 h, respectively, and a hot work steel (AISI H13) was included as a reference. The experimental results show that the corrosion rate of Fe–20Mn–11Al–1.8C–5Cr alloy is ~ 24% of that of H13 steel, suggesting that Fe–20Mn–11Al–1.8C–5Cr alloy in molten aluminum possesses better corrosion resistance than H13 steel. Detailed analysis show that κ-carbide ((Fe, Mn)3AlC x ) and Cr7C3 carbide precipitated in the matrix play a key role in enhancing the corrosion resistance of Fe–20Mn–11Al–1.8C–5Cr alloy in molten aluminum. Both of them show better corrosion resistance than γ-Fe matrix and H13 steel, and can also take on the role of roots in grasping the corrosion product and restrain them from spalling into the molten aluminum.

Tribological and Wear Performance of Nanocomposite PVD Hard Coatings Deposited on Aluminum Die Casting Tool.

In the aluminum die casting process, erosion, corrosion, soldering, and die sticking have a significant influence on tool life and product quality. A number of coatings such as TiN, CrN, and (Cr,Al)N deposited by physical vapor deposition (PVD) have been employed to act as protective coatings due to their high hardness and chemical stability. In this study, the wear performance of two nanocomposite AlTiN and AlCrN coatings with different structures were evaluated. These coatings were deposited on aluminum die casting mold tool substrates (AISI H13 hot work steel) by PVD using pulsed cathodic arc evaporation, equipped with three lateral arc-rotating cathodes (LARC) and one central rotating cathode (CERC). The research was performed in two stages: in the first stage, the outlined coatings were characterized regarding their chemical composition, morphology, and structure using glow discharge optical emission spectroscopy (GDOES), scanning electron microscopy (SEM), and X-ray diffraction (XRD), respectively. Surface morphology and mechanical properties were evaluated by atomic force microscopy (AFM) and nanoindentation. The coating adhesion was studied using Mersedes test and scratch testing. During the second stage, industrial tests were carried out for coated die casting molds. In parallel, tribological tests were also performed in order to determine if a correlation between laboratory and industrial tests can be drawn. All of the results were compared with a benchmark monolayer AlCrN coating. The data obtained show that the best performance was achieved for the AlCrN/Si3N4 nanocomposite coating that displays an optimum combination of hardness, adhesion, soldering behavior, oxidation resistance, and stress state. These characteristics are essential for improving the die mold service life. Therefore, this coating emerges as a novelty to be used to protect aluminum die casting molds.

Temperature dependent cyclic mechanical properties of a hot work steel after time and temperature dependent softening

In this paper, the temperature dependent cyclic mechanical properties of the martensitic hot work tool steel 1.2367 after tempering are investigated. To this end, hardness measurements as well as monotonic and cyclic tests at temperatures in the range from room temperature to 650 °C are performed on material tempered for different tempering times and temperatures. To describe the observed time and temperature dependent softening during tempering a kinetic model for the evolution of the mean size of secondary carbides based on Ostwald ripening is developed. Furthermore, mechanism-based as well as phenomenological relations for the cyclic mechanical properties of the Ramberg-Osgood model depending on carbide size and temperature are introduced. A good overall agreement of the measured and the calculated stress-strain hysteresis loops for different temperatures and heat treatments is obtained using the determined material properties of the kinetic and mechanical model.

Case Study to Illustrate the Potential of Conformal Cooling Channels for Hot Stamping Dies Manufactured Using Hybrid Process of Laser Metal Deposition (LMD) and Milling

Hot stamping dies include cooling channels to treat the formed sheet. The optimum cooling channels of dies and molds should adapt to the shape and surface of the dies, so that a homogeneous temperature distribution and cooling are guaranteed. Nevertheless, cooling ducts are conventionally manufactured by deep drilling, attaining straight channels unable to follow the geometry of the tool. Laser Metal Deposition (LMD) is an additive manufacturing technique capable of fabricating nearly free-form integrated cooling channels and therefore shape the so-called conformal cooling. The present work investigates the design and manufacturing of conformal cooling ducts, which are additively built up on hot work steel and then milled in order to attain the final part. Their mechanical performance and heat transfer capability has been evaluated, both experimentally and by means of thermal simulation. Finally, conformal cooling conduits are evaluated and compared to traditional straight channels. The results show that LMD is a proper technology for the generation of cooling ducts, opening the possibility to produce new geometries on dies and molds and, therefore, new products.

