Parameters which characterize the nitriding atmosphere in the gas nitriding process of steel are: the nitriding potential K N , ammonia dissociation rate α and nitrogen availabilitym N2 . The article discusses the possibilities of utilization of the nitriding atmosphere's nitrogen availability in the design of gas nitriding processes of alloyed steels in atmospheres derived from raw ammonia, raw ammonia diluted with pre-dissociated ammonia, with nitrogen, as well as with both nitrogen and pre-dissociated ammonia. The nitriding processes were accomplished in four series. The parameters selected in the particular processes were: process temperature (T ), time (t), value of nitriding potential (K N ), corresponding to known dissociation rate of the ammonia which dissociates during the nitriding process (α). Variable parameters were: nitrogen availability (m N2 ), composition of the ingoing atmosphere and flow rate of the ingoing atmosphere (F In ).Keywords: Controlled gas nitriding, Nitrogen availability, Nitriding potential, Ammonia dissociation rate, Alloyed steels Parametrami charakteryzującymi atmosferę azotującą w procesie azotowania gazowego stali są: potencjał azotowy K N , stopień dysocjacji amoniaku α oraz rozporządzalność azotu m N2 . W artykule omówiono możliwości wykorzystania rozporzą-dzalności azotu atmosfery azotującej w projektowaniu procesów azotowania gazowego stali stopowych w atmosferach azotują-cych uzyskanych z amoniaku, z amoniaku rozcieńczonego zdysocjowanym amoniakiem, azotem oraz azotem i zdysocjowanym amoniakiem. Procesy azotowania wykonano w czterech seriach. Parametrami ustalonymi w poszczególnych seriach była temperatura procesu (T ), czas (t), wartość potencjału azotowego (K N ), któremu odpowiada określona wartość stopnia dysocjacji amoniaku zdysocjowanego podczas procesu azotowania (α). Parametrami zmiennymi była rozporządzalność azotu (m N2 ), skład atmosfery wlotowej i natężenie przepływu atmosfery azotującej wlotowej (F w ).
Purpose: The effects of laser processing parameters on the dimensions of simple laser
tracks, produced on the previously nitrided layer, were analysed.
Design/methodology/approach: Gas nitriding is one of the most commonly used
thermochemical treatment, resulting in many advantageous properties: high hardness,
enhanced corrosion resistance, improved wear resistance and fatigue strength. However, an
unfavourable increase in the thickness of compound zone (e + g’) close to the surface was
observed after conventional gas nitriding. This was the reason for undesirable embrittlement
and flaking of the layer. Therefore, a controlled gas nitriding was intensively developed,
reducing the percentage of the most brittle e (Fe2-3N) iron nitrides. In this study, the hybrid
surface layer was produced. The controlled gas-nitriding was followed by laser heat
treatment (LHT). Laser modification was carried out using various laser beam powers and
scanning rates. The dimensions of laser tracks (i.e. depths and widths of re-melted zone and
heat-affected zone) were measured. Numerical methods were used in order to formulate a
mathematical model.
Findings: Laser processing parameters (laser beam power and scanning rate) influenced
the microstructure obtained. The microstructure of laser modified nitrided steel with
re-melting consisted of re-melted zone (MZ), heat-affected zone (HAZ), nitrided layer without
visible effects of laser treatment and the substrate. The use of laser beam power of 0.26 kW
resulted in only a partial re-melting of the compound zone. The two characteristic values of
laser beam power were estimated. P0MZ corresponded to the laser beam power at which the
re-melted zone disappeared (i.e. width and depth of MZ were equal to 0). P0HAZ was a value
of laser beam power at which the effects of laser irradiation were invisible in microstructure
(i.e. width and depth of HAZ were equal to 0). The model was proposed in order to predict
the effects of LHT on microstructure.
Research limitations/implications: The presented model was limited to the scanning
rates in the range of 2.24-3.84 m/min. In the future research, this range should be exceeded,
especially, taking into account the lower values of scanning rate.
Practical implications: The presented model could be used in order to control the
microstructure and properties of hybrid surface layers, obtained as a consequence of the
controlled gas-nitriding and LHT.
Originality/value: his work is related to the new conception of laser modification of
nitrided layers. Such a treatment provided the hybrid layers of new advantageous properties.
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