influence of sea water on the fatigue strength and notch sensitivity of a plasma nitrided b–mn...

Materials Science and Engineering A247 (1998) 204–213

Influence of sea water on the fatigue strength and notch sensitivityof a plasma nitrided B–Mn steel

Pedro De la Cruz 1, Torsten Ericsson *Di6ision of Engineering Materials, Department of Mechanical Engineering, Linkoping Uni6ersity, S-581 83 Linkoping, Sweden

Received 9 April 1997; received in revised form 13 October 1997

Abstract

Notched and smooth cylindrical plasma nitrided (PN) and quench and tempered (Q&T) steel specimens made of a B–MnSS2131(:AISI 15B21H) steel have been exposed to constant amplitude plane reversed bending corrosion fatigue tests (R= −1)at 47 Hz in sea water. S–N curves show that sea water suppresses the fatigue limit and reduces fatigue strength (especially at longlives) of smooth and notched Q&T and PN specimens. Plasma nitriding improves the corrosion fatigue resistance of Q&Tspecimens; this is associated with the good corrosion resistance of o and g %-phases, the enhancement of corrosion and fatigue bycompressive residual stresses, and the consumption of H+ ions during reduction of nitrogen. This improvement is more significantfor smooth specimens and for long lives. Notch sensitivity of Q&T and PN specimens decreases with fatigue life. Pitting corrosion,cyclic applied stress and residual stresses due to plasma nitriding are the main topics of this work. An equation to predictcorrosion fatigue strength from air fatigue strength of smooth specimens, has been established. © 1998 Elsevier Science S.A. Allrights reserved.

Keywords: Corrosion fatigue; Fatigue life; Plasma nitriding; Notch sensitivity; B–Mn steel

1. Introduction

Many steel components and structures are subjectedto the simultaneous effect of cyclic loads and aggressiveenvironments, resulting in a high reduction of fatigueresistance, compared with cycling in air or other inertenvironments. That complex phenomenon is called cor-rosion fatigue and is one of the most common causes offracture. It is well known that the air fatigue limit isproportional to the static ultimate tensile strength, andthat the same is true for steels in moderately corrosiveenvironments. However, the corrosion fatigue strengthof steel in aggressive environments, like sea water, isindependent of static strength [1].

Corrosion fatigue is affected by several interdepen-dent mechanical, metallurgical and environmental vari-ables [2–11] which may affect the crack initiationand/or crack propagation. Some mechanisms have been

proposed [4,9–21] to explain the enhanced crack propa-gation rates. They comprise the single or mutual occur-rence of anodic dissolution and/or hydrogenembrittlement at the crack tip. Regarding the effect ofmean stress, it has been shown [1] that the corrosionfatigue strength increases sharply on the compressionside of mean stress, while on the tension side, the effectof mean stress is very small. It has been suggested [9,21]that the introduction of compressive mean stresses dueto surface treatments can enhance the corrosion fatigueresistance.

Corrosion fatigue failures usually start at the metalsurface because of direct interaction with the aggressiveenvironment. Surface treatment is an effective way toimprove the corrosion fatigue resistance and there aretwo general ways to combat corrosion. One is bringingthe potential of the metal surface to a value where themetal is immune to attack and the other one is provid-ing the right conditions to get a slow corrosion rate.Beneficial surface treatments against corrosion fatigueinclude those which introduce compressive residualstresses on the metal surface and/or remove surfaceirregularities acting as stress raisers [9,19–21].

* Corresponding author. Tel.: +46 13 281168; fax: +46 13282505.

1 Permanent address: Departamento de Fısica, Universidad Na-cional de Trujillo, Apartado 315, Trujillo, Peru.

0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.

