Peter Mayr

Combinations of coating and heat treating processes: establishing a system for combined processes and examples

Abstract High resistance of metals against wear, fatigue and corrosion can be achieved by several different treatments, like thermal, mechanical, thermochemical and coating processes. Combining successful single processes into one treatment can result in an even higher resistance of materials against complex loads, e.g. superimposed wear, fatigue and corrosion, because of the addition of the single process advantages. For a substantial choice of technical and economical promising combinations, a classification for combined processes was set up. The single processes were divided into five groups: thermal, mechanical, thermochemical, ion implantation and coating processes. A 5×5 matrix out of these five groups was set up, which contains a large number of possible combined processes. This matrix holds for steels as well as for non-ferrous alloys. The potential of different combined processes was analyzed theoretically and experimentally. Several combined processes for steels and non-ferrous metals were reviewed. Popular combined processes for steels are thermochemical treatment and coating as well as coating and thermal treatment. Examples like carburizing and CVD, nitriding and PVD, CVD and quench hardening, and CVD and induction hardening will be presented. The combined process CVD and quench hardening illustrates the principle of combined processes: The high hardness of the thin CVD-coating is supported by the high strength of the quench hardened steel substrate. Examples for non-ferrous alloys are plasma nitriding and precipitation hardening of aluminum alloys and also nitriding and CVD of titanium alloys. These examples will highlight the great potential of combined processes.

In situ and ex situ examination of plasma-assisted nitriding of aluminium alloys

Abstract Pre-sputtering and plasma-assisted nitriding of pure aluminium and 2024 aluminium alloy were studied in situ by plasma monitoring and ex situ by several surface analysis methods. The influence of the process parameters, time, temperature, pressure and gas composition on mass and energy distributions of ions, as well as on topography and chemical composition, was examined. The chronology of the sputtering process could thus be clarified. An oxide film which is relatively thick compared to the native oxide layer and enriched with magnesium is formed at the initial stage of the sputtering treatment, followed by the removal of this film with a subsequent roughening of the surface. The cleaning effect of the plasma can be enhanced by increasing the argon pressure or by an intermixture of hydrogen. Due to a supplementary chemical etching, hydrogen admixture reduces the surface roughness and effectively decreases the oxygen content of the surface. During the nitriding treatment, nanocrystalline hexagonal aluminium nitride AlN is formed. Growth rate and nitrogen content of the nitride layer were found to strongly depend on the gas composition during sputtering and nitriding. The positive effect of addition of argon and hydrogen, respectively, arises from an intensification of the nitrogen plasma, the production of nitrogen–hydrogen molecular ions and an increase of the ion energy. The nitrogen pressure affects the layer thickness due to higher nitrogen activity and ion energies. The chemical composition of the cathode material influences discharge current and, as a result, the ion energies.

Anisotropic phase transformation strain in forged D2 tool steel

Abstract Heat treatment of forged D2 tool steel produces anisotropic dimensional change. It has been observed that tools have a larger dimensional change along the forging direction than perpendicular to it. This paper investigates anisotropic dimensional change by means of dilatometry. The results show that anisotropic phase transformation strain is produced in forged D2 steel during heat treatment. The anisotropic transformation strain is the main reason for anisotropic distortion in heat treatment of forged D2 steel. Anisotropic transformation strain produced during martensite transformation increases with higher austenitising temperature and is little influenced by cooling rate. A suggested mechanism is that transformation induced plasticity is produced under the internal stresses caused by the anisotropic microstructure (carbide bands) in the steel.

Effect of machining and heating parameters on distortion of AISI 52100 steel bearing rings

In this work, the influences of different heating parameters on distortion of bearing rings (bearing steel AISI 52100, EN 100Cr6) are examined. The rings were machined in two different ways, which led to significant differences in roundness and residual stress distribution after machining. The heating parameters (heating rate, austenitising temperature, and stacking arrangement) were combined in a 23 full factorial matrix. In addition, the influence of preheating and soaking was examined. The cooling rate was very slow in order to avoid additional distortion during the cooling process. Also, distortion after heating and slow cooling is compared to that after quench hardening. The distortion of the rings during austenitising and slow cooling, in terms of change in roundness and change in flatness, is mainly influenced by stacking arrangement. The heating rate has a significant effect on the change in flatness and also influences the change in roundness. In addition, interactions of heating rate and stacking arrangement influence the change in flatness.

Plasma monitoring of plasma-assisted nitriding of aluminium alloys

Abstract To improve the plasma-assisted nitriding process of aluminium alloys it is necessary to obtain knowledge about the underlying reaction mechanisms. A suitable diagnostic tool to clarify these mechanisms is plasma monitoring, which provides mass- and energy-resolved analysis of the ions hitting the substrate surface in a glow discharge. Application of this technique to plasma-assisted nitriding of pure aluminium and the 2024 aluminium alloy is demonstrated. Mass spectra and energy distributions of the major ions were recorded during sputtering in an argon atmosphere and nitriding in pure nitrogen. The energy distributions of the gas ions are mainly determined by resonant charge exchange collisions, whereas the ions of the sputtered metal atoms reach the cathode almost without interaction. The influence of the process parameters, temperature and working pressure, on the discharge characteristic were examined. Changing the substrate temperature did not significantly affect the ion energy distributions, whereas a reduction of the working pressure increased the collision probability of the ions due to a disproportionate elongation of the cathode fall.

