Tribological and vibrational characteristics of AISI 316L tested at elevated temperature and 600 Torr vacuum

K.Arul Rj ,M.P.Arunkumr ,P.K.C.Kniglpul ,M.Krthikeyn

a Department of Mechanical Engineering,Koneru Lakshmaiah Education Foundation,Vaddeswaram,Guntur,Andhra Pradesh,522502,India

b Department of Mechanical Engineering,RajaRajeswari College of Engineering,Bangaluru,Karnataka,India

Keywords:Pin-on-disc Sliding wear Vacuum De-lamination Scanning electron microscope Vibration

A B S T R A C T Friction and wear studies enable the investigation of material interaction between two sliding surfaces in contact.In the present investigation,the coefficient of friction and the wear resistance of AISI 316 L parts were studied under self-mating,dry sliding conditions using a pin-on-disc type configuration.The experiments were conducted at vacuum based high temperature pin-on-disc tribometer.The 4 mm diameter pin and 180 mm diameter disc were subjected to varying sliding velocities(0.5,0.75 and 1.5 m/s)and were operated in 200,400,500 and 580°C temperature at 600 Torr vacuum.The variation of specific wear rates with sliding velocities and different environmental conditions was studied.The morphology of sliding/rubbed surfaces was observed using Scanning Electron Microscope.In summary,it was found that a severe to mild wear transition occurred in sliding under operating conditions.Increased wear rates have been observed for 500 and 580°C with increasing sliding velocity.Adhesive wear has been found to be predominant at 500 and 580°C where as de-lamination has been observed at ambient temperature,200 and 400°C in vacuum.The present paper also carried out the numerical analysis of the vibration behavior of AISI 316 L under thermal environment.Results revealed that at high temperature vibrational amplitude and natural frequency is significantly reduced.This can be attributed to the reduction in stiffness of the material at elevated temperatures.This high amplitude vibration during service can lead to high wear rate.

1.Introduction

In recent years,an interest towards the sliding wear behavior of different types of stainless steels is under research.Among these,austenitic stainless steel is one of the important classifications,which is used in various applications.Industrial applications of aluminium and its alloys are restricted because of their poor tribological properties[1],so it is important to make use of stainless steels.Mainly this type of steelthatis a high-temperature structural material,have been used widely in chemical,power generation plants and military applications,because of its high toughness and superior corrosion resistance.Most specifically the stainless steels are used in missile structurals,aircraft missile bearings,engine,missile casings.So,it is very essential to analyse the wear behavior of such materials used in military applications under thermal environment.Lef fler[2]explains the effect of alloy constituents on steel.The presence of chromium increases the resistance of corrosion and oxidation.It promotes the formation of ferrite structure.Nickel is responsible for the stabilization of the austenite at ambient temperatures.It promotes ductility and toughness and resists corrosion.Molybdenum increases resistance of general and localized corrosion.The carbon increases the mechanical strength as well as increases susceptibility of corrosion.In particular SS316L has a lower proportion of carbon compared to chromium,preventing the formation of chromium carbide,an important corrosion by-product in stainless steels.The oxidative wear of metals can be divided into 2 regimes namely mild and severe wear.At lower loads,mild wear occurs where the debris is fine and mostly consists of oxide particles and in further process,the surface becomes polished.At higher loads,severe wear occurs due to high contact resistance and the debris is coarse and the surface becomes rough[3].The Pioneer work in this field was done by Bowden and Tabor[4]suggested regarding the metals high pressures sliding contact with individual contact spots,causing local/adhesion,were subsequently sheared by relative sliding on the surfaces.If the asperities get sheared at the plane of interaction,no wear particles will be produced,but it may also happen that the transfer of material might occur from one metal to the other.These results in the formation of pockmarked craters due to adhesive debris removal.Adhesive debris is formed when one surface is harder,and then the harder asperities plough a groove on the surface of the softer metal.These theories were extensively acknowledged regarding the friction of metals.Jahanmir et al.[5]conducted wear microscopic observation regarding the formation of sheet through delamination process.The wear tests were conducted on 8 different alloys like AISI 1020 steel and doped AISI 1020 steel using cylindercylinder set up.The surface features and sub-surface deformation were observed using SEM images.The specimens were crosssectioned parallel to the track and were subjected to metallographically polished and etched to resolve the surface beneath the crack.The detached wear sheet region from the wear track,shows the formation of shear dimples(at low speed)and ductile fracture broken needle points.The cracks are nucleated by the coalescence of voids below the surface,which shear up to the surface and give an appearance of ductile fracture.the wear de-lamination theory have been explained by Suh[6]regarding the wear process in sliding contact of metals.This wear necessitated,as the adhesion theory was unable to explain the loose wear debris formation.The theory states that the asperities observed on the softer surface wereflattered by the repeated plastic loading by the harder asperities.When the contact stresses exerted are in between the elastic and yield limit,the softer asperities were exposed to repeated stress cycles,which culminate in the formation of a wear particle.The theory explains that cracks are nucleated at a certain depth below the surface that propagates to the sliding direction.On reaching the surface,wear particles(plate like structures)were formed.Suh et al.[7]conducted sliding wear tests on SS 304 and titanium in self-mating conditions,using a pin on ring geometry,in atmospheric conditions at a speed of 0.5-10 m/s,as a result,the wear rate versus sliding speed is a complex function of coefficient of friction and toughness.Microscopic examination of wear particles and sub-surface of worn specimen indicated that the wear was predominantly by sub-surface deformation,crack nucleation and growth process.At speeds between 1 m/s and 5 m/s,the metal flowed extensively and mushrooming effect was observed.The coefficient of friction was also found to decrease.At higher speeds the wear rate dropped which is attributed to an increase in toughness and dissolution if carbides are formed.It is explained that the wear rate was higher than 1 microns,as there was time for the formation of a sufficient thickness of oxide coating which acts as an abrasive leading to greater wear.If the thickness is less than a critical limit,then the oxidefilm acts as a protective tribo- films and the wear is less.Honey Combe[8]provided useful facts on the properties of austenitic stainless steel at medium and elevated temperatures.If austenitic stainless steels are maintained in the temperature range of 500-800°C and slowly cooled,it will reject carbon from the supersaturated solution,which precipitate as chromium carbides at the grain boundaries resulting in inter granular corrosion.This results in severe wear as strength and stiffness of the material drastically reduces.Hence to combat this corrosion,the author recommends the usage of SS316L having lesser carbon instead of 316.The low carbon content also aids the partial transformation of austenite to martensite at room temperature due to deformation processes.

