Thermal fatigue is an important life-limiting factor in die casting moulds. It is observed as a network of fine cracks on the surface exposed to thermal cycling. The crack network degrades the surface quality of the tool and, consequently, the surface of the casting. Surface engineered materials are today successfully utilised to improve the erosion and corrosion resistance. However, their resistance when exposed to thermal cycling is not fully understood. In this work, surface treatments (boriding and Toyota diffusion to give CrC) and physically vapour deposited (PVD) coatings of CrN, as single-layered or duplex-treated (nitriding followed by PVD coating), on hot work tool steel specimens were compared with the untreated reference material by experimental simulation of thermal fatigue. The test is based on induction heating and surface strain measurements by a non-contact laser speckle technique, which enables studies of the surface strain during thermal cycling. Thermal fatigue cracking of a surface engineered tool steel is influenced by the modification of the mechanical properties of the substrate which occurs during the engineering process. With the exception of duplex-treatment, all variants of surface engineering show a tendency to decrease the resistance to thermal fatigue cracking as compared to the reference steel. However, the fact that the duplex-treated PVD CrN coating increased the resistance to thermal fatigue cracking as well as reduces the density of cracks as compared to the single-layered CrN coating, the potential to improve the life and performance in for example die casting applications still prevails.

In this study, the temperature variations in the surface layer of a hot work tool steel was experimentally evaluated during actual brass die casting. A special method was developed to measure the temperature in the surface layer of the mould at different depths. Untreated, borided, and physical vapour deposition (PVD) coated (CrN) tools were included. Temperature profiles in the surface layer of the mould were recorded and details of the thermal cycling were obtained. Starting with a tool of room temperature, the tool temperature range averages during the initial 20 casting events. Simultaneously, within a surface layer of about 2 mm thickness, the maximum stress is reduced to a constant level. Based on the temperature profiles, the maximum surface temperature of the mould and the thickness of the tool surface layer within which fatigue damage can occur were estimated to about 826 °C and 1.5 mm, respectively. This limited thickness results from the decrease in temperature range and, consequently, stress from the tool surface and inwards. Finally, no notable influence of the investigated surface engineering on the thermal conditions was found.

Thermal fatigue cracking is an important life-limiting failure mechanism in die casting tools. It is observed as a network of fine cracks on the surfaces exposed to thermal cycling. The crack network degrades the surface quality of the tool and, consequently, the surface of the casting. Surface engineered materials are today successfully applied to improve the erosion and corrosion resistance. However, their resistance against thermal fatigue is not fully explored.In this work, a selection of hot work tool steel grades was surface modified and experimentally evaluated in a dedicated thermal fatigue simulation test. The surface modifications included boriding, nitriding, Toyota diffusion (CrC), and physical vapour deposition (PVD) of coatings (CrC, CrN and TiAlN), both as single-layers and deposited after nitriding (duplex treatment). Untreated specimens of each tool steel grade were used as references. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded through a non-contact laser speckle technique.Generally, all surface treatments decreased the resistance against surface cracking as compared to the reference materials. The reason is that the engineering processes influence negatively on the mechanical properties of the tool materials. Of the processes evaluated, duplex treatment was the least destructive. It gave a lower crack density than the reference steel, but the diffusion layer is more susceptible to crack propagation. In addition, the single-layered CrN coating showed almost comparable thermal fatigue cracking resistance as the reference material. Finally, the resistance against thermal crack propagation of surface engineered tool steels is primarily determined by the mechanical properties of the substrate material. (C) 2004 Published by Elsevier B.V.

Tools for die casting of, for example, brass and aluminium alloys are exposed to severe thermal, mechanical, and chemical conditions. The performance and service life of the die components are limited because of reasons such as thermal fatigue cracking, erosion, corrosion, soldering, and gross fracture. To minimise these damages, the dies are normally made of hot work tool steel.This study aims at elucidating the life-limiting failure mechanisms in dies aimed for brass die casting. Two cavity inserts and eight cores of two hot work tool steels, quenched and tempered to different conditions, were examined and evaluated with respect to failure mechanisms after use in actual brass die casting.It was found that the dominating failure mechanism in the investigated tools was thermal fatigue cracking. The thermal fatigue crack initiation is associated to accumulation of the local plastic strain that occurs during each casting cycle, typical of a low-cycle fatigue process. The initial growth of the thermal cracks is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by additional oxidation in combination with crack filling of cast material, and by softening of the tool material. The most striking observation was a mechanism of crack growth promotion that involves a local enrichment of Pb from the brass alloy melt at the crack front. Consequently, in addition to increasing the overall tool steel yield strength and/or oxidation resistance, there is a potential to improve the tool life either by removing or substituting the Pb in the brass casting alloy with any non-harmful element, or using a tool material not susceptible to Pb embrittlement.

Die casting is a very cost efficient method to manufacture near net-shaped and complex cast products. One limitation for further cost reduction is fatigue cracking of the tool due to thermal cycling, which is observed as a crack network on the tool surface. Hot work tool steels are commonly used as die material.In this study, an experimental test machine for simulation of thermal fatigue is described. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded during the thermal cycling through a non-contact laser speckle technique. The applicability of the test is demonstrated on two hot work tool steel grades, hardened and tempered to different conditions, and heat cycled between Tmin 170 °C and Tmax 600–850 °C.It is shown that the test method can simulate surface cracking of tools exposed to thermal fatigue. The surface strain recordings proved to give sufficient information to successfully deduce the strains and stresses behind the mechanism of thermal fatigue surface cracking, without knowledge of the temperature distribution below the surface. It was also found that low-cycle fatigue occurs for the tests with Tmax 600 and 700 °C, although the estimated tensile stress after cooling does not exceed the initial yield strength of the steel. Most probably, the reason is the gradual softening of the tool steels during the thermal cycling. Additionally, the presence of stress concentrators play a critical role during these conditions.