PII S0921-5093(97)00738-7

P. De la Cruz, T. Ericsson / Materials Science and Engineering A247 (1998) 204–213 205

Table 1Chemical composition of SS 2131 B–Mn steel

C Si Mn P S Cr Ni Mo Ti B N

0.00090.25 0.29 1.27 0.024 0.008 0.42 0.09 0.01 0.03 0.004

Nitriding is one of the methods used to improvewear, fatigue and corrosion properties of metal sur-faces. Corrosion studies of conventionally nitrided[22,23] and plasma nitrided (PN) [24–34] steels showthat nitriding increases the corrosion resistance of lowalloy steels but it usually deteriorates the corrosionresistance of stainless steels. It has been shown that thecompound layer has better corrosion resistance thanthe diffusion zone. Despite its technological and scien-tific importance, very few studies [35,36] have dealt withcorrosion fatigue of PN steels. The purpose of thiswork has been to study the effect of sea water andplasma nitriding upon the fatigue strength and notchsensitivity of a B–Mn steel. The effect of plasma nitrid-ing on the air fatigue strength and fracture of the sameB–Mn steel has been studied elsewhere [37].

2. Experimental details

2.1. Material

The base material was a B–Mn steel, SS 2131, and itschemical composition is listed in Table 1. The steel wassupplied as hot rolled bars 20 mm in diameter. The barswere machined and ground in circumferential directionto a surface finish Ra of 0.5 mm in the test section. Twotypes of fatigue specimen geometry were prepared, asmooth one (Kt=1.05) and a notched one (Kt=1.7),

see Fig. 1. The specimens were heat treated in a protec-tive endogas of neutral carbon potential comprisingaustenitisation at 850°C for 1 h, quenching in oil, andtempering at 525°C for 1 h, resulting in a hardness of290 HV, a yield strength (0.2% offset) of 779 MPa, andan ultimate strength of 873 MPa. No geometrical dis-tortion or decarburisation of the specimen was ob-served after heat treatment.

Both types of specimens were PN2 at 480°C for 24 hresulting in a surface hardness of 644 HV5. The mi-crostructure and the microhardness depth profile wasobtained using traditional metallographic methods. Thehardness was measured in the cross section of the gaugelength of the specimens, with a Vicker indentor at 300g (2.9 N) applied load.

Phase analysis and residual stress determination as afunction of depth was conducted with a Seifert PDS3000 X-ray diffractometer using CrKa radiation and ascintillation detector. The procedure is described in anearlier work [37].

The microstructure of the nitrided layer was investi-gated using metallographic analysis.

2.2. Potentiostat polarisation test

The electrochemical behaviour of quench and tem-pered (Q&T) and PN specimens were determined bypotentiostatic polarisation tests, using a Wenking po-tentiostat model 70HC3, in a corrosion cell containingartificial sea water, prepared according the ASTM stan-dard D 1141 86 with pH 8.2. The measured pH agreedwith this value. The electrochemical potentials werereferred to the saturated calomel electrode (SCE).

2.3. Corrosion fatigue testing

The corrosion fatigue test of Q&T and PN smoothand notched specimens was carried out in constantamplitude plane reversed bending with R= −1 at 47Hz and room temperature. A minimum of ten speci-mens were used for each S–N curve. A polynomial wasfitted by the least square method to the data points.The applied strain was monitored continuously duringa fatigue test from permanently mounted strain-gaugeson the beam of the fatigue machine, which were previ-ously calibrated against a smooth standard specimen.

Fig. 1. Geometry of (a) smooth (Kt=1.05) and (b) notched (Kt=1.7)specimens for plane reversed bending fatigue. Measurements in mm.

2 The plasma nitriding procedure was performed by Bruken Hard-verkstader A8 lvsjo, Sweden.

P. De la Cruz, T. Ericsson / Materials Science and Engineering A247 (1998) 204–213206

Fig. 2. Cross sectional optical micrograph of the white layer.

[37]. The microhardness profile for smooth specimensshowed a peak hardness of 680 HV at a depth of 100mm below the surface, and a hardening depth of 0.43mm (base hardness +50 HV). The microhardnessprofile for notched specimens showed a hardness valueof :585 HV at :70 mm below the surface and ahardening depth of 0.30 mm, i.e. the smooth PN speci-mens have a more extended diffusion zone than thenotched PN specimens. The residual stress depth profile[37] showed that, up to :1 mm below the surface,plasma nitriding generates a compressive residual stressprofile. The maximum compressive stress has a value of:−640 MPa at :30 mm below the surface. Theequilibrating tensile residual stresses were distributedover the remaining 4 mm up to the centre of the crosssection.

