PM Tool Steels Research Articles

On the critical roles of initial plastic deformation and material transfer on the sliding friction between metals

It is well known that the friction level rapidly shifts during the initial sliding between two unlubricated metals. Often this change is so rapid that the initial friction level is not even noticed, owing to initial surface contamination or roughening of the contacting surfaces, either due to plastic deformation and wear, or transfer.The present work uses a unique experimental set-up – the Uppsala Load Scanner – to generate detailed information on friction and surface modification in initial sliding contact between pairs of crossed metal cylinders. A wide range of loads, each exclusively coupled to a well-defined contact area on both contacting bodies, is evaluated in a single test. By performing repeated scans, intertwined with high resolution FEG-SEM studies of the contact surfaces, investigations of the couplings between local load, friction, surface modification, topography, transfer, etc. is facilitated and the progress of deformation and friction can be followed. The SEM studies also include local FIB cross sectioning of selected samples to learn more about the sub-surface modifications.The materials selected include a PM tool steel, a case-hardened component steel and an aluminium alloy. All tests were conducted in dry contact.The roles of the large initial surface roughening or flattening, the following material transfer and other gradual surface modifications in determining the friction level, are discussed.

Development of a New PM Tool Steel for Optimization of Cold Working of Advanced High-Strength Steels

In the present study, Uddeholm Vancron SuperClean cold work tool steel was investigated concerning wear resistance and fatigue strength, using laboratory and semi-industrial tests. The Uddeholm Vancron SuperClean was designed with the help of ThermoCalc calculations to contain a high amount of a carbonitride phase, which was suggested to improve tribological performance of this tool steel. In order to investigate the tested steel, galling tests with a slider-on flat-surface tribotester and semi-industrial punching tests were performed on an advanced high-strength steel, CP1180HD. Uddeholm Vanadis 8 SuperClean containing only a carbide phase and Uddeholm Vancron 40 containing a mixture of carbides and carbonitrides were also tested to compare the performance of the tool steels. The microstructure and wear mechanisms were characterized with scanning electron microscopy. It was found that the carbonitrides presented in Uddeholm Vancron SuperClean improved its resistance to material transfer and galling. Semi-industrial punching tests also confirmed that Uddeholm Vancron SuperClean cold work tool steel also possesses enhanced resistance to chipping and fatigue crack nucleation, which confirms the beneficial role of the carbonitride phase in wear resistance of cold work tool steel.

Local Adjustment of Surface Integrity of Forming Tools by Adaptation of Tool Making Process

Within sheet-bulk metal forming manufacturing of functional elements, like e.g. gears, causes distinctive tool stresses. Since highest stresses occur at the surface of active tool components, the tool surface has major influence on the tool performance. Within the present study, the influence of a local surface treatment on the surface integrity of an AISI M3:2 PM tool steel was investigated. In this regard, a laser heat treatment was implemented in a conventional process chain consisting of WEDM and post machining by blasting and shot peening. Due to an enhancement of ductility of the surface region, higher compressive residual stresses can be achieved for the adapted process chains after the final peening step.

Galling resistance and wear mechanisms for cold-work tool steels in lubricated sliding against high strength stainless steel sheets

Tool damage in sheet metal forming of stainless steel is of high concern for the forming industry. In the present work, ingot cast AISI D2 and advanced powder metallurgy tool steel (PM) cold-work tool steels were evaluated and ranked regarding wear mechanisms and galling resistance. Wear tests were performed using a slider-on-flat-surface (SOFS) tribometer in sliding against austenitic–ferritic (duplex) stainless steel sheets at different contact pressures in lubricated conditions. The best galling resistance was observed for the nitrogen alloyed PM tool steels. Abrasive scratching of the tool surfaces and transfer of sheet material due to adhesive wear were the main metal forming tool surface damage mechanisms. By increasing the hardness of one PM sheet metal forming tool grade, the galling resistance was enhanced.

Quantification of Carbide Distribution in PM Tool Steels with Niob Addition

Powder of tool steel STN 19830 with 1.9 wt.% of Nb addition was prepared by atomization. Compact material was prepared using of hot isostatic pressing (HIP) and followed by heat treatment to improve the carbides distribution. The aim of this work was to evaluate and quantify the influence of heat treatment parameters on size, shape and spatial distribution of the carbides. Light optical microscopy and image analysis was used to obtain relevant data. Area fraction of carbides was used as parameter of carbide distribution in steel matrix. Sensitivity and accuracy of this method was compared with technique based on Voronoi tessellation. Result shows that size characteristics of tessellation and area fraction analysis are both sufficiently sensitive to indicate the deviation between different states of the respective material and could be used for quantification of size and spatial distribution of the carbides in PM tool steels. Positive effect of used heat treatment on size and distribution of carbides was verified.

