G. Pradelli

Glow-discharge nitriding of sintered stainless steels

Abstract The glow-discharge nitriding process is particularly suitable to harden the surface of sintered stainless steels components, owing to the high porosity levels of these materials. On wrought austenitic stainless steels this treatment produces a metastable phase, known as supersaturated austenite or S phase, which has shown high hardness values and good corrosion resistance. In the present paper the influence of glow-discharge nitriding process on the microstructural and mechanical properties of AISI 316L austenitic sintered stainless steel has been evaluated and it is compared with the results obtained with ion-nitrided martensitic (AISI 410) and ferritic (AISI 430L) sintered stainless steels. The ion-nitriding treatment, performed at 773 K for 4 and 8 h, produces modified surface layers. The microhardness profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observed on AISI 316L samples, in comparison with AISI 410 and AISI 430L samples. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face centred tetragonal lattice as base for this phase fits well the diffraction spectra, in respect of the ‘traditional’ face centred cubic lattice usually adopted, since the lattice shows a strong distortion, in spite of this, the d -spacing values calculated with a f.c.t. base show a good agreement with literature data, when the used f.c.c. indexing is modified for the f.c.t. lattice.

Effects of load and sliding speed on the tribological behaviour of Ti6Al4V plasma nitrided different temperatures

Abstract Dry sliding tests were carried out under different load and sliding speed conditions on Ti6Al4V alloy plasma nitrided at three temperatures 973, 1073 and 1173 K. The results were interpreted on the basis of the evolution of the frictioc basis of the evolution of the friction coefficient and characterizing the wear debris and worn surfaces in order to understand the acting wear mechanisms. A comparison was also made with the results obtained with untreated specimens, tested under the same conditions. Plasma nitriding can noticeably improve the dry sliding resistance of the Ti6Al4V alloy. The nitriding temperature must be chosen according to the main wear mechanism observed under specific load and sliding speed conditions. When wear is determined by the resistance of the compound layer (low loads and low sliding speeds), the nitriding treatment has to be carried out at 1073 K. In this case, the compound layer has optimal properties with respect to resistance to adhesion and fragmentation. When the material is exposed to delamination (high loads and high sliding speeds), the strength of the diffusion layer has to be maximized. In this case, the nitriding temperature should be as high as possible (1173 K in the present investigation) in order to enhance the hardening of the diffusion layer.

Influence of the treatment atmosphere on the characteristics of glow-discharge treated sintered stainless steels

Abstract The surface hardening of sintered stainless steel components is a major goal, in order to extend the applications of these materials. The aim of the present study was to evaluate the influence of the treatment atmosphere on the microstructure and mechanical properties of AISI 316L austenitic and AISI 410 martensitic sintered stainless steels by performing glow-discharge nitriding and nitrocarburizing treatments. The treatments, performed at 773 K for 8 h, produced modified surface layers. The use of a nitrocarburizing atmosphere is able to promote the formation of e carbonitride [Fe 2−3 (N, C)], especially on AISI 410 martensitic samples. The microhardness profiles of the treated samples show high hardness values in the modified layers and a steep decrease to matrix values. The AISI 316L samples have thinner hardened layers and lower hardness values, in comparison with AISI 410 ones. Thinner hardened layers with a steeper decrease to matrix values are obtained using a nitrocarburizing atmosphere, if compared with those of nitrided samples. On the modified layers of both the nitrided and nitrocarburized AISI 316L samples the metastable S phase is detected. The X-ray diffraction analysis has shown that its lattice parameter values, evaluated by fitting it with a face centred tetragonal lattice, are influenced by the treatment atmosphere and that the amount of this phase increases when a nitrocarburizing atmosphere is used.

B transus temperature influence on α-β titanium alloys behaviour in ion-nitriding process

Abstract A study has been made in order to determine the transition α (h.c.p.) ⇒ β (b.c.c.) temperatures in some two-phase α-β titanium alloys for industrial use: for this aim D.T.A. tests and for comparison, specific metallographic analyses on samples specially heat treated have been carried out. The correlation between β transition temperatures, treatment temperatures and the behaviour of the tested alloys in the ion-nitriding process has been examined: thus, it has been possible to define in which measure the selection of the treatment temperature, with reference to the B transition temperatures, determines the phases present in the nitrided surface layers and their mutual distribution. Finally, the morphological characterization of the hardened surface layers, obtained through the thermochemical nitriding treatment, has been defined: in particular, the nitrogen distribution in the phases present in the surface layers has been studied by means of back scattered electron analysis techniques.