The polarisation curves for PN and Q&T specimensare shown in Fig. 4. The corrosion potential and corro-sion current, obtained from the polarisation curvesderived with the sweep rate of 50 mV (5 min)−1, are:−640 mV (SCE) and 9×10−5 mA mm−2 for Q&Tmaterial and :−580 mV and :6x10−5 mA mm−2

for PN material, that is, the PN material is more noblethan the Q&T base material. It is also seen that Q&TB–Mn steel is subjected to a uniform attack in seawater, while, when it is PN, the general attack isseverely reduced and a tendency to passivation is ob-served. As the corrosion rate is proportional to thecorrosion current, it can be deduced from Fig. 4 thatthe corrosion rate of PN material is much lower than ofQ&T material at lower potentials but tends gradually tothe corrosion rate of Q&T material at high potentials.

The S–N corrosion fatigue curves for smooth andnotched Q&T and PN specimens are shown in Figs. 5and 6, respectively. Fatigue curves in air for smoothand notched specimens [37] are also displayed.

The results obtained from the S–N curves are sum-marised in Table 2, that is, the values of: fatiguestrength at three different fatigue lives, changes incorrosion fatigue strength with respect to the corre-sponding air fatigue strength DCFair, the improvementof corrosion fatigue behaviour due to plasma nitriding,the effective stress concentration factor Kf and thenotch sensitivity defined as Kf/Kt.

It is seen from Figs. 5 and 6 and Table 2 that thefatigue limit has been eliminated for Q&T smooth andnotched specimens by the effect of sea water. PNsmooth specimens show run outs (defined at 2×107

cycles), and the stress amplitude value (180 MPa) corre-sponds to 72% of the fatigue limit in air (635 MPa).The air fatigue limit of PN notched specimens has beenremoved by sea water, Fig. 6. The endurance limit forboth types of Q&T and PN specimens, are also reducedby sea water and the reduction increases as fatigue life,N, increases.

The specimens were surrounded by a cylindrical plasticchamber along the gauge length. The corrosive mediumwas obtained by pumping sea water (pH 7.2) from avessel. Sea water from the Pacific Ocean at Peru’s coast,was brought to the laboratory and fed through a recir-culatory system to the corrosion cell, so that the gaugelength of the specimens was immersed in sea water. Thesea water was renewed frequently.

The effective stress concentration factor Kf and thenotch sensitivity were calculated by using Eq. (1) andEq. (2) respectively [38].

Kf=Ss/Sn (1)

Kf/Kt=Notch sensitivity, (2)

where Ss and Sn are the fatigue strengths of smooth andnotched specimens as read from the polynomial curvesand Kt is the elastic stress concentration factor.

The fracture surfaces were examined using an opticalstereomicroscope and a scanning electron microscope(JEOL 6400) equipped with an EDS system.

3. Results

Identical fatigue specimens have been characterisedin a study on air fatigue [37]. The microstructure of thenitrided B–Mn steel used consists of a white layer witha thickness of 2–6 mm and a diffusion zone, see Fig. 2.Very thin channels are observed at the top of the whitelayer. The results of X-ray diffraction analysis at theouter surface without removing the white layer indicatethe presence of o-Fe2–3N and g %-Fe4N nitrides, and steelsubstrate, Fig. 3(a). After removal of surface material,which leaves a diffusion zone of only g %-phase and steelsubstrate, the peaks from the o-phase disappear at :26mm, Fig. 3(b). From a depth of :78 mm, only the steelsubstrate was observed, Fig. 3(c).

Plasma nitriding introduced a high hardness andcompressive stresses in the surface treated B–Mn steel


Fig. 3. X-ray diffraction pattern from PN layer at different depths: (a) 0, (b) 26, (c) 78 mm.

In the region NB105 cycles the S–N corrosion fa-tigue curve for notched PN specimens tends to thecorresponding S–N air fatigue curve, Fig. 6. The S–Ncorrosion fatigue curve for smooth and notched Q&Tspecimens are slightly above the corresponding air fa-tigue curves.

In the region 2×105BNB106 cycles, there is asignificant reduction of the fatigue strength for bothtypes of PN specimens (see Table 2 and Figs. 5 and 6).Q&T specimens, especially the notched type, showlower corrosion fatigue strength reduction than PN inthat region. The effect of sea water on fatigue reductionincreases with fatigue life. The fatigue strength reduc-tion for PN smooth specimens is a little lower than

those for notched specimens. Plasma nitriding ofsmooth specimens increases the corrosion fatigue resis-tance relative to Q&T treatment while there is nosignificant difference between notched specimens.