Corrosion characteristics of a tungsten alloy die-casting mould material in molten aluminium alloy

To understand the corrosion characteristics of W90 tungsten alloy and compare them with those of conventional hot-worked steel (SKD61), immersion tests were carried out in molten aluminium alloy (ADC12). Severe corrosion occurred in SKD61, whereas the corrosion resistance of W90 was about 40 times greater, with little occurring even after 300 h immersion. However, significant microstructural changes occurred in W90 resulting from penetration of aluminium and silicon through grain boundaries to create WAl5 and WSi2 phases, detectable even after a few hours’ immersion. Relatively soft (hardness 1.5 GPa) WAl5 formed between the W based grains, and hard (10 GPa) WSi2 outside the W based grains. Tensile strength decreased slowly with increasing immersion time, while a significant reduction in fracture strain occurred. These changes in tensile properties were caused by the microstructural changes, e.g. soft and brittle intermetallic compounds. Crack propagated along the WAl5 phases (intergranular fracture), whereas transgranular fracture was dominant for uncorroded or less corroded W90.

Factors Affecting the Wear Resistance of Forging Tools

AbstractThe durability of forging tools is a function of many variables: tool heat treatment, surface quality, temperature, pressure, number of forgings, diffusion layers (nitriding) and many others. The objective of study was to analyze and compare the working conditions of forging tools. For the analysis of selected flat surfaces of tools. Analyzed forging dies subjected to normal use. Presented results of laboratory tests . The effect of temperature and time on the properties of the surface layer of forging tools. The results were compared with the literature data. This article shows the results of microhardness tests for forging dies which have forged the corresponding number of forgings. The results of laboratory studies on microhardness of hot working steel 1.2344 in the furnace at various temperatures and time are also presented. The working conditions of the forging tools are very complex. The most often described in the literature are: thermal fatigue, abrasive wear, mechanical fatigue and cracks. The article discusses the effects of increased temperature on the surface properties of forging tools. Forging dies were made of hot work tool steel 1.2344. FEM modeling of changes in the surface layer should take into account changes in tool hardness as a function of time (number of forgings).

Influence of bias voltage and sputter mode on the coating properties of TiAlSiN

AbstractSilicon offers promising opportunities to improve the characteristics of thin coatings. By adding silicon to TiAlN, the oxidation resistance as well as the tribological properties can be increased and improved. To analyze the influence of the silicon content on the coating properties of TiAlSiN, it is necessary to keep the ratio of the other coating elements constant by using the right target configuration. Within this study, TiAlSiN coatings were deposited on hot work steel AISI H11 by using magnetron sputtering (Cemecon CC800/9 sinox ML). This steel was previously plasma nitrided to increase the hardness and hence the carrying load of the substrate, avoiding shell egg effect during the analysis. Different sputter modes were used to analyze the possibility to produce TiAlSiN by utilizing a pure low conductive silicon target. The bias voltages were systematically varied to see their influence on the structure and chemical compositions of the coating which were investigated by means of scanning electron microscopy and energy dispersive X‐ray spectroscopy (EDX). Furthermore, the roughness of the surface of the coatings was measured by an optical three‐dimensional surface analyzer. The results of this study serve as a basis for further investigations regarding the variation of the silicon content of TiAlSiN coatings.

Effect of unit size on thermal fatigue behavior of hot work steel repaired by a biomimetic laser remelting process

AISI H13 hot work steel with fatigue cracks was repaired by a biomimetic laser remelting (BLR) process in the form of lattice units with different sizes. Detailed microstructural studies and microhardness tests were carried out on the units. Studies revealed a mixed microstructure containing martensite, retained austenite and carbide particles with ultrafine grain size in units. BLR samples with defect-free units exhibited superior thermal fatigue resistance due to microstructure strengthening, and mechanisms of crack tip blunting and blocking. In addition, effects of unit size on thermal fatigue resistance of BLR samples were discussed.

The influence of PVD coating on the low cycle fatigue behaviour of Cr-Mo-V steel at elevated temperatures

The paper reports on findings of an experimental investigation of the low cycle fatigue behaviour of high performance Cr-Mo-V hot working steel, which is used for the forging die inserts in hot forging operations. Fatigue tests of standardised uniform-gauge test specimens with duplex coating (plasma nitrided and hard PVD coating) were carried out in a low cycle regime at stress ratio R≈0.05 at elevated temperatures of 150°C and 500°C. These were determined for typical hot forging operations by means of computational simulations.The experimental results have shown that in a low cycle fatigue regime at stress ranges around yield strength the influence of PVD (Physical Vapour Deposition) surface coating on the fatigue behaviour is negligible. However, in a high cycle fatigue regime at stress ranges in elastic domain the PVD surface coating has a positive effect on the material fatigue behaviour, which results in a longer fatigue life in comparison to the uncoated specimens.The low cycle fatigue parameters for uncoated and duplex coated surface of treated Cr-Mo-V steel for operating temperatures of 150°C and 500°C have been derived from experimental results for the first time. The determined fatigue parameters can be used to predict the fatigue life of similar hot forging tools under low cycle fatigue conditions.