Smith[9]conducted sliding wear tests on a reciprocating test bed for SS316 steel in a self mating configuration,in a variety at atmospheres-air,CO2argon and partial vacuum.In all the atmospheres,the prow formation was predominantly seen due to the breakdown of wear particles.Initially,the formation of prow growth that is the formation of transfer elements by severity interactions and sub-sequent accumulations form plateau.Thus the plateau is the elevated region about shared area,which is in contact with debris particles and act as load bearing areas.These stainless steels are considered as poor galling and wear resistance.But the increasing use of sliding stainless steel interfaces in both the gas cooled nuclear reactor(CAGR)as well as boiler systems,makes it essential to understand the fundamentals regarding the sliding wear behavior.The wear rate decreases when the platelet size decreases.The formation of platelet was explained by the delamination theory.Plasma nitride sliding wear behavior on SS 316 was investigated by Suh et al.[10]using alumina and bearing steel act as counter faces.The untreated wear specimen on steel was occurred in a severe mode which results in the formation of plate like wear debris in a rough metallic plate.These types of wears typically adhesive wear.With the formation of prows and cracks,the worn surface was severely deformed.Van Harpen et al.[11]conducted sliding methodological wear tests at room temperature on SS304L claddings.The wear couple constituted a tube in thick annular ring.Severe oxidative wear was reported where the protective film of chromium carbide was removed and the fresh oxide layer was formed.The oxidefilm breaks off as it reaches its critical thickness as explained by wear kinetics.Significant changes occur in the micro mechanical state.The microstructures show evidence of strain-hardened dark bands,which are mechanical twins on the tube.XRD spectra on the worn area indicated the existence of α′-martensite which was formed from austenite.These two evidences are conclusive to show the occurrence of strain hardening.Li et al.[12]did sliding wear tests of SS316 against cemented tungsten carbide ball as the counter face on a pin-on-disc tribometer.The wear of untreated 316 was severe characterized by strong adhesion,abrasion and plastic deformation.The SEM micrographs of wear tracks suggested the existence of adhesive wear,presence of smeared surface and wear particles(plate like structures).With greater sliding distance,the increased contact resulted in lower contact stresses and higher debris amount in the wear track which results in the rate of wear deduction.An-other reason given is the marten site phase transformation due to work hardening that was confirmed by micro-hardness tests.Jayahari et al.[13]carried out the metallographic studies on ASS-304 S S at different thermal environment under warm deep drawing.They observed,the change in microstructure for deep draw cups at different thermal environment.Raj et al.[14]studied the wear and sliding friction characteristics of stainless steel SS316LN.They found that formation of adhesion layer and the self welding of mating parts took place at temperature above 500°C.It has been concluded that oxygen content load and temperature have an in fluence on friction coefficient.