In the region N\106 cycles, the largest reduction infatigue strength occurs. There is no significant differ-ence between the corrosion fatigue strength of smoothand notched Q&T specimens. The corrosion fatiguestrength of smooth PN specimens is higher than that ofnotched specimens, see Figs. 5 and 6. The highestcorrosion fatigue improvement due to plasma nitridingis observed for smooth specimens in this region.

Fig. 5. Wohler curves of fatigue and corrosion fatigue of Q&T andPN smooth specimens. The air fatigue data are from [37].

Fig. 4. Polarisation curves for Q&T and PN specimens in artificial seawater (pH 8.2).


Fig. 6. Wohler curves of fatigue and corrosion fatigue of Q&T andPN notched specimens. The air fatigue data are from [37].

rms similar findings reported in the literature. However,it is expected that the corresponding values for the seawater (pH 7.2) used in the corrosion fatigue tests areactually higher than the calculated ones, because highcontents of sulphur were measured in the surface layerand fracture surface of corrosion fatigued specimensand it is known that sulphides, always present in pol-luted water, greatly accelerate attack on steel [39]. Noanalysis of the sea water was carried out however.

The higher corrosion resistance exhibited by PN steelis due to the good corrosion resistance of the o andg %-phases present in the compound layer, as has beenwell established. The polarisation curves show maximafor the current density, at :0, +600 (not well defined)and +1000 mV (SCE). Similar maxima has been ob-served in other studies of corrosion of PN steels[23,27,32]. Some of these peaks have been related topartial passivation [23]. Chyou and Shih [30] haveestablished a potential (E)–pH diagram for nitrided4140 steel in various concentrations of NaCl solutionsat 25°C, Fig. 14. It is shown there that the verticalboundary (constant pH) between the general corrosionregion and the perfect and imperfect regions, located atpH 8 for unnitrided steels, is shifted to pH 6; thereforethe region where general corrosion occurs is reduced bythe effect of plasma nitriding. Chyou and Shih foundthat nitrogen atoms in the nitrided layer could bereduced electrochemically to ammonium ions NH+

4 .From the E–pH diagram for unnitrided steel [30], it isseen that for pH 8 and E\−0.77 mV (SCE), generalcorrosion is expected, which agrees well with the corro-sion behaviour of the base material in the presentstudy, (Fig. 4). For pH 8, the E–pH diagram fornitrided steel predicts different corrosion behaviours fordifferent potential ranges: immunity (EB−750 mV(SCE)), general corrosion (−750BEB−460 mV(SCE)), perfect passivity (−460BEB40 mV (SCE))and imperfect passivity (E\+40 mV (SCE)) withpitting corrosion at a pitting potential of :570 mV(SCE) for 3.0% NaCl solutions. By the help of theseE–pH diagrams, the peaks at 0 and +600 mV (SCE)in the polarisation curve (Fig. 4) can be interpreted asthe limits between the perfect–imperfect passivity, andthe pitting potential, respectively. Chyou and Shih [29]report the break down of a predicted passive filmduring corrosion of PN SAE 4140 steel in 0.5% NaCl(pH 2.68) at potentials higher than 600 mV (SCE).

4.2. Crack initiation mechanisms

Fatigue cracks for Q&T and PN specimens are ini-tiated at the surface. General corrosion is observed inQ&T specimens. The corrosion types observed at thesurface of corrosion fatigued PN specimens, (Fig. 14),are intergranular corrosion and pitting corrosion. Fa-tigue cracks have mostly been associated with pits, like

Notch sensitivity of Q&T and PN specimens in seawater decreases with fatigue life, PN specimens beingmore notch sensitive than Q&T specimens, Fig. 7. Atintermediate and long lives, notch sensitivity of Q&Tspecimens in sea water is much smaller than in air,whereas, at intermediate lives, notch sensitivity of PNspecimens is higher in sea water than air, and at longlives the difference is rather small.

