Martensitic Hot Work Tool Steel Research Articles

Tailoring the alloy composition for wire arc additive manufacturing utilizing metal-cored wires in the cold metal transfer process

In the present study, the application and tailoring of the alloy composition of chromium martensitic hot-work steels using metal cored wires (MCW) for wire arc additive manufacturing (WAAM) in a modified short-circuit metal transfer process is demonstrated. The nickel content was varied and the alloys were fabricated as tubular-cored wires with various powder fillings. By recording the material transfer at high speed during processing, evidence was gathered indicating the suitability of the fabricated cored wires for WAAM. Optimized process parameters were identified by taking a Design of Experiment (DoE) approach and additive manufacturing (AM) structures were fabricated from the chromium martensitic hot-work tool steel alloys. The microstructure and mechanical properties of the parts were subsequently characterized. The phase fraction of the polygonally shaped delta ferrite could be reduced and microstructural refinement could be achieved by adding nickel to the investigated hot-work tool steel. In addition to molybdenum-enriched precipitates that covered the grain boundaries, randomly scattered non-metallic inclusions and oxides were observed. Modifying the microstructure by adding nickel also affects the mechanical properties of the product: an increase in hardness, impact toughness and yield strength as the nickel content increased in the AM structures fabricated by WAAM was observed.

Effect of preheating temperature on the porosity and microstructure of martensitic hot work tool steel manufactured with L-PBF

Additive manufacturing (AM) brings the possibility to produce tools with a complex design of the cooling channels to achieve desired properties in the working piece. However, AM of tool steels can be challenging due to the residual stresses and crack formation which may be mitigated by optimizing process parameters. In this study, process parameters development in the laser powder bed fusion (L-PBF) of a martensitic hot work tool steel was under investigation. Having power, scanning speed and hatch distance as variables, a design of experiment was performed to map a process window to produce dense components. In addition, the same sets of process parameters were applied at different preheating temperatures to study and discuss the effect of three preheating temperatures on the porosity, microstructure and hardness of the manufactured components. The characterization of the parts was done using optical and scanning electron microscopy, x-ray diffraction and micro-hardness test. The results showed that the martensitic hot work tool steel can be manufactured with L-PBF with high densities above 99.95%.

Influence of initial defect density on mechanical properties of AISI H13 hot-work tool steel produced by laser powder bed fusion and hot isostatic pressing

ABSTRACT In this study, the microstructure and mechanical properties of martensitic hot-work tool steel AISI H13 produced by laser powder bed fusion (LPBF) under argon atmosphere in different LPBF process times were analysed in the as-built, hardened and tempered (HT) and hot isostatic pressed, hardened and tempered (HIP+HT) condition and compared to conventionally manufactured reference material. Three parameter sets were selected to adjust an initial defect density of 0.6%, 0.9% and 2.2% and thus process time reductions of 17% and 29%. We observed a change in the defect type from almost argon-free microcracks to argon-filled pores with higher scan velocities. Argon embrittlement was evident also in the results of tensile, notched impact toughness and fatigue tests. After HIP+HT a fatigue strength of 972 MPa was obtained for the set with 0.6% microcracks, but only 775 MPa and 718 MPa were obtained for samples with argon porosity.

Influence of thermomechanical fatigue loading conditions on the nanostructure of secondary hardening steels

Abstract Dual hardening steels reach their well-balanced mechanical properties in terms of strength and toughness through the combination of secondary hardening carbide and intermetallic particle precipitation. This characteristic profile makes them well suited for hot-work applications. In this study, out-of-phase thermomechanical fatigue tests, recreating operating conditions present during hot-work applications, were performed on a dual hardening steel and a 5% Cr martensitic hot-work tool steel. Via high resolution analysis utilizing atom probe tomography and transmission electron microscopy, the behaviour of the different precipitate populations under combined thermal and mechanical loading conditions were compared. Coarsening of the different precipitates and partial dissolution of the intermetallic compounds was observed. It could be shown that with rising maximum fatigue test temperature, the dual hardening steel reaches an increased lifetime caused by its higher tempering resistance.

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.

Thermo-mechanical coupling in constitutive modeling of dissipative materials

In the present paper a thermodynamics based constitutive modeling of coupled dissipative phenomena is discussed. In the proposed analysis special attention is paid to the problem of thermo-mechanical coupling in rate equations of thermodynamic conjugated forces. As an example of thermo-plastic coupling, the non-isothermal fatigue behavior of tempered martensitic hot work tool steel is considered. The numerical simulations are performed, to illustrate the qualitative and quantitative influence of temperature rate on the response of constitutive model when cyclic thermo-mechanical loading is applied.

