Plasma Nitrided Tool Steel Research Articles

Effect of surface engineered tool steel and lubrication on aluminium transfer at high temperature

Hot stamping is characterised by severe contact conditions, especially when forming aluminium components. In order to improve the tool lifetime, process economy, and component quality, understanding the initiation mechanisms behind aluminium transfer onto the tool surface at high temperatures is critical. To date, the tribological interaction between tools and aluminium sheets at high temperature has received limited attention. Lubricants, combined with surface engineering techniques (e.g. coatings, nitriding and surface topography control), show great potential for reducing the severity of material transfer at high temperatures. However, there is still, limited knowledge about their interaction and performance in this tribological context. In this study, high temperature tribological tests were carried out to characterise the synergetic effects of surface coatings/treatments with and without lubrication on friction and wear. A commercially available lubricant was evaluated when used in combination with uncoated, nitrided and CrWN- or DLC ta-C-based PVD coated tool steel. The tests were carried out on a hot strip drawing tribometer, employing an open contact configuration representative of the hot stamping contact conditions at two different temperatures. The counter-material was a 6082 aluminium alloy, heated up following a thermal cycle relevant for the hot stamping process. The results showed that the tribological response was highly dependent on the retention of the lubricant in the contact and the type of surface modification technique. The results show that bonding of the lubricant to the tool surface is critical. In the case of lubricant failure, severe adhesive wear and aluminium transfer onto the tool surface occurred, correlated with an increase in friction. The use of different surface engineering methods led to different results: lower friction levels could be reached when combining use of lubricant and PVD coatings compared to using uncoated or plasma nitrided tool steel. In this study, the best combination to minimise aluminium transfer and friction is the association of the lubricant with CrWN PVD coating in this study.

Combined Use of Surface Texturing, Plasma Nitriding and DLC Coating on Tool Steel

In cold rolling, a textured roll can be used to imprint a desired surface topography onto the sheet during rolling. This work proposes the use of diamond-like carbon (DLC) coatings to protect the surface topography of the rolls in replacement of the carcinogenic hard chrome. For that, hydrogenated amorphous carbon (a-C:H) was deposited on plasma nitrided tool steel, both for ground and textured specimens. Changes in surface topography due to DLC coating were assessed using a confocal microscope. Coating adhesion was evaluated using the method VDI 3198. The specimens were characterized using X-ray diffraction (XRD), microhardness test and scanning electron microscopy (SEM). The coating was characterized using Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS). The results showed a soft multilayer coating consisting of a plasma nitrided layer for load support, a Si-rich interlayer to improve adhesion and an a-C:H top layer. DLC deposition reduced the roughness of the textured specimens. The coating resulted in relatively stable friction and good durability, with small damage and negligible wear even under dry sliding.

Material transfer during high temperature sliding of Al-Si coated 22MnB5 steel against PVD coatings with and without aluminium

Press hardening of Al-Si coated 22MnB5 steel is the dominant technology to enable light weight design in automotive applications. Transfer of the Al-Si coating onto the tool surface occurs during hot forming. This affects process economy and quality of produced components. The reported galling mechanisms are adhesion and compaction of wear debris. Surface engineering of forming tools has been proposed to minimise the transfer of Al-Si coating. Plasma nitriding of tool steel surfaces reduces adhesion but has poor abrasive wear resistance. PVD coatings have generally been found to promote galling due to higher chemical affinity but improve abrasive wear resistance. Most studied PVD coatings are transition metal nitrides containing aluminium. The aim of this study is to investigate the role of aluminium in PVD coatings and its effect on transfer of Al-Si coating material during sliding against coated tool steel at high temperatures. This work has focussed on PVD coatings (AlCrN and CrWN) deposited on plasma nitrided tool steel. Their tribological behaviour was studied using a hot strip-drawing tribometer capable of simulating the conditions prevalent in press hardening. The results showed that PVD coatings containing aluminium induce more material transfer. The material transfer is mainly related to chemical affinity since all coatings were polished to a low surface roughness (Sa =~120 nm) to minimise transfer initiated by surface defects. The hardness of the PVD coatings does not seem to influence the material transfer since the softer coating (CrWN, HV0.05 = ~1850) showed less transfer compared to AlCrN (HV0.05 = ~2100). The CrWN coating showed longer running-in compared to AlCrN due to reduced initial material transfer. Formation of thicker transfer layers governs the steady state friction mechanisms. Material transfer of Fe-Al intermetallic compounds occurs at the initial stages of sliding through direct adhesion to the PVD coating. The layers grow to > 5 µm thickness within a few decimetres of sliding.

