The influence of heat treatment on the nitriding layer on austenitic steel

The article will be focused on the analysis of the influence of heat treatment on the nitriding layer, which will be applied on austenitic steel. AISI 304 austenitic steel delivered without heat treatment will be used as experimental material. The nitriding layer will be applied to the austenitic steel samples by plasma nitriding. Then, after plasma nitriding, samples will be subjected to heat treatment. Solution annealing and sensitization will be chosen as heat treatment. Experiments deal with microstructural material analysis, fractographic analysis, mechanical and fatigue tests.


Introduction
Austenitic stainless steels find extensive applications as structural materials across diverse industries encompassing aerospace, nuclear, transportation, chemical, and food sectors [1,2].These steels are especially favored for use in elevated temperature settings owing to their commendable mechanical properties, corrosion resistance, and formability [3].Among the stainless steel grades, AISI 304, an austenitic variant, stands out as one of the most versatile due to its exceptional blend of corrosion resistance, weldability, and cost-effectiveness.Moreover, it remains a viable choice wherever nonmagnetic properties are a requisite [4].
These steels exhibit a drawback in terms of their limited resistance to abrasion and hardness.To bolster their mechanical, tribological, and fatigue properties, contemporary utilization of plasma nitriding technology has become prevalent [5].When nitriding austenitic steels at temperatures exceeding 450°C, the precipitation of chromium nitrides occurs.These nitrides sequester chromium from the material matrix, consequently diminishing its corrosion resistance [6].An experiment conducted by Faltejsek et al. [7] highlighted that plasma nitriding substantially heightens surface hardness, consequently enhancing the abrasion resistance of austenitic steels.This research also affirmed that the presence of molybdenum negatively impacts both hardness and the depth of the nitriding layer, as it retards the diffusion of nitrides.In industrial practice, plasma nitriding is typically employed as the final production step before the component is put into practical use [8].
This study seeks to investigate the impact of heat treatment on the nitriding layer applied to AISI 304 austenitic steel.Following plasma nitriding, the samples will undergo additional heat treatment steps, specifically solution annealing and sensitization.The experiments encompass microstructure analysis, fractographic analysis, as well as mechanical and fatigue tests.Micropulse plasma nitriding technology developed by Rϋbig company was used for nitridation of the samples.The nitriding process was carried out in the temperature range of 520-530 °C for 24 hours.Heat treatment by solution annealing took place on AISI 304 samples after plasma nitriding (AISI 304 PN+SA).Conducting the solution annealing without the use of a protective atmosphere, it was employed an electric resistance furnace.The solution annealing process entailed a temperature of 1050°C maintained for 35 minutes, with subsequent cooling achieved through furnace cooling.Heat treatment by sensitization also took place on AISI 304 samples after plasma nitriding (AISI 304 PN+S).In the sensitization process, it was utilized an electric resistance furnace without a protective atmosphere.The sensitization temperature was set at 750°C, maintained for a duration of 10 hours.
A Neophot 32 light microscope was used to evaluate the microstructure of austenitic steels AISI 304 PN+SA and AISI 304 PN+S.Preparing samples for metallographic examination involved adhering to standard metallographic procedures, followed by etching with Kalling's 2 reagent [10,11].
Utilizing the Vickers method, microhardness measurements were conducted on a Zwick/Roell ZHμ microhardness tester, employing selected loads of HV 0.5 and HV 0.01.
Three-point cyclic bending fatigue tests (figure 1a) were performed on samples of austenitic steel AISI 304 PN+SA and AISI 304 PN+S.Utilizing a Vibrophores Amsler 150 HFP Zwick/Roell fatigue machine, the fatigue tests involved preloading the test specimens at -13 kN, followed by cyclic loading at various amplitudes.The loading frequency was maintained at approximately 100 Hz [12,13].The experimental specimens, free of V-notches, were cut from rectangular bars and sized at 55x10x10 mm (figure 1b).
Employing a Tescan Vega LMUII (SEM), it was conducted a fractographic evaluation of fracture surfaces following fatigue tests.This analysis aimed to identify initiation points, scrutinize the fatigue mechanism, and assess the condition of the nitriding layer after heat treatment [14 -16].

Microstructural analysis
Examining the microstructure of austenitic steel AISI 304 PN+SA (figure 2a), it reveals polyhedral austenite grains of varied sizes.Within this microstructure, a substantial presence of sulfides is evident, oriented in the rolling direction, along with annealing twins (AT) that originated during the manufacturing process.The influence of high temperature and subsequent cooling had a substantial impact on deformation martensite (DM).This deformation martensite, present in the initial austenite state, has been entirely disappeared from the microstructure [17].
Analyzing the microstructure of austenitic steel AISI 304 PN+S (figure 2b), it reveals polyhedral austenite grains of varying sizes.Within this microstructure, a substantial presence of deformation martensite and annealing twins is observed, both of which were formed during the production process and persisted even after sensitization heat treatment.The presence of sulfides on the sample, oriented in the rolling direction, is also discernible.The sensitization heat treatment resulted in the precipitation of Cr23C6 carbides along the grain boundaries.

Measurement of microhardness
Microhardness on nitrided samples of AISI 304 austenitic steel after solution annealing and cooling in the furnace and after sensitization was measured from the edge of the sample to its core (figure 4).Microhardness was also measured on nitriding layers (figure 5).
By comparing the microhardness of both states, it can be concluded that the AISI 304 PN+S samples had a higher course of microhardness along the entire length of the measurement.The minimum microhardness on the AISI 304 PN+SA sample was 138 HV 0.5 and the maximum 182 HV 0.5.On the AISI 304 PN+S sample, the microhardness ranged from 253 HV 0.5 to 364 HV 0.5.The microhardness was increased at the beginning of the measurement (below the surface, due to the nitriding layer).

