CHARACTERISTICS OF AISI 420 STAINLESS STEEL MODIFIED BY COMBINING GAS NITRIDING AND CrN COATING

AISI 420 stainless steel is widely used in applications where good mechanical properties and corrosion resistance are required. However, the heat treatment and nitriding process can reduce the corrosion resistance of this stainless steel. This article focuses on investigating the influence of tempered substrate and efficiency of the gas nitriding at temperatures of 520 °C and 550 °C on some properties of CrN coating. The experiments were carried out to evaluate the surface hardness, microstructure and phase composition of nitrided layers. The coating adhesion and load capacity of the coating were performed according to VDI 3198 standard. Electrochemical testing was performed in a solution of 3.5% NaCl and then using the Tafel method to determine the corrosion current and corrosion potential. The thickness of CrN and CrN/CrN coating was 1.6 μm and 3 μm, respectively. The study showed that the corrosion resistance of coatings fabricated through duplex technology was affected not only by the normal defects but also by the porosity on the nitrided surface. The multilayer duplex coating improved a better corrosion resistance than mono-layer duplex coating due to its ability to cover and reduce pores and pitting defects.


INTRODUCTION
Stainless steel plays an increasingly important role in industries due to its corrosion resistance, hygienic properties, aesthetic appeal, etc. Currently, stainless steel is being studied in diverse directions like metallurgy [1][2][3][4], heat treatment [5][6][7][8][9][10], thermochemical treatment [11][12][13][14][15][16][17][18] and coating technologies [19,20]. Martensitic stainless steels like 410, 420, 440A grade are widely used as moulds for plastic injection and glass moulding, cutting tools, structural parts, automotive components, surgical and dental instruments, where high strength, wear and corrosion resistance are required [11,12]. Bjarbo and Hatterstrand [5] reported that for the steel grade with content of carbon higher than 0.2% and Cr 12-13%, the annealed steel included the carbides M3C, M7C3 and M23C6 (where M is alloy element). The mechanical properties and corrosion resistance depend on the solubility of these carbides into matrix during austenitization and on carbide precipitation during tempering [5,7]. Austenitization temperature of this steel grade at 1000÷1040 °C gave an optimum percentage of carbide distribution in the matrix which resulted in increasing the hardness and corrosion resistance [8,12]. For application required only heat treatment, tempering temperature should be chosen below 370 o C to ensure the corrosion resistance [9]. Kerl-Erick Thelning [21] recommended that martensite stailess steel should be nitrided in the temperature interval from 520 o C to 550 o C, while austenite stailness steel should be nitrided from 570 o C to 620 o C. On the other hand, at this temperature nitriding process generates the chrome nitride CrN and the chromium content was decreased as result, thus corrrosion resistance was also decreased. Pilch et al. [22] performed plasma nitriding at 515 o C and 525 o C for turbocharger stator made of X12Cr13 steel and found that corrosion reistance of untreated surface was better than nitrided one. Other studies also confirmed this phenomenon [14,23]. Many studies tried to conduct nitriding in plasma at a lower temperature (below 500 °C) to improve the corrosion and wear resistance of carbon and low alloy steels [13,24,25]. Unfortunately, the effect of this technique is very limited for cold working steels and martensite stainless steel due to the low productivity [23]. Yang Li et al. [26] performed Active screen plasma nitring for AISI 420 steel at 440 o C and 480 o C for 6 hours, the corrosion resistance was improved but the nitrided layer was too thin (18 µm and 25 µm). Salt bath nitriding (SBN) can form nitrided layer at very low temperature (400 o C). Funatani reported the SBN at 480 o C dies for hot deformation and extrusion [27]. For stainless steels SBN is usually performed at temperature above 520 °C due to melting temperature, fluidity of melted salts and passive layer of chrome oxide. Unlike the SBN, gas-nitriding temperature was recommended at 495 o C ÷ 570 o C [28]. Kolsterising, a carburizing process, is an effective method to treat stainless steel. This technology can produce at 450 o C but it requires a long treatment time (up to 24 hours) [29]. Otherwise, this technology is still not widely spread over the world. Duplex technology characterized by combining nitriding and coating is a very natural idea to promote hardness, corrosion resistance, wear resistance and high temperature resistance of nitrided layer. The coating can be deposited by various methods, such as electroplating, chemical immersing, chemical vapour deposition (CVD), physical vapour deposition (PVD), etc. [20,23,32]. Other studies presented duplex layer-coating technology on the AISI 420 steel [30,31]. From viewpoint of technology investment, gas nitriding is still popular modified surface technology, in particular in developing countries. An important note is that for the martensite stainless steel, the gas nitriding forms a thick compound layer with high porosity. The pores appear not only in the outermost layer of the surface, where there are many nitrogen-rich phases like Fe2-3N or Fe4N but they can be also generated in the diffusion layer because unstable -Fe(N) phase decomposes into -Fe(N)* and nitrogen gas [33,34]. The growth of nitrogen gas leads to growth of pores in the diffusion layer [16]. The number of pores in nitrided layer for gas nitriding was higher than for plasma nitriding due to the low energy of the N atoms, they tend to recombine to form nitrogen gas [34]. AISI 420 martensite stainless steel is a meaningful grade in manufacturing PET (PolyEthylene Terephthalate) die mould [31]. Cause of the operation conditions, the heat treatment is not enough to improve the wear of the die mould. There are not many studies on duplex coating combined gas nitriding and coating on this steel yet. In addition, the effect of surface morphology on duplex coating quality was not much detailed. Therefore, this study focuses on investigating the influence of nitrided layer structure (compound layer thickness) and surface morphology (porosity) on the adhesion and corrosion behaviour of duplex coating fabricated through gas nitriding and CrN coating on the AISI 420 steel.