Thermal fatigue cracking and wear by erosion and corrosion are important life-limiting failure mechanisms in die casting dies. To develop new and more resistant tool materials for this application detailed knowledge of the casting conditions, the failure mechanisms and their aggressiveness are essential. Experimental simulations have successfully been applied to study the failure mechanisms and also to evaluate the resistance of tool steels and surface engineered materials against failure in die casting. This study elucidates the thermal conditions during actual die casting of brass. In addition, thermal fatigue and corrosive wear of surface engineered hot work tool steel specimens were experimentally evaluated. Thermal fatigue cracking was evaluated for the following conditions of a hot work tool steel: quenched and tempered (reference), treated by boriding and Toyota diffusion (CrC), respectively, PVD CrN-coated and duplex-treated topped with a PVD CrN-coating, respectively. A special study of corrosive wear of CrN PVD coatings applied on hot work tool steel specimens after treatment in an aluminium melt was also performed.

Die casting is a very cost-efficient method of forming thin-walled and complex near net-shaped products with close geometric tolerances and good surface finish. A permanent die tool is used to make large quantities of identical products. The performance and tool life are limited by several mechanisms, e.g. thermal fatigue cracking, erosion, and corrosion. To develop new and more resistant tool materials for die casting detailed knowledge of the actual casting conditions and the tool failure mechanisms are essential. This thesis contributes to an increased knowledge of tool failure in die casting by investigating and simulating actual casting conditions and tool failure mechanisms.A method to record the temperature fluctuations in a cavity insert during actual brass die casting was developed, and details of the temperature conditions were obtained. Also, a test method based on cyclic induction heating and internal cooling of hollow cylindrical test rods was developed, where the surface strain during thermal cycling could be measured. This method reproduced the characteristic type of surface cracking observed on die casting tools, and proved to give information of the strains and stresses behind the fatigue failure.In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking. The formation of the cracks is associated to accumulation of the local plastic strain that occurs during each casting cycle. Initial crack growth is facilitated by oxidation of the crack surfaces, and proceeded growth is facilitated by this oxidation in combination with crack filling of cast material, and by softening of the tool material. In addition, local enrichment of Pb at the crack front from the cast alloy melt was also observed to promote the crack growth in die casting of brass.In an investigation of thermal fatigue of two hot work tool steels, quenched and tempered to different conditions, it was found that low-cycle fatigue occurs, although the estimated tensile stress never exceed the initial yield strength of the steel. The reason is a gradual softening of the steel during the thermal cycling, and the presence of stress raising defects. The resistance against thermal cracking improves with initial tool steel hardness, because any initial ranking in hardness among the steels is unaffected by the thermal cycling.Another investigation on a selection of surface engineered tool steels, including common diffusion treatments, PVD coatings and combinations of these, showed that surface engineering generally reduce the resistance against thermal cracking as compared to untreated references, since the engineering processes influence negatively on the mechanical properties of the hot work tool steels.Finally, corrosion tests of CrN PVD-coated tool steels by exposing them to molten aluminium revealed the mechanisms of initiation and progress of liquid metal corrosion of this material combination, and that the corrosion resistance improves with the CrN coating thickness.

Tools for die casting and hot forging applications are exposed to thermal cycling, which may induce stresses high enough to cause plastic deformation during each cycle. The tool material behaviour in thermal fatigue loading is determined by the material properties and the thermal and mechanical load conditions. Coupled studies by experimental and numerical simulations are necessary for an increased understanding of the material behaviour as related to the interaction between the thermal and mechanical conditions. In this study, thermal fatigue testing of a tool steel, 55NiC rMoV7, and numerical simulation of its behaviour have been performed. The experimen- tal test is based on induction heating and surface strain measurements by a non-contact laser speckle technique, which enables studies on the surface strain response during thermal cycling. Thermal cycling up to 600 ◦Cwere included. The numerical model was developed to simulate the thermal con- ditions in the specimen and to evaluate the non-isothermal stress-strain be- haviour under thermal fatigue loading. It is based on a thermal analysis and a mechanical cyclic elasto-visco- plastic constitutive mo del, taking into ac-

Experimental investigation on the influence of surface engineering on thermal fatigue of a hot work tool steel

Erosive and corrosive wear are two major life-limiting factors in die casting dies. To resist the corrosive and erosive attack from molten metal flow the tool surface needs to be hard and chemically inert. It has been indicated that local coating defects rather than intrinsic deficiencies limit the potential gain of coatings on hot work tool steel exposed to liquid aluminium. This motivates a search for suitable protective coatings on the dies. A number of ceramic coatings are of interest. In this work, physically vapour deposited (PVD) CrN coatings were applied on hot work tool steel specimens and treated in an aluminium melt. Substrate temperature and deposition time were varied to give coatings with individual properties. Type and density of defects as well as thickness of the coatings were characterised for all coatings. The influence of substrate temperature and deposition time on coating characteristics and corrosion resistance was studied as well as the mechanism of corrosion damage. It is clearly demonstrated that liquid aluminium corrosion of CrN-coated tool steel is initiated at defects which penetrate through the coating, and localised corrosion pits are formed. Subsequently, the pits coalesce and the corrosive attack aggravates. Consequently, the corrosion resistance is improved by reducing the density of defects through the coating, which, for PVD CrN, was achieved by increasing the coating thickness.