Critical investigation of high temperature gas nitriding of a PM tool steel

AbstractThe nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogenbased case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metallurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230°C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified.

Corrosion of stainless PM tool steels: Interpretation of current density potential curves in acid environments

Abstract For testing the corrosion resistance of metallic materials, the measurement of current density potential curves in acid environments e. g. sulphuric acid is a widely used method. These curves are usually evaluated by means of the current density in the passive area, the maximum current density in the active peak and the width of the passive area. Stainless cold work tool steels however feature a more complex behavior which pushes this approach to a limit. Thus, a more detailed evaluation is necessary to understand the current density potential curves of such multiphase-materials. In this paper the relation of the curve progression and corrosion phenomena on the microscopic scale were investigated in order to contribute to a better understanding of experimental data.

Influence of hard particle addition and chemical interdiffusion on the properties of hot extruded tool steel compounds

Low alloyed steel bars were co-extruded with pre-sintered tool steel powders with the addition of tungsten carbides (W 2C/WC) as hard particles. During the hot extrusion process of these massive and powdery materials, an extrudate is formed consisting of a completely densified wear resistant coating layer and a bulk steel bar as the tough substrate core. This work combines experimental measurements (EPMA) and diffusion calculations (DICTRA™) to investigate the effect of hard particle addition and its dissolution, as well as the formation of M 6C carbides on the properties of two different PM tool steel coatings hot extruded with a 1.2714 steel bar. A carburization effect resulting from the W 2C hard particles is responsible for an increase of the 1.2344 steel matrix hardness. The mechanical properties of the interface region between coating matrix and substrate are influenced by chemical interdiffusion of carbon and other alloying elements occurring during heat treatment.

PM tool steels push the edge in fatigue tests

Austrian researchers fatigue tested PM cold work steel and high speed steels and found that they had remarkable fatigue resistance compared with similar conventional ingot steels. The results they obtained resembled those for high strength bearing and spring steels and they concluded that fatigue limits for these PM tool steels did not exist…

Mechanical properties of co‐extruded wear resistant powder metallurgical layers on steel substrates

AbstractMaterials with high resistance against abrasive wear are of interest for many tool applications. For economical reasons, thick coatings of several millimetres are requested. The cladding of these materials to low alloyed substrates is commonly performed using hot isostatic pressing, being a cost intensive process in particular for long products. Thus, a novel manufacturing route via direct hot extrusion of encapsulated bulk steel bars and presintered tool steel powders was recently developed. In this manner, wear resistant claddings of PM tool steels and wear resistant MMC on steel substrates could be processed.Heating to process temperature leads to presintering of the powder and only a weak bonding between the steel substrate and the powdery layer. However, after direct hot extrusion at 1150 °C an interface free of macroscopic defects is formed between both materials. The quality and strength of this bond zone was investigated by micro tensile, 4 point bending and shear tests for different materials combinations. For high strength substrate materials, failure always occurs in the brittle wear resistant layer and not at the interface. These results are in agreement with microstructural investigations, exhibiting a pore‐ and defect‐free interface dominated by interdiffusion processes.

Damage mechanisms in materials for cold forging dies under loading conditions typical for dies

PM-tool steels are the most widely used materials for cold forging dies. The lifetime of such dies is mainly controlled by fatigue damage, which consists of the fatigue crack initiation and fatigue crack growth. A special technique was developed in order to characterise the fatigue crack initiation under stress ratios which are typical in cold forging dies. The fatigue cracks are simultaneously generated by the cracking of carbides, debonding of carbide/metallic matrix interfaces or by crack initiation in the matrix. Fatigue crack growth of these high-strength tool steels is plasticity-controlled and comparable to that of ductile materials.

Comprehensive Wear Study on Powder Metallurgical Steels for the Plastics Industry, Especially Injection Moulding Machines

M390 microclean® of Böhler Edelstahl is a powder metallurgical plastic mould steel with a high level of corrosion and wear resistance and therefore often used in the plastics processing industry. But as a consequence of rapidly advancing developments in the plastics processing industry the required level of wear resistance of tool steels in this field is constantly rising. For that reason a new PM tool steel with higher hardness values and an increased amount of primary carbides has been developed to improve the resistance against abrasive and adhesive wear. The wear resistance of both steels against adhesive situations for components of the plastification unit of injection moulding machines has been tested with a novel method. In case of processing polyolefins with an injection moulding machine it was found that there is adhesive wear between the check-ring and the flights of the screw tip of the non-return valve under certain circumstances. The temperature in that region was measured with an infrared temperature sensor. The existence of significant peaks of that signal was used as an indicator for an adhesive wear situation.

