Paolo Marcassoli

Hydrogen embrittlement behavior of HSLA line pipe steel under cathodic protection

Abstract This paper deals with hydrogen embrittlement (HE) under cathodic protection of steel used or recently proposed for buried pipelines of the oil and gas industry. High strength low alloy steel with different chemical composition, microstructure and ultimate tensile strength ranging between 530 and 860 MPa were considered to compare new steel with traditional grades. Their behavior was assessed through constant load tests, fracture mechanics tests on modified wedge-opening load specimens, slow strain rate tensile tests and interrupted slow strain rate tensile tests in solutions simulating soil moisture and sea water, at room temperature and cathodic potentials between -2 and -0.8 V vs. saturated calomel electrode reference. All the steel was found immune under static load and HE phenomena were observed only during slow strain rate tests at potentials more negative than a critical value, depending on steel susceptibility and strain rate. Quenched and tempered steel with high tempered martensite and very fine precipitation distribution showed better behavior than the rolled steels with banded microstructure, exhibiting the highest HE resistance. The resistance of steel characterized by banded microstructures increased with ultimate tensile strength until ∼700 MPa and then decreased. The increase of ultimate tensile strength occurred by changing the steel microstructure, from hot rolling with coarse ferrite/pearlite to fine ferrite/pearlite microstructure, and very fine acicular ferrite, yields the resistance of steel to HE. A further increase of ultimate tensile strength related to untempered martensite inside the acicular ferrite based microstructure produces a worsening of resistance.

Mechanical characterization of hydroxiapatite micro/macro-porous ceramics obtained by means of innovative gel-casting process

An innovative gel-casting process was developed in order to obtain macro porous ceramics scaffolds of hydroxyapatite to be used in regenerative medicine for bone tissue reconstruction. Mechanical investigation was carried out on different formulations of dense hydroxyapatite samples in order to evaluate the effect of the gel casting process parameters on the density, the elastic modulus, the tensile and the compressive strength. The fracture critical stress intensity factor (KIC) was also evaluated by means of microhardness measurements. The correlations between KIC and tensile and compressive strength were examined taking into account the average and maximum size of porosity. The mechanical properties of macro and micro-porous HA are in agreement with the model of Gibson and Ashby.

Experimental investigation of fatigue crack growth in the welding nugget of FSW joints of a 6060 Aluminum Alloy

The work investigates the fatigue behavior of friction stir welded butt joints by means of fracture mechanics techniques. FSW joints of artificially aged AA6060 T6 aluminum alloy were studied. The welding was performed on 8 mm thick butt joined sheets by means of a CNC machine tool. Welding speed in the range between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm were considered. Fatigue crack growth tests were performed according to ASTM E647 standard on CT specimens, under constant load amplitude conditions, at 0.1 minimum to maximum load ratio, with the notch placed in the dynamic recrystallization zone of weld nugget, oriented along the welding direction. The comparison of results demonstrates the crack growth rate is always equal or lower than the base material at low values of stress intensity range. At ΔK values above 12 MPa√m, crack growth rate was found to be higher than base material for high feed, low speed and high feed/speed ratio.

Study of the Effect of DCT and PVD Treatments on the Fatigue Behaviour of AISI 302 Stainless Steel

In this paper the influence of DCT (Deep Cryogenic Treatment) and a CrN arc-deposited PVD (Physical Vapor Deposition) coating on the fatigue behaviour of AISI 302 stainless steel was studied. Rotating bending tests were carried out on standard specimens to evaluate the fatigue limit at 300000 load cycles. The single and the combined effects of the two treatments were investigated by addressing untreated, PVD-coated and both PVD-coated and DC-treated specimens to the tests. All the series of specimens were also tested statically and laboratory analyses including fracture surface SEM observations and hardness measurements were performed.