Figs. 8–13, show details of fracture surface andsurface layer appearances for PN smooth and notchedspecimens. Multiple crack initiation points are observedin some specimens. Fatigue cracks are initiated at thesurface and they are associated to hemicircular pits(Figs. 8 and 9), and machining marks. A higher amountof Cr and Mn has been observed inside the pits withrespect to its surroundings. Both, compound zone anddiffusion zone, exhibit a transgranular fracture, (Fig. 8)but some specimens show partially intergranular frac-ture. Short cracks are observed in the white layer. Pitsare also observed on the outer surface; some of themare isolated (Fig. 10), and others are coalesced alongthe machine marks (Figs. 11 and 12). The surface ofcorrosion fatigue specimens tested shows intergranularcorrosion, and pitting corrosion, preferentially alongmachining marks, Fig. 13.

4. Discussion

4.1. Corrosion beha6iour

The corrosion rates (or penetration depths) calcu-lated [39] with the corrosion current obtained for Q&Tand PN material in artificial sea water, are :100×10−3 mm year−1 and :70×10−3 mm year−1, respec-tively. The higher corrosion resistance shown by PNmaterial in comparison with the B–Mn base steel confi-


Table 2Summary of the corrosion fatigue results

N2 (1×106 cycles)N1 (2×105 cycles) N3 (2×107 cycles)Fatigue life

Notched Smooth NotchedSmooth SmoothSpecimen type Notched

CF strength [MPa]460 (465)Q&T 385 (350) 270 (416) 250 (260) 100 (416) 115 (260)

423 (560) 395 (665) 298 (560)604 (765) 180 (635)PN 154 (560)

DCFair (%)−1Q&T +10 −35 −4 −76 −56

−24 −41 −47−21 −72PN −73

+10 (+60) +46 (+60) +19 (+115) +80 (+53) 34 (+115)CF changes due to PN (%) +31 (+65)

Kf

1.08 (1.60)1.19 (1.33) 0.87 (1.60)Q&T1.33 (1.19)PN 1.17 (1.13)1.43 (1.37)

Kf/Kt,mech

0.63 (0.94)0.70 (0.78) 0.51 (0.94)Q&TPN 0.78 (0.70)0.84 (0.81) 0.69 (0.66)

Numbers in parenthesis represent the corresponding air fatigue values [37]. CF, corrosion fatigue; Q&T, quench and tempering; PN, plasmanitriding; DCFair, changes in corrosion fatigue strength with respect to the corresponding air fatigue strength.

the one shown in Figs. 9 and 10. Possible surface sitesfor pit nucleation are slip bands, grain boundaries,channels formed by pores in the outer compound layer,inclusions and machine marks. No nucleation at inclu-sions and slip bands has been observed in this work.

During corrosion fatigue, the corrosion rate deter-mined from potentiostatic measurements is enhanced bythe cyclic applied stress [17,40], but this enhancement iscounteracted by the compressive residual stresses presentin the surface nitrided layer, which shift the electrodepotential to more negative values [9]. The result is ageneral corrosion with relatively low corrosion rate.

Chyou and Shih [30] propose that the channels ob-served in the compound zone are due to coalescence ofvoids formed by decomposition of o-Fe2–3N and g %-Fe4N nitrides near the surface.

Sea water enters the channels and Cl− ions attackthe nitrides o-(Fe,Cr)2–3N and g %-(Fe,Cr)3N4, producinga preferential dissolution of the iron according to thereaction [41]:

2Fe2+ +2H2O+OH−

=Me(OH)+ +2H+ +Me(OH)2(aq) (3)

The corrosion products at pH 8.2 are soluble [41], sothat no protective film is formed, whereas the produc-tion of H+ ions continuously lowers the pH, promot-ing an accelerated corrosion. However, the dissolutionof iron causes an enrichment of Cr and N in thesurroundings of the oxy-nitrided layer, and when thepH is lower than 4, according to the E–pH diagram,Fig. 14, reduction of nitrogen in nitrides starts follow-ing the reaction [30]:

[N]+4H+ +3e− =NH+4 (4)

It is seen from this reaction that a great consumption ofH+ ions occurs, so that the local acidity generated byEq. (3) is avoided, raising the pH and the pittingpotential to higher values. In this way the better corro-sion and corrosion fatigue properties of the PN layerthan those of the core material can be understood. Asa consequence of the above process, the upper part of a

Fig. 7. Notch sensitivity Kf/Kt as a function of fatigue life in air andsea water.


