Advance Complex Liquid Nitriding of Stainless Steel AISI 321 Surface at 430Â Â°C

Advance Complex Liquid Nitriding of Stainless Steel AISI321 Surface at 430 �C

Yuanhua Lin, Jun Wang, Dezhi Zeng, Runbo Huang, and Hongyuan Fan

(Submitted September 28, 2012; in revised form January 17, 2013; published online April 16, 2013)

Liquid nitriding of type 321 austenite stainless steel was conducted at low temperature at 430 �C, using atype of a complex chemical heat-treatment; and the properties of the nitrided surface were evaluated.Experimental results revealed that a modified layer was formed on the surface with the thickness rangingfrom 2 to 30 lm varying with changing treatment time. When the stainless steel subjected to the advancedliquid nitriding less than 8 h at 430 �C, the main phase of the nitrided coating layer was the S phasegenerally. When the treatment time prolonged up to 16 h, S phase formed and partially transformed to CrNsubsequently; and then the fine secondary CrN phase precipitated. All treatments performed in the currentstudy can effectively improve the surface hardness. The nitrided layer thickness changed intensively withthe increasing nitrided time. The growth of the nitride layer took place mainly by nitrogen diffusionaccording to the expected parabolic rate law. The highest hardness value obtained in this experiment wasabout 1400 Hv0.25. Low-temperature nitriding can improve the corrosion resistance of the 321 stainless steelagainst diluted vitriolic acid. The immerse test results revealed that the sample nitrided for 16 h had thebest corrosion resistance than the others. SEM examinations indicated that after nitriding, the corrosionmechanisms of the steel had changed from serious general corrosion of untreated sample to selectivitycorrosion of nitrided samples in the diluted vitriolic acid.

Keywords advanced liquid nitriding, corrosion resistance, hard-ness, microstructure, stainless steel AISI 321

1. Introduction

Austenitic stainless steels AISI 321 are extensively used inthe chemical and food industries due to their excellentcorrosion resistance (Ref 1-4). However, the low surfacehardness and poor wear resistance has restricted their applica-tions in engineering fields (Ref 5-9). Nitriding is an effectiveprocess for improving the surface hardness and anti-wearproperties of stainless steel (Ref 10-18).

Compared to conventional gas nitriding and ion nitriding,the liquid nitriding treatment (called the ‘‘salt bath nitriding’’ insomewhere) was regarded as an effective, low-cost methodwith many advantages, such as low treatment temperature,short treatment time, high degree of shape and dimensionalstabilities, and reproducibility (Ref 17-19). Moreover, Li et al.(Ref 19, 20) claimed that the complex liquid nitriding process isconsidered as an effective engineering technology to improvecorrosion properties of alloy steels. Funatani (Ref 14) insistedthat the liquid nitriding technology is an environment-friendlyprocess and that a combination of high fatigue resistance and

good wear and corrosion resistance can be achieved. Salt bathprocess technology can solve environmental problems and canbe applicable to the hardening of stainless and high alloy steelswith high reaction efficiency (Ref 14). This process is anitrocarburizing process, since the environment of molten saltcontains both carbon and nitrogen. The two elements generallydiffuse into the surface of steel parts, simultaneously (Ref 11).

But austenitic stainless steel is known as a difficult materialfor nitriding (Ref 21, 22). The standard liquid nitridingtechniques fail to improve mechanical properties without losingcorrosion resistance. At these temperatures, such as 550-580 �C, the great mass precipitation of CrN results in adepletion of Cr from the matrix. This induces a strong decreaseof the corrosion resistance that greatly degrades the beneficialeffect of increased hardness by common liquid nitriding (Ref22-26). Some reports insisted that to avoid the precipitation ofCrN, nitriding must be carried out at a low temperature, andthen a so-called S phase with high hardness and better corrosionresistance can be obtained on the surface of the austeniticstainless steel (Ref 1-6). But with the high natural melting pointof the most composition of salt medium, the development oflow-temperature liquid nitriding technology meet with bigobstacles. Fortunately, Hiroyuki et al. (Ref 12) nitrided an SUS304 stainless steel tube in a molten salt (LiCl-KCl-Li3N) at500 �C with different applied potentials and suggested that themolten salt electrochemical process can be applied to speci-mens of various shapes. Hamdy et al. (Ref 10) conducted thenitridation of 316 stainless steel samples in a closed systemcontaining KNO3 salt bath (the nitrate bath) in the presence ofultra pure N2 gas atmosphere at 450 �C. The nitrided stainlesssteels performed the higher electrochemical corrosion resis-tance to chloride ions attack after 2 weeks of immersion in3.5% NaCl solution than the untreated samples.