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.

Effect of plasma nitriding potential on tribological behaviour of AISI D2 cold-worked tool steel

The effect of the nitrogen potential on the micro-abrasive wear resistance of AISI D2 plasma nitrided tool steel was evaluated. Plasma nitriding experiments were conducted in a pulsed plasma reactor using different nitrogen potentials. The results indicate that a nitrided layer, with finely dispersed nitrides, can be formed when shorter nitriding times and higher nitrogen potentials were used. Although the thickness of this layer is small, its wear resistance is higher than that observed at longer nitriding times. Increasing the nitriding time resulted in the formation of thicker layers, albeit with grain boundary nitrides, which are deleterious to wear resistance. For untreated material, three-body abrasive wear rolling was the dominant mechanism, and for nitrided material, the mechanism transition was not clearly observed in spite of its better wear resistance.

Galling mechanisms during interaction of tool steel and Al–Si coated ultra-high strength steel at elevated temperature

Occurrence of galling in hot forming is detrimental to the quality of produced parts and process economy. Material transfer from Al–Si coated work-piece to the tool material has been studied in this work. PVD coatings (AlCrN, TiAlN and DLC) on tool steel substrate have been considered as well as plasma nitriding and their tribological behaviour was compared to the case of an untreated tool steel. Galling initiates through accumulation and compaction of wear debris when untreated tools are used whereas the PVD coatings resulted in increased galling due to adhesion. Plasma nitrided tool steel showed negligible galling due to formation of glaze layers and the formation of such layers depends on the occurrence of wear of the nitrided tool steel.

Isothermal oxidation of plasma-nitrided hot-work tool steel at 750°C

Isothermal oxidation of plasma-nitrided tool steel at 750°C, typical of steel thixoforming process, was investigated. A multi-layered oxide forms on the surface of plasma nitrided X32CrMoV33 hot work tool steel when held at 750°C in an air furnace for 6 hours. The hematite layer at the surface is replaced by magnetite in the middle. The innermost layer is an iron –chromium spinel with approximately 4 at% Cr. These features are typical of oxidised low-Cr steels and suggest that nitriding does not affect the response to thermal exposure of hot work tool steels at elevated temperatures. It is thus concluded that hot-work tool steels are inadequate for tooling in the steel thixoforming process even when plasma-nitrided. The substrate tool material must contain higher levels of Cr to assure a slowly growing, adherent Cr2O3 layer to reduce the overall oxidation rate.

Isothermal oxidation of plasma-nitrided hot-work tool steel at 750°C

Isothermal oxidation of plasma-nitrided hot-work tool steel at 750°C

Residual Stress Field Analysis and Prediction in Nitrided Tool Steel

Residual stresses are present in engineering components as an unintended consequence of manufacturing processes, but they are also deliberately introduced to beneficial effect during surface engineering procedures. Plasma nitriding is a process of particular importance for forming tools and dies, giving significant advantages in wear, and fatigue resistance through the generation of near-surface compressive residual stresses. A precise knowledge of the level and distribution of residual stresses that exist in engineering components is necessary for an accurate design and prediction of a component's fatigue resistance. However, measurement of residual stresses is not always possible, which is especially true for forming tools. Therefore, other methods for residual stress evaluation and prediction are required. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase with nitriding time and temperature. On the other hand, experimental data indicates that the residual stress distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution. A minimum in the microhardness depth profile derived over depth corresponds to the location of the compressive residual stress maximum. Furthermore, the residual stress level can be extrapolated by using reference residual stress data and genetic programming. In this way the residual stress level and distribution can be obtained even for components where measurement is not possible.

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe–Cr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe–Cr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite–spinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N2 gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.

Analysis and Prediction of Residual Stresses in Nitrided Tool Steel

Plasma nitriding is a thermo-chemical process of high importance for engineering components, which through generation of near-surface compressive residual stresses significantly improves wear and fatigue resistance. A precise knowledge of the level and distribution of residual stresses that exist in surface engineered components is necessary for accurate prediction of a component’s fatigue resistance. However, measurement of residual stress is not always possible, especially in the case of industrial tools and dies. Therefore, other methods for residual stress evaluation and prediction are required by industry. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase by nitriding time and temperature. On the other hand, experimental data show that residual steel distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution.

