Cementation Characterization of xNC6 Steel by Micro Magnetiques methods

Increase the hardness, the endurance and the life time of material can be realised by surface treatments and remetallings, the electromagnetic properties of steels depend on their composition, their microstructures and constraints applied. We can use the electric and magnetic parameters to evaluate their microstructure. The object of this work is the characterization of cementation by the non-destructive methods and the determination of physicochemical parameters. Sample of low carbon tenor steel was cemented with differents alternative. We studied these samples by micro magnetic NDT as eddy current (ET), Barkhausen Noise (BN) and hysteresis loop... The treatment of these results will be able to inform us about the physicochemical aspect of these materials .This work allow to determine the surface layers by NDT methods Keyword: NDT, Cementation,carbone, eddy current, remanence, corecitivity Email:[email protected] INTRODUCTION In addition to the defect characterization, actual studies deal with the metallurgical evaluation of materials. The characterization of microstructures, mechanical properties, deformation, damage initiation and growth by Non-Destructive Evaluation (NDE) techniques plays a vital role in various industries because of the growing awareness of the benefits that can be derived by using NDE techniques for assessing the performance of various components The method of reference chosen to compare the NDT results is the micro hardness profile. After cementing the carbon concentration decreases asymptotically of surface while going towards the heart. It is the same for hardness and as it is the profile of the hardness which conditions the behaviour of the parts in service, it is thus naturally with hardness that he is made call for the control of cementing. The objective of this treatment is to obtain on the surface steel a layer from 0.3 to 2 mm from thickness with high percentage of carbon .The Content generally ranging between 0.6 and 0.9 %. This layer has a martensitic structure of great hardness (being able to reach 700 to 900 HV on the surface) and the residual stresses of compression are raised. After introduction of carbon on the surface, the face-hardened part undergoes a heat treatment intended to confer its properties of employment to him [1]. The conditions of hardening are selected to obtain the best compromise between the qualities wished for the layer and the sought characteristics in heart of the part; it must also minimize as much as possible the deformations due to the variations of volume associated with the transformation [2]. NDT APPROACH According the state of art the eddy current testing (ET) has a strong application in the defect detection. The sensitivity to characterize defects and other parameters can be improved by an optimal choice of probes and operation frequency. This work discussed here is a study to materials structure characteristics and especially the cementation using eddy current techniques. The design of eddy current probes was optimized in order to increase their sensitivity and their resolution. Studies of the magnetic fields in the vicinity of a probe are suitable to characterize the field activity and to optimize the controlled measuring process and the probe size [3]. The magnetic Barkhausen noise (MBN) technique has attracted considerable attention in recent years as a possible non-destructive technique for evaluation of microstructural parameters such as grain size, carbide precipitates, inclusions, dislocation and characterizing the magnetic properties of steel. Barkhausen noise shows magnetizing discontinuities of ferromagnetic materials, such as alloys containing iron, Co, Ni [4]. In the application of variable magnetic field on the ferromagnetic material, many processes such as the creation and the annihilation of domain and the domain wall motion’s are observed. Each domain is strained along its direction of magnetisation , the magnetostriction phenomena occurs, if the domain wall motion’s are sufficiently rapid, the abrupt changes in local strain give rise to the generation of elastic waves, This phenomenon is observed for certain crystals when, one records the hysteresis loop B(H) which appears made up of a succession of marches[5 ]. These variations of magnetisation also appear by a signal of noise at the boundaries of a detecting reel placed near material. EXPERIMENTAL APPROACH CEMENTATION Cementing was carried out in three stages. The thermal cycles corresponding at the three stages are represented on the figure 1. carbon Enrichment Oil hardening relaxation Returned 1Diagrammatic representation of the treatments carried out This test enables us to determine the evolution of in-depth hardness from surface. Measures of micro hardness are taken starting from the periphery towards the centre with a step of 50μm. the distance between two test have is of 100μm ( Figure 2). 2Filiation of micro hardness In this study, the steel treated are the 10NC6, 12NC6, 16NC6 and 20NC6 nuances .The time of cementation of each nuances is 5,8,10 and 14 hours. (tab1) 10NiCr6 Maintaining time (h) 5 8 10 12 14 Cementation thickness (μm) 361 734 887 892 1047 hardness at 100 μm across the surface 798 773,9 785 767 811 Hardness at centre 354 357 355 347 362 16NC6 Maintaining time (h) 5 8 10 12 14 Cementation thickness (μm) 473 836 888 89