M. Pellizzari

Effect of deep cryogenic treatment on the mechanical properties of tool steels

Abstract The effect of deep cryogenic treatment (−196°C) on the properties of some tool steels was studied by means of both field tests on real tools and laboratory tests. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50%. A laboratory investigation on an AISI M2 and an AISI H13 steel confirms the possibility of increasing the wear resistance and toughness by carrying out the treatment after the usual heat treatment.

Influence of load and temperature on the dry sliding behaviour of Al-based metal-matrix-composites against friction material

In the present investigation, the effect of load and external heating on the friction and wear behaviour of two Al-based metal-matrix-composites (SiC10 and SiC20, containing 10 and 20 vol.% of reinforcement) dry sliding against a semi-metallic friction material was studied. For loads lower than 200 N wear was by abrasion and adhesion, and friction coefficient was quite high, around 0.45. For loads higher than 200 N, friction decreased with load in both materials, whereas wear increased with load for SiC10 and became negative, because of transfer for SiC20. External heating induced a decrease in wear of both composites (and was negative in both cases) but also an unacceptable decrease in friction and increase in wear of the counterface friction material. The particular friction and wear behaviour displayed by the materials under study was explained making reference to the characteristics of the transfer layer, which forms on the surface of the rotating disc.

Thermal fatigue resistance of gas and plasma nitrided 41CrAlMo7 steel

The influence of gas and plasma nitriding on thermal fatigue resistance of a 41CrAlMo7 steel is considered. The role of compound and diffusion layers is discussed on the basis of the damage observed in service. The thick and porous compound layer, resulting from gas nitriding, is not very effective in preventing thermal crack nucleation and propagation up to the interface with the underlying diffusion layer. On the contrary, the thin and compact compound layer obtained by plasma nitriding is able to prevent crack nucleation. Overnitriding, i.e. excessive nitriding induced embrittlement, has to be avoided in order to obtain high crack arrest fracture toughness in the diffusion layer. This suggests the need to realize surface layers with optimized hardness. A simplified approach to predict the conditions for crack propagation in the diffusion layer as a function of the thermal loading and the properties of the nitride layer has been also proposed.

Tribological properties of surface engineered hot work tool steel for aluminium extrusion dies

Abstract A tribological test aimed at the simulation of aluminium hot extrusion was performed. A 6082-Al disc, induction heated up to a maximum surface temperature of 350°C, is allowed to rotate against a conformal hot work steel block, simulating the extrusion die. The test simulates the wear mechanism observed under in service conditions. After an initial period, the hot plasticised Al progressively adheres to the steel counterpart thus allowing direct Al–Al contact. The high shear stresses given by the strong adhesion produce the nucleation of deep cracks which strongly deteriorate the surface of the steel by delamination. Hot work tool steel (AISI H11) is employed as extrusion dies. Salt bath nitriding (Tenifer) is usually carried out to improve wear resistance. The possible application of PVD (CrN, TiAlN) and CVD (TiC + TiN) hard coatings, exhibiting lower compatibility versus Al, has been evaluated in the present work. The occurrence of two distinct damage regimes was detected. Regime 1 shows minor excursions of the friction coefficient μ and is followed by regime 2 which is characterised by fully plastic Al–Al contact, with large fluctuations of μ. A time to transition tT has been identified as indicative of the ability of a surface layer in delaying the occurrence of regime 2. tT is correlated to the chemical compatibility of the surface layer (compound layer, PVD or CVD coating) versus Al. Furthermore, tT is related to the mechanical stability of the surface layer. A generalised and severe form of wear is displayed by nitrided steel while a localised and less severe form of wear is displayed by duplex treated, nitriding + PVD steel. No wear traces were observed by CVD TiC + TiN coated steel, providing the best performance among the surface treatment investigated.

Development of a Hybrid Tool Steel Produced by Spark Plasma Sintering

The production of a new Powder Metallurgy (PM) tool steel obtained by Spark Plasma Sintering (SPS) powder blends of AISI H13 and AISI M2 is considered. The objective is to obtain a product, the properties of which could be modulated on the basis of the specific application considered. Sintering powders of four blends were obtained (20% H13–80% M2, 40% H13–60% M2, 60% H13–40% M2, and 80% H13–20% M2). Density, hardness, fracture toughness, and wear resistance were evaluated. Increasing the fraction of M2 improves hardness and wear resistance. Fracture toughness is correspondingly lowered. The toughness of H13 rich blends was negatively affected by the high oxygen content in the base powder.

