Carlos Eduardo Pinedo

On the kinetics of plasma nitriding a martensitic stainless steel type AISI 420

Plasma nitriding was employed to treat martensitic stainless steel type AISI 420. The ability to remove the passive film from the surface is an important advantage in this process in order to guarantee a homogeneous surface treatment. The resulting nitrided surface shows the presence of a compound layer and a diffusion zone. The interface between the diffusion zone and the substrate is flat, as a consequence of the high chromium content on the alloy. The precipitation of chromium nitrides is also responsible for the high levels of hardening obtained. A value up to 1350 HV 0.025 constituting a maximum hardness was obtained. Diffusion depth increases with an increase of the nitriding temperature as measured by optical microscopy and verified by the hardness profiles. Calculated diffusion coefficients of nitrogen were very low compared to the data for alpha iron, and allow calculation of the Arrhenius behaviour during the process.

The use of selective plasma nitriding on piston rings for performance improvement

Plasma nitriding process was used in this work for surface treatment of gasoline engine piston rings. The research was conducted to optimise metallurgical properties and to improve the manufacturing route of the rings. The results are compared to the conventional gas nitriding process. Selective nitriding of thin wall gasoline piston rings is very important because the side surfaces are deleterious to the engine performance. Plasma nitriding proved that selective nitriding is possible using a simple mounting system, leading to homogeneous properties on both inner and outer diameter surfaces. Only a diffusion zone, without grain boundaries nitrides, composes the nitrided microstructure. The surface hardness is increased up to 1100 HV0.1. The mechanical tests showed that the performance of selective plasma nitrided rings is superior to the gas nitrided counterpart. The wear rate is 30% higher for the plasma nitrided rings, but acceptable for practical purposes.

Estrutura e propriedades do aço inoxidável austenítico AISI 316L Grau ASTM F138 nitretado sob plasma à baixa temperatura

Austenitic stainless steels cannot be conventionally nitrided at temperatures near 550°C due to the intense precipitation of chromium nitrides in the diffusion zone. The precipitation of chromium nitrides increases the hardness but severely impairs corrosion resistance. Plasma nitriding allows introducing nitrogen in the steel at temperatures below 450°C, forming pre-dominantly expanded austenite (gN), with a crystalline structure best represented by a special triclinic lattice, with a very high nitrogen atomic concentration promoting high compressive residual stresses at the surface, increasing substrate hardness from 4 GPa up to 14 GPa on the nitrided case.

Low temperature plasma carburizing of AISI 316L austenitic stainless steel and AISI F51 duplex stainless steel

O aco inoxidavel austenitico AISI 316L e o aco inoxidavel duplex AISI F51 (EN 1.4462) foram cementados sob plasma-DC na temperatura de 480oC, utilizando-se CH4 como gas de arraste. A cementacao sob plasma a baixa temperatura conduziu a uma elevada supersaturacao do reticulado cristalino em carbono com a formacao de austenita expandida(γC), sem a precipitacao de carbonetos. A dureza do aco 316L, apos a cementacao, atingiu um valor maximo de 1000 HV, devido a supersaturacao de ∼ 13 at% de carbono e a expansao do reticulado cristalino CFC. Para o aco inoxidavel duplex AISI F51, os graos de austenita se transformaram em austenita expandida pelo carbono e os graos de ferrita se transformaram para ferrita expandida com a precipitacao de carbonetos do tipo M23C6, na camada cementada. A dureza da camada cementada, no aco F51, atingiu 1600HV, devido ao efeito combinado da expansao dos reticulados cristalinos da austenita e da ferrita com a precipitacao fina e dispersa de carbonetos M23C6.

On the α to γ' nitride transformation after plasma nitriding and aging a low carbon steel

Phase transformations during nitriding are dependent on the nitrogen content diffused into the ferritic matrix during the thermochemical treatment. According to the iron-nitrogen phase diagram [1] the nitrogen solubility limit in ferrite is 0.1% in mass. The increase in nitrogen content leads to iron nitride precipitation. The equilibrium iron nitrides in the Fe-N system are the Fe4N (γ ′) and Fe2-3N (e). According to Jack [2] for certain substrate compositions and processing parameters a metastable nitride is formed before γ ′ precipitation, the α′′-Fe16N2 face centred tetragonal nitride. This nitride is formed on {001} planes of the ferritic matrix [2–4]. The α′′ nitride precipitates show a plate like morphology, whereas the γ ′ phase precipitates as needles. Precipitation of α′′ occurs in the 100–220 ◦C temperature range, with an activation energy of 115 kJ/mol.

Solid State Alloying by Plasma Nitriding and Diffusion Annealing Treatment for Austenitic Stainless Steel

Nitrogen has been added to stainless steels to improve mechanical strength and corrosion resistance, high Nitrogen steel production is limited by high gas pressure requirements and low nitrogen solubility in the melt. One way to overcome this limitation is the addition of Nitrogen in solid state because of its higher solubility in austenite. However, gas and salt bath nitriding have been done at temperatures around 550°C, where nitrogen solubility in the steel is still very low. High temperature nitriding has been, thus proposed to increase nitrogen contents in the steel but the presence of oxide layers on top of the steel is a barrier to nitrogen intake. In this paper a modified plasma nitriding process is proposed. The first step of this process is a hydrogen plasma sputtering for oxide removal, exposing active steel surface improving nitrogen pickup. This is followed by a nitriding step where high nitrogen contents are introduced in the outermost layer of the steel. Diffusion annealing is then performed in order to allow nitrogen diffusion into the core. AISI 316 austenitic stainless steel was plasma nitrided and diffusion annealed at 1423K, for 6 hours, with 0.2 MPa nitrogen pressure. The nitrided steel presented ∼ 60 μm outermost compact layer of (Fe,Cr) 3 N and (Fe,Cr) 4 N with 11 wt. % N measured by surface depth profiling chemical analysis - GDS system. During the annealing treatment the nitride layer was dissolved and nitrogen diffused to the core of the sample leaving more even nitrogen distribution into the steel. Using this technique one-millimetre thick sample were obtained having high nitrogen content and uniform distribution through the thickness.

NITRETAÇÃO SOB PLASMA POR TELA ATIVA E PLASMA DIRETO/DC DO AÇO INOXIDÁVEL ENDURECÍVEL POR PRECIPITAÇÃO TIPO 17-4 PH

Resumo A nitretação do aço inoxidável endurecível por precipitação do tipo 17-4PH foi estudada comparativamente pelos processos de Plasma-DC e de Tela Ativa. Os resultados mostram que os dois processos são eficientes em promover a formação da camada nitretada de elevada dureza superficial, cerca de 1300 HV0,025. Entretanto, diferenças importantes são observadas. No processo de Plasma-DC a camada nitretada é composta, além da martensita acicular saturada em nitrogênio, de nitretos de ferro e cromo precipitados pela supersaturação. No processo de Tela Ativa a precipitação de nitretos de cromo é praticamente suprimida e predominando os nitretos de ferro. Esta diferença na sequência de precipitação de nitretos é inferida como sendo causada pela oferta de nitrogênio. Em Plasma-DC a oferta de nitrogênio é suficiente para a precipitação do nitreto de cromo, o que não ocorre em Tela Ativa. A maior profundidade de camada e de endurecimento transversal confirma a maior difusão de nitrogênio em Plasma-DC. Ainda, em ambos os processos a dureza de partida do substrato envelhecido é preservada após o ciclo de nitretação. Palavras-chave: Aço 17-4 PH; Nitretação sob plasma; Tela ativa; Endurecimento.