Prédiction de la durée de vie sous sollicitations thermomécaniques des outillages en acier travaillant à chaud

39èmes journées du cercle d'études des métaux, 17-18 mai 2000, Saint-EtienneInternational audienceSurface of hot work tools is damaged by complex thermomechanical-wear-reacti-ve environment interactions. A thermomechanical fatigue (TMF) experiment using tubular spe cimens is developed. Tests are carried out under out-of-phase thermomechanical cycle. The mechanical strain is minimum (εm min) at upper temperature (Tmax) and it is maximum (εm max) at lower temperature (Tmin) of thermal cycle. The behaviour, the damage and the fatigue life of two tempered martensitic steels X38CrMoV5 (AISI H 11) and 55NiCrMoV8 (47 HRC) are assessed. The effect of Tmin and Tmax is examined. Softening is observed for both steels. For a given mechanical strain amplitude (εm = εm m ax-Em min), a drastic dependence on Tmax is demonstrated, while Tmin has less important effect. X38CrMoV5 has a better TMF life. Ductile fatigue striations are observed on the fracture surface of X38CrMoV5 specimens. The fracture surface of 55NiCrMoV8 specimens is covered by oxidation, making difficult to reveal the fatigue striations. Both oxide cracking and spalling are observed on the external surface of specimens. A phenomenological power law uniaxial model, based on the mechanical strain amplitude and Tmax, εm = K(Tmax).Nfα(Tm ax), is proposed to predict the life under non-isothermal fatigue solli citations. The life of some thermal fatigue tests is predicted within a factor of two to three. The capability of the model to predict the critical regions of an industrial hot work tool is reported.La surface des outillages travaillant à chaud s 'endommage p a r des interactions complexes de la sollicitation thermomécanique-usure-environnement réactif. Un essais de fatigue thermomécanique (FTM) utilisant des éprouvettes tubulaires a été mis au point. Les essais ont été menés sous cycles thermomécaniques hors-phases. La déformation mécanique est respectivement minimale (εm min) lorsque la température est maximale (Tmax) et est maximale (εm max) lorsque la température est minimale Tmin). La vitesse de chauffage et de refroidissement est d 'environ 4 ° C/s. Le comportement, l'endommagement et la durée de vie de deux aciers martensitiques revenus X38CrMoV5 (AISI H I I) et 55NiCrMoV8 (47 HRC), sont étudiés. L 'adoucisse ment cyclique est observé dans les deux aciers. Pour un niveau de l 'amplitude de déformation donné (εm = em max, em min), une très forte dépendance de la durée de vie à Tmax est observée, alors que Tmin a un moindre effet. L 'acier X38CrMoV5 a une meilleure durée de vie sous les mêmes conditions de sollicitations. Les stries de fatigue observées sur les faciès de rupture de X38CrMoV5 révèlent donc le caractère ductile de la propagation de fissure. Les faciès de rupture de l 'acier 55NiCrMoV8 sont couverts d 'une couche d 'oxyde qui rend difficile la détection des stries de fatigue. La fissuration de la couche d 'oxyde ainsi que l'écaillage sont observés sur la surface externe des éprouvettes. Un modèle phénoménologique uniaxial, de type puissance, basé sur l 'amplitude de la déformation mécanique et Tmax, εm = K(Tmax). Nf α(Tmax), est proposée pour prédire la durée de vie sous des sollicitations anisothermes. La durée de vie en fatigue thermique ou thermomécanique de certains essais réalisés au laboratoire est prédite à un facteur 2 à 3. La capacité du modèle à prédire et identifier les régions critiques d 'un outil de forge industriel est montré

Cyclic behaviour simulation of X38CRMOV5-47HRC (AISI H11)-tempered martensitic hot-work tool steel

Issu de : 7th International tooling conference on tooling materials and their applications from research to market, Torino, ITALY, 2-5 May 2006International audienceThe cyclic behaviour of X38CrMoV5 (AISI H11) tool steel with a nominal hardness of 47HRC has been predicted. Basically, thermo-elastoplastic and thermo-elastoviscoplastic constitutive laws are investigated. First, various uniaxial isothermal conditions (LCF) with different strain rate, strain amplitude and temperature level are investigated. Then the constitutive laws are examined under various TMF loading conditions. The simulated results by both the approaches are compared with experimental results in terms of stress?strain behaviour and cyclic softening. Some applications of the model for simulation of thermal fatigue sample are shown. Taking into consideration the results of this work, the goal is to further characterise the limitations of these constitutive laws under complex and severe loading conditions, i.e., under variable temperature, variable strain amplitude and thermal fatigue structural specimen

Normalization of fatigue crack growth data in AISI H11 tool steel at room and elevated temperature