Yuanhua Lin and Dezhi Zeng, State Key Lab of Oil and GasReservoir Geology and Exploitation, Southwest Petroleum University,Chengdu 610500, People�s Republic of China; and Jun Wang, RunboHuang, and Hongyuan Fan, School of Manufacturing Science andEngineering, Sichuan University, Chengdu 610065, People�s Republicof China. Contact e-mail: [email protected].

JMEPEG (2013) 22:2567–2573 �ASM InternationalDOI: 10.1007/s11665-013-0545-8 1059-9495/$19.00

Journal of Materials Engineering and Performance Volume 22(9) September 2013—2567

Unfortunately, there is insufficient knowledge about theeffects of nitriding parameters on microstructure and propertieswhen complex liquid nitriding is done on stainless steel AISI321 at low temperature. Therefore, the aim of this study is tomake an attempt to investigate the influences of liquid nitridingprocess on surface microstructure and properties of 321stainless steels, by X-ray diffraction (XRD), micro-hardnesstest, scanning electron microscopy (SEM), transmission elec-tron microscope (TEM), and corrosion resistance experiments.

2. Experiment

The material used in this work is austenite stainless steelAISI 321, and its chemical composition is shown in Table 1.

The specimens were cut from a hot-rolled steel plate. Aftersimple pretreatment of derusting and degreasing, the samples of321 steel were dipped in the molten salt to nitride at 430 �C fordifferent hours and then cooled in air to room temperature.After the treatment, samples were ultrasonically cleaned inalcohol bath for 15 min.

The salt medium for nitrocarburizing was mainly composedof M2CO3 (M denotes elements of K, Na, and Li), CO(NH2)2,and some trace components. CNO� concentration in the saltwas above 40%. The pretreatment to remove passive film of thesteel samples was not necessary because the CNO� in salt bathhas a strong reducing potential.

The nascent nitrogen utilized for nitriding reaction comesfrom the dissociation of CNO�: 4CNO� fi CO3

�2+2CN�+CO + 2[N] (Ref 19). Concentration gradient between samplesurface and nitriding salt bath turned out to be the driving forcefor active nitrogen atom permeating into austenite structure,leading to surface nitride layer formation. In the meantime, afew nascent carbon which comes from the dissociation of CO:2CO fi CO2 + [C] also permeates into austenite structuretogether with nitrogen.

The microstructural transformation in the modified layerwas investigated using cross sections for optical microscopyand the Type JSM5910-LV and Hitachi S4800 scanningelectron microscopy (SEM) with the Oxford EDS tester. X-ray diffractometer type Dmax-1400 with Cu K alpha radiationand a nickel filter were used to determine the phases present inthe modified layer. The nitrided specimens were also preparedfor transmission electron microscopy (TEM) studies using aJEM 2010 operating at 200 kV. Careful mechanical polishingin SiC No. 4000 emery paper was performed down to athickness of about 50 lm. Then, Ar bombardment at 3 keVwas carried out in a model 691Gatan precision ion polishingsystem.

Corrosion tests were performed by immersing the untreatedand nitrided samples in a 15% H2SO4 water solution at roomtemperature for up to 120 h. The back and the side of the disksamples were masked with several layers of lacquer, leavingonly one flat surface, with a theoretical area of 620 mm2, in

contact with the testing solution. After certain time interval,samples were removed from the solution. They were cleanedand dried, and then measured with a balance to an accuracy of0.1 mg, so that the weight loss due to corrosion was obtained.Two batches of tests were performed, and each data reported inthis paper represent the average value of two tests.

Dynamic polarization experiments were performed using acommercial electrochemical system (Model CS310, WuhanCorrTest Instrument Co. Ltd, China). The scan rate was 5 mV/sand the experiments were conducted in the 3.5 wt.% NaClsolution at room temperature (20 �C). The reference electrodewas saturated calomel electrode (SCE), the counter electrodewas platinum plate (Pt), and the samples were connected withthe working electrode. The electrodes were prepared by epoxycold resin mounting of specimens, leaving areas for exposure tothe electrolyte of about 1 cm2.