Induction surface hardening of hard coated steels

Abstract The properties of hard coatings deposited using CVD processes are usually excellent. However, high deposition temperatures negatively influence the substrate properties, especially in the case of low alloyed steels. Therefore, a subsequent heat treatment is necessary to restore the properties of steel substrates. Here, induction surface hardening is used as a method of heat treatment after the deposition of TiN hard coatings on AISI 4140 (DIN42CrMo4) substrates. The influences of the heat treatment on both the coating and the substrate properties are discussed in relation to the parameters of induction heating. Thereby, the heating time, heating atmosphere and the power input into the coating–substrate compounds are varied. As a result of induction surface hardening, the properties of the substrates are improved without losing good coating properties. High hardness values in the substrate near the interface allow the AISI 4140 substrates to support TiN hard coatings very well. Consequently, higher critical loads are measured in scratch tests after the heat treatment. Also, compressive residual stresses in the substrate are generated. In addition, only a very low distortion appears.

Microstructure and property changes caused by diffusion during CVD coating of steels

Abstract The interdiffusion of carbon and titanium between a substrate and coating during chemical vapor deposition (CVD) coating of steels influences the microstructures and properties of the coating/substrate compounds. The CVD parameters atmosphere, temperature and pressure as well as the composition of the steel substrates were investigated systematically to optimize the properties of the coating/substrate compounds. The tool and carbon steels AISI H13 (DIN X40CrMoV5-1), AISI A2 (DIN X100CrMoV5-1), AISI 1045 (DIN Ck45) and ∼AISI 1095 (DIN 100V1) were coated by high-temperature CVD TiN, low-pressure CVD TiN and moderate-temperature CVD TiCN. Chemical compositions of coatings and substrates were measured by glow discharge optical spectroscopy (GDOS) and electron probe microanalysis (EPMA). Substrate and coating microstructures were examined in taper sections by light microscopy. Further, the substrate hardness was investigated in the taper sections. In an earlier work, carbon diffusion near the interface was investigated. Low coating pressures combined with high temperatures and high carbon activities of the substrates promoted the diffusion process. In this work, the investigations were extended to deeper substrate regions and to the titanium diffusion. From the carbon profiles in coatings and substrates, a carbon balance was set up for the diffusion from the substrate to the coating during CVD.

Enhancing Surface Hardness of Titanium Alloy Ti-6Al-4V by Combined Nitriding and CVD Coating

Abstract Titanium alloys show a relatively high strength to density ratio and good corrosion resistance. Their main disadvantage in many applications is the low hardness and hence low resistance to abrasive wear. The wear resistance of titanium alloys can be enhanced by nitriding, but the compound layer thickness is limited. The requirement of thicker compound layers could be fulfilled by a combination process. The duplex process of nitriding plus moderate temperature chemical vapour deposition (MTCVD) TiCN coating of titanium alloys appears promising. Either pressure nitriding or gas nitriding can be applied to Ti-6Al-4V. With pressure nitriding, homogeneous compound layers can be produced on complex shaped components; however, the combined process must be carried out discontinuously in two reactors, due to the different process parameters. When using gas nitriding, the combined process can be performed continuously in the CVD equipment. With the combined nitriding +MTCVD coating route, thick compound layers could be produced in relatively short process times. The surface microstructures consist of a nitrogen diffusion layer, an intermediate layer, a compound layer, and a TiCN coating. The Ti-6Al-4V surface was characterised by high hardness and good layer adhesion.

Investigation of creep damage in advanced martensitic chromium steel weldments using synchrotron X-ray micro-tomography and EBSD

Abstract In recent years, a design concept for the stabilisation of the microstructure by addition of boron and nitrogen was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening by solid solution with precipitation strengthening by finely dispersed nitrides. Welded joints of MARBN steels showed no formation of a uniform fine grained region in the heat affected zone (HAZ) which is in general highly susceptible to Type IV cracking. In this work, the crossweld creep strength of a newly developed MARBN steel was analysed and the evolution of damage was investigated using synchrotron microtomography supported by electron microscopy. Three-dimensional (3D) reconstructions of the tested samples together with electron backscatter diffraction investigations revealed an intense void formation in a restricted area along small grains at prior austenite grain boundaries in the HAZ as the main reason for premature creep failures in the HAZ of welded joints.

The Impact of Weld Metal Creep Strength on the Overall Creep Strength of 9% Cr Steel Weldments

In this work, three joints of a X11CrMoWVNb9-1- (P911) pipe were welded with three filler metals by conventional arc welding. The filler metals varied in creep strength level, so that one overmatched, one undermatched, and one matched the creep strength of the P911 grade pipe base material. The long-term objective of this work was to study the influence of weld metal creep strength on the overall creep behavior of the welded joints and their failure mechanism. Uniaxial creep tests at 600°C and stresses ranging from 70 MPa to 150 MPa were performed on the cross-weld samples of all three welds. A total creep testing time of more than 470,000 h was accumulated. The longest running sample achieved a time-to-rupture of more than 45,000 h. Creep testing revealed that the use of undermatching weld metal led to a premature fracture in the weld metal at higher stress levels. Compared with undermatching weld metal, the use of matching and overmatching filler materials increased the time-to-rupture at high stress levels by 75% and 33% at lowest stress levels. At typical component stresses below 100 MPa, all samples failed in the grain-refined heat-affected zone by characteristic type IV failure. For investigations of the failure modes, cross sections of fractured samples were investigated by optical light microscopy, scanning electron microscopy, and electron backscatter diffraction. The mechanism of weld metal creep failures and type IV creep failures is discussed in detail.