International audienceThe fatigue crack growth rate of materials is shown to be dependent on the testing conditions like load ratio R and testing temperature. Great interest exists in normalizing this data onto a single curve. In this research, some methods commonly used to normalize the effect of R ratio are tested on Fatigue Crack Growth Rate (FCGR) curves of AISI H11 tool steel. These methods are based on either purely mathematical relations or on the effect of crack closure in variable R ratio tests. A model based on the crack tip opening displacement measurements to normalize the effect of R ratio as well as temperature is also used. This model takes into account the material‐hardening coefficient, yield stress, Young's modulus, and the crack tip opening displacement measurements. Crack tip opening displacement measurements have also been directly used to characterize the FCGR. A method is presented to find out crack closure as well as crack tip opening displacement using 2D digital image correlation measurements near the crack tip. At the end, a critical analysis of the four normalization techniques is presented

A phenomenological model of the third body particles circulation in a high temperature contact

International audienceIn high temperature forming processes (forging, rolling, die-casting...) the tool surfaces are the privileged places for mechanical, thermal and physico-chemical solicitations. More precisely, friction and wear play an important part in tool surface damage. The tool steel grades exhibit damages such as local plastic deformation, plastic flow, carbides fragmentation and oxidation. Oxide scales, which depend on the contact temperature, influence wear mechanisms and have to be considered in the wear model development. The aim of this work is to assess the third body particles circulation in a high temperature friction contact. The wear investigations are carried out using a high-temperature pin-on-disc tribometer. The pin is made of X38CrMoV5 steel (AISI H11) and the disc is made of common steel (C38, AISI 1035). The setting temperature is 900 °C. All experiments are performed under constant load and velocity. The special design of the pin surface contributes to the trapping of third body particles and to the identification of the third body flows. Environmental Scanning Electron Microscopy (ESEM, in ambient air) observations and Energy Dispersive Spectrometry (EDS) investigations have revealed that different types of third body particles circulate in the contact. Based on these observations and chemical analyses, a phenomenological model of the third body particles circulation in a high temperature contact is proposed, based on the third body concept approach

Numerical life prediction of mechanical fatigue for hot forging tools

Issu de : ESAFORM 2009 - 12th ESAFORM Conference on material forming, Enschede, THE NETHERLANDS, 27–29 April 2009International audienceIn the forging industry, tools represent an important part in term of production and costs. Enhancing their life cycle is then a challenging issue. Several mechanical and thermal mechanisms are responsible for hot forging tools damage such as wear, thermal and mechanical fatigue. This work will be focused only on the mechanical fatigue life prediction for hot forging tools. Both experimental data analysis and numerical simulation will be discussed in this paper. The aim is to perform qualitative and quantitative indicators of mechanical fatigue. First, experimental data of fatigue tests are used to identify both plastic strain-based Manson Coffin and stress-based Basquin life laws for 2 tool steel grades. These laws are quite classical for fatigue prediction [1-4]. The half-life strain or stress amplitudes are usually used for their identification but these amplitudes are very expensive to obtain from a numerical point of view since it is well known that hot work martensitic steels present a continuous cyclic softening from the first cycle till the rupture. Therefore an important number of cycles have to be simulated to reach these mechanical parameters at half-life. For all theses reasons, an alternative methodology is used [4]. The fatigue life curves are established using the mechanical parameters that are identified from the first hysteresis loops of fatigue experiments. Comparisons are performed with the fatigue laws coming from more classical identification procedure performed at half life cycle. Good agreement is shown between experimental data and the new laws. A lower scattering is even observed in experimental results in comparison to the traditional fatigue laws. Then these new laws are introduced in the commercial software Forge® and are then applied to different industrial cases. A pretty good agreement is observed between predicted tool life and industrial value

Fatigue thermique des aciers à outils pour travail à chaud

International audienceThermal fatigue of X 38 CrMoV 5 (formerly Z38CDV5), a 5% Cr steel, is investiga ted, in as quenched and tempered (47 HRC) as well as annealed conditions. A thermal fatigue rig usinghighfrequency induction heating is developed. Tubularspecimens are used. By modi fying the specimen geometry, various thermal gradients and therefore different thermo-mecha nical loading i.e. mechanical strain versus temperature loops are generated. Finite element calculations of the thermo-mechanical strains and stresses reveal that the stress state in the centre of the external surface of the thermal fatigue specimen is quasi bi-axial and does not change for the different geometries used. Outside of this region, the stress ratio (i.e. hoop stress (σθθ) over axial stress (σzz)) arises progressively to about 2 to 2.5 (uni-axial condition). Depending upon the maximum temperature of the thermal cycle and the amplitude of the mecha nical strain generated by the thermal gradient, bi-axial oxide-scale spalling or heat checking were observed. Heat checking (bi-axial cracking) was predominantly observed in the centre of the specimens while towards to the ends of the specimens, the uni-axial cracking proceeds. Microhardness measurements at room temperature reveal a thermal fatigue softening in as quenchedand temperedsteel (47 HRC). A higher maximum temperature of the thermal cycle and a higher mechanical strain increases thermal fatigue softening. The role of the number of the thermal cycles was overshadowed by the more important effect of the amplitude of the mechanical strain and the temperature. A quasi linear softening is observed over few milli metres beneath the external surface. The hardness achieve then the initial hardness ofthe steel. The softening rate i.e. hardness over the thermo-mechanically affected zone width is controlled by the thermal gradient and thus the thermo-mechanical loading. No softening was observed in annealed steel