The Application-Oriented Heat Treatment of Tool Steels

A review of the principal results obtained in the laboratory of metallurgy of the University of Trento is presented in this article. The correlations between heat treatment, surface engineering, microstructure, and properties of hot work and high speed steels in a wide range of applications were evaluated. The basic damage mechanisms of tools and dies were illustrated. The thermal cracking (heat checking) occurring in hot work tool steels for casting/forging dies and in high speed steel work rolls was studied by means of a customary thermal fatigue test. The corrosion by molten metal in die casting dies has been studied by immersion in liquid aluminium. The tribological properties of dies during aluminium hot extrusion were analyzed using a specific simulation test.

Behavior at Elevated Temperature of 55NiCrMoV7 Tool Steel

The aim of present work is to investigate the behavior at high temperature of 55NiCrMoV7 steel used in mandrels for ring rolling. During service the tool is exposed to complex conditions including thermal fatigue, oxidation, and hot wear. In this work, samples were obtained from a broken mandrel. The steel microstructure in the as-delivered state consisted of overtempered martensite and bainite. The very low hardness (32HRC) was produced by thermal softening during service. A new set of harder samples (41HRC) was obtained after vacuum heat treatment. Hot wear and thermal fatigue resistance for the steel in the two conditions were compared.

Effect of cryogenic treatment on the hardness and tensile behaviour of AISI 4140 steel

This paper deals with the influence of cryogenic treatment on the hardness and tensile behaviour of AISI 4140 steel. The impetus for studying the tensile properties of steels is to ensure that steels used in shafts and gears have sufficient tensile strength to prevent failure when they are subjected to tensile or fatigue loads, and to provide basic design information on the strength of 4140 steel. A comparative study on the effects of deep cryogenic treatment (DCT, 77 K × 5 h) and conventional heat treatment (CHT) was made by means of hardness and tension testing. Fracture features of tensile specimens were also compared. It has been shown that the hardness of shallow and deep cryogenically treated specimens are higher whereas the tensile strength of the shallow and deep cryogenically treated specimens are lower when compared to that of conventionally treated steel. The transformation of retained austenite to martensite is responsible for present result.

Spark Plasma Co-Sintering of Mechanically Milled Tool Steel and High Speed Steel Powders

Hot work tool steel (AISI H13) and high speed steel (AISI M3:2) powders were successfully co-sintered to produce hybrid tool steels that have properties and microstructures that can be modulated for specific applications. To promote co-sintering, which is made difficult by the various densification kinetics of the two steels, the particle sizes and structures were refined by mechanical milling (MM). Near full density samples (>99.5%) showing very fine and homogeneous microstructure were obtained using spark plasma sintering (SPS). The density of the blends (20, 40, 60, 80 wt % H13) was in agreement with the linear rule of mixtures. Their hardness showed a positive deviation, which could be ascribed to the strengthening effect of the secondary particles altering the stress distribution during indentation. A toughening of the M3:2-rich blends could be explained in view of the crack deviation and crack arrest exerted by the H13 particles.

Thermal fatigue properties of hot-work tool steels

The Thermal Fatigue (TF) resistance rates of four martensitic chromium steel grades (DIN 1.2343, 1.2344, 1.2365 and 1.2367) are evaluated. Three parameters were used to describe the resistance to thermal cracking: the mean crack length lm, the maximum crack length lmax and the crack density ρ. A higher damage is shown by DIN 1.2343 and 1.2344 compared to DIN 1.2365 and 1.2367, the latter being characterised by higher yield strength at elevated temperature, higher resistance to thermal softening and lower toughness. An increasing number of cycles is required to nucleate the cracks in different materials, i.e., 60, 120, 180 and 240 for 1.2343, 1.2344, 1.2367 and 1.2365, respectively. During the initial incubation stage, the crack density displays an increase with increasing thermal softening and decreasing hot strength. During the rest of the test, a general increase in maximum crack length can be appreciated with increasing thermal softening and decreasing hot strength.