3. Results and Discussion

3.1 Metallography Analysis

The microstructure produced during liquid nitriding of typestainless steel AISI 321 at 430 �C is shown in Fig. 1. Theetchant is the Marble solution. The modified layer clearlyappears to be ‘‘featureless’’ at a higher magnification underSEM. According to Li�s hypothesis (Ref 24), these phenomenaindicated that the modified layer obtained a possible enhance-ment in corrosion resistance to the harsh etchants by low-temperature nitriding. But the substrate does not. So thenitrided layers are distinguished from the substrate due to thedifference etching degree. Some tiny secondary precipitateswere transformed when the specimens nitrided over 16 h. Thecorrosion resistances of precipitate zone became worse. Theprecipitate zone can be easily observed under the microscopeafter reagent etched.

Figure 2 shows the thickness of the nitrided layer, as afunction of square root of the treated time at 430 �C. It can beseen that the thickness of the treated layer increased withprolonging treated time. In this case, it shows a parabolic ratelaw with time. According to Tsujimura (Ref 12, 13), this resultsuggested that this chemical nitriding was controlled by adiffusion process.

3.2 Phase Analysis of the Nitrided Layer in Stainless SteelAISI 321

The X-ray diffraction patterns at the untreated and nitridedsamples are shown in Fig. 3. According to these patterns, it isobvious that the phase composition of nitrided layers on 321 steeldepends on treatment time. The phases present in the untreated321 sample are dominated by austenite. After complex nitridingtreatment at 430 �C for 1 h, the sample microstructure ischaracterized by the peaks of the expanded austenite, also calledthe ‘‘S phase.’’ When the nitrogen diffuses inward, the grainlattice is supersaturated by nitrogen to such extent that thetransformation of austenite into expended austenite, i.e., S phaseformed. Therefore, nitriding after 1 h at 430 �C produces anoticeable peak at a diffraction angle 2h of 41� and 46�,corresponding with (111) and (200) crystal planes of austenite,which is in line with the earlier observations (Ref 1).

Broad S phase diffraction peaks, which correspond to thenitrogen expanded austenite and which are typical of low-

Table 1 Composition of type austenite stainless steelAISI 321 (mass.%)

Element C Si Mn Cr Ni Ti Fe

Mass.% 0.04 0.54 1.33 17.55 9.00 0.48 Bal.

2568—Volume 22(9) September 2013 Journal of Materials Engineering and Performance

temperature nitriding of stainless steel (Ref 3-7), can be seen inaddition to the austenite reflections from the substrate material.This broadening is probably due to the gradient of nitrogen,residual stresses, and possible defect structure of the nitridedlayers (Ref 21-23). It can be observed that the intensity of thepeaks related to the S phase increases as the nitriding timeincreases. Gradual peak shift can be observed, and especiallythe (200) reflection has moved considerably toward larger dspacing. The layer of 1 h nitriding also showed the evidentexpanded austenite peaks. Table 2 gives the estimated latticeparameters and interplanar spacing from Bragg�s law as afunction of nitriding time. The observation that the latticeparameter becomes larger when nitriding time increases is ingood agreement with the results obtained in the other studies(Ref 9-12). The lattice parameter calculated from (200)becomes considerably higher than lattice parameters calculatedfrom (111) planes. This behavior of the lattice parameter resultsobviously from the gradual distortion of the cubic symmetry ofthe lattice (Ref 7).

According to Christiansen (Ref 3), the nitrogen solubilitiescorresponding to AISI 321 as the occupancy, yN, i.e., the

Fig. 1 Cross-sectional microstructure of sample nitrided at tempera-ture 430 �C (a 4 h; b 8 h; c 16 h; d 16 h in SEM)

Fig. 2 The thickness of the nitrided layer versus processing time

Fig. 3 XRD patterns of nitrided and unnitrided 321 SS for differenttime (at 430 �C)


fraction of the interstitial sublattice occupied by nitrogen atoms,as a function of the nitriding time are presented in Fig. 4.Clearly, yN increases with prolonging the nitriding time. Innitriding 16 h, the peaks of the S phase are shifted to lowerangle though some fine CrN transformed.

The microstructure obtained by SEM and EDS of nitridedlayer of the complex nitrided sample at 430 �C for 16 h isshown in Fig. 5. From the pictures, it can be obviouslyobserved that some tiny zone has discrepant alloy compositionsin the modified layer after nitriding for 16 h; and there is a sub-layer, which is mainly containing Cr near the outer surface.

3.3 Hardness

The microhardness of nitrided layers as a function of thetreatment time is shown in Fig. 6, where the hardness increasedwith the increase of time. The hardness increase with time isdue to the raise of nitrided layer thickness and high nitrogencontent in the layer. For the nitrided at 430 �C for 8 h sample,the hardness of the sample was increased by approximately afactor of 4 compared with the substrate. The highest hardnessvalue obtained in this experiment was about 1400 Hv0.25.