Rôle du poteyage et de la température initiale du moule sur les sollicitations thermomécaniques des moules permanents de fonderie

International audienceIn the casting industry metal moulds or dies are used more and more. The reason is that they allow a fa st cooling rate o f the solidifying part, hence allowing higher productivity, finer microstructure and higher mechanical properties. In most cases the die is made out o f steel and reacts with the liquid cast metal. The usual solution is to cover the moulding surface with a coating or spray. Depending on the casting technology, the coating is sprayed every cycle or every 8 to 10 hours o f production. A second but nonetheless important effect o f the coa ting is its thermal effect. The coating acts as a thermal barrier and protects the die against thermal shocks. The topic o f the present paper is to assess this function o f the coating. During a casting cycle, the coated die and the molten metal are briefly in contact during the very first moments and then an air gap may form and separate them apart. During the first stage, an intense heat is tran sferred from the m elt to the die. H eat flu x den sities fro m 0.5 MW/m2 up to 10 MW/m2 have been reported in literature. The intense heat transfer gene rates high temperature heterogeneity into the die. The corresponding dilatation heterogeneity is responsible fo r internal stresses into the die, so called thermal stresses. They are usually compressive stresses on the hot surface. It will be shown in this paper that the moulding surface o f the die suffers the most stress. The stresses can be high enough to cause yielding o f the steel at high temperature. Because the steels in use fo r dies have a high yield stress at high tempera ture the plastic deformation remains small. However it is a cyclic plasticity because the same phenomenon occurs at every casting cycle. We believe that this plasticity in warm conditions is responsible fo r residual tensile stresses in cold conditions (i.e. nearly isothermal conditions). This phenomenon is rather classical in most thermal stresses problems [ 1, 2], A t the lifetime scale o f the die, the moulding surface is cyclically stressed in traction at low temperatures and compression at high temperatures leading to a fatal cracking. In a first approach we suggest measuring temperatures within the die during a casting cycle. From this measurement it is possible to estim ate the thermal stresses, assuming that the stresses remain below or at least close to the yield stress o f the die materials. This assumption is usually fair. Indeed, if the plastic deformation were large during every cycle, the die would never last much longer than a few thousands cycles. If this ever occurred, it would not be a great advantage fo r the casting factory nor its client. Other materials should be sought as a first priority. From the estimation o f thermal stresses, it would be delicate to foresee the mecha nical behaviour o f the materials o f the die in fatigue condition. Some materials tend to harden (copper alloys, f cc materials) while others tend to soften (heat treated martensite steels [4])[5], Instead o f trying to guess the behaviour, the method that we suggest is to perform a thermo mechanical fatigue test (TMF). This TMF test consists o f applying measured temperature and evaluated strain/stresses history to a mechanical testing sample [6], The most relevant tempe rature and stress history is, o f course, the one corresponding to the moulding surface o f the die. This test will provide information on the materials behaviour and some relevant data about the lifetime o f the die. This paper provides an example o f this method. The thermal data was obtained from a gravity casting experiment [3] that is described in the first part. The second part deals with the evaluation o f the thermal stresses and the third part shows some results from the TMF testing. Throughout the paper the influence o f the coating nature and o f the die initial temperature is examined.Cet article traite de la fatigue thermique subie par les outillages de mise en forme. Pour appréhender le problème d’endommagement des outillages, il est nécessaire de bien connaître les conditions de transfert de chaleur et d’évaluer les contraintes thermo-mécaniques subies par l’outillage. La prévision de la durée de vie et l’évolution de la plasticité cyclique qui précède la fissuration est difficile car certains matériaux ont tendance à s’écrouir alors que d’autres ont tendance à s’adoucir. Nous proposons le recours à des expériences de fatigue thermo-mécanique (TMF) pour appréhender cette prévision. Par un exemple emblématique, nous montrons comment il est possible d’aborder le problème. Le cas considéré est la fonderie gravité en coquille d’acier. Nous mettons en évidence l’influence de paramètres process sur le transfert de chaleur et sur les contraintes thermo-mécaniques subies par le moule pendant un cycle de coulée. Les paramètres retenus sont la température initiale du moule et la nature du poteyage. Des essais de fatigue thermo-mécanique ont été menés et les résultats sur 1000 cycles sont exposés

Sollicitations thermomécaniques des outillages de forge à chaud

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Propagation de fissures dans un acier martensitique 5Cr-Mo-V sous sollicitations thermo-mécaniques

National audienc

Thermo-mechanical fatigue behavior of a tempered martensitic 5 % chromium steel

International audienc