Pore Formation Upon Nitriding Iron and Iron-Based Alloys: The Role of Alloying Elements and Grain Boundaries

Abstract

Pure iron and a series of iron-based Fe-Me alloys (with Me = Al, Si, Cr, Co, Ni, and Ge) were nitrided in a NH 3 /H 2 gas mixture at 923 K (650°C). Different nitriding potentials were applied to investigate the development of pores under ferrite and austenite stabilizing conditions. In all cases, pores developed in the nitrided microstructure, i.e., also and strikingly pure ferritic iron exhibited pore development. The pore development is shown to be caused by the decomposition of (homogeneous) nitrogen-rich Fe(-Me)-N phase into nitrogen-depleted Fe(-Me)-N phase and molecular N 2 gas. The latter, gas phase can be associated with such high pressure that the surrounding iron-based matrix can yield. Thermodynamic assessments indicate that continued decomposition, i.e., beyond the state where yielding is initiated, is possible. Precipitating alloying-element nitrides, i.e., AlN, CrN, or Si 3 N 4 , in the diffusion zone below the surface, hinder the formation of pores due to the competition of alloying-element nitride (Me x N y ) precipitation and pore (N 2 ) development; alloying elements reducing the solubility of nitrogen enhance pore formation. No pore formation was observed upon nitriding a single crystalline pure iron specimen, nitrided under ferrite stabilizing conditions, thereby exhibiting the essential function of grain boundaries for nucleation of pores.

Microstructural development and crystallographic properties of decomposing Fe–N–C compound layers

“…2d. On somes pecimens formation of h or v carbides (as verifiedbyXRD) in conjunction with pores (caused by N 2 development [38]) was observed close to the surface due to decomposition of the original, interstitial-rich e .D uringt he nitrocarburising treatment, spheroidisation (coarsening) of the h lamellae in the substrate, as originallypresent in the untreated material, occurs. Therefore, it is difficult to dissolve, upon nitrocarburising, all initiallypresent h into c or e .The remnants of these h grains are visible as artefacts [39] in the compound layer (see Fig.…”

“…Duringthe secondary annealing treatment in Ar pore formation occurs in the partofthe e layer of highest supersaturation(i. e. the surface adjacent part) [38] leading to loss of nitrogen and the emergence of h .This process becomes visible after approximately 2h,see Fig. 7d.…”

In situ X-ray imaging of defect and molten pool dynamics in laser additive manufacturing

“…1f ). As the melt track cools, the gas solubility reduces significantly in the liquid metal 42 , 43 , forming gas pores (e.g., hydrogen 44 or nitrogen 45 , 46 ) ahead of the solid/liquid interface ( t = 384 ms). These gas pores possibly originated from the gas porosity in the powder 47 or from moisture on the powder surface (or inside the environmental chamber), which can easily transfer into the molten pool during LAM.…”

The Effect of Temperature Distribution during Laser Heat Treatment of Gas-Nitrided 42CrMo4 Steel on the Microstructure and Mechanical Properties

“…Nitriding process is well-known as a technique of surface treatment that improves the mechanical properties of metallic materials [1]. The most common methods of nitriding are as follows: controlled gas nitriding [2][3][4][5][6], plasma nitriding [7][8][9][10][11][12], and low-pressure gas nitriding [13,14]. Controlled gas nitriding and low-pressure gas nitriding is commonly used for constructional and tool steels.…”