Influence of <i>In Situ</i> Formed Tribolayer on Abrasive Wear Reduction

High temperatures (HT) are essential for many production methods. When operating with abrasives from natural sources, e.g. ores, abrasive wear is limiting the lifetime of core components. Material degradation and wear is accelerated in HT environments; wear protective solutions are necessary to minimize maintenance efforts. Especially, cost efficient solutions are needed in the industry, thereto in this research different Fe-based materials with prospective good HT wear pro­perties were chosen: i) austenitic stainless steel, ii) martensitic hot work tool steel, iii) carbide-rich ferrite MMC (metal matrix composite) and iv) complex alloyed hardfacing MMC. The materials were investigated by conducting high stress abrasion tests at temperatures up to 700°C. Wear rates were calculated and wear reducing effects by formation of mechanically mixed layers (MML) were studied. Two different technical approaches were carried out: SEM investigation of the surface coverage by abrasive embedding and optical microscopy analysis on cross-sections to determine the penetration depth of the abrasives. All materials except the hardfacing showed significant MML formation. Results showed increased coverage and penetration depth with ascending temperature. A wear reducing effect predominantly at HT is suspected, as MML forming materials showed shallower wear increase with temperature compared to the hardfacing.

Modeling of a tempered martensitic hot work tool steel behavior in the presence of thermo-viscoplastic coupling

In the present analysis a constitutive model is proposed to describe the fatigue behavior of martensitic hot work tool steel in non-isothermal conditions. 2M1C (2 mechanisms-1 yield criterion) model derived by Cailletaud and Saï (1995) and identified by Velay et al. (2006) to describe the fatigue behavior of AISI L6 steel at different (but constant) temperatures, is extended to account for the effect of temperature change. Two cases are compared: (1) partial coupling, when changing temperature is accounted only in changing material parameters, and (2) full coupling, when additional terms proportional to temperature rate are added in the kinetic equations of thermodynamic conjugate forces. Numerical simulations are performed, which indicate the significant influence of temperature rate on the response of constitutive model when cyclic thermo-mechanical loading is considered.

Atom probe study of the carbon distribution in a hardened martensitic hot-work tool steel X38CrMoV5-1

The microstructure of the hardened common hot-work tool steel X38CrMoV5-1 has been characterized by atom probe tomography with the focus on the carbon distribution. Samples quenched with technically relevant cooling parameters λ from 0.1 (30K/s) to 12 (0.25K/s) have been investigated. The parameter λ is an industrially commonly used exponential cooling parameter, representing the cooling time from 800 to 500°C in seconds divided with hundred. In all samples pronounced carbon segregation to dislocations and cluster formation could be observed after quenching. Carbon enriched interlath films with peak carbon levels of 6–10at.%, which have been identified to be retained austenite by TEM, show a thickness increase with increasing λ. Therefore, the fraction of total carbon staying in the austenite grows. This carbon is not available for the tempering induced precipitation of secondary carbides in the bulk. Through all samples no segregation of any substitutional elements takes place. Charpy impact testing and fracture surface analysis of the hardened samples reveal the cooling rate induced microstructural distinctions.

Surface Alloying of AISI H13 Steel during Electrical Discharge Machining (EDM)

Electrical discharge machining (EDM) is one of the most widely used non-conventional machining processes for the production of moulds/dies, cutting tools and aero-engine parts, such as turbine/compressor blades. The paper details experimental research on the surface alloying/modification of chromium martensitic hot-work tool steel components (AISI H13, 55HRC) during EDM die-sinking operations using powder metallurgy (PM) tool electrodes, as a means of achieving enhanced workpiece wear resistance without resorting to a subsequent coating operation. Tool electrode performance of partially sintered WC/Co electrodes operating in a common hydrocarbon oil dielectric was assessed and subsequently compared with that of conventional electrode materials, such as Cu and Graphite. Surface/subsurface observations by optical and scanning electron microscopy (SEM) showed a recast solidified layer of ~ 8 µm when using WC/Co electrodes. Performed XRD and SEM-EDX analysis indicated that WC and Co contained in the PM tool electrodes, together with C decomposed from the hydrocarbon oil during sparking, were transferred and alloyed to the steel substrate surfaces. EDM surface alloyed layers were hardened over 1200 HK0.025. This hardening is related both to the formation of tungsten carbides with different stoichiometries and to the non-equilibrium microstructure evolution. Thickness of the hardened zone was shown to be dependent on EDM operating parameters, in particular peak-current (A) and pulse on-time (s).

