Olaf Kessler

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.

Dissolution and Precipitation Behaviour during Continuous Heating of Al–Mg–Si Alloys in a Wide Range of Heating Rates

In the present study, the dissolution and precipitation behaviour of four different aluminium alloys (EN AW-6005A, EN AW-6082, EN AW-6016, and EN AW-6181) in four different initial heat treatment conditions (T4, T6, overaged, and soft annealed) was investigated during heating in a wide dynamic range. Differential scanning calorimetry (DSC) was used to record heating curves between 20 and 600 °C. Heating rates were studied from 0.01 K/s to 5 K/s. We paid particular attention to control baseline stability, generating flat baselines and allowing accurate quantitative evaluation of the resulting DSC curves. As the heating rate increases, the individual dissolution and precipitation reactions shift to higher temperatures. The reactions during heating are significantly superimposed and partially run simultaneously. In addition, precipitation and dissolution reactions are increasingly suppressed as the heating rate increases, whereby exothermic precipitation reactions are suppressed earlier than endothermic dissolution reactions. Integrating the heating curves allowed the enthalpy levels of the different initial microstructural conditions to be quantified. Referring to time–temperature–austenitisation diagrams for steels, continuous heating dissolution diagrams for aluminium alloys were constructed to summarise the results in graphical form. These diagrams may support process optimisation in heat treatment shops.

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.

Continuous Cooling Transformation (CCT) Diagram of Aluminium Alloy Al-4.5Zn-1Mg

Age hardening is one of the most important processes to strengthen aluminium alloys. It usually consists of the steps solution annealing, quenching and aging. For heat treatment simulations as well as for the appropriate choice of quenching processes in heat treatment shops, knowledge of the temperature- and time-dependent precipitation behaviour during continuous cooling is required. Quenching should happen as fast as necessary to reach high strengths, but also as slow as possible, to reduce residual stresses and distortion. This optimal quenching rate of an aluminium component depends on its chemical composition, initial microstructure and solution annealing parameters as well as on its dimensions. Unfortunately continuous cooling transformation (CCT) diagrams of aluminium alloys do almost not exist. Instead isothermal transformation (IT) diagrams or given average quenching rates are used to estimate quenching processes, but they are not satisfying neither for heat treatment simulations nor for heat treatment shops. Thermal analysis, especially Differential Scanning Calorimetry (DSC) provides an approach for CCT-diagrams of aluminium alloys, if the relevant quenching rates can be realized in the DSCequipment. The aluminium alloy Al-4.5Zn-1Mg (7020) is known for its relatively low quenching sensitivity as well as for its technical importance. The complete CCT-diagram of 7020 with cooling rates from a few K/min to some 100 K/min has been recorded. Samples have been solution annealed and quenched with different cooling rates in a high speed DSC. The resulting precipitation heat peaks during cooling have been evaluated for temperature and time of precipitation start, as well as their areas as a measure for the precipitate amount. Quenched samples have been further investigated regarding their microstructure by light and electron microscopy, hardness after aging and precipitation behaviour during re-heating in DSC. The CCT-diagram correlated very well with the microstructure, hardness and re-heating results. A critical cooling rate with no detectable precipitation during continuous cooling 155 K/min could be determined for 7020. A model to integrate the CCT-diagram in heat treatment simulation of aluminium alloys is under development.

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.

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.

Continuous cooling precipitation diagram of high alloyed Al-Zn-Mg-Cu 7049A alloy

Abstract The precipitation behaviour during cooling from solution annealing of high alloyed 7049A aluminium alloy was investigated, covering the complete cooling-rate-range of technical interest. This ranges from slow cooling rates close to equilibrium up to rates above complete supersaturation and is covering seven orders of magnitude in cooling rate (0.0005 to 5000 K/s). The continuous cooling precipitation behaviour of 7049A alloy was recorded by combining different differential scanning calorimetry (DSC) techniques and microstructure analysis by SEM and Vickers hardness testing. The high alloyed, high strength and quench sensitive wrought aluminium alloy 7049A was investigated during quenching from solution annealing by conventional DSC in the cooling rate range of 0.0005 to 4 K/s. In this range at least two exothermal precipitation reactions were observed: a high temperature reaction in a narrow temperature interval of 450–430 °C, and a low temperature reaction in a broad temperature interval down to about 200 °C. Intensities of both reactions decreased with increasing cooling rate. Quenching from solution annealing with rates up to 1000 K/s was investigated by differential fast scanning calorimetry (DFSC) and the differential reheating method (DRM). A critical quenching rate to suppress all precipitation reactions of 100–300 K/s was been determined.

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.

Mechanical properties of undercooled aluminium alloys and their implementation in quenching simulation

Abstract The mechanical properties of aluminium alloys with non-equilibrium microstructures necessary for heat treatment simulation are not available. Therefore, compression tests of undercooled aluminium alloys such as AlSi1MgMn and AlZn4·5Mg1 have been performed in a quenching and deformation dilatometer with varying quenching rates and quenching finish temperatures. The compressive load on quenching finish temperature was applied immediately after quenching. The mechanical properties such as yield strength and strain hardening are strongly dependent on quenching rates and finish temperatures of quenching. Flow curves depending on quenching rates and quenching finish temperatures have been implemented in a quenching simulation by the finite element method. For the cooling process of extrusion profiles in water and in a gas nozzle field, the simulation results are presented.