Tesis doctoral de Ingrid Picas Anfruns
On account of environmental and safety related requirements, the majority of the most popular automotive manufacturers convey to introduce ultra high strength steels (uhss) and press hardened steels (phs) in the vehicle body-in-white. Nevertheless, the real success of uhss and phs parts implementation into the upcoming automobiles is being so far restricted by their harsh cold forming and post forming operations, e.G. Shearing. Their high yield stress leads to accelerated damage and premature fracture of forming and cutting tools and this stands as great handicap of their manufacturing process. Tool materials require the maximum hardness ¿ toughness relationship so as to face up to such severe mechanical solicitations. The performance of tool steels during cold forming and shearing applications depends on the mechanical properties resulting from the complex interaction between their main microstructural constituents; i.E. Primary alloy carbides and tempered martensite matrix (which consists in a fine distribution of nanometer-sized alloy carbides, the so-called secondary hardening carbides, precipitated in a martensite matrix).In this regard, the present thesis has analysed the microstructure, macro- and micro-mechanical properties, crack nucleation and growth under monotonic loads, r-curve behaviour and fatigue crack nucleation and propagation mechanisms of four different cold work tool steels: 1.2379, universal, k360 and hws. The mechanical and tribological behaviour of industrial and laboratory tools made of these steels have been studied, as well as the effects of shearing process parameters on their performance and quality of sheet sheared edges. The suitability of linear elastic fracture mechanics (lefm) based models to estimate the performance of tools has been evaluated in order to predict their service lives. Rationalisation of damage on tool steels from a micro-mechanical point of view in laboratory samples as well as in real tools has permitted to identify the aspects that a model should consider in order to successfully predict tool lives. In addition, it has permitted to propose a new approach to estimate tool performances based on the reduction of tool steels mechanical properties with the increase of the number of strokes. 1.2379, universal and k360 are produced following conventional ingot metallurgy routes while hws is obtained by powder metallurgy (pm). As follows from their different chemical compositions and manufacturing routes, these steels show different primary alloy carbide features and distributions in the microstructure. Nucleation of cracks in tool steels is due to the failure of primary carbides by cleavage (and inclusion particles in case of pm steels). Small cracks nucleated from primary carbides in ingot cast steels are initially very shallow, but they tend to grow to semi-circular shapes as they propagate through the matrix bands. Ingot cast tool steels present r-curve behaviour, i.E. Small cracks nucleated in the microstructure have lower toughness values than those determined for long cracks in standard measurement tests. The main failure mechanism of tools for punching and slitting uhss is chipping at the cutting edge. Plastic deformation and nucleation of cracks at the surface of tools occurs as a result of the high frictional forces during sliding against the sheet. Due to the high contact pressures at the surface, carbides within a narrow range of depth beneath the surface are broken and cracks are also nucleated from surface asperities. Chipping is then explained by the nucleation and coalescence of these cracks. In presence of marked grinding grooves, failure of tools is significantly accelerated. Models based on lefm have proved to predict with rather poor accuracy the life of tools. A new approach to predict tool performances is proposed which holds the idea that the micro-mechanical properties of the metallic matrix in tool steels decrease due to the application of repetitive load
Datos académicos de la tesis doctoral «Mechanical behaviour of tools for shearing ultra high-strength steels: influence of the microstructure on fracture and fatigue micro-mechanisms of tool steels and evaluation of micro-mechanical damage in tools«
- Título de la tesis: Mechanical behaviour of tools for shearing ultra high-strength steels: influence of the microstructure on fracture and fatigue micro-mechanisms of tool steels and evaluation of micro-mechanical damage in tools
- Autor: Ingrid Picas Anfruns
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 10/12/2012
Dirección y tribunal
- Director de la tesis
- Daniel Casellas Padro
- Tribunal
- Presidente del tribunal: m. dolores (secretario) Riera colom
- jean Steinmetz (vocal)
- wolfram johannes Volk (vocal)
- (vocal)