Fig. 8. Scanning electron micrograph showing a pit with a corrosionfatigue crack growing from its bottom. The length of the arrowindicates approximately the width of the white layer.

Fig. 10. Scanning electron micrograph showing a isolated pit at thesurface of a PN corrosion fatigued specimen.

Hence, after a while, the bottom of the pit can reachthe interface between the compound and the diffusionzone.

Corrosion attack can occur at plastically deformedareas of a metal, e.g. at the base of the pit in thediffusion zone, with the non deformed area of thecompound zone acting as a cathode. The dissolution ofthe base of the pit is enhanced by the highly unfa-vourable area ratio: large cathodic area to small anodicarea. Therefore, a very high localised attack occurs atthe base of the pit with practically no corrosion of thecompound zone. This selected attack is responsible forthe morphology shown by the pits, Figs. 9 and 10.

Due to the severe localised corrosion, a micro-notchis formed at the base of the pit. Under the appliedstress, the stress concentration at the tip of the micro-notch produces a microcrack. The microcrack can notgrow rapidly because of the high compressive stressacting on it, and its advance is due to dissolution of thediffusion zone along the grain boundaries, see Fig. 9.Eventually, the microcrack reaches the critical size to

pore channel is corroded faster than the deeper part.The dissolution process can become critical for poreslocated on grain boundaries and especially on machinemarks where stress concentration and crevice may oc-cur producing a higher dissolution of the walls of thechannel and eventually a hemicircular pit is formed.

Fig. 9. Scanning electron micrograph of fracture surface of a PNspecimen showing pit (marked with an arrow) and channels in thecompound zone suggesting the corrosion fatigue mechanisms.

Fig. 11. Scanning electron micrograph showing a pit located along acircumferential long crack at the surface of a corrosion fatigued PNspecimen.


Fig. 12. Scanning electron micrograph showing coalescence of pitsalong circumferential direction on the surface of a PN corrosionfatigued specimen.

Fig. 14. Schematic E–pH diagram for (a) unnitrided and (b) nitridedsteel. See Chyou [30].

become a long fatigue crack. For a given externalapplied stress, the long crack grows faster in a corrosiveenvironment than in air because (1) crack growth rate isenhanced by dissolution of the metal at the crack tip,and (2) the formation of pits and microcracks mayproduce relaxation of the compressive residual stresses.

Based on the proposed mechanism for crack initia-tion, the effect of sea water on the corrosion fatiguestrength and notch sensitivity at different life regions isdiscussed below.

4.2.1. NB105 cyclesThe negligible effect of sea water on the fatigue

behaviour of both types of PN specimens observed inthis region can be explained by the relatively shortinteraction time associated with this fatigue life regionand the high corrosion resistance of PN material, seeFig. 4. The higher fatigue resistance shown by the Q&Tand the notched PN specimens when they are fatigued

in sea water may be due to a cooling effect of the seawater in this region with cyclic plastic deformation.

4.2.2. N\105 cyclesHigh fatigue strength reduction occurs at N\105

cycles for both types of PN specimens. The greatestfatigue reduction takes place at very long lives, (smallcyclic applied loads). Long fatigue lives mean that theinteraction time between sea water and compound zoneis enough to incubate pits at the base of the channels,to nucleate the microcrack and to form the long crack,which grows until final fracture occurs. For the sametype of specimens and plasma nitriding treatment, thefatigue crack growth threshold DKth for a surface crackin air was estimated to :2.5 MPa m1/2 and an appliedload of 1000 MPa was necessary for a small crack togrow to a critical size of :20 mm [37]. The propagationof surface microcracks under loads much lower than1000 MPa and the suppression of the fatigue limitsobserved in this study, may be interpreted as a reduc-tion of the air threshold stress intensity value DKth, dueto (1) residual stress relaxation associated with micro-

Fig. 13. Different types of corrosion attack observed on the surfacenitrided layer of corrosion fatigued specimens.


cracks and pit formation, (2) elimination of residualstresses by dissolution of the nitrided layer, and (3)crack propagation enhancement produced by sea waterat the tip of the microcrack.