Duplex surface treatment of the Nb-alloyed PM tool steel

In this work we studied the influence of the nitriding atmosphere composition on hardness, microstructure, phase composition and wear resistance of duplex treated cold work PM tool steel alloyed with niobium (2.6 wt.%). Pulsed-plasma nitriding was carried out at 500 °C for 180 min, the atmosphere composition was N2:H2=1:3 and 1:6. After nitriding, the material was PVD coated (magnetron sputtering) with TiAlN layer. The PVD coat was also applied upon a non-nitrided material. We found out that when both nitriding conditions were applied the wear resistance was significantly higher than the PVD — only coated material. The best wear resistance was achieved when the TiAlN coating was supported by a layer applied by nitriding in a lower-nitrogen atmosphere (N2:H2=1:6). These conditions prevented formation of an undesirable compound layer in the nitrided layer.

Wear and corrosion resistance of a plasma-nitrided PM tool steel alloyed with niobium

The aim of this work was to evaluate the corrosion and abrasive-wear resistance of plasma-nitrided PM tool steel with increased content of niobium (2.6 wt.%). It was found that the wear behaviour was significantly influenced not only by surface hardness, but also by other factors, mainly by surface roughness and by the presence and properties of a compound layer. The maximum wear resistance was achieved by plasma nitriding at 500 °C for 180 min. It was also shown that nitriding led to an increase of the free corrosion potential and that the compound layer on the surface acted as a barrier against corrosion attack. The nitriding conditions that resulted in the maximum wear resistance also provided significantly increased corrosion resistance. In addition, it was observed that the conventional gas nitriding conducted at the same conditions did not produce such significant improvement of the wear resistance.

Transverse rupture strength of a PM tool steel

Powder Metallurgy has been reported as a suitable alternate processing route for the manufacture of tool steels. The advantage of this technique is in being able to obtain a refined and more uniform microstructure that improves properties such high wear resistance and toughness. A molybdenum containing AISI M3:2 tool steel, (trade name Sinter 23), manufactured from spherical gas-atomized powders by hot isostatic pressing followed by hot working was tested in three-point bending tests after various heat treatments. Transverse rupture strength (TRS) samples were cut and heat treated at four distinct austenitizing temperatures. Each austenitizing temperature was combined with three tempering temperatures, giving a total of twelve different hardening conditions. Hardness tests were carried out to establish correlations among the effectiveness of heat treatment, the hardness values and the TRS results. At least five parallel samples were tested in each heat treatment condition.

Pulsed-plasma nitriding of a niobium–alloyed PM tool steel

The aim of this work is to describe the processes during plasma nitriding of Nb-containing, powder metallurgy prepared (PM) tool steel. PM steel was studied containing 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb and 1.0% W, produced by melt atomization and hot isostatic pressing. The hardened and tempered steel was plasma nitrided at temperatures ranging from 470 to 530 °C. The nitriding time was 30–180 min. Optical microscopy, electron microscopy, XRD, EDS, WDS and hardness measurements were used to study the steel's properties, microstructure, phases and chemical composition. The nitriding temperature of 470 °C was found to be unsuitable for practical use because the layers prepared at this temperature were non-uniform. The compound layer formed by M 2,3(C,N) and M 4N was found after nitriding at temperatures of 500 and 530 °C for at least 60 min. The effect of niobium on the formation of this layer was studied.

Thermal treatment of PM-tool steel alloyed with niobium

The work is aimed at describing the influence of thermal treatment on the properties of a tool steel containing (in wt.%) 2.5% C, 3.3% Si, 6.2% Cr, 2.2% Mo, 2.6% V, 2.6% Nb, 1.0% W prepared by the powder metallurgy process. Alloying with niobium positively affects the microstructure, the mechanical properties and the performance of the steel. The microstructure of the hardened steel is composed of a martensitic matrix, retained austenite and various types of carbide. During tempering of the hardened steel at the temperatures above 480 °C precipitation of very fine carbides occurs, which results in secondary hardening. The optimum thermal treatment of the steel is austenitisation at 1050–1100 °C, nitrogen cooling and tempering at 540 °C (3×1 h).

Properties of PM high-speed and tool steels made with the newest technology : R. Rabitsch et al. (Böhler Edelstahl GmbH., Kapfenberg, Austria.)

Properties of PM high-speed and tool steels made with the newest technology : R. Rabitsch et al. (Böhler Edelstahl GmbH., Kapfenberg, Austria.)

Effects of process conditions on structure and abrasive wear of PM tool steels

Effects of process conditions on structure and abrasive wear of PM tool steels

High performance wrought PM tool steels : R.B.Dixon. (Crucible Research, USA.)

High performance wrought PM tool steels : R.B.Dixon. (Crucible Research, USA.)