Chedly Braham

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

Influence of steel type on electrical discharge machined surface integrity

Abstract This paper studies the influence of type of steel on electrical discharge machined (EDM) surface integrity. Tests were performed with two hardenable steels (tool steel type X155CrMoV12 and high carbon content steel type C90) and two non-hardenable steels (austenitic stainless steel type X2CrNiMo17-12-02 and ferritic stainless steel type X6Cr17). Surface integrity was characterized by roughness, micro-hardness, residual stress distribution and near surface damage. In the case of hardenable steels, EDM leads to a structure of three layers, white, martensite quenched and transition layers. High carbon diffusion induces important near surface hardening and a high tensile residual stress level that leading to crack generation. In the case of non-hardenable steels, metallurgical transformations result only on the formation of the recast layer with a dendritic structure and a slight increase in grain size. Near surface hardening due to carbon enrichment depends strongly on the initial structure (BCC or FCC). Finite element method has been used to assess temperature and residual stress fields in the case of X2CrNiMo17-12-02 steel. Experimental and calculated residual stress profiles were compared. It was found that the shapes of the profiles are different in the surface layers and quite similar in the deeper layers, and that the calculated residual stress values are greater than those obtained experimentally.

Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.

Effect of near-surface residual stress and microstructure modification from machining on the fatigue endurance of a tool steel

This study concerns the effect of machining on the fatigue life of an EN X155CrMoV12 tool steel (SAE J438b), with regard to the generation of near-surface residual stress and microstructural modification of the machined surface. Two possible methods for machining tool steels were compared: electro-discharge machining (EDM), a high energy density process, and milling, a more conventional cutting process. Particular attention was given to characterization of the surface roughness, microstructure, and residual stress, using a combination of microstructural analysis, crack observation, scanning electron microscopy (SEM), x-ray diffraction (XRD), and chemical composition changes by energy-dispersive x-ray. A decrease of around 35% in the fatigue limit was observed for the EDM samples, compared with the milled samples. This was attributed to a tensile residual stress state after EDM, combined with significant phase transformation and hydrogen embrittlement. The milled surfaces showed no microstructural transformation or surface cracking and contained compressive residual stresses, all of which contributed to an improved fatigue resistance.

Microstresses in textured polycrystals studied by the multireflection diffraction method and self-consistent model

A new version of the X-ray diffraction method for determining macrostresses and microstresses in textured polycrystalline material is presented. In this method the lattice strains for various orientation of the scattering vector as well as for various crystallographic planes {hkl} are measured. The interpretation of the experimental data is based on the least-squares fitting procedure, in which the diffraction elastic constants and theoretical values of microstresses are used. The diffraction elastic constants and the microstresses are calculated by the self-consistent model. The new method was successfully applied for stress determination in one- and two-phase steels subjected to elastoplastic deformation, and the significant anisotropy of the incompatibility stresses was observed in textured samples.

Corrosion reactivity of laser-peened steel surfaces

Laser-shock processing or laser peening (LP) is a novel process used to reinforce surfaces by generating compressive residual stresses that has been investigated to change the surface mechanical state and modify the electrochemical properties of three commercial steels. The first part of this paper relates to experiments where LP has been applied to G10380 and G41400 steels for corrosion testing in an acid HKSO4-0.3 M solution. Only in the case of G41400 martensitic steel is a reduction of the corrosion current observed, depending on the degree of work hardening and the amplitude of compressive stresses. This indicates a small mechanochemical effect of LP, which seems to be restricted to martensitic structure. Second, the effect of LP on stress corrosion cracking (SCC) of AISI 316L stainless steel is demonstrated by static tests in MgCl2 44% – 153 °C solution. The results confirm the applicability of LP to suppress cracks on all the areas processed without occurrence of any problems in the treated-nontreated transitions zones.