It can be concluded that in this region both pittingcorrosion and mechanical factors are actingsimultaneously.

4.3. Prediction of corrosion fatigue strength

A model is proposed to predict the fatigue strengthof notched PN specimens at long lives, based on thefollowing facts:1. The interaction time between the corrosive medium

(sea water) and the surface of fatigue specimen isenough to form hemicircular pits on the surfacenitrided layer.

2. The pit acts as a mechanical notch.3. Linear relations hold between the fatigue strength S

and the logaritm of the number of cycles to failureN (Basquin’s law) [43].

4. For high notch sensitive materials the effective fa-tigue stress concentration factor for a mechanicalnotch, Kf,mech, can be related to the stress concentra-tion factor, Kt,mech, through the expression [42]:

Kf,mech=K1/(1+n)t,mech (5)

where n is the strain hardening exponent of the mate-rial. The final equation is written:

Scorr,noth=Sair,smooth

CK1/(1+n)t,mech

(N)−m (6)

where Sair,smooth and Scorr,smooth are the air and corrosionfatigue strength of PN smooth specimens respectivelyand C and m are material constants derived from thefatigue curves for smooth specimens and their valuesare C=0.07 and m=0.22. By using Eq. (6) the corro-sion fatigue strength of a notched specimen Scorr,notch

can be predicted from the observed air fatigue strengthof smooth specimens. The value for n=0.19 corre-sponds to the value of the strain hardening exponentreported by Qian and Fatemi [44] for a Q&T SAE 1045steel with hardness 350 BHN. The predicted valuesfound for Scorr,notch agree well with the experimentalvalues, see Fig. 15.

For low notch sensitivity materials, Eq. (6) should bereplaced by the expression [42]:

Kf,mech=K2/(3+n)t (7)

5. Conclusions

The observed suppression of the fatigue limits maybe interpreted as a reduction of the air threshold stressintensity value DKth, due to (1) residual stress relaxation

associated with microcracks and pit formation, (2) elim-ination of residual stresses by dissolution of the nitridedlayer and (3) crack propagation enhancement producedby sea water at the tip of the microcrack.

The corrosion fatigue notch sensitivity of Q&T andPN specimens are reduced with increasing number ofcycles.

In the short life region (relatively high stress ampli-tudes) there is not enough time for sea water to form apit and the mechanical processes are the controllingfactors.

At N�105 cycles, there is time enough for pit nucle-ation and the subsequent corrosion fatigue crack nucle-ation in the channels of the compound zone, thereforea remarkable reduction of corrosion fatigue strengthoccurs. At very long lives, the corrosion fatiguestrength of smooth Q&T and PN is practically the sameand slightly higher than that of the correspondingnotched specimens. In this region the notch sensitivityof Q&T and PN specimens in sea water decreases as apower function of N, the notch sensitivity of PN beingmuch higher than the one for Q&T specimens. In thisregion pitting corrosion and mechanical factors areacting simultaneously.

The improvement of corrosion fatigue by plasmanitriding, observed in this work, is associated to (1) thegood corrosion resistance of the o- and g %-phases form-ing the compound zone, (2) the enhancement of corro-sion and fatigue resistance due to the compressiveresidual stresses existing in the compound and diffusionzone and (3) the large consumption of H+ ions duringthe reduction of nitrogen. This improvement increaseswith fatigue life. In contrast to air fatigue, the corrosionfatigue improvement is much higher for smooth thanfor notched specimens.

The effect of sea water upon the fatigue of PNspecimens has been modelled. It has been assumed that

Fig. 15. Corrosion fatigue strength prediction (using Eq. (6)) fornotched PN specimens.


sea water produces pits, which act as a mechanicalnotch. The derived equation predicts well the corrosionfatigue strength of PN notched specimens from theobserved air fatigue strength of PN smooth specimens.

Acknowledgements

The authors are grateful to the International Pro-gram in the Physical Sciences of Uppsala University forits financial support of this work. The authors arethankful for the help received from Mrs A. Billeniusand Mr N. Larsson during the experimental work.

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influence of sea water on the fatigue strength and notch sensitivity of a plasma nitrided b–mn...

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