Fatigue lifetime
The S-N curve was derived in semi-logarithmic coordinates from the outcomes of the fatigue tests (figure 6).Analyzing the experimental data, particularly the constructed Wöhler curve, reveals a clear trend: the fatigue lifetime increases with decreasing upper stress amplitude.The results show that the AISI 304 PN+S samples showed a higher fatigue limit at a lower number of cycles than the AISI 304 PN+SA samples.The levels of loading stresses applied in the solution annealed condition on the nitrided specimens were not sufficient to fracture the AISI 304 PN+S specimens, so a higher load had to be set, resulting in a lower endurance life for the AISI 304 PN+S steel, i.e. fewer cycles.Both states show a decreasing nature of fatigue when loads are applied.

Fractographic analysis
After the fatigue tests, the experimental materials were subjected to fractographic analysis.From a macrofractographic point of view, fatigue fracture surfaces after three-point cyclic bending can be divided into three areas: fatigue area, transition area and static failure area (figure 7a and figure 7b).In the case of AISI 304 PN+SA steel, approximately 70% of the fatigue failure is attributed to the fatigue fracture, while for AISI 304 PN+S steel, the fatigue fracture contributes to around 25% of the total fatigue failure.The remaining fractures are distributed between the transition zone and the static failure area.The fracture surfaces of the samples, as depicted in figure 8a and figure 8b, provide insights into the initiation or nucleation site of the fatigue crack.In both cases, the fatigue crack occurred in the corners of the samples.From the images, it is also possible to determine the direction of propagation of the fatigue crack, which was directed to the centre of the samples.Fatigue crack initiations probably started from the nitriding layer, as this layer was visibly damaged by the heat treatment in both states.In the fatigue region of AISI 304 PN+SA steel, there were secondary cracks, a large number of sulphide particle pits and fatigue ductile striations, which were observed in almost the entire fatigue region.Finer striations also appeared in some areas, where the transition between individual striations had a smaller distance (figure 9).After solution annealing, intercrystalline facets of the austenitic grain began to appear in the fatigue region.These facets can be attributed to brittle grain decohesion (figure 10).Even the samples that were affected by the temperature and duration of sensitization showed characteristic signs in the fatigue area.Sulphide dimples were more common.Secondary cracks, which are a manifestation of deviation from the propagation of the main crack, were also demonstrated (figure 11a).The striations created by plastic deformation in the fatigue region were evenly spaced from each other (figure 11b).In both AISI 304 PN+SA steel and AISI 304 PN+S steel, the regions of static failure exhibit transcrystalline ductile failure characterized by a dimple morphology.The dimples vary in size, and there are distinct sulphides, predominantly situated at the bottom of these dimples, as illustrated in figure 12a and figure 12b.

Conclusion
Drawing conclusions from the conducted experiments on austenitic steel AISI 304 PN+SA and austenitic steel AISI 304 PN+S, it can be asserted:  In all states, the microstructure was formed by polyhedral austenite grains with different grain sizes.The microstructure contained a large number of non-metallic inclusions (manganese sulphide) and annealing twins.The AISI 304 PN+S sample contained deformation martensite.
In the AISI 304 PN+SA sample, deformation martensite did not occur in the microstructure because it transformed back to austenite at high temperature. The microhardness of AISI 304 PN+SA steel decreased significantly, compared to AISI 304 PN+S steel.The temperatures of solution annealing were also high enough to reduce the microhardness of the nitriding layer. From the evaluation of the fatigue tests of the loaded samples, it follows that the AISI 304 PN+S steel samples endured a smaller number of cycles but at a higher stress than the AISI 304 PN+SA steel samples. In the fatigue areas of both states, there were pits from sulphide particles, striations, secondary cracks and, in the case of AISI 304 PN+SA steel, also intercrystalline cleavage facets.The area of static failure had a characteristic dimple morphology, and sulphide particles were present at the bottom of the dimples. Heat treatment temperatures had an effect on the nitriding layer.

Figure 1 .
A fatigue experiment; a) three-point cyclic bending test; b) the drawing of sample shape and dimensions.

Figure 2 .Figure 3 .
Microstructure; a) of AISI 304 PN+SA; b) of AISI 304 PN+S.Nitriding layers are visible on the surface of both types of samples, but they are degraded by the heat treatment temperature (figure 3).a) b) Nitriding layer; a) of AISI 304 PN+SA; b) of AISI 304 PN+S.

Figure 4 .Figure 5 .
Figure 4. Measurements of the microhardness; a) courses of microhardness; b) microhardness measurement line of AISI 304 PN+SA; c) microhardness measurement line of AISI 304 PN+S.Measurements of the microhardness of the nitriding layer showed that solution annealing had a significant effect on the microhardness of the layer.The microhardness of the nitriding layer of the sample AISI 304 PN+SA ranged from 346 HV 0.01 to 458 HV 0.01.The sample after plasma nitriding and subsequent sensitization (AISI 304 PN+S) reached an average microhardness of 454 HV 0.01 (the minimum was 353 HV 0.01 and the maximum was 554 HV 0.01).

Figure 7 .
Fractography of fatigue surface after fatigue test; a) for AISI 304 PN+SA; b) for AISI 304 PN+S.

Figure 8 .
Fatigue crack initiation sites a directions of propagation of fatigue; a) for AISI 304 PN+SA; b) for AISI 304 PN+S.

Figure 9 .
Fatigue failure area of AISI 304 PN+SA; a) fatigue striations; b) sulphides and secondary cracks.

Figure 12 .
Area of static failure -transcrystalline ductile failure with dimple morphology; a) for AISI 304 PN+SA; b) for AISI 304 PN+S.

Table 1 .
Chemical composition of the austenitic stainless steel (in wt.%).