Effect of post-oxidation treatment on thermal fatigue behaviour of plasma nitrided hot work tool steel at elevated temperatures

The post-oxidation treatment at 500 °C produced a dense, adherent oxide film on the compound layer. This oxide layer, however, failed to improve the oxidation resistance and the plasma nitrided tool steel subsequently given a post-oxidation treatment failed in nearly the same fashion as the tool steel thermally cycled without the post-oxidation treatment. The thermal stresses acting on the surface during much of the thermal cycle lead to the spallation of the oxide scales, generating fresh surfaces for further oxidation. This sequence is repeated throughout thermal cycling leading to substantial material loss from the centre of the front face of the thermally cycled samples. The thermal fatigue test was terminated after only 500 cycles due to extensive surface damage before there was any evidence for thermal fatigue cracking. Much of the section has suffered considerable loss of mechanical strength due to thermal exposure. The tempered martensitic structure was replaced by fine, equiaxed ferritic grains near the surface. The martensite to ferrite transformation was accompanied by dynamic recrystallization. The selective recrystallization of the nitrided surface layer evidences the relatively higher stresses encountered at the surface.

Influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel

The influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel, of commercial name “DC53”, in 3.5% NaCl solution is reported. The specimens were nitrided at 520 °C for different treatment times using a constant [N2 + H2] gaseous mixture by a DC-pulsed plasma system. The microstructure of the nitrided layers was investigated by optical microscopy and X-ray diffraction. The corrosion behavior was evaluated by potentiodynamic polarization experiments. The plasma nitriding process considerably improves the corrosion resistance of material in NaCl environment as compared to the unnitrided DC53 steel. The modified surface layer consisting mainly of e-nitride (Fe2–3N) and a small amount of γ′-nitride (Fe4N) confers this outstanding behavior. The corrosion resistance dependence on specific nitriding processes is reported and the role of the e-nitride is discussed. In particular, the correlation of pitting current density, density of pits, and volume fraction of e-nitride with nitriding time is analyzed. The results denote that the most important parameter for controlling the corrosion resistance of the material is the volume fraction of e-nitride and the nitrided layer thickness. It is expected that a nitrided layer would be thicker and rich in e-nitride phase to achieve a high corrosion resistance.

Tribological performance of surface engineered tool steel at elevated temperatures

Abstract Tribological components operating at elevated temperatures can experience high wear, oxidation, thermal fatigue and changes in mechanical properties. In this work, the friction and wear characteristics of plasma nitrided and surface coated (CrN and TiAlN) tool steel during sliding against AISI52100 bearing steel have been studied at room temperature and 400 °C respectively using a ball on disc machine. Surface profiler and SEM/EDS techniques were used to characterise the surface topography and resulting surface damage of the test specimens. The results show that the friction of plasma nitrided tool steel during sliding against bearing steel ball is very high at room temperature and it drastically drops at 400 °C. The wear is mainly abrasive at room temperature and adhesive at elevated temperatures. In case of CrN coated tool steel the friction is high but its wear is negligible at room temperature. At 400 °C, the friction decreases marginally and transfer of bearing steel to the coated CrN coated disc has been observed. The TiAlN coating has shown relatively lower friction, compared to CrN and negligible wear at room temperature. At 400 °C, the friction is very high and unstable and transfer of TiAlN coating to the mating ball occurs.

Atomic transport and chemical reaction in TiN/Ti nanolayers on plasma nitrided steel

Transport of N, O, and Ti during dc magnetron sputtering deposition of nanoscopic TiN/Ti and TiN structures on plasma nitrided M2 tool steel, as well as transport of metallic species composing the plasma nitrided steel substrates were investigated. N and O depth distributions were determined with subnanometric resolution using narrow resonant nuclear reaction profiling, whereas Ti was profiled, also with subnanometric depth resolution, by medium energy ion scattering. The surface elementary compositions of the TiN/Ti/nitrided steel and TiN/nitrided steel structures were determined by low energy ion scattering. The chemical compounds formed during deposition were accessed by X-ray photoelectron spectroscopy, indicating the presence of TiN, TiO2, Ti oxynitrides, as well as other metallic nitrides and oxynitrides, but no metallic Ti was observed. Owing to the observed intensive atom mobility, the compositions of the deposited films on plasma nitrided steel structures varied continuously on a nanoscopic scale, from the core of the steel substrate to the bulk of the stoichiometric TiN films. The Ti interlayer assists interdiffusion of all species, in contrast to the TiN film layer, which is known to be a diffusion barrier. The improved adhesion of TiN hard coatings to plasma nitrided steel under working conditions is discussed in terms of the gradual compositional change around the interfaces and the atomic mobility during their formation.