Material and surface modification
The samples in the dimension of 12 mm diameter x 7 mm thickness were made from AISI 420 martensitic stainless steel. The chemical composition of steel was verified using Optical Emission Spectrometry ARL3460. The measured results are listed in Table 1. These samples were austenitized in the vacuum furnace at 1030 o C for 1.5 hours, following quenched in nitrogen gas with the gas pressure of 1.5 bar. With this pressure, the cooling rate was about 20 o C/min, the hardness after quenching was 55÷56 HRC. After that, the samples were tempered at 580 o C for 4 hours. The hardness after tempering was 42 HRC. Before gas nitriding, the samples were sequentially ground by abrasive papers with grit size to 2000, then sandblasted to remove the chromium oxide layer. The nitriding atmosphere consisted of ammonia and nitrogen gas with a decomposition ratio of about 24% (nitriding potential KN = 7.2). The gas nitriding process was carried out in two variants of temperature: 520 o C (LN -Low-Temperature Nitriding) and 550 o C (HN -High-Temperature Nitriding) for 5 hours. All tempered and nitrided samples were ground with sandpaper #1200, #1500, and then polished using 3 µm Cr2O3 abrasive suspension. The sample was fixed on a holder which enable to keep in hand to grind. A micrometer with accuracy of 1 µm was used to control the thickness of the sample to remove by grinding and polishing process. The aim of this step is to the oxide layer on the surface which affects the properties of the duplex coating [35] and improve surface roughness after nitriding procees to attain surface of pre-coated sample to have the unique surface roughness.
The sample surface was cleaned in alcohol using the ultrasonic bath. Intermediate Cr coating and CrN coating were conducted on nitrided surfaces using Dreava-400 coating system with the following process parameters: Cr target with a purity of 99.99 %, temperature of 300 o C, vacuum pressure of 2.4×10 -2 Pa, argon gas flow of 20 mL/min, the arc current of 60 A. The bias voltage -40 V was applied to reduce the creation of macroparticles on the surface. The Cr coating was deposited for 60 seconds, then nitrogen gas was added to the chamber with a flow rate of 200 mL/min to form a CrN compound film. The mono-layer coating CrN was deposited for 30 minutes. The symbols H/CrN, LN/CrN and HN/CrN were used to refer mono-layer coating on the tempered, low temperature (520 o C) and high temperature (550 o C) nitrided substrates. The multilayer CrN/CrN coating was performed on mono-layer CrN coating. Before second deposition, the samples were polished again to remove macroparticles, subsequently repeated procedure as the first deposition. Thus, this sample group had a total deposition duration of 60 minutes.
The experimental samples were marked as shown in Table 2.

Characterizations of the modified surface
The surface roughness of samples was measured using a Keyence VHX-700 stereomicroscope. The layer/coating structure was captured and analyzed using Axiovert 40 MAT optical microscope, JSM-7600 scanning electron microscope accompanied with Xmax 50 EDS probe and Panalytical X'Pert Pro X-ray diffractometer. The Marble etching solution (4 g CuSO4 : 25 ml HCl : 25 HNO3) was used to observe the nitrided substrate structure of cross-section samples. The surface hardness HV0.1 were measured using Future -Tech Corp 700 hardness tester. The adhesion of the coatings to the substrates was studied using an HRC indentor with a load of 150 kg.f, then the imprints were captured by microscope and evaluate obtained images according to VDI 3198 standard (the principle illustrated in Fig. 1). In addition, the diameter and depth of the HRC imprint were also measured. Fig. 1 Principle of indentation assessment according to VDI 3198 standard [36] An electrochemical test was performed on the Autolab PGSTAT302N device using a standard three-electrode cell with a platinum counter electrode and a saturated calomel electrode as the reference electrode. The used electrolyte, 3.5 % NaCl solution, was reduced oxygen by plunging nitrogen for 1 hour. Before scanning, the sample was installed in the electrode cell and immersed 30 minutes in the electrolyte, the contact square of the sample was 1 cm 2 . The test was performed with a scan rate of 1 mV/s. After each potentiodynamic polarization test, the corrosion potential (Ecorr) and the corrosion current density (icorr) were determined from the Tafel plot.

