Purpose: The aim of this work was to study the microstructure and wear resistance of
hybrid surface layers, produced by a controlled gas nitriding and laser modification.
Design/methodology/approach: Nitriding is well-known method of thermo-chemical
treatment, applied in order to produce surface layers of improved hardness and wear
resistance. The phase composition and growth kinetics of the diffusion layer can be
controlled using a gas nitriding with changeable nitriding potential. In this study, gas
nitriding was carried out on 42CrMo4 steel at 570°C (843 K) for 4 hours using changeable
nitriding potential in order to limit the thickness of porous e zone. Next, the nitrided layer
was laser-modified using TRUMPF TLF 2600 Turbo CO2 laser. Laser tracks were arranged
as the multiple tracks with scanning rate vl=2.88 m/min and overlapping of about 86% using
the two laser beam powers (P): 0.21 kW and 0.26 kW. Microstructure was observed by an
optical microscope. Phase composition was studied using XRD. Hardness profiles in the
produced hybrid layers was determined using a Vickers method. Wera resistance tests were
performed using MBT-01 tester.
Findings: Gas nitriding resulted in formation of compound zone, consisting of e nitrides
close to the surface and a zone, composed of e + g' nitrides. Below the white compound
zone, the diffusion zone occurred with nitric sorbite and precipitates of g' nitrides. In the
microstructure after laser heat treatment (LHT) of nitrided layer, the zones were observed
as follows: the re-melted zone (MZ) with e nitrides, nitric martensite and non-equilibrium
FeN0.056 phase, the heat-affected zone (HAZ) with nitric martensite and precipitates of g'
phase and the diffusion zone (DZ) without visible effect of laser treatment. Laser beam power
influenced the depth of MZ and HAZ, so the thickness of hardened zone. The hardness of
MZ was slightly decreased compared to the hardness of compound zone after gas nitriding.
However, the significant increase in hardness was observed in HAZ. The formation of hybrid
layers advantageously influenced the tribological properties. The laser-heat treated nitrided
layers were characterized by improved wear resistance compared to the only gas-nitrided
layer.
Research limitations/implications: The effect of LHT on the microstructure and
properties of gas-nitrided layer was limited to the two laser beam powers. In the future
research, this range should be exceeded, especially, taking into account the lower values of
laser beam power. It will result in laser modification without re-melting.
Practical implications: The selection of suitable LHT parameters could provide the
hybrid layers of modified microstructure and improved wear resistance.
Originality/value: This work was related to the new concept of modification of nitrided
layer by laser heat treatment.
Investigation of laser heat treated Monel 400