S20 Prediction of Residual Stress Distribution in Plasma Nitrided Tool Steel

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Formation of a nanostructured CrN layer on nitrided tool steel by low-temperature chromizing

A duplex chromizing process involving the salt bath chromizing of a plasma-nitrided tool steel was carried out at 773 K for 3–24 h. A single-phase CrN layer with a grain size ranging from 20 to 50 nm and an underlying layer consisting of α-Fe and nanosized CrN grains with a grain size ranging from 40 to 80 nm were formed by a 24 h chromizing at 773 K. The duplex chromizing mechanism was discussed.

PVD coatings deposited onto plasma nitrided X37CrMoV5-1 type steel

The purpose of this paper was characterisation of the structure, mechanical and tribological properties of the monolayers CrN, TiN and multilayers TiN/(Ti,Al)N PVD coatings deposited onto plasma nitrided X37CrMoV5-1 type hot work tool steel. Taking into account the results of measurements, one can state that the lowest wear at certain conditions in both room and elevated temperatures displays the TiN/(Ti,Al)N multilayer coating, what can be connected with their good adherence to the substrate materials as well as the highest hardness. The investigation results will provide useful information to applying the PVD coating for the improvement of wear resistance of tools made from hot work steels. The paper contributes to better understanding of the structure, adhesion and the wear resistance at the elevated temperature up to 500°C of CrN and TiN, TiN/(Ti,Al)N PVD coatings deposition onto hot work tool steel X37CrMoV5-1 type. [Received on 25 October 2005; Accepted on 12 February 2007]

Modelling of residual stress profiles in plasma nitrided tool steel

Residual stresses may be present in engineering components as an unintended consequence of manufacturing processes, but they may also be introduced deliberately to beneficial effect during surface engineering procedures. Plasma nitriding and nitrocarburizing are such processes of particular importance for tool steel components used in tool, die and machine applications. These processes afford significant advantages, such as greatly improved wear and corrosion resistance, as well as fatigue strength resulting from the generation of near-surface compressive residual stresses. A precise knowledge of the level and distribution of residual stresses that exist in such engineering components is necessary for product development and quality control purposes, as well as for the accurate prediction of fatigue resistance. If located at the same depth as the Hertzian stress concentrations, compressive residual stresses can greatly improve the fatigue properties of loaded surfaces. Therefore, reliable methods for residual stress determination are required, with residual stress measurement techniques having their limitations. The aim of the present paper was to correlate the residual stress distribution in plasma nitrided and nitrocarburized H11 tool steel with the microhardness depth profile, in order to determine whether a relatively simple microhardness-related parameter could be used to predict the depth of the maximum compressive residual stresses, and to thereby enable optimization of the nitriding parameters for specific contact conditions.

XPS and ion scattering studies on compound formation and interfacial mixing in TiN/Ti nanolayers on plasma nitrided tool steel

The adhesion improvement produced by bright plasma nitriding hardening of tool steel prior to TiN deposition was investigated by angle-resolved X-ray photoelectron spectroscopy and atomic profiling with sub-nanometric depth resolution. Steel hardening is considered to be a consequence of homogeneously distributed, very fine iron and chromium nitride precipitates existing in the N-diffused zone formed below the surface by bright plasma nitriding, a picture confirmed here by the chemical analysis of the near-surface regions of the steel substrate. Besides hardening, analysis of the O1s, N1s, Ti2p, Fe2p, and Cr2p photoelectron groups showed that the finely dispersed chromium and iron nitrides produced by plasma nitriding also replace in part the oxides and hydroxides in the near-surface regions. Ti profiling with medium energy ion scattering and N profiling with narrow resonant nuclear reaction pointed out the interdiffusion of at least these two species near the interfaces. Thus, the enhanced adhesion and improved performance observed in duplex treated tool steel are discussed in light of the present findings: i) stronger chemical bonding at the coating/substrate interface promoted by bright plasma nitriding of the steel substrate prior to TiN deposition and ii) enhanced interdiffusion around the coating/substrate interface owing to the removal of the oxides which constitute diffusion barriers, producing an enlarged contact area and more gradual coating/substrate interfaces.