Many researchers has not carried out the much research on effect of vibration amplitude on wear rate.Chowdhury and Helali[15]shows that the value of friction coefficient increases with the increase of natural frequency of vibration and decreases with increasing amplitude of vibration.The high vibration amplitude will result in high wear rate of the material,so it is very important to study the vibration behavior of the SS316L.Since the study is mainly focused in thermal environment,the vibrational study also carried out in thermal environment.Arunkumar et al.[16-18]studied the free and forced vibration behavior of orthotropic plate and they predicted that vibration behavior is in fluenced by the properties of the materials.The present paper also predicts the vibration behavior of SS316L under thermal environment.The code used to predict the vibration behavior is used in the present manuscript.A lot of research was carried out in the area concerning the sliding wear behaviour of stainless steels.Stainless steel of type AISI 316 L is proposed as one of the suited structural materials in sodium cooled fast reactors.Most of the manufactured stainless steel(SS316L)reactor components design life,which are under contact and sliding,have to overcome different wear types like galling,adhesion,fretting and abrasion.This present investigation describes the Sliding Wear and Friction Characteristics of SS316L at Elevated Temperature and Vacuum Condition.Also in the present paper,vibration behaviour of SS316L thin walled cylinder subjected to harmonic excitation under thermal environment.Since,it is necessary to maintain appropriate level of natural frequency and amplitude of vibration to improve mechanical processes.

2.Experimentation

2.1.Test specimen

The specimen of SS316L grade had been purchased in the form of rod of diameter 20 mm and it is machined to 10 mm as per the mechanics of contact.The chemical composition of AISI 316 L has been given in Table 1.When surface is brought into contact under load,they contact each other at the high spots called asperities.The number of contacts and their distribution depends on the nature of test pins prepared.The exact contact area was designed to be a diameter of 4 mm and 2 mm to a length of 7 mm from one end of the diameter reduced test pin.The disc is prepared from the similar stainless steelwith a diameterof 180 mm and thickness 10 mm.The surface is finely finished in a surface grinding machine.The prepared specimen in the form of pin and disc are shown in Fig.1.

2.2.Wear tests

The sliding wear test has been performed on a Purpose buit Pino-Disc Tribometer.The pins were held securely in a pin holder by two screws.Two holes were provided in the holder to insert thermocouple wires to record the temperature in the vicinity of the pin.The disc was held in a fixture,connected to a magnetically coupled drive,which was driven by a motor.The pin is made to touch.The sliding tests were conducted at high temperatures 200,400,500 and 580°C in vacuum condition.The experiments were conducted at sliding velocities of 0.15,0.75 and 1.5 m/s.

Vacuum was created in the chamber by a roughing pump and a turbo molecular pump.The roughing pump reached the pressure in the chamber to 0.01 mbar after which the molecular pump spurred into action reducing the pressure to 0.00035 mbar.For higher temperatures,an induction furnace was placed in the chamber enclosing the pin and disc.The temperature and heating rate of the furnace was controlled using a potentiometer.The sliding was started only after steady state thermal condition was achieved.A load cell and an linear variable differential transducer(Range:1000μm-2000μm)were used to measure the frictional force measurements and the depth loss.The specific wear rate of the pin is given by Equation 1

where,h is the depth loss measurement by linear variable differential transducer in mm,A is the apparent area of contact in mm2,W is the normal load acting on the pin in N and S is the sliding distance in m.

Table 1 Chemical composition of AISI316L.

3.Results and discussions

3.1.Tribographs

The tribo-graphs below shows the wear resistance behaviour of SS316L at different temperatures with various sliding distances.The variables are sliding velocity and frictional force.The wear rate were studied at various temperatures such as 200,400,500 and 580°C with the velocities of 0.15,0.75 and 1.5 m/s.