Surface hardness and microstructure
Measured surface hardness is presented in Fig. 2  The cross-section structure of the coating on the nitrided substrate is shown in Fig. 4. After grinding and polishing, the case depth of HN sample was 50 µm. Measured results on SEM image showed that the coating thickness of mono-layer coating CrN was about 1.6 μm, the multilayer coating CrN/CrN was about 3 μm. The cracks that appeared on the HN substrate was generated at some locations of the compound layer, where the pores existed. Porosity As reported in [13], the nitrided layer of AISI 420 martensitic stainless steel consisted of a thick compound layer consisting of -Fe2-3N, '-Fe4N phases at both temperatures 480 o C and 560 o C. The uniform compound layer with small porosity exhibited better corrosion resistance than the compound layer with higher porosity. The phase composition of nitrided samples (LN and HN samples) measured by the X-ray diffraction technique is shown in Fig. 5. The ε-Fe2-3N and γ'-Fe4N phases were found. In addition, the Fe2O3 phase also existed on the HN/CrN sample, possibly due to the high porosity, therefore oxygen has penetrated the nitrided layer to create iron oxide. Porosity appeared on and within the outermost layer of the surface, where there are many rich-nitrogen Fe2-3N, Fe4N phases. The porosity was calculated from 5 pictures captured at various postions on the surface. When increasing the nitriding temperature from 520 o C to 550 o C (HN sample), the porosity on surface after grinding and polishing increased from 5% (Fig. 6a) to 15% (Fig. 6b). The different in porosity can be refer to the amount of -Fe2-3N phase: With the increase of temperature, the amount of -Fe2-3N phase in the nitrided surface increases, while the amount of -N and γ′-Fe4N phases decreases significantly [16]. It leads to the higher porosity after high-temperature nitriding.
On the surface of the LN/CrN, it can be found macroparticles.
On the surface of the HN/CrN sample (Fig. 6d), there are macroparticles and pore defects. These defects can be the deepest pore location of the nitrided layer that coating could not completely cover.

Adhesion and load resistance
The test adhesion results of coating on various substrates determined according to the VDI 3198 standard are shown in Table 3 and Fig. 7.  The imprint on the H/CrN sample (Fig. 7a)

Corrosion behavior
The polarization curves of electrochemical measurements are shown in Fig. 8. The corrosion potential Ecorr and corrosion current icorr was determined by extrapolation of Tafel curves as shown in Fig. 9.
The results confirmed the decrease of corrosion resistance of both nitrided LN and HN samples compare to tempered sample as results of chromium concentration decrease in steel and pore formation in the compound layer, as mentioned in [7,8,13].
In addition, the low-temperature nitriding LN showed a better corrosion behaviour than the high-temperature nitriding HN due to lower icorr (4.75 mA/m 2 vs. 8.07 mA/m 2 ) and more positive Ecorr (-0.46 V vs. -0.508 V). This effect is related to the porosity of nitrided substrate detailed above. Likewise, with the same CrN coating process, the LN/CrN and LN/CrN/CrN samples exhibited better corrosion results than HN/CrN and HN/CrN/CrN samples. The pores existed on nitrided surface were covered by CrN coating, but at the defect locations due to holes and pinholes, corrosive agents penetrated deeply into the substrate through the pores to the compound layer, even to the diffusion layer.
By impressively reducing corrosion current density icorr simultaneously increasing corrosion potential Ecorr, the CrN coating exhibited a good ability to improve the corrosion resistance of substrate, not only nitrided samples but also tempered samples. This effect observed on multilayer coating CrN/CrN was better on mono-layer coating CrN due to greater thickness and polished surface before the second deposition.

CONCLUSION
This study presents the formation of a nitrided layer on the tempered substrate, the deposition of mono-layer CrN and multilayer CrN/CrN on the nitrided substrate, the influence of nitrided layer structure (compound layer thickness) and surface morphology (porosity) on the adhesion and corrosion re-sistance of duplex coating fabricated by gas nitriding and cathode arc deposition. Gas nitriding at 520 °C formed the nitrided layer with a similar case depth to nitriding at 550 °C (50 µm vs. 55 µm). The XRD analysis results showed more rich-nitrogen ε-Fe2-3N and γ'-Fe4N phases found on the nitrided surface 550 °C. The amount of epsilon phase explains why the hardness of the HN sample was greater and also more porosity than the LN sample. For the mono-layer coating CrN, the coating-nitrided substrate system improved the adhesion and load capacity compared with the coating-tempered system. In the corrosion test, the cover effect of the CrN coating on tempered and LN substrates was good enough, so the icorr value was decreased from 5 to 7 times. Due to the more porosity of nitrided layer LN, the LN/CrN sample had a slightly decreased icorr value from 8.07 mA/m2 to 4.68 mA/m 2 . For the multilayer coating CrN/CrN, the adhesion and load capacity of the coating-substrate system was similar to the mono-layer coating. The mechanical polishing, intermediate coating Cr even function coating CrN allowed to fill in pores resulted in reducing the defect on the surface substrate and therefore increased corrosion resistance impressively. The icorr value decreased 18.5, 237.5 and 5.8 times compared to corresponded tempered, LN nitriding and HN nitriding substrate.