The typical microstructures of the modified layer of low-temperature molten salt nitriding stainless steel AISI 321 areshown in Fig. 7. The observed results demonstrate that thestacking fault, the dislocation group, and the twin structure areprominent features in the modified layer. This special micro-structure is caused by nitrogen and carbon atom invadingduring nitriding at 430 �C. The deformed twins were inducedby the stress resulting from supersaturating nitrogen. Thestacking fault precipitates or deformed twins have a randomstacking fault arrangement. Xu et al. (Ref 29) also observed the

twin structure during plasma nitriding of AISI 304 stainless atlow temperature.

The extremely high values of the microhardness achievedcan be explained at large by the great mismatch induced stressfields associated with the plenty of dislocation group, twinstructure, and stacking fault caused by the supersaturation ofnitrogen in solid solution. In other words, it is the solutionstrengthening, the dislocations strengthening, and the precipi-tates strengthening that result in greatly hardening of themodified layer.

Fig. 4 The nitrogen solubility of 321 changes with different time(at 430 �C)

Fig. 5 Cross-sectional microstructure and EDS of sample nitridedfor 16 h at 430 �C

Table 2 Lattice parameters and lattice spacing calculated from (111) and (200) planes for different nitriding time

Nitriding time, h a(111), nm a(200), nm d(111), nm d(200), nm Dd/do(111), % Dd/do(200), %

0 0.3603 0.3603 0.20785 0.18037 0 01 0.3774 0.3925 0.21792 0.19625 0.048 0.0884 0.3809 0.3941 0.21995 0.19706 0.058 0.0938 0.3849 0.3983 0.22231 0.19916 0.070 0.104

16 0.3898 0.3998 0.22503 0.19989 0.0827 0.10840 … 0.3974 … 0.19869 … 0.102

Fig. 6 Microhardness of 321 SS salt bath nitrided for various timeat 430 �C


3.4 The Corrosion Behavior

Figure 8(a) shows the corrosion weight losses of theuntreated and nitrided 321 steel after immersion in 15%H2SO4 solution for up to 120 h. To avoid the influence of thesurface black oxide film in the nitrided sample, the oxide filmwas carefully cleared away by the cloth finishing mop beforeimmersed in the solution. The corrosion rate, in a unit of gramsper square meter per day (gmd or g m d) was calculated and isshown in Fig. 8(b). It can be indicated that the longtime liquidnitriding can effectively improve the corrosion resistance of the321 stainless steel. The longer time nitride sample (16 h) hadthe lowest corrosion weight loss and the best corrosion ratesthan others (Fig. 8). During the test, some gas bubblescontinuously emitted from the surface of non-treated sample,

and after the test, the untreated sample surface became veryrough and attached with many dark-colored corrosion products.

Micrographs of the corroded surfaces on the untreated andnitrided 321 steel after immersion in 10% H2SO4 for 120 h areshown in Fig. 9. The SEM examinations indicated that thecorrosion mechanisms changed from the serious generalcorrosion in the untreated samples to selectivity corrosion inthe nitrided samples. For the untreated sample, the seriousuniform corrosion zones had penetrated to a great depth,although dissolution preferably occurred along the grainboundaries and some crystallographic directions (Fig. 9a). Inthe high magnified picture (Fig. 9c), it clearly observed that thesubstrate corroded by harsh acid and resulted in a great numberof tubular structure. In comparison, corrosion of the nitrided16 h sample was more likely to be in a selectivity corrosionformat. Some selectivity corroded zones can indeed beobserved, but their amount and depth were much smaller. Inthis sense, it may be concluded that the longtime nitriding hasmodified the corrosion properties of the 321 stainless steel indiluted vitriolic acid. Lei and Zhang (Ref 16) examined1Cr18Ni9Ti steel after nitriding at 280-480 �C and found thatcN has good corrosion resistance to pitting in 1% NaCl and togeneral corrosion in 0.5 M H2SO4.

Fig. 7 Microstructure of 321 SS salt bath nitrided for 40 h at430 �C

Fig. 8 Corrosion weight loss and corrosion rate of 321 stainlesssteels in 20% H2SO4 water solution. (a) weight loss and (b) corro-sion rate


The polarization corrosion of the different treatment condi-tions of specimens was studied in the cycle volt amperepolarization method. Typical potentiodynamic polarizationcurves of the different conditions obtained in 3.5 mass.% NaClsolution at room temperature are shown in Fig. 10 for compar-ison. Icorr, anodic/cathodic Tafel slopes, and corrosion ratesmeasured for specimens in different conditions with the help ofpotentiodynamic polarization technique are presented in Table 3.