From Fig.2,the wear rate varies from 0.001 to 0.006 mm3/Nm with the velocities from 0.15 to 1.5 m/s.At 200°C the wear rate increases more rapidly with the sliding velocity.Highest wear rate(0.001892 mm3/Nm)is observed for 1.5 m/s sliding velocity at 200°C.The wear curves for the sliding velocities such as 0.15 and 0.75 are entirely different from the curve drawn for 1.5 sliding velocity.At sliding velocity 1.5 m/s the wear rate is varied linearly with temperature up to 425°C,beyond that wear rate is decreased gradually with temperature.But more wear rate is observed when the sliding velocity is 1.5 m/s.Irrespective of sliding distance the wear rate increased at higher temperature.It is clear that the material is undergone adhesive wear.The material is plucked out and transferred to the surface.Fig.3 is drawn between specific wear rate with sliding velocity for different temperature.From the graph with the increase in the sliding velocity,the wear rate increases linearly.Especially at the temperatures such as 400,500 and 580°C the wear rate linearly increases with sliding velocity.

The graphs shown in Fig.4 were drawn with coefficient of friction as a function of sliding distance for specimen tested at different temperatures(200,400,500 and 580°C).As per the law of friction,when applied force is enough and tangential motion occurs,the friction force always acts in an opposite direction to that of the relative velocity of the surfaces.Coefficientof friction reduces at a faster rate just after start(low sliding speed)but the variation decreases as the sliding speed increases.At very low speed,theμis quite high.Again at very high sliding speed,surface melting occurs which works as lubricant and produces a very low coefficient of friction.The friction force has weak dependence on the roughness of the sliding surfaces.The coefficient of friction increases initially and decreases when the experiment continues and reaches constant value.This is observed due to the fact that the material undergoes plastic deformation at high temperatures and due to this there is an increase in area,throwing out and squeezing outof some surface films.Also,at high temperatures,surface melting occurs which works as lubricant and produces low coefficient of friction.The friction force is independent of the apparent area of contact but is dependent on real area of contact.It is noticed from Fig.5 that the load applied is 10 MPa and sliding velocities are 0.15,0.75 and 1.5 m/s.The pin shows more plastic deformation.This behavior is noticed due to formation of loose materials.The loose materials are formed due to the chemical changes and the residual elastic energy in the adherent fragments.In steel materials,the loose wear particles formation is increased as the sliding velocity and temperature is increased.It is also observed that there is an increase in wear,as sliding velocity increases for all environmental condition considered.

3.2.SEM observations

Fig.1.(A)Photograph of AISI 316 L Pin of 10 mm height(contact area 2 mm and 4 mm)(b)Photograph of AISI 316 L Disc of diameter 180 mm and 10 mm thickness.

Fig.2.Specific wear rate as a function of temperature,for sliding velocities 0.15,0.75 and 1.5 m/s.

The surface morphology of the pin was studied by taking SEM micrographs.The pin was sectioned parallel on the sliding direction to observe the sub-surface deformation.It is well known that temperature induces micro structural as well as mechanical changes in metal and alloys.Hordon[13]has said that the hardening of metals depends on temperature.When temperature becomes high,the material loses its hardness.This increased the tendency of asperities to adhere,resulting in higher wear rates.Fig.6(a)shows the pin wear,when the pin slides at 0.15 m/s and at operating temperature 200°C.It was evident from the plough marks available in the worn surface of the pin.It is concluded that the pin shows delaminative wear.The SEM images showed surface contours due to ploughing and the formation of Shear dimples which indicate delaminative wear.Fig.6(b)showed the pin wear,when the pin slides at 0.15 m/s and at operating temperature 400°C.The SEM images show the formation of adhesion craters.It is also evident that there is extensive sub surface deformation due to the adhesion craters and oxide layers formed.

Fig.3.Specific wear rate as a function of sliding velocity,for elevated temperatures 200 °C,400 °C,500 °C and 580 °C.

Fig.4.Coefficient of friction as a function of sliding distance.

Fig.5.Wear as a function of Sliding distance.