Cyclic behaviour simulation of X38CRMOV5-47HRC (AISI H11)-tempered martensitic hot-work tool steel

The cyclic behaviour of X38CrMoV5 (AISI H11) tool steel with a nominal hardness of 47HRC has been predicted. Basically, thermo-elastoplastic and thermo-elastoviscoplastic constitutive laws are investigated. First, various uniaxial isothermal conditions (LCF) with different strain rate, strain amplitude and temperature level are investigated. Then the constitutive laws are examined under various TMF loading conditions. The simulated results by both the approaches are compared with experimental results in terms of stress?strain behaviour and cyclic softening. Some applications of the model for simulation of thermal fatigue sample are shown. Taking into consideration the results of this work, the goal is to further characterise the limitations of these constitutive laws under complex and severe loading conditions, i.e., under variable temperature, variable strain amplitude and thermal fatigue structural specimen.

Cyclic behavior modeling of a tempered martensitic hot work tool steel

In this paper, a non-unified elasto-viscoplastic behavior model based on internal state variables, is investigated in order to describe the thermo-mechanical stress–strain fatigue response of 55NiCrMoV7 tempered martensitic steels (AISI L6). This model considers a quadratic yield criterion to define the elasticity domain. It allows the determination of two inelastic strain mechanisms resulting from two stress states which can be related to the typical continuous cyclic softening of the tempered martensitic steels. This cyclic softening is reproduced through an isotropic component (drag stress). A memory effect is also introduced to take into account the influence of the plastic strain range on the amount of the cyclic softening. The kinematic component (back stress) of the model allows the description of complex load conditions to which tool steels are subjected. Strain recovery (Baushinger effect), time recovery terms (cyclic behavior including tensile dwell times) and ratcheting effects are considered. The numerical implementation is addressed and two integration methods (explicit and implicit) of the constitutive equations are presented. Moreover, the identification methodology of the model parameters from only two sets of experimental data is presented; the coefficients of the kinematic and isotropic parts are determined successively. The main difficulty consists in reaching a good description both of the cyclic behavior for different strain rates and the ratcheting effect. Last, a validation stage of the three dimensional model is investigated from low cycle fatigue tests performed on different notched specimens.

Thermal fatigue testing of chromium martensitic hot-work tool steel after different austenitizing treatments

Tools used in die casting, hot rolling, extrusion and hot forging are exposed to high cyclic temperatures and mechanical loads. Particularly designed hot-work tool steels are frequently used to meet the application requirements, regarding a high thermal fatigue resistance as one important tool property.To increase the tool performance, i.e. tool life length, the tool steel microstructural condition may be optimised. In the present study, the thermal fatigue properties of a new martensitic chromium hot-work tool steel alloy (Uddeholm designation DIEVAR) have been evaluated with regard to the austenitizing treatment. Using four different austenitizing temperatures, the material was quenched and tempered to the same hardness of 470HV. Thermal fatigue was performed using a high frequency induction heating system, designed to simulate hot-work processing conditions, while recording the specimen surface strain with a laser speckle technique.It was found that the thermal fatigue crack growth in the tool steel was strongly dependent on the different austenitizing temperatures, as well as the maximum test temperature. Thermal fatigue resistance was assumed to rely on temper resistance at high maximum cycle temperature and on toughness and ductility at lower. The resistance to thermal fatigue could be increased by optimising the heat treatment. The surface mechanical strain during thermal cycling was estimated from the laser speckle measurements.

Residual macroscopic stress in highly preferentially oriented titanium nitride coatings deposited on various steel types

The residual macroscopic stresses present in 1.5-μm-thick TiN coatings on martensitic hot work tool steel and austenitic stainless steel are studied with a Θ−2Θ decoupled x-ray diffractometer using glancing angle x-ray diffraction optics. All coatings exhibit a preferential orientation which complicates the interpretation of the conventional sin2 Ψ method. To overcome this complication, a method is used, called the fixed crystal orientation (FCO) sin2 Ψ method, in order to determine the mechanical stress in the presence of a symmetrically distributed preferential orientation. It is shown that in contrast to the glancing angle sin2 Ψ method and the conventional sin2 Ψ method interpretational problems resulting from elastic incompatibility can be avoided using the FCO method. In the TiN coatings studied a biaxial stress state can be assumed and different stress values are measured depending on the preferential orientation and the substrate material. The stress values obtained with the FCO sin2 Ψ method for the different samples are compared with the stress values obtained using the glancing angle sin2 Ψ method and the conventional sin2 Ψ method.

Development of hot-work tool steel for high-temperature applications

Five different martensitic hot-work tool-steel alloys, two new alloys and three already established ones, have been investigated and compared with respect to decisive material properties. It is the...