Effect of surface properties on high cycle fatigue behaviour of shot peened ductile steel

Abstract Experimental investigations into shot peened ductile steel have been carried out, applying three surface finishing conditions: as machined, standard shot peening using 100% coverage and severe shot peening with 1000% coverage (high exposure time). The properties of the shot peened surfaces were examined and characterised, and specimens were then submitted to three point bending tests. The fatigue limit was determined for each case. In this way, the dependence of fatigue behaviour on initial surface finishing properties was determined, and a relationship is suggested to describe and correlate fatigue limits with initial surface properties. A phenomenological approach is proposed to characterise and to correlate qualitatively and quantitatively the influence of local shot peened surface properties on fatigue limit of treated specimens. The Crossland multiaxial failure high cycle fatigue criterion is used in this approach to model the influence of each surface property.

Corrosion fatigue crack propagation of a duplex stainless steel X6 Cr Ni Mo Cu 25-6 in air and in artificial sea water

Abstract Fatigue crack growth behavior of a duplex stainless steel has been investigated at a stress ratio of 0.1, in air environment at room temperature and in artificial sea water heated to 70 °C. Growth rates generated under constant load amplitude at a frequency of 0.1 Hz under corrosion fatigue were found to be about 1.7 orders of magnitude higher than those generated in air environment. The study of the crack propagation under both conditions has revealed a significant change in fracture mechanisms between both environments. In air environment, microscopic growth rates in austenite measured by striation spacing in SEM were found to be very close to the macroscopic growth rates whereas in sea water, cyclic cleavage of the ferrite controls the macroscopic growth rates for ΔK between 20 and 60 MPa√m. The cyclic cleavage is induced by hydrogen embrittlement of the ferrite which is produced by an electrochemical reaction between the material and sea water.

Fatigue life improvements of the AISI 304 stainless steel ground surfaces by wire brushing

The surface and subsurface integrity of metallic ground components is usually characterized by an induced tensile residual stress, which has a detrimental effect on the fatigue life of these components. In particular, it tends to accelerate the initiation and growth of the fatigue cracks. In this investigation, to deliberately generate compressive residual stresses into the ground surfaces of the AISI 304 stainless steel (SS), wire brushing was applied. It was found that under the experimental conditions selected in this investigation, while the surface roughness was slightly improved by the brushing process, the surface residual stress shifted from a tensile stress (σ‖=+450 MPa) to a compressive stress (σ‖=−435 MPa). On the other hand, the work-hardened deformation layer was almost two times deeper after wire brushing. Concerning the fatigue life, an improvement of 26% in terms of endurance limit at 2×106 cycles was realized. Scanning electron microscope (SEM) observations of the fatigue fracture location and size were carried out to explain the fatigue life improvement. It was found that the enhancement of the fatigue strength could be correlated with the distribution and location of the fatigue fracture nucleation sites. Concerning the ground surfaces, it was seen that the fatigue cracks initiated at the bottom of the grinding grooves and were particularly long (150–200 µm). However, the fatigue cracks at the brushed surfaces were shorter (20–40 µm) and appeared to initiate sideways to the plowed material caused by the wire brushing. The results of the wire-brushed surface characterization have shown that significant advantages can be realized regarding surface integrity by the application of this low-cost process compared to shot peening.

Fatigue Life Evaluation of Shot Peened Al-Alloys 5083 H11 T-Welded Joints by Experimental and Numerical Approaches

In this investigation, the fatigue life improvements of T-welded joints in aluminium alloy 5083 H11 by shot peening was evaluated. Use was made using both experimental and numerical approaches. The experimental assessment was conducted using four point bending fatigue tests at R = 0.5. The numerical method used in this study, to predict the nucleation fatigue life, was developed on the basis of a local approach, which took into account the stresses induced by the welding process and the residual stresses generated by the shot-peening treatment. The propagation fatigue life was calculated using fracture mechanics concepts. It was found that the numerical results were well correlated with the experimental ones.