Surface–Interface Microstructures and Friction-Wear Performances of Thermal Sprayed FeCrBSi Coatings Obtained by High-Velocity Oxygen Fuel Process

A layer of FeCrBSi coating was prepared on H13 hot work steel using a high-velocity oxygen fuel spraying (HVOF). The morphologies and distribution of chemical elements and phases in the obtained coatings were analyzed using a field emission scanning electron microscopy (FMSEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD), respectively. The friction-wear performance of FeCrBSi coating was examined using a wear test, and the wear mechanism was also discussed. The results show that the coating is primarily composed of Fe, Cr, B, and Si elements, which are uniformly distributed in the coating, enriched in the coating, and poor in the substrate at the coating interface. Among them, the Fe content decreases gradually in the substrate–coating direction, the Fe content of the coating is 40% lower than that of the substrate. The Cr, B, and Si contents in the coating are higher than those in the substrate, which form compounds and diffusion at the interface; as a result, the coating is combined with the substrate in the form of metallurgical bonding. The coating has a good friction reduction and wear resistance, the average COF (coefficient of friction) is 0.2126, the wear rate is 1.5 ∙ 10–6 mm3/sec ∙ N, and the wear mechanism consists abrasive wear and spalling.

Development of a Cu-Sn based brazing system with a low brazing and a high remelting temperature

Objective of the project presented is the development of a joining process for hot working steel components at low brazing temperatures leading to a bond with a much higher remelting temperature. This basically is achieved by the use of a Cu-Sn melt spinning foil combined with a pure Cu foil. During brazing, the Sn content of the foil is decreased by diffusion of Sn into the additional Cu resulting in a homogenious joint with a increased remelting temperature of the filler metal. Within this project specimens were brazed and diffusion annealed in a vacuum furnace at 850 °C varying the processing times (0 – 10 h). The samples prepared were studied metallographically and diffusion profiles of Sn were recorded using EDX line scans. The results are discussed in view of further investigations and envisaged applications.

Surface structuring by laser remelting of metals

Surface structuring by remelting with laser radiation is a new approach to shape metallic surfaces (“WaveShape”). In this structuring process, surface material is reallocated in its molten state instead of being removed, because the process is based on the new active principle of remelting. The surface structure and the microroughness result from a laser-controlled melt pool due to surface tension. Basic research has been conducted with promising results, especially for the hot work steel 1.2343. Since remelting is a thermally driven process, significant differences between metallic materials were expected due to their thermophysical properties such as thermal conductivity, absorption coefficient, viscosity, heat capacity, etc. Therefore, the presented research soughed to expand the spectrum of processable materials. Within the framework of our investigation, we compared the achieved structure height as well as melt pool dimensions for six different materials. Furthermore, we successfully tested new structuring strategies such as not linearly shaped scanning vectors to create innovative surface structures on all materials investigated. The biggest structures were achieved on the titanium alloy Ti6Al4V and the nickel based super alloy IN718. Finally, an approach of reverse structuring was investigated in order to erase existing structures and achieve a smoothed rewriteable metallic surface. The results show that surface structuring by laser remelting is well suited to process a wide range of different metals and to achieve a broad variety of different structures as well as to effectively erase existing surface structures such as milling marks.

Characterisation of cathodic arc evaporated CrTiAlN coatings: Tribological response at room temperature and at 400 °C

In this work, cathodic arc evaporation CrTiAlN coatings have been deposited on H13 hot work steel and the tribological behavior investigated at room temperature and at 400 °C. The microstructure, composition, roughness, indentation hardness and lattice parameter have been measured as a function of the deposition conditions, mainly given by the different Cr and TiAl vapour fluxes coming from the cathode arrangement in the vacuum reactor. The coating microstructures showed dense, compact columnar growth and a good film adhesion. The lattice parameter measured over the (002) diffraction peaks exhibited a quasi lineal correlation with the Ti/Cr+Al atomic ratio of the samples. In addition, the indentation hardness also increased as the lattice parameter increased. The coefficients of friction unveiled the different tribological behavior of the samples depending on the stoichiomentry and the temperature. At 400 °C, the coefficients of friction showed high dispersion, in contrast to the coherent evolution observed at room temperature. The wear damage at 400 °C was more intense than that observed at room temperature in agreement with the friction evolution observed. The coating with a stoichiometry of Cr0.23Ti0.13Al0.22N0.42 showed a good wear performance at 400 °C.