The experiment results revealed that there is a greatdifference in the corrosion rate in different heat treatment

conditions. When nitrided for 8 h, the corrosion rate of thespecimen was 1.64E�03 mm/a. This was less than the rate ofthe untreated sample (3.68E�02 mm/a) obviously. After thelongtime nitriding (40 h), several phases of the compound wereformed on the sample�s surface. Those phases had differentelectrode potentials, once contacted with electrolyte; theprimary battery was formed, which made the corrosionresistance degraded. From the above analysis, a conclusioncan be obtained that the proper low-temperature liquid nitridingtechnology can significantly improve the corrosion resistanceof austenite stainless steel AISI 321 in the 3.5 mass.% NaClsolution.

Several mechanisms have been proposed to explain thebeneficial role of nitrogen in stainless steel (Ref 23-28). Aplausible theory suggested that nitrogen in stainless steel willdissolve during corrosion process and consume the acid in pitby a reaction of: [N] +4H+ +3e� fi NH4. This causes a localneutralizing effect in the acidic pits on the corrosion surface,leading to a decreased growth rate of corrosion. Kuczynska-Wydorska et al. (Ref 11) suggested that it is one of theimportant processes in the mechanism by which invadednitrogen improves the passivation of nitrided sample.

It is common knowledge that the beneficial effect ofnitrogen can only be realized when nitrogen is in solidsolution. Once chemical compounds are transformed, as thatoccurred in nitriding at a relative higher temperature or longertreatment period, chromium in the matrix will be depleted,particularly around the nitrided precipitates. In this case, thelongtime (16 h) low-temperature nitridied samples have apretty thick modified layer in surface. From the immersing testresults, it is obvious that the 16 h sample, with the thickestmodify layer, has the best corrosion resistance in all samples.

Fig. 9 Micrographs of the corroded surfaces on (a) untreated, (b)4 h and (c) 16 h at 430 �C salt nitrided 321 steel after immersion in15% H2SO4 for 120 h

Fig. 10 Comparative polarization curves for different conditions in3.5 mass.% NaCl solution at room temperture

Table 3 Corrosion parameter for various specimensobtained in 3.5% NaCl solution at room temperature bythe cycle volt ampere polarization method

Time, h Icorr, mA/cm2 bA, mV bC, mV Corrosion rate, mm/a

0 3.19E�03 123.81 55.82 3.68E�028 1.42E�04 79.74 74.35 1.64E�0316 4.30E�03 99.73 62.64 4.96E�0240 1.42E�03 109.61 59.28 1.63E�02


The 4 h sample has better corrosion resistance than non-treatment sample. This indicated that the thickness of thesurface-modified file after nitrided play a great role in corrosionresistance. The thin deposition layer on top of the nitrided caseappeared to have mitigated the corrosion attack to a certainextent during immersing tests. This could have related to thedifferent compositions and structure in the deposition layer.Further investigations are needed to verify the mechanisminvolved.

4. Conclusion

From the above-mentioned investigations, it may be con-cluded that when the stainless steel AISI 321 was subjected tocomplex liquid nitriding at 430 �C, themain phase of the nitridedcoating layer was the expanded austenite (S phase) generally.When the treatment time prolonged up to 16 h, S phase is formedand subsequently transforms partially into CrN. Then the finesecondary CrN phase precipitated. All treatments can effectivelyimprove the surface hardness. An expanded austenite layer wasformed on the surface of substrate with the thickness rangingfrom 2 to 30 lm. The nitrided layer depth thickened and changedintensively with the nitriding time. The growth of the nitridelayer takes place mainly by nitrogen diffusion according to theexpected parabolic rate law. The proper low-temperature nitrid-ing can improve the corrosion resistance against the dilutedvitriolic acid. After nitrided for 16 h, the sample has the bestcorrosion resistance than others. SEM examinations indicatedthat after longtime nitriding, the corrosion mechanisms of the321 steel have changed from the general corrosion in non-treatedsample to the selectivity corrosion in the nitrided samples.

Acknowledgments

The authors are very grateful to the National Natural ScienceFoundation of China (Grant No. 50901047) for financial support ofthis research work; and the author (J.W) would like to thank Prof.Luo Defu of Xihua University, People�s Republic of China, for hisvaluable discussions during the course of the research.

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Advance Complex Liquid Nitriding of Stainless Steel AISI 321 Surface at 430Â Â°C