Fig.6(c)showed the pin wears,when the pin slides at 0.15 m/s and at operating temperature 500°C.At 500°C,small granular structures like prows and plateaus have been observed.These could inter granular corrosion products,which decreased the strength of the materials and made them susceptible to severe wear.From Fig.6(d),it was evident that extensive flow coupled with material removal in the form of plateaus and oxide debris.The SEM image revealed that the mechanism of wear was seen to be an adhesive one.In Fig.7(a),SEM micrograph clearly showed the adhesive wear pockets such as scub,debris and plateaus.The wear debris obtained was a mixture of metal debris,which was adhered to the disc material.At higher velocities,a severe to mild wear transaction took place.In Fig.7(b),the specimen pin has a feathery black structure,which is wear debris which were the cause for multilayered delamination.Adhesive wear appeared to be the dominant wear mechanism.At 400°C operating temperature,wear was due to the extensive area contact/junction growth which continued unimpeded in the absence of surface films,resulting in heavy surface extrusion of metal.In Fig.7(c),the SEM micrographs showed the formation of well-defined grooves and ridges due to ploughing action.Flattened ridges were formed due to the successive layers of deformed metal and piled up one above the other.The wear mechanism was suspected to be delamination as it was evident from the sheared dimple like structures with needle like ductile fracture.From Fig.7(d),it was evident that the sub-surface voids may have coalesced to initiate cracks,which was propagated to the surface and sheared off the material.The SEM images showed extensive plastic deformation indicating the high initial wear rates due to oxides formation and severity in plough marks.

Fig.6.SEM image for pin slide at sliding velocity 0.15 m/s and operating temperatures.

Fig.7.SEM image for pin slide at sliding velocity 1.5 m/s and operating temperatures.

4.Free and forced vibration response

In this section,free and forced vibration response of SS316L Cylindrical shell of outer diameter of 20 m,inner diameter 19 m and height 25 m is analyzed.The cylindrical shell is modeled by extracting mid surface of the cylinder.Then the cylinder is descritized by meshing with four noded quadrilateral layered structural shell element(SHELL 181).The material properties of SS 316 L are elastic modulus=2×105GPa,poisson's ratio(ν)=0.265,material density(ρ)=8027 kg/m3and damping ratio=0.02 have been assigned accordingly with options available in ANSYS.

Initially free vibration behavior is predicted by solving the Eigen value problem as given below

where,K represents structural stiffness matrix,M is the structural mass matrix,while ωkis the circular natural frequency and φkis the corresponding mode shape.Further,harmonic response analysis was carried out to find the forced vibration response.The general equation of motion for harmonic analysis is as follows

Where,C is the damping matrix,F(t)the applied load vector(assumed time-harmonic),Ü,U˙,U are the acceleration,velocity and displacement vector of the cylindrical shell.In the present study,regarding the work vibration,the responses were calculated using commercial finite element.SHELL 181 is used to carry out the analysis.

Table 2,shows the effect of temperature(200°C,400°C,600°C and 1000°C)on free vibration response of SS 316 L clamped cylindrical shell(clamped at both bottom and top of the cylinder).

From Table 2,it is clear that effect of temperature on free vibration behavior is significant.This can be attributed to the decrease in stiffness when temperature increases.It is noticed that lower temperature,natural frequency is high.It can be concluded that friction coefficient can decrease at high temperature.In forced vibration study,a magnitude of 1 N/m2dynamic pressure is applied inside the cylinder and its vibration response root mean square velocity is calculated.Fig.8 shows the forced vibration response of SS 316 L clamped cylindrical shell.From Fig.8,it is clear that the amplitude of vibration response increases with increase in temperature.High amplitude of vibration response is seen in high temperatures.This can be associated to the decrease in stiffness of the material at high temperatures.As stiffness reduces,the material is flexible to attain its mode shape easily.

5.Conclusion

In the present work,a pin-on-disc tribometer is used to deduce the wear mechanisms in self-mating SS316L.Also numerical analysis of vibration behavior of SS 316 L cylindrical shell under thermal environment is studied.In the vibration study,interest is on effect of temperature on natural frequency and amplitude,since natural frequency and amplitude of vibration response as direct relation with frictional coefficient of the material.The conclusions that can be drawn from the obtained experimental and numerical data are:

·In mating SS316L pin and SS316L disc,it is evident that there is adhesive-oxidative type of interaction.

·De-lamination wear was found to be predominant in vacuum at 200°C and 400°C whereas ad-hesive wear was observed at 500°C and 580°C.

·Wear rates have been found to be excessive at 0.15 m/s,and at higher velocities in 500°C and 580°C.All these have occurreddue to adhesion.Hence a safe operating range for this steel pair is below steel couple at higher temperatures.

Table 2 In fluence of temperature on natural frequency(Hz).

Fig.8.Forced Vibration response(Vrms Vs Frequency).

·Natural frequency decreases with increase in temperature due to decrease in stiffness.

·Amplitude of forced vibration response at resonant frequency increases with increase in tem-perature due to decrease in stiffness.