The Numerical Analysis of the Hardening Phenomena of the Hot-work Tool Steel

In the paper, to the numerical analysis of quenching phenomena, the complex model of hardening of the hot-work tool steel is used. The numerical algorithm of the thermal phenomena is based on the solving of the heat transfer equation, which takes into account the heat of phase transformations in the solid state, using the finite element method. Model of estimation of phase fractions and their kinetics is based on the continuous cooling diagram (CCT). Phase fractions which occur during the continuous heating and cooling (austenite, pearlite or bainite) are described by Johnson-Mehl-Avrami (JMA) formula. To determine of the formed martensite the modified Koistinen-Marburger (KM) equation is used. The stress and strain are determined by the solution of the equilibrium equations in the rate form using finite element method. The Young's and tangent modulus were dependent on temperature, whereas the yields stress was function of temperature and phase composition of the hardened element. In the model the thermal, structural, plastic strains and transformation plasticity are taken into account. Thermophysical properties occurring in the constitutive relations depended on the temperature and phase composition of the material. To calculate the plastic strains the Huber-Mises plasticity condition with isotropic hardening is used. Whereas to determine transformations induced plasticity the modified Leblond model is applied. Based on the implemented algorithms of the volumetric hardening process (assuming the various temperatures of the cooling medium) the numerical analysis of the phase content, strains and stresses for the hot-work steel (W360) element is performed.

Powder metallurgy opens new ways for tool steels

Powder metallurgy permits to produce tool steels with finer microstructure and improved mechanical properties compared to wrought ones. This major advantage was recently used by the authors in designing new steel grades combining higher strength and toughness. Novel hybrid tool steels, by mixing hot work and high speed steel powders in different proportions, were developed looking at materials with properties tunable for a specific application. Near ultrafine grained materials could be obtained combining mechanical milling and spark plasma sintering, a fast consolidation technique assisted by pulsed current. Particle reinforced tool steel matrix composites have also been produced using the same approach. Finally, in order to look for steels combining high hardness and improved thermal conductivity hybrid tool steel-Cu grades were sintered. The aim of the present paper is to give a general overview of the research activities and the opportunities opened by these new powder metallurgy products.

Powder metallurgy opens new ways for tool steels

Powder metallurgy permits to produce tool steels with finer microstructure and improved mechanical properties compared to wrought ones. This major advantage was recently used by the authors in designing new steel grades combining higher strength and toughness. Novel hybrid tool steels, by mixing hot work and high speed steel powders in different proportions, were developed looking at materials with properties tunable for a specific application. Near ultrafine grained materials could be obtained combining mechanical milling and spark plasma sintering, a fast consolidation technique assisted by pulsed current. Particle reinforced tool steel matrix composites have also been produced using the same approach. Finally, in order to look for steels combining high hardness and improved thermal conductivity hybrid tool steel-Cu grades were sintered. The aim of the present paper is to give a general overview of the research activities and the opportunities opened by these new powder metallurgy products.

Reversible, high temperature softening of plasma-nitrided hot-working steel studied using in situ micro-pillar compression

The high temperature mechanical behaviour of plasma-nitrided tool steel is investigated at variable temperatures using in situ micro-pillar compression. It is observed that the strengthening brought about by nitriding is steadily, but reversibly, reduced when increasing the testing temperature, until it is completely negated at 500°C.

Improving the Corrosion Resistance of Hot-Working Mold Steel against Al Alloy Melt by Coating

In order to reduce the adhesion tendency of aluminum melt on the die casting mold and extend the service life of the die, multi-arc ion plating technique was used to deposit Ti/TiN/CrN multi-layer coating on the surface of 8418 hot-working mold steel. Corrosion resistance of the steel and the Ti/TiN/CrN coating against Al alloy melt was studied. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy were used to study the microstructure, phase constituent and phase composition of the materials. Uniform Ti/TiN/CrN coatings with good bonding interfaces and a total thickness of about 2 μm were obtained. Results showed that Ti/TiN/CrN coating can improve the corrosion resistance of the 8418 hot-working die steel significantly against the aluminum alloy melt.

Wear Characteristics of Mo-W-Type Hot-Work Steel at High Temperature

The friction and wear characteristics of new Mo-W-type hot-work die steel, known as SDCM-S, were studied at high temperature. The results showed that the new Mo-W-type steel had greater wear resistance compared with H13 steel, which was due to its high oxidizability and high-temperature property. The high oxidizability and high temper stability features facilitate the generation, growth, and maintenance of a tribo-oxide layer at high temperature under relatively stable conditions. The thick tribo-oxide layer and high temper stability postpone the transition from mild to severe wear and ensure that conditions of mild oxidative wear are maintained in SDCM-S steel. The M2C and M6C carbides in Mo-W-type steel increase the temper stability and provide sufficient additional support for the formation and growth of a single thick tribo-oxide layer at high temperature. Mild oxidative wear is the dominant wear mechanism for SDCM-S steel between 400 and 700 °C.